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1

Theoretical and Experimental Considerations of the Pseudo-First-Order Approximation in Conventional Kinetic Analysis of IAsys Biosensor Data  

Microsoft Academic Search

The validity of the conventional interpretation of IAsys biosensor profiles in terms of pseudo-first-order kinetic behavior is subjected to closer scrutiny by its application to simulated data for low- and high-affinity interactions between ligate and immobilized ligand. As might reasonably have been expected, analysis of the simulated data for the low-affinity system (association equilibrium constant of 105M?1) in such terms

Damien R. Hall; Nick N. Gorgani; Joseph G. Altin; Donald J. Winzor

1997-01-01

2

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

NASA Technical Reports Server (NTRS)

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.

Keyser, L. F.

1980-01-01

3

Kinetic and mechanistic studies of blue tetrazolium reaction with phenylhydrazines.  

PubMed

The reaction kinetics of blue tetrazolium with selected arylhydrazines were investigated under pseudo-first-order conditions. The reaction rate constants were obtained at various temperatures, and the enthalpy (8.4-11.2 kcal/mole) and entropy (-38--45 eu) of activations were calculated. A Hammett plot yielded a straight line with a slope of 0.52. The reaction was inhibited by atmospheric oxygen and iodine. A free radical mechanism is presented. PMID:660510

Biehl, E R; Wooten, R; Kenner, C T; Graham, R E

1978-07-01

4

Reaction kinetics of resveratrol with tert-butoxyl radicals  

NASA Astrophysics Data System (ADS)

The rate constant for the reaction of t-butoxyl radicals with resveratrol was studied under pseudo-first order conditions. The rate constant was determined by measuring the phenoxyl radical formation rate at 390 nm as function of resveratrol concentration in acetonitrile. The rate constant was determined to be 6.5×108 M-1s-1. This high value indicates the high reactivity consistent with the strong antioxidant activity of resveratrol.

Džeba, Iva; Pedzinski, Tomasz; Mihaljevi?, Branka

2012-09-01

5

Faster rates with less catalyst in template-directed reactions  

NASA Technical Reports Server (NTRS)

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.

Kanavarioti, A.; Baird, E. E.

1995-01-01

6

Accessing reaction rate constants in on-column reaction chromatography: an extended unified equation for reaction educts and products with different response factors.  

PubMed

An extension of the unified equation of chromatography to directly access reaction rate constants k(1) of first-order reaction in on-column chromatography is presented. This extended equation reflects different response factors in the detection of the reaction educt and product which arise from structural changes by elimination or addition, e.g., under pseudo-first-order reaction conditions. The reaction rate constants k(1) and Gibbs activation energies DeltaG(double dagger) of first-order reactions taking place in a chromatographic system can be directly calculated from the chromatographic parameters, i.e., retention times of the educt E and product P (t(R)(A) and t(R)(B)), peak widths at half height (w(A) and w(B)), the relative plateau height (h(p)) of the conversion profile, and the individual response factors f(A) and f(B). The evaluation of on-column reaction gas chromatographic experiments is exemplified by the evaluation of elution profiles obtained by ring-closing metathesis reaction of N,N-diallytrifluoroacetamide in presence of Grubbs second-generation catalyst, dissolved in polydimethylsiloxane (GE SE 30). PMID:19669676

Trapp, Oliver; Bremer, Sabrina; Weber, Sven K

2009-11-01

7

Reaction kinetics and transformation products of 1-naphthol by Mn oxide-mediated oxidative-coupling reaction  

Microsoft Academic Search

In this study, the transformation of 1-naphthol via oxidative-coupling reaction was investigated using Mn oxides. 1-Naphthol was transformed completely by birnessite, which is one of the natural Mn oxides present in soil. The surface area-normalized specific rate constant, ksurf, for 1-naphthol was determined to be 9.66×10?4L\\/m2min using observed pseudo-first-order rate constants with respect to birnessite loading. The transformation of 1-naphthol

Hyun-Sang Shin; Dong-Min Lim; Doo-Hee Lee; Ki-Hoon Kang

2009-01-01

8

Unified equation for access to rate constants of first-order reactions in dynamic and on-column reaction chromatography.  

PubMed

A unified equation to evaluate elution profiles of reversible as well as irreversible (pseudo-) first-order reactions in dynamic chromatography and on-column reaction chromatography has been derived. Rate constants k1 and k(-1) and Gibbs activation energies are directly obtained from the chromatographic parameters (retention times tR(A) and tR(B) of the interconverting or reacting species A and B, the peak widths at half-height wA and wB, and the relative plateau height h(p)), the initial amounts A0 and B0 of the reacting species, and the equilibrium constant K(A/B). The calculation of rate constants requires only a few iterative steps without the need of performing a computationally extensive simulation of elution profiles. The unified equation was validated by comparison with a data set of 125,000 simulated elution profiles to confirm the quality of this equation by statistical means and to predict the minimal experimental requirements. Surprisingly, the recovery rate from a defined data set is on average 35% higher using the unified equation compared to the evaluation by iterative computer simulation. PMID:16383327

Trapp, O

2006-01-01

9

Applications of Reaction Rate  

ERIC Educational Resources Information Center

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…

Cunningham, Kevin

2007-01-01

10

QSARS for predicting biotic and abiotic reductive transformation rate constants of halogenated hydrocarbons in anoxic sediment systems  

Microsoft Academic Search

Quantitative structure-activity relationships (QSARs) are developed relating biotic and abiotic pseudo-first-order disappearance rate constants of halogenated hydrocarbons in anoxic sediments to a number of readily available molecular descriptors. Based upon knowledge of the underlying reaction mechanisms, four descriptors were selected: carbon halogen bond strength, the summation of the Hammett (aromatics) and Taft (aliphatics) sigma constants and the inductive constants (aromatics)

W. J. G. M. Peijnenburg; M. J. t Hart; H. A. den Hollander; D. van de Meent; H. H. Verboom

1991-01-01

11

Polychlorinated ethane reaction with zero-valent zinc: pathways and rate control  

NASA Astrophysics Data System (ADS)

Efficient design of zero-valent metal permeable `barriers' for the reduction of organohalides requires information regarding the pertinent reaction rates as well as an understanding of the resultant distribution of products. In this study, the pathways and kinetics for reaction of polychlorinated ethanes with Zn(0) have been examined in batch reactors. Reductive ?-elimination was the only route through which hexachloroethane (HCA), 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA), 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA), 1,1,2-trichloroethane (1,1,2-TCA) and 1,2-dichloroethane (1,2-DCA) reacted. Pentachloroethane (PCA) reacted via concurrent reductive ?-elimination (93%) and hydrolysis (7%). As previously demonstrated, 1,1,1-trichloroethane (1,1,1-TCA) and 1,1-dichloroethane (1,1-DCA) reacted predominantly via reductive ?-elimination. Attempts to correlate BET surface area-normalized rate constants ( kSA-BET) with one-electron reduction potential ( E1) met with limited success, as HCA, PCA, 1,1,1,2-TeCA, and 1,1,1-TCA reacted at nearly identical rates despite substantial differences in E1 values. Comparison of the pseudo-first-order rate constants ( kobs) for these species with rate constants ( kLa) obtained from a correlation for mass transfer to suspended particles revealed that the reaction of these species was mass transfer limited even though reaction rates were unaffected by mixing speed. Calculations suggest that mass transfer limitations may also play a role in the design of treatment systems for highly reactive species, with overall rate constants predicted to increase with flow velocity.

Arnold, William A.; Ball, William P.; Roberts, A. Lynn

1999-12-01

12

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

PubMed

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): PMID:24817270

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

2014-06-28

13

What Is a Reaction Rate?  

ERIC Educational Resources Information Center

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.

Schmitz, Guy

2005-01-01

14

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

PubMed

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 ?-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 ?-site. The two data sets were used to directly infer the branching ratio for the reaction at the ?-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 ?-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. PMID:24405356

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

2014-02-01

15

Inverse phase transfer catalysis: kinetics of the pyridine 1-oxide-catalyzed two-phase reactions of fluoro- and butyl-benzoyl chlorides and benzoate ions  

Microsoft Academic Search

The substitution reactions of XC6H4COCl (X=2-F, 3-F, 4-F, or 4-(CH3)3C) and YC6H4COONa (Y=2-F, 3-F, 4-F, or H) in a two-phase H2O\\/CH2Cl2 medium using pyridine 1-oxide (PNO) as an inverse phase transfer catalyst were investigated. Under suitable reaction conditions, the kinetics of the reaction follows a pseudo-first-order rate law, with a constant observed rate being a linear function of the concentration

Shu-Mei Hung; Jing-Jer Jwo

2000-01-01

16

Diffusion-controlled chemical reactions modeled by continuous-time random walks  

Microsoft Academic Search

In the kinetic theory of pseudo-first-order diffusion-controlled chemical reactions, the diffusion motion of one component can be suppressed as long as the relative diffusion coefficient is assigned to the other component. The continuous-time-random-walk (CTWR) treatment of the pseudo-first-order reactions reaffirmed this well-known notion in the case of an exponential jump time distribution. In the case of a long-tail jump time

W. P. Helman; K. Funabashi

1979-01-01

17

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

NASA Technical Reports Server (NTRS)

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.

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

2004-01-01

18

Overall rate constant measurements of the reaction of hydroxy- and chloroalkylperoxy radicals derived from methacrolein and methyl vinyl ketone with nitric oxide.  

PubMed

The overall rate constants of the reactions of NO with hydroxy- and chloroalkylperoxy radicals, derived from the OH- and Cl-initiated oxidation of methacrolein and methyl vinyl ketone, respectively, were directly determined for the first time using the turbulent-flow technique and pseudo-first-order kinetics conditions with high-pressure chemical ionization mass spectrometry for the direct detection of peroxy radical reactants. The individual 100 Torr, 298 K hydroxyalkylperoxy + NO rate constants for the methacrolein [(0.93 +/- 0.12) (2sigma) x 10(-11) cm3 molecule(-1) s(-1)] and methyl vinyl ketone [(0.84 +/- 0.10) x 10(-11) cm3 molecule(-1) s(-1)] systems were found to be identical within the 95% confidence interval associated with each separate measurement, as were the chloroalkylperoxy + NO rate constants for both methacrolein [(1.17 +/- 0.11) x 10(-11) cm3 molecule(-1) s(-1)] and methyl vinyl ketone [(1.14 +/- 0.14) x 10(-11) cm3 molecule(-1) s(-1)]. However, the difference in the rate constants between the hydroxyperoxy + NO and chloroalkylperoxy + NO systems was found to be statistically significant, with the chloroalkylperoxy + NO rate constants about 30% higher than the corresponding hydroxyalkylperoxy + NO rate constants. This substituent effect was rationalized via a frontier molecular orbital model approach. PMID:17249751

Hsin, Hong Yuan; Elrod, Matthew J

2007-02-01

19

Reaction networks, kinetics, and inhibition in the hydroprocessing of simulated heavy coal liquids (Volumes I and II)  

SciTech Connect

The goals of this research included measurement of (1) rates of simultaneous disappearance of 5-8 reactants in mixtures representative of SRC-II heavy distillate, (2) rates of formation of each product, allowing determination of quantitative reaction networks, and (3) determination of effects of inhibitors on these networks. The mixture include quinoline, acridine, indole, 5,6,7,8-tetrahydro-1-naphthol, dibenzofuran, phenanthrene, fluoranthene, and pyrene. The experiments were conducted with a fixed-bed flow microreactor at 171 atm and 350 C using a commercial pre-sulfided Ni-Mo catalyst with a stoichiometric excess of hydrogen and low concentrations (ca. 0.25 wt%) of organic reactants in cyclohexane solvent. Quinoline represents the only significant inhibitor among the different functional groups. In its absence, the hydrodeoxygenation (HDO) of 5,6,7,8-tetrahydro-1-naphthol proceeds with a pseudo first-order rate constant greater than 10 l/(g of catalyst {center dot}h), requiring large hydrogen consumptions. The pseudo first-order rate constants for all the hydrocarbon hydrogenations are 0.4-1.8 l/(g of catalyst {center dot}). Fluoranthene is hydrogenated more rapidly than phenanthrene or pyrene. Dibenzothiophene HDS has a pseudo first-order rate constant of 0.260 l/(g of catalyst {center dot}h); dibenzofuran HDO is one order of magnitude slower. The quninoline feed concentration was varied by one order of magnitude to provide data for modeling the inhibitions. The reaction networks for phenanthrene, fluoranthene and dibenzothiophene coupled with the quinoline network were modeled with Langmuir-Hinshelwood-Hougen-Watson rate expressions. Such a characterization provides the basis for powerful predictive models for the hydroprocessing kinetics of heavy fossil fuels.

Girgis, M.J.

1988-01-01

20

QSARs for Predicting Biotic and Abiotic Reductive Transformation Rate Constants of Halogenated Hydrocarbons in Anoxic Sediment Systems.  

National Technical Information Service (NTIS)

Quantitative structure-activity relationships (QSARs) are developed relating biotic and abiotic pseudo-first-order disappearance rate constants of halogenated hydrocarbons in anoxic sediments to a number of readily available molecular descriptors. Based u...

W. J. G. M. Peijnenburg M. J. 't Hart H. A. den Hollander D. van de Meent H. H. Verboom

1991-01-01

21

QSARS FOR PREDICTING BIOTIC AND ABIOTIC REDUCTIVE TRANSFORMATION RATE CONSTANTS OF HALOGENATED HYDROCARBONS IN ANOXIC SEDIMENT SYSTEMS  

EPA Science Inventory

Quantitative structure-activity relationships (QSARs) are developed relating biotic and abiotic pseudo-first-order disappearance rate constants of halogenated hydrocarbons in anoxic sediments to a number of readily available molecular descriptors. ased upon knowledge of the under...

22

Kinetics of some reactions of atmospheric interest: hydrolysis of carbonyl sulfide, and reaction rates of thermal chlorine atoms with carbonyl sulfide, methanol, hydrogen sulfide, and methyl chloride  

SciTech Connect

The hydrolysis appeared pseudo first order in OCS and showed a rate constant of (1.04 /times/ 10/sup 11/) exp(/minus/10800/T) sec /sup /minus/1/. The hydrolysis of OCS can be increased in rate by the presence of OH/sup /minus// concentration and showed a bimolecular rate constant of (8.12 /times/ 10/sup 9/) exp(/minus/6040/T) 1 mole/sup /minus/1/ sec /sup /minus/1/. The rate corresponds to a lifetime for dissolved OCS of 1.7 days. A two-layer diffusion model with accurate Henry's Law constant and hydrolysis rate constant can obtain air-sea exchange flux. The lifetime for transport of OCS from the troposphere to the ocean with respect to this flux is about 12 years. The ocean is most likely a net source of OCS rather than a sink, but hydrolysis of OCS still occurs in the ocean. The relative rate constants for the gas phase reactions of thermal chlorine atoms have been measured with H/sub 2/S, CH/sub 3/OH, C/sub 2/H/sub 6/, and CH/sub 2/ = CHBr over the temperature range from 232 to 359 K. These data for Cl atoms reaction with H/sub 2/S at a pressure of 4000 Torr of CClF/sub 3/ are consistent with a temperature-dependent rate constant of (10.5 /+-/ 0.4) /times/ 10/sup /minus/11/ cm/sup 3/ molecule/sup /minus/1/ sec/sup /minus/1/. The absolute rate constant for Cl atoms reaction with CH/sub 3/OH is determined to be (9.14 /+-/ 1.02) /times/ 10/sup /minus/11/ cm/sup 3/ molecule/sup /minus/1/ sec/sup /minus/1/ at temperature 295 K. We have carried out a series of competitive runs of OCS with CH/sub 2/ = CHBr and trans-2,3-DCHF-2B. The results of these runs do not indicate any rapid removal of /sup 38/Cl through reaction with OCS with or without the presence of O/sub 2/. An upper limit of 1 /times/ 10/sup /minus/15/ cm/sup 3/ molecule/sup /minus/1/ sec/sup /minus/1/ for the reaction of /sup 38/Cl with OCS.

Lu, E.C.C.

1987-01-01

23

Bimolecular rate constant and product branching ratio measurements for the reaction of C2H with ethene and propene at 79 K.  

PubMed

The reactions of the ethynyl radical (C(2)H) with ethene (C(2)H(4)) and propene (C(3)H(6)) are studied under low temperature conditions (79 K) in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by 193 nm photolysis of acetylene (C(2)H(2)) and the reactions are studied in nitrogen as a carrier gas. Reaction products are sampled and subsequently photoionized by the tunable vacuum ultraviolet radiation of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The product ions are detected mass selectively and time-resolved by a quadrupole mass spectrometer. Bimolecular rate coefficients are determined under pseudo-first-order conditions, yielding values in good agreement with previous measurements. Photoionization spectra are measured by scanning the ALS photon energy while detecting the ionized reaction products. Analysis of the photoionization spectra yields-for the first time-low temperature isomer resolved product branching ratios. The reaction between C(2)H and ethene is found to proceed by H-loss and yields 100% vinylacetylene. The reaction between C(2)H and propene results in (85 ± 10)% C(4)H(4) (m/z = 52) via CH(3)-loss and (15 ± 10)% C(5)H(6) (m/z = 66) by H-loss. The C(4)H(4) channel is found to consist of 100% vinylacetylene. For the C(5)H(6) channel, analysis of the photoionization spectrum reveals that (62 ± 16)% is in the form of 4-penten-1-yne, (27 ± 8)% is in the form of cis- and trans-3-penten-1-yne and (11 ± 10)% is in the form of 2-methyl-1-buten-3-yne. PMID:22429068

Bouwman, Jordy; Goulay, Fabien; Leone, Stephen R; Wilson, Kevin R

2012-04-19

24

Investigating Factors Influencing Rates of Chemical Reactions  

NSDL National Science Digital Library

This activity is a lab investigation in which students observe the rate of generation of hydrogen gas from a reaction, and then modify the procedure to compare another variable affecting the rate of this reaction.

Derickson, Paula

25

Reaction rate and isomer-specific product branching ratios of C2H + C4H8: 1-butene, cis-2-butene, trans-2-butene, and isobutene at 79 K.  

PubMed

The reactions of C2H radicals with C4H8 isomers 1-butene, cis-2-butene, trans-2-butene, and isobutene are studied by laser photolysis-vacuum ultraviolet mass spectrometry in a Laval nozzle expansion at 79 K. Bimolecular-reaction rate constants are obtained by measuring the formation rate of the reaction product species as a function of the reactant density under pseudo-first-order conditions. The rate constants are (1.9 ± 0.5) × 10(-10), (1.7 ± 0.5) × 10(-10), (2.1 ± 0.7) × 10(-10), and (1.8 ± 0.9) × 10(-10) cm(3) s(-1) for the reaction of C2H with 1-butene, cis-2-butene, trans-2-butene, and isobutene, respectively. Bimolecular rate constants for 1-butene and isobutene compare well to values measured previously at 103 K using C2H chemiluminescence. Photoionization spectra of the reaction products are measured and fitted to ionization spectra of the contributing isomers. In conjunction with absolute-ionization cross sections, these fits provide isomer-resolved product branching fractions. The reaction between C2H and 1-butene yields (65 ± 10)% C4H4 in the form of vinylacetylene and (35 ± 10)% C5H6 in the form of 4-penten-1-yne. The cis-2-butene and trans-2-butene reactions yield solely 3-penten-1-yne, and no discrimination is made between cis- and trans-3-penten-1-yne. Last, the isobutene reaction yields (26 ± 15)% 3-penten-1-yne, (35 ± 15)% 2-methyl-1-buten-3-yne, and (39 ± 15)% 4-methyl-3-penten-1-yne. The branching fractions reported for the C2H and butene reactions indicate that these reactions preferentially proceed via CH3 or C2H3 elimination rather than H-atom elimination. Within the experimental uncertainties, no evidence is found for the formation of cyclic species. PMID:23701666

Bouwman, Jordy; Fournier, Martin; Sims, Ian R; Leone, Stephen R; Wilson, Kevin R

2013-06-20

26

Thermochemical pretreatment of lignocellulose to enhance methane fermentation. I. Monosaccharide and furfurals hydrothermal decomposition and product formation rates  

SciTech Connect

Over a pH range 1-4 and temperatures from 170 to 230/sup 0/C, the decomposition rates of xylose, galactose, mannose, glucose, 2-furfural, and 5-hydroxymethyl-2-furfural (5-HMF) were pseudo first order. The effect of temperature and pH on the pseudo first-order decomposition rate constants was modeled using the Arrhenius equation and acid-base catalysis, respectively. Decomposition rates of the monosaccharides were minimum at a pH 2-2.5. Above pH 2.5, the monosaccharide decomposition was base catalyzed, with acid catalysis occurring at a pH of less than 2 for glucose. The furfurals were subject to acid catalysis at below ca. pH 3.5. The hydrothermal conversion of glucose to its decomposition products during thermochemical pretreatment can be modeled as a combination of series and parallel reactions. The formation rates of identified soluble products from glucose decomposition, 5-HMF and levulinic acid, were also functions of temperature and pH. The rate of 5-HMF formation relative to glucose decomposition decreased as the pH increased from 2.0 to 4.0, with levulinic acid formation only detected when the pH was 2.5 or less. For glucose decomposition, humic solids accounted for ca. 20% of the decomposition products.

Baugh, K.D.; McCarty, P.L.

1988-01-01

27

Method of controlling fusion reaction rates  

DOEpatents

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.

Kulsrud, Russell M. (Princeton, NJ); Furth, Harold P. (Princeton, NJ); Valeo, Ernest J. (Princeton Junction, NJ); Goldhaber, Maurice (Bayport, NY)

1988-01-01

28

COMPUTERIZED EXTRAPOLATION OF HYDROLYSIS RATE DATA  

EPA Science Inventory

The program RATE was developed to aid in the extrapolation and interpretation of hydrolysis rate data to a format that is useful for environmental risk assessment. ydrolysis data typically are reported in the literature as pseudo-first-order rate constants at the temperature and ...

29

Reaction Order Ambiguity in Integrated Rate Plots  

ERIC Educational Resources Information Center

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…

Lee, Joe

2008-01-01

30

pH & Rate of Enzymatic Reactions.  

ERIC Educational Resources Information Center

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…

Clariana, Roy B.

1991-01-01

31

Factors influencing the mechanism of surfactant catalyzed reaction of vitamin C-ferric chloride hexahydrate system  

NASA Astrophysics Data System (ADS)

The kinetics of vitamin C by ferric chloride hexahydrate has been investigated in the aqueous ethanol solution of basic surfactant viz. octadecylamine (ODA) under pseudo-first order conditions. The critical micelle concentration (CMC) of surfactant was determined by surface tension measurement. The effect of pH (2.5-4.5) and temperature (15-35°C) in the presence and absence of surfactant were investigated. Activation parameters, ? E a, ? H #, ? S #, ? G ?, for the reaction were calculated by using Arrhenius and Eyring plot. Surface excess concentration (?max), minimum area per surfactant molecule ( A min), average area occupied by each molecule of surfactant ( a), surface pressure at the CMC (?max), Gibb's energy of micellization (? G M°), Gibb's energy of adsorption (? G ad°), were calculated. It was found that the reaction in the presence of surfactant showed faster oxidation rate than the aqueous ethanol solution. Reaction mechanism has been deduced in the presence and absence of surfactant.

Farrukh, Muhammad Akhyar; Kauser, Robina; Adnan, Rohana

2013-09-01

32

Astrophysical Reaction Rates Obtained By Indirect Techniques  

SciTech Connect

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.

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. [Cyclotron Institute, Texas A and M University, College Station, TX 77843 (United States)

2010-08-12

33

Efficient rate enhancement due to intramolecular general base (IGB) assistance in the hydrolysis of N-(o-hydroxyphenyl)phthalimide.  

PubMed

The rates of the hydrolyses of N-(o-hydroxyphenyl)phthalimide (1) and N-(o-methoxyphenyl)phthalimide (2), studied at different pH, show that the hydrolysis of 1 involves intramolecular general base (IGB) assistance where the o-O- group of ionized 1 acts as IGB and H2O as the reactant. The rate enhancement due to the IGB-assisted reaction of H2O with ionized 1 is>8x10(4)-fold. Pseudo-first-order rate constant for the reaction of water with 2 is approximately 2x10(3)-fold smaller than the first-order rate constant (0.10 s-1) for pH-independent hydrolysis of 1 within the pH range of 9.60-10.10. Second-order rate constants (kOH) for hydroxide ion-assisted hydrolysis of ionized 1 and 2 are 3.0 and 29.1 M-1 s-1, respectively. The solvent deuterium kinetic isotope effect (dKIE) on the rate of alkaline hydrolysis of 1 and 2 reveals that the respective values of kOH/kOD are 0.84 and 0.78, where kOD represents the second-order rate constant for DO--assisted cleavage of these imides (1 and 2). The value of kwH2O/kdD2O is 2.04, with kwH2O and kdD2O representing pseudo-first-order rate constants for the reactions of ionized 1 with H2O and D2O, respectively. PMID:17341117

Sim, Yoke-Leng; Ariffin, Azhar; Khan, M Niyaz

2007-03-30

34

Reaction Rate Calculations in Dense Stellar Matter  

NASA Astrophysics Data System (ADS)

Pycnonuclear fusion processes take place in the extreme density conditions anticipated to exist in the cores of white dwarfs and the crusts of neutron stars. Accurate modelling of pycnonuclear reactions is essential for understanding energy generation and transport in neutron stars. Calculation of pycnonuclear fusion rates strongly relies on knowledge of reaction cross sections. In most cases relevant cross section data is not availble and as such, cross section models must be used that are validated when ever possible by experimental measurements. Recently a formalism for calculating pycnonuclear reactions was developed for isotopes between boron and silicon. The pycnonuclear reaction rates will be presented and the results of the nucleosynthesis simulations will be discussed. This work was supported by the Joint Institute for Nuclear Astrophysics (NSF-PHY-0822648).

Beard, Mary; Brown, E.; Gasques, L.; Lau, R.; Schatz, H.; Wiescher, M.; Yakovlev, D.

2012-05-01

35

Reaction rate constant of methane clathrate formation  

Microsoft Academic Search

Particle size distribution measurements were performed during the growth stage of methane hydrate formation in a semi-batch stirred tank crystallizer. Experiments were carried out at temperatures between 275.1 and 279.2K and pressures ranging from 3873 to 5593kPa. The reaction rate constant of methane hydrate formation was determined using the model of Bergeron and Servio (AIChE J 2008;54:2964). The experimental reaction

Sebastien Bergeron; Juan G. Beltrán; Phillip Servio

2010-01-01

36

Shock tube measurements of the tert-butanol + OH reaction rate and the tert-C4H8OH radical ?-scission branching ratio using isotopic labeling.  

