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Sample records for reaction rate constant

  1. Rate constant for the Cl + Chloroform reaction

    SciTech Connect

    Brahan, K.M.; Hewitt, A.D.; Boone, G.D.

    1995-12-01

    The rate constant for the reaction of Cl with chloroform has been measured relative to the reaction of Cl with methane, methyl chloride, and carbon monoxide using long path FTIR spectroscopy. Our measured rate constant at 298 K and 1 atm of (1.2{plus_minus}0.1)x10{sup -13} cm{sup 3}molecule{sup -1}sec{sup -1} is 50% higher than that recommended for stratospheric modeling; however, it is within experimental error of the values measured by Clyne and Walker and by Watson. The implications of this reaction on the budgets of chloroform and phosgene in the troposphere will be discussed.

  2. Rate constant for reaction of atomic hydrogen with germane

    NASA Technical Reports Server (NTRS)

    Nava, David F.; Payne, Walter A.; Marston, George; Stief, Louis J.

    1990-01-01

    Due to the interest in the chemistry of germane in the atmospheres of Jupiter and Saturn, and because previously reported kinetic reaction rate studies at 298 K gave results differing by a factor of 200, laboratory measurements were performed to determine the reaction rate constant for H + GeH4. Results of the study at 298 K, obtained via the direct technique of flash photolysis-resonance fluorescence, yield the reaction rate constant, k = (4.08 + or - 0.22) x 10(exp -12) cu cm/s.

  3. Reaction sintering of zinc ferrite during constant rates of heating

    SciTech Connect

    Rahaman, M.N. . Ceramic Engineering Dept.); De Jonghe, L.C. . Lawrence Berkeley Lab.)

    1993-07-01

    The reaction sintering of equimolar quantities of zinc oxide and ferric oxide was investigated under conditions of constant rates of heating and the data were compared with those for a calcined, single-phase zinc ferrite powder. For the heating rate of 1C/min, the densifications of the reaction-sintered sample and the calcined sample were approximately the same. However, as the heating rate increased, the density at any temperature increased slightly for the reaction-sintered sample but decreased slightly for the calcined powder. The factors responsible for this slight difference in sintering between the reaction-sintered sample and the calcined sample are discussed. For the constant heating rates used, the reaction was completed prior to any significant densification. Relative densities of >95% were obtained for both the reaction-sintered sample and the calcined sample under identical sintering conditions (1-10C/min to 1350C). Reaction sintering in a steep temperature gradient produced a nearly fully dense body prior to complete reaction; a composite microstructure consisting of fine zinc oxide grains in matrix of zinc ferrite was obtained.

  4. Reaction rate constant for radiative association of CF+

    NASA Astrophysics Data System (ADS)

    Ã-ström, Jonatan; Bezrukov, Dmitry S.; Nyman, Gunnar; Gustafsson, Magnus

    2016-01-01

    Reaction rate constants and cross sections are computed for the radiative association of carbon cations (C+) and fluorine atoms (F) in their ground states. We consider reactions through the electronic transition 11Π → X1Σ+ and rovibrational transitions on the X1Σ+ and a3Π potentials. Semiclassical and classical methods are used for the direct contribution and Breit-Wigner theory for the resonance contribution. Quantum mechanical perturbation theory is used for comparison. A modified formulation of the classical method applicable to permanent dipoles of unequally charged reactants is implemented. The total rate constant is fitted to the Arrhenius-Kooij formula in five temperature intervals with a relative difference of <3%. The fit parameters will be added to the online database KIDA. For a temperature of 10-250 K, the rate constant is about 10-21 cm3 s-1, rising toward 10-16 cm3 s-1 for a temperature of 30 000 K.

  5. Prediction of Rate Constants for Catalytic Reactions with Chemical Accuracy.

    PubMed

    Catlow, C Richard A

    2016-08-01

    Ex machina: A computational method for predicting rate constants for reactions within microporous zeolite catalysts with chemical accuracy has recently been reported. A key feature of this method is a stepwise QM/MM approach that allows accuracy to be achieved while using realistic models with accessible computer resources.

  6. Rate constants for reactions of perhaloalkylperoxyl radicals with alkenes

    SciTech Connect

    Alfassi, Z.B.; Huie, R.E.; Neta, P. )

    1993-07-01

    Trichloro- and tribromomethylperoxyl radicals were produced by radiolytic reduction of CCl[sub 4] and CBr[sub 4] in various aerated solvents. Rate constants for the reactions of these radicals with unsaturated organic compounds were determined in methanol solutions by competition kinetics, using mainly chloropromazine as a reference. The rate constants were in the range k = 10[sup 4]-10[sup 7] L mol[sup [minus]1] s[sup [minus]1], and a good correlation was obtained between log k and the Taft substituent constants [sigma]*. Steric effects were also noticeable in some cases. The reactivities of the radicals were in the order CBr[sub 3]O[sub 2][center dot] < CCl[sub 3]O[sub 2][center dot] < C[sub 4]F[sub 9]O[sub 2][center dot], which is in contrast with the order observed in electron-transfer reactions. The rate constants for reaction of CCl[sub 3]O[sub 2][center dot] with 2,3-dimethyl-2-butene (DMB) and cyclohexene (CH) were studied in 12 solvents. In all cases DMB is considerably more reactive than CH, indicating that at least in the case of DMB the reaction is predominantly via addition. The rate constant for addition of CCl[sub 3]O[sub 2][center dot] to DMB varies by a factor of 7 in the different solvents. No correlation was found between these rate constants and thermodynamic or polarity parameters of the solvents, although viscosity appears to have an effect. 19 refs., 2 figs., 2 tabs.

  7. Semiclassical Calculation of Reaction Rate Constants for Homolytical Dissociations

    NASA Technical Reports Server (NTRS)

    Cardelino, Beatriz H.

    2002-01-01

    There is growing interest in extending organometallic chemical vapor deposition (OMCVD) to III-V materials that exhibit large thermal decomposition at their optimum growth temperature, such as indium nitride. The group III nitrides are candidate materials for light-emitting diodes and semiconductor lasers operating into the blue and ultraviolet regions. To overcome decomposition of the deposited compound, the reaction must be conducted at high pressures, which causes problems of uniformity. Microgravity may provide the venue for maintaining conditions of laminar flow under high pressure. Since the selection of optimized parameters becomes crucial when performing experiments in microgravity, efforts are presently geared to the development of computational OMCVD models that will couple the reactor fluid dynamics with its chemical kinetics. In the present study, we developed a method to calculate reaction rate constants for the homolytic dissociation of III-V compounds for modeling OMCVD. The method is validated by comparing calculations with experimental reaction rate constants.

  8. Semiclassical Calculation of Reaction Rate Constants for Homolytical Dissociations

    NASA Technical Reports Server (NTRS)

    Cardelino, Beatriz H.

    2002-01-01

    There is growing interest in extending organometallic chemical vapor deposition (OMCVD) to III-V materials that exhibit large thermal decomposition at their optimum growth temperature, such as indium nitride. The group III nitrides are candidate materials for light-emitting diodes and semiconductor lasers operating into the blue and ultraviolet regions. To overcome decomposition of the deposited compound, the reaction must be conducted at high pressures, which causes problems of uniformity. Microgravity may provide the venue for maintaining conditions of laminar flow under high pressure. Since the selection of optimized parameters becomes crucial when performing experiments in microgravity, efforts are presently geared to the development of computational OMCVD models that will couple the reactor fluid dynamics with its chemical kinetics. In the present study, we developed a method to calculate reaction rate constants for the homolytic dissociation of III-V compounds for modeling OMCVD. The method is validated by comparing calculations with experimental reaction rate constants.

  9. On reaction rate constants IO+NO, IO+CO

    NASA Astrophysics Data System (ADS)

    Larin, Igor; Spasskiy, Alexandr; Trofimova, Elen

    2013-04-01

    A method, allowing to define, whether an investigated chemical reaction in a gas phase or on a surface of a reactor for those cases when dependence of the reaction rate on concentration of reagents for heterogeneous and for homogeneous reaction are identical, has been suggested. The method has been used for investigation of reactions of IO with nitric oxide and carbon monoxide. Experiments have been run in the flow reactor with using a method of resonance fluorescence to record atoms of iodine. It has been proved that the reaction of radical IO with nitric oxide proceeds in the gas phase, whereas the reaction of radical IO with carbon monoxide proceeds on the surface of the reactor. It has been obtained that reaction rate constant of IO with nitrogen monoxide in the temperature range 298-353 K is kNO(T)=(1,01±0,2)•10^-11•exp((210±80)/T) cm3*molecule*s-1 and the same for reaction of IO with carbon monoxide is kCO(T)=(1,14±0,64)•10^-14•exp((612±41)/T) cm3*molecule*s-1.

  10. Reaction rate constant for radiative association of CF{sup +}

    SciTech Connect

    Öström, Jonatan Gustafsson, Magnus; Bezrukov, Dmitry S.; Nyman, Gunnar

    2016-01-28

    Reaction rate constants and cross sections are computed for the radiative association of carbon cations (C{sup +}) and fluorine atoms (F) in their ground states. We consider reactions through the electronic transition 1{sup 1}Π → X{sup 1}Σ{sup +} and rovibrational transitions on the X{sup 1}Σ{sup +} and a{sup 3}Π potentials. Semiclassical and classical methods are used for the direct contribution and Breit–Wigner theory for the resonance contribution. Quantum mechanical perturbation theory is used for comparison. A modified formulation of the classical method applicable to permanent dipoles of unequally charged reactants is implemented. The total rate constant is fitted to the Arrhenius–Kooij formula in five temperature intervals with a relative difference of <3%. The fit parameters will be added to the online database KIDA. For a temperature of 10–250 K, the rate constant is about 10{sup −21} cm{sup 3} s{sup −1}, rising toward 10{sup −16} cm{sup 3} s{sup −1} for a temperature of 30 000 K.

  11. Reaction rate constant for radiative association of CF(.).

    PubMed

    Öström, Jonatan; Bezrukov, Dmitry S; Nyman, Gunnar; Gustafsson, Magnus

    2016-01-28

    Reaction rate constants and cross sections are computed for the radiative association of carbon cations (C(+)) and fluorine atoms (F) in their ground states. We consider reactions through the electronic transition 1(1)Π → X(1)Σ(+) and rovibrational transitions on the X(1)Σ(+) and a(3)Π potentials. Semiclassical and classical methods are used for the direct contribution and Breit-Wigner theory for the resonance contribution. Quantum mechanical perturbation theory is used for comparison. A modified formulation of the classical method applicable to permanent dipoles of unequally charged reactants is implemented. The total rate constant is fitted to the Arrhenius-Kooij formula in five temperature intervals with a relative difference of <3%. The fit parameters will be added to the online database KIDA. For a temperature of 10-250 K, the rate constant is about 10(-21) cm(3) s(-1), rising toward 10(-16) cm(3) s(-1) for a temperature of 30,000 K.

  12. Rate Constant for the OH + CO Reaction at Low Temperatures.

    PubMed

    Liu, Yingdi; Sander, Stanley P

    2015-10-01

    Rate constants for the reaction of OH + CO → products (1) have been measured using laser photolysis/laser-induced fluorescence (LP/LIF) over the temperature range 193–296 K and at pressures of 50–700 Torr of Ar and N2. The reaction was studied under pseudo-first-order conditions, monitoring the decay of OH in the presence of a large excess of CO. The rate constants can be expressed as a combination of bimolecular and termolecular components. The bimolecular component was found to be temperature-independent with an expression given by kbi(T) = (1.54 ± 0.14) × 10(–13)[e(–(13±17)/T)] cm(3) molecule(–1) s(–1), with an error of one standard deviation. The termolecular component was fitted to the expression, kter = k0(T)[M]/[1 + (k0(T)[M]/k∞(T)] × 0.6({1+[log10(k0(T)[M]/k∞(T))]2}−1) where k0(T) = k0(300)(T/300)(−n) and k∞(T) = k∞(300)(T/300)(−m). The parameters for k0(T) were determined to be k0(300) = (6.0±0.5) × 10(−33) cm(6) molecule(–2) s(–1) in N2 and k0(300) = (3.4 ± 0.3) × 10(–33) cm(6) molecule(–2) s(–1) in Ar, with n = 1.9±0.5 and 2.0±0.4 in N2 and Ar, respectively. These parameters were determined using k0(T) and m from the NASA kinetics data evaluation (JPL Publication No. 10-6) since the experimental pressure range was far from the high-pressure limit. Addition of low concentrations of O2 had no discernible effect on the mechanism of the OH + CO reaction but resulted in secondary reactions which regenerated OH.

  13. A Priori Estimation of Rate Constants for Unimolecular Decomposition Reactions

    DTIC Science & Technology

    1979-02-01

    priori theoretical predictions for the decomposition rate of the formyl and the methoxy radicals have been made by application of the Rice-Ramsperger...Kassel-Marcus Theory. An ArrheniLus rate coefficient expression is derived for the formyl radical decomposition, and a modified Arrhenius type rate...9 IV. A PREDICTED RATE CONSTANT FOR FORMYL RADICAL DECOMPOSITION. .. .... .................... ........19 V. SUMMtARY

  14. The temperature dependence of the rate constant for the reaction of hydroxyl radicals with nitric acid

    NASA Technical Reports Server (NTRS)

    Kurylo, M. J.; Cornett, K. D.; Murphy, J. L.

    1982-01-01

    The rate constant for the reaction of hydroxyl radicals with nitric acid in the 225-443 K temperature range has been measured by means of the flash photolysis resonance fluorescence technique. Above 300 K, the rate constant levels off in a way that can only be explained by the occurrence of two reaction channels, of which one, operative at low temperatures, proceeds through the formation of an adduct intermediate. The implications of these rate constant values for stratospheric reaction constants is discussed.

  15. The temperature dependence of the rate constant for the reaction of hydroxyl radicals with nitric acid

    NASA Technical Reports Server (NTRS)

    Kurylo, M. J.; Cornett, K. D.; Murphy, J. L.

    1982-01-01

    The rate constant for the reaction of hydroxyl radicals with nitric acid in the 225-443 K temperature range has been measured by means of the flash photolysis resonance fluorescence technique. Above 300 K, the rate constant levels off in a way that can only be explained by the occurrence of two reaction channels, of which one, operative at low temperatures, proceeds through the formation of an adduct intermediate. The implications of these rate constant values for stratospheric reaction constants is discussed.

  16. Rate constant and reaction channels for the reaction of atomic nitrogen with the ethyl radical

    SciTech Connect

    Stief, L.J.; Nesbitt, F.L.; Payne, W.A. ); Kuo, S.C.; Tao, W.; Klemm, R.B. )

    1995-04-01

    The absolute rate constant and primary reaction products have been determined at [ital T]=298 K for the atom--radical reaction N([sup 4][ital S])+C[sub 2]H[sub 5] in a discharge flow system with collision-free sampling to a mass spectrometer. The rate constant measurements employed low energy electron impact ionization while the product study used dispersed synchrotron radiation as the photoionization source. The rate constant was determined under pseudo-first-order conditions by monitoring the decay of C[sub 2]H[sub 5] or C[sub 2]D[sub 5] as a function of time in the presence of excess N atoms. The result is [ital k]=(1.1[plus minus]0.3)[times]10[sup [minus]10] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1]. For the reaction product experiments using photoionization mass spectrometry, products observed at 114 nm (10.9 eV) were CD[sub 3], D[sub 2]CN and C[sub 2]D[sub 4] for the N+C[sub 2]D[sub 5] reaction. The product identification is based on the unambiguous combination of product [ital m]/[ital z] values, the shift of the [ital m]/[ital z] peaks observed for the N+C[sub 2]D[sub 5] reaction products with respect to the N+C[sub 2]H[sub 5] reaction products and the photoionization threshold measured for the major products. The observed products are consistent with the occurrence of the reaction channels D[sub 2]CN+CD[sub 3](2a) and C[sub 2]D[sub 4]+ND(2c). Formation of C[sub 2]D[sub 4] product via channel (2c) accounts for approximately 65% of the C[sub 2]D[sub 5] reacted. Most, if not all, of the remaining 35% is probably accounted for by channel (2a). These rate constant and product results are compared with those for the N+CH[sub 3] reaction as well as other atom+C[sub 2]H[sub 5] reactions. The role of the N+C[sub 2]H[sub 5] reaction in the formation of HCN in the atmospheres of Titan and Neptune is briefly considered. (Abstract Truncated)

  17. Rate constants measured for hydrated electron reactions with peptides and proteins

    NASA Technical Reports Server (NTRS)

    Braams, R.

    1968-01-01

    Effects of ionizing radiation on the amino acids of proteins and the reactivity of the protonated amino group depends upon the pK subscript a of the group. Estimates of the rate constants for reactions involving the amino acid side chains are presented. These rate constants gave an approximate rate constant for three different protein molecules.

  18. The effect of temperature fluctuations of reaction rate constants in turbulent reacting flows

    NASA Technical Reports Server (NTRS)

    Chinitz, W.; Antaki, P. J.; Kassar, G. M.

    1981-01-01

    Current models of turbulent reacting flows frequently use Arrhenius reaction rate constants obtained from static or laminar flow theory and/or experiments, or from best fits of static, laminar, and turbulent data. By treating the reaction rate constant as a continuous random variable which is temperature-dependent, the present study assesses the effect of turbulent temperature fluctuations on the reaction rate constant. This model requires that a probability density function (PDF) describing the nature of the fluctuations be specified. Three PDFs are examined: the clipped Gaussian, the beta PDF, and the ramp model. All the models indicate that the reaction rate constant is greater in a turbulent flow field than in an equivalent laminar flow. In addition, an amplification ratio, which is the ratio of the turbulent rate constant to the laminar rate constant, is defined and its behavior as a function of the mean temperature fluctuations is described

  19. The effect of temperature fluctuations of reaction rate constants in turbulent reacting flows

    NASA Technical Reports Server (NTRS)

    Chinitz, W.; Antaki, P. J.; Kassar, G. M.

    1981-01-01

    Current models of turbulent reacting flows frequently use Arrhenius reaction rate constants obtained from static or laminar flow theory and/or experiments, or from best fits of static, laminar, and turbulent data. By treating the reaction rate constant as a continuous random variable which is temperature-dependent, the present study assesses the effect of turbulent temperature fluctuations on the reaction rate constant. This model requires that a probability density function (PDF) describing the nature of the fluctuations be specified. Three PDFs are examined: the clipped Gaussian, the beta PDF, and the ramp model. All the models indicate that the reaction rate constant is greater in a turbulent flow field than in an equivalent laminar flow. In addition, an amplification ratio, which is the ratio of the turbulent rate constant to the laminar rate constant, is defined and its behavior as a function of the mean temperature fluctuations is described

  20. Rate Constants for the Reactions of Hydroxyl Radical with Several Alkanes, Cycloalkanes, and Dimethyl Ether

    NASA Technical Reports Server (NTRS)

    DeMore, W.; Bayes, K.

    1998-01-01

    Relative rate experiements were used to measure rate constants and temperature denpendencies of the reactions of OH with propane, n-butane, n-pentane, n-hexane, cyclopropane, cyclobutane, cyclopentane, and dimethyl ether.

  1. Rate Constants for the Reactions of Hydroxyl Radical with Several Alkanes, Cycloalkanes, and Dimethyl Ether

    NASA Technical Reports Server (NTRS)

    DeMore, W.; Bayes, K.

    1998-01-01

    Relative rate experiements were used to measure rate constants and temperature denpendencies of the reactions of OH with propane, n-butane, n-pentane, n-hexane, cyclopropane, cyclobutane, cyclopentane, and dimethyl ether.

  2. Rate constants of the reaction of C2-C4 peroxy radicals with OH radicals

    NASA Astrophysics Data System (ADS)

    Assaf, Emmanuel; Tanaka, Shisei; Kajii, Yoshizumi; Schoemaecker, Coralie; Fittschen, Christa

    2017-09-01

    The rate constants for the reaction of three peroxy radicals with OH have been measured using different precursors. Peroxy radicals have been prepared by either the reaction of Cl-atoms with C2H6, C3H8 and n-C4H10 or through photolysis of the corresponding alkyliodide. Using Cl-atoms, the following rate constants have been measured: Experiments using 248 nm photolysis of the corresponding alkyliodides as precursor enhances the rate constants by a factor of two, bringing up the suspicion of a fast reaction between I-atoms and OH radicals.

  3. The Hydroxyl Radical Reaction Rate Constant and Products of Dimethyl Succinate

    DTIC Science & Technology

    2008-03-01

    AFRL-RX-TY-TR-2008-4522 THE HYDROXYL RADICAL REACTION RATE CONSTANT AND PRODUCTS OF DIMETHYL SUCCINATE Sheryl E. Calidonna and...the gas-phase hydroxyl radical reaction with dimethyl succinate. Additional reports and publications resulting from this work unit are not described...herein but are listed below for reference. Baxley, J.Steven and J.R. Wells, “The Hydroxyl Radical Rate Constant and Atmospheric Transforamtion

  4. Measuring kinetic rate constants of multiple-component reactions with optical biosensors.

    PubMed

    Edwards, David A; Evans, Ryan M; Li, Wenbin

    2017-09-15

    One may measure the kinetic rate constants associated with biochemical reactions using an optical biosensor: an instrument in which ligand molecules are convected through a flow cell over a surface to which receptors are immobilized. If there are multiple reactants, one is faced with the problem of fitting multiple kinetic rate constants to one signal, since data from all of the reacting species is lumped together. Even in the presence of ambiguous data, one may use a series of experiments to accurately determine the rate constants. Moreover, the true set of rate constants may be identified by either postprocessing the signals or adjusting the ligand inflow concentrations. Published by Elsevier Inc.

  5. Development of a group contribution method to predict aqueous phase hydroxyl radical (HO*) reaction rate constants.

    PubMed

    Minakata, Daisuke; Li, Ke; Westerhoff, Paul; Crittenden, John

    2009-08-15

    The hydroxyl radical (HO*) is a strong oxidant that reacts with electron-rich sites of organic compounds and initiates complex chain mechanisms. In order to help understand the reaction mechanisms, a rule-based model was previously developed to predict the reaction pathways. For a kinetic model, there is a need to develop a rate constant estimator that predicts the rate constants for a variety of organic compounds. In this study, a group contribution method (GCM) is developed to predict the aqueous phase HO* rate constants for the following reaction mechanisms: (1) H-atom abstraction, (2) HO* addition to alkenes, (3) HO* addition to aromatic compounds, and (4) HO* interaction with sulfur (S)-, nitrogen (N)-, or phosphorus (P)-atom-containing compounds. The GCM hypothesizes that an observed experimental rate constant for a given organic compound is the combined rate of all elementary reactions involving HO*, which can be estimated using the Arrhenius activation energy, E(a), and temperature. Each E(a) for those elementary reactions can be comprised of two parts: (1) a base part that includes a reactive bond in each reaction mechanism and (2) contributions from its neighboring functional groups. The GCM includes 66 group rate constants and 80 group contribution factors, which characterize each HO* reaction mechanism with steric effects of the chemical structure groups and impacts of the neighboring functional groups, respectively. Literature-reported experimental HO* rate constants for 310 and 124 compounds were used for calibration and prediction, respectively. The genetic algorithms were used to determine the group rate constants and group contribution factors. The group contribution factors for H-atom abstraction and HO* addition to the aromatic compounds were found to linearly correlate with the Taft constants, sigma*, and electrophilic substituent parameters, sigma+, respectively. The best calibrations for 83% (257 rate constants) and predictions for 62% (77

  6. Reaction mechanisms and rate constants of waste degradation in landfill bioreactor systems with enzymatic-enhancement.

    PubMed

    Jayasinghe, P A; Hettiaratchi, J P A; Mehrotra, A K; Kumar, S

    2014-06-01

    Augmenting leachate before recirculation with peroxidase enzymes is a novel method to increase the available carbon, and therefore the food supply to microorganisms at the declining phase of the anaerobic landfill bioreactor operation. In order to optimize the enzyme-catalyzed leachate recirculation process, it is necessary to identify the reaction mechanisms and determine rate constants. This paper presents a kinetic model developed to ascertain the reaction mechanisms and determine the rate constants for enzyme catalyzed anaerobic waste degradation. The maximum rate of reaction (Vmax) for MnP enzyme-catalyzed reactors was 0.076 g(TOC)/g(DS).day. The catalytic turnover number (k(cat)) of the MnP enzyme-catalyzed was 506.7 per day while the rate constant (k) of the un-catalyzed reaction was 0.012 per day. Copyright © 2014 Elsevier Ltd. All rights reserved.

  7. Reaction mechanism and rate constants of the CH+CH4 reaction: a theoretical study

    NASA Astrophysics Data System (ADS)

    Ribeiro, Joao Marcelo; Mebel, Alexander M.

    2015-07-01

    Ab initio and density functional calculations have been performed to elucidate the mechanism of CH radical insertion into methane. The results show that the reaction can be viewed to occur via two stages. On the first stage, the CH radical approaches methane without large structural changes to acquire proper positioning for the subsequent stage, where H-migration occurs from CH4 to CH, along with a C-C bond formation. Where the first stage ends and the second begins, a tight transition state was located using the B3LYP/6-311G(d,p) and MP4(SDQ)/6-311++G(d,p) methods. Using a rigid rotor - harmonic oscillator approach within transition state theory, we show that at the MP5/6-311++G(d,p)//MP4(SDQ)/6-311++G(d,p) level the calculated rate constants are in a reasonably good agreement with experiment in a broad temperature range of 145-581 K. Even at low temperatures, the insertion reaction bottleneck is found about the location of the tight transition state, rather than at long separations between the CH and CH4 reactants. In addition, high level CCSD(T)-F12/CBS calculations of the remainder of the C2H5 potential energy surface predict the CH+CH4 reaction to proceed via the initial insertion step to the ethyl radical which then can emit a hydrogen atom to form highly exothermic C2H4+H products.

  8. On the ambiguity of the reaction rate constants in multivariate curve resolution for reversible first-order reaction systems.

    PubMed

    Schröder, Henning; Sawall, Mathias; Kubis, Christoph; Selent, Detlef; Hess, Dieter; Franke, Robert; Börner, Armin; Neymeyr, Klaus

    2016-07-13

    If for a chemical reaction with a known reaction mechanism the concentration profiles are accessible only for certain species, e.g. only for the main product, then often the reaction rate constants cannot uniquely be determined from the concentration data. This is a well-known fact which includes the so-called slow-fast ambiguity. This work combines the question of unique or non-unique reaction rate constants with factor analytic methods of chemometrics. The idea is to reduce the rotational ambiguity of pure component factorizations by considering only those concentration factors which are possible solutions of the kinetic equations for a properly adapted set of reaction rate constants. The resulting set of reaction rate constants corresponds to those solutions of the rate equations which appear as feasible factors in a pure component factorization. The new analysis of the ambiguity of reaction rate constants extends recent research activities on the Area of Feasible Solutions (AFS). The consistency with a given chemical reaction scheme is shown to be a valuable tool in order to reduce the AFS. The new methods are applied to model and experimental data. Copyright © 2016 Elsevier B.V. All rights reserved.

  9. Critical evaluation and rate constants of chemoselective ligation reactions for stoichiometric conjugations in water.

    PubMed

    Saito, Fumito; Noda, Hidetoshi; Bode, Jeffrey W

    2015-04-17

    Chemoselective ligation reactions have contributed immensely to the development of organic synthesis and chemical biology. However, the ligation of stoichiometric amounts of large molecules for applications such as protein-protein conjugates is still challenging. Conjugation reactions need to be fast enough to proceed under dilute conditions and chemoselective in the presence of unprotected functional groups; the starting materials and products must be stable under the reaction conditions. To compare known ligation reactions for their suitability under these conditions, we determined the second-order rate constants of ligation reactions using peptide substrates with unprotected functional groups. The reaction conditions, the chemoselectivity of the reactions, and the stability of the starting materials and products were carefully evaluated. In some cases, the stability could be improved by modifying the substrate structure. These data obtained under the ligation conditions provide a useful guide to choose an appropriate ligation reaction for synthesis of large molecules by covalent ligation reactions of unprotected substrates in water.

  10. Absolute rate constant for the reaction of Cl(/sup 2/P) with CINO

    SciTech Connect

    Nesbitt, F.L.; Nava, D.F.; Payne, W.A.; Stief, L.J.

    1987-09-24

    The room temperature rate constant for the reaction Cl + CINO yields Cl/sub 2/ + NO has been measured by the method of discharge flow mass spectrometry. The rate constant was determined from the decay of CINO in the presence of an excess of Cl atoms at a total pressure of 1 Torr. The rate constant obtained was (7.6 +/- 0.8) X 10/sup -11/ cm/sup 3/ s/sup -1/. This result is compared with previous determinations, the values of which ranged by more than an order of magnitude and all of which depended on knowledge of the absolute concentration of CINO. The authors suggest that the lack of agreement is attributable principally to uncertainties in (CINO) resulting from absorption of this reactive species on glass and metal surfaces. Our result does not depend directly on (CINO) and supports the highest values published for this rate constant.

  11. Temperature Dependence of the Rate Constants of Charge Recombination Reactions in Bacterial Reaction Centers

    NASA Astrophysics Data System (ADS)

    Thuy, T. T.; Yen, V. T. H.; Thao, T. T.; Viet, Nguyen Ai

    The bacterial reaction center couples light-induced electron transfer via a tightly bound ubiquinone (QA) to a mobile ubiquinone (QB). Based on the electron transfer theory by Marcus, we have investigated the rate of charge recombination reactions from Rhodopseudomonas viridis and Rhodobacter sphaeroides, by mean of finding an approximation formula. The results obtained are verified for not only at high and low temperature as the previous works but also at the medium temperature range.

  12. Quantitative rate constants for radical reactions in the nanopores of cotton.

    PubMed

    Hunt, Paula; Worrall, David R; Wilkinson, Frank; Batchelor, Stephen N

    2002-07-24

    The understanding of radical reactions in nanostructured materials is important for developing new synthetic procedures and controlling degradation reactions. To develop this area, an easy method for measuring quantitative rate constants of some radical reactions in nanostructures is required. A simple method for measuring the rate constant of dye bleaching, kdye, by organic radicals in such materials is introduced, involving the measurement of microsecond bleaching kinetics by diffuse reflectance spectroscopy, following laser flash creation of the radicals. Using wet and dry cotton as model substrates, we obtained kdye of 2-hydroxy-2-propyl and 1-hydroxy-1-cyclohexyl radicals with reactive red 3 and reactive orange 4 and compared them to solution-phase values. Surprisingly, the reactions in cotton follow simple liquid-phase kinetics and are diffusion-controlled. A cage effect in cotton is also found.

  13. Ab Initio Calculation of Rate Constants for Molecule–Surface Reactions with Chemical Accuracy

    PubMed Central

    Piccini, GiovanniMaria; Alessio, Maristella

    2016-01-01

    Abstract The ab initio prediction of reaction rate constants for systems with hundreds of atoms with an accuracy that is comparable to experiment is a challenge for computational quantum chemistry. We present a divide‐and‐conquer strategy that departs from the potential energy surfaces obtained by standard density functional theory with inclusion of dispersion. The energies of the reactant and transition structures are refined by wavefunction‐type calculations for the reaction site. Thermal effects and entropies are calculated from vibrational partition functions, and the anharmonic frequencies are calculated separately for each vibrational mode. This method is applied to a key reaction of an industrially relevant catalytic process, the methylation of small alkenes over zeolites. The calculated reaction rate constants (free energies), pre‐exponential factors (entropies), and enthalpy barriers show that our computational strategy yields results that agree with experiment within chemical accuracy limits (less than one order of magnitude). PMID:27008460

  14. Rate constant for the reaction of atomic chlorine with formaldehyde from 200 to 500 K

    NASA Technical Reports Server (NTRS)

    Michael, J. V.; Nava, D. F.; Payne, W. A.

    1979-01-01

    The absolute rate constant for the reaction of atomic chlorine with formaldehyde has been measured from 200 to 500 K using the flash photolysis-resonance fluorescence technique. The results were independent of substantial variations in (H2CO), total pressure (Ar), and flash intensity (i.e., initial (Cl)). The rate constant was shown to be invariant with temperature, the best representation for this temperature range being k-sub-1 = (7.48 + or - 0.50) times 10 to the -11 cu cm/molecule sec, where the error is one standard deviation. This result is compared with the only previous determination of k-sub-1, which was a relative value obtained at 298 K. The rate constant is theoretically discussed, and the potential importance of the reaction in stratospheric chemistry is considered.

  15. Rate constant for the reaction of atomic chlorine with formaldehyde from 200 to 500 K

    NASA Technical Reports Server (NTRS)

    Michael, J. V.; Nava, D. F.; Payne, W. A.

    1979-01-01

    The absolute rate constant for the reaction of atomic chlorine with formaldehyde has been measured from 200 to 500 K using the flash photolysis-resonance fluorescence technique. The results were independent of substantial variations in (H2CO), total pressure (Ar), and flash intensity (i.e., initial (Cl)). The rate constant was shown to be invariant with temperature, the best representation for this temperature range being k-sub-1 = (7.48 + or - 0.50) times 10 to the -11 cu cm/molecule sec, where the error is one standard deviation. This result is compared with the only previous determination of k-sub-1, which was a relative value obtained at 298 K. The rate constant is theoretically discussed, and the potential importance of the reaction in stratospheric chemistry is considered.

  16. Rate constant calculations of H-atom abstraction reactions from ethers by HȮ2 radicals.

    PubMed

    Mendes, Jorge; Zhou, Chong-Wen; Curran, Henry J

    2014-02-27

    In this work, we detail hydrogen atom abstraction reactions from six ethers by the hydroperoxyl radical, including dimethyl ether, ethyl methyl ether, propyl methyl ether, isopropyl methyl ether, butyl methyl ether, and isobutyl methyl ether, in order to test the effect of the functional group on the rate constant calculations. The Møller-Plesset (MP2) method with the 6-311G(d,p) basis set has been employed in the geometry optimizations and frequency calculations of all of the species involved in the above reaction systems. The connections between each transition state and the corresponding local minima have been determined by intrinsic reaction coordinate calculations. Energies are reported at the CCSD(T)/cc-pVTZ level of theory and include the zero-point energy corrections. As a benchmark in the electronic energy calculations, the CCSD(T)/CBS extrapolation was used for the reactions of dimethyl ether + HȮ2 radicals. A systematic calculation of the high-pressure limit rate constants has been performed using conventional transition-state theory, including asymmetric Eckart tunneling corrections, in the temperature range of 500-2000 K. The one dimensional hindrance potentials obtained at MP2/6-311G(d,p) for the reactants and transition states have been used to describe the low frequency torsional modes. Herein, we report the calculated individual, average, and total rate constants. A branching ratio analysis for every reaction site has also been performed.

  17. Rate constants of the reactions of ozone with nitriles, acrylates and terpenes in gas phase

    NASA Astrophysics Data System (ADS)

    Munshi, Hushider B.; Rao, K. V. S. Rama; Iyer, R. Mahadeva

    Rate constants for the reactions of acrylonitrile, methacrylonitrile, methyl and ethyl acrylate with O 3 have been determined with the help of a flow system coupled to an u.v. spectrophotometer. The rate constants obtained have enabled an estimation of the tropospheric lifetimes with respect to O 3 and have also offered a clue to the nature of the initial ozone attack on the unsaturates. Non-stoichiometry observed in the case of acrylonitrile and ethyl acrylate has been rationalized by secondary free radical reactions. Absorptivities of the four unsaturates in vapour as well as in the solution phase have been reported. A brief attempt to study ozone-terpene reactions is also described.

  18. Rate constant for the reaction of O(3P) with diacetylene from 210 to 423 K

    NASA Technical Reports Server (NTRS)

    Mitchell, M. B.; Nava, D. F.; Stief, L. J.

    1986-01-01

    The absolute rate constant for the reaction of O(3P) with diacetylene (C4H2) has been measured as a function of pressure and temperature by the flash-photolysis/resonance-fluorescence method. At 298 K and below, no pressure dependence of the rate constant was observed, but at 423 K a moderate (factor-of-2) increase was detected in the range 3 to 75 torr Ar.Results at or near the high-pressure limit are represented by an Arrhenius expression over the temperature range 210 to 423 K. The results are compared with previous determinations, all of which employed the discharge-flow/mass-spectrometry technique. The mechanism of the reaction is considered, including both primary and secondary processes. The heats of formation of the reactants, adducts, and products for the O(3P) + C4H2 reaction are discussed and contrasted with those for O(3P) + C2H2.

  19. Determination of relative rate constants for in vitro RNA processing reactions by internal competition.

    PubMed

    Lin, Hsuan-Chun; Yandek, Lindsay E; Gjermeni, Ino; Harris, Michael E

    2014-12-15

    Studies of RNA recognition and catalysis typically involve measurement of rate constants for reactions of individual RNA sequence variants by fitting changes in substrate or product concentration to exponential or linear functions. A complementary approach is determination of relative rate constants by internal competition, which involves quantifying the time-dependent changes in substrate or product ratios in reactions containing multiple substrates. Here, we review approaches for determining relative rate constants by analysis of both substrate and product ratios and illustrate their application using the in vitro processing of precursor transfer RNA (tRNA) by ribonuclease P as a model system. The presence of inactive substrate populations is a common complicating factor in analysis of reactions involving RNA substrates, and approaches for quantitative correction of observed rate constants for these effects are illustrated. These results, together with recent applications in the literature, indicate that internal competition offers an alternate method for analyzing RNA processing kinetics using standard molecular biology methods that directly quantifies substrate specificity and may be extended to a range of applications. Copyright © 2014 Elsevier Inc. All rights reserved.

  20. Formulation of a universal first-order rate constant for enzymatic reactions.

    PubMed

    Imoto, Taiji

    2013-01-01

    It is a common practice to employ k(cat)[E]₀/K(m) as a first-order rate constant for the analysis of an enzymatic reaction, where [E]₀ is the total enzyme concentration. I describe in this report a serious shortcoming in analyzing enzymatic reactions when kcat[E]₀/K(m) is employed and show that k(cat)[E]₀/K(m) can only be applied under very limited conditions. I consequently propose the use of a more universal first-order rate constant, k(cat)[ES](K)/[S]₀, where [ES](K) is the initial equilibrium concentration of the ES-complex derived from [E]₀, [S]₀ and K(m). Employing k(cat)[ES](K)/[S]₀ as the first-order rate constant enables all enzymatic reactions to be reasonably simulated under a wide range of conditions, and the catalytic and binding contributions to the rate constant of any enzyme can be determined under any and all conditions.

  1. Theoretical and Shock Tube Study of the Rate Constants for Hydrogen Abstraction Reactions of Ethyl Formate.

    PubMed

    Wu, Junjun; Khaled, Fethi; Ning, Hongbo; Ma, Liuhao; Farooq, Aamir; Ren, Wei

    2017-08-24

    We report a systematic chemical kinetics study of the H atom abstractions from ethyl formate (EF) by H, O((3)P), CH3, OH, and HO2 radicals. The geometry optimization and frequency calculation of all the species were conducted using the M06 method and the cc-pVTZ basis set. The one-dimensional hindered rotor treatment of the reactants and transition states and the intrinsic reaction coordinate analysis were also performed at the M06/cc-pVTZ level of theory. The relative electronic energies were calculated at the CCSD(T)/cc-pVXZ (where X = D, T) level of theory and further extrapolated to the complete basis set limit. Rate constants for the tittle reactions were calculated over the temperature range 500-2500 K by the transition state theory (TST) in conjunction with the asymmetric Eckart tunneling effect. In addition, the rate constants of H-abstraction by hydroxyl radical were measured in shock tube experiments at 900-1321 K and 1.4-2.0 atm. Our theoretical rate constants of OH + EF → products agree well with the experimental results within 15% over the experimental temperature range of 900-1321 K. Branching ratios for the five types of H-abstraction reactions were also determined from their individual site-specific rate constants.

  2. Perturbation theory in the catalytic rate constant of the Henri-Michaelis-Menten enzymatic reaction.

    PubMed

    Bakalis, Evangelos; Kosmas, Marios; Papamichael, Emmanouel M

    2012-11-01

    The Henry-Michaelis-Menten (HMM) mechanism of enzymatic reaction is studied by means of perturbation theory in the reaction rate constant k (2) of product formation. We present analytical solutions that provide the concentrations of the enzyme (E), the substrate (S), as well as those of the enzyme-substrate complex (C), and the product (P) as functions of time. For k (2) small compared to k (-1), we properly describe the entire enzymatic activity from the beginning of the reaction up to longer times without imposing extra conditions on the initial concentrations E ( o ) and S ( o ), which can be comparable or much different.

  3. Rate constants for chemical reactions in high-temperature nonequilibrium air

    NASA Technical Reports Server (NTRS)

    Jaffe, R. L.

    1986-01-01

    In the nonequilibrium atmospheric chemistry regime that will be encountered by the proposed Aeroassisted Orbital Transfer Vehicle in the upper atmosphere, where air density is too low for thermal and chemical equilibrium to be maintained, the detailed high temperature air chemistry plays a critical role in defining radiative and convective heating loads. Although vibrational and electronic temperatures remain low (less than 15,000 K), rotational and translational temperatures may reach 50,000 K. Attention is presently given to the effects of multiple temperatures on the magnitudes of various chemical reaction rate constants, for the cases of both bimolecular exchange reactions and collisional excitation and dissociation reactions.

  4. Derivation of a valid momentary first-order rate constant for kinetic and energetic analyses of enzymatic reactions.

    PubMed

    Imoto, Taiji

    2016-12-01

    To analyze enzymatic reactions energetically for comparison with non-enzymatic reactions (first order) under the same dimension, a method to derive valid momentary first-order rate constants for enzymatic reactions was developed. The momentary first-order rate constant, k enz0 = k cat[E'S']e,0/[S]0, was derived for an enzymatic reaction under a certain condition. It was shown that this rate constant is applicable for a wide range of enzymatic reactions. Utilizing this constant, one can conduct reliable kinetic and energetic analyses of enzymatic reactions.

  5. Calorimetric determination of rate constants and enthalpy changes for zero-order reactions.

    PubMed

    Almeida e Sousa, Luis; Beezer, Anthony E; Hansen, Lee D; Clapham, David; Connor, Joseph A; Gaisford, Simon

    2012-06-07

    Calorimetry is a general method for determination of the rates of zero-order processes, but analysis of the data for the rate constant and reaction enthalpy is difficult because these occur as a product in the rate equation so evaluation of one requires knowledge of the other. Three methods for evaluation of both parameters, without prior knowledge, are illustrated with examples and compared with literature data. Method 1 requires the reaction to be studied in two buffers with different enthalpies of ionization. Method 2 is based on calculation of reaction enthalpy from group additivity functions. Method 3 applies when reaction progresses to completion. The methods are applied to the enzymatic hydrolysis of urea, the hydrolysis of acetylsalicylic acid, and the photodegradation of nifedipine, respectively.

  6. Rate constant for the reaction of atomic chlorine with acetaldehyde from 210 to 343 K

    SciTech Connect

    Payne, W.A.; Nava, D.F.; Nesbitt, F.L.; Stief, L.J. )

    1990-09-06

    The absolute rate constant for the reaction of atomic chlorine with acetaldehyde has been measured from 210 to 343 K by use of the flash photolysis-resonance fluorescence technique. The rate constant is shown to be independent of variations in (CH{sub 3}CHO), total pressure (Ar), and, to a lesser extent, flash intensity (initial (Cl)). The rate constant is also independent of temperature over the range studied. The average of all experiments yields k{sub 1} = (6.6 {plus minus} 1.4) {times} 10{sup {minus}11} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}, where the error limit is two standard deviations. This result is compared with previous measurements of k{sub 1}, all of which were relative measurements at 298 K.

  7. The rate constant for the reaction of oxygen /3P/ atoms with dichlorine monoxide

    NASA Technical Reports Server (NTRS)

    Miziolek, A. W.; Molina, M. J.

    1978-01-01

    A fast flow discharge apparatus was used to measure the rate constant for the reaction of ground state oxygen atoms with dichlorine monoxide in the temperature range 236-295 K. The air afterflow technique (NO2 chemiluminescence) was used for detection of oxygen atoms. The Arrhenius expression for the rate constant was found to be 2.7 plus or minus 0.3 times 10 to the -11th power exp(-560 plus or minus 80/T) cu cm per molecule per sec. At 295 K the rate constant is 4.1 plus or minus 0.5 times 10 to the -12th power cu cm per molecule per sec.

  8. Rate constant for the reaction SO + BrO yields SO2 + Br

    NASA Technical Reports Server (NTRS)

    Brunning, J.; Stief, L.

    1986-01-01

    The rate of the radical-radical reaction SO + BrO yields SO2 + Br has been determined at 298 K in a discharge flow system near 1 torr pressure with detection of SO and BrO via collision-free sampling mass spectrometry. The rate constant was determined using two different methods: measuring the decay of SO radicals in the presence of an excess of BrO and measuring the decay of BrO radicals in excess SO. The results from the two methods are in reasonable agreement and the simple mean of the two values gives the recommended rate constant at 298 K, k = (5.7 + or - 2.0) x 10 to the -11th cu cm/s. This represents the first determination of this rate constant and it is consistent with a previously derived lower limit based on SO2 formation. Comparison is made with other radical-radical reactions involving SO or BrO. The reaction SO + BrO yields SO2 + Br is of interest for models of the upper atmosphere of the earth and provides a potential coupling between atmospheric sulfur and bromine chemistry.

  9. Effects of the anion salt nature on the rate constants of the aqueous proton exchange reactions.

    PubMed

    Paredes, Jose M; Garzon, Andres; Crovetto, Luis; Orte, Angel; Lopez, Sergio G; Alvarez-Pez, Jose M

    2012-04-28

    The proton-transfer ground-state rate constants of the xanthenic dye 9-[1-(2-methyl-4-methoxyphenyl)]-6-hydroxy-3H-xanthen-3-one (TG-II), recovered by Fluorescence Lifetime Correlation Spectroscopy (FLCS), have proven to be useful to quantitatively reflect specific cation effects in aqueous solutions (J. M. Paredes, L. Crovetto, A. Orte, J. M. Alvarez-Pez and E. M. Talavera, Phys. Chem. Chem. Phys., 2011, 13, 1685-1694). Since these phenomena are more sensitive to anions than to cations, in this paper we have accounted for the influence of salts with the sodium cation in common, and the anion classified according to the empirical Hofmeister series, on the proton transfer rate constants of TG-II. We demonstrate that the presence of ions accelerates the rate of the ground-state proton-exchange reaction in the same order than ions that affect ion solvation in water. The combination of FLCS with a fluorophore undergoing proton transfer reactions in the ground state, along with the desirable feature of a pseudo-dark state when the dye is protonated, allows one unique direct determination of kinetic rate constants of the proton exchange chemical reaction.

  10. The Rate Constant for the Reaction H + C2H5 at T = 295 - 150K

    NASA Technical Reports Server (NTRS)

    Pimentel, Andre S.; Payne, Walter A.; Nesbitt, Fred L.; Cody, Regina J.; Stief, Louis J.

    2004-01-01

    The reaction between the hydrogen atom and the ethyl (C2H3) radical is predicted by photochemical modeling to be the most important loss process for C2H5 radicals in the atmospheres of Jupiter and Saturn. This reaction is also one of the major sources for the methyl radicals in these atmospheres. These two simplest hydrocarbon radicals are the initial species for the synthesis of larger hydrocarbons. Previous measurements of the rate constant for the H + C2H5 reaction varied by a factor of five at room temperature, and some studies showed a dependence upon temperature while others showed no such dependence. In addition, the previous studies were at higher temperatures and generally higher pressures than that needed for use in planetary atmospheric models. The rate constant for the reaction H + C2H5 has been measured directly at T = 150, 202 and 295 K and at P = 1.0 Torr He for all temperatures and additionally at P = 0.5 and 2.0 Torr He at T = 202 K. The measurements were performed in a discharge - fast flow system. The decay of the C2H5 radical in the presence of excess hydrogen was monitored by low-energy electron impact mass spectrometry under pseudo-first order conditions. H atoms and C2H5 radicals were generated rapidly and simultaneously by the reaction of fluorine atoms with H2 and C2H6, respectively. The total rate constant was found to be temperature and pressure independent. The measured total rate constant at each temperature are: k(sub 1)(295K) = (1.02+/-0.24)x10(exp -10), k(sub 1)(202K) = (1.02+/-0.22)x10(exp -10) and k(sub 1)(150K) = (0.93+/-0.21)x10(exp -10), all in units of cu cm/molecule/s. The total rate constant derived from all the combined measurements is k(sub 1) = (l.03+/-0.17)x10(exp -10) cu cm/molecule/s. At room temperature our results are about a factor of two higher than the recommended rate constant and a factor of three lower than the most recently published study.

  11. Evaluation of canonical and microcanonical nonadiabatic reaction rate constants by using the Zhu-Nakamura formulas.

    PubMed

    Zhao, Yi; Mil'nikov, Gennady; Nakamura, Hiroki

    2004-11-08

    We consider a problem of calculating both thermal and microcanonical rate constants for nonadiabatic chemical reactions. Instead of using the conventional transition state theory, we use a generalized seam surface and introduce a concept of a coordinate dependent effective nonadiabatic transition probability based on the Zhu-Nakamura theory which can treat the nonadiabatic tunneling properly. The present approach can be combined with Monte Carlo method so as to be applicable to chemical reactions in complicated systems. The method is demonstrated to work well in wide energy and temperature range. Numerical tests also show that it is very essential for accurate evaluation of the thermal rate constant to use the generalized seam surface and take into account the nonadiabatic tunneling effect.

  12. Rate constant for formation of chlorine nitrate by the reaction ClO + NO2 + M

    NASA Technical Reports Server (NTRS)

    Leu, M. T.; Lin, C. L.; Demore, W. B.

    1977-01-01

    The pseudo-first-order decay of ClO in a large excess of NO2 was monitored in a discharge flow/mass-spectrometer apparatus in order to measure the rate constant of the reaction ClO + NO2 + M yields ClONO2 + M for M = He, Ar, and N2 over the temperature range from 248 to 417 K. Numerical results are given for He at 248, 299, 360, and 417 K (1 to 9 torr); for Ar at 298 K (1 to 4 torr); and for N2 at 299, 360, and 417 K (1 to 6 torr). Systematic errors are estimated, and identification of the reaction product is discussed. The results obtained are shown to be in excellent agreement with other recent measurements of the same rate constant.

  13. Rate constants for reactions of ClO/x/ of atmospheric interest

    NASA Technical Reports Server (NTRS)

    Watson, R. T.

    1977-01-01

    Chemical kinetics measurements on 82 gas phase reactions of chlorine containing species are reviewed. Recommended rate constants are given. The principal species of interest are Cl, Cl2, ClO, Cl2O, ClOO, OClO, CINO, HCl and halo derivatives of methane and ethane. Absorption spectra are given for 21 species. In addition the chemical kinetics methods used to obtain these data are discussed with regard to their applicability and reliability.

  14. Quantification of rate constants for successive enzymatic reactions with DNP hyperpolarized MR.

    PubMed

    Allouche-Arnon, Hyla; Hovav, Yonatan; Friesen-Waldner, Lanette; Sosna, Jacob; Moshe Gomori, J; Vega, Shimon; Katz-Brull, Rachel

    2014-06-01

    A kinetic model is provided to obtain reaction rate constants in successive enzymatic reactions that are monitored using NMR spectroscopy and hyperpolarized substrates. The model was applied for simulation and analysis of the successive oxidation of choline to betaine aldehyde, and further to betaine, by the enzyme choline oxidase. This enzymatic reaction was investigated under two different sets of conditions: two different choline molecular probes were used, [1,1,2,2-D4 , 1-(13) C]choline chloride and [1,1,2,2-D4 , 2-(13) C]choline chloride, in different MR systems (clinical scanner and high-resolution spectrometer), as well as in different reactors and reaction volumes (4.8 and 0.7 mL). The kinetic analysis according to the model yielded similar results in both set-ups, supporting the robustness of the model. This was achieved despite the complex and negating influences of reaction kinetics and polarization decay, and in the presence of uncontrolled mixing characteristics, which may introduce uncertainties in both effective timing and effective pulses. The ability to quantify rate constants using hyperpolarized MR in the first seconds of consecutive enzyme activity is important for further development of the utilization of dynamic nuclear polarization-MR for biological determinations.

  15. Rate constant for the reaction of atomic oxygen with phosphine at 298 K

    NASA Technical Reports Server (NTRS)

    Stief, L. J.; Payne, W. A.; Nava, D. F.

    1987-01-01

    The rate constant for the reaction of atomic oxygen with phosphine has been measured at 298 K using flash photolysis combined with time-resolved detection of O(3P) via resonance fluorescence. Atomic oxygen was produced by flash photolysis of N2O or NO highly diluted in argon. The results were shown to be independent of (PH3), (O), total pressure and the source of O(3P). The mean value of all the experiments is k1 = (3.6 + or -0.8) x 10 to the -11th cu cm/s (1 sigma). Two previous measurements of k1 differed by more than an order of magnitude, and the results support the higher value obtained in a discharge flow-mass spectrometry study. A comparison with rate data for other atomic and free radical reactions with phosphine is presented, and the role of these reactions in the aeronomy or photochemistry of Jupiter and Saturn is briefly considered.

  16. Rate constant for the reaction of atomic oxygen with phosphine at 298 K

    NASA Technical Reports Server (NTRS)

    Stief, L. J.; Payne, W. A.; Nava, D. F.

    1987-01-01

    The rate constant for the reaction of atomic oxygen with phosphine has been measured at 298 K using flash photolysis combined with time-resolved detection of O(3P) via resonance fluorescence. Atomic oxygen was produced by flash photolysis of N2O or NO highly diluted in argon. The results were shown to be independent of (PH3), (O), total pressure and the source of O(3P). The mean value of all the experiments is k1 = (3.6 + or -0.8) x 10 to the -11th cu cm/s (1 sigma). Two previous measurements of k1 differed by more than an order of magnitude, and the results support the higher value obtained in a discharge flow-mass spectrometry study. A comparison with rate data for other atomic and free radical reactions with phosphine is presented, and the role of these reactions in the aeronomy or photochemistry of Jupiter and Saturn is briefly considered.

  17. Reaction rate constants of H-abstraction by OH from large ketones: measurements and site-specific rate rules.

    PubMed

    Badra, Jihad; Elwardany, Ahmed E; Farooq, Aamir

    2014-06-28

    Reaction rate constants of the reaction of four large ketones with hydroxyl (OH) are investigated behind reflected shock waves using OH laser absorption. The studied ketones are isomers of hexanone and include 2-hexanone, 3-hexanone, 3-methyl-2-pentanone, and 4-methl-2-pentanone. Rate constants are measured under pseudo-first-order kinetics at temperatures ranging from 866 K to 1375 K and pressures near 1.5 atm. The reported high-temperature rate constant measurements are the first direct measurements for these ketones under combustion-relevant conditions. The effects of the position of the carbonyl group (C=O) and methyl (CH3) branching on the overall rate constant with OH are examined. Using previously published data, rate constant expressions covering, low-to-high temperatures, are developed for acetone, 2-butanone, 3-pentanone, and the hexanone isomers studied here. These Arrhenius expressions are used to devise rate rules for H-abstraction from various sites. Specifically, the current scheme is applied with good success to H-abstraction by OH from a series of n-ketones. Finally, general expressions for primary and secondary site-specific H-abstraction by OH from ketones are proposed as follows (the subscript numbers indicate the number of carbon atoms bonded to the next-nearest-neighbor carbon atom, the subscript CO indicates that the abstraction is from a site next to the carbonyl group (C=O), and the prime is used to differentiate different neighboring environments of a methylene group):

  18. Rate constants of chemical reactions from semiclassical transition state theory in full and one dimension.

    PubMed

    Greene, Samuel M; Shan, Xiao; Clary, David C

    2016-06-28

    Semiclassical Transition State Theory (SCTST), a method for calculating rate constants of chemical reactions, offers gains in computational efficiency relative to more accurate quantum scattering methods. In full-dimensional (FD) SCTST, reaction probabilities are calculated from third and fourth potential derivatives along all vibrational degrees of freedom. However, the computational cost of FD SCTST scales unfavorably with system size, which prohibits its application to larger systems. In this study, the accuracy and efficiency of 1-D SCTST, in which only third and fourth derivatives along the reaction mode are used, are investigated in comparison to those of FD SCTST. Potential derivatives are obtained from numerical ab initio Hessian matrix calculations at the MP2/cc-pVTZ level of theory, and Richardson extrapolation is applied to improve the accuracy of these derivatives. Reaction barriers are calculated at the CCSD(T)/cc-pVTZ level. Results from FD SCTST agree with results from previous theoretical and experimental studies when Richardson extrapolation is applied. Results from our implementation of 1-D SCTST, which uses only 4 single-point MP2/cc-pVTZ energy calculations in addition to those for conventional TST, agree with FD results to within a factor of 5 at 250 K. This degree of agreement and the efficiency of the 1-D method suggest its potential as a means of approximating rate constants for systems too large for existing quantum scattering methods.

  19. Absolute rate constants for the reaction of atomic hydrogen with ketene from 298 to 500 K

    NASA Technical Reports Server (NTRS)

    Michael, J. V.; Nava, D. F.; Payne, W. A.; Stief, L. J.

    1979-01-01

    Rate constants for the reaction of atomic hydrogen with ketene have been measured at room temperature by two techniques, flash photolysis-resonance fluorescence and discharge flow-resonance fluorescence. The measured values are (6.19 + or - 1.68) x 10 to the -14th and (7.3 + or - 1.3) x 10 to the -14th cu cm/molecule/s, respectively. In addition, rate constants as a function of temperature have been measured over the range 298-500 K using the FP-RF technique. The results are best represented by the Arrhenius expression k = (1.88 + or - 1.12) x 10 to the -11th exp(-1725 + or - 190/T) cu cm/molecule/s, where the indicated errors are at the two standard deviation level.

  20. Absolute rate constants of alkoxyl radical reactions in aqueous solution. [Tert-butyl hydroperoxide

    SciTech Connect

    Erben-Russ, M.; Michel, C.; Bors, W.; Saran, M.

    1987-04-23

    The pulse radiolysis technique was used to generate the alkoxyl radical derived from tert-butyl hydroperoxide (/sup t/BuOOH) in aqueous solution. The reactions of this radical with 2,2'-azinobis(3-ethyl-6-benzothiazolinesulfonate) (ABTS) and promethazine were monitored by kinetic spectroscopy. The unimolecular decay rate constant of the tert-butoxyl radical (/sup t/BuO) was determined to be 1.4 x 10/sup 6/ s/sup -1/. On the basis of this value, the rate constants for /sup t/BuO attack on quercetin, crocin, crocetin, ascorbate, isoascorbate, trolox c, glutathione, thymidine, adenosine, guanosine, and unsaturated fatty acids were determined. In addition, the reaction of /sup t/BuO with the polyunsaturated fatty acids (PUFA) was observed by directly monitoring the formation of the fatty acid pentadienyl radicals. Interestingly, the attack of /sup t/BuO on PUFA was found to be faster by about one order of magnitude as compared to the same reaction in a nonpolar solvent.

  1. Shock tube measurements of the rate constant for the reaction ethanol + OH.

    PubMed

    Stranic, Ivo; Pang, Genny A; Hanson, Ronald K; Golden, David M; Bowman, Craig T

    2014-02-06

    The overall rate constant for the reaction ethanol + OH → products was determined experimentally from 900 to 1270 K behind reflected shock waves. Ethan(18)ol was utilized for these measurements in order to avoid the recycling of OH radicals following H-atom abstraction at the β-site of ethanol. Similar experiments were also performed with unlabeled ethan(16)ol in order to infer the rate constant that excludes reactivity at the β-site. The two data sets were used to directly infer the branching ratio for the reaction at the β-site. Experimental data in the current study and in previous low-temperature studies for the overall rate constant are best fit by the expression koverall = 5.07 × 10(5) T[K](2.31) exp(608/T[K]) cm(3) mol(-1) s(-1), valid from 300 to 1300 K. Measurements indicate that the branching ratio of the β-site is between 20 and 25% at the conditions studied. Pseudo-first-order reaction conditions were generated using tert-butylhydroperoxide (TBHP) as a fast source of (16)OH with ethanol in excess. (16)OH mole fraction time-histories were measured using narrow-line width laser absorption near 307 nm. Measurements were performed at the linecenter of the R22(5.5) transition in the A-X(0,0) band of (16)OH that does not overlap with any absorption features of (18)OH, thus producing a measurement of the (16)OH mole fraction that is insensitive to the presence of (18)OH.

  2. Determination of reaction rate constants for alkylation of 4-(p-nitrobenzyl) pyridine by different alkylating agents.

    PubMed

    Walles, S A

    1980-02-01

    The rate constants have been determined for the reaction between some different alkylating agents and 4-(p-nitrobenzyl) pyridine (NBP) in methanol. These constants have been compared with those for alkylation of aniline in water. All the constants were lower in methanol than in water but in different degrees. The rate constants of the different alkylating agents have been calculated at a nucleophilic strength n=2. The genetic risk defined as the degree of alkylation of a nucleophile (n=2) is equivalent to the rate constant kn=2 and the target dose. The dependence of the genetic risk on the rate constant (kn=2) is discussed.

  3. Determination of the Temperature Dependence of the Rate Constants for HO2/Acetonylperoxy Reaction and Acetonylperoxy Self-Reaction

    NASA Astrophysics Data System (ADS)

    Darby, E. C.; Grieman, F. J.; Hui, A. O.; Okumura, M.; Sander, S. P.

    2014-12-01

    Reactions of hydroperoxy radical, HO2, with carbonyl containing RO2 can play an important role in the oxidation chemistry of the troposphere. Discovered radical product channels in addition to radical termination channels have resulted in increased study of these important reactions. In our continued study of HO2 reactions with acetonylperoxy and acetylperoxy radicals, we report here our first results on the kinetics of the acetonylperoxy system. Previous studies have resulted in conflicting results and no temperature dependence of the rate constants. Using the Infrared Kinetic Spectroscopy (IRKS) method in which a temperature-controlled slow-flow tube apparatus and laser flash photolysis of Cl2 are used to produce HO2 and CH3C(O)CH2O2 from methanol and acetone, respectively, we studied the chemical kinetics involved over the temperature range of 295 to 240 K. Rates of chemical reaction were determined by monitoring the HO2 concentration as a function of time by sensitive near-IR diode laser wavelength modulation spectroscopy while simultaneously measuring the disappearance of [CH3C(O)CH2O2] in the ultraviolet at 300 nm. The simultaneous fits resulted in the determination of the temperature dependence of the rate constants for the HO2/acetonylperoxy reaction and the acetonylperoxy self-reaction. At the lower temperatures, the reactions of HO2 and CH3C(O)CH2O2 with the adducts HO2•CH3OH and HO2•CH3C(O)CH3 formed in significant concentrations needed to be included in the fitting models.

  4. Application of the compensated Arrhenius formalism to explain the dielectric constant dependence of rates for Menschutkin reactions.

    PubMed

    Petrowsky, Matt; Glatzhofer, Daniel T; Frech, Roger

    2013-11-21

    The dependence of the reaction rate on solvent dielectric constant is examined for the reaction of trihexylamine with 1-bromohexane in a series of 2-ketones over the temperature range 25-80 °C. The rate constant data are analyzed using the compensated Arrhenius formalism (CAF), where the rate constant assumes an Arrhenius-like equation that also contains a dielectric constant dependence in the exponential prefactor. The CAF activation energies are substantially higher than those obtained using the simple Arrhenius equation. A master curve of the data is observed by plotting the prefactors against the solvent dielectric constant. The master curve shows that the reaction rate has a weak dependence on dielectric constant for values approximately less than 10 and increases more rapidly for dielectric constant values greater than 10.

  5. α-Terpineol reactions with the nitrate radical: Rate constant and gas-phase products

    NASA Astrophysics Data System (ADS)

    Jones, Brian T.; Ham, Jason E.

    The bimolecular rate constant of k rad +α-terpineol (16 ± 4) × 10 -12 cm 3 molecule -1 s -1 was measured using the relative rate technique for the reaction of the nitrate radical (NO 3rad ) with α-terpineol (2-(4-methyl-1-cyclohex-3-enyl)propan-2-ol) at 297 ± 3 K and 1 atmosphere total pressure. To more clearly define part of α-terpineol's indoor environment degradation mechanism, the products of α-terpineol + NO 3rad reaction were investigated. The identified reaction products were: acetone, glyoxal (HC( dbnd O)C( dbnd O)H), and methylglyoxal (CH 3C( dbnd O)C( dbnd O)H). The use of derivatizing agents O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) and N, O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) were used to propose the other major reaction products: 6-hydroxyhept-5-en-2-one, 4-(1-hydroxy-1-methylethyl)-1-methyl-2-oxocyclohexyl nitrate, 5-(1-hydroxy-1-methylethyl)-2-oxocyclohexyl nitrate, 1-formyl-5-hydroxy-4-(hydroxymethyl)-1,5-dimethylhexyl nitrate, and 1,4-diformyl-5-hydroxy-1,5-dimethylhexyl nitrate. The elucidation of these products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible α-terpineol + NO 3rad reaction mechanisms based on previously published volatile organic compound + NO 3rad gas-phase mechanisms. The additional gas-phase products (2,6,6-trimethyltetrahydro-2 H-pyran-2,5-dicarbaldehyde and 2,2-dimethylcyclohexane-1,4-dicarbaldehyde) are proposed to be the result of cyclization through a reaction intermediate.

  6. Rate constants for the slow Mu + propane abstraction reaction at 300 K by diamagnetic RF resonance.

    PubMed

    Fleming, Donald G; Cottrell, Stephen P; McKenzie, Iain; Ghandi, Khashayar

    2015-08-14

    The study of kinetic isotope effects for H-atom abstraction rates by incident H-atoms from the homologous series of lower mass alkanes (CH4, C2H6 and, here, C3H8) provides important tests of reaction rate theory on polyatomic systems. With a mass of only 0.114 amu, the most sensitive test is provided by the rates of the Mu atom. Abstraction of H by Mu can be highly endoergic, due to the large zero-point energy shift in the MuH bond formed, which also gives rise to high activation energies from similar zero-point energy corrections at the transition state. Rates are then far too slow near 300 K to be measured by conventional TF-μSR techniques that follow the disappearance of the spin-polarised Mu atom with time. Reported here is the first measurement of a slow Mu reaction rate in the gas phase by the technique of diamagnetic radio frequency (RF) resonance, where the amplitude of the MuH product formed in the Mu + C3H8 reaction is followed with time. The measured rate constant, kMu = (6.8 ± 0.5) × 10(-16) cm(3) s(-1) at 300 K, is surprisingly only about a factor of three slower than that expected for H + C3H8, indicating a dominant contribution from quantum tunneling in the Mu reaction, consistent with elementary transition state theory calculations of the kMu/kH kinetic isotope effect.

  7. Theoretical study and rate constant calculation of the CH2O+CH3 reaction

    NASA Astrophysics Data System (ADS)

    Liu, Jing-yao; Li, Ze-sheng; Wu, Jia-yan; Wei, Zhi-gang; Zhang, Gang; Sun, Chia-chung

    2003-10-01

    The potential energy surface of the CH2O+CH3 reaction is explored at the MP2/6-311++G(d,p), MP4SDQ/6-311G(d,p), and QCISD(T)/6-311+G(3df,2p) (single point) levels of theory. Theoretical calculations suggest that the major product channel (R1) is the hydrogen abstraction leading to the product P1 CHO+CH4 (R1), while the addition process leading to P2H+CH3CHO (R2) appears to be negligibly small. The calculated enthalpies and dissociation activation energies for CH3CH2O and CH3OCH2 radicals involved in the reaction are in line with the experimental values. Dual-level dynamics calculation is carried out for the direct hydrogen abstraction channel. The energy profile of (R1) is refined with the interpolated single-point energies (ISPE) method at the QCISD(T)//MP2 level. The rate constants, which are evaluated by canonical variational transition-state theory (CVT) including small-curvature tunneling (SCT) correction, are in good agreement with the available experimental data. It is shown that tunneling effect plays a significant role in the rate constant calculation; and as a result, the CVT/SCT rate constants exhibit typical non-Arrhenius behavior over a wide temperature range 300-2000 K. The three parameter expression is k=6.35×10-26 T4.4 exp(-2450/T) cm3 molecule-1 s-1.

  8. Product distributions and rate constants for ion-molecule reactions in water, hydrogen sulfide, ammonia, and methane

    NASA Technical Reports Server (NTRS)

    Huntress, W. T., Jr.; Pinizzotto, R. F., Jr.

    1973-01-01

    The thermal energy, bimolecular ion-molecule reactions occurring in gaseous water, hydrogen sulfide, ammonia, and methane have been identified and their rate constants determined using ion cyclotron resonance methods. Absolute rate constants were determined for the disappearance of the primary ions by using the trapped ion method, and product distributions were determined for these reactions by using the cyclotron ejection method. Previous measurements are reviewed and compared with the results using the present methods. The relative rate constants for hydrogen-atom abstraction, proton transfer, and charge transfer are also determined for reactions of the parent ions.

  9. Product distributions and rate constants for ion-molecule reactions in water, hydrogen sulfide, ammonia, and methane

    NASA Technical Reports Server (NTRS)

    Huntress, W. T., Jr.; Pinizzotto, R. F., Jr.