PubMed

The overall rate constant for the reaction tert-butanol + OH ? products was determined experimentally behind reflected shock waves by using (18)O-substituted tert-butanol (tert-butan(18)ol) and tert-butyl hydroperoxide (TBHP) as a fast source of (16)OH. The data were acquired from 900 to 1200 K near 1.1 atm and are best fit by the Arrhenius expression 1.24 × 10(-10)?exp(-2501/T [K]) cm(3) molecule(-1) s(-1). The products of the title reaction include the tert-C4H8OH radical that is known to have two major ?-scission decomposition channels, one of which produces OH radicals. Experiments with the isotopically labeled tert-butan(18)ol also lead to an experimental determination of the branching ratio for the ?-scission pathways of the tert-C4H8OH radical by comparing the measured pseudo-first-order decay rate of (16)OH in the presence of excess tert-butan(16)ol with the respective decay rate of (16)OH in the presence of excess tert-butan(18)ol. The two decay rates of (16)OH as a result of reactions with the two forms of tert-butanol differ by approximately a factor of 5 due to the absence of (16)OH-producing pathways in experiments with tert-butan(18)ol. This indicates that 80% of the (16)OH molecules that react with tert-butan(16)ol will reproduce another (16)OH molecule through ?-scission of the resulting tert-C4H8(16)OH radical. (16)OH mole fraction time histories were measured using narrow-line-width laser absorption near 307 nm. Measurements were performed at the line center of the R22(5.5) transition in the A-X(0,0) band of (16)OH, a transition that does not overlap with any absorption features of (18)OH, hence yielding a measurement of (16)OH mole fraction that is insensitive to any production of (18)OH. PMID:23683356

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

2013-06-13

37

Thermonuclear Reaction Rates for Solar Neutrino Processes  

SciTech Connect

Using the formalism for the effective cross-section factor S{sub eff} we calculate the thermonuclear reaction rates and neutrino fluxes for the pp-chain processes. We adopt the recent LUNA experimental data for the nuclear cross-section S-factor and we compute improved and updated solar neutrino fluxes which we compare with those previously obtained.

Karathanou, G. I.; Tsikoudi, V.; Kosmas, T. S. [Theoretical Physics Section, University of Ioannina, GR 45110 Ioannina (Greece); Liolios, Th. [Hellenic Army Academy, Department of Physical Sciences and Applications, Vari, GR 16673, Attica (Greece)

2008-01-24

38

Enhanced aqueous photochemical reaction rates after freezing.  

PubMed

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. PMID:17918916

Grannas, Amanda M; Bausch, Alexandra R; Mahanna, Kendell M

2007-11-01

39

Kinetics and mechanism of oxygen transfer in the reaction of p-cyano-N,N-dimethylaniline N-oxide with metalloporphyrin salts. 5. The influence of imidazole ligation of (meso-tetrakis(2,6-dimethylphenyl)porphinato)manganese(III) chloride on the rates of oxygen transfer from N-oxide to metalloporphyrin salt and the efficiency of oxidations by the intermediate higher valent manganese-oxo species  

SciTech Connect

Equilibrium constants for mono- and bisligation of imidazole (ImH) with (meso-tetrakis(2,6-dimethylphenyl)porphinato)manganese(III) chloride ((Me/sub 8/TPP)Mn/sup III/Cl) have been determined so that the concentrations (dry CH/sub 2/Cl/sub 2/) of the three species (Me/sub 8/TPP)Mn/sup III/Cl, ((Me/sub 8/TPP)Mn/sup III/(ImH))Cl, and ((Me/sub 8/TPP)Mn/sup III/(ImH)/sub 2/)Cl may be calculated at different ImH concentrations. The equilibrium constants for ligation of the one and two imidazoles are K/sub 1/ = 245 M/sup -1/ and ..beta../sub 2/ = 1.80 x 10/sup 5/ M/sup -1/. The reaction of p-cyano-N,N-dimethylaniline N-oxide (NO) with the manganese(III)porphyrin (under the pseudo-first-order conditions of (NO)/sub i/ >> ((Me/sub 8/TPP)Mn/sup III/Cl)/sub i/ and in the presence and absence of ImH) is first order in both NO and manganese(III) porphyrin, and the rate-controlling step involves oxygen transfer with formation of higher valent manganese-oxo porphyrin species plus p-cyano-N,N-dimethylaniline (DA). From the dependence of the pseudo-first-order rate constants (k/sub obsd/) upon (ImH)/sub i/ and a knowledge of the equilibrium constants for imidazole ligation there has been calculated the second-order rate constants for the kinetic terms k/sub 1/(NO)((Me/sub 8/TPP)Mn/sup III/Cl), k/sub 2/(NO)((Me/sub 8/TPP)Mn/sup III/(ImH))Cl, and k/sub 3/(NO)((Me/sub 8/TPP)Mn/sup III/(ImH)/sub 2/)Cl). Comparison of the second-order rate constants (k/sub 1/ = 3.3 x 10/sup -2/ M/sup -1/ s/sup -1/, k/sub 2/ = 5.53 M/sup -1/ s/sup -1/, and k/sub 4/ = 7.32 x 10/sup -2/ M/sup -1/ s/sup -1/) establishes that ligation by one imidazole increases the rate of reaction of the manganese(III) porphyrin with NO by approx. 166-fold.

Wong, W.H.; Ostovic, D.; Bruice, T.C.

1987-05-27

40

PhET Simulation: Reactions & Rates  

NSDL National Science Digital Library

This applet is designed for study of the kinetics of chemical reactions. It uses a model gas of three atomic species to illustrate the formation and disassociation of the different possible diatomic molecules. The user can run experiments with either one of each atom type or a gas of many atoms, controlling the density of the various species, the temperature, and energetics of the reaction. Data collection tools are available to calculate rate coefficients. Tips for teachers, ideas and activities are included. The direct link to the simulation is given as a mirror URL. This is part of a larger collection developed by the Physics Education Technology project (PhET).

2008-11-03

41

Rate coefficients for the gas-phase reaction of OH with (Z)-3-hexen-1-ol, 1-penten-3-ol, (E)-2-penten-1-ol, and (E)-2-hexen-1-ol between 243 and 404 K  

NASA Astrophysics Data System (ADS)

Rate coefficients, k, for the gas-phase reaction of the OH radical with (Z)-3-hexen-1-ol (Z)-CH3CH2CH = CHCH2CH2OH) (k1), 1-penten-3-ol (CH3CH2CH(OH)CH = CH2) (k2), (E)-2-penten-1-ol ((E)-CH3CH2CH = CHCH2OH) (k3), and (E)-2-hexen-1-ol ((E)-CH3CH2CH2CH = CHCH2OH) (k4), unsaturated alcohols that are emitted into the atmosphere following vegetation wounding, are reported. Rate coefficients were measured under pseudo-first-order conditions in OH over the temperature range 243-404 K at pressures between 20 and 100 Torr (He) using pulsed laser photolysis (PLP) to produce OH radicals and laser induced fluorescence (LIF) to monitor the OH temporal profile. The obtained rate coefficients were independent of pressure with negative temperature dependences that are well described by the Arrhenius expressions k1(T) = (1.3 ± 0.1) × 10-11 exp[(580 ± 10)/T]; k1(297 K) = (1.06 ± 0.12) × 10-10 k2(T) = (6.8 ± 0.7) × 10-12 exp[(690 ± 20)/T]; k2(297 K) = (7.12 ± 0.73) × 10-11 k3(T) = (6.8 ± 0.8) × 10-12 exp[(680 ± 20)/T]; k3(297 K) = (6.76 ± 0.70) × 10-11 k4(T) = (5.4 - 0.6) × 10-12 exp[(690 ± 20)/T]; k4(297 K) = (6.15 ± 0.75) × 10-11 (in units of cm3 molecule-1 s-1). The quoted uncertainties are at the 2? (95% confidence) level and include estimated systematic errors. The rate coefficients obtained in this study are compared with literature values where possible.

Davis, M. E.; Burkholder, J. B.

2011-04-01

42

Pycnonuclear reaction rates for binary ionic mixtures  

NASA Astrophysics Data System (ADS)

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.

Ichimaru, S.; Ogata, S.; van Horn, H. M.

1992-12-01

43

Direct measurement and theoretical calculation of the rate coefficient for Cl+CH3 in the range from T=202-298 K.  

PubMed

The rate coefficient has been measured under pseudo-first-order conditions for the Cl+CH3 association reaction at T=202, 250, and 298 K and P=0.3-2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH3 were generated rapidly and simultaneously by reaction of F with HCl and CH4, respectively. Fluorine atoms were produced by microwave discharge in an approximately 1% mixture of F2 in He. The decay of CH3 was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH3 concentration ([Cl]0/[CH3]0=9-67). Small corrections were made for both axial and radial diffusion and minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range of pressures studied. For example, at T=202 K, the rate coefficient increases from 8.4x10(-12) at P=0.30 Torr He to 1.8x10(-11) at P=2.00 Torr He, both in units of cm3 molecule-1 s-1. A combination of ab initio quantum chemistry, variational transition-state theory, and master-equation simulations was employed in developing a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonable empirical representations of energy transfer and of the effect of spin-orbit interactions yield a temperature- and pressure-dependent rate coefficient that is in excellent agreement with the present experimental results. The high-pressure limiting rate coefficient from the RRKM calculations is k2=6.0x10(-11) cm3 molecule-1 s-1, independent of temperature in the range from 200 to 300 K. PMID:17253663

Parker, James K; Payne, Walter A; Cody, Regina J; Nesbitt, Fred L; Stief, Louis J; Klippenstein, Stephen J; Harding, Lawrence B

2007-02-15

44

INITIAL TEST OF THE BENCHMARK CHEMICAL APPROACH FOR PREDICTING MICROBIAL TRANSFORMATION RATES IN AQUATIC ENVIRONMENTS  

EPA Science Inventory

Using 2,4-dichlorophenoxyacetic acid methyl ester (2,4-DME) as a benchmark chemical, we determined relative pseudo-first-order rate coefficients for butoxyethyl ester of 2,4-dichlorophenoxyacetic acid (2,4-DBE), methyl parathion, and methyl-3-chlorobenzoate in a diversity of micr...

45

Elementary Reactions of Electronically Excited Molecules. Pt. 6. Quenching of O sub 2 ( Sup 1 delta sub(G)) by Amines, Furanes, Pyrroles and Thiophenes.  

National Technical Information Service (NTIS)

The object of this report is the investigation of the reactions of O sub 2 ( sup 1 delta sub(g)) with a large excess of 2,5 dimethylfuran, pyrroles, thiophenes and for comparison with trimethylamine in a flow system under pseudo first order conditions. So...

W. Hack C. Voehringer H. G. Wagner

1985-01-01

46

Kinetic study on the reaction of cisplatin with metallothionein.  

PubMed

The binding of cisplatin to metallothionein (MT) was investigated at 37 degrees C in 10 mM Tris-NO3 (pH approximately 7.4) and 4.62 mM NaCl. The conditions were chosen to mimic passage of clinical concentrations of cisplatin through the cytosol. The reactions were monitored by high-performance liquid chromatography (HPLC), atomic absorption spectroscopy, and ultraviolet (UV) absorption spectroscopy. The UV data showed that several reactions occur, the first of which does not affect the absorbance (no Pt-sulfur bond formation). They also suggested that if [cisplatin] is large compared with [MT], the rate of subsequent reaction is between first and second order in [cisplatin] and between zeroth and first order in [MT]. HPLC eluates with 24 < retention time (tR) < 27 min contained undialyzable Pt, which increased with reaction time and corresponded to Pt-thionein product. Eluates with 3 < tR < 7 min corresponded to unbound cisplatin and allowed determination of second-order rate constants (k), using the second-order rate equation. The k value for cisplatin reacting with apo-MT was approximately 0.14 M-1 s-1, Cd/Zn-MT approximately 0.75 M-1 s-1, Cd7-MT approximately 0.53 M-1 s-1, and Zn7-MT approximately 0.65 M-1 s-1. Thus, cisplatin displaced Cd and Zn equally well. Leukocyte MT concentration was approximately 1.0 mM, so that the kinetics of cisplatin binding to cellular MT is pseudo-first order (pseudo-first-order rate constant, approximately 0.63 x 10-3 s-1; half-life, approximately 18 min). With [cisplatin] = 10 microM, the rate of cisplatin reaction with MT is approximately 6.3 micromol s-1 cm-3. We conclude that cellular MT can trap significant amounts of cisplatin and may efficiently contribute to cisplatin resistance. PMID:12814969

Hagrman, Douglas; Goodisman, Jerry; Dabrowiak, James C; Souid, Abdul-Kader

2003-07-01

47

Random walk theory of reaction kinetics in zeolites  

Microsoft Academic Search

We discuss application of the continuous-time random walk (CTRW) model to studying the kinetics of pseudo-first-order reactions in zeolites. The model includes distance-dependent reaction mechanism, details of the zeolite structure, and dynamics of migration of guest molecules between adsorption sites. Diffuse-reflectance transient-absorption study of triplet anthracene quenching by azulene in NaY zeolite shows that quenching can occur when reactants are

A. V. Barzykin; S. Hashimoto

2000-01-01

48

A simple reaction-rate model for turbulent diffusion flames  

NASA Technical Reports Server (NTRS)

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.

Bangert, L. H.

1975-01-01

49

Rate coefficients for the gas-phase reaction of the hydroxyl radical with CH2=CHF and CH2=CF2.  

PubMed

Rate coefficients, k, for the gas-phase reaction of the OH radical with CH(2)=CHF (k(1)) and CH(2)=CF(2) (k(2)) were measured under pseudo-first-order conditions in OH using pulsed laser photolysis to produce OH and laser-induced fluorescence (PLP-LIF) to detect it. Rate coefficients were measured over a range of temperature (220-373 K) and bath gas pressure (20-600 Torr; He, N(2)). The rate coefficients were found to be independent of pressure. The measured rate coefficient for reaction 1 at room temperature was k(1)(296 K) = (5.18 +/- 0.50) x 10(-12) cm(3) molecule(-1) s(-1), independent of pressure, and the temperature dependence is given by the Arrhenius expression k(1)(T) = (1.75 +/- 0.20) x 10(-12) exp[(316 +/- 25)/T] cm(3) molecule(-1) s(-1); the rate coefficients for reaction 2 were k(2)(296 K) = (2.79 +/- 0.25) x 10(-12) cm(3) molecule(-1) s(-1) and k(2)(T) = (1.75 +/- 0.20) x 10(-12) exp[(140 +/- 20)/T] cm(3) molecule(-1) s(-1). The quoted uncertainties are 2sigma (95% confidence level) and include estimated systematic errors. The fall-off parameters for reaction 2 of k(infinity) = 3 x 10(-12) cm(3) molecule(-1) s(-1) and k(0)(296 K) = 1.8 x 10(-28) cm(6) molecule(-2) s(-1) with F(c) = 0.6 reproduce the room temperature data obtained in this study combined with the low pressure rate coefficient data from Howard (J. Chem. Phys. 1976, 65, 4771). OH radical formation was observed for reactions 1 and 2 in the presence of O(2), and the mechanism was investigated using (18)OH and OD rate coefficient measurements with CH(2)=CHF and CH(2)=CF(2) over a range of temperature (260-373 K) and pressure (20-100 Torr, He). Quantum chemical calculations using density functional theory (DFT) were used to determine the geometries and energies of the reactants and adducts formed in reactions 1 and 2 and the peroxy radicals formed following the addition of O(2). The atmospheric lifetimes of CH(2)=CHF and CH(2)=CF(2) due to loss by reaction with OH are approximately 2 and 4 days, respectively. Infrared absorption spectra of CH(2)=CHF and CH(2)=CF(2) were measured, and global warming potentials (GWP) values of 0.7 for CH(2)=CHF and 0.9 for CH(2)=CF(2) were obtained for the 100 year time horizon. PMID:20225809

Baasandorj, Munkhbayar; Knight, Gary; Papadimitriou, Vassileios C; Talukdar, Ranajit K; Ravishankara, A R; Burkholder, James B

2010-04-01

50

Ring-closing metathesis reactions: interpretation of conversion-time data.  

PubMed

Conversion-time data were recorded for various ring-closing metathesis (RCM) reactions that lead to five- or six-membered cyclic olefins by using different precatalysts of the Hoveyda type. Slowly activated precatalysts were found to produce more RCM product than rapidly activated complexes, but this comes at the price of slower product formation. A kinetic model for the analysis of the conversion-time data was derived, which is based on the conversion of the precatalyst (Pcat) into the active species (Acat), with the rate constant k(act), followed by two parallel reactions: 1) the catalytic reaction, which utilizes Acat to convert reactants into products, with the rate k(cat), and 2) the conversion of Acat into the inactive species (Dcat), with the rate k(dec). The calculations employ two experimental parameters: the concentration of the substrate (c(S)) at a given time and the rate of substrate conversion (-dc(S)/dt). This provides a direct measure of the concentration of Acat and enables the calculation of the pseudo-first-order rate constants k(act), k(cat), and k(dec) and of k(S) (for the RCM conversion of the respective substrate by Acat). Most of the RCM reactions studied with different precatalysts are characterized by fast k(cat) rates and by the k(dec) value being greater than the k(act) value, which leads to quasistationarity for Acat. The active species formed during the activation step was shown to be the same, regardless of the nature of different Pcats. The decomposition of Acat occurs along two parallel pathways, a unimolecular (or pseudo-first-order) reaction and a bimolecular reaction involving two ruthenium complexes. Electron-deficient precatalysts display higher rates of catalyst deactivation than their electron-rich relatives. Slowly initiating Pcats act as a reservoir, by generating small stationary concentrations of Acat. Based on this, it can be understood why the use of different precatalysts results in different substrate conversions in olefin metathesis reactions. PMID:24127335

Thiel, Vasco; Wannowius, Klaus-Jürgen; Wolff, Christiane; Thiele, Christina M; Plenio, Herbert

2013-11-25

51

Let the microbes explicitly carry out the reactions: a new paradigm to the modeling of soil carbon-nutrient dynamics (Invited)  

NASA Astrophysics Data System (ADS)

Soil carbon-nutrient dynamics are carried out by a broad spectrum of microbial functional guilds through a network of different reactions, all impacted by abiotic factors, including temperature, moisture, pH, redox, and mineralogy. Few existing soil biogeochemical models explicitly represent microbes and the related biotic and abiotic chemical reactions. Rather, these models usually assume microbes are perfectly adaptive such that their ecological impacts on carbon and nutrient dynamics can be sufficiently described by pseudo-first order decay parameters. These decay parameters are empirically formulated as functions of organic matter composition (e.g., lignocellulos index, CN ratio), soil temperature, and soil moisture that are obtained by ad hoc regression fitting to experimental and observational measurements. These pseudo-first order decay models claim predictability at large spatial and long temporal scales; however, their lack of explicit microbial ecology and microbe relevant biogeochemical and biogeophysical interactions have limited these models' capability to mechanistically explain some fundamental patterns, such as the divergent temperature response of decomposition, episodic emissions in drying-rewetting cycles, and predator-prey effects on organic matter decomposition. We here propose a new modeling paradigm that explicitly considers microbial ecology and reactions in modeling soil carbon-nutrient dynamics. We show this approach requires a new formulation of substrate kinetics to properly account for the intrinsic network structure of the biogeochemical processes and that the classical Michaelis-Menten kinetics is of limited use for this purpose. With a simple model based on this new paradigm, we mechanistically explained lignin dynamics in litter decomposition experiments without the ad hoc use of a lignin shielding effect. We further show, using existing experimental results, that decomposition dynamics are co-shaped by litter chemistry and associated microbial ecology. We also show that interactions between soil mineralogy and organic substrates can significantly reduce the microbial decomposition rate. Finally, we will discuss a few open questions and the data necessary to make such a model structure feasible for practical applications.

Tang, J.; Riley, W. J.; Bouskill, N.; Brodie, E.

2013-12-01

52

Shock tube measurements of rate coefficients of elementary gas reactions  

Microsoft Academic Search

The primary objective of this paper is to review the current status of rate coefficient determinations of elementary gas reactions using modern shock tube measurement technques. The major inaccuracies in these determinations arise from uncertainties in reaction conditions due to gas dynamic effects, uncertainties introduced by interfering rate processes such as vibrational relaxation, competing reactions, or reactions involving impurities, and

Craig T. Bowman; Ronald K. Hanson

1979-01-01

53

Representing Rate Equations for Enzyme-Catalyzed Reactions  

ERIC Educational Resources Information Center

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…

Ault, Addison

2011-01-01

54

Distribution function, reaction rates and spectral line profile  

Microsoft Academic Search

Quantum mechanics predicts the existence of power law “tails” in the distribution function over momentum in dense media even at the condition of thermodynamics equilibrium. Generalized expressions for the threshold process reaction rates show that reaction rates with released energy (e.g. vibrational-translational relaxation, chemical reaction, fusion rates) may be substantially increased due to quantum corrections. Spectral line profile also may

A. N Starostin; A. G Leonov; Yu. V Petrushevich; Vl. K Roerich

2004-01-01

55

Distribution function, reaction rates and spectral line profile  

Microsoft Academic Search

Quantum mechanics predicts the existence of power law ``tails'' in the distribution function over momentum in dense media even at the condition of thermodynamics equilibrium. Generalized expressions for the threshold process reaction rates show that reaction rates with released energy (e.g. vibrational-translational relaxation, chemical reaction, fusion rates) may be substantially increased due to quantum corrections. Spectral line profile also may

A. N. Starostin; A. G. Leonov; Yu. V. Petrushevich; Vl. K. Roerich

2004-01-01

56

Modeling of DNA zipper reaction rates  

NASA Astrophysics Data System (ADS)

DNA zippers are a thermodynamically driven system consisting of three DNA oligonucleotides. Two of the strands are designed to create a small helix the third is designed to invade and separated the helix. A zipper system consisting of a normal strand (N), a weak strand (W), and an opening strand (O). N is made up of normal DNA bases, while W is engineered with inosine bases substituted for guanine. Inosine forms one less hydrogen bond with cytosine than guanine. By varying the number and order of inosine, W is engineered to provide less than natural bonding affinities to N in forming the [N:W] helix. When O is introduced (a natural complement of N), it competitively displaces W from [N:W] and forms [N:O]. DNA zippers have been used to create new DNA devices such as springs and tweezers and to create functionalized DNA origami structures. Currently, The basic principles and interactions of DNA zippers are not well understood. Here we will report the results on an investigation of several different DNA zipper constructs designed to aid in the creation of a mathematical prediction of the reaction rate for DNA zippers.

Landon, Preston; Sanchez, Casey; Mo, Alexander; Lal, Ratnesh

2012-02-01

57

Reaction of Tris(2-chloroethyl)phosphate with Reduced Sulfur Species  

PubMed Central

Tris(2-chloroethyl)phosphates (TCEP) is a widely used flame retardant in the U.S. It has recently been identified as one of the most frequently detected contaminants in U.S. streams. This contaminant is of toxicological concern in sensitive coastal ecosystems such as estuaries and salt marshes. It is likely that reactions with reduced sulfur species such as polysulfides (Sn2?), bisulfide (HS?), and thiophenolate (PhS?) present in anoxic subregions of coastal water bodies could have a significant impact on rates of removal of such a contaminant. The kinetics of reaction of reduced sulfur species with tris(2-chloroethyl)phosphate have been determined in well-defined aqueous solutions under anoxic conditions. Reactions were monitored at varying concentrations of reduced sulfur species to obtain the second-order rate constants from the observed pseudo-first order rate constants. The determined second-order rate constant for the reaction of TCEP with polysulfide at 25°C is 5.0 (± 1.4) × 10?4 M?1 s?1, with thiophenolate at 50 °C is 34 (± 2) × 10?4 M?1 s?1 and with bisulfide at 50 °C is 0.9 × 10?4 M?1 s?1, respectively. In addition, the degradation products of hydrolysis and the reactions with polysulfides, thiophenolate, and bisulfide with TCEP were studied with GC-FID and LC-MS-MS and were quantified.

Hilaire, Dickens Saint; Ismail, Kamal Z.; Jans, Urs

2014-01-01

58

Reaction of tris(2-chloroethyl)phosphate with reduced sulfur species.  

PubMed

Tris(2-chloroethyl)phosphates (TCEP) is a widely used flame retardant in the US. It has recently been identified as one of the most frequently detected contaminants in US streams. This contaminant is of toxicological concern in sensitive coastal ecosystems such as estuaries and salt marshes. It is likely that reactions with reduced sulfur species such as polysulfides (S(n)(2-)), bisulfide (HS(-)), and thiophenolate (PhS(-)) present in anoxic subregions of coastal water bodies could have a significant impact on rates of removal of such a contaminant. The kinetics of reaction of reduced sulfur species with tris(2-chloroethyl)phosphate have been determined in well-defined aqueous solutions under anoxic conditions. Reactions were monitored at varying concentrations of reduced sulfur species to obtain the second-order rate constants from the observed pseudo-first-order rate constants. The determined second-order rate constant for the reaction of TCEP with polysulfide at 25°C is 5.0 (±1.4)×10(-4) M(-1) s(-1), with thiophenolate at 50°C is 34 (±2)×10(-4) M(-1) s(-1) and with bisulfide at 50°C is 0.9×10(-4) M(-1) s(-1), respectively. In addition, the degradation products of hydrolysis and the reactions with polysulfides, thiophenolate, and bisulfide with TCEP were studied with GC-FID and LC-MS-MS and were quantified. PMID:21419471

Saint-Hilaire, Dickens; Ismail, Kamal Z; Jans, Urs

2011-05-01

59

Reaction Rate and Photochemical Data for Atmospheric Chemistry - 1977.  

National Technical Information Service (NTIS)

A table of data for gas phase chemical reactions and photochemistry of neutral species is presented. Specifically, it gives preferred values for reaction rate constants, photoabsorption cross sections, and quantum yields of primary photochemical processes...

D. Garvin R. F. Hampson

1978-01-01

60

Catalytic hydroprocessing of aromatic compounds: Effects of nickel and vanadium sulfide deposits on reactivities and reaction networks  

SciTech Connect

Ni-Mo/{gamma}-Al{sub 2}O{sub 3} hydroprocessing catalysts enriched in nickel and vanadium by contacting with solutions of the respective metal naphthenates were sulfided and tested for hydroprocessing of naphthalene, dibenzothiophene, and quinoline in a batch reactor at 350 C and 165 atm. Approximately reaction networks were determined for each reactant, and the data showed the dependence of the pseudo-first-order rate constants on the catalyst nickel and vanadium contents. The nickel sulfide deposits only slightly affected the rate constants for hydrogenation, but the vanadium sulfide deposits led to decreases in the rate constants for hydrogenation reactions in the naphthalene network and to increases in those for hydrogenation reactions in the dibenzothiophene network. Nickel sulfide deposits led to almost no change in the rate constants for hydrogenolysis of dibenzothiophene, but vanadium sulfide deposits led to decreased rate constant for this reaction. The nickel sulfide deposits have little activity for reactions giving lower-molecular-weight (cracking) products, but the vanadium sulfide deposits have a relatively high activity for cracking, which suggests that they are acidic; the effects are reversed by the presence of the basic quinoline in the reactants. The results indicate a need for representing the nickel and vanadium sulfide deposits separately in process models for heavy oil hydroprocessing.

Yumoto, Mitsugu [Univ. of Delaware, Newark, DE (United States)] [Univ. of Delaware, Newark, DE (United States); [Cosmo Research Inst., Satte, Saitama (Japan); Kukes, S.G. [Amoco Research Center, Naperville, IL (United States)] [Amoco Research Center, Naperville, IL (United States); Klein, M.T. [Univ. of Delaware, Newark, DE (United States)] [Univ. of Delaware, Newark, DE (United States); Gates, B.C. [Univ. of Delaware, Newark, DE (United States)] [Univ. of Delaware, Newark, DE (United States); [Univ. of California, Davis, CA (United States). Dept. of Chemical Engineering and Materials Science

1996-09-01

61

Reaction of NO(2) with selected conjugated alkenes.  