    1973-01-01

    The thermal energy, bimolecular ion-molecule reactions occurring in gaseous water, hydrogen sulfide, ammonia, and methane have been identified and their rate constants determined using ion cyclotron resonance methods. Absolute rate constants were determined for the disappearance of the primary ions by using the trapped ion method, and product distributions were determined for these reactions by using the cyclotron ejection method. Previous measurements are reviewed and compared with the results using the present methods. The relative rate constants for hydrogen-atom abstraction, proton transfer, and charge transfer are also determined for reactions of the parent ions.

  10. Prediction of Chain Propagation Rate Constants of Polymerization Reactions in Aqueous NIPAM/BIS and VCL/BIS Systems.

    PubMed

    Kröger, Leif C; Kopp, Wassja A; Leonhard, Kai

    2017-04-06

    Microgels have a wide range of possible applications and are therefore studied with increasing interest. Nonetheless, the microgel synthesis process and some of the resulting properties of the microgels, such as the cross-linker distribution within the microgels, are not yet fully understood. An in-depth understanding of the synthesis process is crucial for designing tailored microgels with desired properties. In this work, rate constants and reaction enthalpies of chain propagation reactions in aqueous N-isopropylacrylamide/N,N'-methylenebisacrylamide and aqueous N-vinylcaprolactam/N,N'-methylenebisacrylamide systems are calculated to identify the possible sources of an inhomogeneous cross-linker distribution in the resulting microgels. Gas-phase reaction rate constants are calculated from B2PLYPD3/aug-cc-pVTZ energies and B3LYPD3/tzvp geometries and frequencies. Then, solvation effects based on COSMO-RS are incorporated into the rate constants to obtain the desired liquid-phase reaction rate constants. The rate constants agree with experiments within a factor of 2-10, and the reaction enthalpies deviate less than 5 kJ/mol. Further, the effect of rate constants on the microgel growth process is analyzed, and it is shown that differences in the magnitude of the reaction rate constants are a source of an inhomogeneous cross-linker distribution within the resulting microgel.

  11. Estimation of rate constants of PCB dechlorination reactions using an anaerobic dehalogenation model.

    PubMed

    Karakas, Filiz; Imamoglu, Ipek

    2017-02-15

    This study aims to estimate anaerobic dechlorination rate constants (km) of reactions of individual PCB congeners using data from four laboratory microcosms set up using sediment from Baltimore Harbor. Pathway km values are estimated by modifying a previously developed model as Anaerobic Dehalogenation Model (ADM) which can be applied to any halogenated hydrophobic organic (HOC). Improvements such as handling multiple dechlorination activities (DAs) and co-elution of congeners, incorporating constraints, using new goodness of fit evaluation led to an increase in accuracy, speed and flexibility of ADM. DAs published in the literature in terms of chlorine substitutions as well as specific microorganisms and their combinations are used for identification of pathways. The best fit explaining the congener pattern changes was found for pathways of Phylotype DEH10, which has the ability to remove doubly flanked chlorines in meta and para positions, para flanked chlorines in meta position. The range of estimated km values is between 0.0001-0.133d(-1), the median of which is found to be comparable to the few available published biologically confirmed rate constants. Compound specific modelling studies such as that performed by ADM can enable monitoring and prediction of concentration changes as well as toxicity during bioremediation.

  12. Rate constant for the reaction C2H5 + HBr → C2H6 + Br.

    PubMed

    Golden, David M; Peng, Jingping; Goumri, A; Yuan, J; Marshall, Paul

    2012-06-21

    RRKM theory has been employed to analyze the kinetics of the title reaction, in particular, the once-controversial negative activation energy. Stationary points along the reaction coordinate were characterized with coupled cluster theory combined with basis set extrapolation to the complete basis set limit. A shallow minimum, bound by 9.7 kJ mol(-1) relative to C(2)H(5) + HBr, was located, with a very small energy barrier to dissociation to Br + C(2)H(6). The transition state is tight compared to the adduct. The influence of vibrational anharmonicity on the kinetics and thermochemistry of the title reaction were explored quantitatively. With adjustment of the adduct binding energy by ∼4 kJ mol(-1), the computed rate constants may be brought into agreement with most experimental data in the literature, including new room-temperature results described here. There are indications that at temperatures above those studied experimentally, the activation energy may switch from negative to positive.

  13. The Br+HO 2 reaction revisited: Absolute determination of the rate constant at 298 K

    NASA Astrophysics Data System (ADS)

    Laverdet, G.; Le Bras, G.; Mellouki, A.; Poulet, G.

    1990-09-01

    The absolute determination of the rate constant for the reaction Br+HO 2→HBr+O 2 has been done at 298 K using the discharge-flor EPR method. The value k1 = (1.5±0.2) × 10 -12 cm 3 molecule -1 s -1 was obtained. Previous indirect measurements of k1 from a discharge-flow, LIF/mass spectrometric study of the Br/H 2CO/O 2 system have been reinterpreted, leading to values for k1 ranging from 1.0 × 10 -12 to 2.2 × 10 -12 cm 3 molecule -1 s -1 at 298 K. These results are discussed and compared with other literature values.

  14. Relative rate constants for the reactions of atomic oxygen with HO2 anad OH radicals

    NASA Technical Reports Server (NTRS)

    Keyser, L. F.

    1983-01-01

    Relative rate constants for the reactions O + HO2 - OH + O2 (1) and O + OH - H + O2 (2) were obtained by using the discharge-flow resonance fluorescence technique at 2 torr total pressure and 299 K. HO2 radicals were generated by reacting atomic hydrogen with an excess of O2. Quasi-steady-state concentrations of OH and HO2 were established in the presence of excess atomic oxygen. Observed concentration ratios, namely the ratio of the OH concentration to the HO2 concentration, resulted in a value of 1.7 + or 0.2 for k1/k2. The error limits are twice the standard deviation obtained from the data analysis. Overall experimental error is estimated to be + or - 25 percent. This result confirms earlier direct measurements of k1 and k2 which required knowledge of absolute radical or atomic oxygen concentrations.

  15. CORAL: QSPR modeling of rate constants of reactions between organic aromatic pollutants and hydroxyl radical.

    PubMed

    Toropov, A A; Toropova, A P; Rasulev, B F; Benfenati, E; Gini, G; Leszczynska, D; Leszczynski, J

    2012-09-05

    The rate constants (K(OH)) of reactions between 78 organic aromatic pollutants and hydroxyl radical were examined. Simplified molecular input line entry system was used as representation of the molecular structure of the pollutants. Quantitative structure-property relationships was developed using CORAL software (http://www.insilico.eu/CORAL) for four random splits of the data into the subtraining, calibration, and test sets. The obtained results reveal good predictive potential of the applied approach: correlation coefficients (r(2)) for the test sets of the four random splits are 0.75, 0.91, 0.84, and 0.80. Using the Monte Carlo method CORAL software generated the optimal descriptors for one-variable models. The reproducibility of each model was tested performing three runs of the Monte Carlo optimization. The current data were compared to previous results and discussed. Copyright © 2012 Wiley Periodicals, Inc.

  16. Relative rate constants for the reactions of atomic oxygen with HO2 anad OH radicals

    NASA Technical Reports Server (NTRS)

    Keyser, L. F.

    1983-01-01

    Relative rate constants for the reactions O + HO2 - OH + O2 (1) and O + OH - H + O2 (2) were obtained by using the discharge-flow resonance fluorescence technique at 2 torr total pressure and 299 K. HO2 radicals were generated by reacting atomic hydrogen with an excess of O2. Quasi-steady-state concentrations of OH and HO2 were established in the presence of excess atomic oxygen. Observed concentration ratios, namely the ratio of the OH concentration to the HO2 concentration, resulted in a value of 1.7 + or 0.2 for k1/k2. The error limits are twice the standard deviation obtained from the data analysis. Overall experimental error is estimated to be + or - 25 percent. This result confirms earlier direct measurements of k1 and k2 which required knowledge of absolute radical or atomic oxygen concentrations.

  17. Direct Measurement of the Effective Rate Constant for Primary Charge Separation in Isolated Photosystem II Reaction Centers

    SciTech Connect

    Greenfield, S. R.; Seibert, M.; Govindjee; Wasielewski, M. R.

    1997-03-27

    Transient absorption measurements of the pheophytin a anion band and Qx band bleach region using preferential excitation of P680 are performed on isolated photosystem II reaction centers to determine the effective rate constant for charge separtion. A novel analysis of the Qx band bleach region explicity takes the changing background into account in order to directly measure the rate of growth of the bleach. Both spectral regions reveal biphasic kinetics, with a ca. (8 ps)-1 rate constant for the faster component, and a ca. (50 ps)-1 rate constant for the slower component. We propose that the fster component corresponds to the effective rate constant for charge separation from within the equilibrated reaction center core and provides a lower limit for the intrinsic rate constant for charge separation. The slower component corresponds to charge separation that is limited by slow energy transfer from a long-wavelength accessory chlorophyll a.

  18. Atmospheric Lifetimes of Very Short Lived Substances: OH Reaction Rate Constants of Stereoisomers and Pressure Dependent Reactions.

    NASA Astrophysics Data System (ADS)

    Orkin, Vladimir; Khamaganov, Victor; Kurylo, Michael

    2017-04-01

    The accuracy of OH kinetic data is of primary importance for the comprehensive modeling of any compound's atmospheric behavior and of its environmental impacts, such as its atmospheric residence time and its potential roles in stratospheric ozone depletion, global warming, and local pollution. Rate constants of OH reactions with hydrocarbons and halogenated hydrocarbons can now be determined with an accuracy of 2-3% over the temperature range 220 K to 370 K. This has been demonstrated in studies of OH reactions with various halogenated and non-substituted organics including alkanes, alkenes, alcohols, and ethers. Lower data scattering and higher measurement accuracy allow for rigorous statistical analysis of the results and can reveal often-missing details about the reactivity, such as the weak dependencies of the rate constant on the temperature, pressure dependences, and the effect of molecular geometry on the reactivity. This presentation will provide examples of several of these measurement results that were recently obtained in our laboratory. The estimated atmospheric lifetimes and global warming potentials will be reported.

  19. Potential chlorofluorocarbon replacements: OH reaction rate constants between 250 and 315 K and infrared absorption spectra

    SciTech Connect

    Garland, N.L.; Medhurst, L.J.; Nelson, H.H.

    1993-12-20

    The authors measured the rate constant for reactions of the OH radical with several potential chlorofluorocarbon replacements over the temperature range 251-314 K using laser photolysis laser-induced fluorescence techniques. The compounds studied and Arrhenius parameters determined from fits to the measured rate constants are as follows: CHF{sub 2}OCHF{sub 2} (E 134), k(T) = (5.4 {+-} 3.5) x 10{sup {minus}13} cm{sup 3} s{sup {minus}1} exp [({minus}3.1 {+-} 0.4 kcal mol{sup {minus}1})/RT]; CF{sub 3}CH{sub 2}CF{sub 3} (FC 236fa), k(T) = (2.0 {+-} 1.0) x 10{sup {minus}14} cm{sup 3} s{sup {minus}1} exp [({minus}1.8 {+-} 0.3 kcal mol{sup {minus}1})/RT]; CF{sub 3}CHFCHF{sub 2} (FC 236ea), k(T) = (2.0 {+-} 0.9) x 10{sup {minus}13} cm{sup 3} s{sup {minus}1} exp [({minus}2.0 {+-} 0.3 kcal mol{sup {minus}1})/RT]; and CF{sub 3}CF{sub 2}CH{sub 2}F (FC 236cb), k(T) = (2.6 {+-} 1.6) x 10{sup {minus}13} cm{sup 3} s{sup {minus}1} exp [({minus}2.2 {+-} 0.4 kcal mol{sup {minus}1})/RT]. The measured activation energies (2-3 kcal mol{sup {minus}1}) are consistent with a mechanism of H atom abstraction. The tropospheric lifetimes, estimated from the measured OH reaction rates, and measured integrated infrared absorption cross sections over the range 770 to 1430 cm{sup {minus}1} suggest that E 134 and FC 236fa may have significant global warming potential, while FC 236ea and FC 236cb do not. 17 refs., 4 figs., 3 tabs.

  20. Evaluation of rate constants for enzyme-catalysed reactions by the jackknife technique. Application to liver alcohol dehydrogenase.

    PubMed Central

    Cornish-Bowden, A; Wong, J T

    1978-01-01

    Steady-state measurements of enzyme-catalysed reactions are capable of providing more information about the rate constants of the individual steps than is commonly obtained. We have applied a combination of the jackknife and non-linear regression techniques to measurements of the rate of oxidation of ethanol by NAD+, catalysed by alcohol dehydrogenase from horse liver. This has permitted values and confidence intervals to be assigned to the eight rate constants that characterize the binding of ethanol and NAD+ in random order to the enzyme, and to the net rate constant kcat. for the breakdown of the ternary complex. PMID:743242

  1. Theoretical study of the mechanism and rate constant of the B + CO2 reaction.

    PubMed

    Poully, Benjamin; Bergeat, Astrid; Hannachi, Yacine

    2008-09-04

    The different stationary points on the potential energy surface relative to the title reaction have been reinvestigated at the B3LYP/aug-cc-pVDZ level with relative energies computed at the CCSD(T)/aug-cc-pVTZ level with B3LYP/aug-cc-pVDZ optimized geometries and by using the G3B3 composite method. Two entrance channels have been identified. The first one corresponds to boron addition at one of the oxygen atoms of the CO 2 molecule leading to trans-BOCO, which is found to be about 27 kcal/mol exothermic with a potential energy barrier of 16.4 kcal/mol (G3B3). The second channel, which has not been identified in previous theoretical works, corresponds to a direct insertion of the boron atom into a CO bond and leads to OBCO. The B + CO 2 --> OBCO step is found to be about 84 kcal/mol exothermic and needs to overcome a potential energy barrier of only 3.6 kcal/mol (G3B3). The rate constant at 300 K of the insertion step, calculated by using TST theory with G3B3 calculated activation energy value, is 5.4 10 (-14) cm (3) molecule (-1) s (-1), in very good agreement with the experimental data ((7.0 +/- 2.8) 10 (-14) cm (3) molecule (-1) s (-1), DiGiuseppe, T. G.; Davidovits, P. J. Chem. Phys. 1981, 74, 3287). The one corresponding to the addition process is found to be several orders of magnitude smaller because of a much higher potential energy barrier. The addition channel would not contribute to the title reaction even at high temperature. A modified Arrhenius equation has been fitted in the 300-1000 K temperature range, which might be useful for chemical models.

  2. Rate constant measurements for the overall reaction of OH + 1-butanol → products from 900 to 1200 K.

    PubMed

    Pang, Genny A; Hanson, Ronald K; Golden, David M; Bowman, Craig T

    2012-03-15

    The rate constant for the overall reaction OH + 1-butanol → products was determined in the temperature range 900 to 1200 K from measurements of OH concentration time histories in reflected shock wave experiments of tert-butyl hydroperoxide (TBHP) as a fast source of OH radicals with 1-butanol in excess. Narrow-linewidth laser absorption was employed for the quantitative OH concentration measurement. A detailed kinetic mechanism was constructed that includes updated rate constants for 1-butanol and TBHP kinetics that influence the near-first-order OH concentration decay under the present experimental conditions, and this mechanism was used to facilitate the rate constant determination. The current work improves upon previous experimental studies of the title rate constant by utilizing a rigorously generated kinetic model to describe secondary reactions. Additionally, the current work extends the temperature range of experimental data in the literature for the title reaction under combustion-relevant conditions, presenting the first measurements from 900 to 1000 K. Over the entire temperature range studied, the overall rate constant can be expressed in Arrhenius form as 3.24 × 10(-10) exp(-2505/T [K]) cm(3) molecule(-1) s(-1). The influence of secondary reactions on the overall OH decay rate is discussed, and a detailed uncertainty analysis is performed yielding an overall uncertainty in the measured rate constant of ±20% at 1197 K and ±23% at 925 K. The results are compared with previous experimental and theoretical studies on the rate constant for the title reaction and reasonable agreement is found when the earlier experimental data were reinterpreted.

  3. Mercury chemistry in the MBL: Modeling results including Hg + halogen atom reaction rate constants

    NASA Astrophysics Data System (ADS)

    Hedgecock, I. M.; Pirrone, N.

    2003-04-01

    The inclusion, of recently published kinetic data for the reactions between gas phase elemental Hg and halogen atoms and molecules, in a photochemical box model including aerosols of the Marine Boundary Layer (MBL), suggests that the cycling of Hg over the world's oceans may be much more dynamic than was once thought, as a direct result of halogen activation from marine aerosols. The rate of gas phase oxidation of Hg(0) in the model leads to high concentrations of gas phase oxidised Hg (Hg(II)), which is deposited to the sea surface either directly from the gas phase or indirectly via scavenging by sea salt and non-sea-salt sulphate aerosol particles and subsequent deposition. The model base run predicts Hg(II) concentrations higher than those measured in the marine atmosphere, and a lifetime for Hg(0) of a matter of days, rather than months as has been generally assumed. In light of previous measurements and the known stability of the hemispherical background concentration of Hg(0) the influence of liquid water content (the number of deliquescent aerosol droplets), cloud optical depth at the top of the boundary layer, and the Henry's Law constants for HgCl2 and HgBr2 have been investigated. In order to maintain a stable background concentration of Hg(0) a source strength (for emission from the sea, or entrainment from the free troposphere) of at least 15 ng m-2 hr-1 is required, which seems most unlikely considering results from flux chamber experiments. The model therefore either overestimates the rate of gas phase oxidation or lacks a fundamental reduction process. The evidence from studies of mercury depletion events in the Arctic troposphere lend support to the fast reaction between Hg(0) and Br containing radicals which have been included in the model, it is necessary therefore to investigate homogeneous and heterogeneous mechanisms for the reduction of Hg(II) to Hg(0) in order to explain the measured Hg(II) concentrations in the MBL and the stable

  4. Temperature and pressure dependence of the absolute rate constant for the reactions of NH2 radicals with acetylene and ethylene

    NASA Technical Reports Server (NTRS)

    Bosco, S. R.; Nava, D. F.; Brobst, W. D.; Stief, L. J.

    1984-01-01

    The absolute rate constants for the reaction between the NH2 free radical and acetylene and ethylene is measured experimentally using a flash photolysis technique. The constant is considered to be a function of temperature and pressure. At each temperature level of the experiment, the observed pseudo-first-order rate constants were assumed to be independent of flash intensity. The results of the experiment indicate that the bimolecular rate constant for the NH2 + C2H2 reaction increases with pressure at 373 K and 459 K but not at lower temperatures. Results near the pressure limit conform to an Arrhenius expression of 1.11 (+ or -) 0.36 x 10 to the -13th over the temperature range from 241 to 459 K. For the reaction NH2 + C2H4, a smaller rate of increase in the bimolecular rate constant was observed over the temperature range 250-465 K. The implications of these results for current theoretical models of NH2 + C2H2 (or H4) reactions in the atmospheres of Jupiter and Saturn are discussed.

  5. USE OF ROUGH SETS AND SPECTRAL DATA FOR BUILDING PREDICTIVE MODELS OF REACTION RATE CONSTANTS

    EPA Science Inventory

    A model for predicting the log of the rate constants for alkaline hydrolysis of organic esters has been developed with the use of gas-phase min-infrared library spectra and a rule-building software system based on the mathematical theory of rough sets. A diverse set of 41 esters ...

  6. USE OF ROUGH SETS AND SPECTRAL DATA FOR BUILDING PREDICTIVE MODELS OF REACTION RATE CONSTANTS

    EPA Science Inventory

    A model for predicting the log of the rate constants for alkaline hydrolysis of organic esters has been developed with the use of gas-phase min-infrared library spectra and a rule-building software system based on the mathematical theory of rough sets. A diverse set of 41 esters ...

  7. Evaluation of the reaction rate constants for the gas-phase Al-CH4-air combustion chemistry

    NASA Astrophysics Data System (ADS)

    Sharipov, A. S.; Titova, N. S.; Starik, A. M.

    2012-10-01

    The most likely reaction pathways and reaction products in the Al-CH4-O2-N2 system are investigated using density functional theory and ab initio calculations. The B3LYP functional with extended 6-311+G(3df,2p) basis set as well as the CBS-QB3 composite method are mainly utilised. Theoretical analysis of corresponding reaction rate constants is also performed with the use of simple theoretical models. A critical overview of current knowledge on combustion-relevant reactions with aluminium compounds is given. On the basis of critical comparison of available experimental kinetic data with theoretical calculations, the approximations for rate constants for 44 reversible elementary reactions involving Al-containing species are recommended for use in combustion issues.

  8. Rate Constant Change of Photo Reaction of Bacteriorhodopsin Observed in Trimeric Molecular System.

    PubMed

    Tsujiuchi, Yutaka; Masumoto, Hiroshi; Goto, Takashi

    2016-04-01

    To elucidate the time evolution of photo reaction of bacteriorhodopsin in glycerol mixed purple membrane at around 196 K under irradiation by red light, a kinetic model was constructed. The change of absorption with irradiation at times of 560 nm and 412 nm was analyzed for the purpose of determining reaction rates of photo reaction of bacteriorhodopsin and its product M intermediate. In this study it is shown that reaction rates of conversion from bacteriorhodopsin to the M intermediate can be explained by a set of linear differential equations. This model analysis concludes that bacteriorhodopsin in which constitutes a trimer unit with other two bacteriorhodopsin molecules changes into M intermediates in the 1.73 of reaction rate, in the initial step, and according to the number of M intermediate in a trimer unit, from three to one, the reaction rate of bacteriorhodopsin into M intermediates smaller as 1.73, 0.80, 0.19 which caused by influence of inter-molecular interaction between bacteriorhodopsin.

  9. Rate constant measurements for the reaction Cl + CH2O yields HCl + CHO Implications regarding the removal of stratospheric chlorine

    NASA Technical Reports Server (NTRS)

    Anderson, P. C.; Kurylo, M. J.

    1979-01-01

    The flash photolysis resonance fluorescence technique was employed to investigate the rate constant for the reaction Cl + CH2O yields HCl + CHO from 223 to 323 K. An Arrhenius fit of the data gives a rate constant equal to (1.09 + or - 0.40) x 10 to the -10th exp/-(131 + or - 98)/T/ in units of cu cm/molecule per sec. The results are compared to two very recent kinetic studies and are assessed in view of the reaction's role in disrupting the Cl-ClO stratospheric ozone depletion chain.

  10. Rate constant measurements for the reaction Cl + CH2O yields HCl + CHO Implications regarding the removal of stratospheric chlorine

    NASA Technical Reports Server (NTRS)

    Anderson, P. C.; Kurylo, M. J.

    1979-01-01

    The flash photolysis resonance fluorescence technique was employed to investigate the rate constant for the reaction Cl + CH2O yields HCl + CHO from 223 to 323 K. An Arrhenius fit of the data gives a rate constant equal to (1.09 + or - 0.40) x 10 to the -10th exp/-(131 + or - 98)/T/ in units of cu cm/molecule per sec. The results are compared to two very recent kinetic studies and are assessed in view of the reaction's role in disrupting the Cl-ClO stratospheric ozone depletion chain.

  11. Application of an Artificial Neural Network to the Prediction of OH Radical Reaction Rate Constants for Evaluating Global Warming Potential.

    PubMed

    Allison, Thomas C

    2016-03-03

    Rate constants for reactions of chemical compounds with hydroxyl radical are a key quantity used in evaluating the global warming potential of a substance. Experimental determination of these rate constants is essential, but it can also be difficult and time-consuming to produce. High-level quantum chemistry predictions of the rate constant can suffer from the same issues. Therefore, it is valuable to devise estimation schemes that can give reasonable results on a variety of chemical compounds. In this article, the construction and training of an artificial neural network (ANN) for the prediction of rate constants at 298 K for reactions of hydroxyl radical with a diverse set of molecules is described. Input to the ANN consists of counts of the chemical bonds and bends present in the target molecule. The ANN is trained using 792 (•)OH reaction rate constants taken from the NIST Chemical Kinetics Database. The mean unsigned percent error (MUPE) for the training set is 12%, and the MUPE of the testing set is 51%. It is shown that the present methodology yields rate constants of reasonable accuracy for a diverse set of inputs. The results are compared to high-quality literature values and to another estimation scheme. This ANN methodology is expected to be of use in a wide range of applications for which (•)OH reaction rate constants are required. The model uses only information that can be gathered from a 2D representation of the molecule, making the present approach particularly appealing, especially for screening applications.

  12. Kinetics analysis for development of a rate constant estimation model for ultrasonic degradation reaction of methylene blue.

    PubMed

    Kobayashi, Daisuke; Honma, Chiemi; Matsumoto, Hideyuki; Takahashi, Tomoki; Kuroda, Chiaki; Otake, Katsuto; Shono, Atsushi

    2014-07-01

    Ultrasound has been used as an advanced oxidation method for wastewater treatment. Sonochemical degradation of organic compounds in aqueous solution occurs by pyrolysis and/or reaction with hydroxyl radicals. Moreover, kinetics of sonochemical degradation has been proposed. However, the effect of ultrasonic frequency on degradation rate has not been investigated. In our previous study, a simple model for estimating the apparent degradation rate of methylene blue was proposed. In this study, sonochemical degradation of methylene blue was performed at various frequencies. Apparent degradation rate constant was evaluated assuming that sonochemical degradation of methylene blue was a first-order reaction. Specifically, we focused on effects of ultrasonic frequency and power on rate constant, and the applicability of our proposed model was demonstrated. Using this approach, maximum sonochemical degradation rate was observed at 490 kHz, which agrees with a previous investigation into the effect of frequency on the sonochemical efficiency value evaluated by KI oxidation dosimetry. Degradation rate increased with ultrasonic power at every frequency. It was also observed that threshold power must be reached for the degradation reaction to progress. The initial methylene blue concentration and the apparent degradation rate constant have a relation of an inverse proportion. Our proposed model for estimating the apparent degradation rate constant using ultrasonic power and sonochemical efficiency value can apply to this study which extended the frequency and initial concentration range. Copyright © 2013 Elsevier B.V. All rights reserved.

  13. Pressure dependence of the absolute rate constant for the reaction OH + C2H2 from 228 to 413 K

    NASA Technical Reports Server (NTRS)

    Michael, J. V.; Nava, D. F.; Payne, W. A.; Stief, L. J.; Borkowski, R. P.

    1980-01-01

    The pressure dependence of the absolute rate constant for the reaction of the hydroxyl radical with acetylene, important in both atmospheric and combustion chemistry, is determined for temperatures between 228 and 413 K. The flash photolysis-resonance fluorescence technique was employed at five temperatures over wide ranges of pressure and acetylene concentrations, with the OH produced by water photolysis and hydroxyl resonance fluorescent photons measured by multiscaling techniques. Results indicate that, except at the lowest temperature, the bimolecular rate constant for the reaction depends strongly on total pressure, with the pressure effect becoming more pronounced with increasing temperature. At limiting high pressures, the rate constant is found to be equal to 6.83 + or - 1.19 x 10 to the -12th exp (-646 + or - 47/T) cu cm/molecule per sec, where T is the temperature. Results thus demonstrate the importance of environmental conditions in theoretical studies of atmospheric and combustion product compositions

  14. Determination of rate and equilibrium constants for the reactions between electron transfer mediators and proteins by linear sweep voltammetry.

    PubMed

    Parker, V D; Roddick, A; Seefeldt, L C; Wang, H; Zheng, G

    1997-07-01

    Redox proteins undergo measurable charge transfer at electrodes only under special circumstances, while they readily take part in electron transfer reactions with mediators in solution. Advantage was taken of the latter fact to develop a new method to study the kinetics and equilibria of protein-mediator electron transfer reactions. It was shown that rate and equilibrium constants for the electron exchange between electron transfer mediator and the protein can be obtained from the analysis of the perturbation of the linear sweep voltammetry (LSV) response of the mediator due to the presence of the protein. The experiments were carried out under conditions where the protein does not interact with the electrode. Theoretical data obtained by digital simulation are presented to show the conditions under which rate and equilibrium constants are accessible by the LSV technique. The electron transfer reactions between ferri- and ferrocytochrome c and N,N,N',N'-tetramethylphenylenediamine and the corresponding radical cation in phosphate-buffered saline (0.04 M phosphate, pH 7.4, 0.1 M NaCl) buffer were selected to demonstrate the technique. These studies resulted in an equilibrium constant equal to 1.0 and forward and reverse rate constants equal to 1.6 x 10(4) M-1 s-1. The data available from this method include forward and reverse rate constants for electron transfer and the formal potential for the protein redox couple.

  15. Rate constants for aqueous-phase reactions of hydroxyl radical ({center_dot}OH) with aldehydes and ketones

    SciTech Connect

    Allen, J.M.; Allen, S.K.

    1995-12-31

    A wide variety of aldehydes and ketones are formed in the troposphere by the gas-phase oxidation of hydrocarbons. These compounds are expected to readily partition into cloud, fog, and aquated aerosol drops where they can participate in a variety of aqueous-phase reactions. It has been previously demonstrated by other researchers that aqueous-phase photochemical reactions involving aromatic aldehydes and ketones may lead to the formation of hydrogen peroxide. Hydrogen peroxide is an important oxidant for S(IV) and is also an {center_dot}OH precursor. Aldehydes and ketones may also participate in other aqueous-phase reactions within atmospheric water drops including reactions with {center_dot}OH. Rate constants for reactions involving {center_dot}OH in aqueous solutions have been reported for only a limited number of tropospheric aldehydes and ketones. The authors have measured the rate constants for aqueous-phase reactions of {center_dot}OH with several tropospheric aldehydes and ketones by the technique of competition kinetics. Hydroxyl radicals were generated by continuous illumination at 313 nm of an aqueous acidified solution containing Fe(ClO{sub 4}){sub 3}, an {center_dot}OH scavenger, the aldehyde or ketone whose rate constant was to be measured, and a standard for which the rate constant for reaction with {center_dot}OH is well known. Nitrobenzene was used as the standard in all experiments. Loss of the aldehyde or ketone and the standard were monitored by HPLC. Losses attributable to direct photolysis and dark reactions were minimal.

  16. The use of the magnetic field effect for studying a chemiluminescent chemical reaction in aqueous solution. Reaction rate constants and lifetimes of intermediate molecules

    NASA Astrophysics Data System (ADS)

    Triebel, Michael M.; Totrov, Maxim M.; Zorinyants, George E.; Frankevich, Eugene L.

    1993-11-01

    The phase shift magnetic field effect technique is applied for investigation of the chemiluminescent (ChL) reaction of luminol oxidation by potassium ferricyanide in aqueous alkali solution. The external modulated magnetic field changed the rate constant of recombination of luminol radicals. Rate constants of intermediate stages of the reaction are obtained: 10 8 M -1 s -1 for diazaquinone reaction with hydrogen peroxide, 2 X 10 6 M -1 s -1 for diazaquinone hydrolysis and 2 X 10 5 s -1 for the decomposition of hydroperoxide, which is a precursor of the light emitter.

  17. Rate constant for the reaction NH2 + NO from 216 to 480 K

    NASA Technical Reports Server (NTRS)

    Stief, L. J.; Brobst, W. D.; Nava, D. F.; Borkowski, R. P.; Michael, J. V.

    1982-01-01

    The absolute rate constant was measured by the technique of flash photolysis-laser induced fluorescence (FP-LIF). NH2 radicals were produced by the flash photolysis of ammonia and the fluorescent NH2 photons were measured by multiscaling techniques. At each temperature, the results were independent of variations in total pressure, and flash intensity. The results are compared with previous determinations using the techniques of mass spectrometry, absorption spectroscopy, laser absorption spectroscopy, and laser induced fluorescence. The implications of the results are discussed with regard to combustion, post combustion, and atmospheric chemistry. The results are also discussed theoretically.

  18. Reaction rate constant for OH + HOONO/sub 2/. -->. products over the temperature range 246 to 324 K

    SciTech Connect

    Trevor, P.L.; Black, G.; Barker, J.R.

    1982-04-29

    Absolute bimolecular reaction rate constants for the title reaction have been determined for temperatures ranging from 246 to 324 K. The laser flash-photolysis resonance-fluorescence (LFPRF) technique was used to generate O(/sup 1/D) which reacted with H/sub 2/ and/or H/sub 2/O to produce OH radicals. The bimolecular rate constants for the title reaction showed no dependence on total (He) pressure over the range approx. 3 to 15 torr, and they did not depend upon initial (OH) or upon its mode of formation. The H/sub 2/O/sub 2/ impurity was explicitly measured in all experiments, and the rate constants were corrected for its contribution. A weighted least-squares analysis of the data obtained at nine temperatures (226 data points) gave the Arrhenius expression (k +- 1sigma) = (8.05 +- 5.69) x 10/sup -12/ exp (-193 +- 194/T) cm/sup 3/ s/sup -1/ with covariance 1.098 x 10/sup -9/. A simple weighted average (temperature independent) fits the data just as well, and when the effects of systematic errors are taken into account, our recommended rate constant is (k +- 2sigma) = (4.0 +- 1.6) x 10/sup -12/ cm/sup 3/ s/sup -1/.

  19. High-level theoretical study of the reaction between hydroxyl and ammonia: Accurate rate constants from 200 to 2500 K

    DOE PAGES

    Nguyen, Thanh Lam; Stanton, John F.

    2017-06-02

    Hydrogen abstraction from NH3 by OH to produce H2O and NH2 — an important reaction in combustion of NH3 fuel — was studied with a theoretical approach that combines high level quantum chemistry and advanced chemical kinetics methods. Thermal rate constants calculated from first principles agree well (within 5 to 20%) with available experimental data over a temperature range that extends from 200 to 2500 K. Here, quantum mechanical tunneling effects were found to be important; they lead to a decided curvature and non-Arrhenius behavior for the rate constant.

  20. Absolute rate constant of the reaction between chlorine /2P/ atoms and hydrogen peroxide from 298 to 424 K

    NASA Technical Reports Server (NTRS)

    Keyser, L. F.

    1980-01-01

    The absolute rate constant of the reaction between chlorine (2P) atoms and hydrogen peroxide was determined from 298 to 424 K, using the discharge flow resonance fluorescence technique. Pseudo-first-order conditions were used with hydrogen peroxide in large excess. A fast flow-sampling procedure limited hydrogen peroxide decomposition to less than 5% over the temperature range studied. At 298 K, the rate constant is (4.1 plus or minus 0.2) x 10 to the minus 13th cu cm/molecule-sec.

  1. High-level theoretical study of the reaction between hydroxyl and ammonia: Accurate rate constants from 200 to 2500 K

    NASA Astrophysics Data System (ADS)

    Nguyen, Thanh Lam; Stanton, John F.

    2017-10-01

    Hydrogen abstraction from NH3 by OH to produce H2O and NH2—an important reaction in combustion of NH3 fuel—was studied with a theoretical approach that combines high level quantum chemistry and advanced chemical kinetics methods. Thermal rate constants calculated from first principles agree well (within 5%-20%) with available experimental data over a temperature range that extends from 200 to 2500 K. Quantum mechanical tunneling effects were found to be important; they lead to a decided curvature and non-Arrhenius behavior for the rate constant.

  2. Rate constants and isotope effects for the reaction of H-atom abstraction from RH substrates by PINO radicals

    NASA Astrophysics Data System (ADS)

    Opeida, I. A.; Litvinov, Yu. E.; Kushch, O. V.; Kompanets, M. A.; Shendrik, A. N.; Matvienko, A. G.; Novokhatko, A. A.

    2016-11-01

    The kinetics of the reactions of hydrogen atom abstraction from the C-H bonds of substrates of different structures by phthalimide- N-oxyl radicals is studied. The rate constants of this reaction are measured and the kinetic isotope effects are determined. It is shown that in addition to the thermodynamic factor, Coulomb forces and donor-acceptor interactions affect the reaction between phthalimide- N-oxyl radicals and substrate molecules, altering the shape of the transition state. This favors the tunneling of hydrogen atoms and leads to a substantial reduction in the activation energy of the process.

  3. High Accuracy OH Reaction Rate Constant Measurements: Studies of Pressure Dependencies and of the Reactivities of Stereoisomers.

    NASA Astrophysics Data System (ADS)

    Orkin, V. L.; Khamaganov, V. G.; Kurylo, M. J., III

    2016-12-01

    The accuracy of OH kinetic data is of primary importance for the comprehensive modeling of any compound's atmospheric behavior and of its environmental impacts, such as its atmospheric residence time and its potential roles in stratospheric ozone depletion, global warming, and local pollution. Rate constants of OH reactions with hydrocarbons and halogenated hydrocarbons can now be determined with an accuracy of 2-3% over the temperature range 220 K to 370 K. This has been demonstrated in studies of OH reactions with various halogenated and non-substituted organics including alkanes, alkenes, alcohols, and ethers. Lower data scattering and higher measurement accuracy allow for rigorous statistical analysis of the results and can reveal often-missing details about the reactivity, such as the (small) dependencies of the rate constant on the temperature, pressure, and reactant molecular structure. Such details include the temperature dependence of both H-abstraction and OH-addition reactions exhibiting a noticeable curvature in the Arrhenius plot that cannot be attributed to experimental artifacts, determinations of the temperature dependencies of OH-addition rate constants that are approaching the collision frequency, determination of both low-pressure and high-pressure limiting rate constants from data obtained over a limited pressure range, observations of unexpected pressure dependencies for OH-addition reaction rate constants (such as in case of C4 alkenes), and determinations of substantially different reactivities and their temperature dependencies as well as IR absorption spectra for the stereoisomers of unsaturated compounds. This presentation will provide examples of several of these measurement results that were recently obtained in our laboratory.

  4. Channel specific rate constants for reactions of O(1D) with HCl and HBr

    NASA Technical Reports Server (NTRS)

    Wine, P. H.; Wells, J. R.; Ravishankara, A. R.

    1986-01-01

    The absolute rate coefficients and product yields for reactions of O(1D) with HCl(1) and HBr(2) at 287 K are presently determined by means of the time-resolved resonance fluorescence detection of O(3P) and H(2S) in conjunction with pulsed laser photolysis of O3/HX/He mixtures. Total rate coefficients for O(1D) removal are found to be, in units of 10 to the -10th cu cm/molecule per sec, k(1) = 1.50 + or - 0.18 and k(2) 1.48 + or - 0.16; the absolute accuracy of these rate coefficients is estimated to be + or - 20 percent.

  5. Rate Constants and Activation Energies for Gas‐Phase Reactions of Three Cyclic Volatile Methyl Siloxanes with the Hydroxyl Radical

    PubMed Central

    Safron, Andreas; Strandell, Michael; Kierkegaard, Amelie

    2015-01-01

    ABSTRACT Reaction with hydroxyl radicals (OH) is the major pathway for removal of cyclic volatile methyl siloxanes (cVMS) from air. We present new measurements of second‐order rate constants for reactions of the cVMS octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) with OH determined at temperatures between 313 and 353 K. Our measurements were made using the method of relative rates with cyclohexane as a reference substance and were conducted in a 140‐mL gas‐phase reaction chamber with online mass spectrometry analysis. When extrapolated to 298 K, our measured reaction rate constants of D4 and D5 with the OH radical are 1.9 × 10−12 (95% confidence interval (CI): (1.7–2.2) × 10−12) and 2.6 × 10−12 (CI: (2.3–2.9) × 10−12) cm3 molecule−1 s−1, respectively, which are 1.9× and 1.7× faster than previous measurements. Our measured rate constant for D6 is 2.8 × 10−12 (CI: (2.5–3.2) × 10−12) cm3 molecule−1 s−1 and to our knowledge there are no comparable laboratory measurements in the literature. Reaction rates for D5 were 33% higher than for D4 (CI: 30–37%), whereas the rates for D6 were only 8% higher than for D5 (CI: 5–10%). The activation energies of the reactions of D4, D5, and D6 with OH were not statistically different and had a value of 4300 ± 2800 J/mol. PMID:27708500

  6. Rate Constants and Activation Energies for Gas-Phase Reactions of Three Cyclic Volatile Methyl Siloxanes with the Hydroxyl Radical.

    PubMed

    Safron, Andreas; Strandell, Michael; Kierkegaard, Amelie; Macleod, Matthew

    2015-07-01

    Reaction with hydroxyl radicals (OH) is the major pathway for removal of cyclic volatile methyl siloxanes (cVMS) from air. We present new measurements of second-order rate constants for reactions of the cVMS octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) with OH determined at temperatures between 313 and 353 K. Our measurements were made using the method of relative rates with cyclohexane as a reference substance and were conducted in a 140-mL gas-phase reaction chamber with online mass spectrometry analysis. When extrapolated to 298 K, our measured reaction rate constants of D4 and D5 with the OH radical are 1.9 × 10(-12) (95% confidence interval (CI): (1.7-2.2) × 10(-12)) and 2.6 × 10(-12) (CI: (2.3-2.9) × 10(-12)) cm(3) molecule(-1) s(-1), respectively, which are 1.9× and 1.7× faster than previous measurements. Our measured rate constant for D6 is 2.8 × 10(-12) (CI: (2.5-3.2) × 10(-12)) cm(3) molecule(-1) s(-1) and to our knowledge there are no comparable laboratory measurements in the literature. Reaction rates for D5 were 33% higher than for D4 (CI: 30-37%), whereas the rates for D6 were only 8% higher than for D5 (CI: 5-10%). The activation energies of the reactions of D4, D5, and D6 with OH were not statistically different and had a value of 4300 ± 2800 J/mol.

  7. Temperature-Dependent Rate Constants and Substituent Effects for the Reactions of Hydroxyl Radicals With Three Partially Fluorinated Ethers

    NASA Technical Reports Server (NTRS)

    Hsu, K.-J.; DeMore, W. B.

    1995-01-01

    Rate constants and temperature dependencies for the reactions of OH with CF3OCH3 (HFOC-143a), CF2HOCF2H (HFOC-134), and CF3OCF2H (HFOC-125) were studied using a relative rate technique in the temperature range 298-393 K. The following absolute rate constants were derived: HFOC-143a, 1.9E-12 exp(-1555/T); HFOC-134, 1.9E-12 exp(-2006/T); HFOC-125, 4.7E-13 exp(-2095/T). Units are cm(exp 3)molecule(exp -1) s(exp -1). Substituent effects on OH abstraction rate constants are discussed, and it is shown that the CF3O group has an effect on the OH rate constants similar to that of a fluorine atom. The effects are related to changes in the C-H bond energies of the reactants (and thereby the activation energies) rather than changes in the preexponential factors. On the basis of a correlation of rate constants with bond energies, the respective D(C-H) bond strengths in the three ethers are found to be 102, 104, and 106 kcal/mol, with an uncertainty of about 1 kcal/mol.

  8. The reaction H + C4H2 - Absolute rate constant measurement and implication for atmospheric modeling of Titan

    NASA Technical Reports Server (NTRS)

    Nava, D. F.; Mitchell, M. B.; Stief, L. J.

    1986-01-01

    The absolute rate constant for the reaction H + C4H2 has been measured over the temperature (T) interval 210-423 K, using the technique of flash photolysis-resonance fluorescence. At each of the five temperatures employed, the results were independent of variations in C4H2 concentration, total pressure of Ar or N2, and flash intensity (i.e., the initial H concentration). The rate constant, k, was found to be equal to 1.39 x 10 to the -10th exp (-1184/T) cu cm/s, with an error of one standard deviation. The Arrhenius parameters at the high pressure limit determined here for the H + C4H2 reaction are consistent with those for the corresponding reactions of H with C2H2 and C3H4. Implications of the kinetic carbon chemistry results, particularly those at low temperature, are considered for models of the atmospheric carbon chemistry of Titan. The rate of this reaction, relative to that of the analogous, but slower, reaction of H + C2H2, appears to make H + C4H2 a very feasible reaction pathway for effective conversion of H atoms to molecular hydrogen in the stratosphere of Titan.

  9. The reaction H + C4H2 - Absolute rate constant measurement and implication for atmospheric modeling of Titan

    NASA Astrophysics Data System (ADS)

    Nava, D. F.; Mitchell, M. B.; Stief, L. J.

    1986-04-01

    The absolute rate constant for the reaction H + C4H2 has been measured over the temperature (T) interval 210-423 K, using the technique of flash photolysis-resonance fluorescence. At each of the five temperatures employed, the results were independent of variations in C4H2 concentration, total pressure of Ar or N2, and flash intensity (i.e., the initial H concentration). The rate constant, k, was found to be equal to 1.39 x 10 to the -10th exp (-1184/T) cu cm/s, with an error of one standard deviation. The Arrhenius parameters at the high pressure limit determined here for the H + C4H2 reaction are consistent with those for the corresponding reactions of H with C2H2 and C3H4. Implications of the kinetic carbon chemistry results, particularly those at low temperature, are considered for models of the atmospheric carbon chemistry of Titan. The rate of this reaction, relative to that of the analogous, but slower, reaction of H + C2H2, appears to make H + C4H2 a very feasible reaction pathway for effective conversion of H atoms to molecular hydrogen in the stratosphere of Titan.

  10. [Determination of rate constants of gas-phase reactions of alpha-pinene and beta-pinene with ozone].

    PubMed

    Liu, Z R; Hu, D

    2001-10-01

    alpha-pinene and beta-pinene are the most dominating species among natural terpenes. Terpenes are mainly emitted from forest trees, flowers and grass. In the lower troposphere terpenes can react fast with OH radical, ozone, NO3 radical and ground state oxygen atom. These reactions may contribute to the occurring of aerosols, peroxides (hydrogen peroxide and organic peroxide), carbon cycle (mainly CO), acid rain (organic acids, NO3- and SO4(2-), ozone and active radicals such as OH radical. Reactions with ozone occur both in the daytime and in the night. The study in this field in China began in the late 1980. The main work focus on the source emission and the experimental simulation has just started. It is most of our group's work. In this paper preliminary experimental simulation of the gas-phase reactions of alpha-pinene and beta-pinene with ozone were carried out in the quartz chamber. The rate constants of these reactions were measured using long-path Fourier transform infra-red combined with relative rate constant method. And the rate constants for the gas-phase reactions of alpha-pinene, beta-pinene with ozone were determined as 2.83 x 10(17) cm3.molecule-1.s-1 and 1.48 x 10(17) cm3.molecule-1.s-1 at 1.0 x 10(5) Pa and 296 +/- 3 K. The results are quite similar to the data from Atkinson group. No cyclohexane was added to the reaction system during the measurement to restrain the formation of OH radical. The formation of OH radical could not be quantified, so that the effect of subsidiary reactions induced by OH radical has not been calculated. In the later simulation study and model this effect should be considered.

  11. Reaction efficiencies and rate constants for the goethite-catalyzed Fenton-like reaction of NAPL-form aromatic hydrocarbons and chloroethylenes.

    PubMed

    Yeh, C Kuei-Jyum; Hsu, Chi-Yu; Chiu, Chuen-Huey; Huang, Kuo-Lin

    2008-03-01

    The contaminants present as nonaqueous phase liquids (NAPLs) in the subsurface are long-term sources for groundwater pollution. Fenton-like reaction catalyzed by natural iron oxides such as goethite in soils is one of the feasible in situ chemical reactions used to remediate contaminated sites. This research evaluated the Fenton-like reaction of five chlorinated ethylenes and three aromatic hydrocarbons using goethite as the catalyst. The reaction efficiencies and rate constants of these compounds in NAPL and dissolved forms were compared. The content of goethite used in batch experiments was in the range similar to those found in subsurfaces. Low H2O2 concentrations (0.05 and 0.1%) were tested in order to represent the low oxidant concentration in the outer region of treatment zone. The results showed that at the tested goethite and H2O2 ranges, the majority of contaminants were removed in the first 120 s. When aromatics and chloroethylenes were present as NAPLs, their removal efficiencies and reaction constants decreased. The removal efficiencies of 0.02 mmol NAPL contaminants were 26-70% less than those of the dissolved. The measured rate constants were in the order of 10(9) M(-1) s(-1) for dissolved chlorinated ethylenes and aromatic hydrocarbons, but were 25-60% less for their NAPL forms. The initial dosage of H2O2 and NAPL surface areas (18.4-38.2 mm2) did not significantly affect reaction efficiencies and rate constants of chlorinated NAPLs. Instead, they were related to the octanol-water partition coefficient of compounds. For both dissolved and NAPL forms, aromatic hydrocarbons were more reactive than chlorinated ethylenes in Fenton-like reaction. These results indicated that the decrease in reaction efficiencies and rate constants of NAPL-form contaminants would pose more negative impacts on the less reactive compounds such as benzene and cis 1,2-DCE during goethite-catalyzed Fenton-like reaction.

  12. Rate constant for the reaction of hydroxyl radical with formaldehyde over the temperature range 228-362 K

    NASA Technical Reports Server (NTRS)

    Stief, L. J.; Nava, D. F.; Payne, W. A.; Michael, J. V.

    1980-01-01

    Absolute rate constants for the reaction OH + H2CO have been measured over the temperature range 228-362 K using the flash photolysis-resonance fluorescence technique. The results were independent of variations in forbidden H2CO, total pressure of forbidden Ar and flash intensity (i.e., initial forbidden OH). The rate constant was found to be invariant with temperature in this range, the best representation being k1 = (1.05 + or - 0.11) x 10 to the -11th cu cm/molecule sec where the error is two standard deviations. This result is compared with previous absolute and relative determinations of k1. The reaction is also discussed from a theoretical point of view.

  13. Rate constant for the reaction of hydroxyl radical with formaldehyde over the temperature range 228-362 K

    NASA Technical Reports Server (NTRS)

    Stief, L. J.; Nava, D. F.; Payne, W. A.; Michael, J. V.

    1980-01-01

    Absolute rate constants for the reaction OH ? H2CO measured over the temperature range 228-362 K using the flash photolysis-resonance fluorescence technique are given. The results are independent of variations in H2CO concentration, total pressure Ar concentration, and flash intensity (i.e., initial OH concentration). The rate constant is found to be invariant with temperature in this range, the best representation being k sub 1 = (1.05 ? or - 0.11) x 10 to the 11th power cu cm molecule(-1) s(-1) where the error is two standard deviations. This result is compared with previous absolute and relative determinations of k sub 1. The reaction is also discussed from a theoretical point of view.

  14. Rate constant for the reaction of hydroxyl radical with formaldehyde over the temperature range 228-362 K

    NASA Technical Reports Server (NTRS)

    Stief, L. J.; Nava, D. F.; Payne, W. A.; Michael, J. V.

    1980-01-01

    Absolute rate constants for the reaction OH + H2CO have been measured over the temperature range 228-362 K using the flash photolysis-resonance fluorescence technique. The results were independent of variations in forbidden H2CO, total pressure of forbidden Ar and flash intensity (i.e., initial forbidden OH). The rate constant was found to be invariant with temperature in this range, the best representation being k1 = (1.05 + or - 0.11) x 10 to the -11th cu cm/molecule sec where the error is two standard deviations. This result is compared with previous absolute and relative determinations of k1. The reaction is also discussed from a theoretical point of view.

  15. Atmospheric reaction of Cl + methacrolein: a theoretical study on the mechanism, and pressure- and temperature-dependent rate constants.

    PubMed

    Sun, Cuihong; Xu, Baoen; Zhang, Shaowen

    2014-05-22

    Methacrolein is a major degradation product of isoprene, the reaction of methacrolein with Cl atoms may play some roles in the degradation of isoprene where these species are relatively abundant. However, the energetics and kinetics of this reaction, which govern the reaction branching, are still not well understood so far. In the present study, two-dimensional potential energy surfaces were constructed to analyze the minimum energy path of the barrierless addition process between Cl and the C═C double bond of methacrolein, which reveals that the terminal addition intermediate is directly formed from the addition reaction. The terminal addition intermediate can further yield different products among which the reaction paths abstracting the aldehyde hydrogen atom and the methyl hydrogen atom are dominant reaction exits. The minimum reaction path for the direct aldehydic hydrogen atom abstraction is also obtained. The reaction kinetics was calculated by the variational transition state theory in conjunction with the master equation method. From the theoretical model we predicted that the overall rate constant of the Cl + methacrolein reaction at 297 K and atmospheric pressure is koverall = 2.3× 10(-10) cm(3) molecule(-1) s(-1), and the branching ratio of the aldehydic hydrogen abstraction is about 12%. The reaction is pressure dependent at P < 10 Torr with the high pressure limit at about 100 Torr. The calculated results could well account for the experimental observations.

  16. The H2 + CO ↔ H2CO Reaction: Rate Constants and Relevance to Hot and Dense Astrophysical Media

    NASA Astrophysics Data System (ADS)

    Vichietti, R. M.; Spada, R. F. K.; da Silva, A. B. F.; Machado, F. B. C.; Haiduke, R. L. A.

    2016-07-01

    A theoretical thermochemical and kinetic investigation of the thermal H2 + CO ↔ H2CO reaction was performed for a temperature range from 200 to 4000 K. Geometries and vibrational frequencies of reactants, product, and transition state (TS) were obtained at CCSD/cc-pVxZ (x = T and Q) levels and scaling factors were employed to consider anharmonicity effects on vibrational frequencies, zero-point energies, and thermal corrections provided by these methodologies. Enthalpies Gibbs energies, and rate constants for this reaction were determined by including a complete basis set extrapolation correction for the electronic properties calculated at CCSD(T)/cc-pVyZ (y = Q and 5) levels. Our study indicates that enthalpy changes for this reaction are highly dependent on temperature. Moreover, forward and reverse (high-pressure limit) rate constants were obtained from variational TS theory with quantum tunneling corrections. Thus, modified Arrhenius’ equations were fitted by means of the best forward and reverse rate constant values, which provide very reliable estimates for these quantities within the temperature range between 700 and 4000 K. To our knowledge, this is the first kinetic study done for the forward H2 + CO \\to H2CO process in a wide temperature range. Finally, these results can be used to explain the formaldehyde abundance in hot and dense interstellar media, possibly providing data about the physical conditions associated with H2CO masers close to massive star-forming regions.

  17. Non-steady state mass action dynamics without rate constants: dynamics of coupled reactions using chemical potentials

    NASA Astrophysics Data System (ADS)

    Cannon, William R.; Baker, Scott E.

    2017-10-01

    Comprehensive and predictive simulation of coupled reaction networks has long been a goal of biology and other fields. Currently, metabolic network models that utilize enzyme mass action kinetics have predictive power but are limited in scope and application by the fact that the determination of enzyme rate constants is laborious and low throughput. We present a statistical thermodynamic formulation of the law of mass action for coupled reactions at both steady states and non-stationary states. The formulation uses chemical potentials instead of rate constants. When used to model deterministic systems, the method corresponds to a rescaling of the time dependent reactions in such a way that steady states can be reached on the same time scale but with significantly fewer computational steps. The relationships between reaction affinities, free energy changes and generalized detailed balance are central to the discussion. The significance for applications in systems biology are discussed as is the concept and assumption of maximum entropy production rate as a biological principle that links thermodynamics to natural selection.

  18. Non-steady state mass action dynamics without rate constants: dynamics of coupled reactions using chemical potentials.

    PubMed

    Cannon, William R; Baker, Scott E

    2017-08-16

    Comprehensive and predictive simulation of coupled reaction networks has long been a goal of biology and other fields. Currently, metabolic network models that utilize enzyme mass action kinetics have predictive power but are limited in scope and application by the fact that the determination of enzyme rate constants is laborious and low throughput. We present a statistical thermodynamic formulation of the law of mass action for coupled reactions at both steady states and non-stationary states. The formulation uses chemical potentials instead of rate constants. When used to model deterministic systems, the method corresponds to a rescaling of the time dependent reactions in such a way that steady states can be reached on the same time scale but with significantly fewer computational steps. The relationships between reaction affinities, free energy changes and generalized detailed balance are central to the discussion. The significance for applications in systems biology are discussed as is the concept and assumption of maximum entropy production rate as a biological principle that links thermodynamics to natural selection.

  19. Rapid gas-phase reactions - The reaction of ammonia and the methylamines with boron trifluoride. III - Pressure dependence of rate constant.

    NASA Technical Reports Server (NTRS)

    Glicker, S.

    1973-01-01

    A steady-state flow apparatus, shown earlier to be suitable for the study of rapid gas reactions, has been used to determine the pressure dependence of the relative quasi-bimolecular rate constants for the reactant pairs ammonia (AM)-trimethylamine (TMA) and dimethylamine (DMA)-trimethylamine in reaction with boron trifluoride. The pressure dependence of the former was determined in the range 0.03-75 torr and that of the latter in the range 0.02-720 torr.

  20. Measuring Rate Constants for Reactions of the Simplest Criegee Intermediate CH_2OO by Monitoring the OH Radical

    NASA Astrophysics Data System (ADS)

    Liu, Yingdi; Bayes, Kyle D.; Sander, Stanley P.

    2014-06-01

    Criegee radicals are important atmospheric intermediates formed from ozonolysis of alkenes. It potentially contributes to the atmospheric oxidation cycle mainly by generating OH radicals through unimolecular decomposition. In this work, we focus on studying the unimolecular decomposition reaction of the smallest Criegee intermediate (CH2OO), which was generated by reacting CH2I with O2. While generating the CH2OO molecule by reacting CH2I with O2, significant amounts of the OH radical were observed by laser-induced fluorescence. The addition of molecules known to react with CH2OO increased the observed decay rates of the OH signal. Using the OH signals as a proxy for the CH2OO concentration, the rate constant for the reaction of hexafluoroacetone with CH2OO was determined. The rate constant for the reaction of SO2 with CH2OO showed no pressure dependence over the range of 50 to 200 Torr. This work provides the direct experimental evidence for the unimolecular decomposition of CH2OO, and possible mechanisms of CH2OO have been investigated by this multidimensional study.

  1. Measuring rate constants for reactions of the simplest Criegee intermediate (CH2OO) by monitoring the OH radical.

    PubMed

    Liu, Yingdi; Bayes, Kyle D; Sander, Stanley P

    2014-01-30

    While generating the CH2OO molecule by reacting CH2I with O2, significant amounts of the OH radical were observed by laser-induced fluorescence. At least two different processes formed OH. A fast process was probably initiated by a reaction of vibrationally hot CH2I radicals. The second process appeared to be associated with the decay of the CH2OO molecule. The addition of molecules known to react with CH2OO increased the observed decay rates of the OH signal. Using the OH signals as a proxy for the CH2OO concentration, the rate constant for the reaction of hexafluoroacetone with CH2OO was determined to be (3.33 ± 0.27) × 10(-11) cm(3) molecule(-1) s(-1), in good agreement with the value measured by Taatjes et al.1 The rate constant for the reaction of SO2 with CH2OO, (3.53 ± 0.29) × 10(-11) cm(3) molecule(-1) s(-1), showed no pressure dependence over the range of 50-200 Torr and was in agreement with the value at 4 Torr reported by Welz et al.

  2. Rate constants for the reactions of benzyl and methyl-substituted benzyl radicals with O 2 and NO

    NASA Astrophysics Data System (ADS)

    Ebata, Takayuki; Obi, Kin-ichi; Tanaka, Ikuzo

    1981-02-01

    Rate constants for reactions of benzyl, o-niethylbenzyl and p-meihylbenzyl radicals with O 2 and NO have been measured at room temperature. The radicals were generated by UV flash photolysis and the time decay measured by absorption at ≈ 300 nm. The rate constants are: benzyl (0.99 ± 0.07 and 9.5 ± 1.2), o-methylbenzyl (1.2 ± 0.07 and 8.6 ± 0.8) and p-mithyl-benzyl (1.1= 0.10 and 8.9 = 0.9) for O 2 and NO respectively in units of 10 -12 cm 3 molecule -1 s -1.

  3. Muonium Addition Reactions and Kinetic Isotope Effects in the Gas Phase: k∞ Rate Constants for Mu + C2H2.

    PubMed

    Arseneau, Donald J; Garner, David M; Reid, Ivan D; Fleming, Donald G

    2015-07-16

    The kinetics of the addition reaction of muonium (Mu) to acetylene have been studied in the gas phase at N2 moderator pressures mainly from ∼800 to 1000 Torr and over the temperature range from 168 to 446 K, but also down to 200 Torr at 168 K and over a much higher range of pressures, from 10 to 44 bar at 295 K, demonstrating pressure-independent rate constants, kMu(T). Even at 200 Torr moderator pressure, the kinetics for Mu + C2H2 addition behave as if effectively in the high-pressure limit, giving k∞ = kMu due to depolarization of the muon spin in the MuC2H2 radical formed in the addition step. The rate constants kMu(T) exhibit modest Arrhenius curvature over the range of measured temperatures. Comparisons with data and with calculations for the corresponding H(D) + C2H2 addition reactions reveal a much faster rate for the Mu reaction at the lowest temperatures, by 2 orders of magnitude, in accord with the propensity of Mu to undergo quantum tunneling. Moreover, isotopic atom exchange, which contributes in a major way to the analogous D atom reaction, forming C2HD + H, is expected to be unimportant in the case of Mu addition, a consequence of the much higher zero-point energy and hence weaker C-Mu bond that would form, meaning that the present report of the Mu + C2H2 reaction is effectively the only experimental study of kinetic isotope effects in the high-pressure limit for H-atom addition to acetylene.

  4. Hydrogen Abstraction Reactions from Phenolic Compounds by Peroxyl Radicals: Multireference Character and Density Functional Theory Rate Constants.

    PubMed

    Galano, Annia; Muñoz-Rugeles, Leonardo; Alvarez-Idaboy, Juan Raul; Bao, Junwei Lucas; Truhlar, Donald G

    2016-07-14

    An assessment of multireference character in transition states is considered to be an important component in establishing the expected reliability of various electronic structure methods. In the present work, the multireference characters of the transition states and the forming and breaking of bonds for a large set of hydrogen abstraction reactions from phenolic compounds by peroxyl radicals have been analyzed using the T1, M, B1, and GB1 diagnostics. The extent of multireference character depends on the system and on the conditions under which the reaction takes place, and some systematic trends are observed. In particular, the multireference character is found to be reduced by solvation, the size of the phenolic compound, and deprotonation in aqueous solution. However, the deviations of calculated rate constants from experimental ones are not correlated with the extent of multireference character. The performance of single-determinant density functional theory was investigated for the kinetics of these reactions by comparing calculated rate constants to experimental data; the results from these analyses showed that the M05 functional performs well for the task at hand.

  5. Experimental and theoretical investigations of the rate constant for the reaction of the hydroxyl radical with methyl ethyl ketone

    NASA Astrophysics Data System (ADS)

    Vimal, D.; Stevens, P. S.

    2007-12-01

    Methyl ethyl ketone (MEK) or 2-butanone is a high-volume industrial solvent with a production rate greater than 70 million lbs yr-1. It is also a photo-oxidation product of several volatile organic compounds (VOCs) in the atmosphere. MEK is removed from the atmosphere primarily by its reaction with hydroxyl (OH) radical. As a result, knowledge of the chemical mechanism and temperature dependence of this reaction is important as MEK may be transported to the upper troposphere and influence the chemistry of this region of the atmosphere. We present absolute measurements of the rate constant and the kinetic isotope effect for the reaction of MEK with OH radicals at low pressure and over the temperature range 263-388 K using a discharge-flow technique coupled with resonance fluorescence detection of OH radicals. Theoretical studies of the potential energy surface suggest that the reaction of MEK and OH proceeds by H-abstraction mediated by the formation of a 7- membered hydrogen-bonded complex. This mechanism is similar to that of several other atmospherically relevant oxygenated VOCs such as acetone, acetic acid and hydroxyacetone. The influence of the pre-reactive complex on the temperature dependence for this reaction will be discussed.

  6. Rate constants for the reactions of free radicals with oxygen in solution

    SciTech Connect

    Maillard, B.; Ingold, K.U.; Scaiano, J.C.

    1983-07-27

    The kinetics of the rections of several free radicals with oxygen have been examined in solution at 300 K using laser flash photolysis techniques. The reactions of resonance-stabilized radicals are only slightly slower than those of nonstabilized radicals: for example, for tert-butyl (in cyclohexane), 4.93 x 10/sup 9/; benzyl, 2.36 x 10/sup 9/ (in cyclohexane); cyclohexadienyl (in benzene), 1.64 x 10/sup 9/ M/sup -1/ s/sup -1/. The reaction of butyl-tin (n-Bu/sub 3/Sn.) radicals is unusually fast (7.5 x 10/sup 9/ M/sup -1/ s/sup -1/), a fact that has been tentatively attributed to a relaxation of spin selection rules due to heavy atom effects. 1 table.

  7. Evaluation of Unimolecular Rate Constants for Some Outer-Sphere Electrochemical Reactions.

    DTIC Science & Technology

    1985-01-01

    using the Debye - Huckel model.7 Nevertheless, Haim and ii p coworkers have demonstrated that ket values may be determined directly for homogeneous outer...Chapman theory . Analogous considerations apply to bimolecular outer-sphere reactions in homogeneous solutions, where it is customary to estimate K by...4RT) (where d is calculated by including both reactant and supporting electrolyte ions) and K is the reciprocal Debye length. (This GCrelation assumes

  8. Pressure dependence of the absolute rate constant for the reaction OH + C2H2 from 228 to 413K

    NASA Technical Reports Server (NTRS)

    Michael, J. V.; Nava, D. F.; Borokowski, R. P.; Payne, W. A.; Stief, L. J.

    1980-01-01

    The pressure dependence of absolute rate constants for the reaction of OH + C2H2 yields products has been examined at five temperatures ranging from 228 to 413 K. The experimental techniques which was used is flash photolysis-resonance fluoresence. OH was produced by water photolysis and hydroxyl resonance fluorescent photons were measured by multiscaling techniques. The results indicate that the low pressure bimolecular rate constant is 4 x 10 the the minus 13th power cu cm molecule (-1) s(-1) over the temperature range studied. A substantial increase in the bimolecular rate constant with an increase in pressure was observed at all temperatures except 228 K. This indicates the importance of initial adduct formation and subsequent stablization. The high pressure results are well represented by the Arrhenius expression (k sub bi) sub infinity = (6.83 + or - 1.19) x 10 to the minus 12th power exp(-646 + or - 47/T)cu cm molecule (-1) s(-1). The results are compared to previous investigated and are theoretically discussed. The implications of these results on modeling of terrestrial and planetary atmospheres and also in combustion chemistry are discussed.

  9. MORATE 6.5: A new version of a computer program for direct dynamics calculations of chemical reaction rate constants

    NASA Astrophysics Data System (ADS)

    Hu, Wei-Ping; Lynch, Gillian C.; Liu, Yi-Ping; Rossi, Ivan; Stewart, James J. P.; Steckler, Rozeanne; Garrett, Bruce C.; Isaacson, Alan D.; Lu, Da-hong; Melissas, Vasilios S.; Truhlar, Donald G.

    1995-08-01

    MORATE (Molecular Orbital RATE calculations) is a computer program for direct dynamics calculations of unimolecular and bimolecular rate constants of gas-phase chemical reactions involving atoms, diatoms, or polyatomic species. The dynamical methods used are conventional or variational transition state theory and multidimensional semiclassical approximations for tunneling and nonclassical reflection. Variational transition states are found by a one-dimensional search of generalized-transition-state dividing surfaces perpendicular to the minimum-energy path, and tunneling probabilities are evaluated by multidimensional semiclassical algorithms, including the small-curvature and large-curvature tunneling approximations and the microcanonical optimized multidimensional tunneling approximation. The computer program is a conventiently interfaced package consisting of the POLYRATE program, version 6.5, for dynamical rate constant calculations, and the MOPAC program, version 5.05mn, for semiempirical electronic structure computations. In single-level mode, the potential energies, gradients, and higher derivatives of the potential are computed whenever needed by electronic structure calculations employing semiempirical molecular orbital theory without the intermediary of a global or semiglobal fit. All semiempirical methods available in MOPAC, in particular MINDO/3, MNDO, AM1, and PM3, can be called on to calculate the potential, gradient, or Hessian, as required at various steps of the dynamics calculations, and, in addition, the code has flexible options for electronic structure calculations with neglect of diatomic differential overlap and specific reaction parameters (NDDO-SRP). In dual-level mode, MINDO/3, MNDO, AM1, PM3, or NDDO-SRP is used as a lower level to calculate the reaction path, and interpolated corrections to energies and frequencies are added; these corrections are based on higher-level data read from an external file.

  10. Pressure dependence of the absolute rate constant for the reaction Cl + C2H2 from 210-361 K

    NASA Technical Reports Server (NTRS)

    Brunning, J.; Stief, L. J.

    1985-01-01

    In recent years, considerable attention has been given to the role of chlorine compounds in the catalytic destruction of stratospheric ozone. However, while some reactions have been studied extensively, the kinetic data for the reaction of Cl with C2H2 is sparse with only three known determinations of the rate constant k3. The reactions involved are Cl + C2H2 yields reversibly ClC2H2(asterisk) (3a) and ClC2H2(asterisk) + M yields ClC2H2 + M (3b). In the present study, flash photolysis coupled with chlorine atomic resonance fluorescence have been employed to determine the pressure and temperature dependence of k3 with the third body M = Ar. Room temperature values are also reported for M = N2. The pressure dependence observed in the experiments confirms the expectation that the reaction involves addition of Cl to the unsaturated C2H2 molecule followed by collisional stabilization of the resulting adduct radical.