PubMed

The gas phase reactions of selected alkenes (isoprene, myrcene, ocimene, and 1,3-cyclohexadiene) with NO2 under dark condition have been investigated at T ? 298 K and P ? 760 Torr of purified air. The kinetic studies were performed under pseudo-first-order conditions using a large excess of NO2 concentration to those of the alkenes. The rate coefficients (in 10(-19) cm(3) molecule(-1) s(-1)) obtained are 1.1 ± 0.2 for isoprene, 2.5 ± 0.3 for myrcene, 8.5 ± 1.2 for ocimene, and 15 ± 1 for 1,3-cyclohexadiene. Several products were identified by using in situ Fourier transform infrared (FT-IR) spectrometry, and acetone was found to be the major product from the reactions of NO2 with myrcene and ocimene, with a formation yield of 22 ± 3% and 26 ± 7%, respectively. The oxidation products from the reactions of NO2 with isoprene and 1,3-cyclohexadiene were found to be mainly nitro compounds identified by FT-IR spectroscopy. Reaction mechanisms were proposed to account for the products observed. PMID:24320548

Bernard, François; Cazaunau, Mathieu; Mu, Yujing; Wang, Xinming; Daële, Véronique; Chen, Jianmin; Mellouki, Abdelwahid

2013-12-27

62

Field Based Constraints on Reaction Rates in the Crust  

NASA Astrophysics Data System (ADS)

Modern research in plate boundary processes involving metamorphism frequently employs complex physical models. Such models require some quantification (or assumption) of the rate at which metamorphic reactions, or chemical exchange, proceed in natural systems. Here, a compilation of available quantitative field-based constraints on high temperature reaction rates will be presented. These include quantifications based on isotopic exchange, porphyroblast and reaction corona growth models, geochronology, and textural analysis. Additionally, natural strain rates provide an important upper bound on simultaneous reaction rates by virtue of a direct mechanistic link between reaction and strain that applies in most situations within the deforming crust. These data show that reaction rates attending regional metamorphism are 4-7 orders of magnitude slower than most laboratory-based predictions. A general rate law for regional metamorphic reactions has been derived which best describes these field-based data: log10(Rnet) = .0029T-9.6±1, where Rnet is the net reaction rate in g/cm2/yr and T is temperature (C) (Baxter 2003, JGSL). Reaction rates attending contact metamorphism differ from laboratory-based predictions by less than 2 orders of magnitude, and are in closest agreement at higher temperatures. Regional metamorphic reaction rates may be limited by comparatively lesser (or transient) availability of aqueous fluid in the intergranular medium, slower heat input, and smaller deviations from equilibrium. Implications of slow natural metamorphic reaction rates may include a delay in the completion of metamorphic reactions which release (or take in) volatiles, and transform the mineralogy of the crust in dynamic plate boundary settings such as subduction zones.

Baxter, E. F.

2004-12-01

63

Heterogeneous reactions between NO 2 and anthracene adsorbed on SiO 2 and MgO  

NASA Astrophysics Data System (ADS)

The heterogeneous reactions of NO 2 with anthracene adsorbed on SiO 2 and MgO were investigated under dark conditions at 298 K using DRIFTS, GCMS, and UV-Vis. The pseudo-first-order rate constants of anthracene on SiO 2 and MgO were obtained by fitting the exponential decay of adsorbed PAH concentrations versus reaction time. The reaction on SiO 2 was almost two times faster than on MgO when the concentration of NO 2 was 3.69 × 10 12 molecules cm -3. Both 9-nitroanthracene and 9,10-anthraquinone were the products of the nitration of anthracene adsorbed on SiO 2 whereas 9,10-anthraquinone was the only product formed by NO 2 reaction with anthracene adsorbed on MgO. These results suggest that mineral oxides not only control the reaction kinetics of PAHs with NO 2 but also alter the reaction pathway for the heterogeneous reaction of PAHs with NO 2. The difference in the heterogeneous reactivity of NO 2 with anthracene adsorbed on SiO 2 and MgO was ascribed to the formation of HNO 3 on SiO 2, which can catalyze the nitration of PAHs by NO 2. Due to the formation of nitro-anthracene and oxy-anthracene, the heterogeneous reactions of NO 2 with anthracene also altered the optical properties of the mineral oxides on which anthracene were adsorbed.

Ma, Jinzhu; Liu, Yongchun; He, Hong

2011-02-01

64

DSMC predictions of non-equilibrium reaction rates.  

SciTech Connect

A set of Direct Simulation Monte Carlo (DSMC) chemical-reaction models recently proposed by Bird and based solely on the collision energy and the vibrational energy levels of the species involved is applied to calculate nonequilibrium chemical-reaction rates for atmospheric reactions in hypersonic flows. The DSMC non-equilibrium model predictions are in good agreement with theoretical models and experimental measurements. The observed agreement provides strong evidence that modeling chemical reactions using only the collision energy and the vibrational energy levels provides an accurate method for predicting non-equilibrium chemical-reaction rates.

Gallis, Michail A.; Bond, Ryan Bomar; Torczynski, John Robert

2010-04-01

65

Determination of (n,?) reaction rates at s-process branching points via their inverse reactions  

Microsoft Academic Search

We present our method to determine the (n,?) reaction rates of s-process branching point nuclei using data of the inverse (?,n) reaction. As an example, we selected 95Zr. The analysis of the data of the reaction 96Zr(?,n)95Zr is still in progress. Therefore, we describe the characteristics of this reaction and first preliminary results.

K. Sonnabend; A. Mengoni; P. Mohr; T. Rauscher; K. Vogt; A. Zilges

2003-01-01

66

Upscaling geochemical reaction rates using pore-scale network modeling  

Microsoft Academic Search

Geochemical reaction rate laws are often measured using crushed minerals in well-mixed laboratory systems that are designed to eliminate mass transport limitations. Such rate laws are often used directly in reactive transport models to predict the reaction and transport of chemical species in consolidated porous media found in subsurface environments. Due to the inherent heterogeneities of porous media, such use

Li Li; Catherine A. Peters; Michael A. Celia

2006-01-01

67

Evaluated Chemical Kinetic Rate Constants for Various Gas Phase Reactions  

Microsoft Academic Search

The available information, up to mid-1972, for the rate constants of a series of gas phase chemical reactions has been evaluated critically. For each reaction, relevant thermodynamic data are presented and values for the equilibrium constant expressed in mathematical form. Kinetic data are presented in tabular and graphical form together with a discussion of the pertinent details. Recommended rate constant

Keith Schofield

1973-01-01

68

A new approach to determining gas-particle reaction probabilities and application to the heterogeneous reaction of deliquesced sodium chloride particles with gas-phase hydroxyl radicals.  

PubMed

The reaction kinetics for gaseous hydroxyl radicals (OH) with deliquesced sodium chloride particles (NaCl(aq)) were investigated using a novel experimental approach. The technique utilizes the exposure of substrate-deposited aerosol particles to reactive gases followed by chemical analysis of the particles using computer-controlled scanning electron microscopy with energy-dispersive analysis of X-rays (CCSEM/EDX) capability. Experiments were performed at room temperature and atmospheric pressure with deliquesced NaCl particles in the micron size range at 70-80% RH and with OH concentrations in the range of 1 to 7 x 10(9) cm(-3). The apparent, pseudo first-order rate constant for the reaction was determined from measurements of changes in the chloride concentration of individual particles upon reaction with OH as a function of the particle loading on the substrate. Quantitative treatment of the data using a model that incorporates both diffusion and reaction kinetics yields a lower limit to the net reaction probability of gamma(net) > or = 0.1, with an overall uncertainty of a factor of 2. PMID:16956244

Laskin, Alexander; Wang, Hai; Robertson, William H; Cowin, James P; Ezell, Michael J; Finlayson-Pitts, Barbara J

2006-09-14

69

A New Approach to Determining Gas-Particle Reaction Probabilities and Application to the Heterogeneous Reaction of Deliquesced Sodium Chloride Particles with Gas-Phase Hydroxyl Radicals  

SciTech Connect

The reaction kinetics for gaseous hydroxyl radicals (OH) with deliquesced sodium chloride particles (NaClaq) were investigated using a novel experimental approach. The technique utilizes the exposure of substrate-deposited aerosol particles to reactive gases followed by chemical analysis of the particles using computer-controlled scanning electron microscopy with energy-dispersive analysis of X-rays (CCSEM/EDX) capability. Experiments were performed at room temperature and atmospheric pressure with deliquesced NaCl particles in the micron size range at 70-80% RH and with OH concentrations in the range of 1 to 7?109 cm-3. The apparent, pseudo first-order rate constant for the reaction was determined from measurements of changes in the chloride concentration of individual particles upon reaction with OH as a function of the particle loading on the substrate. Quantitative treatment of the data using a model that incorporates both diffusion and reaction kinetics yields a lower-limit to the net reaction probability of ?net > 0.1, with an overall uncertainty of a factor of two.

Laskin, Alexander; Wang, Hai; Robertson, William H.; Cowin, James P.; Ezell, Michael J.; Finlayson-Pitts, Barbara J.

2006-09-14

70

Reaction rate modeling of PBXN-110  

NASA Astrophysics Data System (ADS)

The reactive rate model for Navy explosive PBXN-110 has been determined. The rate parameters for the Lee-Tarver model were evaluated by comparing the results of DYNA2D hydrocode simulations to the embedded gauge data of gas-gun tests in which the shock loading is mostly one-dimensional. The model parameters were refined such that the failure diameter of the explosive could be reproduced in the calculations. The model was used to simulate a series of Navy sensitivity tests. These are reported here and include detonation curvature, detonation velocity dependency on charge diameter, Modified Gap, and Underwater Sensitivity tests.

Miller, P. J.; Sutherland, G. T.

1996-05-01

71

Heterogeneous photochemical reaction of ozone with anthracene adsorbed on mineral dust  

NASA Astrophysics Data System (ADS)

The heterogeneous reactions of O3 with anthracene adsorbed on TiO2 and on Asian dust storm particles were investigated in the dark and in the presence of light. The reaction rate constants of the heterogeneous reaction between O3 and anthracene adsorbed on TiO2 were increased by a factor of 1.5 in the presence of light compared to the dark conditions. Anthraquinone, which was identified as the main surface product of anthracene reacted with O3 in the dark, can react with O3 quickly in the presence of light. The reactions on Asian dust storm particles exhibited pseudo-first-order kinetics for anthracene loss, and the reactions between O3 and anthracene adsorbed on Asian dust storm particles proceed by the Langmuir–Hinshelwood mechanism in the dark and in the presence of light. At extremely high ozone concentrations, the degradation of anthracene is enhanced by a factor of 3 in the presence of light compared to the dark conditions.

Ma, Jinzhu; Liu, Yongchun; Ma, Qingxin; Liu, Chang; He, Hong

2013-06-01

72

Estimating the Backup Reaction Wheel Orientation Using Reaction Wheel Spin Rates Flight Telemetry from a Spacecraft  

NASA Technical Reports Server (NTRS)

A report describes a model that estimates the orientation of the backup reaction wheel using the reaction wheel spin rates telemetry from a spacecraft. Attitude control via the reaction wheel assembly (RWA) onboard a spacecraft uses three reaction wheels (one wheel per axis) and a backup to accommodate any wheel degradation throughout the course of the mission. The spacecraft dynamics prediction depends upon the correct knowledge of the reaction wheel orientations. Thus, it is vital to determine the actual orientation of the reaction wheels such that the correct spacecraft dynamics can be predicted. The conservation of angular momentum is used to estimate the orientation of the backup reaction wheel from the prime and backup reaction wheel spin rates data. The method is applied in estimating the orientation of the backup wheel onboard the Cassini spacecraft. The flight telemetry from the March 2011 prime and backup RWA swap activity on Cassini is used to obtain the best estimate for the backup reaction wheel orientation.

Rizvi, Farheen

2013-01-01

73

Rate constant for reaction of atomic hydrogen with germane  

NASA Technical Reports Server (NTRS)

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.

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

1990-01-01

74

Shock tube kinetic study of the CH{sub 3} + H{sub 2} {r_equilibrium} H + CH{sub 4} reaction and the methane dissociation reaction  

SciTech Connect

In this kinetic study of (1) the reaction of CH{sub 3} radicals with H{sub 2} and (2) the thermal dissociation of methane, primary product H atoms were monitored directly using the sensitive atomic resonance absorption detection technique. The detection limit for the [H] was about 3{times}10{sup 10} atoms cm{sup {minus}3}. Rate constants for both reactions were obtained under pseudo-first-order conditions. In addition, computer simulations verified that kinetic complications were avoided. For the reaction of CH{sub 3}+H{sub 2}, experiments were performed using either acetone or ethane to generate CH{sub 3} radicals rapidly by thermal dissociation in argon. Twenty-four experiments were performed over the temperature range 1346K to 1793K and a rate constant expression derived using linear least-squares analysis: k{sub {minus}2}(T) = (6.0{plus_minus}0.7){times}10{sup {minus}12} exp ({minus}5920{plus_minus}190K/T) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. 46 refs., 5 figs., 5 tabs.

Klemm, R.B.; Sutherland, J.W.; Tao, Wen

1992-02-01

75

Shock tube kinetic study of the CH sub 3 + H sub 2 r equilibrium H + CH sub 4 reaction and the methane dissociation reaction  

SciTech Connect

In this kinetic study of (1) the reaction of CH{sub 3} radicals with H{sub 2} and (2) the thermal dissociation of methane, primary product H atoms were monitored directly using the sensitive atomic resonance absorption detection technique. The detection limit for the (H) was about 3{times}10{sup 10} atoms cm{sup {minus}3}. Rate constants for both reactions were obtained under pseudo-first-order conditions. In addition, computer simulations verified that kinetic complications were avoided. For the reaction of CH{sub 3}+H{sub 2}, experiments were performed using either acetone or ethane to generate CH{sub 3} radicals rapidly by thermal dissociation in argon. Twenty-four experiments were performed over the temperature range 1346K to 1793K and a rate constant expression derived using linear least-squares analysis: k{sub {minus}2}(T) = (6.0{plus minus}0.7){times}10{sup {minus}12} exp ({minus}5920{plus minus}190K/T) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. 46 refs., 5 figs., 5 tabs.

Klemm, R.B.; Sutherland, J.W.; Tao, Wen.

1992-01-01

76

Revisiting catalytic model reaction p-nitrophenol/NaBH4 using metallic nanoparticles coated on polymeric spheres.  

PubMed

The early reported pseudo-first-order reaction kinetics of the polymer-supported metallic nanocatalysts for the model reaction of p-nitrophenol (p-NP)/NaBH4 were probably oversimplified. Here a detailed study of p-NP reduction by NaBH4 in the presence of the raspberry-like poly(allylamine hydrochloride)-modified polymer poly(glycidyl methacrylate) composite sub-microspheres with tunable gold nanoparticles (PGMA@PAH@AuNPs) was presented. Effects of polyelectrolyte concentration, the ratio of polymer spheres to gold nanoparticles, and the solution pH value for composite synthesis on the induction period, reaction time, average reaction rate and average turnover frequency were systematically investigated. Experimental results in all cases of our study revealed an n(th) order (n > 1) of the p-NP/NaBH4 catalytic reaction by the prepared polymer composite particles. The apparent order of reaction, n, is dependent on the total surface area of the coated gold nanoparticles on the polymer spheres, which can be closely correlated with the tunable gold nanoparticle surface coverage. The mechanism of the observed catalytic activity enhancement was proposed based on active epoxy groups of the polymer spheres and a large adsorption of p-nitrophenolate anions onto the positively-charged spheres. PMID:24129942

Li, Maolin; Chen, Guofang

2013-12-01

77

Revisiting catalytic model reaction p-nitrophenol/NaBH4 using metallic nanoparticles coated on polymeric spheres  

NASA Astrophysics Data System (ADS)

The early reported pseudo-first-order reaction kinetics of the polymer-supported metallic nanocatalysts for the model reaction of p-nitrophenol (p-NP)/NaBH4 were probably oversimplified. Here a detailed study of p-NP reduction by NaBH4 in the presence of the raspberry-like poly(allylamine hydrochloride)-modified polymer poly(glycidyl methacrylate) composite sub-microspheres with tunable gold nanoparticles (PGMA@PAH@AuNPs) was presented. Effects of polyelectrolyte concentration, the ratio of polymer spheres to gold nanoparticles, and the solution pH value for composite synthesis on the induction period, reaction time, average reaction rate and average turnover frequency were systematically investigated. Experimental results in all cases of our study revealed an nth order (n > 1) of the p-NP/NaBH4 catalytic reaction by the prepared polymer composite particles. The apparent order of reaction, n, is dependent on the total surface area of the coated gold nanoparticles on the polymer spheres, which can be closely correlated with the tunable gold nanoparticle surface coverage. The mechanism of the observed catalytic activity enhancement was proposed based on active epoxy groups of the polymer spheres and a large adsorption of p-nitrophenolate anions onto the positively-charged spheres.

Li, Maolin; Chen, Guofang

2013-11-01

78

A compilation of charged-particle induced thermonuclear reaction rates  

Microsoft Academic Search

Low-energy cross section data for 86 charged-particle induced reactions involving light (1 ? Z ? 14), mostly stable, nuclei are compiled. The corresponding Maxwellian-averaged thermonuclear reaction rates of relevance in astrophysical plasmas at temperatures in the range from 106 K to 1010 K are calculated. These evaluations assume either that the target nuclei are in their ground state, or that

C. Angulo; M. Arnould; M. Rayet; P. Descouvemont; D. Baye; C. Leclercq-Willain; A. Coc; S. Barhoumi; P. Aguer; C. Rolfs; R. Kunz; J. W. Hammer; A. Mayer; T. Paradellis; S. Kossionides; C. Chronidou; K. Spyrou; S. Degl'Innocenti; G. Fiorentini; B. Ricci; S. Zavatarelli; C. Providencia; H. Wolters; J. Soares; C. Grama; J. Rahighi; A. Shotter; M. Lamehi Rachti

1999-01-01

79

Rates of membrane-associated reactions: reduction of dimensionality revisited  

PubMed Central

The hypothesis that reactions associated with intracellular membranes enjoy a kinetic advantage from a reduced dimensionality for diffusion is inconsistent with available data on lateral diffusion rates, membrane-substrate affinities, and endogenous concentrations of enzymes and their aqueous substrates.

1986-01-01

80

Non-resonant triple alpha reaction rate at low temperature  

NASA Astrophysics Data System (ADS)

Our experimental goal is to study the non-resonant triple alpha reaction rate at low temperture (T < 108 K). The 13C(p,d) reaction at 66 MeV has been used to probe the alpha-unbound continuum state in 12C just below the 2nd 0+ state at 7.65 MeV. The transition strength to the continuum state is predicted to be sensitive to the non-resonant triple alpha reaction rate. The experiment has been performed at iThemba LABS. We report the present status of the experiment.

Itoh, T.; Tamii, A.; Aoi, N.; Carter, J.; Donaldson, L.; Fujita, H.; Furuno, T.; Hashimoto, T.; Kawabata, T.; Kamimura, M.; Miki, K.; Nemulodi, F.; Neveling, R.; Ogata, K.; Sideras-Haddad, E.; Smit, F. D.; Swarts, C.

2014-05-01

81

Reaction rate uncertainties and the ? p-process  

NASA Astrophysics Data System (ADS)

Current hydrodynamical simulations of core collapse supernovae find proton-rich early ejecta. At the same time, the models fail to eject neutron-rich matter, thus leaving the origin of the main r-process elements unsolved. However, the proton-rich neutrino-driven winds from supernovae have been identified as a possible production site for light n-capture elements beyond iron (such as Ge, Sr, Y, Zr) through the ?p-process. The detailed nucleosynthesis patterns of the ?p-process depend on the hydrodynamic conditions and the nuclear reaction rates of key reactions. We investigate the impact of reaction rate uncertainties on the ?p-process nucleosynthesis.

Fröhlich, C.; Rauscher, T.

2012-11-01

82

Kinetic parameters for the elimination reaction catalyzed by triosephosphate isomerase and an estimation of the reaction's physiological significance.  

PubMed

Kinetic parameters for triosephosphate isomerase catalysis of the elimination reaction of an equilibrium mixture of dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde-3-phosphate (DGAP) to form methylglyoxal and phosphate ion are reported for the enzyme from rabbit muscle. Pseudo-first-order rate constants for the disappearance of substrate (kelim) were determined for reactions at [Enzyme] much greater than [Substrate]. The second-order rate constant kEnz = 10.1 M-1 s-1 was determined from a plot of kelim against enzyme concentration. The kinetic parameters, determined from a steady-state kinetic analysis at [Substrate] much greater than [Enzyme], are kcat = 0.011 s-1, Km = 0.76 mM, and kcat/Km = 14 M-1 s-1. The estimated rate-constant ratio for partitioning of the enzyme-bound intermediate between protonation at carbon 2 and elimination, 1,000,000, is much larger than the ratio of 6.5 determined for the reaction of the enediolate phosphate in a loose complex with quinuclidinonium cation, a small buffer catalyst. There is a 10(5)-10(8)-fold decrease in the rate constant for the elimination reaction of the enediolate phosphate when this species binds to triosephosphate isomerase. The kinetic parameters for the elimination reaction catalyzed by the native triosephosphate isomerase and for the reaction catalyzed by a mutant form of the enzyme, which is missing a segment that forms hydrogen bonds with the phosphate group of substrate [Pompliano, D. L., Peyman, A., & Knowles, J. R. (1990) Biochemistry 29, 3186-3194] are similar.(ABSTRACT TRUNCATED AT 250 WORDS) PMID:2021650

Richard, J P

1991-05-01

83

Rate of reaction between molecular hydrogen and molecular oxygen  

NASA Technical Reports Server (NTRS)

The shock tube data of Jachimowski and Houghton were rigorously analyzed to obtain rate constants for the candidate initiation reactions H2 + O2 yields H + HO2, H2 + O2 yields H2O + O, and H2 + O2 yields OH + OH. Reaction (01) is probably not the initiation process because the activation energy obtained is less than the endothermicity and because the derived rates greatly exceed values inferred in the literature from the reverse of reaction (01). Reactions (02) and (03) remain as possibilities, with reaction (02) slightly favored on the basis of steric and statistical considerations. The solution of the differential equations is presented in detail to show how the kinetics of other ignition systems may be solved.

Brokaw, R. S.

1973-01-01

84

Hydro-Reactive Computations with a Temperature Dependent Reaction Rate  

NASA Astrophysics Data System (ADS)

Hydro-reactive computations are usually performed with a reaction model containing a pressure dependent reaction rate (PDRR). A well-known example is the ``Ignition & Growth'' (I&G) reaction model introduced by Lee and Tarver some twenty years ago. Performing such computations it has become evident that in many cases the results obtained seem unreliable. For these cases using a temperature dependent reaction rate (TDRR) may produce better results. We're using a surface burn reaction model that we've developed some twenty years ago. Originally we used it with a TDRR. Some years ago we introduced it into the PISCES code and used it with a PDRR. In this work we reintroduce the TDRR into the model with the purpose of comparing the performance of the two reaction rates. We first calibrate the rates to reproduce the pop-plot of PBX-9502. We then run the code with the two rates for several 1D and 2D situations. The resulting differences are qualitatively as expected, but previously we could not have estimated them quantitatively.

Partom, Y.

2002-07-01

85

Hydro-Reactive Computations with a Temperature Dependent Reaction Rate  

NASA Astrophysics Data System (ADS)

Hydro-reactive computations are usually performed with a reaction model that contains a pressure dependent reaction rate (PDRR). A well known example is the so called "Ignition and Growth" reaction model introduced by Lee and Tarver some twenty years ago. Performing such hydro-reactive computations for many shock initiation and detonation situations it has become evident that in many cases, the results obtained are inappropriate. For those cases it seems that using a temperature dependent reaction rate (TDRR) may produce more appropriate results. We're using a surface burn reaction model that we've developed some twenty years ago. Originally we used it with a TDRR. Several years ago we introduced it into the PISCES code and used it with a PDRR. In this work we add a TDRR to the model in PISCES with the purpose of comparing the performance of the two reaction rates. We first calibrate the two rates to reproduce the pop-plot of PBX-9502. Then we run the code with the two rates for several 1D and 2D situations such as: short shock initiation; double shock initiation; detonation in a rod and corner turning. The resulting differences are qualitatively as expected, but previousely we could not have estimated them quantatively.

Partom, Yehuda

2001-06-01

86

Stopped flow kinetic studies on reductive half-reaction of histamine dehydrogenase from Nocardioides simplex with histamine.  

PubMed

Histamine dehydrogenase from Nocardioides simplex (HmDH) which catalyzes the oxidative deamination of histamine is an iron-sulphur-containing flavoprotein. For our further understanding on the intramolecular electron transfer process, the reductive half reaction of HmDH with histamine has been studied by stopped flow spectrophotometry at pH 7.5 and 10. The reaction at pH 7.5 is found to be analysed on a kinetic model composed of three sequential first-order reactions. The first fast phase, of which the rate constant shows a hyperbolic dependence on the histamine concentration, is assigned to a direct two-electron reduction of the oxidized flavin (CFMN(O)) by histamine with no involvement of the semiquinone form of the flavin (CFMN(S)). The second moderate process is the substrate-independent intramolecular single-electron transfer from the reduced flavin to the oxidized iron-sulphur cluster. The third slow process is considered to reflect the second binding of histamine to CFMN(S), which is responsible for the substrate inhibition. At pH 10, the reaction is analysed with one pseudo-first-order reaction phase which is substrate-dependent two-electron reduction of CFMN(O) coupled with the subsequent fast intersubunit single-electron transfer. The UV-vis spectroscopy of HmDH suggests the deprotonation of Tyr residues, which seems to cause the switching of the electron transfer property. PMID:20305273

Tsutsumi, Maiko; Tsujimura, Seiya; Shirai, Osamu; Kano, Kenji

2010-07-01

87

Temperature Effects on Lithium-Nitrogen Reaction Rates.  

National Technical Information Service (NTIS)

A series of experiments have been run with the aim of measuring the reaction rate of lithium and nitrogen over a wide spectrum of lithium pool temperatures. In these experiments, pure nitrogen was blown at a controlled flow rate over a preheated lithium p...

W. J. Ijams M. S. Kazimi

1985-01-01

88

Nonuniform reaction rate distribution for the generalized Fisher equation: ignition ahead of the reaction front.  

PubMed

We have addressed the problem of wavefront propagation for the generalized Fisher equation involving diffusion transport with a finite velocity and a spatially nonuniform reaction rate. We have considered in detail the phenomenon of wavefront jump or ignition ahead of the reaction front for a piecewise constant reaction rate. By using a relativistic mechanics technique based on the principle of least action we have found the criterion under which the ignition phenomenon happens and the time of appearance of the new front ahead of the initial reaction front. PMID:11970364

Fedotov, S

1999-10-01

89

Extension of a Kinetic-Theory Approach for Computing Chemical-Reaction Rates to Reactions with Charged Particles  

NASA Technical Reports Server (NTRS)

Recently introduced molecular-level chemistry models that predict equilibrium and nonequilibrium reaction rates using only kinetic theory and fundamental molecular properties (i.e., no macroscopic reaction rate information) are extended to include reactions involving charged particles and electronic energy levels. The proposed extensions include ionization reactions, exothermic associative ionization reactions, endothermic and exothermic charge exchange reactions, and other exchange reactions involving ionized species. The extensions are shown to agree favorably with the measured Arrhenius rates for near-equilibrium conditions.

Liechty, Derek S.; Lewis, Mark J.

2010-01-01

90

Reaction and Anomalous Transport: Effective Reaction Rate Coefficients from the Multirate Model  

NASA Astrophysics Data System (ADS)

One cause of anomalous transport is a distribution of exchange rates between fast- and slow-moving water. Slow-moving water is frequently the preferred location of the most significant reactions, such as in biofilms. We express transport and reaction using the multirate model and we assume first-order reaction from parent to daughter. In this case, the effective rate coefficient can be expressed ? _a = int_0^{infty } frac{alpha p(alpha) ? }{alpha +? } dalpha where alpha is the mass transfer rate coefficient, ?mda is the reaction rate coefficient, and p(alpha) is the density function of mass transfer rate coefficients in the multirate model. Exchange rates much slower than ?mda contribute relatively little to the effective rate coefficient, while exchange rates similar to or faster than ?mda contribute most to the effective rate coefficient. Consequently, a conservative solute may experience anomalous transport, but a reacting solute may exhibit behavior that is not significantly different from Fickian transport with reaction.

Haggerty, R.

2012-12-01

91

Glow Fast, Glow Slow: Alter the Rate of a Reaction!  

NSDL National Science Digital Library

Learners investigate one factor affecting reaction rates: temperature. In a darkened room, two identical lightsticks are placed in water -- one in hot water and one in cold water. The lightstick in the hot water glows brighter than the lightstick in the cold water since the light-producing reaction is faster in hot water. This activity, located on page 2 of the PDF, is one of three activities in the Take Home guide for the Chemical Reactions unit in OMSI's Chemistry Lab. Also included on the PDF are two additional activities: Cloudy Globs and Gas Production.

Industry, Oregon M.

1997-01-01

92

Estimation of the rate of volcanism on Venus from reaction rate measurements  

NASA Technical Reports Server (NTRS)

Laboratory rate data for the reaction between SO2 and calcite to form anhydrite are presented. If this reaction rate represents the SO2 reaction rate on Venus, then all SO2 in the Venusian atmosphere will disappear in 1.9 Myr unless volcanism replenishes the lost SO2. The required volcanism rate, which depends on the sulfur content of the erupted material, is in the range 0.4-11 cu km of magma erupted per year. The Venus surface composition at the Venera 13, 14, and Vega 2 landing sites implies a volcanism rate of about 1 cu km/yr. This geochemically estimated rate can be used to determine if either (or neither) of two discordant geophysically estimated rates is correct. It also suggests that Venus may be less volcanically active than the earth.

Fegley, Bruce, Jr.; Prinn, Ronald G.

1989-01-01

93

Kinetics of heterogeneous reaction of CaCO3 particles with gaseous HNO3 over a wide range of humidity.  

PubMed

Heterogeneous reaction kinetics of gaseous nitric acid (HNO3) with calcium carbonate (CaCO3) particles was investigated using a particle-on-substrate stagnation flow reactor (PS-SFR). This technique utilizes the exposure of substrate deposited, isolated, and narrowly dispersed particles to a gas mixture of HNO3/H2O/N2, followed by microanalysis of individual reacted particles using computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM/EDX). The first series of experiments were conducted at atmospheric pressure, room temperature and constant relative humidity (40%) with a median dry particle diameter of Dp = 0.85 mum, particle loading densities 2 x 104 pseudo first-order rate constant for the reaction was determined from oxygen enrichment in individual particles as a function of particle loading. Quantitative treatment of the data using a diffusion-kinetic model yields a lower limit to the net reaction probability gammanet >/= 0.06 (x3//2). In a second series of experiments, HNO3 uptake on CaCO3 particles of the same size was examined over a wide range of relative humidity, from 10 to 80%. The net reaction probability was found to increase with increasing relative humidity, from gammanet >/= 0.003 at RH = 10% to 0.21 at 80%. PMID:18232670

Liu, Y; Gibson, E R; Cain, J P; Wang, H; Grassian, V H; Laskin, A

2008-02-21

94

Kinetics of Heterogeneous Reaction of CaCO3 Particles with Gaseous HNO3 Over a Wide Range of Humidity  

SciTech Connect

Heterogeneous reaction kinetics of gaseous nitric acid (HNO3) with calcium carbonate (CaCO3) particles was investigated using the Particle-on-Substrate Stagnation Flow Reactor (PS-SFR). The technique utilizes the exposure of substrate deposited, isolated, and narrowly dispersed particles to a gas mixture of HNO3/H2O/N2 followed by microanalysis of individual reacted particles using computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM/EDX). The first series of experiment was conducted at atmospheric pressure, room temperature and constant relative humidity (40%) with a median dry particle diameter pD = 0.85 ?m, particle loading densities 2×104 ? Ns ? 6×106 cm–2 and free stream HNO3 concentrations of 7, 14 and 25 ppb. The apparent, pseudo first-order rate constant for the reaction was determined from oxygen enrichment in individual particles as a function of particle loading. Quantitative treatment of the data using a diffusion-kinetic model yields lower limit to the net reaction probability ?net ? 0.06 (×3/÷2). In the second series of experiments, HNO3 uptake on CaCO3 of the same particle size was examined over a wide range of relative humidity, from 10 to 80%. The lower limit for the net reaction probability was found to increase with an increase in the relative humidity, from ?net ? 0.003 at RH = 10% to 0.21 at 80%.

Liu, Yong; Gibson, Elizabeth R.; Cain, Jeremy P.; Wang, Hai; Grassian, Vicki H.; Laskin, Alexander

2008-02-21

95

Sequence-Specific Cu(II)-Dependent Peptide Bond Hydrolysis: Similarities and Differences with the Ni(II)-Dependent Reaction.  

PubMed

Potentiometry and UV-vis and circular dichroism spectroscopies were applied to characterize Cu(II) coordination to the Ac-GASRHWKFL-NH2 peptide. Using HPLC and ESI-MS, we demonstrated that Cu(II) ions cause selective hydrolysis of the Ala-Ser peptide bond in this peptide and characterized the pH and temperature dependence of the reaction. We found that Cu(II)-dependent hydrolysis occurs solely in 4N complexes, in which the equatorial coordination positions of the Cu(II) ion are saturated by peptide donor atoms, namely, the pyridine-like nitrogen of the His imidazole ring and three preceding peptide bond nitrogens. Analysis of the reaction products led to the conclusion that Cu(II)-dependent hydrolysis proceeds according to the mechanism demonstrated previously for Ni(II) ions ( Kopera , E. ; Kr??el , A. ; Protas , A. M. ; Belczyk , A. ; Bonna , A. ; Wys?ouch-Cieszy?ska , A. ; Pozna?ski , J. ; Bal , W. Inorg. Chem. 2010 , 49 , 6636 - 6645 ). However, the pseudo-first-order reaction rate found for Cu(II) is, on average, 100 times lower than that for Ni(II) ions. The greater ability of Cu(II) ions to form 4N complexes at lower pH partially compensates for this difference in rates, resulting in similar hydrolytic activities for the two ions around pH 7. PMID:24735221

Belczyk-Ciesielska, Agnieszka; Zawisza, Izabela A; Mital, Mariusz; Bonna, Arkadiusz; Bal, Wojciech

2014-05-01

96

The Influence of Particle Charge on Heterogeneous Reaction Rate Coefficients  

NASA Technical Reports Server (NTRS)

The effects of particle charge on heterogeneous reaction rates are presented. Many atmospheric particles, whether liquid or solid are charged. This surface charge causes a redistribution of charge within a liquid particle and as a consequence a perturbation in the gaseous uptake coefficient. The amount of perturbation is proportional to the external potential and the square of the ratio of debye length in the liquid to the particle radius. Previous modeling has shown how surface charge affects the uptake coefficient of charged aerosols. This effect is now included in the heterogeneous reaction rate of an aerosol ensemble. Extension of this analysis to ice particles will be discussed and examples presented.

Aikin, A. C.; Pesnell, W. D.

2000-01-01

97

Microphotometric measurement of initial maximum reaction rates in quantitative enzyme histochemistry in situ  

Microsoft Academic Search

Summary  Final reaction product formation was recorded microphotometrically for succinate dehydrogenase in cross-sectioned muscle fibres at initial rate conditions and during prolonged incubations. Incubations with gel films and aqueous reaction medium both showed a decline of reaction rates. Maximum reaction rates could only be determined at initial rate conditions during the first minute of the incubation. Reaction rates recorded in different

Dirk Pette; Universitfit Konstanz

1981-01-01

98

Suitable combination of promoter and micellar catalyst for kilo fold rate acceleration on benzaldehyde to benzoic acid conversion in aqueous media at room temperature: A kinetic approach  

NASA Astrophysics Data System (ADS)

The kinetics of oxidation of benzaldehyde by chromic acid in aqueous and aqueous surfactant (sodium dodecyl sulfate, SDS, alkyl phenyl polyethylene glycol, Triton X-100 and N-cetylpyridinium chloride, CPC) media have been investigated in the presence of promoter at 303 K. The pseudo-first-order rate constants (kobs) were determined from a logarithmic plot of absorbance as a function time. The rate constants were found to increase with introduction of heteroaromatic nitrogen base promoters such as Picolinic acid (PA), 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen). The product benzoic acid has been characterized by conventional melting point experiment, NMR, HRMS and FTIR spectral analysis. The mechanism of both unpromoted and promoted reaction path has been proposed for the reaction. In presence of the anionic surfactant SDS, cationic surfactant CPC and neutral surfactant TX-100 the reaction can undergo simultaneously in both aqueous and micellar phase with an enhanced rate of oxidation in the micellar phase. Both SDS and TX-100 produce normal micellar effect whereas CPC produce reverse micellar effect in the presence of benzaldehyde. The observed net enhancement of rate effects has been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. SDS and bipy combination is the suitable one for benzaldehyde oxidation.

Ghosh, Aniruddha; Saha, Rumpa; Ghosh, Sumanta K.; Mukherjee, Kakali; Saha, Bidyut

2013-05-01

99

Suitable combination of promoter and micellar catalyst for kilo fold rate acceleration on benzaldehyde to benzoic acid conversion in aqueous media at room temperature: a kinetic approach.  

PubMed

The kinetics of oxidation of benzaldehyde by chromic acid in aqueous and aqueous surfactant (sodium dodecyl sulfate, SDS, alkyl phenyl polyethylene glycol, Triton X-100 and N-cetylpyridinium chloride, CPC) media have been investigated in the presence of promoter at 303 K. The pseudo-first-order rate constants (kobs) were determined from a logarithmic plot of absorbance as a function time. The rate constants were found to increase with introduction of heteroaromatic nitrogen base promoters such as Picolinic acid (PA), 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen). The product benzoic acid has been characterized by conventional melting point experiment, NMR, HRMS and FTIR spectral analysis. The mechanism of both unpromoted and promoted reaction path has been proposed for the reaction. In presence of the anionic surfactant SDS, cationic surfactant CPC and neutral surfactant TX-100 the reaction can undergo simultaneously in both aqueous and micellar phase with an enhanced rate of oxidation in the micellar phase. Both SDS and TX-100 produce normal micellar effect whereas CPC produce reverse micellar effect in the presence of benzaldehyde. The observed net enhancement of rate effects has been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. SDS and bipy combination is the suitable one for benzaldehyde oxidation. PMID:23501718

Ghosh, Aniruddha; Saha, Rumpa; Ghosh, Sumanta K; Mukherjee, Kakali; Saha, Bidyut

2013-05-15

100

Analysis of the Reaction Rate Coefficients for Slow Bimolecular Chemical Reactions  

NASA Astrophysics Data System (ADS)

Simple bimolecular reactions A_1+A_2A_3+A_4 are analyzed within the framework of the Boltzmann equation in the initial stage of a chemical reaction with the system far from chemical equilibrium. The Chapman-Enskog methodology is applied to determine the coefficients of the expansion of the distribution functions in terms of Sonine polynomials for peculiar molecular velocities. The results are applied to the reaction H_2 +ClHCl+H, and the influence of the non-Maxwellian distribution and of the activation-energy dependent reactive cross sections upon the forward and reverse reaction rate coefficients are discussed.

Kremer, Gilberto M.; Silva, Tiago G.

2012-12-01

101

A compilation of charged-particle induced thermonuclear reaction rates  

NASA Astrophysics Data System (ADS)

Low-energy cross section data for 86 charged-particle induced reactions involving light (1 <=Z <=14), mostly stable, nuclei are compiled. The corresponding Maxwellian-averaged thermonuclear reaction rates of relevance in astrophysical plasmas at temperatures in the range from 106 K to 1010 K are calculated. These evaluations assume either that the target nuclei are in their ground state, or that the target states are thermally populated following a Maxwell-Boltzmann distribution, except in some cases involving isomeric states. Adopted values complemented with lower and upper limits of the rates are presented in tabular form. Analytical approximations to the adopted rates, as well as to the inverse/direct rate ratios, are provided.

Angulo, C.; Arnould, M.; Rayet, M.; Descouvemont, P.; Baye, D.; Leclercq-Willain, C.; Coc, A.; Barhoumi, S.; Aguer, P.; Rolfs, C.; Kunz, R.; Hammer, J. W.; Mayer, A.; Paradellis, T.; Kossionides, S.; Chronidou, C.; Spyrou, K.; degl'Innocenti, S.; Fiorentini, G.; Ricci, B.; Zavatarelli, S.; Providencia, C.; Wolters, H.; Soares, J.; Grama, C.; Rahighi, J.; Shotter, A.; Lamehi Rachti, M.

1999-08-01

102

Test of Weak Reaction Rates of Importance for Late Stellar Evolution Using Charge-exchange Reactions.  

NASA Astrophysics Data System (ADS)

Weak reactions (electron captures and beta decays) on nuclei play an important role in the evolution of pre-supernovae stars and their eventual core collapse. The rates are usually predicted in shell-model and mean-field calculations. Experimental information on Gamow-Teller strength distributions is needed to test these calculations. At the NSCL, the (t,^3He) charge-exchange reaction at 115 MeV/nucleon is employed for such studies. In addition, the (^3He,t) reaction at 140 MeV/nucleon (at RCNP, Osaka) is used for detailed studies of the charge-exchange reaction mechanism, which is important for understanding the uncertainties in the experimentally extracted Gamow-Teller strength distributions. Besides a comparison between experimental and theoretical Gamow-Teller strength distributions, the work also includes the calculation of electron-capture rates, so that the sensitivity of the rates on differences between theoretical and experimental rates can be gauged. A significant fraction of the nuclei relevant for late stellar evolution are unstable. Hence, to ensure accurate weak reaction rates for such nuclei, charge-exchange experiments on rare isotopes are crucial, but the techniques have to be developed. At the NSCL, a program has been initiated to study charge-exchange reactions on rare isotopes.

Zegers, R. G. T.

2009-05-01

103

Semiclassical Calculation of Reaction Rate Constants for Homolytical Dissociations  

NASA Technical Reports Server (NTRS)

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.

Cardelino, Beatriz H.

2002-01-01

104

Rates of membrane-associated reactions: reduction of dimensionality revisited  

Microsoft Academic Search

The hypothesis that reactions associated with intracellular membranes enjoy a kinetic advan- tage from a reduced dimensionality for diffusion is inconsistent with available data on lateral diffusion rates, membrane-substra te affinities, and endogenous concentrations of enzymes and their aqueous sub- strates.

Michael A. McCloskey; Mu-ming Pop

1986-01-01

105

Bimolecular reaction rates from ring polymer molecular dynamics  

NASA Astrophysics Data System (ADS)

We describe an efficient procedure for calculating the rates of bimolecular chemical reactions in the gas phase within the ring polymer molecular dynamics approximation. A key feature of the procedure is that it does not require that one calculate the absolute quantum mechanical partition function of the reactants or the transition state: The rate coefficient only depends on the ratio of these two partition functions which can be obtained from a thermodynamic integration along a suitable reaction coordinate. The procedure is illustrated with applications to the three-dimensional H+H2, Cl+HCl, and F+H2 reactions, for which well-converged quantum reactive scattering results are computed for comparison. The ring polymer rate coefficients agree with these exact results at high temperatures and are within a factor of 3 of the exact results at temperatures in the deep quantum tunneling regime, where the classical rate coefficients are too small by several orders of magnitude. This is probably already good enough to encourage future applications of the ring polymer theory to more complex chemical reactions, which it is capable of treating in their full dimensionality. However, there is clearly some scope for improving on the ring polymer approximation at low temperatures, and we end by suggesting a way in which this might be accomplished.

Collepardo-Guevara, Rosana; Suleimanov, Yury V.; Manolopoulos, David E.

2009-05-01

106

EXPERIMENTAL PROTOCOL FOR DETERMINING PROTOLYSIS REACTION RATE CONSTANTS  

EPA Science Inventory

An experimental protocol to determine photolysis rates of chemicals which photolyze relatively rapidly in the gas phase has been developed. This procedure provides a basis for evaluating the relative importance of one atmospheric reaction pathway (i.e., photolysis) for organic su...

107

TURBULENT REACTION RATE IN TWO-PHASE FLOW FLAMES  

Microsoft Academic Search

The evaluation of the mean reaction rate within two-phase flow turbulent flames is the main subject of the present paper. First, the discussion emphasizes the utmost importance of the atomization (and especially the primary break-up) process but also the dispersion process. Then, a general approach is proposed to deal with the complex features of turbulent flames that are likely to

A. Mura; F. X. Demoulin; R. Borghi

108

EXPERIMENTAL PROTOCOL FOR DETERMINING OZONE REACTION RATE CONSTANTS  

EPA Science Inventory

An experimental protocol for the determination of room temperature rate constants for the reactions of ozone with chemicals in the gas phase has been developed and is described in detail. This protocol will thus provide a basis for evaluating the relative importance of one atmosp...

109

Bimolecular reaction rates from ring polymer molecular dynamics.  

PubMed

We describe an efficient procedure for calculating the rates of bimolecular chemical reactions in the gas phase within the ring polymer molecular dynamics approximation. A key feature of the procedure is that it does not require that one calculate the absolute quantum mechanical partition function of the reactants or the transition state: The rate coefficient only depends on the ratio of these two partition functions which can be obtained from a thermodynamic integration along a suitable reaction coordinate. The procedure is illustrated with applications to the three-dimensional H + H(2), Cl + HCl, and F + H(2) reactions, for which well-converged quantum reactive scattering results are computed for comparison. The ring polymer rate coefficients agree with these exact results at high temperatures and are within a factor of 3 of the exact results at temperatures in the deep quantum tunneling regime, where the classical rate coefficients are too small by several orders of magnitude. This is probably already good enough to encourage future applications of the ring polymer theory to more complex chemical reactions, which it is capable of treating in their full dimensionality. However, there is clearly some scope for improving on the ring polymer approximation at low temperatures, and we end by suggesting a way in which this might be accomplished. PMID:19425804

Collepardo-Guevara, Rosana; Suleimanov, Yury V; Manolopoulos, David E

2009-05-01

110

Reaction rates of graphite with ozone measured by etch decoration  

NASA Technical Reports Server (NTRS)

Etch-decoration technique of detecting vacancies in graphite has been used to determine the reaction rates of graphite with ozone in the directions parallel and perpendicular to the layer planes. It consists essentially of peeling single atom layers off graphite crystals without affecting the remainder of the crystal.

Hennig, G. R.; Montet, G. L.

1968-01-01

111

Assessment of reaction-rate predictions of a collision-energy approach for chemical reactions in atmospheric flows.  

SciTech Connect

A recently proposed approach for the Direct Simulation Monte Carlo (DSMC) method to calculate chemical-reaction rates is assessed for high-temperature atmospheric species. The new DSMC model reproduces measured equilibrium reaction rates without using any macroscopic reaction-rate information. Since it uses only molecular properties, the new model is inherently able to predict reaction rates for arbitrary non-equilibrium conditions. DSMC non-equilibrium reaction rates are compared to Park's phenomenological nonequilibrium reaction-rate model, the predominant model for hypersonic-flow-field calculations. For near-equilibrium conditions, Park's model is in good agreement with the DSMC-calculated reaction rates. For far-from-equilibrium conditions, corresponding to a typical shock layer, significant differences can be found. The DSMC predictions are also found to be in very good agreement with measured and calculated non-equilibrium reaction rates, offering strong evidence that this is a viable and reliable technique to predict chemical reaction rates.

Gallis, Michail A.; Bond, Ryan Bomar; Torczynski, John Robert

2010-06-01

112

Enhancing the reaction rates of Morita-Baylis-Hillman reaction in heterocyclic aldehydes by substitutions.  

PubMed

The molecular origin of the experimentally observed pronounced difference in the rates of Morita-Baylis-Hillman (MBH) reaction in heterocyclic aldehydes, depending on the position of the formyl group, is investigated herein by using DFT-based mechanistic studies and free energy computations. These calculations are based on the 1,4-diazobicyclo[2.2.2]octane (DABCO)-catalyzed MBH reaction of methyl acrylate with substituted 4- and 5-isoxazolecarbaldehyde, which are slow- and fast-reacting substrates, respectively. As a result of this study, we propose that by tailoring ring substitutions the reactivity of the formyl group for MBH reactions may be enhanced in slow-reacting heterocyclic aldehydes. This proposition is demonstrated by enhancing the rate of the MBH reaction in 4-isoxazolecarbaldehyde more than 10(4) -fold by installing an ester substitution at the C-3 position. Similarly, the reactivity of the formyl group towards the MBH reaction in substituted 3-pyrazolecarbaldehyde and pyridinecarbaldehyde is shown to be increased several-fold by a halo substitution. We also confirm that the reasons for different reactivities of heterocyclic aldehydes and the proposed scheme for improving the reaction rates remains valid for all the three mechanisms proposed for the MBH reaction, namely, Hill-Isaacs, McQuade, and Aggarwal. PMID:22887878

Kunnikuruvan, Sooraj; Batra, Sanjay; Nair, Nisanth N

2012-11-12

113

Rate of reaction of OH with HNO3  

NASA Technical Reports Server (NTRS)

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.

Wine, P. H.; Ravishankara, A. R.; Kreutter, N. M.; Shah, R. C.; Nicovich, J. M.; Thompson, R. L.; Wuebbles, D. J.

1981-01-01

114

Nuclear fusion reaction rates for strongly coupled ionic mixtures  

SciTech Connect

We analyze the effect of plasma screening on nuclear reaction rates in dense matter composed of atomic nuclei of one or two types. We perform semiclassical calculations of the Coulomb barrier penetrability taking into account a radial mean-field potential of plasma ions. The mean-field potential is extracted from the results of extensive Monte Carlo calculations of radial pair distribution functions of ions in binary ionic mixtures. We calculate the reaction rates in a wide range of plasma parameters and approximate these rates by an analytical expression that is expected to be applicable to multicomponent ion mixtures. Also, we analyze Gamow-peak energies of reacting ions in various nuclear burning regimes. For illustration, we study nuclear burning in {sup 12}C-{sup 16}O mixtures.

Chugunov, A. I.; DeWitt, H. E. [Ioffe Physical-Technical Institute, Politekhnicheskaya 26, RU-194021 St. Petersburg (Russian Federation); Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)

2009-07-15

115

Reaction rate and products for the reaction O/3P/ + H2CO  

NASA Technical Reports Server (NTRS)

A study of reaction kinetics of O + H2CO in a discharge-flow system using mass spectrometric detection of reactants and products is presented. It was performed under both oxygen-atom-rich and formaldehyde-rich conditions over the 296 to 437 K range, showing that the global bimolecular rate constant is in agreement with other studies. This study differs from others in that the reaction products can be observed, and a substantial yield of a primary reaction product was measured with a mass spectral peak at m/e=44. This suggests that the global reaction rate probably consists of combination, as well as of simple abstraction. For the combination, one hypothesis is that triplet dioxymethylene is formed which polymerizes to triplet formic acid; the vibrationally excited triplet formic acid may decompose to form several sets of products, including HCO + OH and HCO2 + H.

Chang, J. S.; Barker, J. R.

1979-01-01

116

Noncatalytic kinetic study on site-selective H/D exchange reaction of phenol in sub- and supercritical water  

NASA Astrophysics Data System (ADS)

The site-selective H/D exchange reaction of phenol in sub- and supercritical water is studied without added catalysts. In subcritical water in equilibrium with steam at 210-240 °C, the H/D exchange proceeds both at the ortho and para sites in the phenyl ring, with no exchange observed at the meta site. The pseudo-first-order rate constants are of the order of 10-4 s-1 50% larger for the ortho than for the para site. In supercritical water, the exchange is observed also at the meta site with the rate constant in the range of 10-6-10-4 s-1. As the bulk density decreases, the exchange slows down and the site selectivity toward the ortho is enhanced. The enhancement is due to the phenol-water interaction preference at the atomic resolution. The site selectivity toward the ortho is further enhanced when the reaction is carried out in benzene/water solution. Using such selectivity control and the reversible nature of the hydrothermal deuteration/protonation process, it is feasible to synthesize phenyl compounds that are deuterated at any topological combination of ortho, meta, and para sites.

Kubo, Masahito; Takizawa, Takeyuki; Wakai, Chihiro; Matubayasi, Nobuyuki; Nakahara, Masaru

2004-07-01

117

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

NASA Astrophysics Data System (ADS)

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.

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

118

Preparation of stabilized nano zero-valent iron particles via a rheological phase reaction method and their use in dye decolourization.  

PubMed

In this study, sodium carboxymethyl cellulose (NaCMC)-stabilized nano zero-valent iron (C-nZVI) was synthesized using a rheological phase reaction method. The orthogonal method was used to evaluate the factors influencing C-nZVI properties and this showed that the reaction time, solid-liquid ratio (w/v), grinding time and NaCMC concentration were all important factors. Characterization with scanning electron microscopy validated the hypothesis that the introduction of CMC led to a decrease in aggregation of iron nanoparticles. X-ray diffraction confirmed the existence of Fe(0) and the strong antioxidant activity of the iron particles. Batch decolourization experiments exhibited that solution pH, C-nZVI dosage and reaction time have significant effects on dye decolourization. A high decolourization efficiency (94.5%) was obtained within 30 min for 100 mg/L of reactive blue-19 at the optimal pH value of 5 and C-nZVI loading of 6 g/L at room temperature. The decolourization rates followed modified pseudo-first-order kinetic equations with respect to dye concentration. The observed removal rate constant was 0.0447 min(-1) for the C-nZVI loading of 6 g/L. PMID:23530358

Cheng, Yue; Lu, Mang; Jiao, Chuang; Liu, Hai-Jiang

2013-01-01

119

Rate of reaction of OH with CS/sub 2/  

SciTech Connect

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.

Wine, P.H.; Shah, R.C.; Ravishankara, A.R.

1980-10-02

120

Boron atom reaction rate constants with the epoxides. 2  

SciTech Connect

Rates have been measured for the gas-phase reaction of boron atoms with a second set of epoxides. The measurements were performed in a flow tube apparatus at 300 K. The bimolecular rate constants in units of cm/sup 3/ molecule /sup - -/exclamation s/sup -1/ are as follows: ethylene oxide (3.0 x 10/sup -1/), epifluorohydrin (7.7 x 10/sup -11/), epibromohydrin (4.1 x 10/sup -11/), cyclopentene oxide (4.4 x 10/sup -11/), styrene oxide (6.9 x 10/sup -11/), cyclohexene oxide (4.5 x 10/sup -11/), vinyloxabicyclohexane (2.1 x 10/sup 11/). These studies together with the previously published work show that the reactivity of the oxygen in the epoxide is substantially affected by various substituents and configurations. MNDO calculations performed on the boron epoxide systems suggest that the reaction of boron atoms with the epoxides proceeds via a bound complex.