  11. Theoretical study on the rate constants for the C2H5 + HBr --> C2H6 + Br reaction.

    PubMed

    Sheng, Li; Li, Ze-Sheng; Liu, Jing-Yao; Xiao, Jing-Fa; Sun, Chia-Chung

    2004-02-01

    The reaction C(2)H(5) + HBr --> C(2)H(6) + Br has been theoretically studied over the temperature range from 200 to 1400 K. The electronic structure information is calculated at the BHLYP/6-311+G(d,p) and QCISD/6-31+G(d) levels. With the aid of intrinsic reaction coordinate theory, the minimum energy paths (MEPs) are obtained at the both levels, and the energies along the MEP are further refined by performing the single-point calculations at the PMP4(SDTQ)/6-311+G(3df,2p)//BHLYP and QCISD(T)/6-311++G(2df,2pd)//QCISD levels. The calculated ICVT/SCT rate constants are in good agreement with available experimental values, and the calculate results further indicate that the C(2)H(5) + HBr reaction has negative temperature dependence at T < 850 K, but clearly shows positive temperature dependence at T > 850 K. The current work predicts that the kinetic isotope effect for the title reaction is inverse in the temperature range from 200 to 482 K, i.e., k(HBr)/k(DBr) < 1.

  12. Pressure dependence of the absolute rate constant for the reaction Cl + C2H2 from 210-361 K

    NASA Technical Reports Server (NTRS)

    Brunning, J.; Stief, L. J.

    1985-01-01

    In recent years, considerable attention has been given to the role of chlorine compounds in the catalytic destruction of stratospheric ozone. However, while some reactions have been studied extensively, the kinetic data for the reaction of Cl with C2H2 is sparse with only three known determinations of the rate constant k3. The reactions involved are Cl + C2H2 yields reversibly ClC2H2(asterisk) (3a) and ClC2H2(asterisk) + M yields ClC2H2 + M (3b). In the present study, flash photolysis coupled with chlorine atomic resonance fluorescence have been employed to determine the pressure and temperature dependence of k3 with the third body M = Ar. Room temperature values are also reported for M = N2. The pressure dependence observed in the experiments confirms the expectation that the reaction involves addition of Cl to the unsaturated C2H2 molecule followed by collisional stabilization of the resulting adduct radical.

  13. Direct rate constant measurements for the reaction of ground-state atomic oxygen with ethylene, 244-1052 K

    SciTech Connect

    Klemm, R.B.; Nesbitt, F.L.; Skolnik, E.G.; Lee, J.H.; Smalley, J.F.

    1987-03-12

    The rate constant for the reaction of ground-state atomic oxygen with ethylene was determined by using two techniques: flash photolysis-resonance fluorescence (FP-RF, 244-1052 K) and discharge flow-resonance fluorescence (DF-RF, 298-1017 K). Kinetic complications due to the presence of molecular oxygen in the FP-RF experiments at high temperatures (T > 800 K) were overcome by using NO as the photolytic source of the O atoms. The rate constant, k/sub 1/ (T), derived in this study exhibits extreme non-Arrhenius behavior, but it can be successfully fit to the sum of exponentials expression, 244-1052 K, k/sub 1/(T) = (1.02 +/- 0.06) x 10/sup -11/ exp(-753 +/- 17 K/T) + (2.75 +/- 0.26) x 10/sup -10/ exp(-4220 +/- 550 K/T), in units of cm/sup 3/ molecule/sup -1/ s/sup -1/. Additionally, a fit of the results of this work to a simple transition-state theory expression and the comparison of these results with those of other workers are discussed.

  14. Quasiclassical trajectory study of the F + H 2 system. Rate constants, kinetic isotope effects and energy partitioning among reaction products

    NASA Astrophysics Data System (ADS)

    Rosenman, Efrat; Persky, Avigdor

    1995-06-01

    Quasiclassical trajectory calculations were carried out for the reactions F + H 2, F + D 2, and F + HD, using two potential energy surfaces T5A and 6SEC. The results which include rate constants and kinetic isotope effects as a function of temperature, isotopic branching ratios for F + HD as a function off collision energy and the energy partitioning and vibrational state distributions of the products at room temperature, are compared with experimental data. For most of the quantities under study, the results for the 6SEC surface are in qualitative agreement with experiment, as opposed to the results for the T5A surface. The conclusions from the present study concerning the quality of the 6SEC surface are consistent with the conclusions of Aoiz et al. which are based mainly on calculations of vibrationally state resolved differential cross sections and vibrational distributions of products, for specific collision energies.

  15. Theoretical rate constants and kinetic isotope effects in the reaction of methane with H, D, T, and Mu atoms.

    PubMed

    Espinosa-García, J

    2008-03-07

    The rate constants and kinetic isotope effects of the reaction of methane with four isotopes of hydrogen, protium (H), deuterium (D), tritium (T), and muonium (Mu), were studied using variational transition state theory with multidimensional tunneling on an analytical potential energy surface, PES-2002, previously constructed by our group. For the four isotopes, our kinetics results agree reasonably with available experimental measurements, improving previous theoretical results that used different potential energy surfaces and/or theoretical approaches. In the comparison of the reactivity between protium and muonium, which is the most severe test of the surface and theoretical method due to the large mass difference between the two isotopes, some sources of discrepancy between theory and experiment were analyzed. These were the zero-point energy, tunneling effect, and the role of the reactivity from methane excited vibrational states.

  16. The reaction O((3)P) + HOBr: Temperature dependence of the rate constant and importance of the reaction as an HOBr stratospheric loss process

    NASA Technical Reports Server (NTRS)

    Nesbitt, F. L.; Monks, P. S.; Payne, W. A.; Stief, L. J.; Toumi, R.

    1995-01-01

    The absolute rate constant for the reaction O((3)P) + HOBr has been measured between T = 233K and 423K using the discharge-flow kinetic technique coupled to mass spectrometric detection. The value of the rate coefficient at room temperature is (2.5 +/- 0.6) x 10(exp -11)cu cm/molecule/s and the derived Arrhenius expression is (1.4 +/- 0.5) x 10(exp -10) exp((-430 +/- 260)/T)cu cm/molecule/s. From these rate data the atmospheric lifetime of HOBr with respect to reaction with O((3)P) is about 0.6h at z = 25 km which is comparable to the photolysis lifetime based on recent measurements of the UV cross section for HOBr. Implications for HOBr loss in the stratosphere have been tested using a 1D photochemical box model. With the inclusion of the rate parameters and products for the O + HOBr reaction, calculated concentration profiles of BrO increase by up to 33% around z = 35 km. This result indicates that the inclusion of the O + HOBr reaction in global atmospheric chemistry models may have an impact on bromine partitioning in the middle atmosphere.

  17. The reaction O((3)P) + HOBr: Temperature dependence of the rate constant and importance of the reaction as an HOBr stratospheric loss process

    NASA Technical Reports Server (NTRS)

    Nesbitt, F. L.; Monks, P. S.; Payne, W. A.; Stief, L. J.; Toumi, R.

    1995-01-01

    The absolute rate constant for the reaction O((3)P) + HOBr has been measured between T = 233K and 423K using the discharge-flow kinetic technique coupled to mass spectrometric detection. The value of the rate coefficient at room temperature is (2.5 +/- 0.6) x 10(exp -11)cu cm/molecule/s and the derived Arrhenius expression is (1.4 +/- 0.5) x 10(exp -10) exp((-430 +/- 260)/T)cu cm/molecule/s. From these rate data the atmospheric lifetime of HOBr with respect to reaction with O((3)P) is about 0.6h at z = 25 km which is comparable to the photolysis lifetime based on recent measurements of the UV cross section for HOBr. Implications for HOBr loss in the stratosphere have been tested using a 1D photochemical box model. With the inclusion of the rate parameters and products for the O + HOBr reaction, calculated concentration profiles of BrO increase by up to 33% around z = 35 km. This result indicates that the inclusion of the O + HOBr reaction in global atmospheric chemistry models may have an impact on bromine partitioning in the middle atmosphere.

  18. Rate constants and temperature dependences for the reactions of hydroxyl radical with several halogenated methanes, ethanes, and propanes by relative rate measurements

    NASA Technical Reports Server (NTRS)

    Hsu, K.-J.; DeMore, W. B.

    1995-01-01

    Rate constants of 15 OH reactions with halogen-substituted alkanes, C1 to C3, were studied using a relative rate technique in the temperature range 283-403 K. Compounds studied were CHF2Cl (22), CHF2Br (22B), CH3F (41), CH2F2 (32), CHF3 (23), CHClFCCl2F (122a), CHCl2CF3 (123), CHClFCF3 (124), CH3CF3 (143a), CH3CH2F (161), CF3CHFCF3 (227ea), CF3CH2CF3 (236fa), CF3CHFCHF2 (236ea), and CHF2CF2CH2F (245ca). Using CH4, CH3CCl3, CF3CF2H, and C2H6 as primary reference standards (JPL 92-20 rate constants), absolute rate constants are derived. Results are in good agreement with previous experimental results for six of the compounds studied, including CHF2Cl, CHF2Br, CH2F2, CH3CF3, CHFClCFCl2, and CF3CHFCF3. For the remainder the relative rate constants are lower than those derived from experiments in which OH loss was used to measure the reaction rate. Comparisons of the derived Arrhenius A factors with previous literature transition-state calculations show order of magnitude agreement in most cases. However, the experimental A factors show a much closer proportionality to the number of H atoms in the molecule than is evident from the transition state calculations. For most of the compounds studied, an A factor of (8 +/- 3)E-13 cm(exp 3)/(molecule s) per C-H bond is observed. A new measurement of the ratio k(CH3CCl3)/k(CH4) is reported that is in good agreement with previous data.

  19. Rate constant for the termolecular reaction of OH+toluene+helium in the fall-off range below 10 Torr

    NASA Astrophysics Data System (ADS)

    Bourmada, N.; Devolder, P.; Sochet, L.-R.

    1988-08-01

    We have measured the title rate constant by the discharge flow technique associated with detection of OH by resonance fluorescence and photon counting. The experimental conditions are as follows: pressure range 0.4 to 9.8 Tort; temperature range 297 to 353 K. From room temperature results, the Troe parameters k0 (low-pressure limiting rate constant) and k∞ (high-pressure limiting rate constant) are derived: k0 = (4.0 ± 0.5 ) × 10 -28 cm 6 molecule -2 s -1 ; k∞ = (6.0±0.7) × 10 -12 cm 3 molecule -1 s -1.

  20. The Effect of Low Frequency Vibrations in Methane on the Rate Constant for the Reactions of Diatomic Oxygen(+) (X square)II sub g, v=0)) with Methane

    DTIC Science & Technology

    1992-01-01

    Frequency VibrAatns in Hon the Rate Constant for the Reaction of (X2 1I’ v-0) With PR 2310 C114 Mu nY I TA G2 )v. c M WU 30 A.A. Viggiano , Robert K...92-2033 The effect of low frequency vibrations in CH 4 on the rate constant for the reaction of 0 + (X 2][1 , v=0) with CH4 A. A. Viggiano , Robert A...of PhySics 275 276 Viggiano eta.: Reaction of O with CH, with CH4 using the Phillips Laboratory variable tempera- over the range 85-570 K. All

  1. Development of a QSAR model for predicting aqueous reaction rate constants of organic chemicals with hydroxyl radicals.

    PubMed

    Luo, Xiang; Yang, Xianhai; Qiao, Xianliang; Wang, Ya; Chen, Jingwen; Wei, Xiaoxuan; Peijnenburg, Willie J G M

    2017-03-22

    Reaction with hydroxyl radicals (˙OH) is an important removal pathway for organic pollutants in the aquatic environment. The aqueous reaction rate constant (kOH) is therefore an important parameter for fate assessment of aquatic pollutants. Since experimental determination fails to meet the requirement of being able to efficiently handle numerous organic chemicals at limited cost and within a relatively short period of time, in silico methods such as quantitative structure-activity relationship (QSAR) models are needed to predict kOH. In this study, a QSAR model with a larger and wider applicability domain as compared with existing models was developed. Following the guidelines for the development and validation of QSAR models proposed by the Organization for Economic Co-operation and Development (OECD), the model shows satisfactory performance. The applicability domain of the model has been extended and contained chemicals that have rarely been covered in most previous studies. The chemicals covered in the current model contain functional groups including [double bond splayed left]C[double bond, length as m-dash]C[double bond splayed right], -C[triple bond, length as m-dash]C-, -C6H5, -OH, -CHO, -O-, [double bond splayed left]C[double bond, length as m-dash]O, -C[double bond, length as m-dash]O(O)-, -COOH, -C[triple bond, length as m-dash]N, [double bond splayed left]N-, -NH2, -NH-C(O)-, -NO2, -N[double bond, length as m-dash]C-N[double bond splayed right], [double bond splayed left]N-N[double bond splayed right], -N[double bond, length as m-dash]N-, -S-, -S-S-, -SH, -SO3, -SO4, -PO4, and -X (F, Cl, Br, and I).

  2. Catalytic antibody light chain capable of cleaving a chemokine receptor CCR-5 peptide with a high reaction rate constant.

    PubMed

    Mitsuda, Yukie; Hifumi, Emi; Tsuruhata, Kumi; Fujinami, Hiroko; Yamamoto, Naoki; Uda, Taizo

    2004-04-20

    A monoclonal antibody (MAb), ECL2B-2, was obtained by immunizing a peptide possessing a part of a sequence of a chemokine receptor, CCR-5, which is present as a membrane protein on the macrophage surface, and which plays an important role in human immunodeficiency virus (HIV) infection. From the DNA and the deduced amino acid sequences of the light and heavy chains of ECL2B-2 MAb, molecular modeling was conducted to calculate the steric conformation of the antibody. Modeling suggested that the structure of ECL2B-2 could possess one or two catalytic triad(s), composed of Asp(1), Ser(27a) (or Ser(27e)), and His(93) (or His(27d)), in the light chain of ECL2B-2. The three amino acid residues, Asp(1), Ser(27a), and His(93), are identical to those of catalytic antibody light chains such as VIPase and i41SL1-2. The light chain of ECL2B-2 MAb degraded the antigenic peptide CCR-5 within about 100 h. Surprisingly, the light chain had a very high catalytic reaction rate constant (k(cat)) of 2.23 min(-1), which is greater by factors of tens to hundreds than those of natural catalytic antibodies obtained previously. The heavy chain of ECL2B-2 MAb, which has no catalytic triad because of a lack of His residue, did not degrade the CCR-5 peptide.

  3. The chemistry of bromine in the stratosphere: Influence of a new rate constant for the reaction BrO + HO2

    NASA Technical Reports Server (NTRS)

    Pirre, Michel; Marceau, Francois J.; Lebras, Georges; Maguin, Francoise; Poulet, Gille; Ramaroson, Radiela

    1994-01-01

    The impact of new laboratory data for the reaction BrO + HO2 yields HOBr + O2 in the depletion of global stratospheric ozone has been estimated using a one-dimensional photochemical model taking into account the heterogeneous reaction on sulphate aerosols which converts N2O5 into HNO3. Assuring an aerosol loading 2 times as large as the 'background' and a reaction probability of 0.1 for the above heterogeneous reaction, the 6 fold increase in the measured rate constant for the reaction of BrO with HO2 increases the computed depletion of global ozone produced by 20 ppt of total bromine from 2.01 percent to 2.36 percent. The use of the higher rate constant increases the HOBr mixing ratio and makes the bromine partitioning and the ozone depletion very sensitive to the branching ratio of the potential channel forming HBr in the BrO + HO2 reaction.

  4. Low temperature rate constants for the N + CN → N2 + C reaction: two-dimensional quantum capture calculations on an accurate potential energy surface.

    PubMed

    Ma, Jianyi; Guo, Hua; Dawes, Richard

    2012-09-21

    The title reaction is thought to be responsible for the production of molecular nitrogen in interstellar clouds. In this work, we report quantum capture calculations on a new two-dimensional potential energy surface determined by interpolating high-level ab initio data. The low-temperature rate constant calculated using a capture model is quite large and has a positive temperature dependence, in agreement with a recent experiment. The origin of the aforementioned behaviors of the rate constant is analyzed.

  5. RATE CONSTANTS FOR THE GAS-PHASE REACTIONS OF A SERIES OF C3-C6 ALDEHYDES WITH OH AND NO3 RADICALS. (R825252)

    EPA Science Inventory

    By using relative rate methods, rate constants for the gas-phase reactions of OH and NO3 radicals with propanal, butanal, pentanal, and hexanal have been measured at 296 ? 2 K and atmospheric pressure of air. By using methyl vinyl ketone as the reference compound, the ...

  6. Nonadiabatic rate constants for proton transfer and proton-coupled electron transfer reactions in solution: Effects of quadratic term in the vibronic coupling expansion

    SciTech Connect

    Soudackov, Alexander V.; Hammes-Schiffer, Sharon

    2015-11-21

    Rate constant expressions for vibronically nonadiabatic proton transfer and proton-coupled electron transfer reactions are presented and analyzed. The regimes covered include electronically adiabatic and nonadiabatic reactions, as well as high-frequency and low-frequency proton donor-acceptor vibrational modes. These rate constants differ from previous rate constants derived with the cumulant expansion approach in that the logarithmic expansion of the vibronic coupling in terms of the proton donor-acceptor distance includes a quadratic as well as a linear term. The analysis illustrates that inclusion of this quadratic term in the framework of the cumulant expansion framework may significantly impact the rate constants at high temperatures for proton transfer interfaces with soft proton donor-acceptor modes that are associated with small force constants and weak hydrogen bonds. The effects of the quadratic term may also become significant in these regimes when using the vibronic coupling expansion in conjunction with a thermal averaging procedure for calculating the rate constant. In this case, however, the expansion of the coupling can be avoided entirely by calculating the couplings explicitly for the range of proton donor-acceptor distances sampled. The effects of the quadratic term for weak hydrogen-bonding systems are less significant for more physically realistic models that prevent the sampling of unphysical short proton donor-acceptor distances. Additionally, the rigorous relation between the cumulant expansion and thermal averaging approaches is clarified. In particular, the cumulant expansion rate constant includes effects from dynamical interference between the proton donor-acceptor and solvent motions and becomes equivalent to the thermally averaged rate constant when these dynamical effects are neglected. This analysis identifies the regimes in which each rate constant expression is valid and thus will be important for future applications to proton

  7. Nonadiabatic rate constants for proton transfer and proton-coupled electron transfer reactions in solution: Effects of quadratic term in the vibronic coupling expansion

    PubMed Central

    Soudackov, Alexander V.; Hammes-Schiffer, Sharon

    2015-01-01

    Rate constant expressions for vibronically nonadiabatic proton transfer and proton-coupled electron transfer reactions are presented and analyzed. The regimes covered include electronically adiabatic and nonadiabatic reactions, as well as high-frequency and low-frequency proton donor-acceptor vibrational modes. These rate constants differ from previous rate constants derived with the cumulant expansion approach in that the logarithmic expansion of the vibronic coupling in terms of the proton donor-acceptor distance includes a quadratic as well as a linear term. The analysis illustrates that inclusion of this quadratic term in the framework of the cumulant expansion framework may significantly impact the rate constants at high temperatures for proton transfer interfaces with soft proton donor-acceptor modes that are associated with small force constants and weak hydrogen bonds. The effects of the quadratic term may also become significant in these regimes when using the vibronic coupling expansion in conjunction with a thermal averaging procedure for calculating the rate constant. In this case, however, the expansion of the coupling can be avoided entirely by calculating the couplings explicitly for the range of proton donor-acceptor distances sampled. The effects of the quadratic term for weak hydrogen-bonding systems are less significant for more physically realistic models that prevent the sampling of unphysical short proton donor-acceptor distances. Additionally, the rigorous relation between the cumulant expansion and thermal averaging approaches is clarified. In particular, the cumulant expansion rate constant includes effects from dynamical interference between the proton donor-acceptor and solvent motions and becomes equivalent to the thermally averaged rate constant when these dynamical effects are neglected. This analysis identifies the regimes in which each rate constant expression is valid and thus will be important for future applications to proton

  8. Theoretical derivation for reaction rate constants of H abstraction from thiophenol by the H/O radical pool

    PubMed Central

    Batiha, Marwan; Altarawneh, Mohammednoor; Al-Harahsheh, Mohammad; Altarawneh, Ibrahem; Rawadieh, Saleh

    2011-01-01

    Reaction and activation energy barriers are calculated for the H abstraction reactions (C6H5SH + X• → C6H5S + XH, X = H, OH and HO2) at the BB1K/GTLarge level of theory. The corresponding reactions with H2S and CH3SH are also investigated using the G3B3 and CBS-QB3 methods in order to demonstrate the accuracy of BB1K functional in finding activation barriers for hydrogen atom transfer reactions. Arrhenius parameters for the title reactions are fitted in the temperature range of 300 K–2000 K. The calculated reaction enthalpies are in good agreement with their corresponding experimental reaction enthalpies. It is found that H abstraction by OH radicals from the thiophenol molecule proceed in a much slower rate in reference to the analogous phenol molecule. ΔfH298o of thiophenoxy radical is calculated to be 63.3 kcal/mol. Kinetic parameters presented herein should be useful in describing the decomposition rate of thiophenol; i.e., one of the major aromatic sulfur carriers, at high temperatures. PMID:22485200

  9. Absolute rate constant for the reaction of atomic chlorine with hydrogen peroxide vapor over the temperature range 265-400 K

    NASA Technical Reports Server (NTRS)

    Michael, J. V.; Whytock, D. A.; Lee, J. H.; Payne, W. A.; Stief, L. J.

    1977-01-01

    Rate constants for the reaction of atomic chlorine with hydrogen peroxide were measured from 265-400 K using the flash photolysis-resonance fluorescence technique. Analytical techniques were developed to measure H2O2 under reaction conditions. Due to ambiguity in the interpretation of the analytical results, the data combine to give two equally acceptable representations of the temperature dependence. The results are compared to previous work at 298 K and are theoretically discussed in terms of the mechanism of the reaction. Additional experiments on the H + H2O2 reaction at 298 and 359 K are compared with earlier results from this laboratory and give a slightly revised bimolecular rate constant.

  10. Energy disposal and thermal rate constants for the OH + HBr and OH + DBr reactions: quasiclassical trajectory calculations on an accurate potential energy surface.

    PubMed

    de Oliveira-Filho, Antonio G S; Ornellas, Fernando R; Bowman, Joel M

    2014-12-26

    We report reaction cross sections, energy disposal, and rate constants for the OH + HBr → Br + H2O and OH + DBr → Br + HDO reactions from quasiclassical trajectory calculations using an ab initio potential energy surface [ de Oliveira-Filho , A. G. S. ; Ornellas , F. R. ; Bowman , J. M. J. Phys. Chem. Lett. 2014 , 5 , 706 - 712 ]. Comparison with available experiments are made and generally show good agreement.

  11. Room-temperature and temperature-dependent QSRR modelling for predicting the nitrate radical reaction rate constants of organic chemicals using ensemble learning methods.

    PubMed

    Gupta, S; Basant, N; Mohan, D; Singh, K P

    2016-07-01

    Experimental determinations of the rate constants of the reaction of NO3 with a large number of organic chemicals are tedious, and time and resource intensive; and the development of computational methods has widely been advocated. In this study, we have developed room-temperature (298 K) and temperature-dependent quantitative structure-reactivity relationship (QSRR) models based on the ensemble learning approaches (decision tree forest (DTF) and decision treeboost (DTB)) for predicting the rate constant of the reaction of NO3 radicals with diverse organic chemicals, under OECD guidelines. Predictive powers of the developed models were established in terms of statistical coefficients. In the test phase, the QSRR models yielded a correlation (r(2)) of >0.94 between experimental and predicted rate constants. The applicability domains of the constructed models were determined. An attempt has been made to provide the mechanistic interpretation of the selected features for QSRR development. The proposed QSRR models outperformed the previous reports, and the temperature-dependent models offered a much wider applicability domain. This is the first report presenting a temperature-dependent QSRR model for predicting the nitrate radical reaction rate constant at different temperatures. The proposed models can be useful tools in predicting the reactivities of chemicals towards NO3 radicals in the atmosphere, hence, their persistence and exposure risk assessment.

  12. The reaction of atomic hydrogen with germane - Temperature dependence of the rate constant and implications for germane photochemistry in the atmospheres of Jupiter and Saturn

    NASA Technical Reports Server (NTRS)

    Nava, David F.; Payne, Walter A.; Marston, George; Stief, Louis J.

    1993-01-01

    Studies of the formation and loss processes for GeH4 are required in order to provide data to help determine the major chemical form in which germanium exists in the atmospheres of Jupiter and Saturn. The reaction of hydrogen atoms with germane is one of the most important of these reactions. The absolute rate constant for this reaction as a function of temperature and pressure is studied. Flash photolysis of dilute mixtures of GeH4 in argon, combined with time-resolved detection of H atoms via Lyman alpha resonance fluorescence, is employed to measure the reaction rate. The reaction is shown to be moderately rapid, independent of total pressure, but possessing a positive temperature dependence.

  13. Understanding supported reactions in spherical compartments: a general algorithm to model and determine rate constants, diffusion coefficients, and spatial product distributions.

    PubMed

    Egelhaaf, Hans-Joachim; Rademann, Jörg

    2005-01-01

    A general algorithm allowing the numerical modeling of the time and space dependence of product formation in spherical reaction volumes is described. The algorithm is described by the complete set of mass balance equations. On the basis of these equations, the effects of the diffusion coefficient, reaction rate, bead size, reagent excess, and packing density of the resin beads on the overall reaction rates are determined for second-order reactions. Experimental data of reaction progress are employed to calculate reaction rates and diffusion coefficients in polymer-supported reactions. In addition, the conditions for shell-like product formation are determined, and various strategies for the radial patterning of resin beads are compared. The effect of diffusion on polymer-supported enzyme-catalyzed reactions of the Michaelis-Menten type is treated, as well. Finally, the effects of typical nonideal solid-phase phenomena, namely, the inhomogeneity of rate constants and the concentration dependence of diffusion coefficients, on overall rates are discussed.

  14. Accurate prediction of rate constants of Diels-Alder reactions and application to design of Diels-Alder ligation.

    PubMed

    Tang, Shi-Ya; Shi, Jing; Guo, Qing-Xiang

    2012-04-07

    Bioorthogonal reactions are useful tools to gain insights into the structure, dynamics, and function of biomolecules in the field of chemical biology. Recently, the Diels-Alder reaction has become a promising and attractive procedure for ligation in bioorthogonal chemistry because of its higher rate and selectivity in water. However, a drawback of the previous Diels-Alder ligation is that the widely used maleimide moiety as a typical Michael acceptor can readily undergo Michael addition with nucleophiles in living systems. Thus, it is important to develop a nucleophile-tolerant Diels-Alder system in order to extend the scope of the application of Diels-Alder ligation. To solve this problem, we found that the theoretical protocol M06-2X/6-31+G(d)//B3LYP/6-31G(d) can accurately predict the activation free energies of Diels-Alder reactions with a precision of 1.4 kcal mol(-1) by benchmarking the calculations against the 72 available experimental data. Subsequently, the electronic effect and ring-strain effect on the Diels-Alder reaction were studied to guide the design of the new dienophiles. The criteria of the design is that the designed Diels-Alder reaction should have a lower barrier than the Michael addition, while at the same time it should show a similar (or even higher) reactivity as compared to the maleimide-involving Diels-Alder ligation. Among the designed dienophiles, three substituted cyclopropenes (i.e. 1,2-bis(trifluoromethyl)-, 1,2-bis(hydroxylmethyl)- and 1,2-bis(hydroxylmethyl)-3-carboxylcyclopropenes) meet our requirements. These substituted cyclopropene analogs could be synthesized and they are thermodynamically stable. As a result, we propose that 1,2-bis(trifluoromethyl)-, 1,2-bis(hydroxylmethyl)- and 1,2-bis(hydroxylmethyl)-3-carboxylcyclopropenes may be potential candidates for efficient and selective Diels-Alder ligation in living systems.

  15. Rate constant for the reaction of NO sub 2 with sulfur(IV) over the pH range 5. 3-13

    SciTech Connect

    Clifton, C.L.; Altstein, N.; Hule, R.E. )

    1988-05-01

    Rate constants have been determined for the reactions of NO{sub 2} with SO{sub 3}{sup 2{minus}} and HSO{sub 3}{sup {minus}} in aqueous solutions. A pulse radiolysis apparatus with signal averaging, which has allowed us to monitor the decay of NO{sub 2} directly and to measure rate constants for the reaction of NO{sub 2} with SO{sub 3}{sup 2{minus}} and HSO{sub 3}{sup {minus}} over the pH range 5.3-13. The rate constant increases from about 1.2 {times} 10{sup 7} M{sup {minus}1} s{sup {minus}1} near pH 5 to 2.9 {times} 10{sup 7} M{sup {minus}1} s{sup {minus}1} at pH 13. The reaction appears to involve the formation of an intermediate complex that may undergo subsequent reaction with NO{sub 2} to yield the ultimate products or may react with other substrates present. The formation of a long-lived intermediate would have implications on the chemistry of flue gas scrubbers and on luminol-based NO{sub 2} detectors.

  16. Nonadiabatic rate constants for proton transfer and proton-coupled electron transfer reactions in solution: Effects of quadratic term in the vibronic coupling expansion

    SciTech Connect

    Soudackov, Alexander; Hammes-Schiffer, Sharon

    2015-11-17

    Rate constant expressions for vibronically nonadiabatic proton transfer and proton-coupled electron transfer reactions are presented and analyzed. The regimes covered include electronically adiabatic and nonadiabatic reactions, as well as high-frequency and low-frequency regimes for the proton donor-acceptor vibrational mode. These rate constants differ from previous rate constants derived with the cumulant expansion approach in that the logarithmic expansion of the vibronic coupling in terms of the proton donor-acceptor distance includes a quadratic as well as a linear term. The analysis illustrates that inclusion of this quadratic term does not significantly impact the rate constants derived using the cumulant expansion approach in any of the regimes studied. The effects of the quadratic term may become significant when using the vibronic coupling expansion in conjunction with a thermal averaging procedure for calculating the rate constant, however, particularly at high temperatures and for proton transfer interfaces with extremely soft proton donor-acceptor modes that are associated with extraordinarily weak hydrogen bonds. Even with the thermal averaging procedure, the effects of the quadratic term for weak hydrogen-bonding systems are less significant for more physically realistic models that prevent the sampling of unphysical short proton donor-acceptor distances, and the expansion of the coupling can be avoided entirely by calculating the couplings explicitly for the range of proton donor-acceptor distances. This analysis identifies the regimes in which each rate constant expression is valid and thus will be important for future applications to proton transfer and proton-coupled electron transfer in chemical and biological processes. We are grateful for support from National Institutes of Health Grant GM056207 (applications to enzymes) and the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy

  17. Textbook Deficiencies: Ambiguities in Chemical Kinetics Rates and Rate Constants

    NASA Astrophysics Data System (ADS)

    Quisenberry, Keith T.; Tellinghuisen, Joel

    2006-03-01

    Balanced chemical reactions often have at least some stoichiometry coefficients that are not unity. To avoid ambiguity in defining the kinetics rate for a reaction, the IUPAC has established the convention, rate = (1/ν i )/(d[A i ]/d t ) relating the reaction rate to the rate of change of concentration of any reactant or product A i and its stoichiometry number ν i (negative for reactants, positive for products). The rate is a product of the rate constant k and some function of the concentrations of reactants and products that must be determined experimentally. While most general chemistry textbooks correctly state this convention, most also proceed to ignore it in subsequent development, particularly in the use of integrated rate laws and the definition of the reaction half-life. We recommend that in future editions, authors make it clear that (i) the reaction rate and rate constant cannot be defined unambiguously without explicitly stating the reaction for which they apply and therefore (ii) the relation between the half-life, which is a physical property of the reaction system, and the rate constant depends upon how the reaction is written. The errors have arisen in part because most texts simply state the integrated rate expressions for first- and second-order reactions without deriving them. It is both appropriate and easy to include such derivations in texts oriented toward students intending careers in science, engineering, and medicine.

  18. Quantum mechanical calculations of state-to-state cross sections and rate constants for the F + DCl → Cl + DF reaction.

    PubMed

    Bulut, Niyazi; Kłos, Jacek; Roncero, Octavio

    2015-06-07

    We present accurate state-to-state quantum wave packet calculations of integral cross sections and rate constants for the title reaction. Calculations are carried out on the best available ground 1(2)A' global adiabatic potential energy surface of Deskevich et al. [J. Chem. Phys. 124, 224303 (2006)]. Converged state-to-state reaction cross sections have been calculated for collision energies up to 0.5 eV and different initial rotational and vibrational excitations, DCl(v = 0, j = 0 - 1; v = 1, j = 0). Also, initial-state resolved rate constants of the title reaction have been calculated in a temperature range of 100-400 K. It is found that the initial rotational excitation of the DCl molecule does not enhance reactivity, in contract to the reaction with the isotopologue HCl in which initial rotational excitation produces an important enhancement. These differences between the isotopologue reactions are analyzed in detail and attributed to the presence of resonances for HCl(v = 0, j), absent in the case of DCl(v = 0, j). For vibrational excited DCl(v = 1, j), however, the reaction cross section increases noticeably, what is also explained by another resonance.

  19. Quantum mechanical calculations of state-to-state cross sections and rate constants for the F + DCl → Cl + DF reaction

    SciTech Connect

    Bulut, Niyazi; Kłos, Jacek; Roncero, Octavio

    2015-06-07

    We present accurate state-to-state quantum wave packet calculations of integral cross sections and rate constants for the title reaction. Calculations are carried out on the best available ground 1{sup 2}A′ global adiabatic potential energy surface of Deskevich et al. [J. Chem. Phys. 124, 224303 (2006)]. Converged state-to-state reaction cross sections have been calculated for collision energies up to 0.5 eV and different initial rotational and vibrational excitations, DCl(v = 0, j = 0 − 1; v = 1, j = 0). Also, initial-state resolved rate constants of the title reaction have been calculated in a temperature range of 100-400 K. It is found that the initial rotational excitation of the DCl molecule does not enhance reactivity, in contract to the reaction with the isotopologue HCl in which initial rotational excitation produces an important enhancement. These differences between the isotopologue reactions are analyzed in detail and attributed to the presence of resonances for HCl(v = 0, j), absent in the case of DCl(v = 0, j). For vibrational excited DCl(v = 1, j), however, the reaction cross section increases noticeably, what is also explained by another resonance.

  20. Temperature Dependence of the Rate Constant for the CH3 Recombination Reaction: A Loss Process in Outer Planet Atmospheres

    NASA Technical Reports Server (NTRS)

    Cody, R. J.; Payne, W. A.; Thorn, R. P., Jr.; Romani, P. N.; Stief, L. J.; Nesbitt, F. L.; Iannone, M. A.; Tardy, D. C.

    2002-01-01

    The methyl free radical (CH3) has been observed in the atmospheres of Saturn and Neptune by the ISO satellite. There are discrepancies between the column densities for the CH3 radical derived from the ISO observations and the column densities derived from atmospheric photochemical models. For Neptune the model column density is 1.5 times that derived from ISO. For Saturn the model is 6 times that from ISO. The recombination of methyl radicals is the major loss process for methyl in these atmospheres. The serious disagreement between observed and calculated levels of CH3 has led to suggestions that the atmospheric models greatly underestimated the loss of CH3 due to poor knowledge of the rate of the reaction (1) CH3 + CH3 + M goes to C2H6 + M at the low temperatures and pressures of these atmospheric systems. Although the reaction CH3 + CH3 + M goes to C2H6 + M has been extensively studied both theoretically and experimentally, the laboratory conditions have been, with only a few exceptions, higher temperatures (T greater than 298K), higher pressures (P greater than or equal to 10 Torr - 13.3 mbar) or M=Ar rather than H2 or He as the bath gas.

  1. Rate constant and arrhenius parameter determination for the reaction of the hydrated electron with iodomethane, iodoethane, 1-iodopropane and 2-iodopropane in aqueous solution

    NASA Astrophysics Data System (ADS)

    Mezyk, Stephen P.

    1997-04-01

    The techniques of electron pulse radiolysis and absorption spectroscopy have been used to directly measure rate constants for the hydrated electron reaction with CH 3I, C 2H 5I, 1-C 3H 7I and 2-C 3H 7I. At 25°C, specific values of (1.78 ± 0.05) ×x 10 10, (1.57 ± 0.07) × 10 10, (1.05 ± 0.06) × 10 10 and (1.05 ± 0.03) × 10 10 dm 3 mol -1 s -1 were determined for these compounds respectively, with corresponding overall activation energiesof 16.9 ± 1.3 (3.2-35.5°C), 19.5 ± 1.1 (3.3-51.6°C), 16.60 ± 0.65 (2.5-85.1°C) and 12.94 ± 0.44 (2.5-84.5°C) kJ mol -1. The slight curvature observed in the Arrhenius plots for the three larger iodoalkanes suggests that these rate constants are influenced by both diffusion and chemical processes. By fitting the measured temperature-dependent rate constants to the general equation 1/ kobs = 1 / kdiff + 1/ kreact, where kobs, is the measured rate constant, kdiff is the encounter rate constant of the two reacting species and kreact is the rate constant that would be measured if diffusion of the species was not rate influencing, specific Arrhenius parameters for only the chemical component have also been determined.

  2. Ratiometric analysis in hyperpolarized NMR (I): test of the two-site exchange model and the quantification of reaction rate constants.

    PubMed

    Li, Lin Z; Kadlececk, Stephen; Xu, He N; Daye, Dania; Pullinger, Benjamin; Profka, Harrilla; Chodosh, Lewis; Rizi, Rahim

    2013-10-01

    Conventional methods for the analysis of in vivo hyperpolarized (13) C NMR data from the lactate dehydrogenase (LDH) reaction usually make assumptions on the stability of rate constants and/or the validity of the two-site exchange model. In this study, we developed a framework to test the validity of the assumption of stable reaction rate constants and the two-site exchange model in vivo via ratiometric fitting of the time courses of the signal ratio L(t)/P(t). Our analysis provided evidence that the LDH enzymatic kinetics observed by hyperpolarized NMR are in near-equilibrium and satisfy the two-site exchange model for only a specific time window. In addition, we quantified both the forward and reverse exchange rate constants of the LDH reaction for the transgenic and mouse xenograft models of breast cancer using the ratio fitting method developed, which includes only two modeling parameters and is less sensitive to the influence of instrument settings/protocols, such as flip angles, degree of polarization and tracer dosage. We further compared the ratio fitting method with a conventional two-site exchange modeling method, i.e. the differential equation fitting method, using both the experimental and simulated hyperpolarized NMR data. The ratio fitting method appeared to fit better than the differential equation fitting method for the reverse rate constant on the mouse tumor data, with less relative errors on average, whereas the differential equation fitting method also resulted in a negative reverse rate constant for one tumor. The simulation results indicated that the accuracy of both methods depends on the width of the transport function, noise level and rate constant ratio; one method may be more accurate than the other based on the experimental/biological conditions aforementioned. We were able to categorize our tumor models into specific conditions of the computer simulation and to estimate the errors of rate quantification. We also discussed possible

  3. Rate constants for the thermal decomposition of ethanol and its bimolecular reactions with OH and D: reflected shock tube and theoretical studies.

    PubMed

    Sivaramakrishnan, R; Su, M-C; Michael, J V; Klippenstein, S J; Harding, L B; Ruscic, B

    2010-09-09

    The thermal decomposition of ethanol and its reactions with OH and D have been studied with both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for ethanol have been measured at high T in reflected shock waves using OH optical absorption and high-sensitivity H-atom ARAS detection. The three dissociation processes that are dominant at high T are C2H5OH--> C2H4+H2O (A) -->CH3+CH2OH (B) -->C2H5+OH (C).The rate coefficient for reaction C was measured directly with high sensitivity at 308 nm using a multipass optical White cell. Meanwhile, H-atom ARAS measurements yield the overall rate coefficient and that for the sum of reactions B and C , since H-atoms are instantaneously formed from the decompositions of CH(2)OH and C(2)H(5) into CH(2)O + H and C(2)H(4) + H, respectively. By difference, rate constants for reaction 1 could be obtained. One potential complication is the scavenging of OH by unreacted ethanol in the OH experiments, and therefore, rate constants for OH+C2H5OH-->products (D)were measured using tert-butyl hydroperoxide (tBH) as the thermal source for OH. The present experiments can be represented by the Arrhenius expression k=(2.5+/-0.43) x 10(-11) exp(-911+/-191 K/T) cm3 molecule(-1) s(-1) over the T range 857-1297 K. For completeness, we have also measured the rate coefficient for the reaction of D atoms with ethanol D+C2H5OH-->products (E) whose H analogue is another key reaction in the combustion of ethanol. Over the T range 1054-1359 K, the rate constants from the present experiments can be represented by the Arrhenius expression, k=(3.98+/-0.76) x10(-10) exp(-4494+/-235 K/T) cm3 molecule(-1) s(-1). The high-pressure rate coefficients for reactions B and C were studied with variable reaction coordinate transition state theory employing directly determined CASPT2/cc-pvdz interaction energies. Reactions A , D , and E were studied with conventional transition state theory

  4. Rate constants for the reaction, O + H sub 2 O yields OH + OH, over the temperature range, 1500--2400 K, by the flash photolysis-shock tube technique: A further consideration of the back reaction

    SciTech Connect

    Lifshitz, A.; Michael, J.V.

    1990-01-01

    Rate constants for the reaction, O + H{sub 2}O {yields} OH + OH, have been measured by the Flash Photolysis-Shock Tube (FP-ST) technique over the temperature range, 1500--2400 K. This technique combines stock heating with flash photolysis in the reflected shock wave regime, and the transient species, O-atoms in this case, are monitored by atomic resonance absorption spectroscopy (aras). Additional experiments were performed with N{sub 2}O as a thermal source of O-atoms, and the formation and depletion of (O) were followed by the aras technique. These results require that the decomposition rate behavior of N{sub 2}O be known. The results obtained by this technique are compared to those obtained by the FP-ST technique and are found to be corroborative. Hence, the combined results are used to describe the rate constants for the title reaction. The experimental results are compared to earlier work, and rate constants for the title reaction are additionally calculated from published results for the reverse reaction, OH + OH, and the well known equilibrium constant. All results are combined, and the rate behavior for the title reaction is evaluated. Lastly, the results for both forward and reverse reactions are compared to the theoretical calculations presented recently by Harding and Wagner. It is concluded that theory and experiment are in agreement within experimental error.

  5. Radiosensitization of DNA by Cisplatin Adducts Results from an Increase in the Rate Constant for the Reaction with Hydrated Electrons and Formation of PtI

    PubMed Central

    Behmand, B.; Marignier, J.-L.; Mostafavi, M.; Wagner, J. R.; Hunting, D. J.; Sanche, L.

    2015-01-01

    Pulse radiolysis measurements of the decay of hydrated electrons in solutions containing different concentrations of the oligonucleotide GTG with and without a cisplatin adduct show that the presence of a cisplatin moiety accelerates the reaction between hydrated electrons and the oligonucleotide. The rate constant of the reaction is found to be 2.23 × 1010 mol−1 L s−1, which indicates that it is diffusion controlled. In addition, we show for the first time the formation of a PtI intermediate as a result of the reaction of hydrated electrons with GTG-cisplatin. A putative reaction mechanism is proposed, which may form the basis of the radiosensitization of cancer cells in concomitant chemoradiation therapy with cisplatin. PMID:26098937

  6. Rotational state dependence of rate constants for the reaction of ions with asymmetric top molecules at very low temperatures: application to the N+/H2O system

    NASA Astrophysics Data System (ADS)

    Dubernet, M. L.; McCarroll, R.

    1990-12-01

    The adiabatic rotational state method is applied to the study of reactions between ions and polar asymmetric top molecules at very low temperatures. Detailed results of the calculated rate coefficients for the reaction of N+ with H2O are presented. A strong dependence of the rate coefficients on the initial rotational state is observed at low temperatures. In the case of a thermal distribution of rotational states, where the rate constants are summed over a Boltzman distribution, the replacement of the asymmetric top by an average symmetric top, which leads to a considerable simplification of the calculations, appears to be satisfactory. On the other hand, for a non thermal distribution, no such simplifying assumption can be made. In particular, the rate coefficient for a specific initial rotational state is quite sensitive to the orientation of the dipole moment.

  7. Low-energy reaction rate constants for the Ni+-assisted decomposition of acetaldehyde: observation of C-H and C-C activation.

    PubMed

    Dee, S Jason; Castleberry, Vanessa A; Villarroel, Otsmar J; Laboren, Ivanna E; Bellert, Darrin J

    2010-02-04

    Rate constants for the low-energy Ni(+)-assisted dissociative reaction of acetaldehyde have been measured under jet-cooled conditions in the gas phase. The rate constants are acquired through monitoring the time dependence of fragment Ni(+)CO formation. The decomposition of the precursor Ni(+)-acetaldehyde cluster ion proceeds via consecutive, parallel reaction coordinates that originate with the Ni(+)-assisted cleavage of either a C-C or an aldehyde C-H bond. The energies used to initiate these reactions are well below that required to cleave sigma-bonds in the isolated acetaldehyde molecule. Direct measurement of the reaction kinetics over a range of energies indicates that the rate-limiting step in the dissociative mechanism changes at cluster ion internal energies = 17,200 +/- 400 cm(-1). Arguments are presented that this energy marks the closure of the dissociative coordinate that initiates with C-H sigma-bond activation and thus provides a measure of the activation energy of this dissociative pathway.

  8. Equilibrium and rate constants, and reaction mechanism of the HF dissociation in the HF(H2O)7 cluster by ab initio rare event simulations.

    PubMed

    Elena, Alin Marin; Meloni, Simone; Ciccotti, Giovanni

    2013-12-12

    We perform restrained hybrid Monte Carlo (MC) simulations to compute the equilibrium constant of the dissociation reaction of HF in HF(H2O)7. We find that the HF is a stronger acid in the cluster than in the bulk, and its acidity is higher at lower T. The latter phenomenon has a vibrational entropic origin, resulting from a counterintuitive balance of intra- and intermolecular terms. We find also a temperature dependence of the reactions mechanism. At low T (≤225 K) the dissociation reaction follows a concerted path, with the H atoms belonging to the relevant hydrogen bond chain moving synchronously. At higher T (300 K), the first two hydrogen atoms move together, forming an intermediate metastable state having the structure of an eigen ion (H9O4(+)), and then the third hydrogen migrates completing the reaction. We also compute the dissociation rate constant, kRP. At very low T (≤75 K) kRP depends strongly on the temperature, whereas it gets almost constant at higher T’s. With respect to the bulk, the HF dissociation in the HF(H2O)7 is about 1 order of magnitude faster. This is due to a lower free energy barrier for the dissociation in the cluster.

  9. Direct measurements of the total rate constant of the reaction NCN + H and implications for the product branching ratio and the enthalpy of formation of NCN.

    PubMed

    Fassheber, Nancy; Dammeier, Johannes; Friedrichs, Gernot

    2014-06-21

    The overall rate constant of the reaction (2), NCN + H, which plays a key role in prompt-NO formation in flames, has been directly measured at temperatures 962 K < T < 2425 K behind shock waves. NCN radicals and H atoms were generated by the thermal decomposition of NCN3 and C2H5I, respectively. NCN concentration-time profiles were measured by sensitive narrow-line-width laser absorption at a wavelength of λ = 329.1302 nm. The obtained rate constants are best represented by the combination of two Arrhenius expressions, k2/(cm(3) mol(-1) s(-1)) = 3.49 × 10(14) exp(-33.3 kJ mol(-1)/RT) + 1.07 × 10(13) exp(+10.0 kJ mol(-1)/RT), with a small uncertainty of ±20% at T = 1600 K and ±30% at the upper and lower experimental temperature limits.The two Arrhenius terms basically can be attributed to the contributions of reaction channel (2a) yielding CH + N2 and channel (2b) yielding HCN + N as the products. A more refined analysis taking into account experimental and theoretical literature data provided a consistent rate constant set for k2a, its reverse reaction k1a (CH + N2 → NCN + H), k2b as well as a value for the controversial enthalpy of formation of NCN, ΔfH = 450 kJ mol(-1). The analysis verifies the expected strong temperature dependence of the branching fraction ϕ = k2b/k2 with reaction channel (2b) dominating at the experimental high-temperature limit. In contrast, reaction (2a) dominates at the low-temperature limit with a possible minor contribution of the HNCN forming recombination channel (2d) at T < 1150 K.

  10. Review of Rate Constants and Exploration of Correlations of the Halogen Transfer Reaction of Tri-substituted Carbon-centered Radicals with Molecular Halogens

    SciTech Connect

    Poutsma, Marvin L

    2012-01-01

    Rate constants for the reaction (R 3C + X2 R 3CX + X ; X = F, Cl, Br, and I) are reviewed. Because of curved Arrhenius plots and negative EX values, empirical structure-reactivity correlations are sought for log kX,298 rather than EX. The well-known poor correlation with measures of reaction enthalpy is demonstrated. The best quantitative predictor for R 3C is p, the sum of the Hammett p constants for the three substituents, R . Electronegative substituents with lone pairs, such as halogen or oxygen, thus appear to destabilize the formation of a polarized pre-reaction complex and/or TS ( +R---X---X -) by -inductive/field electron withdrawal while simultaneously stabilizing them by -resonance electron donation. The best quantitative predictor of the reactivity order of the halogens, I2 > Br2 >> Cl2 F2, is the polarizability of the halogen, (X-X). For the data set of 60 rate constants which span 6.5 orders of magnitude, a modestly successful correlation of log kX,298 is achieved with only two parameters, p and (X-X), with a mean unsigned deviation of 0.59 log units. How much of this residual variance is the result of inaccuracies in the data compared with over-simplification of the correlation approach remains to be seen.

  11. Structural effects on the beta-scission reaction of alkoxyl radicals. Direct measurement of the absolute rate constants for ring opening of benzocycloalken-1-oxyl radicals.

    PubMed

    Bietti, Massimo; Lanzalunga, Osvaldo; Salamone, Michela

    2005-02-18

    [reaction: see text] The absolute rate constants for beta-scission of a series of benzocycloalken-1-oxyl radicals and of the 2-(4-methylphenyl)-2-butoxyl radical have been measured directly by laser flash photolysis. The benzocycloalken-1-oxyl radicals undergo ring opening with rates which parallel the ring strain of the corresponding cycloalkanes. In the 1-X-indan-1-oxyl radical series, ring opening is observed when X = H, Me, whereas exclusive C-X bond cleavage occurs when X = Et. The factors governing the fragmentation regioselectivity are discussed.

  12. Calculated rate constants for the reaction ClO + O yields Cl + O2 between 220 and 1000 deg K. [molecular trajectories and stratospheric ozone

    NASA Technical Reports Server (NTRS)

    Jaffee, R. L.

    1978-01-01

    Classical trajectory calculations are presented for the reaction ClO + O yields Cl + O2, a reaction which is an important step in the chlorine-catalyzed destruction of ozone which is thought to occur in the 220 and 1000 K. The calculated rate constant is 4.36 x 10 to the minus 11th power exp (-191/T)cu cm molecule (-1)s(-1) and its value at 300 K is 2.3 plus or minus 10 to the 11th power cu cm molecule (-1)s(-1), about a factor of 2 lower than recent experimental data. The empirical potential energy surface used in the calculations was constructed to fit experimental data for ClO, O2 and ClOO molecules. Other important features of this potential surface, such as the barrier to reaction, were varied systematically and calculations were performed for a range of conditions to determine the best theoretical rate constants. Results demonstrate the utility of classical trajectory methods for determining activation energies and other kinetic data for important atmospheric reactions.

  13. Reaction rate constant of CH2O + H = HCO + H2 revisited: a combined study of direct shock tube measurement and transition state theory calculation.

    PubMed

    Wang, Shengkai; Dames, Enoch E; Davidson, David F; Hanson, Ronald K

    2014-11-06

    The rate constant of the H-abstraction reaction of formaldehyde (CH2O) by hydrogen atoms (H), CH2O + H = H2 + HCO, has been studied behind reflected shock waves with use of a sensitive mid-IR laser absorption diagnostic for CO, over temperatures of 1304-2006 K and at pressures near 1 atm. C2H5I was used as an H atom precursor and 1,3,5-trioxane as the CH2O precursor, to generate a well-controlled CH2O/H reacting system. By designing the experiments to maintain relatively constant H atom concentrations, the current study significantly boosted the measurement sensitivity of the target reaction and suppressed the influence of interfering reactions. The measured CH2O + H rate constant can be expressed in modified Arrhenius from as kCH2O+H(1304-2006 K, 1 atm) = 1.97 × 10(11)(T/K)(1.06) exp(-3818 K/T) cm(3) mol(-1)s(-1), with uncertainty limits estimated to be +18%/-26%. A transition-state-theory (TST) calculation, using the CCSD(T)-F12/VTZ-F12 level of theory, is in good agreement with the shock tube measurement and extended the temperature range of the current study to 200-3000 K, over which a modified Arrhenius fit of the rate constant can be expressed as kCH2O+H(200-3000 K) = 5.86 × 10(3)(T/K)(3.13) exp(-762 K/T) cm(3) mol(-1)s(-1).

  14. Ab initio and RRKM calculations for multichannel rate constants of the C{sub 2}H{sub 3}+O{sub 2} reaction

    SciTech Connect

    Mebel, A.M.; Diau, E.W.G.; Lin, M.C.; Morokuma, K.

    1996-10-09

    A potential energy surface for the reaction of vinyl radical with molecular oxygen has been studied using the ab initio G2M(RCC, MP2) method. The most favorable reaction pathway leading to the major CHO+CH{sub 2}O products is described. The C{sub 2}H{sub 3}O+O products can be formed by elimination of the oxygen atom from C{sub 2}H{sub 3}OO via TS 23, which is by 7.8 kcal/mol lower in energy than the reactants, but by 6.5 kcal/mol higher than TS 9`. The hydrogen migration in 1` gives rise to another significant product channel: C{sub 2}H{sub 3}+O{sub 2} {yields} 1` {yields} TS 25` {yields} C{sub 2}H{sub 2}+O{sub 2}H, with TS 25` lying below C{sub 2}H{sub 3}+O{sub 2} by 3.5 kcal/mol. Multichannel RRKM calculations have been carried out for the total and individual rate constants for various channels using the G2M(RCC, MP2) energetics and molecular parameters of the intermediates and transition states. The computed low pressure reaction rate constant is in quantitative agreement with experiment. At atmospheric pressure, the title reaction is dominated by the stabilization of vinylperoxy radical C{sub 2}H{sub 3}OO at room temperature. In the 500-900 K temperature range, the CHO+CH{sub 2}O channel has the highest rate constant, and at T >= 900 K, C{sub 2}H{sub 3}O+O are the major products. At very high temperatures, the channel producing C{sub 2}H{sub 2} + O{sub 2}H becomes competitive. 15 refs., 3 figs., 4 tabs.

  15. Absolute rate constant of the reaction OH + H2O2 yields HO2 + H2O from 245 to 423 K

    NASA Technical Reports Server (NTRS)

    Keyser, L. F.

    1980-01-01

    The absolute rate constant of the reaction between the hydroxyl radical and hydrogen peroxide was measured by using the discharge-flow resonance fluorescence technique at total pressure between 1 and 4 torr. At 298 K the result is (1.64 + or - 0.32) x 10 to the -12th cu cm/molecule s. The observed rate constant is independent of pressure, surface-to-volume ratio, the addition of vibrational quenchers, and the source of OH. The temperature dependence has also been determined between 245 and 423 K; the resulting Arrhenius expression is k cu cm/molecule s is equal to (2.51 + or - 0.6) x 10 to the -12th exp(-126 + or - 76/T).

  16. New ab initio potential energy surface for BrH2 and rate constants for the H + HBr → H2 + Br abstraction reaction.

    PubMed

    Jiang, Bin; Xie, Changjian; Xie, Daiqian

    2011-03-21

    A global potential energy surface (PES) for the electronic ground state of the BrH(2) system was constructed based on the multireference configuration interaction (MRCI) method including the Davidson's correction using a large basis set. In addition, the spin-orbit correction were computed using the Breit-Pauli Hamiltonian and the unperturbed MRCI wavefunctions in the Br + H(2) channel and the transition state region. Adding the correction to the ground state potential, the lowest spin-orbit correlated adiabatic potential was obtained. The characters of the new potential are discussed. Accurate initial state specified rate constants for the H + HBr → H(2) + Br abstraction reaction were calculated using a time-dependent wave packet method. The predicted rate constants were found to be in excellent agreement with the available experimental values and much better than those obtained from a previous PES.

  17. Upper bound and probable value of the rate constant of the reaction OH + HO2 yields H2O + O2

    NASA Technical Reports Server (NTRS)

    Chang, J. S.; Kaufman, F.

    1978-01-01

    Discharge-flow experiments at 295 K of the OH + HO2 reaction are described in which excess O3 is added to OH through a movable injector; OH concentration is measured downstream by resonance fluorescence with and without addition of excess NO just upstream of the detector so that both OH concentration and OH concentration + HO2 concentration are determined independently. The data are compared with a computer model involving 12 reaction steps. A simple sensitivity analysis is carried out to establish how errors in the other 'known' rate constants affect the data fit. Best agreement is obtained between computer calculations and experiments when k1 for the OH + HO2 reaction is in the range 2-3 x 10 to the -11th cu cm/s. Values above about 5 x 10 to the -11th are very unlikely whereas those below about 1 x 10 to the -11th are less strongly excluded.

  18. The Low-Energy Unimolecular Reaction Rate Constants for the Gas Phase, Ni+-Mediated Dissociation of the C-C σ Bond in Acetone

    NASA Astrophysics Data System (ADS)

    Castleberry, Vanessa A.; Dee, S. Jason; Villarroel, Otsmar J.; Laboren, Ivanna E.; Frey, Sarah E.; Bellert, Darrin J.

    2009-09-01

    The time dependence of the gaseous unimolecular decomposition of the jet-cooled adduct ion, Ni+-OC(CH3)2, was monitored through selective detection of the Ni+CO fragment ion. Various resolved amounts of energy in the range 15600-18800 cm-1 were supplied to initiate the dissociation reaction through absorption of laser photons by the title molecular complex. First-order rate constants, k(E), ranged from 113000 to 55000 s-1 and decreased with decreasing amounts of internal excitation. The energy used to initiate the reaction is well below that required to fragment C-C σ bonds and indicates the necessity of the Ni+ cation to induce bond activation and fragmentation. These measurements are carried out in a unique apparatus and represent the first direct kinetic study of such catalytic type reactions.

  19. The hydrogen abstraction reaction mechanism and rate constants from 200 K to 2000 K between sevoflurane and chlorine atom: A theoretical investigation

    NASA Astrophysics Data System (ADS)

    Ren, Hongjiang; Yang, Xiaohui; Li, Xiaojun; Liu, Yan; Wei, Xiao

    2014-06-01

    The H abstraction reaction mechanisms between sevoflurane and Cl atom were investigated with DFT method. The geometries of all the species were optimized at wB97XD/6-311++G(d,p) level. Thermochemistry properties were calculated with the accurate model chemistry method G3MP2 combined with the standard statistical thermodynamics. Gibbs free energies were used for analyzing reaction pathways. Three pathways correspond to the H9, H11 and H10 reactions with the barriers of 55.01, 32.50 and 32.50 kJ·mol-1, respectively. The rate constants for all the pathways over a wide temperature range of 200-2000 K were calculated, and the result is in good agreement with the experimental values.

  20. Upper bound and probable value of the rate constant of the reaction OH + HO2 yields H2O + O2

    NASA Technical Reports Server (NTRS)

    Chang, J. S.; Kaufman, F.

    1978-01-01

    Discharge-flow experiments at 295 K of the OH + HO2 reaction are described in which excess O3 is added to OH through a movable injector; OH concentration is measured downstream by resonance fluorescence with and without addition of excess NO just upstream of the detector so that both OH concentration and OH concentration + HO2 concentration are determined independently. The data are compared with a computer model involving 12 reaction steps. A simple sensitivity analysis is carried out to establish how errors in the other 'known' rate constants affect the data fit. Best agreement is obtained between computer calculations and experiments when k1 for the OH + HO2 reaction is in the range 2-3 x 10 to the -11th cu cm/s. Values above about 5 x 10 to the -11th are very unlikely whereas those below about 1 x 10 to the -11th are less strongly excluded.

  1. Critical Analysis of Rate Constants and Turnover Frequency in Nucleic Acid-Templated Reactions: Reaching Terminal Velocity.

    PubMed

    Chang, Dalu; Lindberg, Eric; Winssinger, Nicolas

    2017-02-01

    Nucleic acid-templated reactions have attracted significant attention for nucleic acid sensing and imaging. The level of signal amplification obtained from templated reactions is a function of the template turnover, wherein the template acts as the catalyst. Herein, we report the application of a pyridinium linker that immolates upon photocatalytic reduction with a ruthenium complex to yield the fastest nucleic acid templated reaction reported to date. We show that the templated reaction turnover is limited by the duplex dissociation kinetics beyond probes longer than a 6-mer and proceeded fastest for a 5-mer PNA probe. Using a beacon architecture that masks the catalytic template, we show that this methodology can be used for nucleic acid sensing extending the analyte recognition beyond a 5-mer. The system proceeds with a catalytic efficiency of 10(5) M(-1) s(-1) and achieves turnover frequency of >100 h(-1).

  2. Temperature dependence of the rate constant for hydrogen atom reaction with Cl2-• in water by pulse radiolysis of aqueous HCl solution

    NASA Astrophysics Data System (ADS)

    Kazmierczak, L.; Szala-Bilnik, J.; Wolszczak, M.; Swiatla-Wojcik, D.

    2015-12-01

    The temperature dependence of the rate constant for the reaction of dichloride anion radical (Cl2- •) with atomic hydrogen (H•) in water up to 75 °C has been determined by pulse radiolysis of deaerated 0.1 M HCl solution. The room temperature value is (6.1±0.6)×109 M-1 s-1. The activation energy of (13.2±0.6) kJ mol-1 is less than 16.7 kJ mol-1, expected for the diffusion-controlled reaction. Based on the temperature dependence of the rate constant for the reactions H•+Cl2- • and H•+Cl2, derived in this work, and on that reported earlier (Szala-Bilnik et al., 2014) for Cl2- • + Cl2- • , we show that a value of (10±2) M-1 s-1 determined by Hartig and Getoff (1982) for k (H•+H2O) in water at 25 °C is overestimated by at least two orders of magnitude.

  3. Reaction dynamics with the multi-layer multi-configurational time-dependent Hartree approach: H + CH4 → H2 + CH3 rate constants for different potentials.

    PubMed

    Welsch, Ralph; Manthe, Uwe

    2012-12-28

    The multi-layer extension of the multi-configurational time-dependent Hartree (MCTDH) approach is applied to the investigation of elementary bimolecular chemical reactions. Cumulative reaction probabilities and thermal rate constants of the H + CH(4) → H(2) + CH(3) reaction are calculated using flux correlation functions and the quantum transition state concept. Different coordinate systems and potential energy surfaces (PESs) are studied. The convergence properties of different layerings are investigated and the efficiency of multi-layer MCTDH approach is compared to the standard MCTDH approach. It is found that the multi-layer approach can decrease the numerical effort by more than an order of magnitude. The increased efficiency resulting from the multi-layer MCTDH approach is crucial for quantum dynamical calculations on recent global H + CH(4) → H(2) + CH(3) PESs, e.g., the ZBB3-PES [Z. Xie, J. M. Bowman, and X. Zhang, J. Chem. Phys. 125, 133120 (2006)] based on permutational invariant polynomials, which are numerically more demanding than earlier PESs. The results indicate that an accurate description of all transition state frequencies is important to obtain accurate thermal rate constants.

  4. Oxidation of CO by N/sub 2/O between 1076 and 1228 K: determination of the rate constant of the exchange reaction

    SciTech Connect

    Loirat, H.; Caralp, F.; Destriau, M.; Lesclaux, R.

    1987-12-17

    New measurements of the rate constant of the direct reaction of CO with N/sub 2/O are reported with the principal purpose of removing some of the remaining discrepancies on its value. Experiments were performed at lower temperatures (1076-1228 K) and lower pressure (approx. 15 Torr) than those prevailing in most of previous works, by using a static reactor. It is shown that, under these experimental conditions, the reaction proceeds essentially according to the direct reaction CO + N/sub 2/O ..-->.. CO/sub 2/ + N/sub 2/ (1). The previously proposed wet mechanism is not significant under our experimental conditions. It has to be taken into account, however, to describe the observed production and consumption of molecular oxygen. The Arrhenius expression derived from these experiments is k/sub 1/ = 10/sup 14.4 +/- 0.3 exp(-(46 +- 2) kcal mol/sup -1/RT) cm/sup 3/ mol/sup -1/ s/sup -1/. A detailed analysis of the results shows that the uncertainties in side reactions do not greatly influence the value of k/sub 1/. A critical discussion of the data reported in the literature is presented. In spite of remaining uncertainties in the reaction mechanism, the present results, obtained in a low-temperature range, show that the low activation energy values of reaction 1, reported in several works performed at higher temperatures, are highly unlikely

  5. Virtual Instrument for Determining Rate Constant of Second-Order Reaction by pX Based on LabVIEW 8.0.

    PubMed

    Meng, Hu; Li, Jiang-Yuan; Tang, Yong-Huai

    2009-01-01

    The virtual instrument system based on LabVIEW 8.0 for ion analyzer which can measure and analyze ion concentrations in solution is developed and comprises homemade conditioning circuit, data acquiring board, and computer. It can calibrate slope, temperature, and positioning automatically. When applied to determine the reaction rate constant by pX, it achieved live acquiring, real-time displaying, automatical processing of testing data, generating the report of results; and other functions. This method simplifies the experimental operation greatly, avoids complicated procedures of manual processing data and personal error, and improves veracity and repeatability of the experiment results.

  6. Determination of absolute thermal rate constants for the charge-transfer reaction DBr+(2∏I,v+) + HBr --> HBr+(2∏I',v'+) + DBr

    NASA Astrophysics Data System (ADS)

    Xie, Jinchun; Zare, Richard N.

    1992-03-01

    The charge transfer reaction DBr+(2∏I,v+,J+)+HBr→HBr+(2∏I',v'+,J'+)+DBr is studied in a state-to-state manner under thermal conditions in a slowly flowing gas mixture of HBr and DBr. The DBr+ reagent is prepared in a selected vibronic level by using (2+1) resonance-enhanced multiphoton ionization. The HBr+ product is detected in a quantum-state-specific manner using laser-induced fluorescence. From the measurements of the molecular density and the populations of both HBr+ product and DBr+ reagent, the absolute thermal rate constants k(i,v+→i',v'+) are determined for this charge-transfer process. The rate constants for near-resonant charge transfer in which Δv+=0 and Δi=0 are much large than charge-transfer channels in which either Δi≠0; the smallest rate constants are for those channels in which both Δi≠0 and Δv+≠0. The rotational distribution of the HBr+(i',v'+) products fits a temperature well in each case. For near-resonant charge transfer, the rotational temperature is slightly warmer than thermal, whereas for nonresonant charge transfer, the rotational temperature is much hotter than thermal. A model in which the excess energy of a charge-transfer process is statistically partitioned among all the degrees of freedom of the complex is able to predict closely the observed rotational temperature.

  7. Calculated rate constants of the chemical reactions involving the main byproducts SO2F, SOF2, SO2F2 of SF6 decomposition in power equipment

    NASA Astrophysics Data System (ADS)

    Fu, Yuwei; Rong, Mingzhe; Yang, Kang; Yang, Aijun; Wang, Xiaohua; Gao, Qingqing; Liu, Dingxin; Murphy, Anthony B.

    2016-04-01

    SF6 is widely used in electrical equipment as an insulating gas. In the presence of an electric arc, partial discharge (PD) or spark, SF6 dissociation products (such as SF2, SF3 and SF4) react with the unavoidable gas impurities (such as water vapor and oxygen), electrodes and surrounding solid insulation materials, forming several toxic and corrosive byproducts. The main stable decomposition products are SO2F, SO2F2 and SOF2, which have been confirmed experimentally to have a direct relationship with discharge faults, and are thus expected to be useful in the fault diagnosis of power equipment. Various studies have been performed of the main SF6 decomposition species and their concentrations under different types of faults. However, most of the experiments focused on the qualitative analysis of the relationship between the stable products and discharge faults. Although some theoretical research on the formation of main SF6 derivatives have been carried out using chemical kinetics models, the basic data (chemical reactions and their rate constants) adopted in the model are inaccurate and incomplete. The complex chemical reactions of SF6 with the impurities are ignored in most cases. The rate constants of some reactions obtained at ambient temperature or in a narrow temperature range are adopted in the models over a far greater range, for example up to 12 000 K, due to the difficulty in the experimental measurement and theoretical estimation of rate coefficients, particularly at high temperatures. Therefore, improved theoretical models require not only the consideration of additional SF6 decomposition reactions in the presence of impurities but also on improved values of rate constants. This paper is devoted to determining the rate constants of the chemical reactions relating to the main byproducts of SF6 decomposition in SF6 gas-insulated power equipment: SO2F, SOF2 and SO2F2. Quantum chemistry calculations with density functional theory, conventional

  8. Rate Constant and RRKM Product Study for the Reaction Between CH3 and C2H3 at T = 298K

    NASA Technical Reports Server (NTRS)

    Thorn, R. Peyton, Jr.; Payne, Walter A., Jr.; Chillier, Xavier D. F.; Stief, Louis J.; Nesbitt, Fred L.; Tardy, D. C.