Tabacco, M.D.; Stanton, C.T.; Davidovits, P.

1986-06-05

121

A model for reaction rates in turbulent reacting flows  

NASA Technical Reports Server (NTRS)

To account for the turbulent temperature and species-concentration fluctuations, a model is presented on the effects of chemical reaction rates in computer analyses of turbulent reacting flows. The model results in two parameters which multiply the terms in the reaction-rate equations. For these two parameters, graphs are presented as functions of the mean values and intensity of the turbulent fluctuations of the temperature and species concentrations. These graphs will facilitate incorporation of the model into existing computer programs which describe turbulent reacting flows. When the model was used in a two-dimensional parabolic-flow computer code to predict the behavior of an experimental, supersonic hydrogen jet burning in air, some improvement in agreement with the experimental data was obtained in the far field in the region near the jet centerline. Recommendations are included for further improvement of the model and for additional comparisons with experimental data.

Chinitz, W.; Evans, J. S.

1984-01-01

122

Triple-{alpha} reaction rate constrained by stellar evolution models  

SciTech Connect

We investigate the quantitative constraint on the triple-{alpha} reaction rate based on stellar evolution theory, motivated by the recent significant revision of the rate proposed by nuclear physics calculations. Targeted stellar models were computed in order to investigate the impact of that rate in the mass range of 0.8{<=}M/M{sub Circled-Dot-Operator }{<=}25 and in the metallicity range between Z= 0 and Z= 0.02. The revised rate has a significant impact on the evolution of low-and intermediate-mass stars, while its influence on the evolution of massive stars (M > 10M{sub Circled-Dot-Operator }) is minimal. We find that employing the revised rate suppresses helium shell flashes on AGB phase for stars in the initial mass range 0.8{<=}M/M{sub Circled-Dot-Operator }{<=}6, which is contradictory to what is observed. The absence of helium shell flashes is due to the weak temperature dependence of the revised triple-{alpha} reaction cross section at the temperature involved. In our models, it is suggested that the temperature dependence of the cross section should have at least {nu} > 10 at T = 1-1.2 Multiplication-Sign 10{sup 8}K where the cross section is proportional to T{sup {nu}}. We also derive the helium ignition curve to estimate the maximum cross section to retain the low-mass first red giants. The semi-analytically derived ignition curves suggest that the reaction rate should be less than {approx} 10{sup -29} cm{sup 6} s{sup -1} mole{sup -2} at Almost-Equal-To 10{sup 7.8} K, which corresponds to about three orders of magnitude larger than that of the NACRE compilation.

Suda, Takuma; Hirschi, Raphael; Fujimoto, Masayuki Y. [National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588 (Japan); Keele University, Keele, Staffordshire ST5 5BG (United Kingdom) and Institute for the Physics and Mathematics of the Universe, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8583 (Japan); Hokkaido University, Kita 10 Nishi 8, Kita-ku, Sapporo 060-0810 (Japan)

2012-11-12

123

Determination of palladium--aluminum reaction propagation rates and temperatures  

Microsoft Academic Search

Three experimental techniques--thermometry, framing and streaking photography, and dynamic radiography--were employed to determine the reaction propagation rates and temperatures of various mixtures of palladium and aluminum. Pellets of 94\\/6, 90\\/10, and 80\\/20 Pd\\/Al mixtures with an average density of 5 x 10³ kg\\/m³ were prepared and tested in an insulating sleeve. Pretest radiography was used to determine the density in

Birnbaum

1978-01-01

124

Influence of matrix properties on alkali silica reaction rates  

Microsoft Academic Search

Matrices with different cracking and confinement characteristics are utilized to isolate the important parameters that influence\\u000a gel initiation and alkali silica reaction rate. The paper is divided into two parts. In Part I, the matrix exhibits crack\\u000a growth resistance behavior which induces compressive stresses on the alkali silica gel and in addition prevents the alkali\\u000a silica gel from leaving the

H. Song; K. Cheng; C. P. Ostertag

2008-01-01

125

Electrostatic effects on the rates of DNA-catalyzed reactions  

NASA Astrophysics Data System (ADS)

Diol-epoxide metabolites of many genotoxic polycyclic aromatic hydrocarbons are hydrolyzed to tetraols in a detoxification reaction. The hydrolysis reaction has both spontaneous and acid-catalyzed components; moreover, the reaction rate increases in the presence of DNA. The best studied of these diol epoxide metabolites are the trans 7,8 diol-9,10 epoxides of benzo[a]pyrene: anti-BPDE, the proximate carcinogen, in which the oxirane ring is anti with respect to the 7-hydroxyl group, and its syn diastereomer, syn-BPDE. Jerina and coworkers have studied the kinetics of the hydrolysis of syn- and anti-BPDE as a function of pH and DNA concentration and have measured the equilibrium constant for the formation of a noncovalent complex with DNA. They constructed a two-state model in which the diol epoxide is either free or statically bound: the free fraction is hydrolyzed with the same kinetics as it exhibits in solution without DNA; the bound diol epoxide reacts at faster rates. In this model, the dependence of the observed hydrolysis rates on both DNA concentration and pH is explained by using the rate constants, k0 and kH, for reactions of the free diol epoxide, the rate constants, kcat0 and kcatH, for the bound molecule, and the binding constant, Keq. The present work uses an acidic-domain interpretation of the two-state model to explain the catalytic effect of DNA on the acid-catalyzed hydrolysis of syn- and anti-BPDE. Postulating that the rate enhancement is a result of acidic domains at the DNA surface, we assumed the relationship k catH = k H [H +] b, where [H +] b is the hydrogen ion concentration near the bound molecule. Using numerical solutions to the Poisson-Boltzmann equation, the pH dependence of acidic domains at the surface of the polyelectrolyte, DNA, was calculated. Energy-minimization calculations were used to estimate the conformations of diol epoxide-DNA intercalation complexes. Poisson-Boltzmann (PB) calculations on these structures yielded hydrogen-ion concentrations near the epoxide group consistent with the k catH/k H ratio over a range of added-salt concentrations. The results strongly suggest that DNA catalysis of diol-epoxide hydrolysis is a polyelectrolyte effect. The mechanisms and rate constants observed for the acid-catalyzed hydrolysis in the absence of DNA are consistent with the increase in the rate constant induced by DNA. It may be concluded that the catalysis is primarily an effect of the acidic domains in the surface grooves of the nucleic acid.

Pack, George R.; Wong, Linda

1996-04-01

126

Application of semiclassical methods to reaction rate theory  

SciTech Connect

This work is concerned with the development of approximate methods to describe relatively large chemical systems. This effort has been divided into two primary directions: First, we have extended and applied a semiclassical transition state theory (SCTST) originally proposed by Miller to obtain microcanonical and canonical (thermal) rates for chemical reactions described by a nonseparable Hamiltonian, i.e. most reactions. Second, we have developed a method to describe the fluctuations of decay rates of individual energy states from the average RRKM rate in systems where the direct calculation of individual rates would be impossible. Combined with the semiclassical theory this latter effort has provided a direct comparison to the experimental results of Moore and coworkers. In SCTST, the Hamiltonian is expanded about the barrier and the ``good`` action-angle variables are obtained perturbatively; a WKB analysis of the effectively one-dimensional reactive direction then provides the transmission probabilities. The advantages of this local approximate treatment are that it includes tunneling effects and anharmonicity, and it systematically provides a multi-dimensional dividing surface in phase space. The SCTST thermal rate expression has been reformulated providing increased numerical efficiency (as compared to a naive Boltzmann average), an appealing link to conventional transition state theory (involving a ``prereactive`` partition function depending on the action of the reactive mode), and the ability to go beyond the perturbative approximation.

Hernandez, R.

1993-11-01

127

Rates of reaction of propyl radicals with molecular oxygen  

SciTech Connect

The rates of reaction of 1-propyl and 2-propyl radicals with O/sub 2/ have been measured in the gas phase at room temperature. The rate constants are k/sub 1/(1-C/sub 3/H/sub 7/+O/sub 2/) = (5.5 +/- 0.9) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/ and k/sub 2/(2-C/sub 3/H/sub 7/+O/sub 2/) = (14.1 +/- 2.4) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/. Limited pressure studies suggest that both of these reactions are at or near their high-pressure limits at pressures of a few torr. The logarithms of these and previously measured rate constants show an excellent correlation with the ionization potential of the alkyl radical. Adiabatic channel model calculations have been carried out for these reactions with various long-range potential functions. It is concluded that some mixing of the charge-transfer surface (R/sup +/ + O/sub 2//sup -/) into the neutral surface (R + O/sub 2/) is responsible for the correlation of log (k) with ionization potential.

Ruiz, R.P.; Bayes, K.D.

1984-06-07

128

Neutron detectors for fusion reaction-rate measurements  

SciTech Connect

Fusion reactions in an inertial-confinement fusion (ICF) target filled with deuterium or a deuterium/tritium fuel release nearly monoenergetic neutrons. Because most the neutrons leave the compressed target without collision, they preserve reaction-rate information as they travel radially outward from their point of origin. Three fast, neutron detector techniques, each capable of measuring the fusion reaction-rate of ICF targets, have been demonstrated. The most advanced detector is based on the fast rise-time of a commercial plastic scintillator material (BC-422) which acts as a neutron-to-light converter. Signals, which are recorded with a fast optical streak camera, have a resolution of 25 ps. Good signals can be recorded for targets producing only 5 x 10{sup 7} DT neutrons. Two other detectors use knock-on collisions between neutrons and protons in a thin polyethylene (CH{sub 2}) converter. In one, the converter is placed in front of the photocathode of an x-ray streak camera. Recoil protons pass through the photocathode and knock out electrons which are accelerated and deflected to produce a signal. Resolutions < 25 ps are possible. In the other, the converter is placed in front of a microchannel plate (MCP) with a gated microstrip. Recoil protons eject electrons from the gold layer forming the microstrip. If a gate pulse is present, the signal is amplified. Present gate times are about 80 ps.

Lerche, R.A.; Phillion, D.W.; Landen, O.L.; Murphy, T.J. [Lawrence Livermore National Lab., CA (United States); Jaanimagi, P.A. [Univ. of Rochester, NY (United States). Laboratory for Laser Energetics

1994-02-10

129

Thermonuclear reaction rate of 18O(p,?)19F  

NASA Astrophysics Data System (ADS)

For stars with 0.8 M? ? M ? 8.0 M?, nucleosynthesis enters its final phase during the asymptotic giant branch (AGB) stage. During this evolutionary period, grain condensation occurs in the stellar atmosphere, and the star experiences significant mass loss. The production of presolar grains can often be attributed to this unique stellar environment. A subset of presolar oxide grains features dramatic 18O depletion that cannot be explained by the standard AGB star burning stages and dredge-up models. An extra mixing process, referred to as cool bottom processing (CBP), was proposed for low-mass AGB stars. The 18O depletion observed within certain stellar environments and within presolar grain samples may result from the 18O+p processes during CBP. We report here on a study of the 18O(p,?)19F reaction at low energies. Based on our new results, we found that the resonance at ERlab=95 keV has a negligible affect on the reaction rate at the temperatures associated with CBP. We also determined that the direct capture S factor is almost a factor of 2 lower than the previously recommended value at low energies. An improved thermonuclear reaction rate for 18O(p,?)19F is presented.

Buckner, M. Q.; Iliadis, C.; Cesaratto, J. M.; Howard, C.; Clegg, T. B.; Champagne, A. E.; Daigle, S.

2012-12-01

130

Neutron detectors for fusion reaction-rate measurements  

NASA Astrophysics Data System (ADS)

Fusion reactions in an inertial-confinement fusion (ICF) target filled with deuterium or a deuterium/tritium fuel release nearly monoenergetic neutrons. Because most the neutrons leave the compressed target without collision, they preserve reaction-rate information as they travel radially outward from their point of origin. Three fast, neutron detector techniques, each capable of measuring the fusion reaction-rate of ICF targets, have been demonstrated. The most advanced detector is based on the fast rise-time of a commercial plastic scintillator material (BC-422) which acts as a neutron-to-light converter. Signals, which are recorded with a fast optical streak camera, have a resolution of 25 ps. Good signals can be recorded for targets producing only 5×107 DT neutrons. Two other detectors use knock-on collisions between neutrons and protons in a thin polyethylene (CH2) converter. In one, the converter is placed in front of the photocathode of an x-ray streak camera. Recoil protons pass through the photocathode and knock out electrons which are accelerated and deflected to produce a signal. Resolutions <25 ps are possible. In the other, the converter is placed in front of a microchannel plate (MCP) with a gated microstrip. Recoil protons eject electrons from the gold layer forming the microstrip. If a gate pulse is present, the signal is amplified. Present gate times are about 80 ps.

Lerche, R. A.; Phillion, D. W.; Landen, O. L.; Murphy, T. J.; Jaanimagi, P. A.

1994-10-01

131

Neutron detectors for fusion reaction-rate measurements  

NASA Astrophysics Data System (ADS)

Fusion reactions in an inertial-confinement fusion (ICF) target filled with deuterium or a deuterium/tritium fuel release nearly monoenergetic neutrons. Because most the neutrons leave the compressed target without collision, they preserve reaction-rate information as they travel radially outward from their point of origin. Three fast neutron detector techniques, each capable of measuring the fusion reaction-rate of ICF targets, have been demonstrated. The most advanced detector is based on the fast rise-time of a commercial plastic scintillator material (BC-422) which acts as a neutron-to-light converter. Signals, which are recorded with a fast optical streak camera, have a resolution of 25 ps. Good signals can be recorded for targets producing only 5 x 10(exp 7) DT neutrons. Two other detectors use knock-on collisions between neutrons and protons in a thin polyethylene (CH2) converter. In one, the converter is placed in front of the photocathode of an x-ray streak camera. Recoil protons pass through the photocathode and knock out electrons which are accelerated and deflected to produce a signal. Resolutions less than 25 ps are possible. In the other, the converter is placed in front of a microchannel plate (MCP) with a gated microstrip. Recoil protons eject electrons from the gold layer forming the microstrip. If a gate pulse is present, the signal is amplified. Present gate times are about 80 ps.

Lerche, R. A.; Phillion, D. W.; Landen, O. L.; Murphy, T. J.; Jaanimagi, P. A.

1994-02-01

132

Primordial lithium: New reaction rates, new abundances, new constraints  

SciTech Connect

Newly measured nuclear reaction rates for /sup 3/H(..cap alpha..,..gamma..)/sup 7/Li (higher than previous values) and /sup 7/Li(p,..cap alpha..)/sup 4/He (lower than previous values) are shown to increase the /sup 7/Li yield from big bang nucleosynthesis for lower baryon to photon ratio (eta less than or equal to 4 x 10/sup -10/); the yield for higher eta is not affected. New, independent determinations of Li abundances in extreme Pop II stars are in excellent agreement with the earlier work of the Spites and give continued confidence in the use of /sup 7/Li in big bang baryon density determinations. The new /sup 7/Li constraints imply a lower limit on eta of 2 x 10/sup -10/ and an upper limit of 5 x 10/sup -10/. This lower limit to eta is concordant with that obtained from considerations of D + /sup 3/He. The upper limit is consistent with, but even more restrictive than, the D bound. With the new rates, any observed primordial Li/H ratio below 10/sup -10/ would be inexplicable by the standard big bang nucleosynthesis. A review is made of the strengths and possible weaknesses of utilizing conclusions drawn from big bang lithium considerations. An appendix discusses the null effect of a factor of 32 increase in the experimental rate for the D(d,..gamma..)/sup 4/He reaction. 28 refs., 1 fig.

Kawano, L.; Schramm, D.; Steigman, G.

1986-12-01

133

Reaction Rate of Small Diffusing Molecules on a Cylindrical Membrane  

NASA Astrophysics Data System (ADS)

Biomembranes consist of a lipid bi-layer into which proteins are embedded to fulfill numerous tasks in localized regions of the membrane. Often, the proteins have to reach these regions by simple diffusion. Motivated by the observation that IP3 receptor channels (IP3R) form clusters on the surface of the endoplasmic reticulum (ER) during ATP-induced calcium release, the reaction rate of small diffusing molecules on a cylindrical membrane is calculated based on the Smoluchowski approach. In this way, the cylindrical topology of the tubular ER is explicitly taken into account. The problem can be reduced to the solution of the diffusion equation on a finite cylindrical surface containing a small absorbing hole. The solution is constructed by matching appropriate `inner' and `outer' asymptotic expansions. The asymptotic results are compared with those from numerical simulations and excellent agreement is obtained. For realistic parameter sets, we find reaction rates in the range of experimentally measured clustering rates of IP3R. This supports the idea that clusters are formed by a purely diffusion limited process.

Straube, Ronny; Ward, Michael J.; Falcke, Martin

2007-10-01

134

Drug Effects on Heart Rate and Heart Rate Variability During a Prolonged Reaction Task.  

National Technical Information Service (NTIS)

The effects of an amphetamine and a barbiturate on heart rate were investigated during long term performance (three hours in a serial reaction task). Besides the interbeat interval (IBI) derived from the successive R-tops of the ECG, the variability of IB...

A. W. K. Gaillard D. A. Trumbo A. J. Krul

1975-01-01

135

Metal-silicon reaction rates - The effects of capping  

NASA Technical Reports Server (NTRS)

Evidence is presented showing that the presence of the commonly used anti-reflection coating material Ta2O5 on the free surface of contact metallization can either suppress or enhance, depending on the system, the interaction that takes place at elevated temperatures between the metallization and the underlying Si. The cap layer is shown to suppress both the generation and annihilation of vacancies at the free surface of the metal which are necessary to support metal-Si interactons. Evidence is also presented indicating that the mechanical condition of the free metal surface has a significant effect on the metal-silicon reaction rate.

Weizer, Victor G.; Fatemi, Navid S.

1989-01-01

136

Rate-Controlled Constrained-Equilibrium Theory of Chemical Reactions  

NASA Astrophysics Data System (ADS)

The Rate-Controlled Constrained-Equilibrium (RCCE) method for simplifying the treatment of reactions in complex systems is summarized and the selection of constraints for both close-to and far-from equilibrium systems is discussed. Illustrative examples of RCCE calculations of carbon monoxide concentrations in the exhaust products of an internal combustion engine and ignition delays for methane-oxygen mixtures in a constant volume adiabatic chamber are given and compared with ``detailed'' calculations. The advantages of RCCE calculations over ``detailed'' calculations are discussed.

Keck, James C.

2008-08-01

137

RPMDRATE: Bimolecular chemical reaction rates from ring polymer molecular dynamics  

NASA Astrophysics Data System (ADS)

We present RPMDRATE, a computer program for the calculation of gas phase bimolecular reaction rate coefficients using the ring polymer molecular dynamics (RPMD) method. The RPMD rate coefficient is calculated using the Bennett-Chandler method as a product of a static (centroid density quantum transition state theory (QTST) rate) and a dynamic (ring polymer transmission coefficient) factor. The computational procedure is general and can be used to treat bimolecular polyatomic reactions of any complexity in their full dimensionality. The program has been tested for the H+H2, H+CH4, OH+CH4 and H+C2H6 reactions. Program summaryProgram title: RPMDrate Catalogue identifier: AENW_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AENW_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: MIT license No. of lines in distributed program, including test data, etc.: 94512 No. of bytes in distributed program, including test data, etc.: 1395674 Distribution format: tar.gz Programming language: Fortran 90/95, Python (version 2.6.x or later, including any version of Python 3, is recommended). Computer: Not computer specific. Operating system: Any for which Python, Fortran 90/95 compiler and the required external routines are available. Has the code been vectorized or parallelized?: The program can efficiently utilize 4096+ processors, depending on problem and available computer. At low temperatures, 110 processors are reasonable for a typical umbrella integration run with an analytic potential energy function and gradients on the latest x86-64 machines. RAM: 256 Mb Classification: 16.12. External routines: NumPy (http://numpy.scipy.org, version 1.5.0 or later is recommended). FFTW3 (http://www.fftw.org, version 3.3 or later is recommended). Nature of problem: The RPMDrate program calculates thermal bimolecular rate coefficients of thermally activated atom-diatom and more complex bimolecular chemical reactions in the gas phase. Solution method: The RPMD rate is calculated using the Bennett-Chandler factorization as a product of a static (centroid density quantum transition state theory (QTST) rate) and a dynamic (transmission coefficient) factor. A key feature of this procedure is that it does not require that one calculate the absolute quantum mechanical partition function of the reactants or the transition state. The centroid density QTST rate is calculated from the potential of mean force along the reaction coordinate using umbrella integration. The reaction coordinate is taken to be an interpolating function that connects two dividing surfaces: one located in the asymptotic reactant valley and the other located in the transition state region. The Hessian of the collective reaction coordinate is obtained analytically. The transmission coefficient is calculated from the RPMD simulations with the hard constraint along the reaction coordinate. Restrictions: The applicability of RPMDrate is restricted to global potential energy surfaces with gradients. In the current release, they should be provided by Python callable objects. Unusual features: Simple and user-friendly input system provided by Python syntax. Additional comments: Test calculations for the H+H2 reactions were performed using the Boothroyd-Keogh-Martin-Peterson-2 (BKMP2) potential energy surface (PES) [1]. PESs for the H+CH4,OH+CH4 and H+C2H6 reactions are taken from the online POTLIB library [2]. PESs are included within the distribution package as Fortran subroutines. Implementations of the colored-noise, generalized Langevin equation (GLE) thermostats [3-5] have been included in the current release. The distribution contains example data and a detailed manual describing the use of RPMDrate. Running time: Highly dependent on the input parameters. The running time of RPMDrate depends mainly on the complexity of the potential energy surface and number of ring polymer beads. For the H+H2, H+CH4, and OH+CH4 test calculations given (with 128 ring polymer beads and analytic gradients), the running time is

Suleimanov, Yu. V.; Allen, J. W.; Green, W. H.

2013-03-01

138

Reaction rate constants and mechanistic detail of the Ni+ + butanone reaction.  

PubMed

The unimolecular decomposition kinetics of the jet-cooled Ni(+)-butanone cluster ion has been monitored over a range of internal energies (16000-18800 cm?¹). First-order rate constants are acquired for the precursor ion dissociation into three product channels. The temporal growth of each fragment ion is selectively monitored in a custom instrument and yields similar valued rate constants at a common ion internal energy. The decomposition reaction is proposed to proceed along two parallel reaction coordinates. Each dissociative pathway is rate-limited by the initial Ni(+) oxidative addition into either the C-CH? or C-C?H? ?-bond in the butanone molecule. Ratios of integrated product ion intensities as well as the measured rate constants are used to determine values for each ?-bond activation rate constant. The lowest energy measurement presented in this study occurs when the binary complex ion possesses an internal energy of 16000 cm?¹. Under this condition, the Ni(+) assisted decomposition of the butanone molecule is rate limited by k(act)(C-C?H?) = (0.92 ± 0.08) × 10? s?¹ and k(act)(C-CH?) = (0.37 ± 0.03) × 10? s?¹. The relative magnitudes of the two rate constants reflect the greater probability for reaction to occur along the C-C?H? ?-bond insertion pathway, consistent with thermodynamic arguments. DFT calculations at the B3LYP/6-311++G(d,p) level of theory suggest the most likely geometries and relative energies of the reactants, intermediates, and products. PMID:21338169

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

2011-03-17

139

Self-inhibited rate in gas-solid noncatalytic reactions. The shrinking core model  

Microsoft Academic Search

The shrinking core model is examined for gas-solid noncatalytic reactions with a self-inhibited rate form and it is shown that multiple reaction pathways are possible for solid particles reacted under identical conditions. The observed reaction rate can have up to two discontinuities (jumps) during reaction for particles of spherical and cylindrical shape. The geometric instability analysis reveals that the reaction

Henry F. Erk; Milorad P. Dudukovic

1984-01-01

140

Primordial lithium - New reaction rates, new abundances, new constraints  

NASA Technical Reports Server (NTRS)

Newly measured nuclear reaction rates for H-3(alpha, gamma)Li-7 (higher than previous values) and Li-7(p, alpha)He-4 (lower than previous values) are shown to increase the Li-7 yield from big band nucleosynthesis for lower baryon-to-photon ratio (less than about 4 x 10 to the 10th). Recent revisions in the He-3(alpha, gamma)Be-7 and the D(p, gamma)He-3 rates enhance the high (greater than 4 x 10 to the 10th) Li-7(Be) production. New, independent determinations of Li abundances in extreme population II stars are in excellent agreement with the work of Spites and give continued confidence in the use of Li-7 in big bang baryon density determinations.

Kawano, Lawrence; Schramm, David; Steigman, Gary

1988-01-01

141

Constraining kinetic rates of mineral reactions using reactive transport models  

NASA Astrophysics Data System (ADS)

We use a reactive transport model to better understand results of experiments to obtain kinetic rates of mineral reactions in closed systems. Closed system experiments pose special challenges in that secondary minerals may form that modify the fluid composition evolution and may grow on the dissolving minerals thus armoring the surface. Even so, such closed system experiments provide critical data for what minerals would actually form in field applications and how coupled dissolution and precipitation mineral reactions are strongly linked. Comparing to experimental observations can test the reactive transport model, and the experimental observations can be better understood by comparing the results to the modeling. We apply a 0D end member of the model to understand the dissolution of single crystals of forsterite in a variety of settings (low pH, high pH, or NaHCO3 initial fluids, at 100 C and 1 bar, or 200 C and 150 bar). Depending on the initial conditions, we observe the precipitation of talc, brucite, amorphous silica, chrysotile, or magnesite, in various combinations. We compare simulation results to fluid compositions and the presence of secondary minerals experimentally sampled at various times. Insight from the simulations helped create an inverse model to extract the rates of forsterite dissolution and to create a simple forward model useful for exploring the influence of system size, secondary mineral surface areas, etc. Our reactive transport model allows secondary minerals to armor the forsterite surface, which can strongly decrease the dissolution rate as the system evolves. Tuning our model with experimentally derived rates and assuring relevant processes are included so as to reproduce experimental observations is necessary before upscaling to heterogeneous field conditions. The reactive transport model will be used for field-scale sequestration simulations and coupled with a geomechanical model that includes the influence of deformation.

Bolton, E. W.; Wang, Z.; Ague, J.; Bercovici, D.; Cai, Z.; Karato, S.; Oristaglio, M. L.; Qiu, L.

2012-12-01

142

Spatial rock-paper-scissors models with inhomogeneous reaction rates.  