    2000-01-01

    The total rate constant k1 has been determined at P = 1 Torr nominal pressure (He) and at T = 298 K for the vinyl-methyl cross-radical reaction CH3 + C2H3 yields products. The measurements were performed in a discharge flow system coupled with collision-free sampling to a mass spectrometer operated at low electron energies. Vinyl and methyl radicals were generated by the reactions of F with C2H4 and CH4, respectively. The kinetic studies were performed by monitoring the decay of C2H3 with methyl in excess, 6 < |CH3|(sub 0)/|C2H3|(sub 0) < 21. The overall rate coefficient was determined to be k1(298 K) = (1.02 +/- 0.53)x10(exp -10) cubic cm/molecule/s with the quoted uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C2H3 + H and C2H3 + C2H3. The present result for k1 at T = 298 K is compared to two previous studies at high pressure (100-300 Torr He) and to a very recent study at low pressure (0.9-3.7 Torr He). Comparison is also made with the rate constant for the similar reaction CH3 + C2H5 and with a value for k1 estimated by the geometric mean rule employing values for k(CH3 + CH3) and k(C2H3 + C2H3). Qualitative product studies at T = 298 K and 200 K indicated formation of C3H6, C2H2, and C2H5 as products of the combination-stabilization, disproportionation, and combination-decomposition channels, respectively, of the CH3 + C2H3 reaction. We also observed the secondary C4H8 product of the subsequent reaction of C3H5 with excess CH3; this observation provides convincing evidence for the combination-decomposition channel yielding C3H5 + H. RRKM calculations with helium as the deactivator support the present and very recent experimental observations that allylic C-H bond rupture is an important path in the combination reaction. The pressure and temperature dependencies of the branching fractions are also predicted.

  9. Rate Constant and RRKM Product Study for the Reaction Between CH3 and C2H3 at T = 298K

    NASA Technical Reports Server (NTRS)

    Thorn, R. Peyton, Jr.; Payne, Walter A., Jr.; Chillier, Xavier D. F.; Stief, Louis J.; Nesbitt, Fred L.; Tardy, D. C.

    2000-01-01

    The total rate constant k1 has been determined at P = 1 Torr nominal pressure (He) and at T = 298 K for the vinyl-methyl cross-radical reaction CH3 + C2H3 yields products. The measurements were performed in a discharge flow system coupled with collision-free sampling to a mass spectrometer operated at low electron energies. Vinyl and methyl radicals were generated by the reactions of F with C2H4 and CH4, respectively. The kinetic studies were performed by monitoring the decay of C2H3 with methyl in excess, 6 < |CH3|(sub 0)/|C2H3|(sub 0) < 21. The overall rate coefficient was determined to be k1(298 K) = (1.02 +/- 0.53)x10(exp -10) cubic cm/molecule/s with the quoted uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C2H3 + H and C2H3 + C2H3. The present result for k1 at T = 298 K is compared to two previous studies at high pressure (100-300 Torr He) and to a very recent study at low pressure (0.9-3.7 Torr He). Comparison is also made with the rate constant for the similar reaction CH3 + C2H5 and with a value for k1 estimated by the geometric mean rule employing values for k(CH3 + CH3) and k(C2H3 + C2H3). Qualitative product studies at T = 298 K and 200 K indicated formation of C3H6, C2H2, and C2H5 as products of the combination-stabilization, disproportionation, and combination-decomposition channels, respectively, of the CH3 + C2H3 reaction. We also observed the secondary C4H8 product of the subsequent reaction of C3H5 with excess CH3; this observation provides convincing evidence for the combination-decomposition channel yielding C3H5 + H. RRKM calculations with helium as the deactivator support the present and very recent experimental observations that allylic C-H bond rupture is an important path in the combination reaction. The pressure and temperature dependencies of the branching fractions are also predicted.

  10. Rate constant for the reaction of OH with methyl iodide, a re-determination by flash photolysis of water vapour and time resolved resonance fluorescence of OH

    NASA Astrophysics Data System (ADS)

    Zhang, Shaoliang; Strekowski, Rafal; Zetzsch, Cornelius

    2010-05-01

    Methyl iodide is a major source gas for atmospheric iodine, and it is mainly emitted from the ocean. Aqueous-phase reactions, such as hydrolysis and exchange reactions with chloride control its emissions to the atmosphere, where its lifetime is limited to less than a week, mainly by photolysis. A minor contribution to the loss processes in the troposphere is the gas-phase reaction with OH radicals, that has been investigated by several authors. On the other hand, this reaction turned out to be uncertain in spite of interest in nuclear safety after the International Phebus Fission Product programme, initiated in 1988. Some of the most important observed phenomena with regard to the chemistry of iodine were not predicted, clearly showing the need for carrying out rate constant determinations for the reactions of I2 and CH3I with OH, which is a major oxidant product from the air radiolysis under accident conditions. We have measured the rate constant for the reaction OH + CH3I - H2O + CH2I in He at 260 mbar in the temperature range from 298 to 362 K. OH radicals were produced by flash photolysis of H2O in the vacuum-UV at wavelengths > 115 nm using a Xe flash lamp with a MgF2 window. Time profiles of OH radicals are monitored by resonance fluorescence of the A2 Σ - X2 Π transition at 308 nm, induced by the emission from a microwave discharge of a flow of He and H2O, a few Torr each. The signal is monitored by photon counting and multichannel scaling, collecting the counts from 50 flashes each, obtaind by pulsed photolysis of various mixtures of H2O and CH3I under slow-flow conditions. Decays of OH in the presence of CH3I are observed to be exponential, and the decay rates are found to be linearly dependent on the concentration of CH3I. Rate constants, k ± 2σ (in units of 10-14 cm3 s-1) of 4.14±0.20, 6.33±0.68, 7.31±1.18 and 8.24±1.60 at 298, 326, 352 and 362 K, respectively, are obtained from linear regressions and lead to an Arrhenius expression of k = 1.5

  11. Rate and equilibrium constant of the reaction of 1-methylvinoxy radicals with O2: CH3COCH2 + O2<--> CH3COCH2O2.

    PubMed

    Hassouna, Melynda; Delbos, Eric; Devolder, Pascal; Viskolcz, Bela; Fittschen, Christa

    2006-06-01

    The reaction of 1-methylvinoxy radicals, CH3COCH2, with molecular oxygen has been investigated by experimental and theoretical methods as a function of temperature (291-520 K) and pressure (0.042-10 bar He). Experiments have been performed by laser photolysis coupled to a detection of 1-methylvinoxy radicals by laser-induced fluorescence LIF. The potential energy surface calculations were performed using ab inito molecular orbital theory at the G3MP2B3 and CBSQB3 level of theory based on the density function theory optimized geometries. Derived molecular properties of the characteristic points of the potential energy surface were used to describe the mechanism and kinetics of the reaction under investigation. At 295 K, no pressure dependence of the rate constant for the association reaction has been observed: k(1,298K) = (1.18 +/- 0.04) x 10(-12) cm3 s(-1). Biexponential decays have been observed in the temperature range 459-520 K and have been interpreted as an equilibrium reaction. The temperature-dependent equilibrium constants have been extracted from these decays and a standard reaction enthalpy of deltaH(r,298K) = -105.0 +/- 2.0 kJ mol(-1) and entropy of deltaS(r,298K) = -143.0 +/- 4.0 J mol(-1) K(-1) were derived, in excellent agreement with the theoretical results. Consistent heats of formation for the vinoxy and the 1-methylvinoxy radical as well as their O2 adducts are recommended based on our complementary experimental and theoretical study deltaH(f,298K) = 13.0 +/- 2.0, -32. 9+/- 2.0, -85.9 +/- 4.0, and -142.1 +/- 4.0 kJ mol(-1) for CH2CHO, CH3COCH2 radicals, and their adducts, respectively.

  12. Kinetics of the NH[sub 2] + NO reaction. Effects of temperature on the total rate constant and the OH/H[sub 2]O branching ratio

    SciTech Connect

    Diau, E.W.; Yu, T.; Wagner, M.A.G.; Lin, M.C. )

    1994-04-14

    The rate constant for the reaction of NH[sub 2] with NO has been measured between 297 and 673 K using the cavity-ring-down technique to monitor the disappearance of the NH[sub 2] radical. The measured bimolecular rate constant can be effectively represented by the expression k[sup II] = (2.2 [+-] 0.7) x 10[sup [minus]12] exp[525 [+-] 80/T] cm[sup 3]/s, which agrees reasonably well with the results of several other recent measurements employing various diagnostic methods. A multichannel RRKM calculation has been carried out to account for the observed negative temperature dependence and the product branching ratio, OH/H[sub 2]O, based on Walch's recent potential energy surface data for various transition states and stable intermediates leading to the formation of the OH and H[sub 2]O products. The predicted temperature dependencies agree reasonably well with experimental observations. We have also performed kinetic modeling using a set of of reactions involving H, NH[sub 3], NH[sub 2], NO, and their anticipated products. The result of the modeling aided by sensitivity analysis suggests that the unknown [open quotes]third channel[close quotes] responsible for the decline of the ([OH] + [H[sub 2]O])/[NH[sub 2

  13. Mechanism and Kinetics of Methane Combustion, Part I: Thermal Rate Constants for Hydrogen-Abstraction Reaction of CH4 + O((3)P).

    PubMed

    Peng, Ya; Jiang, Zhong'an; Chen, Jushi

    2017-03-23

    The mechanism and kinetics of gas-phase hydrogen-abstraction by the O((3)P) from methane are investigated using ab initio calculations and dynamical methods. Not only are the electronic structure properties including the optimized geometries, relative energies, and vibrational frequencies of all the stationary points obtained from state-averaged complete active space self-consistent field calculations, but also the single-point energies for all points on the intrinsic reaction coordinate are evaluated using the internally contracted multireference configuration interaction approach with modified optimized cc-pCVDZ basis sets. Our calculations give a fairly accurate description of the regions around the (3)A″ transition state in the O((3)P) attacking a near-collinear H-CH3 direction with a barrier height of 12.53 kcal/mol, which is lower than those reported before. Subsequently, thermal rate constants for this hydrogen-abstraction are calculated using the canonical unified statistical theory method with the temperature ranging from 298 K to 1000 K. These calculated rate constants are in agreement with experiments. The present work reveals the reaction mechanism of hydrogen-abstraction by the O((3)P) from methane, and it is helpful for the understanding of methane combustion.

  14. Thermal rate constants for the O(3P) + HBr and O(3P) + DBr reactions: transition-state theory and quantum mechanical calculations.

    PubMed

    de Oliveira-Filho, Antonio G S; Ornellas, Fernando R; Peterson, Kirk A; Mielke, Steven L

    2013-12-05

    The O((3)P) + HBr → OH + Br and O((3)P) + DBr → OD + Br reactions are studied on a recent high-quality ab initio-based potential energy surface. Thermal rate constants over the 200-1000 K temperature range, calculated using variational transition-state theory (VTST) with the small-curvature tunneling (SCT) correction and quantum mechanical methods with the J-shifting approximation (QM/JS) for zero total angular momentum (J = 0), are reported. These results are compared to the available experimental data, which lie in the ranges of 221-554 and 295-419 K for O + HBr and O + DBr, respectively. The rate constants, in cm(3) molecule(-1) s(-1) and at 298 K, for the O + HBr reaction are 3.66 × 10(-14) for VTST, 3.80 × 10(-14) for QM/JS, and 3.66 × 10(-14) for the average of eight experimental measurements.

  15. Review of rate constants and exploration of correlations of the halogen transfer reaction of trisubstituted carbon-centered radicals with molecular halogens.

    PubMed

    Poutsma, Marvin L

    2012-03-16

    Rate constants for the reaction (R'(3)C(•) + X(2) → R'(3)CX + X(•); X = F, Cl, Br, I) are reviewed. Because of curved Arrhenius plots and negative E(X) values, empirical structure-reactivity correlations are sought for log k(X,298) rather than E(X). The well-known poor correlation with measures of reaction enthalpy is demonstrated. The best quantitative predictor for R'(3)C(•) is Σσ(p), the sum of the Hammett σ(p) constants for the three substituents, R'. Electronegative substituents with lone pairs, such as halogen and oxygen, thus appear to destabilize the formation of a polarized prereaction complex and/or TS ((δ+)R- - -X- - -X(δ-)) by σ inductive/field electron withdrawal while simultaneously stabilizing them by π resonance electron donation. The best quantitative predictor of the reactivity order of the halogens, I(2) > Br(2) ≫ Cl(2) ≈ F(2), is the polarizability of the halogen, α(X(2)). For the data set of 60 rate constants which span 6.5 orders of magnitude, a modestly successful correlation of log k(X,298) is achieved with only two parameters, Σσ(p) and α(X(2)), with a mean unsigned deviation of 0.59 log unit. How much of this residual variance is the result of inaccuracies in the data in comparison with oversimplification of the correlation approach remains to be seen.

  16. On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer. V. Comparison and Properties of Electrochemical and Chemical Rate Constants

    DOE R&D Accomplishments Database

    Marcus, R. A.

    1962-01-01

    Using a theory of electron transfers which takes cognizance of reorganization of the medium outside the inner coordination shell and of changes of bond lengths inside it, relations between electrochemical and related chemical rate constants are deduced and compared with the experimental data. A correlation is found, without the use of arbitrary parameters. Effects of weak complexes with added electrolytes are included under specified conditions. The deductions offer a way of coordinating a variety of data in the two fields, internally as well as with each those in another. For example, the rate of oxidation or reduction of a series of related reactants by one reagent is correlated with that of another and with that of the corresponding electrochemical oxidation-reduction reaction, under certain specified conditions. These correlations may also provide a test for distinguishing an electron from an atom transfer mechanism. (auth)

  17. Cross Section of OH Radical Overtone Transition near 7028 cm(-1) and Measurement of the Rate Constant of the Reaction of OH with HO2 Radicals.

    PubMed

    Assaf, Emmanuel; Fittschen, Christa

    2016-09-15

    The absorption cross section of an overtone transition of OH radicals at 7028.831 cm(-1) has been measured using an improved experimental setup coupling laser photolysis to three individual time-resolved detection techniques. Time-resolved relative OH radical profiles were measured by laser-induced fluorescence (LIF), and their absolute profiles have been obtained by cw-cavity ring-down spectroscopy (cw-CRDS). HO2 radicals were quantified simultaneously at the well-characterized absorption line at 6638.21 cm(-1) by a second cw-CRDS absorption path. Initial OH concentrations and thus their absorption cross sections have been deduced from experiments of 248 nm photolysis of H2O2: OH and HO2 profiles have been fitted to a simple kinetic model using well-known rate constants. The rate constant of the reaction between OH and HO2 radicals turned out to be sensitive to the deduction of the initial OH concentration and has been revisited in this work: OH decays have been observed in the presence of varying excess HO2 concentrations. A rate constant of (1.02 ± 0.06) × 10(-10) cm(3) s(-1) has been obtained, in good agreement with previous measurements and recent recommendations. An absorption cross section of σOH = (1.54 ± 0.1) × 10(-19) cm(2) at a total pressure of 50 Torr helium has been obtained from consistent fitting of OH and HO2 profiles in a large range of concentrations.

  18. Relationship between reaction rate constants of organic pollutants and their molecular descriptors during Fenton oxidation and in situ formed ferric-oxyhydroxides.

    PubMed

    Jia, Lijuan; Shen, Zhemin; Su, Pingru

    2016-05-01

    Fenton oxidation is a promising water treatment method to degrade organic pollutants. In this study, 30 different organic compounds were selected and their reaction rate constants (k) were determined for the Fenton oxidation process. Gaussian09 and Material Studio software sets were used to carry out calculations and obtain values of 10 different molecular descriptors for each studied compound. Ferric-oxyhydroxide coagulation experiments were conducted to determine the coagulation percentage. Based upon the adsorption capacity, all of the investigated organic compounds were divided into two groups (Group A and Group B). The percentage adsorption of organic compounds in Group A was less than 15% (wt./wt.) and that in the Group B was higher than 15% (wt./wt.). For Group A, removal of the compounds by oxidation was the dominant process while for Group B, removal by both oxidation and coagulation (as a synergistic process) took place. Results showed that the relationship between the rate constants (k values) and the molecular descriptors of Group A was more pronounced than for Group B compounds. For the oxidation-dominated process, EHOMO and Fukui indices (f(0)x, f(-)x, f(+)x) were the most significant factors. The influence of bond order was more significant for the synergistic process of oxidation and coagulation than for the oxidation-dominated process. The influences of all other molecular descriptors on the synergistic process were weaker than on the oxidation-dominated process.

  19. Calculation of kinetic rate constants from thermodynamic data

    NASA Technical Reports Server (NTRS)

    Marek, C. John

    1995-01-01

    A new scheme for relating the absolute value for the kinetic rate constant k to the thermodynamic constant Kp is developed for gases. In this report the forward and reverse rate constants are individually related to the thermodynamic data. The kinetic rate constants computed from thermodynamics compare well with the current kinetic rate constants. This method is self consistent and does not have extensive rules. It is first demonstrated and calibrated by computing the HBr reaction from H2 and Br2. This method then is used on other reactions.

  20. Rate constants for the reactions ofCH{3}O with CH{2}O, CH{3}CHO and {i}-C{4}H10

    NASA Astrophysics Data System (ADS)

    Fittschen, C.; Delcroix, B.; Gomez, N.; Devolder, P.

    1998-11-01

    The rate constants for the reactions of CH3O radicals with 3 reactants have been measured by two different techniques: laser photolysis (LP) and fast flow reactor (FF), both coupled with a detection of CH3O radicals by laser induced fluorescence. The reaction with formaldehyde CH2O has been measured in the temperature range 295 - 450 K. Both sets of results (LP and FF) are in excellent agreement and lead to a pressure independent rate constant of k_1=(1.1± 0.3)×10-12exp(9.6±1.0 kJ mol-1/RT) cm3 s-1. The reaction with acetaldehyde CH3CHO has been measured between 286 and 493 K. Both sets of experiments are again in excellent agreement and lead to a pressure independent rate constant of k_2=(5.7± 1.3)×10-13exp(-5.2±0.7 kJ mol-1/RT) cm3 s-1. No reaction could be detected with isobutane i-C4H{10} at temperature up to 356 K, leading to an upper limit for k_3≤2.5×10-15 cm3 s-1. Les constantes de vitesse des réactions du radical méthoxyde (CH3O) avec 3 réactifs ont été mesurées par deux techniques différentes : la photolyse laser (LP) et le réacteur à écoulement rapide (FF), toutes deux associées à une détection de CH3O par fluorescence induite par laser (LIF). La réaction avec le formaldéhyde CH2O a été étudiée dans la gamme de températures 295 450 K. Ces deux techniques conduisent à des valeurs de k1 bien cohérentes et représentées par l'expression k_1=(1,1± 0,3)×10-12exp(9,6± 1,0 kJ mol-1/RT) cm3 s-1, indépendante de la pression. La réaction avec l'acétaldéhyde CH3CHO a été étudiée dans la gamme de températures 286 493 K. L'ensemble des valeurs est bien représenté par k_2=(5,7± 1,3)×10-13exp(-5,2±0.7 kJ mol-1/RT) cm3 s-1, indépendante de la pression. Avec la technique de LP aucune réaction n'a pu être détectée jusqu'à 356 K pour la réaction de CH3O avec l'isobutane i-C4H{10} (k3), d'où la valeur limite : k_3≤2.5×10-15 cm3 s-1.

  1. The use of digital simulation to improve the cyclic voltammetric determination of rate constants for homogeneous chemical reactions following charge transfers.

    PubMed

    Mozo, J D; Carbajo, J; Sturm, J C; Núñez-Vergara, L J; Moscoso, R; Squella, J A

    2011-08-05

    Cyclic voltammetry (CV) is a very useful electrochemical tool used to study reaction systems that include chemical steps that are coupled to electron transfers. This type of system generally involves the chemical reaction of an electrochemically generated free radical. Published methods exist that are used to determine the kinetics of electrochemically initiated chemical reactions from the measurements of the peak current ratio (i(pa)/i(pc)) of a cyclic voltammogram. The published method requires working curves to relate a kinetic parameter to the peak current ratio. In the presented work, a digital simulation package was used to obtain improved working curves for specific working conditions. The curves were compared with the published results for the first- and second-order chemical reactions following the charge transfer step mechanisms. According to the presented results, the previously published working curve is reliable for a mechanism with a first-order chemical reaction; however, a change in the switching potential requires a recalculation of the curve. In the case of mechanisms with a second-order step (dimerisation and disproportionation), several different views exist on how the second-order chemical term should be expressed so that different values of the constant are obtained. Parameters such as electrode type, electrode area, electroactive species concentration, switching potential, scan rate and method for peak current ratio calculation modify the working curves and must always be specified. We propose a standardised method to obtain the most reliable kinetic constant values. The results of this work will permit researchers who handle simulation software to construct their own working curves. Additionally, those who do not have the simulation software could use the working curves described here. The revelations of the presented experiments may be useful to a broad chemistry audience because this study presents a simple and low-cost procedure for the

  2. Rate constants and H atom branching ratios of the gas-phase reactions of methylidyne CH(X2Pi) radical with a series of alkanes.

    PubMed

    Loison, Jean-Christophe; Bergeat, Astrid; Caralp, Françoise; Hannachi, Yacine

    2006-12-21

    The reactions of the CH radical with several alkanes were studied, at room temperature, in a low-pressure fast-flow reactor. CH(X2Pi, v = 0) radicals were obtained from the reaction of CHBr(3) with potassium atoms. The overall rate constants at 300 K are (0.76 +/- 0.20) x 10(-10) [Fleurat-Lessard, P.; Rayez, J. C.; Bergeat, A.; Loison, J. C. Chem. Phys. 2002, 279, 87],1 (1.60 +/- 0.60) x 10(-10)[Galland, N.; Caralp, F.; Hannachi, Y.; Bergeat, A.; Loison, J.-C. J. Phys. Chem. A 2003, 107, 5419],2 (2.20 +/- 0.80) x 10(-10), (2.80 +/- 0.80) x 10(-10), (3.20 +/- 0.80) x 10(-10), (3.30 +/- 0.60) x 10(-10), and (3.60 +/- 0.80) x 10(-10) cm3 molecule(-1) s(-1), (errors refer to +/-2sigma) for methane, ethane, propane, n-butane, n-pentane, neo-pentane, and n-hexane respectively. The experimental overall rate constants correspond to those obtained using a simple classical capture theory. Absolute atomic hydrogen production was determined by V.U.V. resonance fluorescence, with H production from the CH + CH4 reaction being used as a reference. Observed H branching ratios were for CH4, 1.00[Fleurat-Lessard, P.; Rayez, J. C.; Bergeat, A.; Loison, J. C. Chem. Phys. 2002, 279, 87];1 C(2)H(6), 0.22 +/- 0.08 [Galland, N.; Caralp, F.; Hannachi, Y.; Bergeat, A.; Loison, J.-C. J. Phys. Chem. A 2003, 107, 5419];2 C(3)H(8), 0.19 +/- 0.07; C(4)H(10) (n-butane), 0.14 +/- 0.06; C(5)H(12) (n-pentane), 0.52 +/- 0.08; C(5)H(12) (neo-pentane), 0.51 +/- 0.08; C(5)H(12) (iso-pentane), 0.12 +/- 0.06; C(6)H(14) (n-hexane), 0.06 +/- 0.04.

  3. Fully Coriolis-coupled quantum studies of the H + O2 (upsilon i = 0-2, j i = 0,1) --> OH + O reaction on an accurate potential energy surface: integral cross sections and rate constants.

    PubMed

    Lin, Shi Ying; Sun, Zhigang; Guo, Hua; Zhang, Dong Hui; Honvault, Pascal; Xie, Daiqian; Lee, Soo-Y

    2008-01-31

    We present accurate quantum calculations of the integral cross section and rate constant for the H + O2 --> OH + O combustion reaction on a recently developed ab initio potential energy surface using parallelized time-dependent and Chebyshev wavepacket methods. Partial wave contributions up to J = 70 were computed with full Coriolis coupling, which enabled us to obtain the initial state-specified integral cross sections up to 2.0 eV of the collision energy and thermal rate constants up to 3000 K. The integral cross sections show a large reaction threshold due to the quantum endothermicity of the reaction, and they monotonically increase with the collision energy. As a result, the temperature dependence of the rate constant is of the Arrhenius type. In addition, it was found that reactivity is enhanced by reactant vibrational excitation. The calculated thermal rate constant shows a significant improvement over that obtained on the DMBE IV potential, but it still underestimates the experimental consensus.

  4. Rate constants for the reaction of OH radicals with n-propyl, n-butyl, iso-butyl and tert-butyl vinyl ethers

    NASA Astrophysics Data System (ADS)

    Thiault, G.; Mellouki, A.

    Rate constants for the reaction of OH radicals with n-propyl vinyl ether (PVE, CH 3CH 2CH 2OCH dbnd CH 2), n-butyl vinyl ether (BVE, CH 3CH 2CH 2CH 2OCH dbnd CH 2), iso-butyl vinyl ether (IBVE, (CH 3) 2CHCH 2OCH dbnd CH 2) and tert-butyl vinyl ether (TBVE, (CH 3) 3COCH dbnd CH 2), have been measured in the temperature and pressure ranges 232-373 K and 30-300 Torr using the pulsed laser photolysis-laser-induced fluorescence method, and at 298 K and 760 Torr using the relative method. The obtained results are k1=(9.3±0.6)×10 -12 exp[(708±20)/ T], k2=(1.5±0.2)×10 -11 exp[(572±42)/ T], k3=(1.6±0.1)×10 -11 exp[(567±20)/ T], k4=(1.7±0.2)×10 -11 exp[(549±25)/ T] cm 3 molecule -1 s -1. The values at 298 K are k1=(1.0±0.1)×10 -10, k2=(1.0±0.1)×10 -10, k3=(1.1±0.1)×10 -10, k4=(1.1±0.1)×10 -10 cm 3 molecule -1 s -1. The deduced tropospheric lifetimes of these ethers for reaction with OH are of the order of 1 h and they are comparable to those for reaction with ozone.

  5. The load dependence of rate constants.

    PubMed

    Walcott, Sam

    2008-06-07

    As experimental techniques in biophysics have progressed at the single molecule level, there has been considerable interest in understanding how external mechanical influences (such as load) affect chemical reactions. The majority of biophysical studies investigating load-dependent kinetics use an equation where the rate constant exponentially depends on force, which is sometimes called Bell's equation. This equation requires the determination of two parameters that describe the potential energy-strain function: k(0), which is the reaction rate in the absence of load, and x(c), which is the difference in strain between the reactant and transition states. However, there have been theoretical studies based on Kramers' theory suggesting that the rate constant should have load-dependent pre-exponential terms and nonlinear load-dependent terms in the exponential. Kramers' theory requires an exact knowledge of the potential energy-strain function, which is in general not known for an experimental system. Here, we derive a general approximation of Kramers' theory where the potential energy-strain function is described by five parameters, which can, for small loads, be reduced to four-, three-, and finally to two parameters (Bell's equation). We then use an idealized physical system to validate our approximations to Kramers' theory and show how they can predict parameters of interest (such as k(0) and x(c)) better than Bell's equation. Finally, we show previously published experimental data that are not well fitted by Bell's equation but are adequately fitted by these more exact equations.

  6. Correction for instrument time constant in determination of reaction kinetics.

    SciTech Connect

    Chilton, Marie; Clark, Jared; Thomas, Nathan; Nicholson, Allen; Hansen, Lee D.; Hansen, Clifford W.; Hansen, Jaron

    2010-02-01

    Rates of reactions can be expressed as dn/dt = kcf(n) where n is moles of reaction, k is a rate constant, c is a proportionality constant, and f(n) is a function of the properties of the sample. When the instrument time constant, ?, and k are sufficiently comparable that measured rates are significantly affected by instrument response, correction for instrument response must be done to obtain accurate reaction kinetics. Correction for instrument response has previously been done by truncating early data or by use of the Tian equation. Both methods can lead to significant errors. We describe a method for simultaneous determination of ?, k, and c by fitting equations describing the combined instrument response and rate law to rates observed as a function of time. The method was tested with data on the heat rate from acid-catalyzed hydrolysis of sucrose.

  7. Absolute rate constant measurements of CF(X{sup 2}II) reactions. 1. Reactions with O{sub 2}, F{sub 2}, Cl{sub 2}, and NO

    SciTech Connect

    Peeters, J.; Van Hoeymissen, J.; Vanhaelemeersch, S.

    1992-02-06

    Rate coefficients of the elementary reactions of CF(X{sup 2}II, v=O) with O{sub 2}, F{sub 2}, Cl{sub 2}, and NO at T = 294 K and p = 2-10 Torr (He or Ar bath gas) have been determined for the first time, using laser photodissociation/laser induced induced fluorescence (LIF) techniques. The CF(X{sup 2}II) radicals were generated by excimer laser photolysis of CF{sub 2}Br{sub 2} at 248 nm. Exponential decays of CF(X, v=0) were monitored by LIF (A{sup 2}{Sigma}{sup +}, v{prime}=1{l_arrow}X{sup 2}II, v{double_prime}=0 transition) at varying concentrations of the reactant, in very large excess over CF. The rate constant for each of the title reactions was found to be independent of pressure in the 2-10-Torr range: k(O{sub 2}) = (1.6 {+-} 0.2) x 10{sup -12}; k(F{sub 2}) = (3.9 {+-}0.4) x 10{sup -12}; k(Cl{sub 2}) = (1.7 {+-} 0.2) x 10{sup -11}, and k(NO) = (2.1 {+-} 0.2) x 10{sup -11} cm{sup 3} molecule{sup -1} s{sup -1}. Besides, an upper limit K {le} 10{sup 14} was established for the reactions of CF(X) with H{sub 2}, CH{sub 4}, C{sub 2}H{sub 2}, CF{sub 4}, and CO{sub 2}. In general, the rate coefficients obtained for the investigated CF(X{sup 2}II) reactions are close to literature data on CCl and CBr analogues but far below reported rate constants of corresponding CH(X{sup 2}II) reactions are close to literature data on CCl and CBr analogues but far below reported rate constants of corresponding CH(X{sup 2}II) processes, specifically regarding reactions with closed-shell molecules. The exceptional stability and concomitantly low reactivity of CF-relative to CH-can be attributed to strong back-donation from one of the fluorine 2p{pi} orbitals into the vacant carbon 2p{pi} orbital. 33 refs., 8 figs., 1 tab.

  8. When Rate Constants Are Not Enough.

    PubMed

    Barker, John R; Frenklach, Michael; Golden, David M

    2015-07-16

    Real-world chemical systems consisting of multiple isomers and multiple reaction channels often react significantly prior to attaining a steady state energy distribution (SED). Detailed elementary reaction models, which implicitly require SED conditions, may be invalid when non-steady-state energy distributions (NSED) exist. NSED conditions may result in reaction rates and product yields that are different from those expected for SED conditions, although this problem is to some extent reduced by using phenomenological models and rate constants. The present study defines pragmatic diagnostics useful for identifying NSED conditions in stochastic master equation simulations. A representative example is presented for each of four classes of common combustion species: RO2 radicals, aliphatic hydrocarbons, alkyl radicals, and polyaromatic radicals. An example selected from the seminal work of Tsang et al. demonstrates that stochastic simulations and eigenvalue methods for solving the master equation predict the same NSED effects. NSED effects are common under relatively moderate combustion conditions, and accurate simulations may require a master equation analysis.

  9. Multivariate curve resolution-alternating least squares and kinetic modeling applied to near-infrared data from curing reactions of epoxy resins: mechanistic approach and estimation of kinetic rate constants.

    PubMed

    Garrido, M; Larrechi, M S; Rius, F X

    2006-02-01

    This study describes the combination of multivariate curve resolution-alternating least squares with a kinetic modeling strategy for obtaining the kinetic rate constants of a curing reaction of epoxy resins. The reaction between phenyl glycidyl ether and aniline is monitored by near-infrared spectroscopy under isothermal conditions for several initial molar ratios of the reagents. The data for all experiments, arranged in a column-wise augmented data matrix, are analyzed using multivariate curve resolution-alternating least squares. The concentration profiles recovered are fitted to a chemical model proposed for the reaction. The selection of the kinetic model is assisted by the information contained in the recovered concentration profiles. The nonlinear fitting provides the kinetic rate constants. The optimized rate constants are in agreement with values reported in the literature.

  10. Cheap but accurate calculation of chemical reaction rate constants from ab initio data, via system-specific, black-box force fields

    NASA Astrophysics Data System (ADS)

    Steffen, Julien; Hartke, Bernd

    2017-10-01

    Building on the recently published quantum-mechanically derived force field (QMDFF) and its empirical valence bond extension, EVB-QMDFF, it is now possible to generate a reliable potential energy surface for any given elementary reaction step in an essentially black box manner. This requires a limited and pre-defined set of reference data near the reaction path and generates an accurate approximation of the reference potential energy surface, on and off the reaction path. This intermediate representation can be used to generate reaction rate data, with far better accuracy and reliability than with traditional approaches based on transition state theory (TST) or variational extensions thereof (VTST), even if those include sophisticated tunneling corrections. However, the additional expense at the reference level remains very modest. We demonstrate all this for three arbitrarily chosen example reactions.

  11. Cheap but accurate calculation of chemical reaction rate constants from ab initio data, via system-specific, black-box force fields.

    PubMed

    Steffen, Julien; Hartke, Bernd

    2017-10-28

    Building on the recently published quantum-mechanically derived force field (QMDFF) and its empirical valence bond extension, EVB-QMDFF, it is now possible to generate a reliable potential energy surface for any given elementary reaction step in an essentially black box manner. This requires a limited and pre-defined set of reference data near the reaction path and generates an accurate approximation of the reference potential energy surface, on and off the reaction path. This intermediate representation can be used to generate reaction rate data, with far better accuracy and reliability than with traditional approaches based on transition state theory (TST) or variational extensions thereof (VTST), even if those include sophisticated tunneling corrections. However, the additional expense at the reference level remains very modest. We demonstrate all this for three arbitrarily chosen example reactions.

  12. Temperature dependence for the absolute rate constant for the reaction CH/sub 2/OH + O/sub 2/. -->. HO/sub 2/ + H/sub 2/CO from 215 to 300 K

    SciTech Connect

    Nesbitt, F.L.; Payne, W.A.; Stief, L.J.

    1988-07-14

    The absolute rate constant for the reaction CH/sub 2/OH + O/sub 2/ ..-->.. HO/sub 2/ + H/sub 2/CO has been determined from 215 to 300 K in a discharge flow system at 1-Torr pressure. The decay of CH/sub 2/OH was monitored in excess O/sub 2/ by collision-free sampling mass spectrometry. The temperature dependence observed is quite different from the usual Arrhenius form. For the 215-250 K interval, an activation energy of 3.4 kcal/mol is obtained. Above 250 K, the rate constant increases at a slower rate; previous high-temperature studies (1000-2000 K) again show a faster increase in the rate constant. A possible explanation for these observations is that the reaction mechanism changes from addition at low temperature to direct abstraction at high temperatures.

  13. Transient state kinetics of enzyme IICBGlc, a glucose transporter of the phosphoenolpyruvate phosphotransferase system of Escherichia coli: equilibrium and second order rate constants for the glucose binding and phosphotransfer reactions.

    PubMed

    Meadow, Norman D; Savtchenko, Regina S; Nezami, Azin; Roseman, Saul

    2005-12-23

    During translocation across the cytoplasmic membrane of Escherichia coli, glucose is phosphorylated by phospho-IIA(Glc) and Enzyme IICB(Glc), the last two proteins in the phosphotransfer sequence of the phosphoenolpyruvate:glucose phosphotransferase system. Transient state (rapid quench) methods were used to determine the second order rate constants that describe the phosphotransfer reactions (phospho-IIA(Glc) to IICB(Glc) to Glc) and also the second order rate constants for the transfer from phospho-IIA(Glc) to molecularly cloned IIB(Glc), the soluble, cytoplasmic domain of IICB(Glc). The rate constants for the forward and reverse phosphotransfer reactions between IIA(Glc) and IICB(Glc) were 3.9 x 10(6) and 0.31 x 10(6) m(-1) s(-1), respectively, and the rate constant for the physiologically irreversible reaction between [P]IICB(Glc) and Glc was 3.2 x 10(6) m(-1) s(-1). From the rate constants, the equilibrium constants for the transfer of the phospho-group from His90 of [P]IIA(Glc) to the phosphorylation site Cys of IIB(Glc) or IICB(Glc) were found to be 3.5 and 12, respectively. These equilibrium constants signify that the thiophospho-group in these proteins has a high phosphotransfer potential, similar to that of the phosphohistidinyl phosphotransferase system proteins. In these studies, preparations of IICB(Glc) were invariably found to contain endogenous, firmly bound Glc (estimated K'(D) approximately 10(-7) m). The bound Glc was kinetically competent and was rapidly phosphorylated, indicating that IICB(Glc) has a random order, Bi Bi, substituted enzyme mechanism. The equilibrium constant for the binding of Glc was deduced from differences in the statistical goodness of fit of the phosphotransfer data to the kinetic model.

  14. Rate constant for the reaction H + NO2 from 195 to 400 K with FP-RF and DF-RF techniques. [Flash Photolysis and Discharge Flow-Resonance Fluorescence

    NASA Technical Reports Server (NTRS)

    Michael, J. V.; Nava, D. F.; Payne, W. A.; Lee, J. H.; Stief, L. J.

    1979-01-01

    Measurements are made of the rate constant for the reaction H + NO2 yielding OH + NO over significant temperature ranges with the flash photolysis-resonance fluorescence (FP-RF) technique and also with the discharge flow-resonance fluorescence (DF-RF) technique. Since it is important to study chemical reactions with more than one technique, the above reaction is investigated with the FP-RF technique over the temperature range 230-400 K. The results reported do not agree with earlier determinations. Accordingly, a separate set of measurements is made with the DF-RF technique over the temperature range 195-368 K; the results are also reported. The reaction is also considered theoretically, especially with regard to the question of temperature dependence and absolute magnitude of the rate constant.

  15. Rate constants for the reactions of OH with HFC-134a (CH[sub 3]CH[sub 2]F) and HFC-134 (CHF[sub 2]CHF[sub 2])

    SciTech Connect

    DeMore, W.B.

    1993-07-09

    This paper presents the results of experimental measurements of the rate constants for the reaction of OH with HFC-134a (CF[sub 3]CH[sub 2]F) and with HFC-134 (CF[sub 2]HCF[sub 2]H). The rate constants were measured relative to reaction rates with methane, methyl chloroform (CH[sub 3]CCl[sub 3]), and HFC-125 (CF[sub 3]CHF[sub 2]). The measurements were done at atmospheric pressure, in the temperature range 298-358 K, in a slow-flow photochemical reactor. These constants are important for predicting the atmospheric lifetimes of these compounds, which are being considered as substitutes for chlorine containing CFC compounds for use in refrigeration systems.

  16. Applications of Reaction Rate

    ERIC Educational Resources Information Center

    Cunningham, Kevin

    2007-01-01

    This article presents an assignment in which students are to research and report on a chemical reaction whose increased or decreased rate is of practical importance. Specifically, students are asked to represent the reaction they have chosen with an acceptable chemical equation, identify a factor that influences its rate and explain how and why it…

  17. The reactions CHnD4-n+OH-->P and CH4+OD-->CH3+HOD as a test of current direct dynamics multicoefficient methods to determine variational transition state rate constants. II

    NASA Astrophysics Data System (ADS)

    Masgrau, Laura; González-Lafont, Àngels; Lluch, José M.

    2001-09-01

    In this paper we have carried out a test of current multilevel electronic structure methods to give accurate rate constants for the reactions CHnD4-n+OH→P and for the reaction of methane with OD. These multilevel methods are single-point energy techniques designed as general parametrizations for extrapolation to the full configuration interaction limit and, in some cases, to attain also the infinite basis set limit. By means of variational transition state theory including multidimensional tunneling corrections, the rate constants for these reactions, over a wide range of temperatures, have been computed using two recently developed multicoefficient schemes for extrapolating correlated electronic structure calculations: multicoefficient scaling all correlation (MCSAC) and multicoefficient correlation methods (MCCM). For comparison purposes, we have also evaluated the same rate constants using two other multilevel extrapolation techniques, namely, the multicoefficient quadratic configuration interaction (MC-QCISD) method and the complete basis set extrapolation model for free radicals (CBS-RAD). Two dual-level direct dynamics techniques have been employed within the scheme of variational transition state theory: the interpolated single-point energy corrections (ISPE) and the interpolated optimized corrections (IOC), with the purpose to analyze the importance of correcting a low level potential energy surface with the optimizations of the stationary points carried out at the highest computational level affordable. We have shown that the so-called MCCM-CCSD(T)-1sc multilevel scheme provides the best results for the set of reactions studied. A slight difference from the experimental rate constants still persists, specially at the lowest temperatures, although we think that the best theoretical rate constants of the present paper are accurate enough for most of the practical applications. However, the kinetic isotope effects (KIEs) are not so well reproduced because

  18. The Rate Constant for Fluorescence Quenching

    ERIC Educational Resources Information Center

    Legenza, Michael W.; Marzzacco, Charles J.

    1977-01-01

    Describes an experiment that utilizes fluorescence intensity measurements from a Spectronic 20 to determine the rate constant for the fluorescence quenching of various aromatic hydrocarbons by carbon tetrachloride in an ethanol solvent. (MLH)

  19. The Rate Constant for Fluorescence Quenching

    ERIC Educational Resources Information Center

    Legenza, Michael W.; Marzzacco, Charles J.

    1977-01-01

    Describes an experiment that utilizes fluorescence intensity measurements from a Spectronic 20 to determine the rate constant for the fluorescence quenching of various aromatic hydrocarbons by carbon tetrachloride in an ethanol solvent. (MLH)

  20. Dependence of rate constants on vibrational temperatures - An Arrhenius description

    NASA Technical Reports Server (NTRS)

    Ford, D. I.; Johnson, R. E.

    1988-01-01

    An interpretation of the variation of rate constants with vibrational temperature is proposed which introduces parameters analogous to those of the classical Arrhenius expression. The constancy of vibrational activation energy is studied for the dissociaton of NO, the ion-molecular reaction of O(+) with N2, and the atom exchange reaction of I with H2. It is found that when a Boltzmann distribution for vibrational states is applicable, the variation of the rate constant with the vibrational temperature can be used to define a vibrational activation energy. The method has application to exchange reactions where a vibrational energy threshold exists.

  1. Dependence of rate constants on vibrational temperatures - An Arrhenius description

    NASA Technical Reports Server (NTRS)

    Ford, D. I.; Johnson, R. E.

    1988-01-01

    An interpretation of the variation of rate constants with vibrational temperature is proposed which introduces parameters analogous to those of the classical Arrhenius expression. The constancy of vibrational activation energy is studied for the dissociaton of NO, the ion-molecular reaction of O(+) with N2, and the atom exchange reaction of I with H2. It is found that when a Boltzmann distribution for vibrational states is applicable, the variation of the rate constant with the vibrational temperature can be used to define a vibrational activation energy. The method has application to exchange reactions where a vibrational energy threshold exists.

  2. Reaction Rate Constants of CH4(ads) ⇌ CH3(ads) + H(ads) on Ni(111): The Effect of Lattice Motion.

    PubMed

    Wang, Wenji; Zhao, Yi

    2015-12-31

    Methane dissociation on metal surfaces is of great commercial importance. The dissociation and recombination rate constants of CH4 on Ni(111) are calculated using the quantum instanton approach with the path integral Monte Carlo method. The Ni(111) lattice is treated rigidly, classically, and quantum mechanically to reveal the effects of lattice motion and quantum tunneling. For the dissociation of CH4, the rates have the smallest value on the rigid lattice, while they possess the largest value on the quantum lattice. For instance, at 300 K, the rates on the classical and quantum lattices are 5 and 12 times larger than that on the rigid lattice, respectively. The curve of the Arrhenius plot for the dissociation rates on the rigid lattice demonstrates that the quantum tunneling effect of the ruptured H atom is remarkable, while the nearly invariable dissociation rates at low temperatures on the quantum lattice confirm that the thermally assisted tunneling should be dominant at low temperatures. For the recombination of CH4, the quantum lattice still has rates that are much larger than that of the rigid lattice. For instance, the ratio of the recombination rates on the quantum and rigid lattices is 12 at 300 K. The quantum tunneling effect seems to play a minor role in the recombination rates on the rigid lattice; however, the thermally assisted tunneling is still very significant for the recombination process.

  3. Quasiclassical Trajectory Calculations of the Rate Constant of the OH + HBr → Br + H2O Reaction Using a Full-Dimensional Ab Initio Potential Energy Surface Over the Temperature Range 5 to 500 K.

    PubMed

    de Oliveira-Filho, Antonio G S; Ornellas, Fernando R; Bowman, Joel M

    2014-02-20

    We report a permutationally invariant, ab initio potential energy surface (PES) for the OH + HBr → Br + H2O reaction. The PES is a fit to roughly 26 000 spin-free UCCSD(T)/cc-pVDZ-F12a energies and has no classical barrier to reaction. It is used in quasiclassical trajectory calculations with a focus on the thermal rate constant, k(T), over the temperature range 5 to 500 K. Comparisons with available experimental data over the temperature range 23 to 416 K are made using three approaches to treat the OH rotational and associated electronic partition function. All display an inverse temperature dependence of k(T) below roughly 160 K and a nearly constant temperature dependence above 160 K, in agreement with experiment. The calculated rate constant with no treatment of spin-orbit coupling is overall in the best agreement with experiment, being (probably fortuitously) within 20% of it.

  4. Beyond transition state theory: accurate description of nuclear quantum effects on the rate and equilibrium constants of chemical reactions using Feynman path integrals.

    PubMed

    Vanícek, Jirí

    2011-01-01

    Nuclear tunneling and other nuclear quantum effects have been shown to play a significant role in molecules as large as enzymes even at physiological temperatures. I discuss how these quantum phenomena can be accounted for rigorously using Feynman path integrals in calculations of the equilibrium and kinetic isotope effects as well as of the temperature dependence of the rate constant. Because these calculations are extremely computationally demanding, special attention is devoted to increasing the computational efficiency by orders of magnitude by employing efficient path integral estimators.

  5. Direct and Indirect Determinations of Elementary Rate Constants H + O2: Chain Branching; the Dehydration of tertiary-Butanol; the Retro Diels-Alder Reaction of Cyclohexene; the Dehydration of Isopropanol

    NASA Astrophysics Data System (ADS)

    Heyne, Joshua S.

    Due to growing environmental concern over the continued use of fossil fuels, methods to limit emissions and partially replace fossil fuel use with renewable biofuels are of considerable interest. Developing chemical kinetic models for the chemistry that affects combustion properties is important to understanding how new fuels affect combustion energy conversion processes in transportation devices. This thesis reports the experimental study of several important reactions (the H + O2 branching reaction, the key decomposition reactions of tertiary-butanol, the dehydration reaction of isopropanol, and the retro Diels-Alder reaction of cyclohexene) and develops robust analysis methods to estimate the absolute uncertainties of specific elementary rate constants derived from the experimental data. In the study of the above reactions, both a direct and indirect rate constant determination technique with associated uncertainty estimation methodologies are developed. In the study of the decomposition reactions, a direct determination technique is applied to experimental data gathered in preparation of this thesis. In the case of the dehydration reaction of tertiary-butanol and the retro Diels-Alder reaction of cyclohexene, both of which are used as internal standards for relative rate studies (Herzler et al. 1997) and chemical thermometry (Rosado-Reyes et al. 2013) , analysis showed an ˜20 K difference in the reaction rate between the reported results and the previous recommendations. In light of these discrepancies, an uncertainty estimation of previous recommendations illuminated an uncertainty of at least 20 K for the dehydration reaction of tertiary-butanol and the retro Diels-Alder reaction of cyclohexene, thus resolving the discrepancies. The determination of the H + O2 branching reaction and decomposition reactions of isopropanol used an indirect determination technique. The uncertainty of the H + O2 branching reaction rate is shown to be underestimated by previous

  6. RATE CONSTANTS FOR THE REACTIONS OF OH RADICALS AND CL ATOMS WITH DI-N-PROPYL ETHER AND DI-N-BUTYL ETHER AND THEIR DEUTERATED ANALOGS. (R825252)

    EPA Science Inventory

    Using relative rate methods, rate constants for the gas-phase reactions of OH radicals and Cl atoms with di-n-propyl ether, di-n-propyl ether-d14, di-n-butyl ether and di-n-butyl ether-d18 have been measured at 296 ? 2 K and atmos...

  7. Measurements in N2-CH4(C2H2) discharges of reaction rates and thermochemical constants for Titan atmosphere study.

    PubMed

    Ricard, A; Cernogora, G; Fitaire, M; Hochard, L; Kouassi, N; Speller, C; Vacher, J R

    1995-01-01

    The kinetic reactions in N2-xCH4(C2H2) gas discharges with x less than 1% have been studied by emission spectroscopy in the afterglow of D.C. discharges and by mass spectroscopy from radiolysis ionization using alpha particles. The pressure range is from several Torr to 100 Torr. At the end of N2 D.C. discharges at room temperature, for a residence time of about 10(-2) s, the dominant active species are the N atoms with density of 10(14)-10(15) cm-3 for N2 density of about 10(17) cm-3 (3 Torr), the N2(X,V) vibrational molecules with for example [N2(X,V = 10)] approximately 10(14) cm-3 and the electronic metastable molecules N2(A 3 sigma u +) with a density of 10(12) cm-3. In such conditions, the following kinetic reactions have been studied: N2(A) + N2(A) --> N2(C,B,V') + N2(X), N2(A) + N2(X,V>5) --> N2(X) + N2(B,V') in pure N2 post-discharges and N2(A) + CH4 --> products, C + N + M2 --> CN(B,V') + M2, N2(X,V>4) + CN --> N2(X) + CN(B,A,V'), in N2-1% CH4 post-discharges. The clustering reactions of N2-(1-5%)CH4(C2H2) gas mixtures after radiolysis ionization have been studied for the H2CN+ nN2 ions and the equilibrium constants have been determined in the temperature range T = 140-300 K.

  8. Rate constants for 1,5- and 1,6-hydrogen atom transfer reactions of mono-, di-, and tri-aryl-substituted donors, models for hydrogen atom transfers in polyunsaturated fatty acid radicals.

    PubMed

    DeZutter, Christopher B; Horner, John H; Newcomb, Martin

    2008-03-06

    Rate constants for 1,5- and 1,6-hydrogen atom transfer reactions in models of polyunsaturated fatty acid radicals were measured via laser flash photolysis methods. Photolyses of PTOC (pyridine-2-thioneoxycarbonyl) ester derivatives of carboxylic acids gave primary alkyl radicals that reacted by 1,5-hydrogen transfer from mono-, di-, and tri-aryl-substituted positions or 1,6-hydrogen transfer from di- and tri-aryl-substituted positions to give UV-detectable products. Rate constants for reactions in acetonitrile at room temperature ranged from 1 x 10(4) to 4 x 10(6) s(-1). The activation energies for a matched pair of 1,5- and 1,6-hydrogen atom transfers giving tri-aryl-substituted radicals were approximately equal, as were the primary kinetic isotope effects, but the 1,5-hydrogen atom transfer reaction was 1 order of magnitude faster at room temperature than the 1,6-hydrogen atom transfer reaction due to a less favorable entropy of activation for the 1,6-transfer reaction. Solvent effects on the rate constants for the 1,5-hydrogen atom transfer reaction of the 2-[2-(diphenylmethyl)phenyl]ethyl radical at ambient temperature were as large as a factor of 2 with the reaction increasing in rate in lower polarity solvents. Hybrid density functional theory computations for the 1,5- and 1,6-hydrogen atom transfers of the tri-aryl-substituted donors were in qualitative agreement with the experimental results.

  9. Inflation with a constant rate of roll

    SciTech Connect

    Motohashi, Hayato; Starobinsky, Alexei A.; Yokoyama, Jun'ichi E-mail: alstar@landau.ac.ru

    2015-09-01

    We consider an inflationary scenario where the rate of inflaton roll defined by {sup ··}φ/H φ-dot remains constant. The rate of roll is small for slow-roll inflation, while a generic rate of roll leads to the interesting case of 'constant-roll' inflation. We find a general exact solution for the inflaton potential required for such inflaton behaviour. In this model, due to non-slow evolution of background, the would-be decaying mode of linear scalar (curvature) perturbations may not be neglected. It can even grow for some values of the model parameter, while the other mode always remains constant. However, this always occurs for unstable solutions which are not attractors for the given potential. The most interesting particular cases of constant-roll inflation remaining viable with the most recent observational data are quadratic hilltop inflation (with cutoff) and natural inflation (with an additional negative cosmological constant). In these cases even-order slow-roll parameters approach non-negligible constants while the odd ones are asymptotically vanishing in the quasi-de Sitter regime.

  10. Inflation with a constant rate of roll

    NASA Astrophysics Data System (ADS)

    Motohashi, Hayato; Starobinsky, Alexei A.; Yokoyama, Jun'ichi

    2015-09-01

    We consider an inflationary scenario where the rate of inflaton roll defined by ̈phi/H dot phi remains constant. The rate of roll is small for slow-roll inflation, while a generic rate of roll leads to the interesting case of 'constant-roll' inflation. We find a general exact solution for the inflaton potential required for such inflaton behaviour. In this model, due to non-slow evolution of background, the would-be decaying mode of linear scalar (curvature) perturbations may not be neglected. It can even grow for some values of the model parameter, while the other mode always remains constant. However, this always occurs for unstable solutions which are not attractors for the given potential. The most interesting particular cases of constant-roll inflation remaining viable with the most recent observational data are quadratic hilltop inflation (with cutoff) and natural inflation (with an additional negative cosmological constant). In these cases even-order slow-roll parameters approach non-negligible constants while the odd ones are asymptotically vanishing in the quasi-de Sitter regime.

  11. Rate Constants for the Reactions of OH with CO, NO and NO2, and of HO2 with NO2 in the Presence of Water Vapour at Lower-Tropospheric Conditions

    NASA Astrophysics Data System (ADS)

    Rolletter, Michael; Fuchs, Hendrik; Novelli, Anna; Ehlers, Christian; Hofzumahaus, Andreas

    2016-04-01

    Recent studies have shown that the chemistry of gaseous nitrous acid (HONO) in the lower troposphere is not fully understood. Aside from heterogenous reactions, the daytime HONO formation in the gas-phase is not well understood (Li et al., Science, 2014). For a better understanding of HONO in the gas-phase, we have reinvestigated the reaction rate constants of important tropospheric reactions of the HOx radical family (OH and HO2) with nitrogen oxides at realistic conditions of the lower troposphere (at ambient temperature/pressure and in humid air). In this study we apply a direct pump and probe technique with high accuracy, using small radical concentrations to avoid secondary chemistry. Pulsed laser photolysis/laser-induced fluorescence (LP/LIF) was used to investigate the reaction rate constants of OH with CO, NO, NO2, and HO2 with NO2 in synthetic air at different water vapor concentrations (up to 5 x 1017 molecules cm-3). Photolysis of ozone in the presence of gaseous water was the source of OH. The reactions took place in a flow-tube at room temperature and atmospheric pressure. The chemical decay of the radicals was monitored by laser-induced fluorescence detection in a low-pressure cell, which sampled air continuously from the end of the flow-tube. Knowing the reactant concentrations subsequently allowed to calculate the bimolecular reaction rate constants at 1 atm from the pseudo-first-order decays. In order to observe HO2 reactions, OH was converted into HO2 with an excess of CO in the flow-tube. The newly measured rate constants for OH with CO, NO and NO2 agree very well with current recommendations by NASA/JPL and IUPAC and have an improved accuracy (uncertainty < 5%). These rate coefficients are independent of the presence of water vapour. The measured rate constant of HO2 with NO2 was found to depend significantly on the water-vapour concentration (probably due to formation of HO2*H2O complexes) and to exceed current recommendations by NASA/JPL and

  12. Radical scavenging ability of gallic acid toward OH and OOH radicals. Reaction mechanism and rate constants from the density functional theory.

    PubMed

    Marino, Tiziana; Galano, Annia; Russo, Nino

    2014-09-04

    Gallic acid is a ubiquitous compound, widely distributed in the vegetal kingdom and frequently found in the human diet. In the present work, its primary antioxidant activity has been investigated using the density functional theory (DFT), and the quantum mechanics-based test for overall free radical scavenging activity (QM-ORSA) protocol. It was found that gallic acid is a better antioxidant than the reference compound, Trolox, regardless of the polarity of the environment. In addition, gallic acid is predicted to be among the best peroxyl radical scavengers identified so far in nonpolar (lipid) media. This compound is capable of scavenging hydroxyl radicals at diffusion-limited rates, and hydroperoxyl radicals with rate constants in the order of 10(5) M(-1) s(-1). The deprotonation of gallic acid, in aqueous solution, is predicted to increase the protective action of this compound against oxidative stress. Gallic acid was also identified as a versatile scavenger, capable of rapidly deactivating a wide variety of reactive oxygen species (ROS) and reactive nitrogen species (RNS) via electron transfer at physiological pH.

  13. Photochemical properties of trans-1-chloro-3,3,3-trifluoropropene (trans-CHCl═CHCF3): OH reaction rate constant, UV and IR absorption spectra, global warming potential, and ozone depletion potential.

    PubMed

    Orkin, Vladimir L; Martynova, Larissa E; Kurylo, Michael J

    2014-07-17

    Measurements of the rate constant for the gas-phase reactions of OH radicals with trans-1-chloro-3,3,3-trifluoropropene (trans-CHCl═CHCF3) were performed using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The reaction rate constant exhibits a noticeable curvature of the temperature dependence in the Arrhenius plot, which can be represented by the following expression: kt-CFP (220-370 K) = 1.025 × 10(-13) × (T/298)(2.29) exp(+384/T) cm(3 )molecule(-1) s(-1). The room-temperature rate constant was determined to be kt-CFP (298 K) = (3.29 ± 0.10) × 10(-13) cm(3) molecule(-1) s(-1), where the uncertainty includes both two standard errors (statistical) and the estimated systematic error. For atmospheric modeling purposes, the rate constant below room temperature can be represented by the following expression: kt-CFP (220-298 K) = (7.20 ± 0.46) × 10(-13) exp[-(237 ± 16)/T] cm(3) molecule(-1) s(-1). There was no difference observed between the rate constants determined at 4 kPa (30 Torr) and 40 kPa (300 Torr) at both 298 and 370 K. The UV and IR absorption cross sections of this compound were measured at room temperature. The atmospheric lifetime, global warming potential, and ozone depletion potential of trans-CHCl═CHCF3 were estimated.

  14. On determining dose rate constants spectroscopically

    SciTech Connect

    Rodriguez, M.; Rogers, D. W. O.

    2013-01-15

    Purpose: To investigate several aspects of the Chen and Nath spectroscopic method of determining the dose rate constants of {sup 125}I and {sup 103}Pd seeds [Z. Chen and R. Nath, Phys. Med. Biol. 55, 6089-6104 (2010)] including the accuracy of using a line or dual-point source approximation as done in their method, and the accuracy of ignoring the effects of the scattered photons in the spectra. Additionally, the authors investigate the accuracy of the literature's many different spectra for bare, i.e., unencapsulated {sup 125}I and {sup 103}Pd sources. Methods: Spectra generated by 14 {sup 125}I and 6 {sup 103}Pd seeds were calculated in vacuo at 10 cm from the source in a 2.7 Multiplication-Sign 2.7 Multiplication-Sign 0.05 cm{sup 3} voxel using the EGSnrc BrachyDose Monte Carlo code. Calculated spectra used the initial photon spectra recommended by AAPM's TG-43U1 and NCRP (National Council of Radiation Protection and Measurements) Report 58 for the {sup 125}I seeds, or TG-43U1 and NNDC(2000) (National Nuclear Data Center, 2000) for {sup 103}Pd seeds. The emitted spectra were treated as coming from a line or dual-point source in a Monte Carlo simulation to calculate the dose rate constant. The TG-43U1 definition of the dose rate constant was used. These calculations were performed using the full spectrum including scattered photons or using only the main peaks in the spectrum as done experimentally. Statistical uncertainties on the air kerma/history and the dose rate/history were Less-Than-Or-Slanted-Equal-To 0.2%. The dose rate constants were also calculated using Monte Carlo simulations of the full seed model. Results: The ratio of the intensity of the 31 keV line relative to that of the main peak in {sup 125}I spectra is, on average, 6.8% higher when calculated with the NCRP Report 58 initial spectrum vs that calculated with TG-43U1 initial spectrum. The {sup 103}Pd spectra exhibit an average 6.2% decrease in the 22.9 keV line relative to the main peak when

  15. Combined valence bond-molecular mechanics potential-energy surface and direct dynamics study of rate constants and kinetic isotope effects for the H +C2H6 reaction

    NASA Astrophysics Data System (ADS)

    Chakraborty, Arindam; Zhao, Yan; Lin, Hai; Truhlar, Donald G.

    2006-01-01

    This article presents a multifaceted study of the reaction H +C2H6→H2+C2H5 and three of its deuterium-substituted isotopologs. First we present high-level electronic structure calculations by the W1, G3SX, MCG3-MPWB, CBS-APNO, and MC-QCISD/3 methods that lead to a best estimate of the barrier height of 11.8±0.5kcal/mol. Then we obtain a specific reaction parameter for the MPW density functional in order that it reproduces the best estimate of the barrier height; this yields the MPW54 functional. The MPW54 functional, as well as the MPW60 functional that was previously parametrized for the H +CH4 reaction, is used with canonical variational theory with small-curvature tunneling to calculate the rate constants for all four ethane reactions from 200 to 2000 K. The final MPW54 calculations are based on curvilinear-coordinate generalized-normal-mode analysis along the reaction path, and they include scaled frequencies and an anharmonic C-C bond torsion. They agree with experiment within 31% for 467-826 K except for a 38% deviation at 748 K; the results for the isotopologs are predictions since these rate constants have never been measured. The kinetic isotope effects (KIEs) are analyzed to reveal the contributions from subsets of vibrational partition functions and from tunneling, which conspire to yield a nonmonotonic temperature dependence for one of the KIEs. The stationary points and reaction-path potential of the MPW54 potential-energy surface are then used to parametrize a new kind of analytical potential-energy surface that combines a semiempirical valence bond formalism for the reactive part of the molecule with a standard molecular mechanics force field for the rest; this may be considered to be either an extension of molecular mechanics to treat a reactive potential-energy surface or a new kind of combined quantum-mechanical/molecular mechanical (QM/MM) method in which the QM part is semiempirical valence bond theory; that is, the new potential

  16. Combined valence bond-molecular mechanics potential-energy surface and direct dynamics study of rate constants and kinetic isotope effects for the H + C2H6 reaction.

    PubMed

    Chakraborty, Arindam; Zhao, Yan; Lin, Hai; Truhlar, Donald G

    2006-01-28

    This article presents a multifaceted study of the reaction H+C(2)H(6)-->H(2)+C(2)H(5) and three of its deuterium-substituted isotopologs. First we present high-level electronic structure calculations by the W1, G3SX, MCG3-MPWB, CBS-APNO, and MC-QCISD/3 methods that lead to a best estimate of the barrier height of 11.8+/-0.5 kcal/mol. Then we obtain a specific reaction parameter for the MPW density functional in order that it reproduces the best estimate of the barrier height; this yields the MPW54 functional. The MPW54 functional, as well as the MPW60 functional that was previously parametrized for the H+CH(4) reaction, is used with canonical variational theory with small-curvature tunneling to calculate the rate constants for all four ethane reactions from 200 to 2000 K. The final MPW54 calculations are based on curvilinear-coordinate generalized-normal-mode analysis along the reaction path, and they include scaled frequencies and an anharmonic C-C bond torsion. They agree with experiment within 31% for 467-826 K except for a 38% deviation at 748 K; the results for the isotopologs are predictions since these rate constants have never been measured. The kinetic isotope effects (KIEs) are analyzed to reveal the contributions from subsets of vibrational partition functions and from tunneling, which conspire to yield a nonmonotonic temperature dependence for one of the KIEs. The stationary points and reaction-path potential of the MPW54 potential-energy surface are then used to parametrize a new kind of analytical potential-energy surface that combines a semiempirical valence bond formalism for the reactive part of the molecule with a standard molecular mechanics force field for the rest; this may be considered to be either an extension of molecular mechanics to treat a reactive potential-energy surface or a new kind of combined quantum-mechanical/molecular mechanical (QM/MM) method in which the QM part is semiempirical valence bond theory; that is, the new potential

  17. Estimation of hydrolysis rate constants for carbamates ...

    EPA Pesticide Factsheets

    Cheminformatics based tools, such as the Chemical Transformation Simulator under development in EPA’s Office of Research and Development, are being increasingly used to evaluate chemicals for their potential to degrade in the environment or be transformed through metabolism. Hydrolysis represents a major environmental degradation pathway; unfortunately, only a small fraction of hydrolysis rates for about 85,000 chemicals on the Toxic Substances Control Act (TSCA) inventory are in public domain, making it critical to develop in silico approaches to estimate hydrolysis rate constants. In this presentation, we compare three complementary approaches to estimate hydrolysis rates for carbamates, an important chemical class widely used in agriculture as pesticides, herbicides and fungicides. Fragment-based Quantitative Structure Activity Relationships (QSARs) using Hammett-Taft sigma constants are widely published and implemented for relatively simple functional groups such as carboxylic acid esters, phthalate esters, and organophosphate esters, and we extend these to carbamates. We also develop a pKa based model and a quantitative structure property relationship (QSPR) model, and evaluate them against measured rate constants using R square and root mean square (RMS) error. Our work shows that for our relatively small sample size of carbamates, a Hammett-Taft based fragment model performs best, followed by a pKa and a QSPR model. This presentation compares three comp

  18. Development of a chemical kinetic measurement apparatus and the determination of the reaction rate constants for lithium-lead/water interaction

    SciTech Connect

    Biney, P.O.

    1993-04-01

    An experimental set-up for accurate measurement of hydrogen generation rate in Lithium-Lead (Li[sub 17]Pb[sub 83]) Steam or water interactions has been designed. The most important features of the design include a pneumatic actuated quick opening and closing high temperature all stainless steel valve used to control the reaction time and the placement of most measuring devices below a water line to minimize leakage of the hydrogen collected. A PC based data acquisition and control system provides remote process sequencing, acquisition and control of all major components of the set-up. Initial tests indicate that the first design objective of maintaining leakproof gas collection chamber has been achieved. Initial pressure tests indicated that the pressure drop over a time span of 30 minutes was within the tolerance of the pressure transducer used to measure the pressure (within 0.690 kPa) at a nominal system pressure of 685 kPa. The experimental system hardware, data acquisition and control programs and data analysis program have been completed, tested and are currently functional.

  19. High-level direct-dynamics variational transition state theory calculations including multidimensional tunneling of the thermal rate constants, branching ratios, and kinetic isotope effects of the hydrogen abstraction reactions from methanol by atomic hydrogen.

    PubMed

    Meana-Pañeda, Rubén; Truhlar, Donald G; Fernández-Ramos, Antonio

    2011-03-07

    We report a detailed theoretical study of the hydrogen abstraction reaction from methanol by atomic hydrogen. The study includes the analysis of thermal rate constants, branching ratios, and kinetic isotope effects. Specifically, we have performed high-level computations at the MC3BB level together with direct dynamics calculations by canonical variational transition state theory (CVT) with the microcanonically optimized multidimensional tunneling (μOMT) transmission coefficient (CVT/μOMT) to study both the CH(3)OH+H→CH(2)OH+H(2) (R1) reaction and the CH(3)OH+H→CH(3)O+H(2) (R2) reaction. The CVT/μOMT calculations show that reaction R1 dominates in the whole range 298≤T (K)≤2500 and that anharmonic effects on the torsional mode about the C-O bond are important, mainly at high temperatures. The activation energy for the total reaction sum of R1 and R2 reactions changes substantially with temperature and, therefore, the use of straight-line Arrhenius plots is not valid. We recommend the use of new expressions for the total R1 + R2 reaction and for the R1 and R2 individual reactions. © 2011 American Institute of Physics.

  20. The Kinetic Rate Law for Autocatalytic Reactions.

    ERIC Educational Resources Information Center

    Mata-Perez, Fernando; Perez-Benito, Joaquin F.

    1987-01-01

    Presented is a method of obtaining accurate rate constants for autocatalytic reactions. The autocatalytic oxidation of dimethylamine by permanganate ion in aqueous solution is used as an example. (RH)

  1. The reaction NH2 + PH3 yields NH3 + PH2 - Absolute rate constant measurement and implication for NH3 and PH3 photochemistry in the atmosphere of Jupiter

    NASA Technical Reports Server (NTRS)

    Bosco, S. R.; Brobst, W. D.; Nava, D. F.; Stief, L. J.

    1983-01-01

    The rate constant is measured over the temperature interval 218-456 K using the technique of flash photolysis-laser-induced fluorescence. NH2 radicals are produced by the flash photolysis of ammonia highly diluted in argon, and the decay of fluorescent NH2 photons is measured by multiscaling techniques. For each of the five temperatures employed in the study, the results are shown to be indepenent of variations in PH3 concentration, total pressure (argon), and flash intensity. It is found that the rate constant results are best represented for T between 218 and 456 K by the expression k = (1.52 + or - 0.16) x 10 to the -12th exp(-928 + or - 56/T) cu cm per molecule per sec; the error quoted is 1 standard deviation. This is the first determination of the rate constant for the reaction NH2 + PH3. The data are compared with an estimate made in order to explain results of the radiolysis of NH3-PH3 mixtures. The Arrhenius parameters determined here for NH2 + PH3 are then contrasted with those for the corresponding reactions of H and OH with PH3.

  2. The reaction NH2 + PH3 yields NH3 + PH2 - Absolute rate constant measurement and implication for NH3 and PH3 photochemistry in the atmosphere of Jupiter

    NASA Technical Reports Server (NTRS)

    Bosco, S. R.; Brobst, W. D.; Nava, D. F.; Stief, L. J.

    1983-01-01

    The rate constant is measured over the temperature interval 218-456 K using the technique of flash photolysis-laser-induced fluorescence. NH2 radicals are produced by the flash photolysis of ammonia highly diluted in argon, and the decay of fluorescent NH2 photons is measured by multiscaling techniques. For each of the five temperatures employed in the study, the results are shown to be indepenent of variations in PH3 concentration, total pressure (argon), and flash intensity. It is found that the rate constant results are best represented for T between 218 and 456 K by the expression k = (1.52 + or - 0.16) x 10 to the -12th exp(-928 + or - 56/T) cu cm per molecule per sec; the error quoted is 1 standard deviation. This is the first determination of the rate constant for the reaction NH2 + PH3. The data are compared with an estimate made in order to explain results of the radiolysis of NH3-PH3 mixtures. The Arrhenius parameters determined here for NH2 + PH3 are then contrasted with those for the corresponding reactions of H and OH with PH3.

  3. The reaction NH2 + PH3 yields NH3 + PH2: Absolute rate constant measurement and implication for NH3 and PH3 photochemistry in the atmosphere of Jupiter

    NASA Technical Reports Server (NTRS)

    Bosco, S. R.; Brobst, W. D.; Nava, D. F.; Stief, L. J.

    1983-01-01

    The rate constant is measured over the temperature interval 218-456 K using the technique of flash photolysis-laser-induced fluorescence. NH2 radicals are produced by the flash photolysis of ammonia highly diluted in argon, and the decay of fluorescent NH2 photons is measured by multiscaling techniques. For each of the five temperatures employed in the study, the results are shown to be independent of variations in PH3 concentration, total pressure (argon), and flash intensity. It is found that the rate constant results are best represented for T between 218 and 456 K by the expression k = (1.52 + or - 0.16) x 10 to the -12th exp(-928 + or - 56/T) cu cm per molecule per sec; the error quoted is 1 standard deviation. This is the first determination of the rate constant for the reaction NH2 + PH3. The data are compared with an estimate made in order to explain results of the radiolysis of NH3-PH3 mixtures. The Arrhenius parameters determined here for NH2 + PH3 are then constrasted with those for the corresponding reactions of H and OH with PH3.

  4. Flame Chemiluminescence Rate Constants for Quantitative Microgravity Combustion Diagnostics

    NASA Technical Reports Server (NTRS)

    Luque, Jorge; Smith, Gregory P.; Jeffries, Jay B.; Crosley, David R.; Weiland, Karen (Technical Monitor)

    2001-01-01

    Absolute excited state concentrations of OH(A), CH(A), and C2(d) were determined in three low pressure premixed methane-air flames. Two dimensional images of chemiluminescence from these states were recorded by a filtered CCD camera, processed by Abel inversion, and calibrated against Rayleigh scattering, Using a previously validated 1-D flame model with known chemistry and excited state quenching rate constants, rate constants are extracted for the reactions CH + O2 (goes to) OH(A) + CO and C2H + O (goes to) CH(A) + CO at flame temperatures. Variations of flame emission intensities with stoichiometry agree well with model predictions.

  5. Title: Elucidation of Environmental Fate of Artificial Sweeteners (Aspartame, Acesulfame K and Saccharin) by Determining Bimolecular Rate Constants with Hydroxyl Radical at Various pH and Temperature Conditions and Possible Reaction By-Products

    NASA Astrophysics Data System (ADS)

    Teraji, T.; Arakaki, T.; Suzuka, T.

    2012-12-01

    Use of artificial sweeteners in beverages and food has been rapidly increasing because of their non-calorie nature. In Japan, aspartame, acesulfame K and sucralose are among the most widely used artificial sweeteners. Because the artificial sweeteners are not metabolized in human bodies, they are directly excreted into the environment without chemical transformations. We initiated a study to better understand the fate of artificial sweeteners in the marine environment. The hydroxyl radical (OH), the most potent reactive oxygen species, reacts with various compounds and determines the environmental oxidation capacity and the life-time of many compounds. The steady-state OH concentration and the reaction rate constants between the compound and OH are used to estimate the life-time of the compound. In this study, we determine the bimolecular rate constants between aspartame, acefulfame K and saccharin and OH at various pH and temperature conditions using a competition kinetics technique. We use hydrogen peroxide as a photochemical source of OH. Bimolecular rate constant we obtained so far for aspartame was (2.6±1.2)×109 M-1 s-1 at pH = 3.0 and (4.9±2.3)×109 M-1 s-1 at pH = 5.5. Little effect was seen by changing the temperatures between 15 and 40 oC. Activation energy (Ea) was calculated to be -1.0 kJ mol-1 at pH = 3.0, +8.5 kJ mol-1 at pH = 5.5, which could be regarded as zero. We will report bimolecular rate constants at different pHs and temperatures for acesulfame K and saccharin, as well. Possible reaction by-products for aspartame will be also reported. We will further discuss the fate of aspartame in the coastal environment.

  6. Ion-Molecule Rate Constants and Branching Ratios for the Reaction of N(3)+ + O(2) From 120 to 1400 K

    DTIC Science & Technology

    2004-11-15

    Fernandez, 2303 A.A. Viggiano , Peng Zhang**, and K. Morokuma 5e. TASK NUMBER BM 5f. WORK UNIT NUMBER Al 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS...ducing the exothermicity of the reaction to 299 kJ mo1-1, or Air plasmas at elevated pressures are currently the sub- production of N+NO, which would...nonequilibrium plasmas have heavy particle temperatures been obtained by combining the average of the experimental below 1000 K and average electron

  7. Reinvestigation of the laser-initiated Cl/sub 2//HBr chain reaction: absolute rate constants and the v = 2/v = 1 ratio from Cl + HBr. -->. HCl(v) + Br

    SciTech Connect

    Dolson, D.A.; Leone, S.R.

    1987-06-18

    The Cl/sub 2//HBr chain reaction is reinvestigated by using real time state-selected observations of ..delta..v = -1 chemiluminescence from the HCl(V) products following pulsed laser photolysis of Cl/sub 2/. These state-selected observations are analyzed with a more complete kinetic treatment to obtain room temperature rate constants for the chain propagation steps and the vibrational deactivation of HCl(V = 1,2) by HBr. The chain propagation rate constants are k/sub R1/ = (1.02 +/- 0.15) x 10/sup -11/ and k/sub R2/ = (1.1 +/- 0.4) x 10/sup -15/ cm/sup 3/ molecule/sup -1/ s/sup -1/, respectively, for Cl + HBr ..-->../sup kR1/ HCl(v) + Br and Br + Cl/sub 2/ ..-->../sup kR2/ BrCl + Cl. Rate constants for vibrational deactivation of HCl(v=1) and HCl(v=2) by HBr are k/sub V1/ = (1.06 +/- 0.16) x 10/sup -12/ and k/sub V2/ = (2.09 +/- 0.50) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/. Relative intensity measurements of the HCl v = 2 ..-->.. 1 and 1 ..-->.. 0 vibrational fluorescence are used to obtain an HCl(v) product branching ratio, N/sub v=2//N/sub v=1/ = 0.40 +/- 0.06. The kinetic analysis indicates that broad-band observations of infrared chemiluminescence may lead to erroneous rate constant determinations because of vibrational cascade, whereas the detection of individual vibrational states leads to correct results.

  8. The rate constant for formation of HCl through radiative association

    NASA Astrophysics Data System (ADS)

    Kathir, R. K.; Nyman, Gunnar; Gustafsson, Magnus

    2017-09-01

    Formation of HCl in its electronic ground state through radiative association is studied. We ignore spin-orbit couplings and then the formation can happen through two dipole-allowed reactions, one involving an electronic transition and one where the H and Cl atoms approach and remain in the ground electronic molecular state. The radiative association rate constant is computed, through a combination of classical and quantum methods, for use in modelling of interstellar chemistry.

  9. Transition state theory thermal rate constants and RRKM-based branching ratios for the N((2)D) + CH(4) reaction based on multi-state and multi-reference ab initio calculations of interest for the Titan's chemistry.

    PubMed

    Ouk, Chanda-Malis; Zvereva-Loëte, Natalia; Scribano, Yohann; Bussery-Honvault, Béatrice

    2012-10-30

    Multireference single and double configuration interaction (MRCI) calculations including Davidson (+Q) or Pople (+P) corrections have been conducted in this work for the reactants, products, and extrema of the doublet ground state potential energy surface involved in the N((2)D) + CH(4) reaction. Such highly correlated ab initio calculations are then compared with previous PMP4, CCSD(T), W1, and DFT/B3LYP studies. Large relative differences are observed in particular for the transition state in the entrance channel resolving the disagreement between previous ab initio calculations. We confirm the existence of a small but positive potential barrier (3.86 ± 0.84 kJ mol(-1) (MR-AQCC) and 3.89 kJ mol(-1) (MRCI+P)) in the entrance channel of the title reaction. The correlation is seen to change significantly the energetic position of the two minima and five saddle points of this system together with the dissociation channels but not their relative order. The influence of the electronic correlation into the energetic of the system is clearly demonstrated by the thermal rate constant evaluation and it temperature dependance by means of the transition state theory. Indeed, only MRCI values are able to reproduce the experimental rate constant of the title reaction and its behavior with temperature. Similarly, product branching ratios, evaluated by means of unimolecular RRKM theory, confirm the NH production of Umemoto et al., whereas previous works based on less accurate ab initio calculations failed. We confirm the previous findings that the N((2)D) + CH(4) reaction proceeds via an insertion-dissociation mechanism and that the dominant product channels are CH(2)NH + H and CH(3) + NH. Copyright © 2012 Wiley Periodicals, Inc.

  10. Rate constant for the reaction of OH with CH3CCl2F (HCFC-141b) determined by relative rate measurements with CH4 and CH3CCl3

    NASA Technical Reports Server (NTRS)

    Huder, Karin; Demore, William B.

    1993-01-01

    Determination of accurate rate constants for OH abstraction is of great importance for the calculation of lifetimes for HCFCs and their impact on the atmosphere. For HCFC-141b there has been some disagreement in the literature for absolute measurements of this rate constant. In the present work rate constant ratios for HCFC-141b were measured at atmospheric pressure in the temperature range of 298-358 K, with CH4 and CH3CCl3 as reference gases. Ozone was photolyzed at 254 nm in the presence of water vapor to produce OH radicals. Relative depletions of 141b and the reference gases were measured by FTIR. Arrhenius expressions for 141b were derived from each reference gas and found to be in good agreement with each other. The combined expression for HCFC-141b which we recommend is 1.4 x 10 exp -12 exp(-1630/T) with k at 298 K being 5.9 x 10 exp -15 cu cm/molec-s. This value is in excellent agreement with the JPL 92-20 recommendation.

  11. Path integral calculation of thermal rate constants within the quantum instanton approximation: application to the H + CH4 --> H2 + CH3 hydrogen abstraction reaction in full Cartesian space.

    PubMed

    Zhao, Yi; Yamamoto, Takeshi; Miller, William H

    2004-02-15

    The quantum instanton approximation for thermal rate constants of chemical reactions [Miller, Zhao, Ceotto, and Yang, J. Chem. Phys. 119, 1329 (2003)], which is modeled after the earlier semiclassical instanton approach, is applied to the hydrogen abstraction reaction from methane by a hydrogen atom, H + CH4 --> H2 + CH3, using a modified and recalibrated version of the Jordan-Gilbert potential surface. The quantum instanton rate is evaluated using path integral Monte Carlo approaches based on the recently proposed implementation schemes [Yamamoto and Miller, J. Chem. Phys. 120, 3086 (2004)]. The calculations were carried out using the Cartesian coordinates of all the atoms (thus involving 18 degrees of freedom), thereby taking explicit account of rotational effects of the whole system and also allowing the equivalent treatment of the four methane hydrogens. To achieve such a treatment, we present extended forms of the path integral estimators for relevant quantities that may be used for general N-atom systems with any generalized reaction coordinates. The quantum instanton rates thus obtained for the temperature range T = 200-2000 K show good agreement with available experimental data, which gives support to the accuracy of the underlying potential surface used.

  12. Oligomer formation of the bacterial second messenger c-di-GMP: reaction rates and equilibrium constants indicate a monomeric state at physiological concentrations.

    PubMed

    Gentner, Martin; Allan, Martin G; Zaehringer, Franziska; Schirmer, Tilman; Grzesiek, Stephan

    2012-01-18

    Cyclic diguanosine-monophosphate (c-di-GMP) is a bacterial signaling molecule that triggers a switch from motile to sessile bacterial lifestyles. This mechanism is of considerable pharmaceutical interest, since it is related to bacterial virulence, biofilm formation, and persistence of infection. Previously, c-di-GMP has been reported to display a rich polymorphism of various oligomeric forms at millimolar concentrations, which differ in base stacking and G-quartet interactions. Here, we have analyzed the equilibrium and exchange kinetics between these various forms by NMR spectroscopy. We find that the association of the monomer into a dimeric form is in fast exchange (constant of about 1 mM. At concentrations above 100 μM, higher oligomers are formed in the presence of cations. These are presumably tetramers and octamers, with octamers dominating above about 0.5 mM. Thus, at the low micromolar concentrations of the cellular environment and in the absence of additional compounds that stabilize oligomers, c-di-GMP should be predominantly monomeric. This finding has important implications for the understanding of c-di-GMP recognition by protein receptors. In contrast to the monomer/dimer exchange, formation and dissociation of higher oligomers occurs on a time scale of several hours to days. The time course can be described quantitatively by a simple kinetic model where tetramers are intermediates of octamer formation. The extremely slow oligomer dissociation may generate severe artifacts in biological experiments when c-di-GMP is diluted from concentrated stock solution. We present a simple method to quantify c-di-GMP monomers and oligomers from UV spectra and a procedure to dissolve the unwanted oligomers by an annealing step.

  13. Oxidation kinetics of zinc sulfide: determination of intrinsic rate constant

    SciTech Connect

    Prabhu, G.M.

    1983-06-01

    An initial reaction rate study was done with the help of a thermogravimetric technique. Energy dispersive x-ray analyses on partially oxidized zinc sulfide pellets with a sintered porosity of 72.4% indicated flat sulfur intensity profiles within pellets reacted below 560/sup 0/C, which suggested a homogeneous reaction mechanism. Therefore, reaction temperatures below 600/sup 0/C were chosen for the reaction rate studies. Initial reaction rate studies on 72.4, 58.2, and 34.0% porous, cylindrical zinc sulfide pellets and the corresponding Arrhenius plot suggested chemical control in the temperature range from 480 to 565/sup 0/C. The corresponding intrinsic rate constant is correlated as k = 3.45 x 10/sup 17/ exp (- 86051/RT) cm/s. The variation in sulfur intensity within sintered pellets having a porosity of 72.4% that were reacted at temperatures above 560/sup 0/C indicated that the pore diffusion resistance gradually became comparable to the chemical reaction resistance leading to a mixed control mechanism above 570/sup 0/C. The critical temperature at which this shift occurred, increased with pellet porosity.

  14. The lowest-lying electronic singlet and triplet potential energy surfaces for the HNO-NOH system: energetics, unimolecular rate constants, tunneling and kinetic isotope effects for the isomerization and dissociation reactions.

    PubMed

    Bozkaya, Uğur; Turney, Justin M; Yamaguchi, Yukio; Schaefer, Henry F

    2012-04-28

    The lowest-lying electronic singlet and triplet potential energy surfaces (PES) for the HNO-NOH system have been investigated employing high level ab initio quantum chemical methods. The reaction energies and barriers have been predicted for two isomerization and four dissociation reactions. Total energies are extrapolated to the complete basis set limit applying focal point analyses. Anharmonic zero-point vibrational energies, diagonal Born-Oppenheimer corrections, relativistic effects, and core correlation corrections are also taken into account. On the singlet PES, the (1)HNO → (1)NOH endothermicity including all corrections is predicted to be 42.23 ± 0.2 kcal mol(-1). For the barrierless decomposition of (1)HNO to H + NO, the dissociation energy is estimated to be 47.48 ± 0.2 kcal mol(-1). For (1)NOH → H + NO, the reaction endothermicity and barrier are 5.25 ± 0.2 and 7.88 ± 0.2 kcal mol(-1). On the triplet PES the reaction energy and barrier including all corrections are predicted to be 7.73 ± 0.2 and 39.31 ± 0.2 kcal mol(-1) for the isomerization reaction (3)HNO → (3)NOH. For the triplet dissociation reaction (to H + NO) the corresponding results are 29.03 ± 0.2 and 32.41 ± 0.2 kcal mol(-1). Analogous results are 21.30 ± 0.2 and 33.67 ± 0.2 kcal mol(-1) for the dissociation reaction of (3)NOH (to H + NO). Unimolecular rate constants for the isomerization and dissociation reactions were obtained utilizing kinetic modeling methods. The tunneling and kinetic isotope effects are also investigated for these reactions. The adiabatic singlet-triplet energy splittings are predicted to be 18.45 ± 0.2 and 16.05 ± 0.2 kcal mol(-1) for HNO and NOH, respectively. Kinetic analyses based on solution of simultaneous first-order ordinary-differential rate equations demonstrate that the singlet NOH molecule will be difficult to prepare at room temperature, while the triplet NOH molecule is viable with respect to isomerization and dissociation reactions up to

  15. What Is a Reaction Rate?

    ERIC Educational Resources Information Center

    Schmitz, Guy

    2005-01-01

    The definition of reaction rate is derived and demonstrations are made for the care to be taken while using the term. Reaction rate can be in terms of a reaction property, the extent of reaction and thus it is possible to give a definition applicable in open and closed systems.

  16. THE C({sup 3}P) + NH{sub 3} REACTION IN INTERSTELLAR CHEMISTRY. II. LOW TEMPERATURE RATE CONSTANTS AND MODELING OF NH, NH{sub 2}, AND NH{sub 3} ABUNDANCES IN DENSE INTERSTELLAR CLOUDS

    SciTech Connect

    Hickson, Kevin M.; Loison, Jean-Christophe; Bourgalais, Jérémy; Capron, Michael; Picard, Sébastien D. Le; Goulay, Fabien; Wakelam, Valentine

    2015-10-20

    A continuous supersonic flow reactor has been used to measure rate constants for the C({sup 3}P) + NH{sub 3} reaction over the temperature range 50–296 K. C({sup 3}P) atoms were created by the pulsed laser photolysis of CBr{sub 4}. The kinetics of the title reaction were followed directly by vacuum ultra-violet laser induced fluorescence of C({sup 3}P) loss and through H({sup 2}S) formation. The experiments show unambiguously that the reaction is rapid at 296 K, becoming faster at lower temperatures, reaching a value of (1.8 ± 0.2) × 10{sup −10} cm{sup 3} molecule{sup −1} s{sup −1} at 50 K. As this reaction is not currently included in astrochemical networks, its influence on interstellar nitrogen hydride abundances is tested through a dense cloud model including gas–grain interactions. In particular, the effect of the ortho-to-para ratio of H{sub 2}, which plays a crucial role in interstellar NH{sub 3} synthesis, is examined.

  17. Quantum instanton evaluation of the thermal rate constants and kinetic isotope effects for SiH4+H →SiH3+H2 reaction in full Cartesian space

    NASA Astrophysics Data System (ADS)

    Wang, Wenji; Feng, Shulu; Zhao, Yi

    2007-03-01

    The quantum instanton calculations of thermal rate constants for the gas-phase reaction SiH4+H →SiH3+H2 and its deuterated analogs are presented, using an analytical potential energy surface. The quantum instanton approximation is manipulated by full dimensionality in Cartesian coordinate path integral Monte Carlo approach, thereby taking explicitly into account the effects of the whole rotation, the vibrotational coupling, and anharmonicity of the reaction system. The rates and kinetic isotope effects obtained for the temperature range of 200-1000K show good agreements with available experimental data, which give support to the accuracy of the underlying potential surface used. In order to investigate the sole quantum effect to the rates, the authors also derive the classical limit of the quantum instanton and find that it can be exactly expressed as the classical variation transition state theory. Comparing the quantum quantities with their classical analogs in the quantum instanton formula, the authors demonstrate that the quantum correction of the prefactor is more important than that of the activation energy at the transition state.

  18. Rate constant calculations of the GeH4 + OH/OD → GeH3 + H2O/HOD reactions using an ab initio based full-dimensional potential energy surface.

    PubMed

    Espinosa-Garcia, J; Rangel, C; Corchado, J C

    2016-06-22

    We report an analytical full-dimensional potential energy surface for the GeH4 + OH → GeH3 + H2O reaction based on ab initio calculations. It is a practically barrierless reaction with very high exothermicity and the presence of intermediate complexes in the entrance and exit channels, reproducing the experimental evidence. Using this surface, thermal rate constants for the GeH4 + OH/OD isotopic reactions were calculated using two approaches: variational transition state theory (VTST) and quasi-classical trajectory (QCT) calculations, in the temperature range 200-1000 K, and results were compared with the only experimental data at 298 K. Both methods showed similar values over the whole temperature range, with differences less than 30%; and the experimental data was reproduced at 298 K, with negative temperature dependence below 300 K, which is associated with the presence of an intermediate complex in the entrance channel. However, while the QCT approach reproduced the experimental kinetic isotope effect, the VTST approach underestimated it. We suggest that this difference is associated with the harmonic approximation used in the treatment of vibrational frequencies.

  19. The H + HeH(+) → He + H2(+) reaction from the ultra-cold regime to the three-body breakup: exact quantum mechanical integral cross sections and rate constants.

    PubMed

    De Fazio, Dario

    2014-06-21

    In this work, we present a quantum mechanical scattering study of the title reaction from 1 mK to 2000 K. Total integral cross sections and thermal rate constants are compared with previous theoretical and experimental data and with simpler theoretical models to understand the range of validity of the approximations used in the previous studies. The obtained quantum reactive observables have been found to be nearly insensitive to the roto-vibrational energy of the reactants at high temperatures. More sensitive to the reactant's roto-vibrational energy are the data in the cold and ultra-cold regimes. The implications of the new data presented here in the early universe scenario are also discussed and analyzed.

  20. Calculation of individual isotope equilibrium constants for geochemical reactions

    USGS Publications Warehouse

    Thorstenson, D.C.; Parkhurst, D.L.

    2004-01-01

    Theory is derived from the work of Urey (Urey H. C. [1947] The thermodynamic properties of isotopic substances. J. Chem. Soc. 562-581) to calculate equilibrium constants commonly used in geochemical equilibrium and reaction-transport models for reactions of individual isotopic species. Urey showed that equilibrium constants of isotope exchange reactions for molecules that contain two or more atoms of the same element in equivalent positions are related to isotope fractionation factors by ?? = (Kex)1/n, where n is the number of atoms exchanged. This relation is extended to include species containing multiple isotopes, for example 13C16O18O and 1H2H18O. The equilibrium constants of the isotope exchange reactions can be expressed as ratios of individual isotope equilibrium constants for geochemical reactions. Knowledge of the equilibrium constant for the dominant isotopic species can then be used to calculate the individual isotope equilibrium constants. Individual isotope equilibrium constants are calculated for the reaction CO2g = CO2aq for all species that can be formed from 12C, 13C, 16O, and 18O; for the reaction between 12C18 O2aq and 1H218Ol; and among the various 1H, 2H, 16O, and 18O species of H2O. This is a subset of a larger number of equilibrium constants calculated elsewhere (Thorstenson D. C. and Parkhurst D. L. [2002] Calculation of individual isotope equilibrium constants for implementation in geochemical models. Water-Resources Investigation Report 02-4172. U.S. Geological Survey). Activity coefficients, activity-concentration conventions for the isotopic variants of H2O in the solvent 1H216Ol, and salt effects on isotope fractionation have been included in the derivations. The effects of nonideality are small because of the chemical similarity of different isotopic species of the same molecule or ion. The temperature dependence of the individual isotope equilibrium constants can be calculated from the temperature dependence of the fractionation

  1. Effective reaction rates for diffusion-limited reaction cycles

    NASA Astrophysics Data System (ADS)

    Nałecz-Jawecki, Paweł; Szymańska, Paulina; Kochańczyk, Marek; Miekisz, Jacek; Lipniacki, Tomasz

    2015-12-01

    Biological signals in cells are transmitted with the use of reaction cycles, such as the phosphorylation-dephosphorylation cycle, in which substrate is modified by antagonistic enzymes. An appreciable share of such reactions takes place in crowded environments of two-dimensional structures, such as plasma membrane or intracellular membranes, and is expected to be diffusion-controlled. In this work, starting from the microscopic bimolecular reaction rate constants and using estimates of the mean first-passage time for an enzyme-substrate encounter, we derive diffusion-dependent effective macroscopic reaction rate coefficients (EMRRC) for a generic reaction cycle. Each EMRRC was found to be half of the harmonic average of the microscopic rate constant (phosphorylation c or dephosphorylation d), and the effective (crowding-dependent) motility divided by a slowly decreasing logarithmic function of the sum of the enzyme concentrations. This implies that when c and d differ, the two EMRRCs scale differently with the motility, rendering the steady-state fraction of phosphorylated substrate molecules diffusion-dependent. Analytical predictions are verified using kinetic Monte Carlo simulations on the two-dimensional triangular lattice at the single-molecule resolution. It is demonstrated that the proposed formulas estimate the steady-state concentrations and effective reaction rates for different sets of microscopic reaction rates and concentrations of reactants, including a non-trivial example where with increasing diffusivity the fraction of phosphorylated substrate molecules changes from 10% to 90%.

  2. Effective reaction rates for diffusion-limited reaction cycles.

    PubMed

    Nałęcz-Jawecki, Paweł; Szymańska, Paulina; Kochańczyk, Marek; Miękisz, Jacek; Lipniacki, Tomasz

    2015-12-07

    Biological signals in cells are transmitted with the use of reaction cycles, such as the phosphorylation-dephosphorylation cycle, in which substrate is modified by antagonistic enzymes. An appreciable share of such reactions takes place in crowded environments of two-dimensional structures, such as plasma membrane or intracellular membranes, and is expected to be diffusion-controlled. In this work, starting from the microscopic bimolecular reaction rate constants and using estimates of the mean first-passage time for an enzyme-substrate encounter, we derive diffusion-dependent effective macroscopic reaction rate coefficients (EMRRC) for a generic reaction cycle. Each EMRRC was found to be half of the harmonic average of the microscopic rate constant (phosphorylation c or dephosphorylation d), and the effective (crowding-dependent) motility divided by a slowly decreasing logarithmic function of the sum of the enzyme concentrations. This implies that when c and d differ, the two EMRRCs scale differently with the motility, rendering the steady-state fraction of phosphorylated substrate molecules diffusion-dependent. Analytical predictions are verified using kinetic Monte Carlo simulations on the two-dimensional triangular lattice at the single-molecule resolution. It is demonstrated that the proposed formulas estimate the steady-state concentrations and effective reaction rates for different sets of microscopic reaction rates and concentrations of reactants, including a non-trivial example where with increasing diffusivity the fraction of phosphorylated substrate molecules changes from 10% to 90%.

  3. Robust Biased Brownian Dynamics for Rate Constant Calculation

    PubMed Central

    Zou, Gang; Skeel, Robert D.

    2003-01-01

    A reaction probability is required to calculate the rate constant of a diffusion-dominated reaction. Due to the complicated geometry and potentially high dimension of the reaction probability problem, it is usually solved by a Brownian dynamics simulation, also known as a random walk or path integral method, instead of solving the equivalent partial differential equation by a discretization method. Building on earlier work, this article completes the development of a robust importance sampling algorithm for Brownian dynamics—i.e., biased Brownian dynamics with weight control—to overcome the high energy and entropy barriers in biomolecular association reactions. The biased Brownian dynamics steers sampling by a bias force, and the weight control algorithm controls sampling by a target weight. This algorithm is optimal if the bias force and the target weight are constructed from the solution of the reaction probability problem. In reality, an approximate reaction probability has to be used to construct the bias force and the target weight. Thus, the performance of the algorithm depends on the quality of the approximation. Given here is a method to calculate a good approximation, which is based on the selection of a reaction coordinate and the variational formulation of the reaction probability problem. The numerically approximated reaction probability is shown by computer experiments to give a factor-of-two speedup over the use of a purely heuristic approximation. Also, the fully developed method is compared to unbiased Brownian dynamics. The tests for human superoxide dismutase, Escherichia coli superoxide dismutase, and antisweetener antibody NC6.8, show speedups of 17, 35, and 39, respectively. The test for reactions between two model proteins with orientations shows speedups of 2578 for one set of configurations and 3341 for another set of configurations. PMID:14507681

  4. Interpretation of the temperature dependence of equilibrium and rate constants.

    PubMed

    Winzor, Donald J; Jackson, Craig M

    2006-01-01

    The objective of this review is to draw attention to potential pitfalls in attempts to glean mechanistic information from the magnitudes of standard enthalpies and entropies derived from the temperature dependence of equilibrium and rate constants for protein interactions. Problems arise because the minimalist model that suffices to describe the energy differences between initial and final states usually comprises a set of linked equilibria, each of which is characterized by its own energetics. For example, because the overall standard enthalpy is a composite of those individual values, a positive magnitude for DeltaH(o) can still arise despite all reactions within the subset being characterized by negative enthalpy changes: designation of the reaction as being entropy driven is thus equivocal. An experimenter must always bear in mind the fact that any mechanistic interpretation of the magnitudes of thermodynamic parameters refers to the reaction model rather than the experimental system. For the same reason there is little point in subjecting the temperature dependence of rate constants for protein interactions to transition-state analysis. If comparisons with reported values of standard enthalpy and entropy of activation are needed, they are readily calculated from the empirical Arrhenius parameters.

  5. Microfabricated microengine with constant rotation rate

    DOEpatents

    Romero, Louis A.; Dickey, Fred M.

    1999-01-01

    A microengine uses two synchronized linear actuators as a power source and converts oscillatory motion from the actuators into constant rotational motion via direct linkage connection to an output gear or wheel. The microengine provides output in the form of a continuously rotating output gear that is capable of delivering drive torque at a constant rotation to a micromechanism. The output gear can have gear teeth on its outer perimeter for directly contacting a micromechanism requiring mechanical power. The gear is retained by a retaining means which allows said gear to rotate freely. The microengine is microfabricated of polysilicon on one wafer using surface micromachining batch fabrication.

  6. Microfabricated microengine with constant rotation rate

    SciTech Connect

    Romero, L.A.; Dickey, F.M.

    1999-09-21

    A microengine uses two synchronized linear actuators as a power source and converts oscillatory motion from the actuators into constant rotational motion via direct linkage connection to an output gear or wheel. The microengine provides output in the form of a continuously rotating output gear that is capable of delivering drive torque at a constant rotation to a micromechanism. The output gear can have gear teeth on its outer perimeter for directly contacting a micromechanism requiring mechanical power. The gear is retained by a retaining means which allows said gear to rotate freely. The microengine is microfabricated of polysilicon on one wafer using surface micromachining batch fabrication.

  7. Rate constants for diffusive processes by partial path sampling

    NASA Astrophysics Data System (ADS)

    Moroni, Daniele; Bolhuis, Peter G.; van Erp, Titus S.

    2004-03-01

    We introduce a path sampling method for the computation of rate constants for complex systems with a highly diffusive character. Based on the recently developed transition interface sampling (TIS) algorithm this procedure increases the efficiency by sampling only parts of complete transition trajectories. The algorithm assumes the loss of memory for diffusive progression along the reaction coordinate. We compare the new partial path technique to the TIS method for a simple diatomic system and show that the computational effort of the new method scales linearly, instead of quadratically, with the width of the diffusive barrier. The validity of the memory loss assumption is also discussed.

  8. 18 CFR 806.12 - Constant-rate aquifer testing.

    Code of Federal Regulations, 2011 CFR

    2011-04-01

    ... 18 Conservation of Power and Water Resources 2 2011-04-01 2011-04-01 false Constant-rate aquifer... COMMISSION REVIEW AND APPROVAL OF PROJECTS Application Procedure § 806.12 Constant-rate aquifer testing. (a... withdraw or increase a withdrawal of groundwater shall perform a constant-rate aquifer test in...

  9. Effects of single and double mutations in plastocyanin on the rate constant and activation parameters for the rearrangement gating the electron-transfer reaction between the triplet state of zinc cytochrome c and cupriplastocyanin.

    PubMed

    Ivković-Jensen, M M; Ullmann, G M; Young, S; Hansson, O; Crnogorac, M M; Ejdebäck, M; Kostić, N M

    1998-06-30

    The unimolecular rate constant for the photoinduced electron-transfer reaction 3Zncyt/pc(II) --> Zncyt+/pc(I) within the electrostatic complex of zinc cytochrome c and spinach cupriplastocyanin is kF. We report the effects on kF of the following factors, all at pH 7.0: 12 single mutations on the plastocyanin surface (Leu12Asn, Leu12Glu, Leu12Lys, Asp42Asn, Asp42Lys, Glu43Asn, Glu59Gln, Glu59Lys, Glu60Gln, Glu60Lys, Gln88Glu, and Gln88Lys), the double mutation Glu59Lys/Glu60Gln, temperature (in the range 273.3-302.9 K), and solution viscosity (in the range 1. 00-116.0 cP) at 283.2 and 293.2 K. We also report the effects of the plastocyanin mutations on the association constant (Ka) and the corresponding free energy of association (DeltaGa) with zinc cytochrome c at 298.2 K. Dependence of kF on temperature yielded the activation parameters DeltaH, DeltaS, and DeltaG. Dependence of kF on solution viscosity yielded the protein friction and confirmed the DeltaG values determined from the temperature dependence. The aforementioned intracomplex reaction is not a simple electron-transfer reaction because donor-acceptor electronic coupling (HAB) and reorganizational energy (lambda), obtained by fitting of the temperature dependence of kF to the Marcus equation, deviate from the expectations based on precedents and because kF greatly depends on viscosity. This last dependence and the fact that certain mutations affect Ka but not kF are two lines of evidence against the mechanism in which the electron-transfer step is coupled with the faster, but thermodynamically unfavorable, rearrangement step. The electron-transfer reaction is gated by the slower, and thus rate determining, structural rearrangement of the diprotein complex; the rate constant kF corresponds to this rearrangement. Isokinetic correlation of DeltaH and DeltaS parameters and Coulombic energies of the various configurations of the Zncyt/pc(II) complex consistently show that the rearrangement is a facile

  10. Measurement of both the equilibrium constant and rate constant for electronic energy transfer by control of the limiting kinetic regimes.

    PubMed

    Vagnini, Michael T; Rutledge, W Caleb; Wagenknecht, Paul S

    2010-02-01

    Electronic energy transfer can fall into two limiting cases. When the rate of the energy transfer back reaction is much faster than relaxation of the acceptor excited state, equilibrium between the donor and acceptor excited states is achieved and only the equilibrium constant for the energy transfer can be measured. When the rate of the back reaction is much slower than relaxation of the acceptor, the energy transfer is irreversible and only the forward rate constant can be measured. Herein, we demonstrate that with trans-[Cr(d(4)-cyclam)(CN)(2)](+) as the donor and either trans-[Cr([15]ane-ane-N(4))(CN)(2)](+) or trans-[Cr(cyclam)(CN)(2)](+) as the acceptor, both limits can be obtained by control of the donor concentration. The equilibrium constant and rate constant for the case in which trans-[Cr([15]ane-ane-N(4))(CN)(2)](+) is the acceptor are 0.66 and 1.7 x 10(7) M(-1) s(-1), respectively. The equilibrium constant is in good agreement with the value of 0.60 determined using the excited state energy gap between the donor and acceptor species. For the thermoneutral case in which trans-[Cr(cyclam)(CN)(2)](+) is the acceptor, an experimental equilibrium constant of 0.99 was reported previously, and the rate constant has now been measured as 4.0 x 10(7) M(-1) s(-1).

  11. Divided Saddle Theory: A New Idea for Rate Constant Calculation.

    PubMed

    Daru, János; Stirling, András

    2014-03-11

    We present a theory of rare events and derive an algorithm to obtain rates from postprocessing the numerical data of a free energy calculation and the corresponding committor analysis. The formalism is based on the division of the saddle region of the free energy profile of the rare event into two adjacent segments called saddle domains. The method is built on sampling the dynamics within these regions: auxiliary rate constants are defined for the saddle domains and the absolute forward and backward rates are obtained by proper reweighting. We call our approach divided saddle theory (DST). An important advantage of our approach is that it requires only standard computational techniques which are available in most molecular dynamics codes. We demonstrate the potential of DST numerically on two examples: rearrangement of alanine-dipeptide (CH3CO-Ala-NHCH3) conformers and the intramolecular Cope reaction of the fluxional barbaralane molecule.

  12. Rate Constant for the Reaction CH3 + CH3 Yields C2H6 at T = 155 K and Model Calculation of the CH3 Abundance in the Atmospheres of Saturn and Neptune

    NASA Technical Reports Server (NTRS)

    Cody, Regina J.; Romani, Paul N.; Nesbitt, Fred L.; Iannone, Mark A.; Tardy, Dwight C.; Stief, Louis J.

    2003-01-01

    The column abundances of CH3 observed by the Infrared Space Observatory (ISO) satellite on Saturn and Neptune were lower than predicted by atmospheric photochemical models, especially for Saturn. It has been suggested that the models underestimated the loss of CH3 due to poor knowledge of the rate constant k of the CH3 + CH3 self-reaction at the low temperatures and pressures of these atmospheres. Motivated by this suggestion, we undertook a combined experimental and photochemical modeling study of the CH3 + CH3 reaction and its role in determining planetary CH3 abundances. In a discharge flow-mass spectrometer system, k was measured at T = 155 K and three pressures of He. The results in units of cu cm/molecule/s are k(0.6 Torr) = 6.82 x 10(exp -11), k(1.0 Torr) = 6.98 x 10(exp -11), and k(1.5 Torr) = 6.91 x 10(exp -11). Analytical expressions for k were derived that (1) are consistent with the present laboratory data at T = 155 K, our previous data at T = 202 K and 298 K, and those of other studies in He at T = 296-298 K and (2) have some theoretical basis to provide justification for extrapolation. The derived analytical expressions were then used in atmospheric photochemical models for both Saturn and Neptune. These model results reduced the disparity with observations of Saturn, but not with observations of Neptune. However, the disparity for Neptune is much smaller. The solution to the remaining excess CH3 prediction in the models relative to the ISO observations lies, to a large extent, elsewhere in the CH3 photochemistry or transport, not in the CH3 + CH3 rate.

  13. The behavior of constant rate aerosol reactors

    SciTech Connect

    Friedlander, S.K.

    1982-01-01

    An aerosol reactor is a gaseous system in which fine particles are formed by chemical reaction in either a batch or flow process. The particle sizes of interest range from less than 10 angstrom (molecular clusters) to 10 ..mu..m. Such reactors may be operated to study the aerosol formation process, as in a smog reactor, or to generate a product such as a pigment or a catalytic aerosol. Aerosol reactors can be characterized by three temporal or spatial zones or regions of operation for batch and flow reactors, respectively. In zone I, chemical reaction results in the formation of condensable molecular products which nucleate and form very high concentrations of small particles. The number density depends on the concentration of preexisting aerosol. Zone II is a transition region in which the aerosol number concentration levels off as a result of hetergeneous condensation by the stable aerosol. In zone III coagulation becomes sufficiently rapid to reduce the particle number concentration. There may be a zone IV in which agglomerates form.

  14. Representing Rate Equations for Enzyme-Catalyzed Reactions

    ERIC Educational Resources Information Center

    Ault, Addison

    2011-01-01

    Rate equations for enzyme-catalyzed reactions are derived and presented in a way that makes it easier for the nonspecialist to see how the rate of an enzyme-catalyzed reaction depends upon kinetic constants and concentrations. This is done with distribution equations that show how the rate of the reaction depends upon the relative quantities of…

  15. Representing Rate Equations for Enzyme-Catalyzed Reactions

    ERIC Educational Resources Information Center

    Ault, Addison

    2011-01-01

    Rate equations for enzyme-catalyzed reactions are derived and presented in a way that makes it easier for the nonspecialist to see how the rate of an enzyme-catalyzed reaction depends upon kinetic constants and concentrations. This is done with distribution equations that show how the rate of the reaction depends upon the relative quantities of…

  16. Empirical rate formula for ion-dipolar molecule reactions

    NASA Astrophysics Data System (ADS)

    Sato, Shin

    2017-04-01

    New empirical rate formula for the low temperature reactions is proposed. The formula proposed previously has been simplified by using incomplete gamma function. A few examples of temperature dependence of rate constants for the reactions of ions with dipolar molecules and that for the reaction F +H2 →HF +H are demonstrated by using new rate formula.

  17. Rate constants from instanton theory via a microcanonical approach

    NASA Astrophysics Data System (ADS)

    McConnell, Sean R.; Löhle, Andreas; Kästner, Johannes

    2017-02-01

    Microcanonical instanton theory offers the promise of providing rate constants for chemical reactions including quantum tunneling of atoms over the whole temperature range. We discuss different rate expressions, which require the calculation of stability parameters of the instantons. The traditional way of obtaining these stability parameters is shown to be numerically unstable in practical applications. We provide three alternative algorithms to obtain such stability parameters for non-separable systems, i.e., systems in which the vibrational modes perpendicular to the instanton path couple to movement along the path. We show the applicability of our algorithms on two molecular systems: H2 + OH → H2O + H using a fitted potential energy surface and HNCO + H → NH2CO using a potential obtained on-the-fly from density functional calculations.

  18. Atom tunnelling in the reaction NH3 + + H2 → NH4 + + H and its astrochemical relevance† †Electronic supplementary information (ESI) available: Rate constants, energies of the benchmark, coordinates of stationary points. See DOI: 10.1039/c6fd00096g Click here for additional data file.

    PubMed Central

    Álvarez-Barcia, Sonia; Russ, Marie-Sophie; Meisner, Jan

    2016-01-01

    The title reaction is involved in the formation of ammonia in the interstellar medium. We have calculated thermal rates including atom tunnelling using different rate theories. Canonical variational theory with microcanonically optimised multidimensional tunnelling was used for bimolecular rates, modelling the gas-phase reaction and also a surface-catalysed reaction of the Eley–Rideal type. Instanton theory provided unimolecular rates, which model the Langmuir–Hinshelwood type surface reaction. The potential energy was calculated on the CCSD(T)-F12 level of theory on the fly. We report thermal rates and H/D kinetic isotope effects. The latter have implications for observed H/D fractionation in molecular clouds. Tunnelling causes rate constants to be sufficient for the reaction to play a role in interstellar chemistry even at cryogenic temperature. We also discuss intricacies and limitations of the different tunnelling approximations to treat this reaction, including its pre-reactive minimum. PMID:27711847

  19. 18 CFR 806.12 - Constant-rate aquifer testing.

    Code of Federal Regulations, 2014 CFR

    2014-04-01

    ... 18 Conservation of Power and Water Resources 2 2014-04-01 2014-04-01 false Constant-rate aquifer testing. 806.12 Section 806.12 Conservation of Power and Water Resources SUSQUEHANNA RIVER BASIN COMMISSION REVIEW AND APPROVAL OF PROJECTS Application Procedure § 806.12 Constant-rate aquifer testing. (a...

  20. 18 CFR 806.12 - Constant-rate aquifer testing.

    Code of Federal Regulations, 2012 CFR

    2012-04-01

    ... 18 Conservation of Power and Water Resources 2 2012-04-01 2012-04-01 false Constant-rate aquifer testing. 806.12 Section 806.12 Conservation of Power and Water Resources SUSQUEHANNA RIVER BASIN COMMISSION REVIEW AND APPROVAL OF PROJECTS Application Procedure § 806.12 Constant-rate aquifer testing. (a...

  1. 18 CFR 806.12 - Constant-rate aquifer testing.

    Code of Federal Regulations, 2013 CFR

    2013-04-01

    ... 18 Conservation of Power and Water Resources 2 2013-04-01 2012-04-01 true Constant-rate aquifer testing. 806.12 Section 806.12 Conservation of Power and Water Resources SUSQUEHANNA RIVER BASIN COMMISSION REVIEW AND APPROVAL OF PROJECTS Application Procedure § 806.12 Constant-rate aquifer testing. (a...

  2. ESTIMATION OF PHOSPHATE ESTER HYDROLYSIS RATE CONSTANTS - ALKALINE HYDROLYSIS

    EPA Science Inventory

    SPARC (SPARC Performs Automated Reasoning in Chemistry) chemical reactivity models were extended to allow the calculation of alkaline hydrolysis rate constants of phosphate esters in water. The rate is calculated from the energy difference between the initial and transition state...

  3. ESTIMATION OF PHOSPHATE ESTER HYDROLYSIS RATE CONSTANTS - ALKALINE HYDROLYSIS

    EPA Science Inventory

    SPARC (SPARC Performs Automated Reasoning in Chemistry) chemical reactivity models were extended to allow the calculation of alkaline hydrolysis rate constants of phosphate esters in water. The rate is calculated from the energy difference between the initial and transition state...

  4. ESTIMATION OF PHOSPHATE ESTER HYDROLYSIS RATE CONSTANTS. I. ALKALINE HYDROLYSIS

    EPA Science Inventory

    SPARC (SPARC Performs Automated Reasoning in Chemistry) chemical reactivity models were extended to allow the calculation of alkaline hydrolysis rate constants of phosphate esters in water. The rate is calculated from the energy difference between the initial and transition state...

  5. ESTIMATION OF PHOSPHATE ESTER HYDROLYSIS RATE CONSTANTS. I. ALKALINE HYDROLYSIS

    EPA Science Inventory

    SPARC (SPARC Performs Automated Reasoning in Chemistry) chemical reactivity models were extended to allow the calculation of alkaline hydrolysis rate constants of phosphate esters in water. The rate is calculated from the energy difference between the initial and transition state...

  6. Quantum theory of chemical reaction rates

    SciTech Connect

    Miller, W.H. |

    1994-10-01

    If one wishes to describe a chemical reaction at the most detailed level possible, i.e., its state-to-state differential scattering cross section, then it is necessary to solve the Schroedinger equation to obtain the S-matrix as a function of total energy E and total angular momentum J, in terms of which the cross sections can be calculated as given by equation (1) in the paper. All other physically observable attributes of the reaction can be derived from the cross sections. Often, in fact, one is primarily interested in the least detailed quantity which characterizes the reaction, namely its thermal rate constant, which is obtained by integrating Eq. (1) over all scattering angles, summing over all product quantum states, and Boltzmann-averaging over all initial quantum states of reactants. With the proper weighting factors, all of these averages are conveniently contained in the cumulative reaction probability (CRP), which is defined by equation (2) and in terms of which the thermal rate constant is given by equation (3). Thus, having carried out a full state-to-state scattering calculation to obtain the S-matrix, one can obtain the CRP from Eq. (2), and then rate constant from Eq. (3), but this seems like ``overkill``; i.e., if one only wants the rate constant, it would clearly be desirable to have a theory that allows one to calculate it, or the CRP, more directly than via Eq. (2), yet also correctly, i.e., without inherent approximations. Such a theory is the subject of this paper.

  7. The Unimolecular Reactions of CF3CHF2 Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to the 1,2-H-Atom Transfer, 1,1-HF Elimination and 1,2-HF Elimination Reactions and the Dependence of Threshold Energies on the Number of F-Atom Substituents in the Fluoroethane Molecules.

    PubMed

    Smith, Caleb A; Gillespie, Blanton R; Heard, George L; Setser, Donald W; Holmes, Bert E

    2017-09-19

    The recombination of CF3 and CHF2 radicals in a room temperature bath gas was used to prepare CF3CHF2* molecules with 101 kcal mol-1 of vibrational energy. The subsequent 1,2-H-atom transfer, 1,1-HF elimination and 1,2-HF elimination reactions were observed as a function of bath gas pressure by following the CHF3, CF3(F)C: and C2F4 product concentrations by gas chromatography using a mass spectrometer as the detector. The singlet CF3(F)C: concentration was measured by trapping the carbene with trans-2-butene. The experimental rate constants are 3.6 x 10(4), 4.7 x 10(4) and 1.1 x 10(4) s-1 for the 1,2-H-atom transfer, 1,1-HF elimination and 1,2-HF elimination reactions, respectively. These experimental rate constants were matched to statistical RRKM calculated rate constants to assign threshold energies (E0) of 88 ± 2, 88 ± 2 and 87 ± 2 kcal mol-1 to the three reactions. Pentafluoroethane is the only fluoroethane that has a competitive H-atom transfer decomposition reaction, and it is the only example with 1,1-HF elimination being more important than 1,2-HF elimination. The trend of increasing threshold energies for both 1,1-HF and 1,2-HF processes with the number of F-atoms in the fluoroethane molecule is summarized and investigated with electronic-structure calculations. Examination of the Intrinsic Reaction Coordinate (IRC) associated with the 1,1-HF elimination reaction found an adduct between CF3(F)C: and HF in the exit channel with a dissociation energy of about 5 kcal/mol. Hydrogen-bonded complexes between HF and the H-atom migration transition state of CH3(F)C: and the F-atom migration transition state of CF3(F)C: also were found by the calculations. The role that these carbene-HF complexes could play in 1,1-HF elimination reactions is discussed.

  8. The vibrational dependence of dissociative recombination: Rate constants for N{sub 2}{sup +}

    SciTech Connect

    Guberman, Steven L.

    2014-11-28

    Dissociative recombination rate constants are reported with electron temperature dependent uncertainties for the lowest 5 vibrational levels of the N{sub 2}{sup +} ground state. The rate constants are determined from ab initio calculations of potential curves, electronic widths, quantum defects, and cross sections. At 100 K electron temperature, the rate constants overlap with the exception of the third vibrational level. At and above 300 K, the rate constants for excited vibrational levels are significantly smaller than that for the ground level. It is shown that any experimentally determined total rate constant at 300 K electron temperature that is smaller than 2.0 × 10{sup −7} cm{sup 3}/s is likely to be for ions that have a substantially excited vibrational population. Using the vibrational level specific rate constants, the total rate constant is in very good agreement with that for an excited vibrational distribution found in a storage ring experiment. It is also shown that a prior analysis of a laser induced fluorescence experiment is quantitatively flawed due to the need to account for reactions with unknown rate constants. Two prior calculations of the dissociative recombination rate constant are shown to be inconsistent with the cross sections upon which they are based. The rate constants calculated here contribute to the resolution of a 30 year old disagreement between modeled and observed N{sub 2}{sup +} ionospheric densities.

  9. Tunneling chemical reactions in solid parahydrogen: Direct measurement of the rate constants of R+H2→RH+H (R=CD3,CD2H,CDH2,CH3) at 5 K

    NASA Astrophysics Data System (ADS)

    Hoshina, Hiromichi; Fushitani, Mizuho; Momose, Takamasa; Shida, Tadamasa

    2004-02-01

    Tunneling chemical reactions between deuterated methyl radicals and the hydrogen molecule in a parahydrogen crystal have been studied by Fourier transform infrared spectroscopy. The tunneling rates of the reactions R+H2→RH+H (R=CD3,CD2H,CDH2) in the vibrational ground state were determined directly from the temporal change in the intensity of the rovibrational absorption bands of the reactants and products in each reaction in solid parahydrogen observed at 5 K. The tunneling rate of each reaction was found to differ definitely depending upon the degree of deuteration in the methyl radicals. The tunneling rates were determined to be 3.3×10-6 s-1, 2.0×10-6 s-1, and 1.0×10-6 s-1 for the systems of CD3, CD2H, and CDH2, respectively. Conversely, the tunneling reaction between a CH3 radical and the hydrogen molecule did not proceed within a week's time. The upper limit of the tunneling rate of the reaction of the CH3 radical was estimated to be 8×10-8 s-1.

  10. An Introduction to Collision Theory Rate Constants via Distribution Functions.

    ERIC Educational Resources Information Center

    McAlduff, E. J.

    1980-01-01

    Described is an introduction to the collision theory by arriving at the many degrees of freedom rote constant and showing that the (-Ea/RT) is a special case and corresponds to activation in 2 squared terms on the line of center rate constant, which is found through the use of distributed functions. (Author/DS)

  11. ESTIMATION OF CARBOXYLIC ACID ESTER HYDROLYSIS RATE CONSTANTS

    EPA Science Inventory

    SPARC chemical reactivity models were extended to calculate hydrolysis rate constants for carboxylic acid esters from molecular structure. The energy differences between the initial state and the transition state for a molecule of interest are factored into internal and external...

  12. 18 CFR 806.12 - Constant-rate aquifer testing.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... withdraw or increase a withdrawal of groundwater shall perform a constant-rate aquifer test in accordance... groundwater availability analysis to determine the availability of water during a 1-in-10-year recurrence...

  13. ESTIMATION OF CARBOXYLIC ACID ESTER HYDROLYSIS RATE CONSTANTS

    EPA Science Inventory

    SPARC chemical reactivity models were extended to calculate hydrolysis rate constants for carboxylic acid esters from molecular structure. The energy differences between the initial state and the transition state for a molecule of interest are factored into internal and external...

  14. Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation

    PubMed Central

    Warren, Jeffrey J.; Mayer, James M.

    2010-01-01

    Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO•) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold’s model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments are shown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2′-6,6′-tetramethyl-piperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and α-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions. PMID:20215463

  15. Rate constant for the fraction of atomic chlorine with formaldehyde from 200 to 500K

    NASA Technical Reports Server (NTRS)

    Michael, J. V.; Nava, D. F.; Payne, W. A.; Stief, L. J.

    1978-01-01

    A flash photolysis - resonance fluorescence technique was used to measure rate constant. The results were independent of substantial variations in H2CO, total pressure (Ar), and flash intensity (i.e., initial Cl). The rate constant was shown to be invariant with temperature, the best representation for this temperature range being K = (7.48 + or - 0.50) x 10 to the minus 11 power cu cm molecule-1 s-1 where the error is one standard deviation. The rate constant is theoretically discussed and the potential importance of the reaction in stratospheric chemistry is considered.

  16. The development of a chemical kinetic measurement apparatus and the determination of the reaction rate constants for lithium-lead/steam interaction. Final report 9-21-90--3-31-95

    SciTech Connect

    Biney, P.O.

    1995-03-01

    The objective of this research to experimentally determine the hydrogen generation rate during the beginning and subsequent stages of liquid metal (Li{sub 17}Pb{sub 83}) and water reaction. The experimental set-up has been built. It includes a metal sample preparation apparatus, a reaction system, a measurement system and a PC based data acquisition and control system. The most important feature of the reaction system is a pneumatic actuated quick opening and closing high temperature, all stainless steel valve used the system for reaction time control. The PC system provides remote process sequencing, acquisition and control of all the systems except the metal preparation apparatus. Due to the reactivity of the lithium, all the metal sampling, preparation and loading procedures are executed in a glove box under argon protection. The metal temperature was varied between 350{degrees}C-650{degrees}C and water temperature fixed at 60{degrees}C during the experiments. A set of experimental procedures and two analyses methods: (1) thermodynamics method and (2) heat transfer method are discussed. All the measurements and data collections are executed under the PC system control. A data analysis program is used to calculate both the partial pressure of hydrogen and the hydrogen generation rate. The experiment results indicate that the amount of hydrogen generated is relate to the initial liquid metal temperature when the reaction surface is fixed. The mass of hydrogen generated as a function of initial liquid metal temperature and time of reaction is presented, The hydrogen generation over a time period of 240 seconds and the calculated errors are summarized in Table 1.

  17. Determination of rapid chlorination rate constants by a stopped-flow spectrophotometric competition kinetics method.

    PubMed

    Song, Dean; Liu, Huijuan; Qiang, Zhimin; Qu, Jiuhui

    2014-05-15

    Free chlorine is extensively used for water and wastewater disinfection nowadays. However, it still remains a big challenge to determine the rate constants of rapid chlorination reactions although competition kinetics and stopped-flow spectrophotometric (SFS) methods have been employed individually to investigate fast reaction kinetics. In this work, we proposed an SFS competition kinetics method to determine the rapid chlorination rate constants by using a common colorimetric reagent, N,N-diethyl-p-phenylenediamine (DPD), as a reference probe. A kinetic equation was first derived to estimate the reaction rate constant of DPD towards chlorine under a given pH and temperature condition. Then, on that basis, an SFS competition kinetics method was proposed to determine directly the chlorination rate constants of several representative compounds including tetracycline, ammonia, and four α-amino acids. Although Cl2O is more reactive than HOCl, its contribution to the overall chlorination kinetics of the test compounds could be neglected in this study. Finally, the developed method was validated through comparing the experimentally measured chlorination rate constants of the selected compounds with those obtained or calculated from literature and analyzing with Taft's correlation as well. This study demonstrates that the SFS competition kinetics method can measure the chlorination rate constants of a test compound rapidly and accurately.

  18. Charring rate of wood exposed to a constant heat flux

    Treesearch

    R. H. White; H. C. Tran

    1996-01-01

    A critical factor in the fire endurance of a wood member is its rate of charring. Most available charring rate data have been obtained using the time-temperature curves of the standard fire resistance tests (ASTM E 119 and ISO 834) to define the fire exposure. The increased use of heat release calorimeters using exposures of constant heat flux levels has broadened the...

  19. Determination of reaction rate constants for phenol oxidation using SnO2/Ti anodes coupled with activated carbon adsorption in the presence of TiO2 as catalyst.

    PubMed

    Zhao, Yuemin; Ding, Yi; Wang, Lizhang; Wang, Xiao

    2011-01-01

    Series of experiments for phenol degradation with assistance of TiO2 catalyst at pH of 6.5 and temperature of 25 degrees C were conducted using a lab-scale electrochemical reactor constructed in our laboratory. According to the results, at the presence of the TiO2 catalyst the removal of phenol was increased and first-order kinetics could describe the evolution of phenol concentration. For inspecting the relationship between rate constants and dosage of TiO2, two possible kinetics were proposed in this study. Contrasted to the abundant experimental data, a reasonable kinetics was obtained for the estimation of phenol concentration effluent during continuous flow of raw wastewater, especially when the TiO2 dosage was less than 0.5g L(-1). The model obtained from these experiments could employed for the calculation of rate constants at different TiO2 dosage and the necessary dosage of catalyst when a discharge standard was designed.

  20. Kinetic performance limits of constant pressure versus constant flow rate gradient elution separations. Part I: theory.

    PubMed

    Broeckhoven, K; Verstraeten, M; Choikhet, K; Dittmann, M; Witt, K; Desmet, G

    2011-02-25

    We report on a general theoretical assessment of the potential kinetic advantages of running LC gradient elution separations in the constant-pressure mode instead of in the customarily used constant-flow rate mode. Analytical calculations as well as numerical simulation results are presented. It is shown that, provided both modes are run with the same volume-based gradient program, the constant-pressure mode can potentially offer an identical separation selectivity (except from some small differences induced by the difference in pressure and viscous heating trajectory), but in a significantly shorter time. For a gradient running between 5 and 95% of organic modifier, the decrease in analysis time can be expected to be of the order of some 20% for both water-methanol and water-acetonitrile gradients, and only weakly depending on the value of V(G)/V₀ (or equivalently t(G)/t₀). Obviously, the gain will be smaller when the start and end composition lie closer to the viscosity maximum of the considered water-organic modifier system. The assumptions underlying the obtained results (no effects of pressure and temperature on the viscosity or retention coefficient) are critically reviewed, and can be inferred to only have a small effect on the general conclusions. It is also shown that, under the adopted assumptions, the kinetic plot theory also holds for operations where the flow rate varies with the time, as is the case for constant-pressure operation. Comparing both operation modes in a kinetic plot representing the maximal peak capacity versus time, it is theoretically predicted here that both modes can be expected to perform equally well in the fully C-term dominated regime (where H varies linearly with the flow rate), while the constant pressure mode is advantageous for all lower flow rates. Near the optimal flow rate, and for linear gradients running from 5 to 95% organic modifier, time gains of the order of some 20% can be expected (or 25-30% when accounting for

  1. Theoretical Evaluation of the Transient Response of Constant Head and Constant Flow-Rate Permeability Tests

    USGS Publications Warehouse

    Zhang, M.; Takahashi, M.; Morin, R.H.; Esaki, T.

    1998-01-01

    A theoretical analysis is presented that compares the response characteristics of the constant head and the constant flowrate (flow pump) laboratory techniques for quantifying the hydraulic properties of geologic materials having permeabilities less than 10-10 m/s. Rigorous analytical solutions that describe the transient distributions of hydraulic gradient within a specimen are developed, and equations are derived for each method. Expressions simulating the inflow and outflow rates across the specimen boundaries during a constant-head permeability test are also presented. These solutions illustrate the advantages and disadvantages of each method, including insights into measurement accuracy and the validity of using Darcy's law under certain conditions. The resulting observations offer practical considerations in the selection of an appropriate laboratory test method for the reliable measurement of permeability in low-permeability geologic materials.

  2. Improvement of the method to estimate the relative reaction rate constants of hydroxyl radical with polyphenols using ESR spin trap: X-ray irradiation of water with a flowing system.

    PubMed

    Nakagawa, Seiko

    2017-09-05

    UV-photolysis of hydrogen peroxide is a useful technique to produce hydroxyl radical. However, it is not an appropriate method to estimate the reactivity of polyphenols with hydroxyl radicals because many of the polyphenol derivatives also absorb the UV-light to generate hydroxyl radicals. In this study, X-ray irradiation of water with a flowing system was applied to estimate the reactivity of hydroxyl radicals with polyphenols using electron spin resonance (ESR) spin trap. The obtained relative reaction rates reasonably agreed with previous data by pulse radiolysis. This method will be a useful technique to estimate the reactivity of antioxidants including polyphenols with hydroxyl radicals.

  3. Computer Calculation of First-Order Rate Constants

    ERIC Educational Resources Information Center

    Williams, Robert C.; Taylor, James W.

    1970-01-01

    Discusses the computer program used to calculate first-order rate constants. Discussion includes data preparation, weighting options, comparison techniques, infinity point adjustment, least-square fit, Guggenheim calculation, and printed outputs. Exemplifies the utility of the computer program by two experiments: (1) the thermal decomposition of…

  4. Oxygen uptake in maximal effort constant rate and interval running.

    PubMed

    Pratt, Daniel; O'Brien, Brendan J; Clark, Bradley

    2013-01-01

    This study investigated differences in average VO2 of maximal effort interval running to maximal effort constant rate running at lactate threshold matched for time. The average VO2 and distance covered of 10 recreational male runners (VO2max: 4158 ± 390 mL · min(-1)) were compared between a maximal effort constant-rate run at lactate threshold (CRLT), a maximal effort interval run (INT) consisting of 2 min at VO2max speed with 2 minutes at 50% of VO2 repeated 5 times, and a run at the average speed sustained during the interval run (CR submax). Data are presented as mean and 95% confidence intervals. The average VO2 for INT, 3451 (3269-3633) mL · min(-1), 83% VO2max, was not significantly different to CRLT, 3464 (3285-3643) mL · min(-1), 84% VO2max, but both were significantly higher than CR sub-max, 3464 (3285-3643) mL · min(-1), 76% VO2max. The distance covered was significantly greater in CLRT, 4431 (4202-3731) metres, compared to INT and CR sub-max, 4070 (3831-4309) metres. The novel finding was that a 20-minute maximal effort constant rate run uses similar amounts of oxygen as a 20-minute maximal effort interval run despite the greater distance covered in the maximal effort constant-rate run.

  5. Computer Calculation of First-Order Rate Constants

    ERIC Educational Resources Information Center

    Williams, Robert C.; Taylor, James W.

    1970-01-01

    Discusses the computer program used to calculate first-order rate constants. Discussion includes data preparation, weighting options, comparison techniques, infinity point adjustment, least-square fit, Guggenheim calculation, and printed outputs. Exemplifies the utility of the computer program by two experiments: (1) the thermal decomposition of…

  6. VMATc: VMAT with constant gantry speed and dose rate

    NASA Astrophysics Data System (ADS)

    Peng, Fei; Jiang, Steve B.; Romeijn, H. Edwin; Epelman, Marina A.

    2015-04-01

    This article considers the treatment plan optimization problem for Volumetric Modulated Arc Therapy (VMAT) with constant gantry speed and dose rate (VMATc). In particular, we consider the simultaneous optimization of multi-leaf collimator leaf positions and a constant gantry speed and dose rate. We propose a heuristic framework for (approximately) solving this optimization problem that is based on hierarchical decomposition. Specifically, an iterative algorithm is used to heuristically optimize dose rate and gantry speed selection, where at every iteration a leaf position optimization subproblem is solved, also heuristically, to find a high-quality plan corresponding to a given dose rate and gantry speed. We apply our framework to clinical patient cases, and compare the resulting VMATc plans to idealized IMRT, as well as full VMAT plans. Our results suggest that VMATc is capable of producing treatment plans of comparable quality to VMAT, albeit at the expense of long computation time and generally higher total monitor units.

  7. Rate and equilibrium constants for the addition of N-heterocyclic carbenes into benzaldehydes: a remarkable 2-substituent effect.

    PubMed

    Collett, Christopher J; Massey, Richard S; Taylor, James E; Maguire, Oliver R; O'Donoghue, AnnMarie C; Smith, Andrew D

    2015-06-01

    Rate and equilibrium constants for the reaction between N-aryl triazolium N-heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3-(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2-substituent in these reactions and provide insight into the chemoselectivity of cross-benzoin reactions.

  8. Rate and Equilibrium Constants for the Addition of N-Heterocyclic Carbenes into Benzaldehydes: A Remarkable 2-Substituent Effect.

    PubMed

    Collett, Christopher J; Massey, Richard S; Taylor, James E; Maguire, Oliver R; O'Donoghue, AnnMarie C; Smith, Andrew D

    2015-06-01

    Rate and equilibrium constants for the reaction between N-aryl triazolium N-heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3-(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2-substituent in these reactions and provide insight into the chemoselectivity of cross-benzoin reactions.

  9. Weak Acid Ionization Constants and the Determination of Weak Acid-Weak Base Reaction Equilibrium Constants in the General Chemistry Laboratory

    ERIC Educational Resources Information Center

    Nyasulu, Frazier; McMills, Lauren; Barlag, Rebecca

    2013-01-01

    A laboratory to determine the equilibrium constants of weak acid negative weak base reactions is described. The equilibrium constants of component reactions when multiplied together equal the numerical value of the equilibrium constant of the summative reaction. The component reactions are weak acid ionization reactions, weak base hydrolysis…

  10. Weak Acid Ionization Constants and the Determination of Weak Acid-Weak Base Reaction Equilibrium Constants in the General Chemistry Laboratory

    ERIC Educational Resources Information Center

    Nyasulu, Frazier; McMills, Lauren; Barlag, Rebecca

    2013-01-01

    A laboratory to determine the equilibrium constants of weak acid negative weak base reactions is described. The equilibrium constants of component reactions when multiplied together equal the numerical value of the equilibrium constant of the summative reaction. The component reactions are weak acid ionization reactions, weak base hydrolysis…

  11. Rate constants for H abstraction from benzo(a)pyrene and chrysene: a theoretical study.

    PubMed

    Semenikhin, A S; Savchenkova, A S; Chechet, I V; Matveev, S G; Liu, Z; Frenklach, M; Mebel, A M

    2017-09-12

    Density functional B3LYP/6-31G(d) and ab initio G3(MP2,CC) calculations have been carried out to determine thermal rate constants of direct H abstraction reactions from four- and five-ring polycyclic aromatic hydrocarbons (PAH) chrysene and benzo[a]pyrene by various radicals abundant in combustion flames, such as H, CH3, C3H3, and OH, using transition state theory. The results show that the H abstraction reactions with OH have the lowest barriers of ∼4 kcal mol(-1), followed by those with H and CH3 with barriers of 16-17 kcal mol(-1), and then with propargyl radicals with barriers of 24-26 kcal mol(-1). Thus, the OH radical is predicted to be the fastest H abstractor from PAH. Even at 2500 K, the rate constant for H abstraction by H is still 34% lower than the rate constant for H abstraction by OH. The reaction with H is calculated to have rate constants 35-19 times higher than those for the reaction with CH3 due to a more favorable entropic factor. The reactions of H abstraction by C3H3 are predicted to be orders of magnitude slower than the other reactions considered and their equilibrium is strongly shifted toward the reactants, making propargyl an inefficient H abstractor from the aromatics. The calculations showed strong similarity of the reaction energetics in different H abstraction positions of benzo[a]pyrene and chrysene within armchair and zigzag edges in these molecules, but clear distinction between the armchair and zigzag sites. The zigzag sites appear to be more reactive, with H abstraction rate constants by H, CH3, and OH being respectively 37-42%, a factor of 2.1, and factors of 8-9 higher than the corresponding rate constants for the H abstraction reactions from armchair sites. Although the barrier heights for the two types of edges are similar, the entropic factor makes zigzag sites more favorable for H abstraction. Rate expressions have been generated for all studied reactions with the goal to rectify current combustion kinetics mechanisms.