PubMed

We study several variants of the stochastic four-state rock-paper-scissors game or, equivalently, cyclic three-species predator-prey models with conserved total particle density, by means of Monte Carlo simulations on one- and two-dimensional lattices. Specifically, we investigate the influence of spatial variability of the reaction rates and site occupancy restrictions on the transient oscillations of the species densities and on spatial correlation functions in the quasistationary coexistence state. For small systems, we also numerically determine the dependence of typical extinction times on the number of lattice sites. In stark contrast with two-species stochastic Lotka-Volterra systems, we find that for our three-species models with cyclic competition quenched disorder in the reaction rates has very little effect on the dynamics and the long-time properties of the coexistence state. Similarly, we observe that site restriction only has a minor influence on the system's dynamical properties. Our results therefore demonstrate that the features of the spatial rock-paper-scissors system are remarkably robust with respect to model variations, and stochastic fluctuations as well as spatial correlations play a comparatively minor role. PMID:21230502

He, Qian; Mobilia, Mauro; Täuber, Uwe C

2010-11-01

143

Hydrogen peroxide as a reductant of hexacyanoferrate(III) in alkaline solutions: kinetic studies  

Microsoft Academic Search

The kinetics of hexacyanoferrate(III) reduction by hydrogen peroxide in strongly alkaline media leading to hexacyanoferrate(II)\\u000a ion have been studied spectrophotometrically within the wavelength range 300–500 nm. The reaction obeys a simple pseudo-first-order\\u000a rate expression under the applied conditions, namely, a large excess of the reductant and OH? anion concentrations, and a low oxidant concentration. The linear dependences of the pseudo-first-order rate

Anna Katafias; Olga Impert; Przemys?aw Kita

2008-01-01

144

Nucleation and reaction rates controlled by local reaction volume and reaction-induced stress - spinel layer growth as an example  

NASA Astrophysics Data System (ADS)

We observed the growth of spinel sensu stricto (MgAl2O4) between periclase (MgO) and corundum (Al2O3) in thin films deposited by the pulsed laser deposition method on crystallographically oriented single crystal substrates. The starting samples consisted of cut and ultra polished single crystals of either corundum (parallel (0001)) or periclase (parallel (111)) and an amorphous source layer of the respective reactant that in the very first stages of the experiments became polycrystalline. The cutting direction in the substrate minerals ensures that the substrate phases start to react along their close-packed hexagonal oxygen layers which allows topotactical growth of the newly formed spinel. The entire layer setup on the substrate crystals was only a few 100 nm thick. The growth of these spinel product layers was monitored in-situ using a heating attachment and synchrotron X-ray diffraction. From the reacted samples we took electron transparent foils by the focused ion beam method and analysed them ex-situ by TEM. At 1000°C we found a difference in spinel growth rate between one and two orders of magnitude between the two substrates, all other parameters held constant. At 900 and 1000 °C spinel had formed after one hour by 0.004 nm/s (900°C) and 0.034 nm/s (1000°C) on corundum substrate, while on periclase substrate the reaction had gone completely through the Al2O3 source layer transforming it to spinel by at least 15-30 times higher reaction rates (boundary values) and probably even faster. At 800°C no reaction occurred between periclase layers and corundum single crystals, whereas spinel crystallized at a (linearized) rate of 46 nm/h on periclase single crystals. We explain our findings by the local reaction volume at the periclase-corundum interface. Many studies (including this one) have established that spinel grows by cation exchange in a rather immobile oxygen sublattice. This mechanism implies a negative volume change at the Sp-Per interface (by -13 %) and a positive volume change at the Sp-Cor interface (by +17 %). We observed in TEM images formation of porosity at the Sp-Per substrate interface and lattice strain in the polycrystalline corundum substrate at the Sp-Cor substrate interface, both observations being fully in line with the respective local volume changes. We infer that the local stress field between the growing reaction layer and the rigid single crystal substrate is responsible for the dramatic effect on both the nucleation and reaction rates. With respect to minerals growing at dry conditions in the Earth's crust (granulite facies) we infer that rates of reactions might strongly depend on local volume changes at interfaces, that reactions are impeded by compressive reaction-induced stress applied to rigid neighboring grains, and that nucleation of thermodynamically stable phases on grain boundaries might be suppressed by a positive reaction volume.

Götze, Lutz C.; Milke, Ralf; Dohmen, Ralf; Wirth, Richard

2014-05-01

145

Manual choice reaction times in the rate-domain  

PubMed Central

Over the last 150 years, human manual reaction times (RTs) have been recorded countless times. Yet, our understanding of them remains remarkably poor. RTs are highly variable with positively skewed frequency distributions, often modeled as an inverse Gaussian distribution reflecting a stochastic rise to threshold (diffusion process). However, latency distributions of saccades are very close to the reciprocal Normal, suggesting that “rate” (reciprocal RT) may be the more fundamental variable. We explored whether this phenomenon extends to choice manual RTs. We recorded two-alternative choice RTs from 24 subjects, each with 4 blocks of 200 trials with two task difficulties (easy vs. difficult discrimination) and two instruction sets (urgent vs. accurate). We found that rate distributions were, indeed, very close to Normal, shifting to lower rates with increasing difficulty and accuracy, and for some blocks they appeared to become left-truncated, but still close to Normal. Using autoregressive techniques, we found temporal sequential dependencies for lags of at least 3. We identified a transient and steady-state component in each block. Because rates were Normal, we were able to estimate autoregressive weights using the Box-Jenkins technique, and convert to a moving average model using z-transforms to show explicit dependence on stimulus input. We also found a spatial sequential dependence for the previous 3 lags depending on whether the laterality of previous trials was repeated or alternated. This was partially dissociated from temporal dependency as it only occurred in the easy tasks. We conclude that 2-alternative choice manual RT distributions are close to reciprocal Normal and not the inverse Gaussian. This is not consistent with stochastic rise to threshold models, and we propose a simple optimality model in which reward is maximized to yield to an optimal rate, and hence an optimal time to respond. We discuss how it might be implemented.

Harris, Christopher M.; Waddington, Jonathan; Biscione, Valerio; Manzi, Sean

2014-01-01

146

Manual choice reaction times in the rate-domain.  

PubMed

Over the last 150 years, human manual reaction times (RTs) have been recorded countless times. Yet, our understanding of them remains remarkably poor. RTs are highly variable with positively skewed frequency distributions, often modeled as an inverse Gaussian distribution reflecting a stochastic rise to threshold (diffusion process). However, latency distributions of saccades are very close to the reciprocal Normal, suggesting that "rate" (reciprocal RT) may be the more fundamental variable. We explored whether this phenomenon extends to choice manual RTs. We recorded two-alternative choice RTs from 24 subjects, each with 4 blocks of 200 trials with two task difficulties (easy vs. difficult discrimination) and two instruction sets (urgent vs. accurate). We found that rate distributions were, indeed, very close to Normal, shifting to lower rates with increasing difficulty and accuracy, and for some blocks they appeared to become left-truncated, but still close to Normal. Using autoregressive techniques, we found temporal sequential dependencies for lags of at least 3. We identified a transient and steady-state component in each block. Because rates were Normal, we were able to estimate autoregressive weights using the Box-Jenkins technique, and convert to a moving average model using z-transforms to show explicit dependence on stimulus input. We also found a spatial sequential dependence for the previous 3 lags depending on whether the laterality of previous trials was repeated or alternated. This was partially dissociated from temporal dependency as it only occurred in the easy tasks. We conclude that 2-alternative choice manual RT distributions are close to reciprocal Normal and not the inverse Gaussian. This is not consistent with stochastic rise to threshold models, and we propose a simple optimality model in which reward is maximized to yield to an optimal rate, and hence an optimal time to respond. We discuss how it might be implemented. PMID:24959134

Harris, Christopher M; Waddington, Jonathan; Biscione, Valerio; Manzi, Sean

2014-01-01

147

Laser flash photolysis studies of radical-radical reaction kinetics: The HO{sub 2} + IO reaction  

SciTech Connect

Reactive iodine as a potential tropospheric O{sub 3} sink has received considerable attention recently. Laser flash photolysis of Cl{sub 2}/CH{sub 3}OH/O{sub 2}/I{sub 2}/NO{sub 2}/SF{sub 6}N{sub 2} mixtures at 308 nm has been coupled with simultaneous time-resolved detection of HO{sub 2} (by infrared tunable diode laser absorption spectroscopy) and IO (by visible absorption spectroscopy) to investigate the kinetics of the atmospherically important reaction HO{sub 2} + IO {r_arrow} products over the temperature range 274--373 K in N{sub 2} buffer gas at pressures of 12 and 25 Torr. All experiments were performed under near pseudo-first-order conditions with HO{sub 2} in excess over IO. At 298 K, the rate coefficient was determined to be (9.7 {+-} 2.9) {times} 10{sup {minus}11} cm{sup 3} molecule{sup {minus}1}s{sup {minus}1}, with the primary source of uncertainty being knowledge of the infrared line strength(s) required to convert measured HO{sub 2} absorbances to absolute concentrations. The temperature dependence of the HO{sub 2} + IO rate coefficient was found to be adequately described by the Arrhenius expression k = 9.3 {times} 10{sup {minus}12} exp(680/T) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. The results reported in this study are compared with other recent studies of HO{sub 2} + IO kinetics, and the potential roles of this reaction in atmospheric chemistry are discussed.

Cronkhite, J.M.; Stickel, R.E.; Nicovich, J.M.; Wine, P.H. [Georgia Inst. of Tech., Atlanta, GA (United States)] [Georgia Inst. of Tech., Atlanta, GA (United States)

1999-04-29

148

A slow reaction rate in detonations due to carbon clustering  

SciTech Connect

Theoretical calculations have been made to estimate the rate of heat release due to the carbon clustering process in detonations where elemental carbon is a reaction product. The process is assumed to be diffusion limited. Diffusion constants are determined using modified Enskog theory and the Stokes-Einstein relation. The carbon cluster energy is treated by a surface correction to the bulk. The amount of energy yet to be released has an asymptotic time dependence of t/sup -1/3/. For some explosives, this leads to time dependent detonations where the effective CJ pressure is 10-20% above CJ for run distances of the order of centimeters. 9 refs., 3 figs.

Shaw, M.S.; Johnson, J.D.

1987-07-01

149

The decolorization of Acid Orange II in non-homogeneous Fenton reaction catalyzed by natural vanadium-titanium magnetite.  

PubMed

The catalytic activity of natural vanadium-titanium magnetite was investigated in the decolorization of Acid Orange II by non-homogeneous Fenton process. The natural catalysts purified by magnetic separation were characterized using X-ray diffraction (XRD), polarizing microscope, X-ray absorption fine structure (XAFS) analysis and Mössbauer spectroscopy. The obtained results show that the natural samples after magnetic separation mainly contain titanomagnetite, with a small amount of ilmenite and chlorite. Titanomagnetite is doped with vanadium, whose the valency is mainly +3 and occupies the octahedral site. Batch decolorization studies were performed to evaluate the influences of various experimental parameters like initial pH, the amount of catalyst and initial concentration of hydrogen peroxide on the decolorization efficiency of Acid Orange II. The decolorization of the dye mainly relied on degradation. The degradation efficiency was strongly dependent on pH of the medium where it increased as the pH decreased in acid range. The increase of catalyst and hydrogen peroxide could accelerate the degradation. The catalytic property of natural vanadium-titanium magnetite in the degradation of Acid Orange II was stronger than that of synthetic magnetite (Fe(3)O(4)). The catalytic activity of the natural samples was greatly related to the titanomagnetite content. The degradation process was dominated by heterogeneous Fenton reaction, complying with pseudo-first-order rate law. The natural catalyst has a good catalytic stability. PMID:20554111

Liang, Xiaoliang; Zhong, Yuanhong; Zhu, Sanyuan; Zhu, Jianxi; Yuan, Peng; He, Hongping; Zhang, Jing

2010-09-15

150

Night-time tropospheric chemistry of the unsaturated alcohols ( Z)-pent-2-en-1-ol and pent-1-en-3-ol: Kinetic studies of reactions of NO 3 and N 2O 5 with stress-induced plant emissions  

NASA Astrophysics Data System (ADS)

The night-time tropospheric chemistry of two stress-induced volatile organic compounds (VOCs), ( Z)-pent-2-en-1-ol and pent-1-en-3-ol, has been studied at room temperature. Rate coefficients for reactions of the nitrate radical (NO 3) with these pentenols were measured using the discharge-flow technique. Because of the relatively low volatility of these compounds, we employed off-axis continuous-wave cavity-enhanced absorption spectroscopy for detection of NO 3 in order to be able to work in pseudo first-order conditions with the pentenols in large excess over NO 3. The rate coefficients were determined to be (1.53±0.23)×10 -13 and (1.39±0.19)×10 -14 cm 3 molecule -1 s -1 for reactions of NO 3 with ( Z)-pent-2-en-1-ol and pent-1-en-3-ol. An attempt to study the kinetics of these reactions with a relative-rate technique, using N 2O 5 as source of NO 3 resulted in significantly higher apparent rate coefficients. Performing relative-rate experiments in known excesses of NO 2 allowed us to determine the rate coefficients for the N 2O 5 reactions to be (5.0±2.8)×10 -19 cm 3 molecule -1 s -1 for ( Z)-pent-2-en-1-ol, and (9.1±5.8)×10 -19 cm 3 molecule -1 s -1 for pent-1-en-3-ol. We show that these relatively slow reactions can indeed interfere with rate determinations in conventional relative-rate experiments.

Pfrang, Christian; Baeza Romero, Maria T.; Cabanas, Beatriz; Canosa-Mas, Carlos E.; Villanueva, Florentina; Wayne, Richard P.

151

An Experiment To Demonstrate How a Catalyst Affects the Rate of a Reaction.  

ERIC Educational Resources Information Center

Describes a chemistry experiment that allows students to calculate rates of reaction, orders of reaction, and activation energies. The activity demonstrates that to increase a reaction's rate, a catalyst need only provide any additional pathway for the reaction, not necessarily a pathway having lower activation energy. (WRM)

Copper, Christine L.; Koubeck, Edward

1999-01-01

152

Enhancement of D-T Reaction Rate Due to D-T Contact.  

National Technical Information Service (NTIS)

The reaction rate that is appropriate for magnetized nonuniform plasma is numerically calculated to investigate the enhancement of the D-T reaction rate. Spatial separation of the guiding center distributions of D and T enhances the reaction rate. Cases o...

S. Hitoki M. Ogasawara O. Aono

1979-01-01

153

Students' Ideas about Reaction Rate and Its Relationship with Concentration or Pressure  

ERIC Educational Resources Information Center

This cross-sectional study identifies key conceptual difficulties experienced by upper secondary school and pre-service chemistry teachers (N = 191) in the area of reaction rates. Students' ideas about reaction rates were elicited through a series of written tasks and individual interviews. In this paper, students' ideas related to reaction rate

Cakmakci, Gultekin; Leach, John; Donnelly, James

2006-01-01

154

Pressure Dependence of Gas-Phase Reaction Rates  

ERIC Educational Resources Information Center

It is presented that only simple concepts, mainly taken from activated-complex or transition-state theory, are required to explain and analytically describe the influence of pressure on gas-phase reaction kinetics. The simplest kind of elementary gas-phase reaction is a unimolecular decomposition reaction.

De Persis, Stephanie; Dollet, Alain; Teyssandier, Francis

2004-01-01

155

A Kinetic Study of the Heterogeneous Reaction of Deliquesced NaCl Particles with Gaseous Nitric Acid Using Novel Experimental Approach.  

NASA Astrophysics Data System (ADS)

The kinetics of heterogeneous reaction of deliquesced sodium chloride particles (NaClaq) with HNO3 gas and its uptake on NaClaq were measured using a novel experimental approach. The technique utilizes the exposure of substrate deposited aerosol particles to reactive gases followed by chemical analysis of the particles using computer controlled scanning electron microscopy with energy dispersive analysis of X- ray (CCSEM/EDX) capability. Experimental conditions were carefully selected based on fluid dynamics calculation results to ensure that effects of fluid mechanics have minimal effect on the accuracy of kinetic measurements. As we discuss in this presentation, fundamental reaction kinetics data may be obtained from these experiments after a theoretical kinetic-diffusion analysis of effects of gaseous reactant transport from the bulk gas to the substrate surface. Such effects arise from the close proximity of the reacting particles mounted on the substrate, which may result in kinetic rates being diffusion controlled. We show that these effects may be quantified and kinetic rates can be obtained where the transport limitations associated with the substrate experiments are decoupled from the reaction kinetics. Experiments presented in this work were conducted at room temperature, atmospheric pressure and RH=80-82% with dry NaCl particle size and HNO3 concentration ranging from 200 nm to 1000 nm and 20ppb to 80ppb, respectively. The apparent, pseudo first order rate constants for the heterogeneous reaction were obtained based on the changes in the chloride concentration of individual particles upon reaction with HNO3. The dependence of rate constants on particle density on the substrate was also probed in this work. The measured uptake coefficient (?=5×10-3) of HNO3 on deliquesced NaCl particles agrees with data reported previously.

Cain, J.; Liu, Y.; Wang, H.; Laskin, A.

2006-12-01

156

Determination of the (n, ?) reaction rate of unstable 185W in the astrophysical s-process via its inverse reaction  

Microsoft Academic Search

The (?, n) reaction rate of 186W was measured in order to deduce the (n, ?) reaction rate of 185W, an important branching point in the astrophysical s-process. The theoretical predictions have been confirmed and the error range was slightly decreased. The derived Maxwellian averaged cross section (MACS) at 30 keV is (687±110) mb.

K. Sonnabend; A. Mengoni; P. Mohr; T. Rauscher; K. Vogt; A. Zilges

2003-01-01

157

Enhanced reaction rates in NDP analysis with neutron scattering.  

PubMed

Neutron depth profiling (NDP) makes accessible quantitative information on a few isotopic concentration profiles ranging from the surface into the sample a few micrometers. Because the candidate analytes for NDP are few, there is little interference encountered. Furthermore, neutrons have no charge so mixed chemical states in the sample are of no direct concern. There are a few nuclides that exhibit large probabilities for neutron scattering. The effect of neutron scattering on NDP measurements has not previously been evaluated as a basis for either enhancing the reaction rates or as a source of measurement error. Hydrogen is a common element exhibiting large neutron scattering probability found in or around sample volumes being analyzed by NDP. A systematic study was conducted to determine the degree of signal change when neutron scattering occurs during analysis. The relative signal perturbation was evaluated for materials of varied neutron scattering probability, concentration, total mass, and geometry. Signal enhancements up to 50% are observed when the hydrogen density is high and in close proximity to the region of analysis with neutron beams of sub thermal energies. Greater signal enhancements for the same neutron number density are reported for thermal neutron beams. Even adhesive tape used to position the sample produces a measureable signal enhancement. Because of the shallow volume, negligible distortion of the NDP measured profile shape is encountered from neutron scattering. PMID:24784661

Downing, R Gregory

2014-04-01

158

Neutron detector for fusion reaction-rate measurements  

SciTech Connect

We have developed a fast, sensitive neutron detector for recording the fusion reaction-rate history of inertial-confinement fusion (ICF) experiments. The detector is based on the fast rise-time of a commercial plastic scintillator (BC-422) and has a response < 25-ps FWHM. A thin piece of scintillator material acts as a neutron-to- light converter. A zoom lens images light from the scintillator surface to a high-speed (15 ps) optical streak camera for recording. The zoom lens allows the scintillator to be positioned between 1 and 50 cm from a target. The camera simulaneously records an optical fiducial pulse which allows the camera time base to be calibrated relative to the incident laser power. Bursts of x rays formed by focusing 20-ps, 2.5-TW laser pulses onto gold disk targets demonstrate the detector resolution to be < 25 ps. We have recorded burn histories for deuterium/tritium-filled targets producing as few as 3 {times} 10{sup 7} neutrons.

Lerche, R.A.; Phillion, D.W.; Tietbohl, G.L.

1993-09-03

159

Molecule-based approach for computing chemical-reaction rates in upper atmosphere hypersonic flows.  

SciTech Connect

This report summarizes the work completed during FY2009 for the LDRD project 09-1332 'Molecule-Based Approach for Computing Chemical-Reaction Rates in Upper-Atmosphere Hypersonic Flows'. The goal of this project was to apply a recently proposed approach for the Direct Simulation Monte Carlo (DSMC) method to calculate chemical-reaction rates for high-temperature atmospheric species. The new DSMC model reproduces measured equilibrium reaction rates without using any macroscopic reaction-rate information. Since it uses only molecular properties, the new model is inherently able to predict reaction rates for arbitrary nonequilibrium conditions. DSMC non-equilibrium reaction rates are compared to Park's phenomenological non-equilibrium reaction-rate model, the predominant model for hypersonic-flow-field calculations. For near-equilibrium conditions, Park's model is in good agreement with the DSMC-calculated reaction rates. For far-from-equilibrium conditions, corresponding to a typical shock layer, the difference between the two models can exceed 10 orders of magnitude. The DSMC predictions are also found to be in very good agreement with measured and calculated non-equilibrium reaction rates. Extensions of the model to reactions typically found in combustion flows and ionizing reactions are also found to be in very good agreement with available measurements, offering strong evidence that this is a viable and reliable technique to predict chemical reaction rates.

Gallis, Michail A.; Bond, Ryan Bomar; Torczynski, John Robert

2009-08-01

160

Influence of chemical kinetics on postcolumn reaction in a capillary Taylor reactor with catechol analytes and photoluminescence following electron transfer.  

PubMed

Postcolumn derivatization reactions can enhance detector sensitivity and selectivity, but their successful combination with capillary liquid chromatography has been limited because of the small peak volumes in capillary chromatography. A capillary Taylor reactor (CTR), developed in our laboratory, provides simple and effective mixing and reaction in a 25-microm-radius postcolumn capillary. Homogenization of reactant streams occurs by radial diffusion, and a chemical reaction follows. Three characteristic times for a given reaction process can be predicted using simple physical and chemical parameters. Two of these times are the homogenization time, which governs how long it takes the molecules in the analyte and reagent streams to mix, and the reaction time, which governs how long the molecules in a homogeneous solution take to react. The third characteristic time is an adjustment to the reaction time called the start time, which represents an estimate of the average time the analyte stream spends without exposure to reagent. In this study, laser-induced fluorescence monitored the extent of the postcolumn reaction (reduction of Os(bpy)3(3+) by analyte to the photoluminescent Os(bpy)3(2+)) in a CTR. The reaction time depends on the reaction rates. Analysis of product versus time data yielded second-order reaction rate constants between the PFET reagent, tris(2,2'-bipyridine)osmium, and standards ((ferrocenylmethyl)trimethylammonium cation and p-hydroquinone) or catechols (dopamine, epinephrine, norepinephrine, 3, 4-dihydroxyphenylacetic acid. The extent of the reactions in a CTR were then predicted from initial reaction conditions and compared to experimental results. Both the theory and experimental results suggested the reactions of catechols were generally kinetically controlled, while those of the standards were controlled by mixing time (1-2 s). Thus, the extent of homogenization can be monitored in a CTR using the relatively fast reaction of the reagent and p-hydroquinone. Kinetically controlled reactions of catechols, however, could be also completed in a reasonable time at increased reagent concentration. A satisfactory reactor, operating at 1.7 cm/s (2 microL/min) velocity with solutes having diffusion coefficients in the 5 x 10(-6) cm2/s range, can be constructed from 8.0 cm of 25-microm-radius capillary. Slower reactions require longer reaction times, but theoretical calculations expect that a CTR does not broaden a chromatographic peak (N = 14 000) from a 100-microm-capillary chromatography column by 10% if the pseudo-first-order rate constant is larger than 0.1 s(-1). PMID:15858975

Jung, Moon Chul; Weber, Stephen G

2005-02-15

161

Kinetics and mechanism of reactions of the drug tiopronin with platinum(IV) complexes.  

PubMed

Tiopronin, a synthetic thiol-containing drug being used in treatments of cystinuria and certain types of rare arthritis, is also a hepatoprotective and a detoxifying agent. Many analytical methods have been developed based on its redox chemistry with metal ions/complexes, but the kinetic and mechanistic aspects are poorly understood. In this work, the oxidation of tiopronin by cisplatin prodrug and a model compound, cis-[Pt(NH3)2Cl4] and trans-[PtCl2(CN)4](2-), was investigated. The oxidation kinetics was followed by a stopped-flow spectrophotometer over a wide pH range under the pseudo first-order conditions of [Tiopronin]?[Pt(IV)]. Time-resolved spectra were also recorded for both Pt(IV) complexes, enabling to establish an overall second-order rate law: -d[Pt(IV)]/dt=k'[Tiopronin][Pt(IV)], where k' pertains to observed second-order rate constants. Under the kinetic conditions, tiopronin was oxidized to form the tiopronin-disulfide exclusively as identified by mass spectrometry. A reaction mechanism was proposed, involving parallel reductions of the Pt(IV) complexes by the three protolytic tiopronin species as rate-determining steps. The rate constants for the rate-determining steps were derived. The fully deprotonated tiopronin is about 4×10(4) more reactive than its corresponding thiol form for both Pt(IV) complexes; the huge reactivity difference orchestrates closely with the fact that the nucleophilicity of thiolate is much higher than the corresponding thiol. Hence, the attack of the sulfur atom in thiol/thiolate of tiopronin on the axially-coordinated chloride in the Pt(IV) complexes is nucleophilic in nature in the rate-determining steps, resulting in a bridge formation and a subsequent bridged electron-transfer. PMID:23665090

Huo, Shuying; Shi, Hongmei; Liu, Dongzhi; Shen, Shigang; Zhang, Jiong; Song, Changying; Shi, Tiesheng

2013-08-01

162

Upscaling geochemical reaction rates accompanying acidic CO2-saturated brine flow in sandstone aquifers  

NASA Astrophysics Data System (ADS)

Network flow models were used to simulate the flow of CO2-saturated brine in the pore networks corresponding to three different sandstones. The simulations were used to study upscaling of anorthite and kaolinite reaction rates from pore to core scales. Unique to our simulations is the use of computed tomography to capture the mineral distribution in the samples as well as the sample pore network. The upscaled reaction rates determined from these simulations incorporate mass balance principles and microscale reaction rate laws and capture the physical, mineral, and flow heterogeneities in the network. These upscaled rates were compared with upscaled rates predicted by a continuum model and by a volume-averaged-concentration method. For the anorthite reaction, which remains far from equilibrium, the volume-averaged reaction rate exceeded the reaction rate of the network model by 18% to 46%. While the continuum model rate also exceeded the network model rate by -1% to 53%, its predicted values were generally better than the volume-averaged method. The kaolinite reaction is near equilibrium and is heavily influenced by the form of the microscale rate law in the precipitation regime. Three alternate rate laws were tested, which produced significantly different predictions for the bulk reaction rates. For all three rate laws, continuum and volume-averaged reaction rates incorrectly predicted the magnitude of the kaolinite reaction rate (disagreements of -700% to 55%), and the predicted reaction type, dissolution versus precipitation, was also often opposite to that of the network model. Finally, for both anorthite and kaolinite, all upscaled reaction rates showed significant flow rate dependence.

Kim, D.; Peters, C. A.; Lindquist, W. B.

2011-01-01

163

Kinetics of CH2OO reactions with SO2, NO2, NO, H2O and CH3CHO as a function of pressure.  