  12. RELATIVE RATE CONSTANTS OF CONTAMINANT CANDIDATE LIST PESTICIDES WITH HYDROXYL RADICALS

    EPA Science Inventory

    The objective of this study was to establish the rate constants for the reactions of selected pesticides listed on the US EPA Contaminant Candidate List, with UV and hydroxyl radicals (·OH). Batch experiments were conducted in phosphate buffered solution at pH 7. All pestici...

  13. RELATIVE RATE CONSTANTS OF CONTAMINANT CANDIDATE LIST PESTICIDES WITH HYDROXYL RADICALS

    EPA Science Inventory

    The objective of this study was to establish the rate constants for the reactions of selected pesticides listed on the US EPA Contaminant Candidate List, with UV and hydroxyl radicals (·OH). Batch experiments were conducted in phosphate buffered solution at pH 7. All pestici...

  14. Mixed Quantum-Classical Liouville Approach for Calculating Proton-Coupled Electron-Transfer Rate Constants.

    PubMed

    Shakib, Farnaz; Hanna, Gabriel

    2016-07-12

    In this work, we derive a general mixed quantum-classical formula for calculating thermal proton-coupled electron-transfer (PCET) rate constants, starting from the time integral of the quantum flux-flux correlation function. This formula allows for the direct simulation of PCET reaction dynamics via the mixed quantum-classical Liouville approach. Owing to the general nature of the derivation, this formula does not rely on any prior mechanistic assumptions and can be applied across a wide range of electronic and protonic coupling regimes. To test the validity of this formula, we applied it to a reduced model of a condensed-phase PCET reaction. Good agreement with the numerically exact rate constant is obtained, demonstrating the accuracy of our formalism. We believe that this approach constitutes a solid foundation for future investigations of the rates and mechanisms of a wide range of PCET reactions.

  15. Kinetic performance limits of constant pressure versus constant flow rate gradient elution separations. Part II: experimental.

    PubMed

    Verstraeten, M; Broeckhoven, K; Dittmann, M; Choikhet, K; Witt, K; Desmet, G

    2011-02-25

    We report on a first series of experiments comparing the selectivity and the kinetic performance of constant flow rate and constant pressure mode gradient elution separations. Both water-methanol and water-acetonitrile mobile phase mixtures have been considered, as well as different samples and gradient programs. Instrument pressures up to 1200 bar have been used. Neglecting some small possible deviations caused by viscous heating effects, the experiments could confirm the theoretical expectation that both operation modes should lead to identical separation selectivities provided the same mobile phase gradient program is run in reduced volumetric coordinates. Also in agreement with the theoretical expectations, the cP-mode led to a gain in analysis time amounting up to some 17% for linear gradients running from 5 to 95% of organic modifier at ultra-high pressures. Gains of over 25% were obtained for segmented gradients, at least when the flat portions of the gradient program were situated in regions where the gradient composition was the least viscous. Detailed plate height measurements showed that the single difference between the constant flow rate and the constant pressure mode is a (small) difference in efficiency caused by the difference in average flow rate, in turn leading to a different intrinsic band broadening. Separating a phenone sample with a 20-95% water-acetonitrile gradient, the cP-mode leads to gradient plate heights that are some 20-40% smaller than in the cF-mode in the B-term dominated regime, while they are some 5-10% larger in the C-term dominated regime. Considering a separation with sub 2-μm particles on a 350 mm long coupled column, switching to the constant pressure mode allowed to finish the run in 29 instead of in 35 min, while also a larger peak capacity is obtained (going from 334 in the cF-mode to 339 in the cP-mode) and the mutual selectivity between the different peaks is fully retained. Copyright © 2010 Elsevier B.V. All rights

  16. Observational evidence for constant gas accretion rate since z = 5

    NASA Astrophysics Data System (ADS)

    Spring, Eleanor F.; Michałowski, Michał J.

    2017-10-01

    Star formation rate density (SFRD) has not been constant throughout the history of the Universe. The rate at which stars form greatly affects the evolution of the Universe, but the factors which drive SFRD evolution remain uncertain. There must be sufficient amount of gas to fuel the star formation, either as a reservoir within a galaxy, or as inflow from the intergalactic medium (IGM). This work explores how the gas accretion rate onto galaxies over time has affected star formation rate. We propose a novel method of measuring cosmic gas accretion rate. This involves comparing the comoving densities of available Hi and H2 gas and the densities of existing stars at different redshifts. We constrained gas accretion until z = 5, and we found that the gas accretion rate density (GARD) is relatively constant in the range from z = 5 to z = 0. This constancy in the GARD is not reflected by the SFRD, which declines significantly between z = 1.0 and z = 0. This work suggests that the decline is not due to a reduction in GARD.

  17. The Rate Laws for Reversible Reactions.

    ERIC Educational Resources Information Center

    King, Edward L.

    1986-01-01

    Provides background information for teachers on the rate laws for reversible reactions. Indicates that although prediction of the form of the rate law for a reverse reaction given the rate law for the forward reaction is not certain, the number of possibilities is limited because of relationships described. (JN)

  18. Comparison of TID Effects in Space-Like Variable Dose Rates and Constant Dose Rates

    NASA Technical Reports Server (NTRS)

    Harris, Richard D.; McClure, Steven S.; Rax, Bernard G.; Evans, Robin W.; Jun, Insoo

    2008-01-01

    The degradation of the LM193 dual voltage comparator has been studied at different TID dose rate profiles, including several different constant dose rates and a variable dose rate that simulates the behavior of a solar flare. A comparison of results following constant dose rate vs. variable dose rates is made to explore how well the constant dose rates used for typical part testing predict the performance during a simulated space-like mission. Testing at a constant dose rate equal to the lowest dose rate seen during the simulated flare provides an extremely conservative estimate of the overall amount of degradation. A constant dose rate equal to the average dose rate is also more conservative than the variable rate. It appears that, for this part, weighting the dose rates by the amount of total dose received at each rate (rather than the amount of time at each dose rate) results in an average rate that produces an amount of degradation that is a reasonable approximation to that received by the variable rate.

  19. Development of a chemical kinetic measurement apparatus and the determination of the reaction rate constants for lithium-lead/water interaction. Technical status progress report, October 1, 1991--March 15, 1993

    SciTech Connect

    Biney, P.O.

    1993-04-01

    An experimental set-up for accurate measurement of hydrogen generation rate in Lithium-Lead (Li{sub 17}Pb{sub 83}) Steam or water interactions has been designed. The most important features of the design include a pneumatic actuated quick opening and closing high temperature all stainless steel valve used to control the reaction time and the placement of most measuring devices below a water line to minimize leakage of the hydrogen collected. A PC based data acquisition and control system provides remote process sequencing, acquisition and control of all major components of the set-up. Initial tests indicate that the first design objective of maintaining leakproof gas collection chamber has been achieved. Initial pressure tests indicated that the pressure drop over a time span of 30 minutes was within the tolerance of the pressure transducer used to measure the pressure (within 0.690 kPa) at a nominal system pressure of 685 kPa. The experimental system hardware, data acquisition and control programs and data analysis program have been completed, tested and are currently functional.

  20. A New Look at Reaction Rates

    NASA Astrophysics Data System (ADS)

    Cvitas, Tomislav

    1999-11-01

    Both rates of radioactive decays and rates of chemical reactions can be thought of as numbers of transformations per time. The rate of reaction, as an intensive quantity characteristic of the process, is obtained by dividing the amount of chemical transformations per time by the volume of the reaction system. The practical definition of the reaction rate found in the literature can then be derived by defining the stoichiometric numbers as changes in the number of specific molecules taking part in the reaction per chemical transformation. The name concentration of chemical transformations is introduced for what was previously called reaction variable. It is suggested that the conceptual definition of the advancement of reaction and reaction rate be introduced in general chemistry courses.

  1. Method of controlling fusion reaction rates

    DOEpatents

    Kulsrud, Russell M.; Furth, Harold P.; Valeo, Ernest J.; Goldhaber, Maurice

    1988-01-01

    A method of controlling the reaction rates of the fuel atoms in a fusion reactor comprises the step of polarizing the nuclei of the fuel atoms in a particular direction relative to the plasma confining magnetic field. Fusion reaction rates can be increased or decreased, and the direction of emission of the reaction products can be controlled, depending on the choice of polarization direction.

  2. Method of controlling fusion reaction rates

    DOEpatents

    Kulsrud, Russell M.; Furth, Harold P.; Valeo, Ernest J.; Goldhaber, Maurice

    1988-03-01

    A method of controlling the reaction rates of the fuel atoms in a fusion reactor comprises the step of polarizing the nuclei of the fuel atoms in a particular direction relative to the plasma confining magnetic field. Fusion reaction rates can be increased or decreased, and the direction of emission of the reaction products can be controlled, depending on the choice of polarization direction.

  3. Chlorination of Amino Acids: Reaction Pathways and Reaction Rates.

    PubMed

    How, Zuo Tong; Linge, Kathryn L; Busetti, Francesco; Joll, Cynthia A

    2017-05-02

    Chlorination of amino acids can result in the formation of organic monochloramines or organic dichloramines, depending on the chlorine to amino acid ratio (Cl:AA). After formation, organic chloramines degrade into aldehydes, nitriles and N-chloraldimines. In this paper, the formation of organic chloramines from chlorination of lysine, tyrosine and valine were investigated. Chlorination of tyrosine and lysine demonstrated that the presence of a reactive secondary group can increase the Cl:AA ratio required for the formation of N,N-dichloramines, and potentially alter the reaction pathways between chlorine and amino acids, resulting in the formation of unexpected byproducts. In a detailed investigation, we report rate constants for all reactions in the chlorination of valine, for the first time, using experimental results and modeling. At Cl:AA = 2.8, the chlorine was found to first react quickly with valine (5.4 × 10(4) M(-1) s(-1)) to form N-monochlorovaline, with a slower subsequent reaction with N-monochlorovaline to form N,N-dichlorovaline (4.9 × 10(2) M(-1) s(-1)), although some N-monochlorovaline degraded into isobutyraldehyde (1.0 × 10(-4) s(-1)). The N,N-dichlorovaline then competitively degraded into isobutyronitrile (1.3 × 10(-4) s(-1)) and N-chloroisobutyraldimine (1.2 × 10(-4) s(-1)). In conventional drinking water disinfection, N-chloroisobutyraldimine can potentially be formed in concentrations higher than its odor threshold concentration, resulting in aesthetic challenges and an unknown health risk.

  4. Phototransformation rate constants of PAHs associated with soot particles.

    PubMed

    Kim, Daekyun; Young, Thomas M; Anastasio, Cort

    2013-01-15

    Photodegradation is a key process governing the residence time and fate of polycyclic aromatic hydrocarbons (PAHs) in particles, both in the atmosphere and after deposition. We have measured photodegradation rate constants of PAHs in bulk deposits of soot particles illuminated with simulated sunlight. The photodegradation rate constants at the surface (k(p)(0)), the effective diffusion coefficients (D(eff)), and the light penetration depths (z(0.5)) for PAHs on soot layers of variable thickness were determined by fitting experimental data with a model of coupled photolysis and diffusion. The overall disappearance rates of irradiated low molecular weight PAHs (with 2-3 rings) on soot particles were influenced by fast photodegradation and fast diffusion kinetics, while those of high molecular weight PAHs (with 4 or more rings) were apparently controlled by either the combination of slow photodegradation and slow diffusion kinetics or by very slow diffusion kinetics alone. The value of z(0.5) is more sensitive to the soot layer thickness than the k(p)(0) value. As the thickness of the soot layer increases, the z(0.5) values increase, but the k(p)(0) values are almost constant. The effective diffusion coefficients calculated from dark experiments are generally higher than those from the model fitting method for illumination experiments. Due to the correlation between k(p)(0) and z(0.5) in thinner layers, D(eff) should be estimated by an independent method for better accuracy. Despite some limitations of the model used in this study, the fitted parameters were useful for describing empirical results of photodegradation of soot-associated PAHs.

  5. Using the pseudophase kinetic model to interpret chemical reactivity in ionic emulsions: determining antioxidant partition constants and interfacial rate constants.

    PubMed

    Gu, Qing; Bravo-Díaz, Carlos; Romsted, Laurence S

    2013-06-15

    Kinetic results obtained in cationic and anionic emulsions show for the first time that pseudophase kinetic models give reasonable estimates of the partition constants of reactants, here t-butylhydroquinone (TBHQ) between the oil and interfacial region, P(O)(I), and the water and interfacial region, P(W)(I), and of the interfacial rate constant, k(I), for the reaction with an arenediazonium ion in emulsions containing a 1:1 volume ratio of a medium chain length triglyceride, MCT, and aqueous acid or buffer. The results provide: (a) an explanation for the large difference in pH, >4 pH units, required to run the reaction in CTAB (pH 1.54, added HBr) and SDS (pH 5.71, acetate buffer) emulsions; (b) reasonable estimates of PO(I) and k(I) in the CTAB emulsions; (c) a sensible interpretation of added counterion effects based on ion exchange in SDS emulsions (Na(+)/H3O(+) ion exchange in the interfacial region) and Donnan equilibrium in CTAB emulsions (Br(-) increasing the interfacial H3O(+)); and (d) the significance of the effect of the much greater solubility of TBHQ in MCT versus octane, 1000/1, as the oil. These results should aid in interpreting the effects of ionic surfactants on chemical reactivity in emulsions in general and in selecting the most efficient antioxidant for particular food applications.

  6. Determination of acidity constants of curcumin in aqueous solution and apparent rate constant of its decomposition.

    PubMed

    Bernabé-Pineda, Margarita; Ramírez-Silva, María Teresa; Romero-Romo, Mario; González-Vergara, Enrique; Rojas-Hernández, Alberto

    2004-04-01

    The stability of curcumin (H3Cur) in aqueous media is improved when the systems in which it is present are at high pH values (higher than 11.7), fitting a model describable by a pseudo-zero order with a rate constant k' for the disappearance of the Cur3- species of 1.39 (10(-9)) Mmin(-1). There were three acidity constants measured for the curcumin as follows: pKA3 = 10.51 +/- 0.01 corresponding to the equilibrium HCur2- = Cur3- + H+, a pKA2 = 9.88 +/- 0.02 corresponding to the equilibrium H2Cur- = HCur-(2) + H+. These pKA values were attributed to the hydrogen of the phenol part of the curcumin, while the pKA1 = 8.38 +/- 0.04 corresponds to the equilibrium H3Cur = H2Cur- + H+ and is attributed the acetylacetone type group. Formation of quinoid structures play an important role in the tautomeric forms of the curcumin in aqueous media, which makes the experimental values differ from the theoretically calculated ones, depending on the conditions adopted in the study.

  7. Efficient quantum-classical method for computing thermal rate constant of recombination: application to ozone formation.

    PubMed

    Ivanov, Mikhail V; Babikov, Dmitri

    2012-05-14

    Efficient method is proposed for computing thermal rate constant of recombination reaction that proceeds according to the energy transfer mechanism, when an energized molecule is formed from reactants first, and is stabilized later by collision with quencher. The mixed quantum-classical theory for the collisional energy transfer and the ro-vibrational energy flow [M. Ivanov and D. Babikov, J. Chem. Phys. 134, 144107 (2011)] is employed to treat the dynamics of molecule + quencher collision. Efficiency is achieved by sampling simultaneously (i) the thermal collision energy, (ii) the impact parameter, and (iii) the incident direction of quencher, as well as (iv) the rotational state of energized molecule. This approach is applied to calculate third-order rate constant of the recombination reaction that forms the (16)O(18)O(16)O isotopomer of ozone. Comparison of the predicted rate vs. experimental result is presented.

  8. Measurement of the OH reaction rate constants for CH{sub 3}CH{sub 2}OH, CF{sub 3}CF{sub 2}CH{sub 2}OH, and CF{sub 3}CH(OH)CF{sub 3}

    SciTech Connect

    Tokuhashi, Kazuaki; Nagai, Hidekazu; Takahashi, Akifumi; Kaise, Masahiro; Kondo, Shigeo; Sekiya, Akira; Gotoh, Yoshihiko; Takahashi, Mitsuru; Suga, Atsuo

    1999-04-15

    Fully halogenated chlorofluorocarbons (CFCs) cause depletion of the stratospheric ozone layer as well as global warming. Partially fluorinated alcohols are important in relevance with potential substitutes of CFCs. The OH reaction rate constants have been measured for CF{sub 3}CH{sub 2}OH, CF{sub 3}CF{sub 2}CH{sub 2}OH, and CF{sub 3}CH(OH)CF{sub 3} over the temperature range 250--430K. Kinetic measurements have been carried out using the discharge flow, laser photolysis, and flash photolysis methods, combined respectively with the laser-induced fluorescence technique to monitor the OH radical concentrations. The influence of impurities contained in the sample of CF{sub 3}CF{sub 2}CH{sub 2}OH has been investigated by means of sample purification using gas chromatography. No sizable effect of impurities was found on the measured rate constants of these three fluorinated alcohols. The Arrhenius rate constants have been determined from the respective kinetic data as {kappa}(CF{sub 3}CH{sub 2}OH) = (2.00 {+-} 0.37) {times} 10{sup {minus}12} exp[{minus}(890 {+-} 60)/T], {kappa}(CF{sub 3}CF{sub 2}CH{sub 2}OH) = (1.40 {+-} 0.27) {times} 10{sup {minus}12} exp[{minus}(780 {+-} 60)/T], and {kappa}(CF{sub 3}CH(OH)CF{sub 3}) = (6.99 {+-} 1.56) {times} 10{sup {minus}13} exp[{minus}(990 {+-} 70)/T] cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. A method of predicting the OH reaction rate constants for fluorinated alcohols, hydrofluorocarbons, alkanes, and alcohols has been proposed.

  9. Unsteady flow past an airfoil pitched at constant rate

    NASA Technical Reports Server (NTRS)

    Lourenco, L.; Vandommelen, L.; Shib, C.; Krothapalli, A.

    1992-01-01

    The unsteady flow past a NACA 0012 airfoil that is undertaking a constant-rate pitching up motion is investigated experimentally by the PIDV technique in a water towing tank. The Reynolds number is 5000, based upon the airfoil's chord and the free-stream velocity. The airfoil is pitching impulsively from 0 to 30 deg. with a dimensionless pitch rate alpha of 0.131. Instantaneous velocity and associated vorticity data have been acquired over the entire flow field. The primary vortex dominates the flow behavior after it separates from the leading edge of the airfoil. Complete stall emerges after this vortex detaches from the airfoil and triggers the shedding of a counter-rotating vortex near the trailing edge. A parallel computational study using the discrete vortex, random walk approximation has also been conducted. In general, the computational results agree very well with the experiment.

  10. Path-dependent variational effects and multidimensional tunneling in multi-path variational transition state theory: rate constants calculated for the reactions of HO2 with tert-butanol by including all 46 paths for abstraction at C and all six paths for abstraction at O.

    PubMed

    Bao, Junwei Lucas; Sripa, Pattrawan; Truhlar, Donald G

    2016-01-14

    Multi-path variational transition state theory (MP-VTST) provides a conformationally complete framework for calculating gas-phase rate constants. For reactions in which the transition state has distinguishable torsional minima (which include most reactions), there are multiple possible reaction paths. In principle MP-VTST includes the contributions from all the reaction paths, and it should explicitly treat the variational and tunneling effects of each path, but in practice one may need to truncate the number of paths included in MP-VTST calculations in order to achieve a balance between computational cost and accuracy. In this work, we present calculations including all paths for two prototype combustion reactions, namely the two hydrogen abstraction reactions from tert-butanol by HO2 radical. For both reactions we included all the reaction paths. Since abstraction at C has 46 paths, it provided a good opportunity to carry out a case study in which we investigated the errors introduced by truncating the number of paths. For the reaction studied, we found that the variational and multidimensional tunneling transmission coefficients are very different for different reaction paths, which provides new evidence that MP-VTST is necessary for treating path-dependent variational effects and multidimensional tunneling. We found that tunneling transmission coefficients can be much larger for higher-energy paths than for lower-energy ones. Interestingly, the simple hypothesis that higher barriers are narrower does not explain this finding in the present case; we found instead that the effect is due to higher-energy barriers having the possibility of tunneling at energies farther below the barrier top. We also show that a previously applied criterion for judging convergence with respect to the number of paths may not be reliable at low temperature.

  11. On the Values at Equilibrium and Rate Constants in Inter - Conversion Processes.

    PubMed

    Olteanu, Octav

    2016-01-01

    Inter - conversion processes of labile molecules obey similar rules to those of reversible chemical reactions. Solving the corresponding linear differential systems is used along this work, as well as in the preceding version. The main purpose of the present mini revue paper is to recall, improve and correct some mathematical methods in determining the optimal values at equilibrium, and remarkable particular rate constants. This part was not proved correctly in my previous work. In my previous work, the proof of the equality of the concentrations of the main species at equilibrium was not correct. In the current manuscript, we use increasing velocity in order to obtain this first important result. To this aim, one applies Schwarz inequality and the case when equality occurs. In order to determine significant rate constants, we characterize these special values in terms of the norm of the linear operator defined by the matrix of the differential system. In my previous work, the normal probability density function was used. The latter method was not realistic. Increasing the velocity, one obtains equal optimal values of the concentrations at equilibrium. This method represents a patent in the field. Secondly, characterization of remarkable rate constants (which are also equal) is deduced. The optimal solutions are written explicitly. Under suitable conditions, the values at equilibrium and the rate constants are equal. The common value at equilibrium equals the common value of the rate constants.

  12. Rate of reaction of OH with HNO3

    NASA Technical Reports Server (NTRS)

    Wine, P. H.; Ravishankara, A. R.; Kreutter, N. M.; Shah, R. C.; Nicovich, J. M.; Thompson, R. L.; Wuebbles, D. J.

    1981-01-01

    Measurements of the kinetics of the reaction of OH with HNO3, and mechanisms of HNO3 removal from the stratosphere, are reported. Bimolecular rate constants were determined at temperatures between 224 and 366 K by monitoring the concentrations of OH radicals produced by HNO3 photolysis and HNO3 according to their resonance fluorescence and 184.9-nm absorption, respectively. The rate constant measured at 298 K is found to be somewhat faster than previously accepted values, with a negative temperature dependence. Calculations of a one-dimensional transport-kinetic atmospheric model on the basis of the new rate constant indicate reductions in O3 depletion due to chlorofluoromethane release and NOx injection, of magnitudes dependent on the nature of the reaction products.

  13. Theoretical rate constants of super-exchange hole transfer and thermally induced hopping in DNA.

    PubMed

    Shimazaki, Tomomi; Asai, Yoshihiro; Yamashita, Koichi

    2005-01-27

    Recently, the electronic properties of DNA have been extensively studied, because its conductivity is important not only to the study of fundamental biological problems, but also in the development of molecular-sized electronics and biosensors. We have studied theoretically the reorganization energies, the activation energies, the electronic coupling matrix elements, and the rate constants of hole transfer in B-form double-helix DNA in water. To accommodate the effects of DNA nuclear motions, a subset of reaction coordinates for hole transfer was extracted from classical molecular dynamics (MD) trajectories of DNA in water and then used for ab initio quantum chemical calculations of electron coupling constants based on the generalized Mulliken-Hush model. A molecular mechanics (MM) method was used to determine the nuclear Franck-Condon factor. The rate constants for two types of mechanisms of hole transfer-the thermally induced hopping (TIH) and the super-exchange mechanisms-were determined based on Marcus theory. We found that the calculated matrix elements are strongly dependent on the conformations of the nucleobase pairs of hole-transferable DNA and extend over a wide range of values for the "rise" base-step parameter but cluster around a particular value for the "twist" parameter. The calculated activation energies are in good agreement with experimental results. Whereas the rate constant for the TIH mechanism is not dependent on the number of A-T nucleobase pairs that act as a bridge, the rate constant for the super-exchange process rapidly decreases when the length of the bridge increases. These characteristic trends in the calculated rate constants effectively reproduce those in the experimental data of Giese et al. [Nature 2001, 412, 318]. The calculated rate constants were also compared with the experimental results of Lewis et al. [Nature 2000, 406, 51].

  14. Reaction Order Ambiguity in Integrated Rate Plots

    ERIC Educational Resources Information Center

    Lee, Joe

    2008-01-01

    Integrated rate plots are frequently used in reaction kinetics to determine orders of reactions. It is often emphasised, when using this methodology in practice, that it is necessary to monitor the reaction to a substantial fraction of completion for these plots to yield unambiguous orders. The present article gives a theoretical and statistical…

  15. Reaction Order Ambiguity in Integrated Rate Plots

    ERIC Educational Resources Information Center

    Lee, Joe

    2008-01-01

    Integrated rate plots are frequently used in reaction kinetics to determine orders of reactions. It is often emphasised, when using this methodology in practice, that it is necessary to monitor the reaction to a substantial fraction of completion for these plots to yield unambiguous orders. The present article gives a theoretical and statistical…

  16. Universality of Thermodynamic Constants Governing Biological Growth Rates

    PubMed Central

    Corkrey, Ross; Olley, June; Ratkowsky, David; McMeekin, Tom; Ross, Tom

    2012-01-01

    Background Mathematical models exist that quantify the effect of temperature on poikilotherm growth rate. One family of such models assumes a single rate-limiting ‘master reaction’ using terms describing the temperature-dependent denaturation of the reaction's enzyme. We consider whether such a model can describe growth in each domain of life. Methodology/Principal Findings A new model based on this assumption and using a hierarchical Bayesian approach fits simultaneously 95 data sets for temperature-related growth rates of diverse microorganisms from all three domains of life, Bacteria, Archaea and Eukarya. Remarkably, the model produces credible estimates of fundamental thermodynamic parameters describing protein thermal stability predicted over 20 years ago. Conclusions/Significance The analysis lends support to the concept of universal thermodynamic limits to microbial growth rate dictated by protein thermal stability that in turn govern biological rates. This suggests that the thermal stability of proteins is a unifying property in the evolution and adaptation of life on earth. The fundamental nature of this conclusion has importance for many fields of study including microbiology, protein chemistry, thermal biology, and ecological theory including, for example, the influence of the vast microbial biomass and activity in the biosphere that is poorly described in current climate models. PMID:22348140

  17. Analysis of constant false alarm rate sidelobe canceller criterion

    NASA Astrophysics Data System (ADS)

    Reed, I. S.; Brennan, L. E.

    1985-05-01

    In this final report, the constant false alarm rate (CFAR) detection criterion for a sidelobe canceller (SLC) system, introduced in the last quarterly progress report, is found completely and analyzed. This new detection test for radar exhibits the desirable CFAR property that its probability of a false alarm (PFA) is functionally independent of the covariance of the actual noise field encountered. As a consequence, such a CFAR SLC system is ideally suited to cope with the newly evolving smart jammer threat to radar. An important objective, set in the last quarterly progress report, was to find both the false alarm and signal detection probabilities of this test. The first and most important of these two goals has been met. The probability of a false alarm (or PFA) of this CFAR SLC detection criterion is derived in closed form in this report. The success in finding the PFA is due primarily to the use of a generalization of Cochran's theorem.

  18. Multidimensional reaction rate theory with anisotropic diffusion.

    PubMed

    Berezhkovskii, Alexander M; Szabo, Attila; Greives, Nicholas; Zhou, Huan-Xiang

    2014-11-28

    An analytical expression is derived for the rate constant that describes diffusive transitions between two deep wells of a multidimensional potential. The expression, in contrast to the Kramers-Langer formula for the rate constant, is valid even when the diffusion is highly anisotropic. Our approach is based on a variational principle for the reactive flux and uses a trial function for the splitting probability or commitor. The theoretical result is validated by Brownian dynamics simulations.

  19. Redox reaction rates using potentiostatic coulometry

    SciTech Connect

    Ramette, R.W.; Harris, R.Z.; Bengali, A.A.; Noll, R.J.

    1987-01-01

    A new method based on potentiostatic coulometry was used to study the kinetics of the aqueous redox reactions between the ions chlorate/iodide, bromate/iodide, and bromate/bromide. The halogen product was continuously and rapidly reduced back to halide at a large platinum gauze cathode, the current being a direct measure of reaction rate and the accumulated charge serving to measure the extent of reaction. The reactions were studied at several temperatures, and activation entropies and enthalpies were calculated.

  20. Shock Tube Measurement of the High-Temperature Rate Constant for OH + CH3 → Products.

    PubMed

    Wang, Shengkai; Li, Sijie; Davidson, David F; Hanson, Ronald K

    2015-08-20

    The reaction between hydroxyl (OH) and methyl radicals (CH3) is critical to hydrocarbon oxidation. Motivated by the sparseness of its high-temperature rate constant data and the large uncertainties in the existing literature values, the current study has remeasured the overall rate constant of the OH + CH3 reaction and extended the measurement temperature range to 1214-1933 K, using simultaneous laser absorption diagnostics for OH and CH3 radicals behind incident and reflected shock waves. tert-Butyl hydroperoxide and azomethane were used as pyrolytic sources for the OH and CH3 radicals, respectively. The current study bridged the temperature ranges of existing experimental data, and good agreement is seen between the current measurement and some previous experimental and theoretical high-temperature studies. A recommendation for the rate constant expression of the title reaction, based on the weighted average of the high-temperature data from selected studies, is given by k1 = 4.19 × 10(1)(T/K)(3.15) exp(5270 K/T) cm(3) mol(-1) s(-1) ±30%, which is valid over 1000-2500 K.

  1. Quantitative structure-reactivity relationships of hydroxyl radical rate constants for linear and cyclic volatile methylsiloxanes.

    PubMed

    Kim, Jaeshin; Xu, Shihe

    2017-07-18

    An accurate understanding of the fate of volatile methylsiloxanes (VMS) in air is crucial for determining their persistence and concentrations in the environment. Although oxidation by atmospheric hydroxyl radicals (•OH) is considered as a major degradation mechanism for airborne VMS, the existing bimolecular rate constants with •OH measured and modeled for any given VMS compound varied greatly, depending on the approaches used to generate the data. The objectives of the present study were to measure •OH reaction rate constants for 4 cyclic and 4 linear VMS based on a relative rate method using a newly designed atmospheric chamber and to establish structure-reactivity relationships for the kinetics. In the past, the reaction rate constants for VMS were generally recognized to increase with the number of the methyl groups per molecule, the only differential factor in the existing models. However, the new measurements indicated that molecular structure should also be considered in the prediction of the reaction rates. Better empirical models were developed by simple and multiple linear regressions of the measured values from the present study and the literature. A high correlation existed for the reaction rates with the number of the methyl group attached at 2 distinct siloxane structures (i.e., linear and cyclic VMS). Even better correlations were obtained with one or 2 molecular descriptors that are directly related to the size of VMS, which, in turn, not only depend on the number of methyl groups, but the linear/cyclic structures as well for permethylsiloxanes. Environ Toxicol Chem 2017;9999:1-6. © 2017 SETAC. © 2017 SETAC.

  2. Reaction rates for reaction-diffusion kinetics on unstructured meshes

    NASA Astrophysics Data System (ADS)

    Hellander, Stefan; Petzold, Linda

    2017-02-01

    The reaction-diffusion master equation is a stochastic model often utilized in the study of biochemical reaction networks in living cells. It is applied when the spatial distribution of molecules is important to the dynamics of the system. A viable approach to resolve the complex geometry of cells accurately is to discretize space with an unstructured mesh. Diffusion is modeled as discrete jumps between nodes on the mesh, and the diffusion jump rates can be obtained through a discretization of the diffusion equation on the mesh. Reactions can occur when molecules occupy the same voxel. In this paper, we develop a method for computing accurate reaction rates between molecules occupying the same voxel in an unstructured mesh. For large voxels, these rates are known to be well approximated by the reaction rates derived by Collins and Kimball, but as the mesh is refined, no analytical expression for the rates exists. We reduce the problem of computing accurate reaction rates to a pure preprocessing step, depending only on the mesh and not on the model parameters, and we devise an efficient numerical scheme to estimate them to high accuracy. We show in several numerical examples that as we refine the mesh, the results obtained with the reaction-diffusion master equation approach those of a more fine-grained Smoluchowski particle-tracking model.

  3. pH & Rate of Enzymatic Reactions.

    ERIC Educational Resources Information Center

    Clariana, Roy B.

    1991-01-01

    A quantitative and inexpensive way to measure the rate of enzymatic reaction is provided. The effects of different pH levels on the reaction rate of an enzyme from yeast are investigated and the results graphed. Background information, a list of needed materials, directions for preparing solutions, procedure, and results and discussion are…

  4. pH & Rate of Enzymatic Reactions.

    ERIC Educational Resources Information Center

    Clariana, Roy B.

    1991-01-01

    A quantitative and inexpensive way to measure the rate of enzymatic reaction is provided. The effects of different pH levels on the reaction rate of an enzyme from yeast are investigated and the results graphed. Background information, a list of needed materials, directions for preparing solutions, procedure, and results and discussion are…

  5. Rate constant determinations by laser photolysis/diode laser infrared absorption: examples of HCO+O 2→HO 2+CO and CH 2OH+O 2→HCH(O)+HO 2 reactions at 294 K

    NASA Astrophysics Data System (ADS)

    Hanoune, B.; Dusanter, S.; ElMaimouni, L.; Devolder, P.; Lemoine, B.

    2001-08-01

    The reactions of formyl (HCO) and hydroxymethyl (CH 2OH) radicals with O 2 have been studied at room temperature (294±2 K) using pulsed laser photolysis (PLP)/tunable diode laser (TDL) absorption spectroscopy. The formation of the stable molecular product, CO or HCH(O), has been monitored. The derived values, k 1=(5.0±0.7)×10 -12 cm3 molecule-1 s-1 and k 2=(10±1.4)×10 -12 cm3 molecule-1 s-1, are in excellent agreement with previously published values obtained with different techniques.

  6. Rapid estimation of glucosinolate thermal degradation rate constants in leaves of Chinese kale and broccoli (Brassica oleracea) in two seasons.

    PubMed

    Hennig, Kristin; Verkerk, Ruud; Bonnema, Guusje; Dekker, Matthijs

    2012-08-15

    Kinetic modeling was used as a tool to quantitatively estimate glucosinolate thermal degradation rate constants. Literature shows that thermal degradation rates differ in different vegetables. Well-characterized plant material, leaves of broccoli and Chinese kale plants grown in two seasons, was used in the study. It was shown that a first-order reaction is appropriate to model glucosinolate degradation independent from the season. No difference in degradation rate constants of structurally identical glucosinolates was found between broccoli and Chinese kale leaves when grown in the same season. However, glucosinolate degradation rate constants were highly affected by the season (20-80% increase in spring compared to autumn). These results suggest that differences in glucosinolate degradation rate constants can be due to variation in environmental as well as genetic factors. Furthermore, a methodology to estimate rate constants rapidly is provided to enable the analysis of high sample numbers for future studies.

  7. Micro-mechanism and rate constants for OH-initiated degradation of methomyl in atmosphere.

    PubMed

    Wu, Xiuchao; Sun, Xiaomin; Zhang, Chenxi; Gong, Chen; Hu, Jingtian

    2014-07-01

    The atmospheric degradation reactions of the two isomers of methomyl (MTL) initiated by OH radical in the presence of O2, NO and H2O have been investigated by density functional theory (DFT). The calculations were all carried out at MPWB1K level. The geometrical parameters and vibrational frequencies of stationary points were calculated with 6-31+G (d, p) basis sets. Single-point energy calculations were performed with 6-311+G (3df, 2p) basis sets. Profiles of the potential energy surface were constructed and all possible channels involved in the reactions were discussed. The rate constants of main elementary reactions were calculated over a temperature range of 200-400 K and mostly fitted to Arrhenius formulas. The atmospheric lifetimes of reaction species were discussed for the first time, which can be applied to the study on model simulation and management of hazardous materials.

  8. Determining equilibrium constants for dimerization reactions from molecular dynamics simulations.

    PubMed

    De Jong, Djurre H; Schäfer, Lars V; De Vries, Alex H; Marrink, Siewert J; Berendsen, Herman J C; Grubmüller, Helmut

    2011-07-15

    With today's available computer power, free energy calculations from equilibrium molecular dynamics simulations "via counting" become feasible for an increasing number of reactions. An example is the dimerization reaction of transmembrane alpha-helices. If an extended simulation of the two helices covers sufficiently many dimerization and dissociation events, their binding free energy is readily derived from the fraction of time during which the two helices are observed in dimeric form. Exactly how the correct value for the free energy is to be calculated, however, is unclear, and indeed several different and contradictory approaches have been used. In particular, results obtained via Boltzmann statistics differ from those determined via the law of mass action. Here, we develop a theory that resolves this discrepancy. We show that for simulation systems containing two molecules, the dimerization free energy is given by a formula of the form ΔG ∝ ln(P(1) /P(0) ). Our theory is also applicable to high concentrations that typically have to be used in molecular dynamics simulations to keep the simulation system small, where the textbook dilute approximations fail. It also covers simulations with an arbitrary number of monomers and dimers and provides rigorous error estimates. Comparison with test simulations of a simple Lennard Jones system with various particle numbers as well as with reference free energy values obtained from radial distribution functions show full agreement for both binding free energies and dimerization statistics.

  9. Relative rate constants for the heterogeneous reactions of OH, NO 2 and NO radicals with polycyclic aromatic hydrocarbons adsorbed on carbonaceous particles. Part 1: PAHs adsorbed on 1-2 μm calibrated graphite particles

    NASA Astrophysics Data System (ADS)

    Esteve, Williams; Budzinski, Helene; Villenave, Eric

    2004-11-01

    A discharge flow reactor was used to study the heterogeneous reactions of OH, NO2 and NO radicals with 11 polycyclic aromatic hydrocarbons (PAHs) adsorbed on 1-2 μm calibrated graphite particles. Graphite was chosen as a simple model of carbonaceous atmospheric particles. Relative kinetic information was obtained from the decays of particulate-PAH concentration vs. the time, measured using off-line gas chromatography coupled to mass spectrometry. Particulate-anthracene, -benzo(a)pyrene and -perylene are the most reactive with NO2, whereas all PAHs studied present similar reactivities with OH within uncertainties. The reactivity of NO under our experimental conditions could not be distinguished from the PAH desorption.

  10. Reply to the comment of S. Rayne on "QSAR model reproducibility and applicability: A case study of rate constants of hydroxyl radical reaction models applied to polybrominated diphenyl ethers and (benzo-)triazoles".

    PubMed

    Gramatica, Paola; Kovarich, Simona; Roy, Partha Pratim

    2013-07-30

    We appreciate the interest of Dr. Rayne on our article and we completely agree that the dataset of (benzo-)triazoles, which were screened by the hydroxyl radical reaction quantitative structure-activity relationship (QSAR) model, was not only composed of benzo-triazoles but also included some simpler triazoles (without the condensed benzene ring), such as the chemicals listed by Dr. Rayne, as well as some related heterocycles (also few not aromatic). We want to clarify that in this article (as well as in other articles in which the same dataset was screened), for conciseness, the abbreviations (B)TAZs and BTAZs were used as general (and certainly too simplified) notations meaning an extended dataset of benzo-triazoles, triazoles, and related compounds. Copyright © 2013 Wiley Periodicals, Inc.

  11. Evaluated rate constants for selected HCFC's and HFC's with OH and O((sup)1D)

    NASA Technical Reports Server (NTRS)

    Hampson, Robert F.; Kurylo, Michael J.; Sander, Stanley P.

    1990-01-01

    The chemistry of HCFC's and HFC's in the troposphere is controlled by reactions with OH in which a hydrogen atom is abstracted from the halocarbon to form water and a halo-alkyl radical. The halo-alkyl radical subsequently reacts with molecular oxygen to form a peroxy radical. The reactions of HCFC's and HFC's with O(exp1D) atoms are unimportant in the troposphere, but may be important in producing active chlorine of OH in the stratosphere. Here, the rate constants for the reactions of OH and O(exp1D) with many HFC's and HCFC's are evaluated. Recommendations are given for the five HCFC's and three HFC's specified by AFEAS as primary alternatives as well as for all other isomers of C1 and C2 HCFC's and HFC's where rate data exist. In addition, recommendations are included for CH3CCl3, CH2Cl2, and CH4.

  12. Laser Measurements of the H Atom + Ozone Rate Constant at Mesospheric Temperatures.

    PubMed

    Liu, Yingdi; Peng, Jian; Reppert, Kelsey; Callahan, Sara; Smith, Gregory P

    2016-06-09

    The exothermic H + O3 reaction produces OH(v) Meinel band emissions, used to derive mesospheric H concentrations and chemical heating rates. We remeasured its rate constant to reduce its uncertainty and extended the measurements to lower mesospheric temperatures using modern laser-induced fluorescence (LIF) techniques. H atoms were produced by pulsed ultraviolet laser trace photolysis of O3, followed by reaction of O((1)D) with added H2. A second, delayed, frequency-mixed dye laser measured the reaction decay rate with the remaining ozone using LIF. We monitored either the H atom decay by two photon excitation at 205 nm and detection of red fluorescence, or the OH (v = 9) product time evolution with excitation of the B(2)Σ(+)-X(2)Π (0,9) band at 237 nm and emission in the blue B(2)Σ(+)-A(2)Σ(+) (0,7) band. By cooling the enclosed low pressure flow cell we obtained measurements from 140 to 305 K at 20 to 200 Torr in Ar. Small kinetic modeling corrections were made for secondary regeneration of H atoms. The results are consistent with the current NASA JPL recommendation for this rate constant and establish its extrapolation down to the lower temperatures of the mesosphere.

  13. Laser Measurements of the H Atom + Ozone Rate Constant at Atmospheric Temperatures

    NASA Astrophysics Data System (ADS)

    Liu, Y.; Smith, G. P.; Peng, J.; Reppert, K. J.; Callahan, S. L.

    2015-12-01

    The exothermic H + O3 reaction produces OH(v) Meinel band emissions, used to derive mesospheric H concentrations and chemical heating rates. We have remeasured its rate constant to reduce resulting uncertainties and the measurement extend to lower mesospheric temperatures using modern laser techniques. H atoms are produced by pulsed ultraviolet laser trace photolysis of O3, followed by reaction of O(D) with added H2. A second, delayed, frequency-mixed dye laser measures the reaction decay rate with the remaining ozone by laser induced fluorescence. We monitor either the H atom decay by 2 photon excitation at 205 nm and detection of red fluorescence, or the OH(v=9) product time evolution with excitation of the B-X (0,9) band at 237 nm and emission in blue B-A bands. By cooling the enclosed low pressure flow cell we obtained measurements from 146-305 K. Small kinetic modeling corrections are made for secondary regeneration of H atoms. The results fully confirm the current NASA JPL recommendation for this rate constant, and establish its extrapolation down to the lower temperatures of the mesosphere. This work was supported by the NSF Aeronomy Program and an NSF Physics summer REU student grant.

  14. Isothermal titration calorimetry determination of individual rate constants of trypsin catalytic activity.

    PubMed

    Aguirre, César; Condado-Morales, Itzel; Olguin, Luis F; Costas, Miguel

    2015-06-15

    Determination of individual rate constants for enzyme-catalyzed reactions is central to the understanding of their mechanism of action and is commonly obtained by stopped-flow kinetic experiments. However, most natural substrates either do not fluoresce/absorb or lack a significant change in their spectra while reacting and, therefore, are frequently chemically modified to render adequate molecules for their spectroscopic detection. Here, isothermal titration calorimetry (ITC) was used to obtain Michaelis-Menten plots for the trypsin-catalyzed hydrolysis of several substrates at different temperatures (278-318K): four spectrophotometrically blind lysine and arginine N-free esters, one N-substituted arginine ester, and one amide. A global fitting of these data provided the individual rate constants and activation energies for the acylation and deacylation reactions, and the ratio of the formation and dissociation rates of the enzyme-substrate complex, leading also to the corresponding free energies of activation. The results indicate that for lysine and arginine N-free esters deacylation is the rate-limiting step, but for the N-substituted ester and the amide acylation is the slowest step. It is shown that ITC is able to produce quality kinetic data and is particularly well suited for those enzymatic reactions that cannot be measured by absorption or fluorescence spectroscopy.

  15. On the Initial Rate of Fluid-Solid Reactions

    NASA Astrophysics Data System (ADS)

    Sohn, Hong Yong; Fan, De-Qiu

    2017-02-01

    It is argued in this paper that the initial rate should not be used for the measurement or analysis the kinetics of a fluid-solid reaction, especially for a reaction in which the effect of pore diffusion starts appearing even moderately as the reaction proceeds. Even in the absence of external mass transfer effects, it is shown in this work by rigorous mathematical analysis that the range of conditions where the initial rate represents the intrinsic kinetics is very narrow. For an initially non-porous solid in the absence of external mass transfer effects, the very initial rate should mathematically be the intrinsic rate even when pore diffusion becomes important as the reaction proceeds. However, even in this case, the range of conditions for this statement is very limited. For the reaction of an initially porous solid, the rate at time zero is already affected by pore diffusion unless its effect is negligible over the entire range of conversion. Furthermore, the initial reaction rates of porous solids reacting under large values of k/D e ratio (chemical reactivity is much greater than the capacity for pore diffusion) have an apparent rate constant of √ {k \\cdot D_{e} } and thus pore diffusion alone does not control the initial rate no matter how large the effect of pore diffusion is overall.

  16. [Comparison of propofol and propanidid administered at a constant rate].

    PubMed

    Deschodt, J; Lubrano, J F; Peschaud, J L; Eledjam, J J; du Cailar, J

    1988-01-01

    So as to compare the anaesthesia obtained using propofol with that obtained using propanidid, 40 ASA I patients, aged between 18 and 50 years, who were to undergo elective orthopaedic or plastic surgery lasting more than 60 min, were randomly divided into two equal groups, one receiving propofol (PF) and the other propanidid (PD). All the patients received 0.5 mg atropine, 100 mg pethidine and 7.5 mg droperidol (10 mg if weight greater than 60 kg) intramuscularly 45 min before induction. Patients in group PF were then given 2 mg.kg-1 propofol over 1 min and 0.9 microgram.kg-1 fentanyl over 3 min, followed by a constant rate infusion of 5 mg.kg-1.h-1 propofol and 3 micrograms.kg-1.h-1 fentanyl. For PD patients, the doses of fentanyl were identical; they were given 10.6 mg.kg-1 propanidid over 3 min for induction, and 37 mg.kg-1.h-1 for maintenance. All the patients were intubated and ventilated mechanically. The usual anaesthetic parameters were monitored at induction, during surgery, and during recovery. Consciousness was lost more quickly with propofol (p less than 0.05), but the corneal reflex returned more rapidly in group PD (p less than 0.02). The time required for a full return to normal consciousness was identical in both groups. The fall, during induction, and the increase, during recovery, of Pasys were greater in group PD (p less than 0.05 and less than 0.001 respectively). Padia and heart rate were lower in group PF after the 30th min (p less than 0.05 and less than 0.01 respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

  17. Computational study of the rate constants and free energies of intramolecular radical addition to substituted anilines

    PubMed Central

    Seddiqzai, Meriam; Dahmen, Tobias; Sure, Rebecca

    2013-01-01

    Summary The intramolecular radical addition to aniline derivatives was investigated by DFT calculations. The computational methods were benchmarked by comparing the calculated values of the rate constant for the 5-exo cyclization of the hexenyl radical with the experimental values. The dispersion-corrected PW6B95-D3 functional provided very good results with deviations for the free activation barrier compared to the experimental values of only about 0.5 kcal mol−1 and was therefore employed in further calculations. Corrections for intramolecular London dispersion and solvation effects in the quantum chemical treatment are essential to obtain consistent and accurate theoretical data. For the investigated radical addition reaction it turned out that the polarity of the molecules is important and that a combination of electrophilic radicals with preferably nucleophilic arenes results in the highest rate constants. This is opposite to the Minisci reaction where the radical acts as nucleophile and the arene as electrophile. The substitution at the N-atom of the aniline is crucial. Methyl substitution leads to slower addition than phenyl substitution. Carbamates as substituents are suitable only when the radical center is not too electrophilic. No correlations between free reaction barriers and energies (ΔG ‡ and ΔG R) are found. Addition reactions leading to indanes or dihydrobenzofurans are too slow to be useful synthetically. PMID:24062821

  18. Theoretically predicted rate constants for mercury oxidation by hydrogen chloride in coal combustion flue gases.

    PubMed

    Wilcox, Jennifer; Robles, Joe; Marsden, David C J; Blowers, Paul

    2003-09-15

    In this work, theoretical rate constants are estimated for mercury oxidation reactions by hydrogen chloride that may occur in the flue gases of coal combustion. Rate constants are calculated using transition state theory at the quadratic configuration interaction (QCI) level of theory with single and double excitations, and are compared to results obtained from density functional theory, both including high level pseudopotentials for mercury. Thermodynamic and kinetic data from the literature are used to assess the accuracy of the theoretical calculations when possible. Validation of the chosen methods and basis sets is based upon previous and current research on mercury reactions involving chlorine. The present research shows that the QCISD method with the 1992 Stevens et al. basis set leads to the most accurate kinetic and thermodynamic results for the oxidation of mercury via chlorine containing molecules. Also, a comparison of the heats of reaction data for a series of mercury oxidation reactions reveals that the density functional method, B3LYP, with the 1997 Stuttgart basis set provides reasonably accurate results for these large systems.

  19. Shock Tube Measurement for the Dissociation Rate Constant of Acetaldehyde Using Sensitive CO Diagnostics.

    PubMed

    Wang, Shengkai; Davidson, David F; Hanson, Ronald K

    2016-09-08

    The rate constant of acetaldehyde thermal dissociation, CH3CHO = CH3 + HCO, was measured behind reflected shock waves at temperatures of 1273-1618 K and pressures near 1.6 and 0.34 atm. The current measurement utilized sensitive CO diagnostics to track the dissociation of CH3CHO via oxygen atom balance and inferred the title rate constant (k1) from CO time histories obtained in pyrolysis experiments of 1000 and 50 ppm of CH3CHO/Ar mixtures. By using dilute test mixtures, the current study successfully suppressed the interferences from secondary reactions and directly determined the title rate constant as k1(1.6 atm) = 1.1 × 10(14) exp(-36 700 K/T) s(-1) over 1273-1618 K and k1(0.34 atm) = 5.5 × 10(12) exp(-32 900 K/T) s(-1) over 1377-1571 K, with 2σ uncertainties of approximately ±30% for both expressions. Example simulations of existing reaction mechanisms updated with the current values of k1 demonstrated substantial improvements with regards to the acetaldehyde pyrolysis chemistry.

  20. Surface-active ionic liquids in micellar catalysis: impact of anion selection on reaction rates in nucleophilic substitutions† †Electronic supplementary information (ESI) available: Formulae for calculating aggregation parameters and fitting of kinetic constants and copies of NMR spectra. See DOI: 10.1039/c6cp00493h Click here for additional data file.

    PubMed Central

    Cognigni, Alice; Gaertner, Peter; Zirbs, Ronald; Peterlik, Herwig; Prochazka, Katharina; Schröder, Christian

    2016-01-01

    A series of surface-active ionic liquids based on the 1-dodecyl-3-methylimidazolium cation and different anions such as halides and alkylsulfates was synthesized. The aggregation behavior of these ionic liquids in water was characterized by surface tension, conductivity measurements and UV-Vis spectroscopy in order to determine the critical micelle concentration (CMC) and to provide aggregation parameters. The determination of surface activity and aggregation properties of amphiphilic ionic liquids was accompanied by SAXS studies on selected surface-active ionic liquids. The application of these surface-active ionic liquids with different anions was tested in nucleophilic substitution reactions for the degradation of organophosphorus compounds. Kinetic studies via UV-Vis spectrophotometry showed a strong acceleration of the reaction in the micellar system compared to pure water. In addition, an influence of the anion was observed, resulting in a correlation between the anion binding to the micelle and the reaction rate constants, indicating that the careful choice of the surface-active ionic liquid can considerably affect the outcome of reactions. PMID:27121134

  1. Astrophysical Reaction Rates Obtained By Indirect Techniques

    SciTech Connect

    Tribble, R. E.; Al-Abdullah, T.; Alharbi, A.; Banu, A.; Chen, X.; Clark, H. L.; Fu, C.; Gagliardi, C. A.; Hardy, J. C.; Iacob, V. E.; Lui, Y.-W.; McCleskey, M.; Mukhamedzhanov, A.; Nica, N.; Park, H. I.; Roeder, B.; Simmons, E.; Tabacaru, G.; Tokimoto, Y.; Trache, L.

    2010-08-12

    Indirect techniques have been used to obtain information about reaction rates for several proton capture reactions that occur on short-lived nuclei. The techniques used to carry out the measurements are reviewed and the results obtained are presented. Also future prospects for further measurements with a new facility, T-REX are discussed.

  2. A highly sensitive method for time-resolved detection of O(1D) applied to precise determination of absolute O(1D) reaction rate constants and O(3P) yields.

    PubMed

    Carl, Shaun Avondale

    2005-12-21

    We demonstrate detection, in the gas-phase, of O(1D2) at concentrations down to 10(7) cm(-3) and develop this new method for time-resolved kinetic studies allowing both the total removal rate of O(1D2), of up to 1.5 x 10(6) s(-1), and the fraction quenched to O(3P(J)) by species X, k(q)/k(X), to be determined precisely from a single time profile: at 295 K we find, k(O(1D2) + N2O) = (1.43 +/- 0.08) x 10(-10) cm3 s(-1) with k(q)/k(N2O) = 0.056 +/- 0.009; k(O(1D2) + C2H2) = (3.1 +/- 0.2) x 10(-10) cm3 s(-1) with k(q)/k(C2H2) = 0.020 +/- 0.010; k(q)/k(H2O) < 0.003 for O(1D2) + H2O.

  3. Reaction rates of atmospheric gases with lithium(silicon) alloy

    SciTech Connect

    Searcy, J Q; Reinhardt, F W

    1980-12-01

    Reaction rates of small pellets of Li(Si) alloy with atmospheric gases were studied as a prerequisite to specifying how this alloy should be handled during the production of thermal batteries. The results indicate that Li(Si) reacts with oxygen and nitrogen at ambient conditions too slowly to be of concern, but it reacts very rapidly with water vapor. Rate expressions and constants are developed that allow calculating the weight gain of Li(Si) after its reaction with water vapor, and calculated values are given for three water-vapor concentrations that typify those used in dry rooms where thermal batteries are produced.

  4. Atmospheric Chemistry of 1H-Heptafluorocyclopentene (cyc-CF2CF2CF2CF═CH-): Rate Constant, Products, and Mechanism of Gas-Phase Reactions with OH Radicals, IR Absorption Spectrum, Photochemical Ozone Creation Potential, and Global Warming Potential.

    PubMed

    Liu, Dongpeng; Qin, Sheng; Li, Wei; Zhang, Di; Guo, Zhikai

    2016-12-08

    The rate constant for gas-phase reactions of OH radicals with 1H-heptafluorocyclopentene (cyc-CF2CF2CF2CF═CH-) was measured using a relative rate method at 298 K: (5.20 ± 0.09) × 10(-14) cm(3) molecule(-1) s(-1). The quoted uncertainty includes two standard deviations from the least-squares regression, the systematic error from the GC analysis, and the uncertainties of the rate constants of the reference compounds. The OH-radical-initiated oxidation of cyc-CF2CF2CF2CF═CH- gives the main products COF2, CO, and CO2, leading to negligible environmental impact. For consumptions of cyc-CF2CF2CF2CF═CH- of less than 54%, the yield of the formation of ([COF2] + [CO] + [CO2])/5 (based on the conservation of carbon) was 0.99 ± 0.02, which is very close to 100%. A possible degradation mechanism was proposed. The radiative efficiency (RE) of cyc-CF2CF2CF2CF═CH- measured at room temperature was 0.215 W m(-2) ppb(-1). The atmospheric lifetime of cyc-CF2CF2CF2CF═CH- was calculated as 0.61 year, and the photochemical ozone creation potential (POCP) was negligible. The 20-, 100-, and 500-year time horizon global warming potentials (GWPs) were estimated as 153, 42, and 12, respectively.

  5. Calibrating reaction rates for the CREST model

    NASA Astrophysics Data System (ADS)

    Handley, Caroline A.; Christie, Michael A.

    2017-01-01

    The CREST reactive-burn model uses entropy-dependent reaction rates that, until now, have been manually tuned to fit shock-initiation and detonation data in hydrocode simulations. This paper describes the initial development of an automatic method for calibrating CREST reaction-rate coefficients, using particle swarm optimisation. The automatic method is applied to EDC32, to help develop the first CREST model for this conventional high explosive.

  6. Direct rate constant measurement of radical disulphide anion formation for cysteine and cysteamine in aqueous solution

    NASA Astrophysics Data System (ADS)

    Mezyk, Stephen P.

    1995-03-01

    The techniques of pulse radiolysis, laser photolysis and absorption spectroscopy have been used to directly determine rate constants for radical disulphide anion formation for cysteine and cysteamine in aqueous solution. The measured values for cysteine, over the pH range 7-12, allowed calculation of individual rate constants for the constituent reactions RS . + RSH → RSSR -. + H + and RS . + RS - → RSSR -. as (3.39 ± 0.31) × 10 8 and (1.21 ± 0.04) × 10 9 dm 3 mol -1 s -1, respectively. Analogous values for cysteamine were also determined by this technique as (3.06 ± 0.16) × 10 8 and (3.65 ± 0.07) × 10 9 dm 3 mol -1 s -1.

  7. Shock tube measurements of high temperature rate constants for OH with cycloalkanes and methylcycloalkanes.

    SciTech Connect

    Sivaramakrishnan, R.; Michael, J. V.; Chemical Sciences and Engineering Division

    2009-05-01

    High temperature experiments were performed with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308 nm. The present experiments span a wide T-range, 801-1347 K, and represent the first direct measurements of the title rate constants at T>500 K for cyclopentane and cyclohexane and the only high temperature measurements for the corresponding methyl derivatives. The present work utilized 48 optical passes corresponding to a total path length 4.2 m. As a result of this increased path length, the high [OH] detection sensitivity permitted unambiguous analyses for measuring the title rate constants. The experimental rate constants in units, cm3 molecule-1 s-1, can be expressed in Arrhenius form as k{sub OH+Cyclopentane} = (1.90 {+-} 0.30) x 10{sup -10} exp(-1705 {+-} 156 K/T) (813-1341 K), k{sub OH+Cyclohexane} = (1.86 {+-} 0.24) x 10{sup -10} exp(-1513 {+-} 123 K/T) (801-1347 K), k{sub OH+Methylcyclopentane} = (2.02 {+-} 0.19) x 10{sup -10} exp(-1799 {+-} 96 K/T) (859-1344 K), k{sub OH+Methylcyclohexane} = (2.55 {+-} 0.30) x 10{sup -10} exp(-1824 {+-} 114 K/T) (836-1273 K). These results and lower-T experimental data were used to obtain three parameter evaluations of the experimental rate constants for the title reactions over an even wider T-range. These experimental three parameter fits to the rate constants in units, cm{sup 3} molecule{sup -1} s{sup -1}, are k{sub OH+Cyclopentane} = 1.390 x 10{sup -16}T{sup 1.779} exp(97 K/T) cm{sup 3} molecule{sup -1} s{sup -1} (209-1341 K), k{sub OH+Cyclohexane} = 3.169 x 10{sup -16} T{sup 1.679} exp(119 K/T) cm{sup 3} molecule{sup -1} s{sup -1} (225-1347 K), k{sub OH+Methylcyclopentane} = 6.903 x 10{sup -18}T{sup 2.148} exp(536 K/T) cm{sup 3} molecule{sup -1} s{sup -1} (296-1344 K), k{sub OH+Methylcyclohexane} = 2.341 x 10{sup -18}T{sup 2.325} exp(602 K/T) cm{sup 3} molecule{sup -1} s{sup -1} (296-1273 K). High level electronic structure methods were used to characterize the

  8. Shock tube measurements of high temperature rate constants for OH with cycloalkanes and methylcycloalkanes

    SciTech Connect

    Sivaramakrishnan, R.; Michael, J.V.

    2009-05-15

    High temperature experiments were performed with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308 nm. The present experiments span a wide T-range, 801-1347 K, and represent the first direct measurements of the title rate constants at T>500 K for cyclopentane and cyclohexane and the only high temperature measurements for the corresponding methyl derivatives. The present work utilized 48 optical passes corresponding to a total path length {proportional_to}4.2 m. As a result of this increased path length, the high [OH] detection sensitivity permitted unambiguous analyses for measuring the title rate constants. The experimental rate constants in units, cm{sup 3} molecule{sup -1} s{sup -1}, can be expressed in Arrhenius form as k{sub OH+Cyclopentane}=(1.90{+-}0.30) x 10{sup -10}exp(-1705{+-}56 K/T) (813-1341 K), k{sub OH+Cyclohexane}=(1.86{+-}0.24) x 10{sup -10}exp(-1513{+-}123 K/T) (801-1347 K), k{sub OH+Methylcyclopentane}=(2.02{+-}0.19) x 10{sup -10}exp(-1799{+-}96 K/T) (859-1344 K), k{sub OH+Methylcyclohexane}=(2.55{+-}0.30) x 10{sup -10}exp(-1824{+-}114 K/T) (836-1273 K). These results and lower-T experimental data were used to obtain three parameter evaluations of the experimental rate constants for the title reactions over an even wider T-range. These experimental three parameter fits to the rate constants in units, cm{sup 3} molecule{sup -1} s{sup -1}, are k{sub OH+Cyclopentane}=1.390 x 10{sup -16}T{sup 1.779}exp(97 K/T)cm{sup 3} molecule{sup -1}s{sup -1} (209-1341 K), k{sub OH+Cyclohexane}=3.169 x 10{sup -16}T{sup 1.679}exp(119 K/T)cm{sup 3}molecule{sup -1}s{sup -1} (225-1347 K), k{sub OH+Methylcyclopentane}=6.903 x 10{sup -18}T{sup 2.148}exp(536 K/T)cm{sup 3}molecule{sup -1}s{sup -1} (296-1344 K), k{sub OH+Methylcyclohexane}=2.341 x 10{sup -18}T{sup 2.325}exp(602 K/T)cm{sup 3}molecule{sup -1}s{sup -1} (296-1273 K). High level electronic structure methods were used to characterize the first three

  9. The Hydroxyl Radical Reaction Rate Constant and Products of Cyclohexanol

    DTIC Science & Technology

    2007-10-01

    atmospheric degradation mechanism. The observed products and their formation yields were: cyclohexanone (0.55 0.06), hexanedial (0.32 0.15), 3...COL, tridecane, and decane were obtained from Aldrich with a purity of 99%. Pentanal (99%) and cyclohexanone (99%) were purchased through Ultra... Cyclohexanone (CON) was the only OH COL re- action product observed during the initial kinetic ex- periments. Its presence was also detected later using

  10. Enhanced aqueous photochemical reaction rates after freezing.

    PubMed

    Grannas, Amanda M; Bausch, Alexandra R; Mahanna, Kendell M

    2007-11-01

    Sunlit snow/ice is known to play an important role in the processing of atmospheric species, including photochemical production of NO(x), HONO, molecular halogens, alkyl halides, and carbonyl compounds, among others. It has been shown that a liquid-like (quasi-liquid or disordered) layer exists on the surface of pure ice and that this quasi-liquid layer is also found on the surface of ambient snow crystals and ice at temperatures similar to polar conditions. However, it is unclear what role the liquid-like fractions present in and on frozen water play in potential photochemical reactions, particularly with regard to organic substrates. Here, we report a detailed study of enhanced rates of photochemical nucleophilic substitution of p-nitroanisole (PNA) with pyridine, a well-characterized and commonly used actinometer system. Reaction rates were enhanced by a factor of up to approximately 40 when frozen at temperatures between 236 and 272 K. Reaction rates were dependent on temperature and solute concentration, both variables that control the nature of the liquid-like fraction in frozen water. The results obtained indicate that a major portion of the organic solutes is excluded to the liquid-like layer, significantly impacting the rate of the photochemical nucleophilic substitution reaction studied here. Also, the direct comparison of liquid-phase kinetics to reactions occurring in frozen water systems is drawn into question, indicating that a simple extrapolation of liquid-phase mechanisms to snow/ice may not be valid for certain reactions.

  11. Inverse problem analysis for identification of reaction kinetics constants in microreactors for biodiesel synthesis

    NASA Astrophysics Data System (ADS)

    Pontes, P. C.; Naveira-Cotta, C. P.

    2016-09-01

    The theoretical analysis for the design of microreactors in biodiesel production is a complicated task due to the complex liquid-liquid flow and mass transfer processes, and the transesterification reaction that takes place within these microsystems. Thus, computational simulation is an important tool that aids in understanding the physical-chemical phenomenon and, consequently, in determining the suitable conditions that maximize the conversion of triglycerides during the biodiesel synthesis. A diffusive-convective-reactive coupled nonlinear mathematical model, that governs the mass transfer process during the transesterification reaction in parallel plates microreactors, under isothermal conditions, is here described. A hybrid numerical-analytical solution via the Generalized Integral Transform Technique (GITT) for this partial differential system is developed and the eigenfunction expansions convergence rates are extensively analyzed and illustrated. The heuristic method of Particle Swarm Optimization (PSO) is applied in the inverse analysis of the proposed direct problem, to estimate the reaction kinetics constants, which is a critical step in the design of such microsystems. The results present a good agreement with the limited experimental data in the literature, but indicate that the GITT methodology combined with the PSO approach provide a reliable computational algorithm for direct-inverse analysis in such reactive mass transfer problems.

  12. High-Temperature Slow Crack Growth of Silicon Carbide Determined by Constant-Stress-Rate and Constant-Stress Testing

    NASA Technical Reports Server (NTRS)

    Choi, Sung H.; Salem, J. A.; Nemeth, N. N.

    1998-01-01

    High-temperature slow-crack-growth behaviour of hot-pressed silicon carbide was determined using both constant-stress-rate ("dynamic fatigue") and constant-stress ("static fatigue") testing in flexure at 1300 C in air. Slow crack growth was found to be a governing mechanism associated with failure of the material. Four estimation methods such as the individual data, the Weibull median, the arithmetic mean and the median deviation methods were used to determine the slow crack growth parameters. The four estimation methods were in good agreement for the constant-stress-rate testing with a small variation in the slow-crack-growth parameter, n, ranging from 28 to 36. By contrast, the variation in n between the four estimation methods was significant in the constant-stress testing with a somewhat wide range of n= 16 to 32.

  13. Theoretical Prediction of Rate Constants for Hydrogen Abstraction by OH, H, O, CH3, and HO2 Radicals from Toluene.

    PubMed

    Li, Shu-Hao; Guo, Jun-Jiang; Li, Rui; Wang, Fan; Li, Xiang-Yuan

    2016-05-26

    Hydrogen abstraction from toluene by OH, H, O, CH3, and HO2 radicals are important reactions in oxidation process of toluene. Geometries and corresponding harmonic frequencies of the reactants, transition states as well as products involved in these reactions are determined at the B3LYP/6-31G(2df,p) level. To achieve highly accurate thermochemical data for these stationary points on the potential energy surfaces, the Gaussian-4(G4) composite method was employed. Torsional motions are treated either as free rotors or hindered rotors in calculating partion functions to determine thermodynamic properties. The obtained standard enthalpies of formation for reactants and some prodcuts are shown to be in excellent agreement with experimental data with the largest error of 0.5 kcal mol(-1). The conventional transition state theory (TST) with tunneling effects was adopted to determine rate constants of these hydrogen abstraction reactions based on results from quantum chemistry calculations. To faciliate its application in kinetic modeling, the obtained rate constants are given in Arrhenius expression: k(T) = AT(n) exp(-EaR/T). The obtained reaction rate constants also agree reasonably well with available expermiental data and previous theoretical values. Branching ratios of these reactions have been determined. The present reaction rates for these reactions have been used in a toluene combustion mechanism, and their effects on some combustion properties are demonstrated.

  14. Estimating hydraulic properties of volcanic aquifers using constant-rate and variable-rate aquifer tests

    USGS Publications Warehouse

    Rotzoll, K.; El-Kadi, A. I.; Gingerich, S.B.

    2007-01-01

    In recent years the ground-water demand of the population of the island of Maui, Hawaii, has significantly increased. To ensure prudent management of the ground-water resources, an improved understanding of ground-water flow systems is needed. At present, large-scale estimations of aquifer properties are lacking for Maui. Seven analytical methods using constant-rate and variable-rate withdrawals for single wells provide an estimate of hydraulic conductivity and transmissivity for 103 wells in central Maui. Methods based on constant-rate tests, although not widely used on Maui, offer reasonable estimates. Step-drawdown tests, which are more abundantly used than other tests, provide similar estimates as constant-rate tests. A numerical model validates the suitability of analytical solutions for step-drawdown tests and additionally provides an estimate of storage parameters. The results show that hydraulic conductivity is log-normally distributed and that for dike-free volcanic rocks it ranges over several orders of magnitude from 1 to 2,500 m/d. The arithmetic mean, geometric mean, and median values of hydraulic conductivity are respectively 520, 280, and 370 m/d for basalt and 80, 50, and 30 m/d for sediment. A geostatistical approach using ordinary kriging yields a prediction of hydraulic conductivity on a larger scale. Overall, the results are in agreement with values published for other Hawaiian islands. ?? 2007 American Water Resources Association.

  15. Reaction rates for mesoscopic reaction-diffusion kinetics

    DOE PAGES

    Hellander, Stefan; Hellander, Andreas; Petzold, Linda

    2015-02-23

    The mesoscopic reaction-diffusion master equation (RDME) is a popular modeling framework frequently applied to stochastic reaction-diffusion kinetics in systems biology. The RDME is derived from assumptions about the underlying physical properties of the system, and it may produce unphysical results for models where those assumptions fail. In that case, other more comprehensive models are better suited, such as hard-sphere Brownian dynamics (BD). Although the RDME is a model in its own right, and not inferred from any specific microscale model, it proves useful to attempt to approximate a microscale model by a specific choice of mesoscopic reaction rates. In thismore » paper we derive mesoscopic scale-dependent reaction rates by matching certain statistics of the RDME solution to statistics of the solution of a widely used microscopic BD model: the Smoluchowski model with a Robin boundary condition at the reaction radius of two molecules. We also establish fundamental limits on the range of mesh resolutions for which this approach yields accurate results and show both theoretically and in numerical examples that as we approach the lower fundamental limit, the mesoscopic dynamics approach the microscopic dynamics. Finally, we show that for mesh sizes below the fundamental lower limit, results are less accurate. Thus, the lower limit determines the mesh size for which we obtain the most accurate results.« less

  16. Reaction rates for mesoscopic reaction-diffusion kinetics

    SciTech Connect

    Hellander, Stefan; Hellander, Andreas; Petzold, Linda

    2015-02-23

    The mesoscopic reaction-diffusion master equation (RDME) is a popular modeling framework frequently applied to stochastic reaction-diffusion kinetics in systems biology. The RDME is derived from assumptions about the underlying physical properties of the system, and it may produce unphysical results for models where those assumptions fail. In that case, other more comprehensive models are better suited, such as hard-sphere Brownian dynamics (BD). Although the RDME is a model in its own right, and not inferred from any specific microscale model, it proves useful to attempt to approximate a microscale model by a specific choice of mesoscopic reaction rates. In this paper we derive mesoscopic scale-dependent reaction rates by matching certain statistics of the RDME solution to statistics of the solution of a widely used microscopic BD model: the Smoluchowski model with a Robin boundary condition at the reaction radius of two molecules. We also establish fundamental limits on the range of mesh resolutions for which this approach yields accurate results and show both theoretically and in numerical examples that as we approach the lower fundamental limit, the mesoscopic dynamics approach the microscopic dynamics. Finally, we show that for mesh sizes below the fundamental lower limit, results are less accurate. Thus, the lower limit determines the mesh size for which we obtain the most accurate results.

  17. Atmospheric fate of a series of carbonyl nitrates: photolysis frequencies and OH-oxidation rate constants.

    PubMed

    Suarez-Bertoa, R; Picquet-Varrault, B; Tamas, W; Pangui, E; Doussin, J-F

    2012-11-20

    Multifunctional organic nitrates are potential NO(x) reservoirs whose atmospheric chemistry is somewhat little known. They could play an important role in the spatial distribution of reactive nitrogen species and consequently in ozone formation and distribution in remote areas. In this work, the rate constants for the reaction with OH radical and the photolysis frequencies of α-nitrooxyacetone, 3-nitrooxy-2-butanone, and 3-methyl-3-nitrooxy-2-butanone have been determined at room temperature at 1000 mbar total pressure of synthetic air. The rate constants for the OH oxidation were measured using the relative rate technique, with methanol as reference compound. The following rate constants were obtained for the reaction with OH: k(OH) = (6.7 ± 2.5) × 10(-13) cm(3) molecule(-1) s(-1) for α-nitrooxyacetone, (10.6 ± 4.1) × 10(-13) cm(3) molecule(-1) s(-1) for 3-nitrooxy-2-butanone, and (2.6 ± 0.9) × 10(-13) cm(3) molecule(-1) s(-1) for 3-methyl-3-nitrooxy-2-butanone. The corresponding photolysis frequencies extrapolated to typical atmospheric conditions for July first at noon at 40° latitude North were (4.8 ± 0.3) × 10(-5) s(-1), (5.7 ± 0.3) × 10(-5) s(-1), and (7.4 ± 0.2) × 10(-5) s(-1), respectively. The data show that photolysis is a major atmospheric sink for these organic nitrates.

  18. U-shaped temperature dependence of rate constant of intramolecular photoinduced charge separation in zinc-porphyrin-bridge-quinone compounds.

    PubMed

    Kichigina, Anna O; Ionkin, Vladimir N; Ivanov, Anatoly I

    2013-06-20

    The multichannel stochastic point transition model of photoinduced electron transfer from both a vibrationally unrelaxed and a relaxed states involving the vibrational relaxation in donor-acceptor pairs has been elaborated. The U-shaped temperature dependencies of the rate constants of the intramolecular photoinduced charge separation from both the vibrationally unrelaxed and the relaxed states observed in Zn-porphyrin-bridge-quinone compounds in 2-methyltetrahydrofuran solvent have been reproduced in the framework of the proposed model that accounts for the temperature dependencies of the charge separation free energy gap and the medium reorganization energy. This modeling has allowed uncovering the mechanism of such a variation of the rate constant with the temperature. In the high temperature region, 310-125 K, the charge separation proceeds in the solvent controlled regime and its rate constant decreases with decreasing the temperature mirroring the temperature dependence of the medium relaxation rate. Further lowering the temperature leads to a rise of the reaction free energy gap so that it becomes larger than the medium reorganization energy. In this region the dynamic solvent effect is strongly suppressed and the charge separation rate constant becomes independent from the solvent relaxation rate. Although the medium relaxation rate continues to decrease with decreasing the temperature, the charge separation rate constant starts to rise because the reaction proceeds in the barrierless region.

  19. Comparison of the quantitative performance of constant pressure versus constant flow rate gradient elution separations using concentration-sensitive detectors.

    PubMed

    Verstraeten, M; Broeckhoven, K; Lynen, F; Choikhet, K; Dittmann, M; Witt, K; Sandra, P; Desmet, G

    2012-04-06

    This contribution discusses the difference in chromatographic performance when switching from the customary employed constant flow rate gradient elution mode to the recently re-introduced constant pressure gradient elution mode. In this mode, the inlet pressure is maintained at a set value even when the mobile phase viscosity becomes lower than the maximum mobile phase viscosity encountered during the gradient program. This leads to a higher average flow rate compared to the constant flow rate mode and results in a shorter analysis time. When both modes carry out the same mobile phase gradient program in volumetric units, normally identical selectivities are obtained. However, small deviations in selectivity are found due to the differences in pressure and viscous heating effects. These selectivity differences are of the same type as those observed when switching from HPLC to UHPLC and are inevitable when speeding up the analysis by applying a higher pressure. It was also found that, when using concentration-sensitive detectors, the constant pressure elution mode leads to identical peak areas as the constant flow rate mode. Also the linearity is maintained. In addition, the repeatability of the peak area and retention time remains the same when switching between both elution modes.

  20. A Student Experiment for Measuring Rate Constants of Radical Recombination.

    ERIC Educational Resources Information Center

    Bartle, K. D.; And Others

    1978-01-01

    This experiment is designed specifically for students with little knowledge of radical reactions and provides an introduction to the gas-phase chemistry of radicals. Experimental planning based on the student's own observation is an important component of the experiment. (Author/MA)

  1. A Student Experiment for Measuring Rate Constants of Radical Recombination.

    ERIC Educational Resources Information Center

    Bartle, K. D.; And Others

    1978-01-01

    This experiment is designed specifically for students with little knowledge of radical reactions and provides an introduction to the gas-phase chemistry of radicals. Experimental planning based on the student's own observation is an important component of the experiment. (Author/MA)

  2. Second order rate constants for intramolecular conversions: Application to gas-phase NMR relaxation times

    NASA Astrophysics Data System (ADS)

    Bauer, S. H.; Lazaar, K. I.

    1983-09-01

    The usually quoted expression for the second order rate constant, for a unimolecular reaction at the low pressure limit, is valid only for strictly irreversible processes. Its application to isomerization reactions (which are to some extent reversible) is demonstrably in error; corrected expressions have been published. Attention is directed to intramolecular conversions over low barriers, for which the inappropriateness of the unidirectional expression becomes obvious. For such isomerizations we propose a model which incorporates only operationally observable states, so that an essential conceptual ambiguity is avoided. Use of this model is illustrated for the syn⇄anti conversions of methyl nitrite, derived from a gas phase NMR coalescence curve (Mc:Tc). The present data suggest that during isomerization the alkyl nitrites may not be completely ergodic on a time scale of 10-9 s. A regional phase-space model is proposed which has the appropriate formalism to account for this behavior.

  3. Estimation of rate constants of elementary processes - A review of the state of the art.

    NASA Technical Reports Server (NTRS)

    Golden, D. M.

    1973-01-01

    'Thermochemical kinetics,' the codification and extrapolation of empirical observations, as applied to certain elementary reactions of importance to combustion studies, is described. This approach allows the critical scrutiny of experimental data in areas where sufficient previous data exist, while, at the same time, illuminating those key areas where more experimentation is crucial. It is shown that combination of transition-state theory with an understanding of the molecular basis of entropy puts fairly rigid constraints on the values of the Arrhenius A-factor for most reactions. This, in turn, means that the activation energy is often the key datum that is missing, and that such data can be obtained with some degree of confidence, even from measurements of rate constants at only one temperature. In complex mechanisms, it is often possible to distinguish among alternate pathways and pinpoint key processes.

  4. Atmospheric chemistry of toxic contaminants. 1. Reaction rates and atmospheric persistence

    SciTech Connect

    Grosjean, D. )

    1990-10-01

    Using structure-reactivity relationships between reaction rate constants and ionization potentials for structural homologues, estimates are presented for the rate constants of the reactions of ozone, the hydroxyl radical, and the nitrate radical with forty toxic air contaminants for which no or little data are available. These rate constants are in turn used to estimate the atmospheric persistence of saturated aliphatics, unsaturated aliphatics, and aromatic toxic organics. The corresponding atmospheric half-lives for removal by chemical reactions range from a few hours for the most reactive toxics (chloroprene, hexachlorocyclo-pentadiene, cresols, nitrosamines, maleic anhydride) to several months for the least reactive compounds (nitrobenzene, methyl bromide, phosgene).

  5. Reaction rates for a generalized reaction-diffusion master equation

    DOE PAGES

    Hellander, Stefan; Petzold, Linda

    2016-01-19

    It has been established that there is an inherent limit to the accuracy of the reaction-diffusion master equation. Specifically, there exists a fundamental lower bound on the mesh size, below which the accuracy deteriorates as the mesh is refined further. In this paper we extend the standard reaction-diffusion master equation to allow molecules occupying neighboring voxels to react, in contrast to the traditional approach in which molecules react only when occupying the same voxel. We derive reaction rates, in two dimensions as well as three dimensions, to obtain an optimal match to the more fine-grained Smoluchowski model, and show inmore » two numerical examples that the extended algorithm is accurate for a wide range of mesh sizes, allowing us to simulate systems that are intractable with the standard reaction-diffusion master equation. In addition, we show that for mesh sizes above the fundamental lower limit of the standard algorithm, the generalized algorithm reduces to the standard algorithm. We derive a lower limit for the generalized algorithm which, in both two dimensions and three dimensions, is on the order of the reaction radius of a reacting pair of molecules.« less

  6. Reaction rates for a generalized reaction-diffusion master equation.

    PubMed

    Hellander, Stefan; Petzold, Linda

    2016-01-01

    It has been established that there is an inherent limit to the accuracy of the reaction-diffusion master equation. Specifically, there exists a fundamental lower bound on the mesh size, below which the accuracy deteriorates as the mesh is refined further. In this paper we extend the standard reaction-diffusion master equation to allow molecules occupying neighboring voxels to react, in contrast to the traditional approach, in which molecules react only when occupying the same voxel. We derive reaction rates, in two dimensions as well as three dimensions, to obtain an optimal match to the more fine-grained Smoluchowski model and show in two numerical examples that the extended algorithm is accurate for a wide range of mesh sizes, allowing us to simulate systems that are intractable with the standard reaction-diffusion master equation. In addition, we show that for mesh sizes above the fundamental lower limit of the standard algorithm, the generalized algorithm reduces to the standard algorithm. We derive a lower limit for the generalized algorithm which, in both two dimensions and three dimensions, is of the order of the reaction radius of a reacting pair of molecules.

  7. Reaction rates for a generalized reaction-diffusion master equation

    SciTech Connect

    Hellander, Stefan; Petzold, Linda

    2016-01-19

    It has been established that there is an inherent limit to the accuracy of the reaction-diffusion master equation. Specifically, there exists a fundamental lower bound on the mesh size, below which the accuracy deteriorates as the mesh is refined further. In this paper we extend the standard reaction-diffusion master equation to allow molecules occupying neighboring voxels to react, in contrast to the traditional approach in which molecules react only when occupying the same voxel. We derive reaction rates, in two dimensions as well as three dimensions, to obtain an optimal match to the more fine-grained Smoluchowski model, and show in two numerical examples that the extended algorithm is accurate for a wide range of mesh sizes, allowing us to simulate systems that are intractable with the standard reaction-diffusion master equation. In addition, we show that for mesh sizes above the fundamental lower limit of the standard algorithm, the generalized algorithm reduces to the standard algorithm. We derive a lower limit for the generalized algorithm which, in both two dimensions and three dimensions, is on the order of the reaction radius of a reacting pair of molecules.

  8. Determination of Rate Constants and Equilibrium Constants for Solution-Phase Drug–Protein Interactions by Ultrafast Affinity Extraction

    PubMed Central

    2015-01-01

    A method was created on the basis of ultrafast affinity extraction to determine both the dissociation rate constants and equilibrium constants for drug–protein interactions in solution. Human serum albumin (HSA), an important binding agent for many drugs in blood, was used as both a model soluble protein and as an immobilized binding agent in affinity microcolumns for the analysis of free drug fractions. Several drugs were examined that are known to bind to HSA. Various conditions to optimize in the use of ultrafast affinity extraction for equilibrium and kinetic studies were considered, and several approaches for these measurements were examined. The dissociation rate constants obtained for soluble HSA with each drug gave good agreement with previous rate constants reported for the same drugs or other solutes with comparable affinities for HSA. The equilibrium constants that were determined also showed good agreement with the literature. The results demonstrated that ultrafast affinity extraction could be used as a rapid approach to provide information on both the kinetics and thermodynamics of a drug–protein interaction in solution. This approach could be extended to other systems and should be valuable for high-throughput drug screening or biointeraction studies. PMID:24911267

  9. Determination of rate constants and equilibrium constants for solution-phase drug-protein interactions by ultrafast affinity extraction.

    PubMed

    Zheng, Xiwei; Li, Zhao; Podariu, Maria I; Hage, David S

    2014-07-01

    A method was created on the basis of ultrafast affinity extraction to determine both the dissociation rate constants and equilibrium constants for drug-protein interactions in solution. Human serum albumin (HSA), an important binding agent for many drugs in blood, was used as both a model soluble protein and as an immobilized binding agent in affinity microcolumns for the analysis of free drug fractions. Several drugs were examined that are known to bind to HSA. Various conditions to optimize in the use of ultrafast affinity extraction for equilibrium and kinetic studies were considered, and several approaches for these measurements were examined. The dissociation rate constants obtained for soluble HSA with each drug gave good agreement with previous rate constants reported for the same drugs or other solutes with comparable affinities for HSA. The equilibrium constants that were determined also showed good agreement with the literature. The results demonstrated that ultrafast affinity extraction could be used as a rapid approach to provide information on both the kinetics and thermodynamics of a drug-protein interaction in solution. This approach could be extended to other systems and should be valuable for high-throughput drug screening or biointeraction studies.

  10. Rate Constants of Hydroperoxyl Radical Addition to Cyclic Nitrones: A DFT Study

    PubMed Central

    Villamena, Frederick A.; Merle, John K.; Hadad, Christopher M.; Zweier*, Jay L.

    2008-01-01

    Nitrones are potential synthetic antioxidants against the reduction of radical-mediated oxidative damage in cells, and as analytical reagent for the identification of HO2• and other such transient species. In this work, the PCM/B3LYP/6−31+G(d,p)//B3LYP/6−31G(d) and PCM/mPW1K/6−31+G(d,p) density functional theory (DFT) methods were employed to predict the reactivity of HO2• with various functionalized nitrones as spin traps. The calculated second-order rate constants and free energies of reaction at both levels of theory were in the range of 100−103 M−1 s−1 and 1 to −12 kcal mol−1, respectively, and the rate constants for some nitrones are on the same order of magnitude as those observed experimentally. The trend in HO2• reactivity to nitrones could not be explained solely on the basis of the relationship of the theoretical positive charge densities on the nitronyl-C, with their respective ionization potentials, electron affinities, rate constants, or free energies of reaction. However, various modes of intramolecular H-bonding interaction were observed at the transition state (TS) structures of HO2• addition to nitrones. The presence of intramolecular H-bonding interactions in the transition states were predicted and may play a significant role towards a facile addition of HO2• to nitrones. In general, HO2• addition to ethoxycarbonyl- and spirolactam- substituted nitrones, as well as those nitrones without electron-withdrawing substituents, such as 5,5-dimethyl-pyrroline N-oxide (DMPO) and 5-spirocyclopentyl-pyrroline N-oxide (CPPO), are most preferred compared to the methylcarbamoyl-substituted nitrones. This study suggests that the use of specific spin traps for efficient trapping of HO2• could pave the way toward improved radical detection and antioxidant protection. PMID:17845014

  11. Reduction of iron-oxide-carbon composites: part I. Estimation of the rate constants

    SciTech Connect

    Halder, S.; Fruehan, R.J.

    2008-12-15

    A new ironmaking concept using iron-oxide-carbon composite pellets has been proposed, which involves the combination of a rotary hearth furnace (RHF) and an iron bath smelter. This part of the research focuses on studying the two primary chemical kinetic steps. Efforts have been made to experimentally measure the kinetics of the carbon gasification by CO{sub 2} and wustite reduction by CO by isolating them from the influence of heat- and mass-transport steps. A combined reaction model was used to interpret the experimental data and determine the rate constants. Results showed that the reduction is likely to be influenced by the chemical kinetics of both carbon oxidation and wustite reduction at the temperatures of interest. Devolatilized wood-charcoal was observed to be a far more reactive form of carbon in comparison to coal-char. Sintering of the iron-oxide at the high temperatures of interest was found to exert a considerable influence on the reactivity of wustite by virtue of altering the internal pore surface area available for the reaction. Sintering was found to be predominant for highly porous oxides and less of an influence on the denser ores. It was found using an indirect measurement technique that the rate constants for wustite reduction were higher for the porous iron-oxide than dense hematite ore at higher temperatures (> 1423 K). Such an indirect mode of measurement was used to minimize the influence of sintering of the porous oxide at these temperatures.

  12. Decomposition of phenylarsonic acid by AOP processes: degradation rate constants and by-products.

    PubMed

    Jaworek, K; Czaplicka, M; Bratek, Ł

    2014-10-01

    The paper presents results of the studies photodegradation, photooxidation, and oxidation of phenylarsonic acid (PAA) in aquatic solution. The water solutions, which consist of 2.7 g dm(-3) phenylarsonic acid, were subjected to advance oxidation process (AOP) in UV, UV/H2O2, UV/O3, H2O2, and O3 systems under two pH conditions. Kinetic rate constants and half-life of phenylarsonic acid decomposition reaction are presented. The results from the study indicate that at pH 2 and 7, PAA degradation processes takes place in accordance with the pseudo first order kinetic reaction. The highest rate constants (10.45 × 10(-3) and 20.12 × 10(-3)) and degradation efficiencies at pH 2 and 7 were obtained at UV/O3 processes. In solution, after processes, benzene, phenol, acetophenone, o-hydroxybiphenyl, p-hydroxybiphenyl, benzoic acid, benzaldehyde, and biphenyl were identified.

  13. Reaction rates between water and the Karl Fischer reagent.

    PubMed

    Cedergren, A

    1974-04-01

    Reaction rates between water and the Karl Fischer reagent have been determined by potentiometric measurement for various compositions of the Karl Fischer reagent. The study has been made with an iodine complex concentration of 0.3-1.2 mM and sulphur dioxide complex at 0.01-0.5M. The concentration of excess of pyridine had no measurable effect on the rate of the main reaction. The reaction was found to be first-order with respect to iodine complex, to sulphur dioxide complex, and to water. The rate constant was (1.2+/-0.2) x 10(3) 1(2). mole(-2). sec(-1). In an ordinary titration it is therefore essential to keep the sulphur dioxide concentration high for the reaction to go to completion within a reasonable time. The extent of side-reactions was found to be independent of the iodine concentration at low concentrations. The side-reactions increased somewhat with increasing sulphur dioxide pyridine concentrations and decreased to about 60% when the temperature was lowered from 24 degrees to 7 degrees.

  14. Constant rate control algorithm for Wyner-Ziv video codec

    NASA Astrophysics Data System (ADS)

    Jakubowski, Mariusz

    2009-06-01

    In a distributed video coding (DVC) system, the total bit-rate depends on bit-rate of the key frames (Intra frames) and the Wyner-Ziv (WZ) frames. The key frames bit-rate is relatively easy to control since they are encoded with an Intra coding scheme (e.g. H.264/AVC Intra), and there are many proposed solutions in literature which address this issue1, 2. On the other hand, rate control (RC) of the WZ frames at the encoder is more difficult since the bit-rate of WZ frames is difficult to predict and control due to the absence of the side information at the decoder side. In this work, an RC algorithm developed within the VISNET II, the European Network of Excellence, is presented as an efficient solution to achieve and maintain the target bit-rate for the overall Intra frames and WZ bitstream, mainly by changing the degree of compression of the Intra frames which is controlled by quantization parameter (QP). In order to maintain a similar quality for the Intra and WZ frames, the WZ quantization index (QIndex) follows the QP changes. A statistical model is used to describe the relationship between QIndex and the WZ frames bit-rate. Additionally, an analysis of influence of the key frames residuum complexity on WZ frames bitrate was conducted. The proposed algorithm adapted to the VISNET2 WZ video codec3 confirms its efficiency in terms of achieving and maintaining the target bit-rate.

  15. Biodegradation testing of chemicals with high Henry's constants - Separating mass and effective concentration reveals higher rate constants.

    PubMed

    Birch, Heidi; Andersen, Henrik R; Comber, Mike; Mayer, Philipp

    2017-05-01

    During simulation-type biodegradation tests, volatile chemicals will continuously partition between water phase and headspace. This study addressed how (1) this partitioning affects test results and (2) can be accounted for by combining equilibrium partition and dynamic biodegradation models. An aqueous mixture of 9 (semi)volatile chemicals was first generated using passive dosing and then diluted with environmental surface water producing concentrations in the ng/L to μg/L range. After incubation for 2 h to 4 weeks, automated Headspace Solid Phase Microextraction (HS-SPME) was applied directly on the test systems to measure substrate depletion by biodegradation relatively to abiotic controls. HS-SPME was also applied to determine air to water partitioning ratios. Biodegradation rate constants relating to the chemical in the water phase, kwater, were generally a factor 1 to 11 times higher than biodegradation rate constants relating to the total mass of chemical in the test system, ksystem, with one exceptional factor of 72 times for a long chain alkane. True water phase degradation rate constants were found (i) more appropriate for risk assessment than test system rate constants, (ii) to facilitate extrapolation to other air-water systems and (iii) to be better defined input parameters for aquatic exposure and fate models.

  16. Determination of the rate constants for the NH2(X2B1) + NH2(X2B1) and NH2(X2B1) + H Recombination reactions with collision partners CH4, C2H6, CO2, CF4, and SF6 at low pressures and 296 K. Part 2.

    PubMed

    Altinay, Gokhan; Macdonald, R Glen

    2012-03-08

    The recombination rate constants for the reactions NH2(X2B1) + NH2(X2B1) + M → N2H4 + M and NH2(X2B1) + H + M → NH3 + M, where M was CH4, C2H6, CO2, CF4, or SF6, were measured in the same experiment over presseure ranges of 1-20 and 7-20 Torr, respectively, at 296 ± 2 K. The NH2 radical was produced by the 193 nm laser photolysis of NH3. Both NH2 and NH3 were monitored simultaneously following the photolysis laser pulse. High-resolution time-resolved absorption spectroscopy was used to monitor the temporal dependence of both species: NH2 on the (1)2(21) ← (1)3(31) rotational transition of the (0,7,0)A2A1 ← (0,0,0)X2B1 electronic transition near 675 nm and NH3 in the IR on either of the inversion doublets of the qQ3(3) rotational transition of the ν1 fundamental near 2999 nm. The NH2 self-recombination clearly exhibited falloff behavior for the third-body collision partners used in this work. The pressure dependences of the NH2 self-recombination rate constants were fit using Troe’s parametrization scheme, k(inf), k(0), and F(cent), with k(inf) = 7.9 × 10(-11) cm3 molecule(-1) s(-1), the theoretical value calculated by Klippenstein et al. (J. Phys. Chem. A113, 113, 10241). The individual Troe parameters were CH4, k(0)(CH4) = 9.4 × 10(-29) and F(cent)(CH4) = 0.61; C2H6, k(0)(C2H6) = 1.5 × 10(-28) and F(cent)(C2H6) = 0.80; CO2, k(0)(CO2) = 8.6 × 10(-29) and F(cent)(CO2) = 0.66; CF4, k(0)(CF4) = 1.1 × 10(-28) and F(cent)(CF4) = 0.55; and SF6, k(0)(SF6) = 1.9 × 10(-28) and F(cent)(SF6) = 0.52, where the units of k0 are cm6 molecule(-2) s(-1). The NH2 + H + M reaction rate constant was assumed to be in the three-body pressure regime, and the association rate constants were CH4, (6.0 ± 1.8) × 10(-30); C2H6, (1.1 ± 0.41) × 10(-29); CO2, (6.5 ± 1.8) × 10(-30); CF4, (8.3 ± 1.7) × 10(-30); and SF6, (1.4 ± 0.30) × 10(-29), with units cm6 molecule(-1) s,(-1) and the systematic and experimental errors are given at the 2σ confidence level.

  17. Iterative rate-distortion optimization of H.264 with constant bit rate constraint.

    PubMed

    An, Cheolhong; Nguyen, Truong Q

    2008-09-01

    In this paper, we apply the primal-dual decomposition and subgradient projection methods to solve the rate-distortion optimization problem with the constant bit rate constraint. The primal decomposition method enables spatial or temporal prediction dependency within a group of picture (GOP) to be processed in the master primal problem. As a result, we can apply the dual decomposition to minimize independently the Lagrangian cost of all the MBs using the reference software model of H.264. Furthermore, the optimal Lagrange multiplier lambda* is iteratively derived from the solution of the dual problem. As an example, we derive the optimal bit allocation condition with the consideration of temporal prediction dependency among the pictures. Experimental results show that the proposed method achieves better performance than the reference software model of H.264 with rate control.

  18. Thermodynamical vibronic coupling constant and density: Chemical potential and vibronic coupling in reactions

    NASA Astrophysics Data System (ADS)

    Sato, Tohru; Haruta, Naoki; Tanaka, Kazuyoshi

    2016-05-01

    Vibronic coupling constant (VCC) and density (VCD) defined for a pure state, which have been successfully applied for reactions of fullerenes and nanographenes as reactivity indices, are extended for a mixed state. The extended VCC and VCD, thermodynamical vibronic coupling constant (ThVCC) and density (ThVCD), are formulated in the finite-temperature grand-canonical ensemble. ThVCD can be applied for charge transfer of a fractional number of electron. Based on the total differential of chemical potential, the relationship between chemical potential, absolute hardness, and vibronic coupling in a bimolecular reaction is discussed.

  19. Beta-lactamases as fully efficient enzymes. Determination of all the rate constants in the acyl-enzyme mechanism.

    PubMed

    Christensen, H; Martin, M T; Waley, S G

    1990-03-15

    The rate constants for both acylation and deacylation of beta-lactamase PC1 from Staphylococcus aureus and the RTEM beta-lactamase from Escherichia coli were determined by the acid-quench method [Martin & Waley (1988) Biochem. J. 254, 923-925] with several good substrates, and, for a wider range of substrates, of beta-lactamase I from Bacillus cereus. The values of the acylation and deacylation rate constants for benzylpenicillin were approximately the same (i.e. differing by no more than 2-fold) for each enzyme. The variation of kcat./Km for benzylpenicillin with the viscosity of the medium was used to obtain values for all four rate constants in the acyl-enzyme mechanism for all three enzymes. The reaction is partly diffusion-controlled, and the rate constant for the dissociation of the enzyme-substrate complex has approximately the same value as the rate constants for acylation and deacylation. Thus all three first-order rate constants have comparable values. Here there is no single rate-determining step for beta-lactamase action. This is taken to be a sign of a fully efficient enzyme.

  20. The temperature-dependence of elementary reaction rates: beyond Arrhenius.

    PubMed

    Smith, Ian W M

    2008-04-01

    The rates of chemical reactions and the dependence of their rate constants on temperature are of central importance in chemistry. Advances in the temperature-range and accuracy of kinetic measurements, principally inspired by the need to provide data for models of combustion, atmospheric, and astrophysical chemistry, show up the inadequacy of the venerable Arrhenius equation--at least, over wide ranges of temperature. This critical review will address the question of how to reach an understanding of the factors that control the rates of 'non-Arrhenius' reactions. It makes use of a number of recent kinetic measurements and shows how developments in advanced forms of transition state theory provide satisfactory explanations of complex kinetic behaviour (72 references).

  1. Ab-Initio Based Computation of Rate Constants of Spin Forbidden Transitions in (Bio)inorganic Complexes and Metalloproteins

    NASA Astrophysics Data System (ADS)

    Ozkanlar, Abdullah; Rodriguez, Jorge H.

    2009-03-01

    Some (bio)chemical reactions are non-adiabatic processes whereby the total spin angular momentum, before and after the reaction, is not conserved. These are named spin- forbidden reactions. The application of spin density functional theory (SDFT) to the prediction of rate constants is a challenging task of fundamental and practical importance. We apply non-adiabatic transition state theory in conjunction with SDFT to predict the rate constant of the spin- forbidden dihydrogen binding to iron tetracarbonyl. To model the surface hopping probability between singlet and triplet states, the Landau-Zener formalism is used. The lowest energy point for singlet-triplet crossing, known as minimum energy crossing point (MECP), was located and used to compute, in a semi-quantum approach, reaction rate constants at 300 K. The predicted rates are in good agreement with experiment. In addition, we present results which are relevant to the ligand binding reactions of metalloproteins. This work is supported in part by NSF via CAREER award CHE-0349189 (JHR).

  2. Rate constants for the quenching of metastable O2 (1Sigma g +) molecules

    NASA Technical Reports Server (NTRS)

    Kwang, Y. C.; Leu, M.-T.

    1985-01-01

    The O2 (1Sigma g +) rates for CO2, H2, N2, Cl2, CO, O3, and 2,3 DMB-2 are determined by monitoring the 762-nm emission in a fast-flow-discharge chemiluminescence detection system (Leu, 1984; Leu and Smith, 1981). The results are presented in tables and graphs and briefly characterized. The rate constants (in cu cm/s x 10 to the -16th) are 4600 + or - 500 for CO2, 7000 + or - 300 for H2, 17 + or - 1 for N2, 4.5 + or - 0.8 for Cl2, 45 + or - 5 for CO, 220,000 + or - 30,000 for O3, and 6000 + or - 100 for 2,3 DMB-2. The temperature dependence of the CO2 and O3 quenching reactions at 245-362 K is found to be negligible.

  3. Thermonuclear Reaction Rate Parameterization for Nuclear Astrophysics

    NASA Astrophysics Data System (ADS)

    Sharp, Jacob; Kozub, Raymond L.; Smith, Michael S.; Scott, Jason; Lingerfelt, Eric

    2004-10-01

    The knowledge of thermonuclear reaction rates is vital to simulate novae, supernovae, X-ray bursts, and other astrophysical events. To facilitate dissemination of this knowledge, a set of tools has been created for managing reaction rates, located at www.nucastrodata.org. One tool is a rate parameterizer, which provides a parameterization for nuclear reaction rate vs. temperature values in the most widely used functional form. Currently, the parameterizer uses the Levenberg-Marquardt method (LMM), which requires an initial estimate of the best-fit parameters. The initial estimate is currently provided randomly from a preselected pool. To improve the quality of fits, a new, active method of selecting parameters has been developed. The parameters of each set in the pool are altered for a few iterations to replicate the input data as closely as possible. Then, the set which most nearly matches the input data (based on chi squared) is used in the LMM as the initial estimate for the final fitting procedure. A description of the new, active algorithm and its performance will be presented. Supported by the U. S. Department of Energy.

  4. Inversion of Spectral Lineshapes to Yield Collision Rate Constants.

    DTIC Science & Technology

    1981-07-15

    Rates for Highly Vibrationally Excited Molecules", J. Chem. Phys. 74, 5031 (1981). 4. E. Wilczek, J. BelBruno and J. Gelfand, " Voigt Profiles of Spectral ... Lines : Accuracy of Line Parameters as a Function of Peak Transmittance", Applied Spectroscopy , in press (1981). 5. J. BelBruno, M. Zughul, J. Gelfand...and H.Rabitz, "Analysis of Collision- Broadened and Overlapping Spectral Lines to Obtain Individual Line Param- eters", J. Mol. Spec., in press (1981

  5. Two-temperature reaction and relaxation rates

    NASA Astrophysics Data System (ADS)

    Kolesnichenko, E.; Gorbachev, Yu.

    2017-05-01

    Within the method of solving the kinetic equations for gas mixtures with internal degrees of freedom developed by the authors and based on the approximate summational invariants (ASI) concept, gas-dynamic equations for a multi-temperature model for the spatially inhomogeneous case are derived. For the two-temperature case, the expressions for the non-equilibrium reaction and relaxation rates are obtained. Special attention is drawn to corresponding thermodynamic equations. Different possibilities of introducing the gas-dynamic variables related to the internal degrees of freedom are considered. One is based on the choice of quantum numbers as the ASI, while the other is based on the choice of internal (vibrational) energy as the ASI. Limits to a one-temperature situation are considered in all the cases. For the cutoff harmonic oscillator model, explicit expressions for the reaction and relaxation rates are derived.

  6. Rate of reaction of OH with CS/sub 2/

    SciTech Connect

    Wine, P.H.; Shah, R.C.; Ravishankara, A.R.

    1980-10-02

    The flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction OH + CS/sub 2/ ..-->.. products (k/sub 1/) over the temperature range 251-363 K. Complicating secondary reactions involving CS/sub 2/ photofragments were eliminated only when the photoflash was filtered by 10 torr cm of CS/sub 2/ and SF/sub 6/ was used as the buffer gas. The rate constant was found to be much slower than previous measurements indicated. Based on our experiments, upper limits for k/sub 1/ (in units of 10/sup -15/ cm/sup 3/ molecule/sup -1/s/sup -1/) are 9.9 at 251 K, 1.5 at 297 K, and 1.6 at 363 K. Our results suggest that the title reaction is unimportant as a source for COS in the atmosphere.

  7. Thermodynamic limitations on microbially catalyzed reaction rates

    NASA Astrophysics Data System (ADS)

    LaRowe, Douglas E.; Dale, Andrew W.; Amend, Jan P.; Van Cappellen, Philippe

    2012-08-01

    Quantification of global biogeochemical cycles requires knowledge of the rates at which microorganisms catalyze chemical reactions. In order for models that describe these processes to capture global patterns of change, the underlying formulations in them must account for biogeochemical transformations over seasonal and millennial time scales in environments characterized by different energy levels. Building on existing models, a new thermodynamic limiting function is introduced. With only one adjustable parameter, this function that can be used to model microbial metabolism throughout the range of conditions in which organisms are known to be active. The formulation is based on a comparison of the amount of energy available from any redox reaction to the energy required to maintain a membrane potential, a proxy for the minimum amount of energy required by an active microorganism. This function does not require species- or metabolism-specific parameters, and can be used to model metabolisms that capture any amount of energy. The utility of this new thermodynamic rate limiting term is illustrated by applying it to three low-energy processes: fermentation, methanogenesis and sulfate reduction. The model predicts that the rate of fermentation will be reduced by half once the Gibbs energy of the catalyzed reaction reaches -12 kJ (mol e-)-1, and then slowing exponentially until the energy yield approaches zero. Similarly, the new model predicts that the low energy yield of methanogenesis, -4 to -0.5 kJ (mol e-)-1, for a partial pressure of H2 between 11 and 0.6 Pa decreases the reaction rate by 95-99%. Finally, the new function's utility is illustrated through its ability to accurately model sulfate concentration data in an anoxic marine sediment.

  8. Characteristic thermal constant and dimensionless heating rate. The links to optimum heating rate in GC

    PubMed

    Blumberg; Klee

    2000-09-01

    An initial step in the quest of deriving a generalized approach to optimization of a temperature program in gas chromatography is presented. Central to this is the introduction of a dimensionless heating rate, r. As a first step to defining r, a characteristic thermal constant, thetachar, defined as thetachar = -dT/dk at k = 1, where T and k are, respectively, column temperature and solute retention factor, is introduced and evaluated for our own experimental data and for thermodynamic data from the literature. It was determined that, for silicone stationary phases with a phase ratio of 250, thetachar ranged from about 23 degrees C for low molecular weight hydrocarbons such as dimethylpropane to about 45 degrees C for high molecular weight pesticides such as mirex. It was also found that, for a particular solute and a stationary phase type, a 2 orders of magnitude increase in the film thickness caused only about a 2-fold increase in the characteristic thermal constant. Using thetachar as a fundamental temperature unit in GC and void time as a fundamental time unit, a dimensionless heating rate is introduced and its potential utility for the evaluation of the separation-speed tradeoffs in a temperature-programmed GC is demonstrated.

  9. Pycnonuclear reaction rates for binary ionic mixtures

    NASA Technical Reports Server (NTRS)

    Ichimaru, S.; Ogata, S.; Van Horn, H. M.

    1992-01-01

    Through a combination of compositional scaling arguments and examinations of Monte Carlo simulation results for the interparticle separations in binary-ionic mixture (BIM) solids, we have derived parameterized expressions for the BIM pycnonuclear rates as generalizations of those in one-component solids obtained previously by Salpeter and Van Horn and by Ogata et al. We have thereby discovered a catalyzing effect of the heavier elements, which enhances the rates of reactions among the lighter elements when the charge ratio exceeds a critical value of approximately 2.3.

  10. Pycnonuclear reaction rates for binary ionic mixtures

    NASA Technical Reports Server (NTRS)

    Ichimaru, S.; Ogata, S.; Van Horn, H. M.

    1992-01-01

    Through a combination of compositional scaling arguments and examinations of Monte Carlo simulation results for the interparticle separations in binary-ionic mixture (BIM) solids, we have derived parameterized expressions for the BIM pycnonuclear rates as generalizations of those in one-component solids obtained previously by Salpeter and Van Horn and by Ogata et al. We have thereby discovered a catalyzing effect of the heavier elements, which enhances the rates of reactions among the lighter elements when the charge ratio exceeds a critical value of approximately 2.3.

  11. Reaction Rate Parameterization for Nuclear Astrophysics Research

    NASA Astrophysics Data System (ADS)

    Scott, J. P.; Lingerfelt, E. J.; Smith, M. S.; Hix, W. R.; Bardayan, D. W.; Sharp, J. E.; Kozub, R. L.; Meyer, R. A.

    2004-11-01

    Libraries of thermonuclear reaction rates are used in element synthesis models of a wide variety of astrophysical phenomena, such as exploding stars and the inner workings of our sun. These computationally demanding models are more efficient when libraries, which may contain over 60000 rates and vary by 20 orders of magnitude, have a uniform parameterization for all rates. We have developed an on-line tool, hosted at www.nucastrodata.org, to obtain REACLIB parameters (F.-K. Thielemann et al., Adv. Nucl. Astrophysics 525, 1 (1987)) that represent reaction rates as a function of temperature. This helps to rapidly incorporate the latest nuclear physics results in astrophysics models. The tool uses numerous techniques and algorithms in a modular fashion to improve the quality of the fits to the rates. Features, modules, and additional applications of this tool will be discussed. * Managed by UT-Battelle, LLC, for the U.S. D.O.E. under contract DE-AC05-00OR22725 + Supported by U.S. D.O.E. under Grant No. DE-FG02-96ER40955

  12. Computational study on the mechanisms and rate constants of the OH-initiated oxidation of ethyl vinyl ether in atmosphere.

    PubMed

    Han, Dandan; Cao, Haijie; Li, Jing; Li, Mingyue; He, Maoxia; Hu, Jingtian

    2014-09-01

    The hydroxylation reactions of ethyl vinyl ether (EVE) in the present of O2 and NO are analyzed by using MPWB1K/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) level of theory. According to the calculated thermodynamic data, the detailed reaction mechanisms of EVE and OH are proposed. All of the ten possible reaction pathways are discussed. The major products of the title reaction are ethyl formate and formaldehyde, which is in accordance with experimental detection. The rate constants of the primary reactions over the temperature of 250-400K and the pressure range of 100-2000Torr are computed by employing MESMER program. At 298K and 760Torr, OH-addition channels are predominate and the total rate constant is ktot=4.53×10(-11)cm(3)molecule(-1)s(-1). The Arrhenius equation is obtained as ktot=6.27×10(-12)exp(611.5/T), according to the rate constants given at different temperatures. Finally, the atmospheric half life of EVE with respect to OH is estimated to be 2.13h.

  13. Path integral evaluation of the quantum instanton rate constant for proton transfer in a polar solvent

    NASA Astrophysics Data System (ADS)

    Yamamoto, Takeshi; Miller, William H.

    2005-01-01

    The quantum instanton approximation for thermal rate constants, a type of quantum transition state theory (QTST), is applied to a model proton transfer reaction in liquid methyl chloride developed by Azzouz and Borgis. Monte Carlo path integral methods are used to carry out the calculations, and two other closely related QTST's, namely, the centroid-density and Hansen-Andersen QTST, are also evaluated for comparison using the present path integral approach. A technique is then introduced that calculates the kinetic isotope effect directly via thermodynamic integration of the rate with respect to hydrogen mass, which has the practical advantage of avoiding costly evaluation of the activation free energy. The present application to the Azzouz-Borgis problem shows that the above three types of QTST provide very similar results for the rate, within 30% of each other, which is nontrivial considering the totally different derivations of these QTSTs; the latter rates are also in reasonable agreement with some other previous results (e.g., obtained via molecular dynamics with quantum transitions), within a factor of ˜2(7) for the H(D) transfer, thus significantly diminishing the possible range of the exact rates. In addition, it is revealed that a small but nonnegligible inconsistency exists in the parametrization of the Azzouz-Borgis model employed in previous studies, which resulted in the large apparent discrepancy in the calculated rates.

  14. Reaction rates between water and some modified rapidly-reacting Karl Fischer reagents.

    PubMed

    Cedergren, A

    1978-04-01

    Rate constants were determined for the reaction between water and various modified Karl Fischer reagents containing formamide, dimethylformamide or N-methylformamide. It was shown that the reaction rate can be increased by a factor of 100 by using a reagent containing 40% v/v formamide in pyridine compared to that obtained by use of the conventional methanolic reagent.

  15. Rate constants calculation with a simple mixed quantum/classical implementation of the flux-flux correlation function method.

    PubMed

    Palma, Juliana

    2009-03-28

    A simple mixed quantum/classical (mixed-Q/C) implementation of the flux-flux correlation function method has been applied to evaluate rate constants for a two-dimensional model system. The model consists of an Eckart barrier resembling the collinear H + H(2) reaction, linearly coupled to a harmonic oscillator. Results are presented for a broad range of parameters for temperatures between 140 and 300 K. It is found that the mixed-Q/C method gives fairly accurate results as long as the reaction does not involve too many recrossings. This suggests that the methodology could be extended to treat direct polyatomic reactions in gas phase.

  16. An approximate classical unimolecular reaction rate theory

    NASA Astrophysics Data System (ADS)

    Zhao, Meishan; Rice, Stuart A.

    1992-05-01

    We describe a classical theory of unimolecular reaction rate which is derived from the analysis of Davis and Gray by use of simplifying approximations. These approximations concern the calculation of the locations of, and the fluxes of phase points across, the bottlenecks to fragmentation and to intramolecular energy transfer. The bottleneck to fragment separation is represented as a vibration-rotation state dependent separatrix, which approximation is similar to but extends and improves the approximations for the separatrix introduced by Gray, Rice, and Davis and by Zhao and Rice. The novel feature in our analysis is the representation of the bottlenecks to intramolecular energy transfer as dividing surfaces in phase space; the locations of these dividing surfaces are determined by the same conditions as locate the remnants of robust tori with frequency ratios related to the golden mean (in a two degree of freedom system these are the cantori). The flux of phase points across each dividing surface is calculated with an analytic representation instead of a stroboscopic mapping. The rate of unimolecular reaction is identified with the net rate at which phase points escape from the region of quasiperiodic bounded motion to the region of free fragment motion by consecutively crossing the dividing surfaces for intramolecular energy exchange and the separatrix. This new theory generates predictions of the rates of predissociation of the van der Waals molecules HeI2, NeI2 and ArI2 which are in very good agreement with available experimental data.

  17. The reaction mechanism with free energy barriers at constant potentials for the oxygen evolution reaction at the IrO2 (110) surface

    DOE PAGES

    Ping, Yuan; Nielsen, Robert J.; Goddard, William A.

    2016-12-09

    How to efficiently oxidize H2O to O2 (oxygen evolution reaction, OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work, we performed first-principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OER at the IrO2more » (110) surface. We determined the surface phase diagram, explored the possible reaction pathways including kinetic barriers, and computed reaction rates based on the microkinetic models. Furthermore, this allowed us to resolve several long-standing puzzles about the atomistic OER mechanism.« less

  18. The Reaction Mechanism with Free Energy Barriers at Constant Potentials for the Oxygen Evolution Reaction at the IrO2 (110) Surface.

    PubMed

    Ping, Yuan; Nielsen, Robert J; Goddard, William A

    2017-01-11

    How to efficiently oxidize H2O to O2 (oxygen evolution reaction, OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work, we performed first-principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OER at the IrO2 (110) surface. We determined the surface phase diagram, explored the possible reaction pathways including kinetic barriers, and computed reaction rates based on the microkinetic models. This allowed us to resolve several long-standing puzzles about the atomistic OER mechanism.

  19. Theory for rates, equilibrium constants, and Brønsted slopes in F1-ATPase single molecule imaging experiments.

    PubMed

    Volkán-Kacsó, Sándor; Marcus, Rudolph A

    2015-11-17

    A theoretical model of elastically coupled reactions is proposed for single molecule imaging and rotor manipulation experiments on F1-ATPase. Stalling experiments are considered in which rates of individual ligand binding, ligand release, and chemical reaction steps have an exponential dependence on rotor angle. These data are treated in terms of the effect of thermodynamic driving forces on reaction rates, and lead to equations relating rate constants and free energies to the stalling angle. These relations, in turn, are modeled using a formalism originally developed to treat electron and other transfer reactions. During stalling the free energy profile of the enzymatic steps is altered by a work term due to elastic structural twisting. Using biochemical and single molecule data, the dependence of the rate constant and equilibrium constant on the stall angle, as well as the Børnsted slope are predicted and compared with experiment. Reasonable agreement is found with stalling experiments for ATP and GTP binding. The model can be applied to other torque-generating steps of reversible ligand binding, such as ADP and Pi release, when sufficient data become available.

  20. Equilibrium constants in reactions of 2-aminoethanol and ammonia with isophthalaldehyde and terephthalaldehyde.

    PubMed

    Kulla, E; Zuman, P

    2007-12-20

    Equilibria established in reactions between isophthalaldehyde (IPA) and terephthalaldehyde (TPA) on one side and 2-aminoethanol and NH3 on the other side were followed in solutions, where these amines acted both as reagents and as buffers. The equilibrium between TPA and 2-aminoethanol was followed spectrophotometrically; all others, of reactions of TPA with NH3 and of IPA with both NH3 and H2N-CH2CH2-OH, were followed by using polarography. Separate limiting reduction currents of the dialdehyde and its imine enable simultaneous determination of the starting material and the product. The equilibria are shifted more in favor of the imine in reactions of TPA than those of IPA. Equilibrium constants for reactions of both TPA and IPA with 2-aminoethanol are 3 orders of magnitude higher than those observed for the reactions of dialdehydes with NH3, even when the pKa values of these two reagents differ only slightly.

  1. Bayesian Estimation of Thermonuclear Reaction Rates

    NASA Astrophysics Data System (ADS)

    Iliadis, C.; Anderson, K. S.; Coc, A.; Timmes, F. X.; Starrfield, S.

    2016-11-01

    The problem of estimating non-resonant astrophysical S-factors and thermonuclear reaction rates, based on measured nuclear cross sections, is of major interest for nuclear energy generation, neutrino physics, and element synthesis. Many different methods have been applied to this problem in the past, almost all of them based on traditional statistics. Bayesian methods, on the other hand, are now in widespread use in the physical sciences. In astronomy, for example, Bayesian statistics is applied to the observation of extrasolar planets, gravitational waves, and Type Ia supernovae. However, nuclear physics, in particular, has been slow to adopt Bayesian methods. We present astrophysical S-factors and reaction rates based on Bayesian statistics. We develop a framework that incorporates robust parameter estimation, systematic effects, and non-Gaussian uncertainties in a consistent manner. The method is applied to the reactions d(p,γ)3He, 3He(3He,2p)4He, and 3He(α,γ)7Be, important for deuterium burning, solar neutrinos, and Big Bang nucleosynthesis.

  2. Bimolecular Rate Constants for FAD-Dependent Glucose Dehydrogenase from Aspergillus terreus and Organic Electron Acceptors

    PubMed Central

    Tsuruoka, Nozomu; Sadakane, Takuya; Hayashi, Rika; Tsujimura, Seiya

    2017-01-01

    The flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) from Aspergillus species require suitable redox mediators to transfer electrons from the enzyme to the electrode surface for the application of bioelectrical devices. Although several mediators for FAD-GDH are already in use, they are still far from optimum in view of potential, kinetics, sustainability, and cost-effectiveness. Herein, we investigated the efficiency of various phenothiazines and quinones in the electrochemical oxidation of FAD-GDH from Aspergillus terreus. At pH 7.0, the logarithm of the bimolecular oxidation rate constants appeared to depend on the redox potentials of all the mediators tested. Notably, the rate constant of each molecule for FAD-GDH was approximately 2.5 orders of magnitude higher than that for glucose oxidase from Aspergillus sp. The results suggest that the electron transfer kinetics is mainly determined by the formal potential of the mediator, the driving force of electron transfer, and the electron transfer distance between the redox active site of the mediator and the FAD, affected by the steric or chemical interactions. Higher k2 values were found for ortho-quinones than for para-quinones in the reactions with FAD-GDH and glucose oxidase, which was likely due to less steric hindrance in the active site in the case of the ortho-quinones. PMID:28287419

  3. Determination of rate constants of N-alkylation of primary amines by 1H NMR spectroscopy.

    PubMed

    Li, Chenghong

    2013-09-05

    Macromolecules containing N-diazeniumdiolates of secondary amines are proposed scaffolds for controlled nitrogen oxide (NO) release medical applications. Preparation of these compounds often involves converting primary amine groups to secondary amine groups through N-alkylation. However, N-alkylation results in not only secondary amines but tertiary amines as well. Only N-diazeniumdiolates of secondary amines are suitable for controlled NO release; therefore, the yield of secondary amines is crucial to the total NO load of the carrier. In this paper, (1)H NMR spectroscopy was used to estimate the rate constants for formation of secondary amine (k1) and tertiary amine (k2) for alkylation reagents such as propylene oxide (PO), methyl acrylate (MA), and acrylonitrile (ACN). At room temperature, the ratio of k2/k1 for the three reactions was found to be around 0.50, 0.026, and 0.0072.

  4. Reduction of Iron-Oxide-Carbon Composites: Part I. Estimation of the Rate Constants

    NASA Astrophysics Data System (ADS)

    Halder, S.; Fruehan, R. J.

    2008-12-01

    A new ironmaking concept using iron-oxide-carbon composite pellets has been proposed, which involves the combination of a rotary hearth furnace (RHF) and an iron bath smelter. This part of the research focuses on studying the two primary chemical kinetic steps. Efforts have been made to experimentally measure the kinetics of the carbon gasification by CO2 and wüstite reduction by CO by isolating them from the influence of heat- and mass-transport steps. A combined reaction model was used to interpret the experimental data and determine the rate constants. Results showed that the reduction is likely to be influenced by the chemical kinetics of both carbon oxidation and wüstite reduction at the temperatures of interest. Devolatilized wood-charcoal was observed to be a far more reactive form of carbon in comparison to coal-char. Sintering of the iron-oxide at the high temperatures of interest was found to exert a considerable influence on the reactivity of wüstite by virtue of altering the internal pore surface area available for the reaction. Sintering was found to be predominant for highly porous oxides and less of an influence on the denser ores. It was found using an indirect measurement technique that the rate constants for wüstite reduction were higher for the porous iron-oxide than dense hematite ore at higher temperatures (>1423 K). Such an indirect mode of measurement was used to minimize the influence of sintering of the porous oxide at these temperatures.

  5. Ozonation of pharmaceutical compounds: Rate constants and elimination in various water matrices.

    PubMed

    Javier Benitez, F; Acero, Juan L; Real, Francisco J; Roldán, Gloria

    2009-09-01

    The ozonation of four pharmaceuticals (metoprolol, naproxen, amoxicillin, and phenacetin) in ultra-pure (UP) water was studied in the pH range between 2.5 and 9. The experiments allowed the determination of the apparent rate constants for the reactions between ozone and the selected compounds. The values obtained varied depending on the pH, and ranged between 239 and 1.27x10(4)M(-1) s(-1) for metoprolol; 2.62x10(4) and 2.97x10(5)M(-1)s(-1) for naproxen; 2.31x10(3) and 1.21x10(7)M(-1)s(-1) for amoxicillin; and 215 and 1.57x10(3)M(-1)s(-1) for phenacetin. Due to the acidic nature of these substances, the degree of dissociation of each pharmaceutical was determined at every pH of work, and the specific rate constants of the neutral and ionic species formed were evaluated. Additionally, the simultaneous ozonation of the pharmaceuticals in different water matrices was carried out by considering a groundwater, a surface water from a public reservoir, and three secondary effluents from municipal wastewater treatment plants. The influence of the operating conditions (initial ozone dose, nature of pharmaceuticals and type of water) on the pharmaceuticals elimination efficiency was established, and a kinetic model was proposed for the evaluation of the partial contribution to the global oxidation of both, the direct ozonation reaction and the radical pathway.

  6. Algebraic methods for deriving steady-state rate equations. Practical difficulties with mechanisms that contain repeated rate constants.

    PubMed Central

    Cornish-Bowden, A

    1976-01-01

    Methods of deriving rate equations that rely on repetition of terms for identification of redundant or invalid terms give incorrect results if used with mechanisms in which some rate constants appear more than once. PMID:999635

  7. Thermal energy reaction rates of titanium monomer cation

    SciTech Connect

    MacTaylor, R.S.; Vann, W.D.; Castleman, A.W. Jr.

    1996-03-28

    New determinations of quantitative forward rate constants for reactions of titanium monomer cation with C{sub 2}H{sub 6}, C{sub 3}H{sub 8}, H{sub 2}O, NH{sub 3}, and O{sub 2} are accomplished using a high-pressure (multicollision) flow tube reactor. These experiments were prompted by our observation of conflicting reports in the literature. The reaction rates we consider have been determined previously by both single-collision and multicollision methods, with some systems having generated notably contrasting results. An effort is made to reconcile the difference in values by explicitly addressing possible potential sources of error which have been suggested in earlier publications as the most likely reasons for the discrepancies. A `titration` technique is employed which allows controlled collisional quenching of excited state titanium cations with nitrogen gas such that excited state ion effects can be selectively observed and subsequently eliminated. No evidence is found to cast doubt upon the validity of multicollision methods for the determination of thermal energy rate constants. 20 refs., 4 figs., 1 tab.

  8. Kinetic mechanism of phenylalanine hydroxylase: intrinsic binding and rate constants from single-turnover experiments.

    PubMed

    Roberts, Kenneth M; Pavon, Jorge Alex; Fitzpatrick, Paul F

    2013-02-12

    Phenylalanine hydroxylase (PheH) catalyzes the key step in the catabolism of dietary phenylalanine, its hydroxylation to tyrosine using tetrahydrobiopterin (BH(4)) and O(2). A complete kinetic mechanism for PheH was determined by global analysis of single-turnover data in the reaction of PheHΔ117, a truncated form of the enzyme lacking the N-terminal regulatory domain. Formation of the productive PheHΔ117-BH(4)-phenylalanine complex begins with the rapid binding of BH(4) (K(d) = 65 μM). Subsequent addition of phenylalanine to the binary complex to form the productive ternary complex (K(d) = 130 μM) is approximately 10-fold slower. Both substrates can also bind to the free enzyme to form inhibitory binary complexes. O(2) rapidly binds to the productive ternary complex; this is followed by formation of an unidentified intermediate, which can be detected as a decrease in absorbance at 340 nm, with a rate constant of 140 s(-1). Formation of the 4a-hydroxypterin and Fe(IV)O intermediates is 10-fold slower and is followed by the rapid hydroxylation of the amino acid. Product release is the rate-determining step and largely determines k(cat). Similar reactions using 6-methyltetrahydropterin indicate a preference for the physiological pterin during hydroxylation.

  9. Fusion Reaction Rate in an Inhomogeneous Plasma

    SciTech Connect

    S. Son; N.J. Fisch

    2004-09-03

    The local fusion rate, obtained from the assumption that the distribution is a local Maxwellian, is inaccurate if mean-free-paths of fusing particles are not sufficiently small compared with the inhomogeneity length of the plasma. We calculate the first order correction of P0 in terms of the small spatial gradient and obtain a non-local modification of P(sub)0 in a shock region when the gradient is not small. Use is made of the fact that the fusion reaction cross section has a relatively sharp peak as a function of energy.

  10. Stability analysis of 4-species Aβ aggregation model: A novel approach to obtaining physically meaningful rate constants.

    PubMed

    Ghag, G; Ghosh, P; Mauro, A; Rangachari, V; Vaidya, A

    2013-11-01

    Protein misfolding and concomitant aggregation towards amyloid formation is the underlying biochemical commonality among a wide range of human pathologies. Amyloid formation involves the conversion of proteins from their native monomeric states (intrinsically disordered or globular) to well-organized, fibrillar aggregates in a nucleation-dependent manner. Understanding the mechanism of aggregation is important not only to gain better insight into amyloid pathology but also to simulate and predict molecular pathways. One of the main impediments in doing so is the stochastic nature of interactions that impedes thorough experimental characterization and the development of meaningful insights. In this study, we have utilized a well-known intermediate state along the amyloid-β peptide aggregation pathway called protofibrils as a model system to investigate the molecular mechanisms by which they form fibrils using stability and perturbation analysis. Investigation of protofibril aggregation mechanism limits both the number of species to be modeled (monomers, and protofibrils), as well as the reactions to two (elongation by monomer addition, and protofibril-protofibril lateral association). Our new model is a reduced order four species model grounded in mass action kinetics. Our prior study required 3200 reactions, which makes determining the reaction parameters prohibitively difficult. Using this model, along with a linear perturbation argument, we rigorously determine stable ranges of rate constants for the reactions and ensure they are physically meaningful. This was accomplished by finding the ranges in which the perturbations dieout in a five-parameter sweep, which includes the monomer and protofibril equilibrium concentrations and three of the rate constants. The results presented are a proof-of-concept method in determining meaningful rate constants that can be used as a bonafide way for determining accurate rate constants for other models involving complex

  11. Rate Constants for Peroxidation of Polyunsaturated Fatty Acids and Sterols in Solution and in Liposomes

    PubMed Central

    Xu, Libin; Davis, Todd A.; Porter, Ned A.

    2013-01-01

    Rate constants for autoxidation propagation of several unsaturated lipids in benzene solution at 37°C and in phosphatidylcholine liposomes were determined by a linoleate radical clock. This radical clock is based on competition between hydrogen atom abstraction by an intermediate peroxyl radical derived from linoleic acid that leads to a trans,cis-conjugated hydroxyoctadecadienoic product and β–fragmentation of the same peroxyl that gives the trans,trans-product hydroxyoctadecadienoic acid. Rate constants determined by this approach in solution relative to linoleic acid (kp = 62 M−1s−1) were: arachidonic acid (kp = 197 ± 13 M−1s−1), eicosapentaenoic acid (kp = 249 ± 16 M−1s−1), docosahexaenoic acid (kp = 334 ± 37 M−1s−1), cholesterol (kp = 11 ± 2 M−1s−1), and 7-dehydrocholesterol (kp = 2,260 ± 40 M−1s−1). Free radical oxidations of multilamellar and unilamellar liposomes of various mixtures of glycerophosphatidylcholine molecular species were also carried out. In some experiments, cholesterol or 7-dehydrocholesterol was incorporated into the lipid mixture undergoing oxidation. A phosphatidylcholine bearing a linoleate ester at sn-2 was a component of each liposome peroxidation reaction and the ratio of trans,cis/trans,trans (t,c/t,t)-conjugated diene oxidation products formed from this phospholipid was determined for each oxidation reaction. This t,c/t,t-product ratio from linoleate was used to “clock” liposome constituents as hydrogen atom donors in the lipid bilayer. Application of this lipid bilayer radical clock gives relative autoxidation propagation rate constants of arachidonate (20:4), eicosapentaenoate (20:5), docosahexaenoate (22:6), and 7-dehydrocholesterol to be 115 ± 7, 145 ± 8, 172 ± 13, and 832 ± 86, respectively, a reactivity trend that parallels the one in solution. We also conclude from the liposome oxidations that linoleate peroxyl radicals at different positions on the eighteen-carbon chain (at C-9 and C

  12. Solvent dynamics effects on heterogeneous electron transfer rate constants of cobalt tris(bipyridine)

    SciTech Connect

    Pyati, R.; Murray, R.W.

    1996-02-21

    This paper presents microelectrode voltammetry-derived heterogeneous electron transfer kinetic rates k{sub ET} for the redox couple [Co(bpy){sub 3}]{sup 2+/3+} in a series of solvents for which ({tau}{sub L}) longitudinal relaxation values are known (four polar monomeric solvents and four oligomeric polyether solvents, CH{sub 3}O-(CH{sub 2}CH{sub 2}O){sub n}-CH{sub 3} where n=1, 2, 3, and 4) and one, a higher oligomer (n=8, MPEG-400), for which {tau}{sub L} is estimated. {tau}{sub L} ranges from 0.2 to 38 ps. The results show that k{sub ET} varies inversely with {tau}{sub L}, and according to other modes of analysis, as predicted for control of the energy barrier-crossing rate by the dynamics of solvent dipolar relaxation. Additionally, the observed k{sub ET} is proportional to the diffusion coefficient D{sub Co} of [Co(bpy){sub 3}]{sup 2+}, which is rationalized by the mutual connection of D{sub Co} and k{sub ET} to the solvent viscosity. D{sub Co}, k{sub ET}, and viscosity were also measured as a function of electrolyte concentration in MPEG-400 which allowed extension of the overall solvent viscosity range. The [Co(bpy){sub 3}]{sup 2+/3+} rate constant in these media was also proportional to D{sub Co}, indicating solvent dynamics control over a time scale range of ca. 500-fold, larger than any previously reported. Experiments at constant viscosity but varied electrolyte concentration demonstrated the absence of strong double layer or ion pairing influences on the reaction rate. 32 refs., 6 figs., 3 tabs.

  13. Temperature dependence of the rate constants for oxidation of organic compounds by peroxyl radicals in aqueous alcohol solutions

    SciTech Connect

    Alfassi, Z.B.; Huie, R.E.; Kumar, M.; Neta, P.

    1992-01-23

    Rate constants for reactions of chlorinated methylperoxyl radicals with chlorpromazine (2-chloro-10-[3-(dimethylamino)-propyl]phenothiazine), trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), and ascorbate in aqueous alcohol solutions have been measured by pulse radiolysis as a function of temperature, generally between 5 and 75{degrees}C. The rate constants varied between 10{sup 6} and 10{sup 9} M{sup {minus}1} s{sup {minus}1}, the calculated Arrhenius activation energies ranged from 1 to 30 kJ mol{sup {minus}1}, and the preexponential factors also varied considerably, with log A ranging from 7 to 14. In general, room temperature rate constants increase with an increase in the number of chlorine atoms on the radical (increasing its electron affinity and thus the driving force for the reaction) and with an increase in the solvent polarity. The Arrhenius preexponential factor and the activation energy both increased as the proportion of water in the solvent mixture increased; i.e., the increase in rate constant with solvent polarity is a result of two compensating effects. Electron transfer from the organic reductants to the chlorinated methylperoxyl radicals is suggested to take place via an inner-sphere mechanism involving a transient adduct of the peroxyl radical to the reductant. 15 refs., 2 figs., 3 tabs.

  14. Lipase-catalyzed transesterification in organic media: solvent effects on equilibrium and individual rate constants.

    PubMed

    García-Alles, L F; Gotor, V

    1998-09-20

    The kinetics of the immobilized lipase B from Candida antarctica have been studied in organic solvents. This enzyme has been shown to be slightly affected by the water content of the organic media, and it does not seem to be subject to mass transfer limitations. On the other hand, some evidence indicates that the catalytic mechanism of reactions catalyzed by this lipase proceeds through the acyl-enzyme intermediate. Moreover, despite the fact that the immobilization support dramatically enhances the catalytic power of the enzyme, it does not interfere with the intrinsic solvent effect. Consequently, this enzyme preparation becomes optimum for studying the role played by the organic solvent in catalysis. To this end, we have measured the acylation and deacylation individual rate constants, and the binding equilibrium constant for the ester, in several organic environments. Data obtained show that the major effect of the organic solvent is on substrate binding, and that the catalytic steps are almost unaffected by the solvent, indicating the desolvation of the transition state. However, the strong decrease in binding for hydrophilic solvents such as THF and dioxane, compared to the rest of solvents, cannot be easily explained by means of thermodynamic arguments (desolvation of the ester substrate). For this reason, data have been considered as an indication of the existence of an unknown step in the catalytic pathway occurring prior to formation of the acyl-enzyme intermediate.

  15. Gas-phase chemistry of (alpha-terpineol with ozone and OH radical: rate constants and products.

    PubMed

    Wells, J R

    2005-09-15

    A bimolecular rate constant, kOH+alpha-terpineol, of (1.9 +/- 0.5) x 10(-10) cm3 molecule(-1) s(-1) was measured using gas chromatography/mass spectrometry and the relative rate technique for the reaction of the hydroxyl radical (OH) with alpha-terpineol (1-methyl-4-isopropyl-1-cyclohexen-8-ol) at (297 +/- 3) K and 1 atm total pressure. Additionally, a bimolecular rate constant, kO3+alpha-terpineol, of (3.0 +/- 0.2) x 10(-16) cm3 molecule(-1) s(-1) was measured by monitoring the first order decrease in ozone concentration as a function of excess alpha-terpineol. To better understand alpha-terpineol's gas-phase transformation in the indoor environment, the products of the alpha-terpineol + OH and alpha-terpineol + 03 reactions were also investigated. The positively identified alpha-terpineol/OH reaction products were acetone, ethanedial (glyoxal, HC(=O)C(=O)H), and 2-oxopropanal (methyl glyoxal, CH3C(=O)C(=O)H). The positively identified alpha-terpineol/O3 reaction product was 2-oxopropanal (methyl glyoxal, CH3C(=O)C(=O)H). The use of derivatizing agents O-(2,3,4,5,6-pentalfluorobenzyl)hydroxylamine (PFBHA) and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) clearly indicated that several other reaction products were formed. The elucidation of these other reaction products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible alpha-terpineol/OH and alpha-terpineol/O3 reaction mechanisms based on previously published volatile organic compound/ OH and volatile organic compound/O3 gas-phase reaction mechanisms.

  16. Reactivity of H 2O 2 with radiation produced free radicals: Steady state radiolysis methods for estimating the rate constants

    NASA Astrophysics Data System (ADS)

    Kishore, Kamal; Moorthy, P. N.; Rao, K. N.