PubMed

Kinetics of CH2OO Criegee intermediate reactions with SO2, NO2, NO, H2O and CH3CHO and CH2I radical reactions with NO2 are reported as a function of pressure at 295 K. Measurements were made under pseudo-first-order conditions using flash photolysis of CH2I2-O2-N2 gas mixtures in the presence of excess co-reagent combined with monitoring of HCHO reaction products by laser-induced fluorescence (LIF) spectroscopy and, for the reaction with SO2, direct detection of CH2OO by photoionisation mass spectrometry (PIMS). Rate coefficients for CH2OO + SO2 and CH2OO + NO2 are independent of pressure in the ranges studied and are (3.42 ± 0.42) × 10(-11) cm(3) s(-1) (measured between 1.5 and 450 Torr) and (1.5 ± 0.5) × 10(-12) cm(3) s(-1) (measured between 25 and 300 Torr), respectively. The rate coefficient for CH2OO + CH3CHO is pressure dependent, with the yield of HCHO decreasing with increasing pressure. Upper limits of 2 × 10(-13) cm(3) s(-1) and 9 × 10(-17) cm(3) s(-1) are placed on the rate coefficients for CH2OO + NO and CH2OO + H2O, respectively. The upper limit for the rate coefficient for CH2OO + H2O is significantly lower than has been reported previously, with consequences for modelling of atmospheric impacts of CH2OO chemistry. PMID:24287566

Stone, Daniel; Blitz, Mark; Daubney, Laura; Howes, Neil U M; Seakins, Paul

2014-01-21

164

Kinetic catalytic determination of trace Cu(II) in water samples with the thioglycolic/thiolactic acid-chromate reaction.  

PubMed

The use of two novel similar indicator reactions as applied to the kinetic determination of Cu(II) in water is investigated. The methods rely on the catalytic effect of the analyte on the oxidation of thioglycolic (TGA) and thiolactic (TLA) acids by chromate in acidic media. The extent of the reactions was followed spectrophotometrically at 345 nm. Pseudo-first-order rate coefficients, k(obsd), were determined as a function of catalyst concentration. Interference of Fe(III) and Pb(II) was suppressed by complexation with pyrophosphate. For the reaction of TGA, a linear regression for k(obsd) versus [Cu(II)] was obtained for the entire concentration range considered. Although the plot corresponding to TLA oxidation exhibits a sharp change of slope at approximately 1.8x10(-5) M Cu(II), it can still be described effectively by two linear regressions with different slopes. The reaction of TGA is more sensitive than that of TLA at low Cu(II) concentration. The opposite is true for higher catalyst contents. The detection limits were 65 microg L(-1) for TGA and of 80 microg L(-1) for TLA oxidation, respectively. The relative standard deviations, of 0.4% for TGA and 1.1% for TLA oxidation, respectively, were obtained for five replicate runs at 1000 microg L(-1). Samples of river and wastewater from the mining region of Baia-Mare, Northern Romania were analyzed using the more sensitive reaction of thioglycolic acid. Results were compared to those obtained by the officially standardized methods. Good agreement was obtained, even for an untreated sample. Measurements did not require prior separation of interfering species. PMID:15731914

Rustoiu-Csavdari, A; Mihai, D; Bâldea, I

2005-04-01

165

The kinetics and mechanisms of the reaction of iron(III) with gallic acid, gallic acid methyl ester and catechin.  

PubMed

The kinetics and mechanisms of the reactions of a number of pyrogallol-based ligands with iron(III) have been investigated in aqueous solution at 25 degrees C and ionic strength 0.5 M NaClO(4). Mechanisms have been proposed which account satisfactorily for the kinetic data. These are generally consistent with a mechanism in which the 1:1 complex that is formed initially when the metal reacts with the ligand subsequently decays through an electron transfer reaction. There was also some evidence for the formation of a 1:2 ligand-to-metal complex at higher pH values. The kinetics of complex formation were investigated with either the ligand or metal in pseudo-first-order excess. Rate constants for k(1) of 2.83(+/-0.09)x10(3), 1.75(+/-0.045)x10(3) and 3300(+/-200) M(-1) s(-1) and k(-1) of 20(+/-6.0), 35(+/-13) and 25+/-7.6 M(-1) s(-1) have been evaluated for the reaction of Fe(OH)(2+) with gallic acid, gallic acid methyl ester and catechin, respectively. The stability constant of each [Fe(L)](+) complex has been calculated from the kinetic data. The iron(III) assisted decomposition of the initial iron(III) complex formed was investigated. Analysis of the kinetic data yielded both the equilibrium constants for protonation of the iron(III) complexes initially formed together with the rate constants for the intramolecular electron transfers for gallic acid and gallic acid methyl ester. All of the suggested mechanisms and calculated rate constants are supported by calculations carried out using global analysis of time-dependent spectra. PMID:11410233

Hynes, M J; Coinceanainn, M O

2001-06-01

166

Cross sections and reaction rates of relevance to aeronomy  

SciTech Connect

Experimental and theoretical data relevant to models and measurements of the chemical and thermal structures and luminosity of the thermospheres of the earth and planets published during the last four years are surveyed. Among chemical processes, attention is given to ion-molecule reactions, dissociative recombination of molecular ions, and reactions between neutral species. Both reactions between ground state species and species in excited states are considered, including energy transfer and quenching. Measured and calculated cross sections for interactions of solar radiation with atmospheric species, such as photoabsorption, photoionization, and photodissociation and related processes are surveyed.

Fox, J.L. (USAF, Geophysics Laboratory, Hanscom AFB, MA (United States))

1991-01-01

167

"Depletion": A Game with Natural Rules for Teaching Reaction Rate Theory.  

ERIC Educational Resources Information Center

Depletion is a game that reinforces central concepts of reaction rate theory through simulation. Presents the game with a set of follow-up questions suitable for either a quiz or discussion. Also describes student reaction to the game. (MM)

Olbris, Donald J.; Herzfeld, Judith

2002-01-01

168

Reaction-R Constants and Simple RATE Theories Applied to the Mercury-Chloride Laser.  

National Technical Information Service (NTIS)

Several simple theories for reaction-rate constants are applied to the HgCl laser reactions. These theories are discussed and analyzed using existing data. The theories that are discussed are applicable to dissociative recombination, dissociative attachme...

R. P. Summers

1978-01-01

169

Structural and kinetic studies of the polymerization reactions of ?-caprolactone catalyzed by (pyrazol-1-ylmethyl)pyridine Cu(II) and Zn(II) complexes.  

PubMed

The structural and kinetic studies of polymerization reactions of ?-caprolactone (?-CL) using (pyrazolylmethyl)pyridine Cu(II) and Zn(II) complexes as initiators is described. Reactions of 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1) and 2-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L2) with Zn(Ac)2·2H2O or Cu(Ac)2·2H2O produced the corresponding complexes [Zn(Ac)2(L1)] (1), [Cu(Ac)2(L1)] (2), [Zn(Ac)2(L2)] (3) and [Cu2(Ac)4(L2)2] (4) respectively. Solid state structures of 1 and 4 confirmed that complexes 1 and 4 are monomeric and dimeric respectively and that L1 is bidentate in 1 while L2 is monodentate in 4. X-band EPR spectra of 2 and 4 revealed that complex 2 is monomeric both in solid and solution state, while the paddle-wheel structure of 4 is retained in solution. Complexes 1-4 formed active initiators in the ring opening polymerization of ?-CL. Zn(II) complexes 1 and 3 exhibited higher rate constants of 0.044 h(-1) and 0.096 h(-1) respectively compared to rate constants of 0.017 h(-1) and 0.031 h(-1) observed for the corresponding Cu(II) complexes 2 and 4 respectively at 110 °C. All the polymerization reactions follow pseudo first-order kinetic with respect to ?-CL monomer. Initiator 1 showed first-order dependency on the polymerization reactions and utilizes only one active site as the initiating group. The molecular weights of the polymers produced range from 1982 g mol(-1) to 14,568 g mol(-1) and exhibited relatively broad molecular weight distributions associated with transesterification reactions. PMID:23774962

Ojwach, Stephen O; Okemwa, Teddy T; Attandoh, Nelson W; Omondi, Bernard

2013-08-14

170

Kinetic study of the reaction of the CFCl{sub 2}CH{sub 2}O radical with O{sub 2}  

SciTech Connect

The 2-fluoro-2,2-dichloroethoxy radical, CFCl{sub 2}CH{sub 2}O, is an intermediate in the mechanism of the tropospheric degradation of CFCl{sub 2}CH{sub 3} (HCFC-141b), a chlorofluoromethane replacement in some large-scale industrial applications. In this work, the reaction of CFCl{sub 2}CH{sub 2}O with O{sub 2} was investigated at 10-35 Torr and 251-341 K by UV flash photolysis and time-resolved mass spectrometry. CFCl{sub 2}CH{sub 2}O{sub 2} radicals were formed from the photolysis of CFCl{sub 2}CH{sub 3}/N{sub 2}/O{sub 2} mixtures. The CFCl{sub 2}CH{sub 2}O radical is a product of the reactions of CFCl{sub 2}CH{sub 2}O{sub 2} with Cl and ClO and the self-reaction. The rate of formation and decay of CFCl{sub 2}CH{sub 2}O was followed by observations of the fragment ion CClCH{sub 2}O{sup +}. The rate coefficient of the reaction of CFCl{sub 2}CH{sub 2}O with O{sub 2}, determined from CHCl{sub 2}CH{sub 2}O decay in excess O{sub 2} by both pseudo-first-order analysis and nonlinear regression, can be expressed as (2.4{+-}0.5) x 10{sup -15} exp[-(944{+-}55)/T] cm{sup 3} molecule{sup -1} s{sup -1}. The half-life for removal of CFCl{sub 2}CH{sub 2}O radicals from the atmosphere by the title reaction is 6 ms in the mid-troposphere and 1 s in the mid-stratosphere. 43 refs., 6 figs., 5 tabs.

Wu, F.; Carr, R.W. [Univ. of Minnesota, Minneapolis, MN (United States)] [Univ. of Minnesota, Minneapolis, MN (United States)

1996-05-30

171

Integration of reaction equations containing extremely different reaction rates -application to realistic active tracers in the troposphere  

NASA Astrophysics Data System (ADS)

If reaction rates differ by 18 orders of magnitude or more - as it happens with chemical reaction equations for active tracers in the atmosphere - codes just using double precision are bound to produce erroneous results. I apply a novel scaling procedure that allows application of i) double precision routines ii) fast extrapolation schemes(1) when integrating the reaction equations. (1) P. Deuflhard, F.A. Bornemann, Numerische Mathematik II, de Gruyter, Berlin 1994

Lustfeld, H.

2003-04-01

172

Automated Stopped-Flow Systems for Fast Reaction-Rate Methods  

Microsoft Academic Search

The stopped-flow technique for the rapid mixing of chemical reagents has gained widespread importance in the measurement of the rates of rapid chemical reactions. Reaction-rate information can be obtained routinely on reactions with half-lives as short as a few milliseconds by using stopped-flow mixing in conjunction with a rapid reaction monitoring technique, such as UV-visible spectrophotometry. Stopped-flow mixing is most

S. R. Crouch; F. J. Holwr; P. K. Notz; P. M. Beckwith

1977-01-01

173

Ensemble-averaged thermonuclear reaction rates for turbulent convection in stellar interiors  

Microsoft Academic Search

It is well known in combustion research that the interaction between turbulence and chemsitry can substantially modify flame speeds and overall fuel consumption rates compared to the laminar values. An analogous effect exists for thermonuclear reaction rates in turbulent convection zones in stellar interions. We describe a parametric study that estimates the modification of ensemble-averaged thermonuclear reaction rates by turbulence.

Lawrence D. Cloutman

1994-01-01

174

Uncertainty Quantification of Sequentially Reactive Transport Systems: Calibrating First-Order Reaction Rates  

NASA Astrophysics Data System (ADS)

Reaction rates are usually identified at laboratory scale, by comparing measured concentrations and the solution of the corresponding ordinary differential equations. Laboratory-scale reaction rates may not necessarily reflect the reactive transport scenarios at the field scale. One difficulty of reactive transport modeling for field-scale problems is the determination of reaction kinetics and rates. The conventional inversion of reaction rates relies on optimization approaches which require complex mathematical manipulations. In this presentation we describe our application of the uncertainty quantification software package, PSUADE, to the NUFT reactive flow and transport simulator, to identify the first-order reaction rates at the field scale. Sampling-based methods, which treat simulation models as black boxes, are used to construct response surfaces for further sensitivity analysis and parameter calibration. As demonstrated in the applications, the global sensitivities of the objective functions in terms of uncertain reaction rates are provided.

Sun, Y.

2010-12-01

175

Quick and Easy Rate Equations for Multistep Reactions  

ERIC Educational Resources Information Center

Students rarely see closed-form analytical rate equations derived from underlying chemical mechanisms that contain more than a few steps unless restrictive simplifying assumptions (e.g., existence of a rate-determining step) are made. Yet, work published decades ago allows closed-form analytical rate equations to be written quickly and easily for…

Savage, Phillip E.

2008-01-01

176

VALIDATION OF OH RADICAL REACTION RATE CONSTANT TEST PROTOCOL  

EPA Science Inventory

A study was conducted to evaluate the OH rate constant measurement protocol developed by researchers at the University of California at Riverside. The protocol, which is a relative rate method, was used to measure room temperature OH rate constants for a series of low molecular w...

177

Women's Self-Disclosure of HIV Infection: Rates, Reasons, Reactions.  

ERIC Educational Resources Information Center

A survey of 65 ethnically diverse women revealed relatively low rates of disclosure of HIV-positive serostatus to extended family members, somewhat higher rates for immediate family members, and highest rates for lovers or friends. Spanish-speaking Latinas were less likely to disclose their serostatus than English-speaking Latinas, African…

Simoni, Jane M.; And Others

1995-01-01

178

Reaction rate oscillations during catalytic CO oxidation: A brief overview  

NASA Technical Reports Server (NTRS)

It is not the intent here to present a comprehensive review of the dynamic behavior of the catalytic oxidation of CO. This reaction is one of the most widely studied in the field of catalysis. A review paper by Engel and Ertl has examined the basic kinetic and mechanistic aspects, and a comprehensive paper by Razon and Schmitz was recently devoted to its dynamic behavior. Those interested in further study of the subject should consult these reviews and a number of general review papers on catalytic reaction dynamics. The goal is to present a brief overview of certain interesting aspects of the dynamic behavior of this reaction and to discuss a few questions and issues, which are still the subject of study and debate.

Tsotsis, T. T.; Sane, R. C.

1987-01-01

179

Reaction rate for 8Li(?,n0) 11B  

NASA Astrophysics Data System (ADS)

The cross section of the 11B(n, ?)8Li reaction has been measured at En = 7.6-12.6 MeV. The neutron beam was produced via the D(d,n)3He reaction and a BF3 detector was used both as a target and a detector for the delayed ?-activity of 8Li. Using the principle of detailed balance the data were converted to the case of the 8Li(?,n0)11B reaction. The resulting astrophysical S(E) factor is dominated by a resonance at ER = 580 keV, with ?R = 200 keV and S(ER) = 8400 MeVb. The significance of this result for the inhomogeneous Big Bang nucleosynthesis is discussed.

Paradellis, T.

1991-05-01

180

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

NASA Technical Reports Server (NTRS)

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.

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

1982-01-01

181

Reactions of gas phase H atoms with ethylene, acetylene and ethane adsorbed on Ni( 1 1 1 )  

NASA Astrophysics Data System (ADS)

The products of the reaction of the most energetic form of hydrogen, gas phase H atoms, with ethylene, acetylene and ethane adsorbed on a Ni(1 1 1) surface at 60 K are probed. Adsorbed ethylidyne (CCH 3) is identified by high resolution electron energy loss spectroscopy to be the major product (30% yield) in all three cases. Adsorbed acetylene is a minor product (3% yield) and arises as a consequence of a dynamic equilibrium between CCH 3 and C 2H 2 in the presence of gas phase H atoms. The observation of the same product for the reaction of H atoms with all three hydrocarbons implies that CCH 3 is the most stable C 2 species in the presence of coadsorbed hydrogen. The rates of CCH 3 production are measured as a function of the time of exposure of H atoms to each hydrocarbon. A simple kinetic model treating each reaction as a pseudo-first order reaction in the hydrocarbon coverage is fit to these data. A mechanism for the formation of CCH 3 via a CHCH 2 intermediate common to all three reactants is proposed to describe this model. The observed instability of the CH 2CH 3 species relative to C 2H 4 plays a role in the formulation of this mechanism as does the observed stability of CHCH 2 species in the presence of coadsorbed hydrogen. The CH 2CH 3 and the CHCH 2 species are produced by the translational activation of ethane and the dissociative ionization of ethane and ethylene, respectively. In addition, the binding energy and the vibrational spectrum of ethane adsorbed on Ni(1 1 1) are determined and exceptionally high resolution vibrational spectra of adsorbed ethylene and acetylene are presented.

Bürgi, T.; Trautman, T. R.; Gostein, M.; Lahr, D. L.; Haug, K. L.; Ceyer, S. T.

2002-03-01

182

Effect of temperature oscillation on chemical reaction rates in the atmosphere  

NASA Technical Reports Server (NTRS)

The effect of temperature fluctuations on atmospheric ozone chemistry is examined by considering the Chapman photochemical theory of ozone transport to calculate globally averaged ozone production rates from mean reaction rates, activation energies, and recombination processes.

Eberstein, I. J.

1974-01-01

183

Modeling kinetic partitioning of secondary organic aerosol and size distribution dynamics: representing effects of volatility, phase state, and particle-phase reaction  

NASA Astrophysics Data System (ADS)

This paper describes and evaluates a new framework for modeling kinetic gas-particle partitioning of secondary organic aerosol (SOA) that takes into account diffusion and chemical reaction within the particle phase. The framework uses a combination of (a) an analytical quasi-steady-state treatment for the diffusion-reaction process within the particle phase for fast-reacting organic solutes, and (b) a two-film theory approach for slow- and nonreacting solutes. The framework is amenable for use in regional and global atmospheric models, although it currently awaits specification of the various gas- and particle-phase chemistries and the related physicochemical properties that are important for SOA formation. Here, the new framework is implemented in the computationally efficient Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) to investigate the competitive growth dynamics of the Aitken and accumulation mode particles. Results show that the timescale of SOA partitioning and the associated size distribution dynamics depend on the complex interplay between organic solute volatility, particle-phase bulk diffusivity, and particle-phase reactivity (as exemplified by a pseudo-first-order reaction rate constant), each of which can vary over several orders of magnitude. In general, the timescale of SOA partitioning increases with increase in volatility and decrease in bulk diffusivity and rate constant. At the same time, the shape of the aerosol size distribution displays appreciable narrowing with decrease in volatility and bulk diffusivity and increase in rate constant. A proper representation of these physicochemical processes and parameters is needed in the next generation models to reliably predict not only the total SOA mass, but also its composition- and number-diameter distributions, all of which together determine the overall optical and cloud-nucleating properties.

Zaveri, R. A.; Easter, R. C.; Shilling, J. E.; Seinfeld, J. H.

2014-05-01

184

Regulation of biochemical reaction rates by flexible tethers  

NASA Astrophysics Data System (ADS)

We explore how ligand-receptor binding kinetics can be controlled by tethering the receptor to the end of a flexible polymer. The tether confines the diffusive motion of the receptor thus influencing the rate at which it captures ligands that are free in solution. We compute steady-state collision rates between ligand and receptor for this “tethered-capture” mechanism using a combination of analytic and numerical techniques. In doing so, we uncover a dimensionless control parameter, the “opacity,” that determines under what conditions and to what extent a tether regulates the ligand-receptor collision rate. We compute the opacity for a number of different tethering scenarios that appear in biology and use these results to predict the affect of changing the length and flexibility of the tether on the rate at which ligands are captured from solution.

Reeves, Daniel; Cheveralls, Keith; Kondev, Jane

2011-08-01

185

Nonequilibrium effects on the rate of bimolecular chemical reaction in a dilute gas  

NASA Astrophysics Data System (ADS)

The perturbation solution of the Boltzmann equation in a dilute gas shows that the rate constant of chemical reaction A+ A ? B+ B is diminished due to nonequilibrium effects. For the line-of-centers model, the relative decrease ? of the reaction rate can reach even nearly 45% (for a small reduced threshold energy ?* and a large molar fraction xB). For slow reactions this method is valid and ? does not depend on xB. The appropriate equilibrium expressions for reaction rate (with: (a) the temperature of the system T, (b) the nonequilibrium Shizgal-Karplus temperatures TA and TB) are used for this analysis.

Cukrowski, A. S.; Fritzsche, S.; Fort, J.

2001-06-01

186

Upper atmosphere research: Reaction rate and optical measurements  

NASA Technical Reports Server (NTRS)

The objective is to provide photochemical, kinetic, and spectroscopic information necessary for photochemical models of the Earth's upper atmosphere and to examine reactions or reactants not presently in the models to either confirm the correctness of their exclusion or provide evidence to justify future inclusion in the models. New initiatives are being taken in technique development (many of them laser based) and in the application of established techniques to address gaps in the photochemical/kinetic data base, as well as to provide increasingly reliable information.

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

1990-01-01

187

Measurement of the NO + O 3 Reaction Rate at Atmospheric Pressure Using Realistic Mixing Ratios  

Microsoft Academic Search

Accurate values for the rate and temperature dependence of the reaction NO + O3 ? NO2 + O2 are important in the chemical modelling of photochemical processes in the atmosphere. Previous measurements have been made at low total pressures and\\/or with very large mixing ratios relative to those observed in the atmosphere. In this study the reaction rate has been

P. C. Moonen; J. N. Cape; R. L. Storeton-West; R. McCOLM

1998-01-01

188

Putting Reaction Rates and Collision Theory in the Hands of Your Students.  

ERIC Educational Resources Information Center

Describes a simulation that can be used to give concrete analogies of collision theory and the factors that affect reaction rates including temperature, concentration, catalyst, and molecular orientation. The simulation works best if done as an introduction to the concepts to help prevent misconceptions about reaction rates and collision theory.…

Evenson, Andy

2002-01-01

189

RATE CONSTANT FOR THE REACTION OF OH RADICALS WITH DIACETYLENE AT 297 + OR - 2K  

EPA Science Inventory

Using a relative rate technique, the rate constant for the gas phase reaction of OH radicals with diacetylene, a reaction considered to be of importance in fuel-rich acetylene oxidation, has been determined at 297 + or - 2 K and atmospheric pressure....

190

Applications of the Absolute Reaction Rate Theory to Biological Responses in Electric and Magnetic Fields.  

National Technical Information Service (NTIS)

This paper develops a theoretical foundation for the study of biological responses of electric and magnetic fields. The basis of the development is the absolute reaction rate theory and the effects of fields on reaction rates. A simple application to the ...

J. P. Brannen J. R. Wayland

1976-01-01

191

Rate and pathways for the reaction of OH with the biogenic p-cymene, an alkylated aromatic  

NASA Astrophysics Data System (ADS)

Aromatics are known to contribute strongly to tropospheric formation of ozone, and p-cymene (4-isopropyltoluene) is one of only a few biogenic, volatile aromatic hydrocarbons. In spite of its symmetry, this molecule (CH3 - °-CH(CH3)2) has a multitude of potential pathways of its reaction with OH radicals. Addition of OH is well-known to be the predominating primary step in the tropospheric transformation of aromatic hydrocarbons. The addition is expected to occur preferably at a non-occupied position, where four positions are available: two equivalent ones ortho to the methyl group and two equivalent ones ortho to the isopropyl group. Furthermore, various C-H bonds (4 aromatic and 10 aliphatic) are available for abstraction, leading to benzyl-type radicals in two cases. The present study combines theoretical calculations with kinetic experiments in the gas phase. The theoretical calculations are based on electronic quantum chemistry DFT method for the investigation of the possible pathways in the potential energy surface of the reaction. The experiments are carried out by the flash photolysis/resonance fluorescence technique. OH radicals are produced by pulsed vacuum-UV photolysis of H2O (> 115 nm) in the presence of p-cymene in a slow flow of He as carrier gas. Their pseudo-first-order decays are monitored by resonance fluorescence, storing the photon counts by multichannel scaling in a PC and accumulating 50 decays each; see Koch et al. (2007) for details of the technique and evaluation of data. The temperature was varied between room temperature (295 K) and 345K, the He pressure was 250 mbar, and the level of p-cymene was increased stepwise, up to 3 x 1013 molecules/cm3. The decays of OH were observed to be exponential at room temperature, becoming clearly biexponential at higher temperatures, thus indicating reversible addition of OH according to the equilibration OH + p-cymene â?? p-cymene-OH (1, -1) These reactions might be accompanied by various abstraction channels, summarized as OH + p-cymene â?? alkylbenzyl + H2O (2) A value of 1.4 x 10-11 cm3 s-1 at 295 K is obtained for the sum k1 + k2, in good agreement with a value of 1.51 x 10-11 cm3 s-1determined by Corchnoy and Atkinson (1990) in a smog chamber at 295 K. The sum k1 + k2 decreases slightly with increasing temperature, falling below 10-11 cm3 s-1 at 345 K. The Arrhenius plot reveals a curved behaviour with a negative activation energy, approximately 1 x 10-12 exp (60 K/T) cm3 s-1. The biexponential behaviour corresponds to an apparent equilibrium constant of k1/k-1 = 8 x 10-25 exp [(-8500 ± 400) K/T] cm3 s-1. On the other hand, the bond energy of OH in the adduct can hardly be obtained from this biexponential behaviour alone since the abstraction of H atoms from the alkyl groups of p-cymene can be estimated to contribute markedly. Extrapolating the respective abstraction channels of toluene and the xylenes to two methyl substituents would yield k2= 1.6 x 10-18 T2exp (-38 K/T) cm3 s-1 (Atkinson, 1989). This amounts to 1.2x10-12 cm3 s-1 at 295 K (about 9% of the observed reactivity) and 1.7x10-12 cm3 s-1 at 345K (>17% of the observed reactivity) and does not even take the possibly largerreactivity of the isopropyl group (as compared to CH3) into account. The abstraction channel has been found to predominate in the analogous reaction of atomic Cl with p-cymene (Finlayson-Pitts et al, 1999), and further experiments by other methods are required to clarify the reaction channels for OH radicals. References Atkinson, R. (1989) Kinetics and Mechanisms of the Gas-Phase Reactions of the Hydroxyl Radical with Organic Compounds. J. Phys. Chem. Ref. Data, Monograph 1, Am. Chem. Soc./Am. Inst. Phys./NIST, p. 229. Corchnoy, S.B., Atkinson, R. (1990) Kinetics of the gas-phase reactions of OH and NO3 radicals with 2-Carene, 1,8-CineoIe, p-Cymene, and Terpinolene. Environ. Sci. Technol. 24, 1497-1502. Finlayson-Pitts, B. J., Keoshian, C.J., Buehler, B., Ezell, A.A. (1999) Kinetics of reaction of chlorine atoms with some biogenic organics. Int

Strekowski, R.; Rayez, M.-T.; Rayez, J.-C.; Zetzsch, C.

2009-04-01

192

New measurements of astrophysical (?,n) reaction rates using a quasi-thermal photon bath from bremsstrahlung  

NASA Astrophysics Data System (ADS)

The astrophysical reaction rates for the (?, n) photodisintegration reactions of 204Pb, 196,198,204Hg, and 197Au will be determined using a quasi-thermal photon bath which is produced from bremsstrahlung. Here we present the first part of this experiment. The reaction 197Au(?,n)196Au is discussed in detail because this reaction was measured in several experiments using different experimental techniques. .

Vogt, K.; Babilon, M.; Bayer, W.; Denefleh, K.; Enders, J.; Galaviz, D.; Hartmann, T.; Hutter, C.; Mohr, P.; Volz, S.; Zilges, A.

2001-04-01

193

Controlling the emotional heart: heart rate biofeedback improves cardiac control during emotional reactions.  