    A new steady state radiolysis method for the determination of the rate constants of reaction of H 2O 2 with solute radicals derived by reaction with radiation produced OH radicals is described. The values so obtained are comparable with the ones obtained by the method described by Soylemez and Balkas in the past. A method similar to the latter has been employed to determine the rate constant of reaction of H 2O 2 with solute radicals derived from e -aq reaction. In the case of the (CH 3) 2 COH radical which can be generated by either reaction of OH with isopropanol or e -aq with acetone, the three methods give values agreeing with each other within ± 15% (viz. 2.7 ± 0.4 × 10 5 dm 3 mol -1 s -1). The reaction of the solute radicals with H 2O 2 is considered in the light of known one-electron redox potentials, and the limitations of the applicability of the methods described here are discussed.

  17. Temperature dependence of the rate constant for the bimolecular recombination of Cl-2 in water—A pulse radiolysis study

    NASA Astrophysics Data System (ADS)

    Szala-Bilnik, J.; Pierscieniewska, P.; Wolszczak, M.; Swiatla-Wojcik, D.

    2014-04-01

    The rate constant for the disproportionation of Cl-2 in water has been determined for the temperature range 22-87 °C using pulse radiolysis of 0.1 M NaCl+1 mM HClO4 aqueous solution. The rate constant for the decay of Cl-2 has been found to be 2×(7.35±0.53)×108 M-1 s-1 at 22 °C (at zero ionic strength). The determined activation energy, Ea=10.89±0.37 kJ mol-1, is less than expected for diffusion-controlled reactions. A good fit to the Noyes equation (1/kobs=1/kdiff+1/kreact) has been obtained assuming the reaction step is activationless with kreact=A‧T.

  18. Rates and Mechanisms of Complexation Reactions of Cations with Crown Ethers and Related Macrocycles

    DTIC Science & Technology

    1989-01-23

    as stability constants and entropies of solvation t, be of paramount importance. Complexation rate constants alone seldom disclose a great deal more...sample equilibrium to assure detectability of the reaction. Disadvantages include 1) the need for high solute concentrations in order to detect small...ultrasonic techniques. In principle, the stability constants of macrocycles complexing various cations can be deduced from the amplitudes of the experimental

  19. Calibration of model constants in a biological reaction model for sewage treatment plants.

    PubMed

    Amano, Ken; Kageyama, Kohji; Watanabe, Shoji; Takemoto, Takeshi

    2002-02-01

    Various biological reaction models have been proposed which estimate concentrations of soluble and insoluble components in effluent of sewage treatment plants. These models should be effective to develop a better operation system and plant design, but their formulas consist of nonlinear equations, and there are many model constants, which are not easy to calibrate. A technique has been proposed to decide the model constants by precise experiments, but it is not practical for design engineers or process operators to perform these experiments regularly. Other approaches which calibrate the model constants by mathematical techniques should be used. In this paper, the optimal regulator method of modern control theory is applied as a mathematical technique to calibrate the model constants. This method is applied in a small sewage treatment testing facility. Calibration of the model constants is examined to decrease the deviations between calculated and measured concentrations. Results show that calculated values of component concentrations approach measured values and the method is useful for actual plants.

  20. Hydrolysis rate constants and activation parameters for phosphate- and phosphonate-bridged phthalonitrile monomers under acid, neutral and alkali conditions.

    PubMed

    Belsky, Kirill S; Sulimov, Artem V; Bulgakov, Boris A; Babkin, Alexandr V; Kepman, Alexey V

    2017-08-01

    Hydrolysis data for Bis(3-(3,4-dicyanophenoxy)phenyl) phenyl phosphate and Bis(3-(3,4-dicyanophenoxy)phenyl) phenylphosphonate under pH 4, 7 and 10 are presented. Conversion/time plots collected by HPLC analysis, typical chromatograms and NMR spectra of the reactions products are given. Pseudo-first order rate constants are determined for both substrates at 25, 50 and 80 °C. Activation parameters were calculated from Arrhenius equation.

  1. Pore-Scale Process Coupling and Effective Surface Reaction Rates in Heterogeneous Subsurface Materials

    SciTech Connect

    Liu, Chongxuan; Liu, Yuanyuan; Kerisit, Sebastien N.; Zachara, John M.

    2015-09-01

    This manuscript provides a review of pore-scale researches in literature including experimental and numerical approaches, and scale-dependent behavior of geochemical and biogeochemical reaction rates in heterogeneous porous media. A mathematical equation that can be used to predict the scale-dependent behavior of geochemical reaction rates in heterogeneous porous media has been derived. The derived effective rate expression explicitly links the effective reaction rate constant to the intrinsic rate constant, and to the pore-scale variations in reactant concentrations in porous media. Molecular simulations to calculate the intrinsic rate constants were provided. A few examples of pore-scale simulations were used to demonstrate the application of the equation to calculate effective rate constants in heterogeneous materials. The results indicate that the deviation of effective rate constant from the intrinsic rate in heterogeneous porous media is caused by the pore-scale distributions of reactants and their correlation, which are affected by the pore-scale coupling of reactions and transport.

  2. Lateral instabilities of cubic autocatalytic reaction fronts in a constant electric field

    NASA Astrophysics Data System (ADS)

    Tóth, Ágota; Horváth, Dezső; van Saarloos, Wim

    1999-12-01

    The region of instability for planar reaction fronts of cubic autocatalysis between ionic species under constant electric field has been determined accurately. The ratio of diffusion coefficients at the onset of instability δcr is substantially varied by the component-dependent drift and directly proportional to the concentration of the autocatalyst behind the front βs as δcr=2.3002βs. This opens the possibility to use electric field as a control parameter for reaction-front instabilities. The dispersion relation calculated from the linear stability analysis of the full system is in good agreement with the initial evolution of the Fourier modes associated with the slightly perturbed planar reaction front obtained by the direct integration of the governing equations in two spatial dimensions.

  3. Delay discounting of hypothetical and real money: the effect of holding reinforcement rate constant.

    PubMed

    Dixon, Mark R; Lik, Nicholas Mui Ker; Green, Leonard; Myerson, Joel

    2013-01-01

    We examined the effect of holding reinforcement rate constant on delay discounting of hypothetical and real money when delays were actually experienced. In some conditions, participants were required to wait for the delayed rewards, and in some conditions, reinforcement rate was held constant by adding blackout periods after immediate rewards. Typical discounting occurred with the standard procedure and when there were no blackouts, but not when we held rate of reinforcement constant. Real and hypothetical money produced the same outcomes. © Society for the Experimental Analysis of Behavior.

  4. Electron-ion dissociative recombination rate constants relevant to the Titan atmosphere and the Interstellar Medium

    SciTech Connect

    Osborne, David; Lawson, Patrick; Adams, Nigel

    2014-01-21

    Following the arrival of Cassini at Titan in 2004, the Titan atmosphere has been shown to contain large complex polycyclic-aromatic hydrocarbons. Since Cassini has provided a great deal of data, there exists a need for kinetic rate data to help with modeling this atmosphere. One type of kinetic data needed is electron-ion dissociative recombination (e-IDR) rate constants. These data are not readily available for larger compounds, such as naphthalene, or oxygen containing compounds, such as 1,4 dioxane or furan. Here, the rate constants for naphthalene, 1,4 dioxane, and furan have been measured and their temperature dependencies are determined when possible, using the University of Georgia's Variable Temperature Flowing Afterglow. The rate constants are compared with those previously published for other compounds; these show trends which illustrate the effects which multi-rings and oxygen heteroatoms substitutions have upon e-IDR rate constants.

  5. Electron-ion dissociative recombination rate constants relevant to the Titan atmosphere and the Interstellar Medium

    NASA Astrophysics Data System (ADS)

    Osborne, David; Lawson, Patrick; Adams, Nigel

    2014-01-01

    Following the arrival of Cassini at Titan in 2004, the Titan atmosphere has been shown to contain large complex polycyclic-aromatic hydrocarbons. Since Cassini has provided a great deal of data, there exists a need for kinetic rate data to help with modeling this atmosphere. One type of kinetic data needed is electron-ion dissociative recombination (e-IDR) rate constants. These data are not readily available for larger compounds, such as naphthalene, or oxygen containing compounds, such as 1,4 dioxane or furan. Here, the rate constants for naphthalene, 1,4 dioxane, and furan have been measured and their temperature dependencies are determined when possible, using the University of Georgia's Variable Temperature Flowing Afterglow. The rate constants are compared with those previously published for other compounds; these show trends which illustrate the effects which multi-rings and oxygen heteroatoms substitutions have upon e-IDR rate constants.

  6. A simple reaction-rate model for turbulent diffusion flames

    NASA Technical Reports Server (NTRS)

    Bangert, L. H.

    1975-01-01

    A simple reaction rate model is proposed for turbulent diffusion flames in which the reaction rate is proportional to the turbulence mixing rate. The reaction rate is also dependent on the mean mass fraction and the mean square fluctuation of mass fraction of each reactant. Calculations are compared with experimental data and are generally successful in predicting the measured quantities.

  7. Quantification of in Situ Biodegradation Rate Constants Using a Novel Combined Isotope Approach

    NASA Astrophysics Data System (ADS)

    Blum, P.; Sültenfuß, J.; Martus, P.

    2014-12-01

    Numerous studies have shown the enormous potential of the compound-specific isotope analysis (CSIA) for studying the biodegradation of organic compounds such as monoaromatic hydrocarbons (BTEX), polyaromatic hydrocarbons (PAH), chlorinated solvents and other organic contaminants and environmental transformation mechanisms in groundwater. In addition, two-dimensional isotope analysis such as carbon and hydrogen have been successfully studied indicating the potential to also investigate site-specific reaction mechanisms. The main objective of the current study however is to quantify real effective in situ biodegradation rate constants in a coal-tar contaminated aquifer by combining compound-specific isotope analysis (CSIA) and tracer-based (3H-3He) ground-water dating (TGD). Hence, groundwater samples are used to determine groundwater residence times, and carbon and hydrogen stable isotopes are analyzed for selected BTEX and PAH. The results of the hydrogen stable isotopes surprisingly indicate no isotope fractionation and therefore no biodegradation. In contrast, for stable carbon isotopes of selected BTEX such as o-xylene and toluene, isotope shifts are detected indicating active biodegradation under sulfate-reducing conditions. These and previous results of stable carbon isotopes show that only for o-xylene a clear evidence for biodegradation is possible for the studied site. Nevertheless, in combining these results with the groundwater residence times, which range between 1 year for the shallow wells (20 m below surface) and 41 years for the deeper wells (40 m below surface), it is feasible to effectively determine in situ biodegradation rate constants for o-xylene. Conversely, the outcome also evidently demonstrate the major limitations of the novel combined isotope approach for a successful implementation of monitored natural attenuation (MNA) at such field sites.

  8. Determination of equilibrium constants for the reaction between acetone and HO2 using infrared kinetic spectroscopy.

    PubMed

    Grieman, Fred J; Noell, Aaron C; Davis-Van Atta, Casey; Okumura, Mitchio; Sander, Stanley P

    2011-09-29

    The reaction between the hydroperoxy radical, HO(2), and acetone may play an important role in acetone removal and the budget of HO(x) radicals in the upper troposphere. We measured the equilibrium constants of this reaction over the temperature range of 215-272 K at an overall pressure of 100 Torr using a flow tube apparatus and laser flash photolysis to produce HO(2). The HO(2) concentration was monitored as a function of time by near-IR diode laser wavelength modulation spectroscopy. The resulting [HO(2)] decay curves in the presence of acetone are characterized by an immediate decrease in initial [HO(2)] followed by subsequent decay. These curves are interpreted as a rapid (<100 μs) equilibrium reaction between acetone and the HO(2) radical that occurs on time scales faster than the time resolution of the apparatus, followed by subsequent reactions. This separation of time scales between the initial equilibrium and ensuing reactions enabled the determination of the equilibrium constant with values ranging from 4.0 × 10(-16) to 7.7 × 10(-18) cm(3) molecule(-1) for T = 215-272 K. Thermodynamic parameters for the reaction determined from a second-law fit of our van't Hoff plot were Δ(r)H°(245) = -35.4 ± 2.0 kJ mol(-1) and Δ(r)S°(245) = -88.2 ± 8.5 J mol(-1) K(-1). Recent ab initio calculations predict that the reaction proceeds through a prereactive hydrogen-bonded molecular complex (HO(2)-acetone) with subsequent isomerization to a hydroxy-peroxy radical, 2-hydroxyisopropylperoxy (2-HIPP). The calculations differ greatly in the energetics of the complex and the peroxy radical, as well as the transition state for isomerization, leading to significant differences in their predictions of the extent of this reaction at tropospheric temperatures. The current results are consistent with equilibrium formation of the hydrogen-bonded molecular complex on a short time scale (100 μs). Formation of the hydrogen-bonded complex will have a negligible impact on the

  9. DETERMINATION OF HETEROGENEOUS ELECTRON TRANSFER RATE CONSTANTS AT MICROFABRICATED IRIDIUM ELECTRODES. (R825511C022)

    EPA Science Inventory

    There has been an increasing use of both solid metal and microfabricated iridium electrodes as substrates for various types of electroanalysis. However, investigations to determine heterogeneous electron transfer rate constants on iridium, especially at an electron beam evapor...

  10. DETERMINATION OF HETEROGENEOUS ELECTRON TRANSFER RATE CONSTANTS AT MICROFABRICATED IRIDIUM ELECTRODES. (R825511C022)

    EPA Science Inventory

    There has been an increasing use of both solid metal and microfabricated iridium electrodes as substrates for various types of electroanalysis. However, investigations to determine heterogeneous electron transfer rate constants on iridium, especially at an electron beam evapor...

  11. Reaction rate of H atoms with N2O in hot water

    NASA Astrophysics Data System (ADS)

    Sargent, Leanne; Sterniczuk, Marcin; Bartels, David M.

    2017-06-01

    The rate constant of H• atoms with N2O in water has been measured by a competition method up to 300 °C. Radiolysis with 2.5 MeV electrons generated H• atoms, and the HD product from their reaction with deuterated tetrahydrofuran (THF-d8) was measured with mass spectroscopy. The concentration of THF-d8 was changed by an order of magnitude in the presence of 25 mM N2O to obtain the ratio of rate constants. To determine the rate constant of H• with THF-d8, a similar competition vs. 0.2 mM OH- ion was also measured. The reaction rate of H• with OH- has been accurately determined vs. temperature in previous work, allowing the two unknown rate constants to be deduced. Rate constant of H• with THF-d8 follows the Arrhenius law ln(k/M-1s-1)=27.33 - (32.30 kJ/mol)/RT. Rate constant of H• with N2O follows the Arrhenius law ln(k/M-1s-1)=24.50 - (30.42 kJ/mol)/RT. In all likelihood, the N2O reaction proceeds via cis-HNNO• radical intermediate as in the gas phase, but with participation of a bridging water molecule in the 1,3 hydrogen shift to form N2 and •OH products.

  12. Characterization of Iodine Quenching and Energy Transfer Rate Constants for Supersonic Flow Visualization Applications

    DTIC Science & Technology

    2007-09-28

    SUBTITLE Sm. CONTRACTNUBER Characterization of iodine quenching and energy transfer rate FA9550-41-- o3G Sb. GRANT NUMBER constants for supersonic flow...in the nozzle from a chemical oxygen iodine laser (COIL). PLIF images are recorded using laser excitation of the I= B-X transition. Data for the...Preacolbed byANSI Sad Z30.16 20071015188 Final report for the project, "Characterization of iodine quenching and energy transfer rate constants for

  13. Rate constants for a mechanism including intermediates in the interconversion of ternary complexes by horse liver alcohol dehydrogenase

    SciTech Connect

    Sekhar, V.C.; Plapp, B.V. )

    1990-05-08

    Transient kinetic data for partial reactions of alcohol dehydrogenase and simulations of progress curves have led to estimates of rate constants for the following mechanism, at pH 8.0 and 25{degrees}C: E in equilibrium E-NAD+ in equilibrium *E-NAD+ in equilibrium E-NAD(+)-RCH2OH in equilibrium E-NAD+-RCH2O- in equilibrium *E-NADH-RCHO in equilibrium E-NADH-RCHO in equilibrium E-NADH in equilibrium E. Previous results show that the E-NAD+ complex isomerizes with a forward rate constant of 620 s-1. The enzyme-NAD(+)-alcohol complex has a pK value of 7.2 and loses a proton rapidly (greater than 1000 s-1). The transient oxidation of ethanol is 2-fold faster in D{sub 2}O, and proton inventory results suggest that the transition state has a charge of -0.3 on the substrate oxygen. Rate constants for hydride ion transfer in the forward or reverse reactions were similar for short-chain aliphatic substrates (400-600 s-1). A small deuterium isotope effect for transient oxidation of longer chain alcohols is apparently due to the isomerization of the E-NAD+ complex. The transient reduction of aliphatic aldehydes showed no primary deuterium isotope effect; thus, an isomerization of the E-NADH-aldehyde complex is postulated, as isomerization of the E-NADH complex was too fast to be detected. The estimated microscopic rate constants show that the observed transient reactions are controlled by multiple steps.

  14. Rate constant calculations of the C2 + HCN → CCCN+H addition via the Master Equation.

    PubMed

    da Silva, Washington Barbosa; Albernaz, Alessandra F; Barreto, Patricia R P; Correa, Eberth

    2017-04-01

    The addition of C2 to HCN is of relevant interest in astrochemistry. We studied the pathways of this addition to produce CCCN and estimated its reaction rate using the Master Equation in the circumstellar environment. From the results of this study, it was possible to show that a different pathway in the Surface Potential Energy-PES can also be investigated. In a circumstellar envelop environment, with temperatures varying between 1000 K and 2000 K, the abundances of these species are favorable to this kind of addition, and our branching ratio for the rate constant showed that the new pathway is more favorable in comparison with other possibilities for this range of temperatures in this environment, and must be taken into account in any computation of the rate constant. Graphical Abstract Branching ratios of pathways involved in the C2 + HCN → CCCN+H addition, at a temperature range of 1000-2000 K.

  15. On the mechanism of effective chemical reactions with turbulent mixing of reactants and finite rate of molecular reactions

    NASA Astrophysics Data System (ADS)

    Vorotilin, V. P.

    2017-01-01

    A generalization of the theory of chemical transformation processes under turbulent mixing of reactants and arbitrary values of the rate of molecular reactions is presented that was previously developed for the variant of an instantaneous reaction [13]. The use of the features of instantaneous reactions when considering the general case, namely, the introduction of the concept of effective reaction for the reactant volumes and writing a closing conservation equation for these volumes, became possible due to the partition of the whole amount of reactants into "active" and "passive" classes; the reactants of the first class are not mixed and react by the mechanism of instantaneous reactions, while the reactants of the second class approach each other only through molecular diffusion, and therefore their contribution to the reaction process can be neglected. The physical mechanism of reaction for the limit regime of an ideal mixing reactor (IMR) is revealed and described. Although formally the reaction rate in this regime depends on the concentration of passive fractions of the reactants, according to the theory presented, the true (hidden) mechanism of the reaction is associated only with the reaction of the active fractions of the reactants with vanishingly small concentration in the volume of the reactor. It is shown that the rate constant of fast chemical reactions can be evaluated when the mixing intensity of reactants is much less than that needed to reach the mixing conditions in an IMR.

  16. Absolute rate of the reaction of Cl(p-2) with molecular hydrogen from 200 - 500 K

    NASA Technical Reports Server (NTRS)

    Whytock, D. A.; Lee, J. H.; Michael, J. V.; Payne, W. A.; Stief, L. J.

    1976-01-01

    Rate constants for the reaction of atomic chlorine with hydrogen are measured from 200 - 500 K using the flash photolysis-resonance fluorescence technique. The results are compared with previous work and are discussed with particular reference to the equilibrium constant for the reaction and to relative rate data for chlorine atom reactions. Theoretical calculations, using the BEBO method with tunneling, give excellent agreement with experiment.

  17. Method of experimental determination of the kinetic constants in fast polymerization reactions in nonisothermal diffusion conditions

    SciTech Connect

    Prochukhan, Yu.A.; Berlin, A.A.; Enikolopyan, N.S.

    1986-09-01

    A new method for the experimental determination of the kinetic constants k/sub p/ and k/sub t/ in fast polymerization reactions on the example of cationic (under the effect of AlCl/sub 3/, BF/sub 3/, and other catalysts) liquid phase polymerization of isobutylene in a flow was suggested. The study of the macrokinetic features of low-temperature polymerization of isobutylene revealed the specific conditions of the occurrence of the process (quasi-ideal displacement) which are characterized by the relative constancy and uniformity of the distribution of the concentrations of the reacting substances along the flow section.

  18. Equilibrium constant for the reversible reaction ClO + O2 - ClO-O2

    NASA Technical Reports Server (NTRS)

    Demore, W. B.

    1990-01-01

    It is shown here that the equilibrium constant for the reversible reaction ClO + O2 - ClO-O2 at stratospheric temperatures must be at least three orders of magnitude less than the current NASA upper limit. The new upper limit greatly diminishes the possible role of ClO-O2 in the chlorine-photosensitized decomposition of O3. Nevertheless, it does not preclude the possibility that it is a significant reservoir of ClO, as well as a possible reactant, at low temperatures characteristic of polar vortices.

  19. Fluorescence anisotropy-based measurement of Pseudomonas aeruginosa penicillin-binding protein 2 transpeptidase inhibitor acylation rate constants.

    PubMed

    Shapiro, Adam B; Gao, Ning; Gu, Rong-Fang; Thresher, Jason

    2014-10-15

    High-molecular-weight penicillin-binding proteins (PBPs) are essential integral membrane proteins of the bacterial cytoplasmic membrane responsible for biosynthesis of peptidoglycan. They are the targets of antibacterial β-lactam drugs, including penicillins, cephalosporins, and carbapenems. β-Lactams covalently acylate the active sites of the PBP transpeptidase domains. Because β-lactams are time-dependent inhibitors, quantitative assessment of the inhibitory activity of these compounds ideally involves measurement of their second-order acylation rate constants. We previously described a fluorescence anisotropy-based assay to measure these rate constants for soluble constructs of PBP3 (Anal. Biochem. 439 (2013) 37-43). Here we report the expression and purification of a soluble construct of Pseudomonas aeruginosa PBP2 as a fusion protein with NusA. This soluble PBP2 was used to measure second-order acylation rate constants with the fluorescence anisotropy assay. Measurements were obtained for mecillinam, which reacts specifically with PBP2, and for several carbapenems. The assay also revealed that PBP2 slowly hydrolyzed mecillinam and was used to measure the rate constant for this deacylation reaction. Copyright © 2014 Elsevier Inc. All rights reserved.

  20. Rate constant and thermochemistry for K + O2 + N2 = KO2 + N2.

    PubMed

    Sorvajärvi, Tapio; Viljanen, Jan; Toivonen, Juha; Marshall, Paul; Glarborg, Peter

    2015-04-09

    The addition reaction of potassium atoms with oxygen has been studied using the collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) method. KCl vapor was photolyzed with 266 nm pulses and the absorbance by K atoms at 766.5 nm was measured at various delay times with a narrow line width diode laser. Experiments were carried out with O2/N2 mixtures at a total pressure of 1 bar, over 748-1323 K. At the lower temperatures single exponential decays of [K] yielded the third-order rate constant for addition, kR1, whereas at higher temperatures equilibration was observed in the form of double exponential decays of [K], which yielded both kR1 and the equilibrium constant for KO2 formation. kR1 can be summarized as 1.07 × 10(-30)(T/1000 K)(-0.733) cm(6) molecule(-2) s(-1). Combination with literature values leads to a recommended kR1 of 5.5 × 10(-26)T(-1.55) exp(-10/T) cm(6) molecule(-2) s(-1) over 250-1320 K, with an error limit of a factor of 1.5. A van't Hoff analysis constrained to fit the computed ΔS298 yields a K-O2 bond dissociation enthalpy of 184.2 ± 4.0 kJ mol(-1) at 298 K and ΔfH298(KO2) = -95.2 ± 4.1 kJ mol(-1). The corresponding D0 is 181.5 ± 4.0 kJ mol(-1). This value compares well with a CCSD(T) extrapolation to the complete basis set limit, with all electrons correlated, of 177.9 kJ mol(-1).

  1. Development of Theoretical Methods for Predicting Solvent Effects on Reaction Rates in Supercritical Water Oxidation Processes

    DTIC Science & Technology

    2007-11-02

    Tucker, manuscript in preparation. “Examination of Nonequilibrium Solvent Effects on an SN2 Reaction in Supercritical Water,” R. Behera, B...DATES COVERED Final: 7/1/99 - 12/31/02 4. TITLE AND SUBTITLE Development of theoretical methods for predicting solvent effects on reactions ...computational methods for predicting how reaction rate constants will vary with thermodynamic condition in supercritical water (SCW). Towards this

  2. Effects of a constant electric field on the diffusional instability of cubic autocatalytic reaction fronts

    NASA Astrophysics Data System (ADS)

    D'Hernoncourt, J.; De Wit, A.; Merkin, J. H.

    2007-03-01

    An electric field applied in the direction of propagation of a chemical reaction-diffusion front can affect the stability of this front with regard to diffusive instabilities. The influence of an applied constant electric field is investigated by a linear stability analysis and by nonlinear simulations of a simple chemical system based on the cubic autocatalytic reaction A-+2B-→3B-. The diffusional stability of the front is seen to depend on the intensity E and sign of the applied field, and D, the ratio diffusion coefficients of the reactant species. Depending on E, the front can become more or less diffusively unstable for a given value of D. Above a critical value of E, which depends on D, electrophoretic separation of the two fronts is observed.

  3. Systematic angle random walk estimation of the constant rate biased ring laser gyro.

    PubMed

    Yu, Huapeng; Wu, Wenqi; Wu, Meiping; Feng, Guohu; Hao, Ming

    2013-02-27

    An actual account of the angle random walk (ARW) coefficients of gyros in the constant rate biased rate ring laser gyro (RLG) inertial navigation system (INS) is very important in practical engineering applications. However, no reported experimental work has dealt with the issue of characterizing the ARW of the constant rate biased RLG in the INS. To avoid the need for high cost precise calibration tables and complex measuring set-ups, the objective of this study is to present a cost-effective experimental approach to characterize the ARW of the gyros in the constant rate biased RLG INS. In the system, turntable dynamics and other external noises would inevitably contaminate the measured RLG data, leading to the question of isolation of such disturbances. A practical observation model of the gyros in the constant rate biased RLG INS was discussed, and an experimental method based on the fast orthogonal search (FOS) for the practical observation model to separate ARW error from the RLG measured data was proposed. Validity of the FOS-based method was checked by estimating the ARW coefficients of the mechanically dithered RLG under stationary and turntable rotation conditions. By utilizing the FOS-based method, the average ARW coefficient of the constant rate biased RLG in the postulate system is estimated. The experimental results show that the FOS-based method can achieve high denoising ability. This method estimate the ARW coefficients of the constant rate biased RLG in the postulate system accurately. The FOS-based method does not need precise calibration table with high cost and complex measuring set-up, and Statistical results of the tests will provide us references in engineering application of the constant rate biased RLG INS.

  4. Systematic Angle Random Walk Estimation of the Constant Rate Biased Ring Laser Gyro

    PubMed Central

    Yu, Huapeng; Wu, Wenqi; Wu, Meiping; Feng, Guohu; Hao, Ming

    2013-01-01

    An actual account of the angle random walk (ARW) coefficients of gyros in the constant rate biased rate ring laser gyro (RLG) inertial navigation system (INS) is very important in practical engineering applications. However, no reported experimental work has dealt with the issue of characterizing the ARW of the constant rate biased RLG in the INS. To avoid the need for high cost precise calibration tables and complex measuring set-ups, the objective of this study is to present a cost-effective experimental approach to characterize the ARW of the gyros in the constant rate biased RLG INS. In the system, turntable dynamics and other external noises would inevitably contaminate the measured RLG data, leading to the question of isolation of such disturbances. A practical observation model of the gyros in the constant rate biased RLG INS was discussed, and an experimental method based on the fast orthogonal search (FOS) for the practical observation model to separate ARW error from the RLG measured data was proposed. Validity of the FOS-based method was checked by estimating the ARW coefficients of the mechanically dithered RLG under stationary and turntable rotation conditions. By utilizing the FOS-based method, the average ARW coefficient of the constant rate biased RLG in the postulate system is estimated. The experimental results show that the FOS-based method can achieve high denoising ability. This method estimate the ARW coefficients of the constant rate biased RLG in the postulate system accurately. The FOS-based method does not need precise calibration table with high cost and complex measuring set-up, and Statistical results of the tests will provide us references in engineering application of the constant rate biased RLG INS. PMID:23447008

  5. Faster rates with less catalyst in template-directed reactions

    NASA Technical Reports Server (NTRS)

    Kanavarioti, A.; Baird, E. E.

    1995-01-01

    We have recently shown that the polycytidylic acid-directed polymerization of guanosine 5'-monophosphate 2-methylimidazolide (2-MeImpG) is amenable to kinetic study and that rate determinations as a function of 2-MeImpG concentration can reveal much mechanistic detail (Kanavarioti et al. 1993). Here we report kinetic data which show that, once the reaction has been initiated by the formation of dimers, the elongation of dimers to form longer oligomers is accelerated by decreasing polycytidylate (poly(C)) concentration from 0.05 to 0.002 M. This result is consistent with the previously proposed mechanism. The increase in the observed pseudo-first order rate constant for formation of the trimer, k3', and the corresponding constant for formation of oligomers longer than the trimer, ki' (ki' is independent of oligomer length for i > or = 4), with decreasing template concentration for a given monomer concentration is attributed to an increase in template occupancy as template concentration is reduced.

  6. Higher success rate with transcranial electrical stimulation of motor-evoked potentials using constant-voltage stimulation compared with constant-current stimulation in patients undergoing spinal surgery.

    PubMed

    Shigematsu, Hideki; Kawaguchi, Masahiko; Hayashi, Hironobu; Takatani, Tsunenori; Iwata, Eiichiro; Tanaka, Masato; Okuda, Akinori; Morimoto, Yasuhiko; Masuda, Keisuke; Tanaka, Yuu; Tanaka, Yasuhito

    2017-05-05

    During spine surgery, the spinal cord is electrophysiologically monitored via transcranial electrical stimulation of motor-evoked potentials (TES-MEPs) to prevent injury. Transcranial electrical stimulation of motor-evoked potential involves the use of either constant-current or constant-voltage stimulation; however, there are few comparative data available regarding their ability to adequately elicit compound motor action potentials. We hypothesized that the success rates of TES-MEP recordings would be similar between constant-current and constant-voltage stimulations in patients undergoing spine surgery. The objective of this study was to compare the success rates of TES-MEP recordings between constant-current and constant-voltage stimulation. This is a prospective, within-subject study. Data from 100 patients undergoing spinal surgery at the cervical, thoracic, or lumbar level were analyzed. The success rates of the TES-MEP recordings from each muscle were examined. Transcranial electrical stimulation with constant-current and constant-voltage stimulations at the C3 and C4 electrode positions (international "10-20" system) was applied to each patient. Compound muscle action potentials were bilaterally recorded from the abductor pollicis brevis (APB), deltoid (Del), abductor hallucis (AH), tibialis anterior (TA), gastrocnemius (GC), and quadriceps (Quad) muscles. The success rates of the TES-MEP recordings from the right Del, right APB, bilateral Quad, right TA, right GC, and bilateral AH muscles were significantly higher using constant-voltage stimulation than those using constant-current stimulation. The overall success rates with constant-voltage and constant-current stimulations were 86.3% and 68.8%, respectively (risk ratio 1.25 [95% confidence interval: 1.20-1.31]). The success rates of TES-MEP recordings were higher using constant-voltage stimulation compared with constant-current stimulation in patients undergoing spinal surgery. Copyright © 2017

  7. Calculated Third Order Rate Constants for Interpreting the Mechanisms of Hydrolyses of Chloroformates, Carboxylic Acid Halides, Sulfonyl Chlorides and Phosphorochloridates

    PubMed Central

    Bentley, T. William

    2015-01-01

    Hydrolyses of acid derivatives (e.g., carboxylic acid chlorides and fluorides, fluoro- and chloroformates, sulfonyl chlorides, phosphorochloridates, anhydrides) exhibit pseudo-first order kinetics. Reaction mechanisms vary from those involving a cationic intermediate (SN1) to concerted SN2 processes, and further to third order reactions, in which one solvent molecule acts as the attacking nucleophile and a second molecule acts as a general base catalyst. A unified framework is discussed, in which there are two reaction channels—an SN1-SN2 spectrum and an SN2-SN3 spectrum. Third order rate constants (k3) are calculated for solvolytic reactions in a wide range of compositions of acetone-water mixtures, and are shown to be either approximately constant or correlated with the Grunwald-Winstein Y parameter. These data and kinetic solvent isotope effects, provide the experimental evidence for the SN2-SN3 spectrum (e.g., for chloro- and fluoroformates, chloroacetyl chloride, p-nitrobenzoyl p-toluenesulfonate, sulfonyl chlorides). Deviations from linearity lead to U- or V-shaped plots, which assist in the identification of the point at which the reaction channel changes from SN2-SN3 to SN1-SN2 (e.g., for benzoyl chloride). PMID:26006228

  8. Development of the new approach to the diffusion-limited reaction rate theory

    SciTech Connect

    Veshchunov, M. S.

    2012-04-15

    The new approach to the diffusion-limited reaction rate theory, recently proposed by the author, is further developed on the base of a similar approach to Brownian coagulation. The traditional diffusion approach to calculation of the reaction rate is critically analyzed. In particular, it is shown that the traditional approach is applicable only in the special case of reactions with a large reaction radius and the mean inter-particle distances, and become inappropriate in calculating the reaction rate in the case of a relatively small reaction radius. In the latter case, most important for chemical reactions, particle collisions occur not in the diffusion regime but mainly in the kinetic regime characterized by homogeneous (random) spatial distribution of particles on the length scale of the mean inter-particle distance. The calculated reaction rate for a small reaction radius in three dimensions formally (and fortuitously) coincides with the expression derived in the traditional approach for reactions with a large reaction radius, but notably deviates at large times from the traditional result in the planar two-dimensional geometry. In application to reactions on discrete lattice sites, new relations for the reaction rate constants are derived for both three-dimensional and two-dimensional lattices.

  9. Relationship between phenol-induced cytotoxicity and experimental inhibition rate constant or a theoretical parameter.

    PubMed

    Fujisawa, S; Kadoma, Y

    2012-06-01

    We synthesized various dimer forms of 2-methoxyphenols and 2-tert-butylphenols, as dimers such as curcumin exhibit potent antioxidant and anti-inflammatory activity. We investigated the QSARs between the cytotoxicity and independent variables; kinetic parameters (inhibition rate constant (kinh/kp), stoichiometric factor (n)) or DFT-based theoretical parameters (i.e. phenolic O-H bond dissociation enthalpy (BDE), ionization potential according to Koopman's theorem (IP), LUMO, absolute hardness (η), electronegativity (χ) and electrophilicity (ω)) for 2-methoxyphenols and 2- tert- or 2,6-di-tert-butylphenols. The cytotoxicity of these phenols against human tumor cells (HSG, HL60) and/or human gingival fibroblasts (HGF) showed a marked negative linear relationship to kinh/kp, suggesting that the cytotoxicity of phenols may be related to radical reactions. By contrast, a linear relationship between the cytotoxicity and η-term was demonstrated; 2-methoxyphenols showed a negative slope, whereas 2-tert- or 2,6-di-tert-butylphenols showed a positive slope. Also, the cytotoxicity of tert-butylphenols was linearly dependent on the LUMO-term, showing a positive slope. The cytotoxicity of methoxy-substituted monophenols toward both HSG and HGF cells was related to both log P and η- terms. Also, that of X-phenols toward murine L-1210 cells was related to both log P and η or IP-terms, determined from a dataset reported by Zhang et al., 1998. It was concluded that the phenol-induced cytotoxicity was attributable to radical reactions resulting from the terms (kinh/kp, IP, η, and LUMO) in QSAR. The LUMO-dependent cytotoxicity of 2-tert- or 2,6-di-tert-butylphenols may be related to their quinone oxidation products. Experimental and theoretical parameters provide a useful approach for analysis of the cytotoxicity for phenolic compounds.

  10. Competitive kinetics versus stopped flow method for determining the degradation rate constants of steroids by ozonation.

    PubMed

    López-López, Alberto; Flores-Payán, Valentín; León-Becerril, Elizabeth; Hernández-Mena, Leonel; Vallejo-Rodríguez, Ramiro

    2016-01-01

    Steroids are classified as endocrine disrupting chemicals; they are persistent with low biodegradability and are hardly degraded by conventional methods. Ozonation process has been effective for steroids degradation and the determination of the kinetics is a fundamental aspect for the design and operation of the reactor. This study assessed two methods: competitive kinetics and stopped flow, for determining the degradation kinetics of two steroids, estradiol (E2) and ethinylestradiol (EE2) in spiked water. Experiments were performed at pH 6, 21 °C, and using tertbutyl alcohol as scavenger of hydroxyl radicals; competitive kinetics method used sodium phenolate as reference compound. For the stopped flow, the experiments were performed in a BioLogic SFM-3000/S equipment. For both methods, the second order rate constants were in the order of 10(6) and 10(5) M(-1) s(-1) for E2 and EE2 respectively. The competitive kinetics can be applied with assurance and reliability but needing an additional analysis method to measure the residual concentrations. Stopped flow method allows the evaluation of the degradation kinetics in milliseconds and avoids the use of additional analytical methodologies; this method allows determining the reaction times on line. The methods are applicable for degradation of other emerging contaminants or other steroids and could be applied in water treatment at industrial level. Finally, it is important to consider the resources available to implement the most appropriate method, either competitive kinetics or the stopped-flow method.

  11. Estimating spatially-variable first-order rate constants in groundwater reactive transport systems.

    PubMed

    Bailey, R T; Baù, D

    2011-03-25

    Numerical reactive transport models are often used as tools to assess aquifers contaminated with reactive groundwater solutes as well as investigating mitigation scenarios. The ability to accurately simulate the fate and transport of solutes, however, is often impeded by a lack of information regarding the parameters that define chemical reactions. In this study, we employ a steady-state Ensemble Kalman Filter (EnKF), a data assimilation algorithm, to provide improved estimates of a spatially-variable first-order rate constant λ through assimilation of solute concentration measurement data into reactive transport simulation results. The methodology is applied in a steady-state, synthetic aquifer system in which a contaminant is leached to the saturated zone and undergoes first-order decay. Multiple sources of uncertainty are investigated, including hydraulic conductivity of the aquifer and the statistical parameters that define the spatial structure of the parameter field. For the latter scenario, an iterative method is employed to identify the statistical mean of λ of the reference system. Results from all simulations show that the filter scheme is successful in conditioning the λ ensemble to the reference λ field. Sensitivity analyses demonstrate that the estimation of the λ values is dependent on the number of concentration measurements assimilated, the locations from which the measurement data are collected, the error assigned to the measurement values, and the correlation length of the λ fields.

  12. Reaction of singlet oxygen with tryptophan in proteins: a pronounced effect of the local environment on the reaction rate.

    PubMed

    Jensen, Rasmus Lybech; Arnbjerg, Jacob; Ogilby, Peter R

    2012-06-13

    Singlet molecular oxygen, O(2)(a(1)Δ(g)), can influence many processes pertinent to the function of biological systems, including events that result in cell death. Many of these processes involve a reaction between singlet oxygen and a given amino acid in a protein. As a result, the behavior of that protein can change, either because of a structural alteration and/or a direct modification of an active site. Surprisingly, however, little is known about rate constants for reactions between singlet oxygen and amino acids when the latter are in a protein. In this report, we demonstrate using five separate proteins, each containing only a single tryptophan residue, that the rate constant for singlet oxygen reaction with tryptophan depends significantly on the position of this amino acid in the protein. Most importantly, the reaction rate constant depends not only on the accessibility of the tryptophan residue to oxygen, but also on factors that characterize the local molecular environment of the tryptophan in the protein. The fact that the local protein environment can either appreciably inhibit or accelerate the reaction of singlet oxygen with a given amino acid can have significant ramifications for singlet-oxygen-mediated events that perturb cell function.

  13. Coloring Rate of Phenolphthalein by Reaction with Alkaline Solution Observed by Liquid-Droplet Collision.

    PubMed

    Takano, Yuuka; Kikkawa, Shigenori; Suzuki, Tomoko; Kohno, Jun-ya

    2015-06-11

    Many important chemical reactions are induced by mixing two solutions. This paper presents a new way to measure rates of rapid chemical reactions induced by mixing two reactant solutions using a liquid-droplet collision. The coloring reaction of phenolphthalein (H2PP) by a reaction with NaOH is investigated kinetically. Liquid droplets of H2PP/ethanol and NaOH/H2O solutions are made to collide, which induces a reaction that transforms H2PP into a deprotonated form (PP(2-)). The concentration of PP(2-) is evaluated from the RGB values of pixels in the colored droplet images, and is measured as a function of the elapsed time from the collision. The obtained rate constant is (2.2 ± 0.7) × 10(3) M(-1) s(-1), which is the rate constant for the rate-determining step of the coloring reaction of H2PP. This method was shown to be applicable to determine rate constants of rapid chemical reactions between two solutions.

  14. Rate variations of a hetero-Diels--Alder reaction in supercritical fluid CO{sub 2}

    SciTech Connect

    Thompson, R.L.; Glaeser, R.; Bush, D.; Liotta, C.L.; Eckert, C.A.

    1999-11-01

    The hetero-Diels-Alder reaction between anthracene and excess 4-phenyl-1,2,4-triazoline-3,5-dione has been investigated in supercritical CO{sub 2} at 40 C and pressures between 75 and 216 bar. Biomolecular reaction rate constants have been measured via fluorescence spectroscopy by following the decrease in anthracene concentration with reaction time. The reaction rate is elevated in the vicinity of the critical pressure. This difference is consistent with local composition enhancement and can be modeled with the Peng-Robinson equation of state.

  15. In vivo Target Residence Time and Kinetic Selectivity: The Association Rate Constant as Determinant.

    PubMed

    de Witte, Wilhelmus E A; Danhof, Meindert; van der Graaf, Piet H; de Lange, Elizabeth C M

    2016-10-01

    It is generally accepted that, in conjunction with pharmacokinetics, the first-order rate constant of target dissociation is a major determinant of the time course and duration of in vivo target occupancy. Here we show that the second-order rate constant of target association can be equally important. On the basis of the commonly used mathematical models for drug-target binding, it is shown that a high target association rate constant can increase the (local) concentration of the drug, which decreases the rate of decline of target occupancy. The increased drug concentration can also lead to increased off-target binding and decreased selectivity. Therefore, the kinetics of both target association and dissociation need to be taken into account in the selection of drug candidates with optimal pharmacodynamic properties. Copyright © 2016 Elsevier Ltd. All rights reserved.

  16. K{sub Air} and H*(10) Rate Constants for Gamma Emitters

    SciTech Connect

    Vega-Carrillo, H. R.; Juarez, R. Rodriguez; Manzanares-Acuna, E.; Davila, V. M. Hernandez; Mercado, G. A.

    2008-08-11

    Monte Carlo calculations have been carried out to estimate the Air Kerma rate constant and the Ambient dose equivalent rate constant for 139 monoenergetic photon sources. The factor that relates activity to air kerma rate or to ambient dose equivalent is useful to estimate the dose from a photon emitter source. Here 139 point-like and monoenergetic gamma-ray sources, ranging from 0.01 to 10 MeV were utilized in Monte Carlo calculations to estimate both gamma factors. These factors were utilized to calculate the air kerma-and-ambient dose equivalent rate constants for {sup 137}Cs-{sup 137m}Ba, {sup 198}Au, {sup 60}Co, and {sup 131}I, whose values were compared with those published in the literature.

  17. Reaction of atomic bromine with acetylene and loss rate of atmospheric acetylene due to reaction with OH, Cl, O, and Br

    NASA Technical Reports Server (NTRS)

    Payne, W. A.; Nava, D. F.; Brunning, J.; Stief, L. J.

    1986-01-01

    The first-order, diffusion, and bimolecular rate constants for the reaction Br + C2H2 yields C2H3Br are evaluated. The rate constants are measured at 210, 248, 298, and 393 K and at pressures between 15-100 torr Ar using flash photolysis combined with time-resolved detection of atomic bromine via Br resonance radiation. It is observed that the reaction is not affected by pressure or temperature and the bimolecular constant = (4.0 + or - 0.8) x 10 to the -15th cu cm/sec with an error of two standard deviations. The C2H2 + Br reaction rates are compared with reactions of C2H2 with Cl, OH, NH2, and H. The loss rates for atmospheric C2H2 for reactions with OH, Cl, O, and Br are calculated as a function of altitude.

  18. Reaction of atomic bromine with acetylene and loss rate of atmospheric acetylene due to reaction with OH, Cl, O, and Br

    NASA Technical Reports Server (NTRS)

    Payne, W. A.; Nava, D. F.; Brunning, J.; Stief, L. J.

    1986-01-01

    The first-order, diffusion, and bimolecular rate constants for the reaction Br + C2H2 yields C2H3Br are evaluated. The rate constants are measured at 210, 248, 298, and 393 K and at pressures between 15-100 torr Ar using flash photolysis combined with time-resolved detection of atomic bromine via Br resonance radiation. It is observed that the reaction is not affected by pressure or temperature and the bimolecular constant = (4.0 + or - 0.8) x 10 to the -15th cu cm/sec with an error of two standard deviations. The C2H2 + Br reaction rates are compared with reactions of C2H2 with Cl, OH, NH2, and H. The loss rates for atmospheric C2H2 for reactions with OH, Cl, O, and Br are calculated as a function of altitude.

  19. ESTIMATION OF PHOSPHATE ESTER HYDROLYSIS RATE CONSTANTS. II. ACID AND GENERAL BASE CATALYZED HYDROLYSIS

    EPA Science Inventory

    SPARC (SPARC Performs Automated Reasoning in Chemistry) chemical reactivity models were extended to calculate acid and neutral hydrolysis rate constants of phosphate esters in water. The rate is calculated from the energy difference between the initial and transition states of a ...

  20. ESTIMATION OF PHOSPHATE ESTER HYDROLYSIS RATE CONSTANTS. II. ACID AND GENERAL BASE CATALYZED HYDROLYSIS

    EPA Science Inventory

    SPARC (SPARC Performs Automated Reasoning in Chemistry) chemical reactivity models were extended to calculate acid and neutral hydrolysis rate constants of phosphate esters in water. The rate is calculated from the energy difference between the initial and transition states of a ...

  1. A new approach using coagulation rate constant for evaluation of turbidity removal

    NASA Astrophysics Data System (ADS)

    Al-Sameraiy, Mukheled

    2017-06-01

    Coagulation-flocculation-sedimentation processes for treating three levels of bentonite synthetic turbid water using date seeds (DS) and alum (A) coagulants were investigated in the previous research work. In the current research, the same experimental results were used to adopt a new approach on a basis of using coagulation rate constant as an investigating parameter to identify optimum doses of these coagulants. Moreover, the performance of these coagulants to meet (WHO) turbidity standard was assessed by introducing a new evaluating criterion in terms of critical coagulation rate constant (kc). Coagulation rate constants (k2) were mathematically calculated in second order form of coagulation process for each coagulant. The maximum (k2) values corresponded to doses, which were obviously to be considered as optimum doses. The proposed criterion to assess the performance of coagulation process of these coagulants was based on the mathematical representation of (WHO) turbidity guidelines in second order form of coagulation process stated that (k2) for each coagulant should be ≥ (kc) for each level of synthetic turbid water. For all tested turbid water, DS coagulant could not satisfy it. While, A coagulant could satisfy it. The results obtained in the present research are exactly in agreement with the previous published results in terms of finding optimum doses for each coagulant and assessing their performances. On the whole, it is recommended considering coagulation rate constant to be a new approach as an indicator for investigating optimum doses and critical coagulation rate constant to be a new evaluating criterion to assess coagulants' performance.

  2. A new approach using coagulation rate constant for evaluation of turbidity removal

    NASA Astrophysics Data System (ADS)

    Al-Sameraiy, Mukheled

    2015-09-01

    Coagulation-flocculation-sedimentation processes for treating three levels of bentonite synthetic turbid water using date seeds (DS) and alum (A) coagulants were investigated in the previous research work. In the current research, the same experimental results were used to adopt a new approach on a basis of using coagulation rate constant as an investigating parameter to identify optimum doses of these coagulants. Moreover, the performance of these coagulants to meet (WHO) turbidity standard was assessed by introducing a new evaluating criterion in terms of critical coagulation rate constant (kc). Coagulation rate constants (k2) were mathematically calculated in second order form of coagulation process for each coagulant. The maximum (k2) values corresponded to doses, which were obviously to be considered as optimum doses. The proposed criterion to assess the performance of coagulation process of these coagulants was based on the mathematical representation of (WHO) turbidity guidelines in second order form of coagulation process stated that (k2) for each coagulant should be ≥ (kc) for each level of synthetic turbid water. For all tested turbid water, DS coagulant could not satisfy it. While, A coagulant could satisfy it. The results obtained in the present research are exactly in agreement with the previous published results in terms of finding optimum doses for each coagulant and assessing their performances. On the whole, it is recommended considering coagulation rate constant to be a new approach as an indicator for investigating optimum doses and critical coagulation rate constant to be a new evaluating criterion to assess coagulants' performance.

  3. In-cell reaction rate distributions and cell-average reaction rates in fast critical assemblies

    SciTech Connect

    Brumbach, S.B.; Gasidlo, J.M.

    1985-08-01

    Measurements are described for determining average values of fission rates in /sup 235/U, /sup 238/U and /sup 239/Pu and capture rates in /sup 238/U for heterogeneous cells used to construct fast critical assemblies. The measurements are based on irradiations of foils of /sup 238/U, /sup 235/U and /sup 239/Pu with counting of fission and capture products using gamma-ray spectroscopy. Both plate and pin cells are considered. Procedures are described for inferring cell-average reaction rate values from a single foil location based on a cell using a quantity called a cell factor. Cell factors are determined from special measurements in which several foils are irradiated within a cell. Comparisons are presented between cell factors determined by measurements and by Monte Carlo calculations which lend credibility to the measurement procedures.

  4. Extraction of elementary rate constants from global network analysis of E. coli central metabolism

    PubMed Central

    Zhao, Jiao; Ridgway, Douglas; Broderick, Gordon; Kovalenko, Andriy; Ellison, Michael

    2008-01-01

    Background As computational performance steadily increases, so does interest in extending one-particle-per-molecule models to larger physiological problems. Such models however require elementary rate constants to calculate time-dependent rate coefficients under physiological conditions. Unfortunately, even when in vivo kinetic data is available, it is often in the form of aggregated rate laws (ARL) that do not specify the required elementary rate constants corresponding to mass-action rate laws (MRL). There is therefore a need to develop a method which is capable of automatically transforming ARL kinetic information into more detailed MRL rate constants. Results By incorporating proteomic data related to enzyme abundance into an MRL modelling framework, here we present an efficient method operating at a global network level for extracting elementary rate constants from experiment-based aggregated rate law (ARL) models. The method combines two techniques that can be used to overcome the difficult properties in parameterization. The first, a hybrid MRL/ARL modelling technique, is used to divide the parameter estimation problem into sub-problems, so that the parameters of the mass action rate laws for each enzyme are estimated in separate steps. This reduces the number of parameters that have to be optimized simultaneously. The second, a hybrid algebraic-numerical simulation and optimization approach, is used to render some rate constants identifiable, as well as to greatly narrow the bounds of the other rate constants that remain unidentifiable. This is done by incorporating equality constraints derived from the King-Altman and Cleland method into the simulated annealing algorithm. We apply these two techniques to estimate the rate constants of a model of E. coli glycolytic pathways. The simulation and statistical results show that our innovative method performs well in dealing with the issues of high computation cost, stiffness, local minima and uncertainty

  5. Extraction of elementary rate constants from global network analysis of E. coli central metabolism.

    PubMed

    Zhao, Jiao; Ridgway, Douglas; Broderick, Gordon; Kovalenko, Andriy; Ellison, Michael

    2008-05-07

    As computational performance steadily increases, so does interest in extending one-particle-per-molecule models to larger physiological problems. Such models however require elementary rate constants to calculate time-dependent rate coefficients under physiological conditions. Unfortunately, even when in vivo kinetic data is available, it is often in the form of aggregated rate laws (ARL) that do not specify the required elementary rate constants corresponding to mass-action rate laws (MRL). There is therefore a need to develop a method which is capable of automatically transforming ARL kinetic information into more detailed MRL rate constants. By incorporating proteomic data related to enzyme abundance into an MRL modelling framework, here we present an efficient method operating at a global network level for extracting elementary rate constants from experiment-based aggregated rate law (ARL) models. The method combines two techniques that can be used to overcome the difficult properties in parameterization. The first, a hybrid MRL/ARL modelling technique, is used to divide the parameter estimation problem into sub-problems, so that the parameters of the mass action rate laws for each enzyme are estimated in separate steps. This reduces the number of parameters that have to be optimized simultaneously. The second, a hybrid algebraic-numerical simulation and optimization approach, is used to render some rate constants identifiable, as well as to greatly narrow the bounds of the other rate constants that remain unidentifiable. This is done by incorporating equality constraints derived from the King-Altman and Cleland method into the simulated annealing algorithm. We apply these two techniques to estimate the rate constants of a model of E. coli glycolytic pathways. The simulation and statistical results show that our innovative method performs well in dealing with the issues of high computation cost, stiffness, local minima and uncertainty inherent with large

  6. Photocatalytic degradation of water contaminants in multiple photoreactors and evaluation of reaction kinetic constants independent of photon absorption, irradiance, reactor geometry, and hydrodynamics.

    PubMed

    Grčić, Ivana; Li Puma, Gianluca

    2013-12-03

    The literature on photocatalytic oxidation of water pollutants often reports reaction kinetic constants, which cannot be unraveled from photoreactor type and experimental conditions. This study addresses this challenging aspect by presenting a general and simple methodology for the evaluation of fundamental "intrinsic" reaction kinetic constants of photocatalytic degradation of water contaminants, which are independent of photoreactor type, catalyst concentration, irradiance levels, and hydrodynamics. The degradation of the model contaminant, oxalic acid (OA) on titanium dioxide (TiO2) aqueous suspensions, was monitored in two annular photoreactors (PR1 and PR2). The photoreactors with significantly different geometries were operated under different hydrodynamic regimes (turbulent batch mode and laminar flow-through recirculation mode), optical thicknesses, catalyst and OA concentrations, and photon irradiances. The local volumetric rate of photon absorption (LVRPA) was evaluated by the six-flux radiation absorption-scattering model (SFM). The SFM was further combined with a comprehensive kinetic model for the adsorption and photodecomposition of OA on TiO2 to determine local reaction rates and, after integration over the reactor volume, the intrinsic reaction kinetic constants. The model could determine the oxidation of OA in both PR1 and PR2 under a wide range of experimental conditions. This study demonstrates a more meaningful way for determining reaction kinetic constants of photocatalytic degradation of water contaminants.

  7. The reaction mechanism with free energy barriers at constant potentials for the oxygen evolution reaction at the IrO2 (110) surface

    SciTech Connect

    Ping, Yuan; Nielsen, Robert J.; Goddard, William A.

    2016-12-09

    How to efficiently oxidize H2O to O2 (oxygen evolution reaction, OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work, we performed first-principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OER at the IrO2 (110) surface. We determined the surface phase diagram, explored the possible reaction pathways including kinetic barriers, and computed reaction rates based on the microkinetic models. Furthermore, this allowed us to resolve several long-standing puzzles about the atomistic OER mechanism.

  8. QSAR ANALYSIS OF SORPTION-CORRECTED RATE CONSTANTS FOR REDUCTIVE BIOTRANSFORMATION OF HALOGENATED AROMATICS

    EPA Science Inventory

    The inherent coupling among geochemical and microbial reactions may have significant effects on the environmental fate of a containinant. For example, sorption processes may decrease the concentration of an organic compound in solution, thereby reducing the biodegradation rate of...

  9. Relative rate studies of the reactions of chlorine atoms with simple alkanes and the chlorinated methanes

    SciTech Connect

    Wingen, L.; Lee, J.J.; Neavyn, R.

    1995-12-01

    The reactions of chlorine atoms with organics are of interest because atomic chlorine is a potential tropospheric oxidant. Relative rate constants for the reaction of pairs of simple alkanes (ethane/propane, ethane/n-butane, and isobutane/n-butane) and the chlorinated methanes (chloromethane, dichloromethane, and chloroform relative to methane) were measured, using the photolysis of Cl{sub 2} as the source of chlorine atoms and following the loss of the organics by GC-FID. The ratios of the relative rate constants were in excellent agreement with the literature except for ethane/n-butane, where our results are approximately 20% lower than recently published values, and for chloroform/methane, where our value is approximately 50% higher than the values recommended by JPL and JPCRD. Our results will be compard to previously published relative rate studies as well as to the results of absolute rate constant studies, and the differences will be discussed.

  10. A novel path sampling method for the calculation of rate constants

    NASA Astrophysics Data System (ADS)

    van Erp, Titus S.; Moroni, Daniele; Bolhuis, Peter G.

    2003-05-01

    We derive a novel efficient scheme to measure the rate constant of transitions between stable states separated by high free energy barriers in a complex environment within the framework of transition path sampling. The method is based on directly and simultaneously measuring the fluxes through many phase space interfaces and increases the efficiency with at least a factor of 2 with respect to existing transition path sampling rate constant algorithms. The new algorithm is illustrated on the isomerization of a diatomic molecule immersed in a simple fluid.

  11. Laser flash photolysis generation and kinetic studies of porphyrin-manganese-oxo intermediates. Rate constants for oxidations effected by porphyrin-Mn(V)-oxo species and apparent disproportionation equilibrium constants for porphyrin-Mn(IV)-oxo species.

    PubMed

    Zhang, Rui; Horner, John H; Newcomb, Martin

    2005-05-11

    Porphyrin-manganese(V)-oxo and porphyrin-manganese(IV)-oxo species were produced in organic solvents by laser flash photolysis (LFP) of the corresponding porphyrin-manganese(III) perchlorate and chlorate complexes, respectively, permitting direct kinetic studies. The porphyrin systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP), and 5,10,15,20-tetrakis(4-methylpyridinium)porphyrin (TMPyP). The order of reactivity for (porphyrin)Mn(V)(O) derivatives in self-decay reactions in acetonitrile and in oxidations of substrates was (TPFPP) > (TMPyP) > (TPP). Representative rate constants for reaction of (TPFPP)Mn(V)(O) in acetonitrile are k = 6.1 x 10(5) M(-1) s(-1) for cis-stilbene and k = 1.4 x 10(5) M(-1) s(-1) for diphenylmethane, and the kinetic isotope effect in oxidation of ethylbenzene and ethylbenzene-d(10) is k(H)/k(D) = 2.3. Competitive oxidation reactions conducted under catalytic conditions display approximately the same relative rate constants as were found in the LFP studies of (porphyrin)Mn(V)(O) derivatives. The apparent rate constants for reactions of (porphyrin)Mn(IV)(O) species show inverted reactivity order with (TPFPP) < (TMPyP) < (TPP) in reactions with cis-stilbene, triphenylamine, and triphenylphosphine. The inverted reactivity results because (porphyrin)Mn(IV)(O) disproportionates to (porphyrin)Mn(III)X and (porphyrin)Mn(V)(O), which is the primary oxidant, and the equilibrium constants for disproportionation of (porphyrin)Mn(IV)(O) are in the order (TPFPP) < (TMPyP) < (TPP). The fast comproportionation reaction of (TPFPP)Mn(V)(O) with (TPFPP)Mn(III)Cl to give (TPFPP)Mn(IV)(O) (k = 5 x 10(8) M(-1) s(-1)) and disproportionation reaction of (TPP)Mn(IV)(O) to give (TPP)Mn(V)(O) and (TPP)Mn(III)X (k approximately 2.5 x 10(9) M(-1) s(-1)) were observed. The relative populations of (porphyrin)Mn(V)(O) and (porphyrin)Mn(IV)(O) were determined from the ratios of observed rate constants for

  12. NACRE: A European Compilation of Reaction Rates for Astrophysics

    SciTech Connect

    Carmen Angulo

    1999-12-31

    We report on the program and results of the NACRE network (Nuclear Astrophysics Compilation of Reaction rates). We have compiled low-energy cross section data for 86 charged-particle induced reactions involving light (1 {<=} Z {<=} 14) nuclei. The corresponding Maxwellian-averaged thermonuclear reactions rates are calculated in the temperature range from 10{sup 6} K to 10{sup 10} K. The web site, http://pntpm.ulb.ac.be/nacre.htm, including the cross section data base and the reaction rates, allows users to browse electronically all the information on the reactions studied in this compilation.

  13. NACRE: A European Compilation of Reaction rates for Astrophysics

    SciTech Connect

    Angulo, Carmen

    1999-11-16

    We report on the program and results of the NACRE network (Nuclear Astrophysics Compilation of REaction rates). We have compiled low-energy cross section data for 86 charged-particle induced reactions involving light (1{<=}Z{<=}14) nuclei. The corresponding Maxwellian-averaged thermonuclear reactions rates are calculated in the temperature range from 10{sup 6} K to 10{sup 10} K. The web site http://pntpm.ulb.ac.be/nacre.htm, including the cross section data base and the reaction rates, allows users to browse electronically all the information on the reactions studied in this compilation.

  14. Effects of various process parameters on struvite precipitation kinetics and subsequent determination of rate constants.

    PubMed

    Rahaman, M S; Ellis, N; Mavinic, D S

    2008-01-01

    In this paper, struvite (MgNH(4)PO(4).6H(2)O) precipitation kinetics were studied with different operating conditions (varying supersaturation, pH, Mg:P ratio, degree of mixing and seeding conditions) and relevant rate constants were determined by fitting a slightly modified first-order kinetic model to the experimental data obtained. The rate of change of ortho-P concentration in the bulk solutions increases with increasing supersaturation ratio. The estimated rate constants are 2.034, 1.716 and 0.690 hr(-1) for the supersaturation ratio of 9.64, 4.83, and 2.44, respectively. Kinetic parameters were also evaluated for the Mg:P ratio between the ranges of 1.0 and 1.6, indicating higher phosphorus removal efficiency with increasing Mg:P ratio. The rate constants were found to be 0.942, 2.034 and 2.712 hr(-1) for Mg:P ratios of 1.0, 1.3 and 1.6, respectively. The experimental observations for kinetic study of struvite precipitation with different stirrer speeds clearly show that the mixing intensity used had little effect on the intrinsic rate constants. K values found to be 2.034 and 1.902 h(-1) for 100 and 70 rpm, respectively. Seeding, with 250-500 microm of seed crystals during the struvite precipitation kinetics test, was found to have very little effect on the ortho-P removal.

  15. Robust reconstruction of the rate constant distribution using the phase function method.

    PubMed

    Zhou, Yajun; Zhuang, Xiaowei

    2006-12-01

    Many biological processes exhibit complex kinetic behavior that involves a nontrivial distribution of rate constants. Characterization of the rate constant distribution is often critical for mechanistic understandings of these processes. However, it is difficult to extract a rate constant distribution from data measured in the time domain. This is due to the numerical instability of the inverse Laplace transform, a long-standing mathematical challenge that has hampered data analysis in many disciplines. Here, we present a method that allows us to reconstruct the probability distribution of rate constants from decay data in the time domain, without fitting to specific trial functions or requiring any prior knowledge of the rate distribution. The robustness (numerical stability) of this reconstruction method is numerically illustrated by analyzing data with realistic noise and theoretically proved by the continuity of the transformations connecting the relevant function spaces. This development enhances our ability to characterize kinetics and dynamics of biological processes. We expect this method to be useful in a broad range of disciplines considering the prevalence of complex exponential decays in many experimental systems.

  16. Reaction rate theory of radiation exposure:Effects of dose rate on mutation frequency

    NASA Astrophysics Data System (ADS)

    Bando, Masako; Nakamura, Issei; Manabe, Yuichiro

    2014-03-01

    We revisit the linear no threshold (LNT) hypothesis deduced from the prominent works done by H. J. Muller for the DNA mutation induced by the artificial radiation and by W. L. Russell and E. M. Kelly for that of mega-mouse experiments, developing a new kinetic reaction theory. While the existing theoretical models primarily rely on the dependence of the total dose D on the mutation frequency, the key ingredient in our theory is the dose rate d(t) that accounts for decrease in the mutation rate during the time course of the cellular reactions. The general form for the mutation frequency with the constant dose rate d is simply expressed as, dFm(t)/dt = A - BFm(t) , with A =a0 +a1(d +deff) and B =b0 +b1(d +deff) . We discuss the solution for a most likely case with B > 0 ; Fm(t) = [A/B -Fm(0) ] (1 -e-Bt) +Fm(0) with the control value Fm(0) . We show that all the data of mega-mouse experiments by Russel with different dose rates fall on the universal scaling function Φ(τ) ≡ [Fm(τ) -Fm(0) ]/[ A / B -Fm(0) ] = 1 - exp(- τ) with scaled time τ = Bt . The concept of such a scaling rule provides us with a strong tool to study different species in a unified manner.

  17. Crossover from nonclassical to classical chemical kinetics in an initially separated A + B<-->C reaction-diffusion system with arbitrary diffusion constants.

    PubMed

    Sinder, M; Pelleg, J

    2000-05-01

    The asymptotic long-time properties of the reaction front formed in a reversible reaction-diffusion process A + B<-->C with initially separated reactants are investigated. The case of arbitrary nonzero values of the diffusion constants DA, DB, DC of the components A, B, C and the initial concentrations a0 and b0 of A and B is considered. The system is studied in the limit of g-->0, where g is the backward reaction rate constant. In accordance with previous work, the dynamics of the reaction front is described as a crossover between the "irreversible" regime at times t < g-1 and the "reversible" regime at times t > g-1. It is shown that through this crossover the macroscopic properties of the reaction front, such as the global rate of C production, the motion of the reaction zone center, and the concentration profiles of the components outside the reaction front, are unchanged. The concentration profiles of the components inside the reaction zone are described by quasistatic equations. The results of the theoretical consideration are confirmed by computing the mean-field kinetics equations.

  18. Reaction kinetics and critical phenomena: rates of some first order gas evolution reactions in binary solvents with a consolute point.

    PubMed

    Kim, Yeong Woo; Baird, James K

    2005-06-02

    We have measured the rate of carbon dioxide evolution in the aniline catalyzed decomposition of acetone dicarboxylic acid in a mixture of isobutyric acid + water near its consolute point. Within a temperature interval of 1 degrees C, which included the critical solution temperature, the first-order rate constant oscillated in magnitude by about 10% as it passed through three complete cycles of slowing down followed by speeding up. Whereas we can find no ready explanation for the speeding up, we suggest that, because the mixture contained no inert components, the slowing down should belong to the Griffiths-Wheeler class of strong critical effects [Phys. Rev. A 1970, 2, 1047]. As a check on this conclusion, we have measured the rate of the SN1 decomposition of benzene diazonium tetrafluoroborate in 2-butoxyethanol + water near the lower critical solution temperature and also the rate of the acid-catalyzed decomposition of ethyl diazoacetate in isobutyric acid + water near the upper critical solution temperature. Both of these reactions evolve nitrogen. In the first reaction, 2-butoxyethanol is inert, whereas in the second, isobutyric acid is inert. In both cases, because there was one inert component, we regarded the response of the rate constant to temperature in the critical region to be representative of the Griffiths-Wheeler class of weak critical effects. Within our accuracy of measurement of about 2% in the rate constant and about 1 mK in the temperature, we could detect no effect of the critical point on the rates of either of these reactions, suggesting that a weak effect may be too small to be seen with our experimental apparatus. The successful observation of a critical effect in the rate of decomposition of acetone dicarboxylic acid proves, however, that kinetic critical phenomena are observable in heterogeneous reactions.

  19. THEORETICAL ANALYSIS OF THE TRANSIENT PRESSURE RESPONSE FROM A CONSTANT FLOW RATE HYDRAULIC CONDUCTIVITY TEST.

    USGS Publications Warehouse

    Morin, Roger H.; Olsen, Harold W.

    1987-01-01

    Incorporating a flow pump into a conventional triaxial laboratory system allows fluid to be supplied to or withdrawn from the base of a sediment sample at small and constant rates. An initial transient record of hydraulic head versus time is observed which eventually stabilizes to a constant steady state gradient across the sample; values of hydraulic conductivity can subsequently be determined from Darcy's law. In this paper, analytical methods are presented for determining values of specific storage and hydraulic conductivity from the initial transient phase of such a constant flow rate test. These methods are based on a diffusion equation involving pore pressure and are analogous to those used to describe the soil consolidation process and also to interpret aquifer properties from pumping tests.

  20. Shock tube measurements of specific reaction rates in branched chain CH4-CO-O2 system

    NASA Technical Reports Server (NTRS)

    Brabbs, T. A.; Brokaw, R. S.

    1974-01-01

    Rate constants of two elementary bimolecular reactions involved in the oxidation of methane were determined by monitoring the exponential growth of CO flame band emission behind incident shocks in three suitably chosen gas mixtures.