PubMed

When regulating negative emotional reactions, one goal is to reduce physiological reactions. However, not all regulation strategies succeed in doing that. We tested whether heart rate biofeedback helped participants reduce physiological reactions in response to negative and neutral pictures. When viewing neutral pictures, participants could regulate their heart rate whether the heart rate feedback was real or not. In contrast, when viewing negative pictures, participants could regulate heart rate only when feedback was real. Ratings of task success paralleled heart rate. Participants' general level of anxiety, emotion awareness, or cognitive emotion regulation strategies did not influence the results. Our findings show that accurate online heart rate biofeedback provides an efficient way to down-regulate autonomic physiological reactions when encountering negative stimuli. PMID:24373886

Peira, Nathalie; Fredrikson, Mats; Pourtois, Gilles

2014-03-01

194

Absolute rate parameters for the reaction of ground state atomic oxygen with carbonyl sulfide  

NASA Technical Reports Server (NTRS)

The rate parameters for the reaction of O(3P) with carbonyl sulfide, O(3P) + OCS yields CO + SO, have been determined directly by monitoring O(3P) using the flash photolysis-resonance fluorescence technique. The value for reaction rate was measured over a temperature range of 263-502 K and the data were fitted to an Arrhenius expression with good linearity. A comparison of the present results with those from previous studies of this reaction is also presented.

Klemm, R. B.; Stief, L. J.

1974-01-01

195

Reaction rate constants of HO2 + O3 in the temperature range 233-400 K  

NASA Technical Reports Server (NTRS)

The reaction rate constants of HO2 + O3 were measured in the temperature range 233-400 K using a discharge flow system with photofragment emission detection. In the range 233-253 K, the constants are approximately a constant value, and then increase with increasing temperature. This result suggests that the reaction may have two different channels. An expression representing the reaction rate constants is presented.

Wang, Xiuyan; Suto, Masako; Lee, L. C.

1988-01-01

196

Rate Constants for Reactions of •OH with Several Reduced Sulfur Compounds Determined by Relative Rate Constant Method  

Microsoft Academic Search

The rate constants for the reactions between •OH and six reduced sulfur compounds in air, N2, and O2 were measured using relative rate constant method in a 180-L Teflon bag at 298 K and 1.013×105 Pa. These results were compared with previous published data and were discussed in terms of trends in different buffer gases.

Haitao Wang; Yujie Zhang; Yujin Mu

2008-01-01

197

Time-Resolved O3 Chemical Chain Reaction Kinetics Via High-Resolution IR Laser Absorption Methods  

NASA Technical Reports Server (NTRS)

Excimer laser photolysis in combination with time-resolved IR laser absorption detection of OH radicals has been used to study O3/OH(v = 0)/HO2 chain reaction kinetics at 298 K, (i.e.,(k(sub 1) is OH + 03 yields H02 + 02 and (k(sub 2) is H02 + 03 yields OH + 202). From time-resolved detection of OH radicals with high-resolution near IR laser absorption methods, the chain induction kinetics have been measured at up to an order of magnitude higher ozone concentrations ([03] less than or equal to 10(exp 17) molecules/cu cm) than accessible in previous studies. This greater dynamic range permits the full evolution of the chain induction, propagation, and termination process to be temporally isolated and measured in real time. An exact solution for time-dependent OH evolution under pseudo- first-order chain reaction conditions is presented, which correctly predicts new kinetic signatures not included in previous OH + 03 kinetic analyses. Specifically, the solutions predict an initial exponential loss (chain "induction") of the OH radical to a steady-state level ([OH](sub ss)), with this fast initial decay determined by the sum of both chain rate constants, k(sub ind) = k(sub 1) + k(sub 2). By monitoring the chain induction feature, this sum of the rate constants is determined to be k(sub ind) = 8.4(8) x 10(exp -14) cu cm/molecule/s for room temperature reagents. This is significantly higher than the values currently recommended for use in atmospheric models, but in excellent agreement with previous results from Ravishankara et al.

Kulcke, Axel; Blackmon, Brad; Chapman, William B.; Kim, In Koo; Nesbitt, David J.

1998-01-01

198

The gaseous explosive reaction : the effect of pressure on the rate of propagation of the reaction zone and upon the rate of molecular transformation  

NASA Technical Reports Server (NTRS)

This study of gaseous explosive reaction has brought out a number of important fundamental characteristics of the explosive reaction indicating that the basal processes of the transformation are much simpler and corresponds more closely to the general laws and principles of ordinary transformations than is usually supposed. The report calls attention to the point that the rate of molecular transformation within the zone was found in all cases to be proportional to pressure, that the transformation within the zone is the result of binary impacts. This result is of unusual interest in the case of the reaction of heavy hydrocarbon fuels and the reaction mechanism proposed by the recent kinetic theory of chain reactions.

Stevens, F W

1932-01-01

199

The Gaseous Explosive Reaction : the Effect of Pressure on the Rate of Propagation of the Reaction Zone and upon the Rate of Molecular Transformation  

NASA Technical Reports Server (NTRS)

This study of gaseous explosive reaction has brought out a number of important fundamental characteristics of the explosive reaction indicating that the basal processes of the transformation are much simpler and corresponds more closely to the general laws and principles of ordinary transformations than is usually supposed. The report calls attention to the point that the rate of molecular transformation within the zone was found in all cases to be proportional to pressure, that the transformation within the zone is the result of binary impacts. This result is of unusual interest in the case of the reaction of heavy hydrocarbon fuels and the reaction mechanism proposed by the recent kinetic theory of chain reactions.

Stevens, F W

1932-01-01

200

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

NASA Technical Reports Server (NTRS)

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

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

1981-01-01

201

Formation of OH radicals in the gas-phase reaction of propene, isobutene, and isoprene with O{sub 3}: Yields and mechanistic implications  

SciTech Connect

The gas-phase reaction of ozone with alkenes is one of the very few reactions of atmospheric interest that are initiated without free radicals. This tropospheric oxidation pathway for unsaturated compounds has received considerable attention because of the reported formation of OH radicals. OH radicals originating from the alkene-ozone reaction have been proposed as a relevant source of OH radicals in the lower troposphere. Since the reported yields of OH radicals differ considerably, the authors redetermined the OH radical yield for three terminal alkenes by performing a series of pseudo-first-order experiments. Ozonolysis studies were carried out under excess ozone conditions in the presence of different cyclohexane concentrations. The decay rate of the alkene ({kappa}{sub obs}) was followed by long-path FTIR spectroscopy. From the decrease of the effective rate constant ({kappa}{sub obs} = {kappa}{sub eff}[O{sub 3}]) upon addition of cyclohexane, the OH radical yield was determined. The OH radical yields were found to be independent of the concentration of reactants for the Criegee intermediates, which are formed in ozonolysis systems. From these results the authors conclude that OH radicals are formed in a unimolecular process, presumably from the decomposition of the excited Criegee intermediate. Determined yields of OH radical formation in the ozonolysis of propene, isobutene, and isoprene were 0.34, 0.60, and 0.26, respectively. Detailed product studies were performed to verify if the observed stable products can be explained by the assumption that OH radicals are formed via the hydroperoxide channel as proposed by Niki et al. For the isobutene-ozone system, experimental product yields were found to agree well with predictions from a chemical mechanism based on the chemistry of the acetonylperoxy radical CH{sub 3}C(O)CH{sub 2}O{sub 2}, which is formed as an important radical product from the decomposition of the (CH{sub 3}){sub 2}COO Criegee intermediate.

Neeb, P.; Moortgat, G.K.

1999-11-11

202

Neutron absorption measurements using converging beams - increased reaction rate  

NASA Astrophysics Data System (ADS)

Focusing lenses using neutron reflection optics may considerably increase the neutron beam current density over a small area. The technique enables neutron absorption measurements to be taken with higher sensitivity, greater spatial resolution and lower background. Experimental concerns associated with the beam converging onto the sample are considered and compared to those for a quasi-parallel beam. In particular, we show the connection between the interaction rates for the parallel beam and the isotropic field.

Mildner, D. F. R.; Chen-Mayer, H. H.

1999-02-01

203

Reaction Rate and Composition Dependence of the Stability of Thermonuclear Burning on Accreting Neutron Stars  

NASA Astrophysics Data System (ADS)

The stability of thermonuclear burning of hydrogen and helium accreted onto neutron stars is strongly dependent on the mass accretion rate. The burning behavior is observed to change from Type I X-ray bursts to stable burning, with oscillatory burning occurring at the transition. Simulations predict the transition at a 10 times higher mass accretion rate than observed. Using numerical models we investigate how the transition depends on the hydrogen, helium, and CNO mass fractions of the accreted material, as well as on the nuclear reaction rates of 3? and the hot-CNO breakout reactions 15O(?, ?)19Ne and 18Ne(?, p)21Na. For a lower hydrogen content the transition is at higher accretion rates. Furthermore, most experimentally allowed reaction rate variations change the transition accretion rate by at most 10%. A factor 10 decrease of the 15O(?, ?)19Ne rate, however, produces an increase of the transition accretion rate of 35%. None of our models reproduce the transition at the observed rate, and depending on the true 15O(?, ?)19Ne reaction rate, the actual discrepancy may be substantially larger. We find that the width of the interval of accretion rates with marginally stable burning depends strongly on both composition and reaction rates. Furthermore, close to the stability transition, our models predict that X-ray bursts have extended tails where freshly accreted fuel prolongs nuclear burning.

Keek, L.; Cyburt, R. H.; Heger, A.

2014-06-01

204

Self-inhibited rate in gas-solid noncatalytic reactions. The shrinking core model  

SciTech Connect

The shrinking core model is examined for gas-solid noncatalytic reactions with a self-inhibited rate form and it is shown that multiple reaction pathways are possible for solid particles reacted under identical conditions. The observed reaction rate can have up to two discontinuities (jumps) during reaction for particles of spherical and cylindrical shape. The geometric instability analysis reveals that the reaction interface is stable under a very limited set of conditions only for solid particles of slab geometry. For a sphere or cylinder at large Biot numbers the reaction interface is always potentially unstable. This model provides a plausible explanation for gas-solid reactions which exhibit erratic shrinking core behavior.

Erk, H.F.; Dudukovic, M.P.

1984-02-01

205

The 25Al(p,gamma)26Si Reaction Rate in Novae  

SciTech Connect

The production of 26Al in novae is uncertain, in part, because of the uncertain rate of the 25Al(p,g)26Si reaction at novae temperatures. This reaction is thought to be dominated by a longsought 3+ level in 26Si, and the calculated reaction rate varies by orders of magnitude depending on the energy of this resonance. We present evidence concerning the spin of a level at 5.914 MeV in 26Si from the 28Si(p,t)26Si reaction studied at the Holifield Radioactive Beam Facility at ORNL. We find that the angular distribution for this level implies either a 2+ or 3+ assignment, with only a 3+ being consistent with the mirror nucleus, 26Mg. Additionally, we have used the updated 25Al(p,g)26Si reaction rate in a nova nucleosynthesis calculation and have addressed the effects of the remaining uncertainties in the rate on 26Al production.

Bardayan, Daniel W [ORNL; Blackmon, Jeff C [ORNL; Hix, William Raphael [ORNL; Liang, J Felix [ORNL; Smith, Michael Scott [ORNL; Howard, Joshua A [ORNL; Kozub, R. L. [Tennessee Technological University; Brune, Carl [ORNL; Lingerfelt, Eric J [ORNL; Chae, Kyung Yuk [ORNL; Scott, J. P. [Tennessee Technological University; Johnson, Micah [ORNL; Jones, K. L. [University of Tennessee; Pain, S. D. [Rutgers University; Thomas, J. S. [Rutgers University; Livesay, Jake [ORNL; Visser, D. W. [University of North Carolina, Chapel Hill

2007-10-01

206

A simple expression for the apparent reaction rate of large wood char gasification with steam.  

PubMed

A simple expression for the apparent reaction rate of large wood char gasification with steam is proposed. Large char samples were gasified under steam atmosphere using a thermo-balance reactor. The apparent reaction rate was expressed as the product of the intrinsic rate and the effective factor. The effective factor was modified to include the effect of change in char diameter and intrinsic reaction rate during the reaction. Assuming uniform conversion ratio throughout a particle, the simplified reaction scheme was divided into three stages. In the initial stage, the local conversion ratio increases without particle shrinkage. In the middle stage, the particle shrinks following the shrinking core model without change in the local conversion ratio. In the final stage, the local conversion ratio increases without particle shrinkage. The validity of the modified effective value was confirmed by comparison with experimental results. PMID:20144863

Umeki, Kentaro; Roh, Seon-Ah; Min, Tai-Jin; Namioka, Tomoaki; Yoshikawa, Kunio

2010-06-01

207

The 18F(p,?)15O reaction rate for application to nova ?-ray emission  

NASA Astrophysics Data System (ADS)

The 18F(p,?)15O reaction is recognized as one of the most important reaction for nova gamma ray astronomy as it governs the early ? 511 keV emission. However, its rate remains largely uncertain at nova temperatures due to unknown low energy resonance strengths. We report here on our lastest results concerning the study of the D(18F,p?)15N reaction, as well as on the determination of the 18F(p,?)15O reaction rate using the R-matrix theory. Remaining uncertainties are discussed.

de Séréville, N.; Berthoumieux, E.; Coc, A.

2005-07-01

208

Rate variations of a hetero-Diels--Alder reaction in supercritical fluid CO{sub 2}  

SciTech Connect

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.

Thompson, R.L.; Glaeser, R.; Bush, D.; Liotta, C.L.; Eckert, C.A.

1999-11-01

209

Estimation of the prevalence and rate of acute transfusion reactions occurring in Windhoek, Namibia  

PubMed Central

Background Acute transfusion reactions are probably common in sub-Saharan Africa, but transfusion reaction surveillance systems have not been widely established. In 2008, the Blood Transfusion Service of Namibia implemented a national acute transfusion reaction surveillance system, but substantial under-reporting was suspected. We estimated the actual prevalence and rate of acute transfusion reactions occurring in Windhoek, Namibia. Methods The percentage of transfusion events resulting in a reported acute transfusion reaction was calculated. Actual percentage and rates of acute transfusion reactions per 1,000 transfused units were estimated by reviewing patients’ records from six hospitals, which transfuse >99% of all blood in Windhoek. Patients’ records for 1,162 transfusion events occurring between 1st January – 31st December 2011 were randomly selected. Clinical and demographic information were abstracted and Centers for Disease Control and Prevention National Healthcare Safety Network criteria were applied to categorize acute transfusion reactions1. Results From January 1 – December 31, 2011, there were 3,697 transfusion events (involving 10,338 blood units) in the selected hospitals. Eight (0.2%) acute transfusion reactions were reported to the surveillance system. Of the 1,162 transfusion events selected, medical records for 785 transfusion events were analysed, and 28 acute transfusion reactions were detected, of which only one had also been reported to the surveillance system. An estimated 3.4% (95% confidence interval [CI]: 2.3–4.4) of transfusion events in Windhoek resulted in an acute transfusion reaction, with an estimated rate of 11.5 (95% CI: 7.6–14.5) acute transfusion reactions per 1,000 transfused units. Conclusion The estimated actual rate of acute transfusion reactions is higher than the rate reported to the national haemovigilance system. Improved surveillance and interventions to reduce transfusion-related morbidity and mortality are required in Namibia.

Meza, Benjamin P.L.; Lohrke, Britta; Wilkinson, Robert; Pitman, John P.; Shiraishi, Ray W.; Bock, Naomi; Lowrance, David W.; Kuehnert, Matthew J.; Mataranyika, Mary; Basavaraju, Sridhar V.

2014-01-01

210

Atmospheric chemistry of toxic contaminants. 1. Reaction rates and atmospheric persistence  

SciTech Connect

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).

Grosjean, D. (DGA, Inc., Ventura, CA (United States))

1990-10-01

211

Rate constants for the gas-phase reactions of hydroxyl radicals with tetramethyllead and tetraethyllead  

Microsoft Academic Search

Rate constants for the reactions of OH radicals with (CHâ)âPb and (CâHâ)âPb have been determined at 298 {plus minus} 2 K and a total pressure of approximately 1 atm. Rate data were obtained by using both the absolute rate technique of pulse radiolysis combined with kinetic spectroscopy and a conventional photolytic relative rate method. Rate constants were also obtained for

Ole John Nielsen; D. J. OFarrell; Jack J. Treacy; Howard W. Sidebottom

1991-01-01

212

Effect of Liquid Composition on the Slurry Fischer-Tropsch Synthesis. 1. Rate of Reaction  

Microsoft Academic Search

With a reduced fused magnetite catalyst, the rate of reaction in the presence of phenanthrene is nearly twice that observed in the presence of n-octacosane even though the solubility of Hâ and CO is moderately less in phenanthrene. The rate in the presence of triphenylmethane was about the same as in n-octacosane. In the presence of a perfluoropolyether, the rate

C. N. Satterfield; H. G. Stenger

1984-01-01

213

Rate constants measured for hydrated electron reactions with peptides and proteins  

NASA Technical Reports Server (NTRS)

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.

Braams, R.

1968-01-01

214

ACTIVE: a program to calculate and plot reaction rates from ANISN calculated fluxes  

SciTech Connect

The ACTIVE code calculates spatial heating rates, tritium production rates, neutron reaction rates, and energy spectra from particle fluxes calculated by ANISN. ACTIVE has a variety of input options including the capability to plot all calculated spatial distributions. The code was primarily designed for use with fusion first wall/blanket systems, but could be applied to any one-dimensional problem.

Judd, J.L.

1981-12-01

215

Trickle-bed reactor for investigating upgrading reactions  

SciTech Connect

The catalytic processing of heavy petroleum feeds and many of the heavier synthetic fuels is performed in the liquid phase and quite often in trickle-bed reactors. In this paper we present the basis for the design of a microscale trickle-bed reactor for studying the kinetics of reactions related to coal liquefaction processes. This small (10 cm-long) catalyst bed was used to study the hydrogenation of naphthalene to tetralin at 373 K and the hydrogenation of pyrene to dihydropyrene at 573 K. The hydrogenation of naphthalene to tetralin using a palladium (Pd) catalyst has been shown to be a pseudo-first-order irreversible reaction at 373 K, while the hydrogenation of pyrene to dihydropyrene using a nickel molybdenum on alumina catalyst (NiMo/alumina) has been shown to be a pseudo-first-order reversible reaction at 573 K. First-order kinetic plots for the flow reactor experiments show that plug-flow behavior was achieved for both the hydrogenation of naphthalene and pyrene. In addition, plug flow was not perturbed by increasing the hydrogen gas flow by a factor of four. A reactor of such small scale with achievable plug flow behavior is an important tool for catalyst activity testing since it provides the ability to adjust reaction conditions easily and allows catalyst deactivation to be studied more effectively. In addition, a wide variety of hydrotreating reactions including hydrodesulfurization (HDS), hydrodeoxygenation (HDO), and hydrodenitrogenation (HDN), can be studied using this simple reactor design. 10 refs., 6 figs., 1 tab.

Kottenstette, R.J.; Stephens, H.P.

1990-01-01

216

The influence of low-energy resonances on the reaction rate of 18O(?, ?)22Ne  

NASA Astrophysics Data System (ADS)

The 18O(6Li,d)22Ne ?-transfer reaction has been used to study the level structure of 22Ne around the ?-threshold and to identify natural-parity states in that excitation range. The 18O(?, ?)22Ne reaction has been studied in the energy range between 340 and 800 keV to determine the existence of possible resonances and to investigate their influence on the reaction rate of 18O(?, ?)22Ne.

Giesen, U.; Browne, C. P.; Görres, J.; Ross, J. G.; Wiescher, M.; Azuma, R. E.; King, J. D.; Vise, J. B.; Buckby, M.

1994-01-01

217

Matching of experimental and statistical-model thermonuclear reaction rates at high temperatures  

NASA Astrophysics Data System (ADS)

We address the problem of extrapolating experimental thermonuclear reaction rates toward high stellar temperatures (T>1 GK) by using statistical model (Hauser-Feshbach) results. Reliable reaction rates at such temperatures are required for studies of advanced stellar burning stages, supernovae, and x-ray bursts. Generally accepted methods are based on the concept of a Gamow peak. We follow recent ideas that emphasized the fundamental shortcomings of the Gamow peak concept for narrow resonances at high stellar temperatures. Our new method defines the effective thermonuclear energy range (ETER) by using the 8th, 50th, and 92nd percentiles of the cumulative distribution of fractional resonant reaction rate contributions. This definition is unambiguous and has a straightforward probability interpretation. The ETER is used to define a temperature at which Hauser-Feshbach rates can be matched to experimental rates. This matching temperature is usually much higher compared to previous estimates that employed the Gamow peak concept. We suggest that an increased matching temperature provides more reliable extrapolated reaction rates since Hauser-Feshbach results are more trustwhorthy the higher the temperature. Our ideas are applied to 21 (p,?), (p,?), and (?,?) reactions on A=20-40 target nuclei. For many of the cases studied here, our extrapolated reaction rates at high temperatures differ significantly from those obtained using the Gamow peak concept.

Newton, J. R.; Longland, R.; Iliadis, C.

2008-08-01

218

Influence of operating conditions on the observed reaction rate in the single channel monolith reactor  

Microsoft Academic Search

The catalytic hydrogenation of nitrobenzoic acid (NBA) to the aminobenzoic acid was used as a model reaction for a quantitative study of influences of the operating conditions on the observed reaction rate in a single channel monolith reactor operated in Taylor flow regime. A simple mathematical model was derived and used for the analysis of hydrogenation experiments carried out in

Gorazd Ber?i?

2001-01-01

219

Combustion of Bimodal Nano\\/Micro Aluminum Suspension with New Reaction Rate Model  

Microsoft Academic Search

In this study a mathematical model for combustion of bimodal particle in lean flow was developed. The difference between structure of flame in this work and previous ones was that, in those flame was divided by five zones and reaction rate was considered to be constant in reaction zones and also zero in post flame zone. In reality it was

M. Bidabadi; N. Moallemi; I. Shafieenejad; M. Jadidi

220

Reaction Rates for O + HD Yields OH + D and O + HD Yields OD + H,  

National Technical Information Service (NTIS)

We have calculated reaction rates for the reactions O + HD - OH + D and O + DH - Od + H using improved canonical variational transition state theory and least -action ground-state transmission coefficients with an ab initio potential energy surface. The k...

B. C. Garrett D. G. Truhlar

1987-01-01

221

RATE CONSTANTS FOR THE REACTIONS OF O3 AND OH RADICALS WITH A SERIES OF ALKYNES  

EPA Science Inventory

Rate constants for the reactions of O3 and OH radicals with acetylene, propyne and 1-butyne have been determined at room temperature. The data, especially those for the O3 reactions, which are significantly lower than reported literature data, are discussed and compared with the ...

222

ENEA final results for the IRMA campaign: Intercomparison of reaction rate measurements at MASURCA.  

National Technical Information Service (NTIS)

In November 1984, the international campaign, IRMA, for fission reaction rate and capture-to-fission ration measurements in fast reactor lattices was held at the Masurca zero power facility (CEA-Cadarache), under the auspices of NEACPR. Eight laboratories...

M. Angelone P. Moioli

1990-01-01

223

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

PubMed

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 0.076gTOC/gDS.day. The catalytic turnover number (kcat) of the MnP enzyme-catalyzed was 506.7perday while the rate constant (k) of the un-catalyzed reaction was 0.012perday. PMID:24759644

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

2014-06-01

224

Method and apparatus for obtaining enhanced production rate of thermal chemical reactions  

DOEpatents

The present invention is a method and apparatus (vessel) for providing a heat transfer rate from a reaction chamber through a wall to a heat transfer chamber substantially matching a local heat transfer rate of a catalytic thermal chemical reaction. The key to the invention is a thermal distance defined on a cross sectional plane through the vessel inclusive of a heat transfer chamber, reaction chamber and a wall between the chambers. The cross sectional plane is perpendicular to a bulk flow direction of the reactant stream, and the thermal distance is a distance between a coolest position and a hottest position on the cross sectional plane. The thermal distance is of a length wherein the heat transfer rate from the reaction chamber to the heat transfer chamber substantially matches the local heat transfer rate.

Tonkovich, Anna Lee Y [Pasco, WA; Wang, Yong [Richland, WA; Wegeng, Robert S [Richland, WA; Gao, Yufei [Kennewick, WA

2003-04-01

225

Diffusion-controlled reaction rates for two active sites on a sphere  

PubMed Central

Background The diffusion-limited reaction rate of a uniform spherical reactant is generalized to anisotropic reactivity. Previous work has shown that the protein model of a uniform sphere is unsatisfactory in many cases. Competition of ligands binding to two active sites, on a spherical enzyme or cell is studied analytically. Results The reaction rate constant is given for two sites at opposite ends of the species of interest. This is compared with twice the reaction rate for a single site. It is found that the competition between sites lowers the reaction rate over what is expected for two sites individually. Competition between sites does not show up, until the site half angle is greater than 30 degrees. Conclusions Competition between sites is negligible until the site size becomes large. The competitive effect grows as theta becomes large. The maximum effect is given for theta?=?pi/2.

2014-01-01

226

Controlling the rates of biochemical reactions and signaling networks by shape and volume changes  

PubMed Central

In biological systems, chemical activity takes place in micrometer- and nanometer-sized compartments that constantly change in shape and volume. These ever-changing cellular compartments embed chemical reactions, and we demonstrate that the rates of such incorporated reactions are directly affected by the ongoing shape reconfigurations. First, we show that the rate of product formation in an enzymatic reaction can be regulated by simple volume contraction–dilation transitions. The results suggest that mitochondria may regulate the dynamics of interior reaction pathways (e.g., the Krebs cycle) by volume changes. We then show the effect of shape changes on reactions occurring in more complex and structured systems by using biomimetic networks composed of micrometer-sized compartments joined together by nanotubes. Chemical activity was measured by implementing an enzymatic reaction–diffusion system. During ongoing reactions, the network connectivity is changed suddenly (similar to the dynamic tube formations found inside Golgi stacks, for example), and the effect on the reaction is registered. We show that spatiotemporal properties of the reaction–diffusion system are extremely sensitive to sudden changes in network topology and that chemical reactions can be initiated, or boosted, in certain nodes as a function of connectivity.

Lizana, L.; Bauer, B.; Orwar, O.

2008-01-01

227

A Unified Equation for the Reaction Rate in Dense Matter Stars  

NASA Astrophysics Data System (ADS)

We analyze thermonuclear and pycnonuclear reaction rates in multi-component dense stellar plasma. First we describe calculations of the astrophysical S-factor at low energies using the Sa~o Paulo potential on the basis of the barrier penetration model. Then we present a simple phenomenological expression for a reaction rate. The expression contains several fit parameters which we adjust to reproduce the best microscopic calculations available in the literature.

Gasques, L. R.; Wiescher, M.; Yakovlev, D. G.

2007-10-01

228

Reaction of the m-THPC triplet state with the antioxidant Trolox and the anesthetic Propofol: modulation of photosensitization mechanisms relevant to photodynamic therapy?  

PubMed

Antioxidants may affect the outcome of photodynamic therapy (PDT) through the inactivation of reactive oxygen species. Their direct interaction with photosensitizers excited at the triplet state is also worthy of interest. This process is investigated by laser flash photolysis of m-THPC (meso-tetra(3-hydroxyphenyl)chlorin, Foscan) hydroalcoholic solutions added with Trolox (TrOH), a standard antioxidant or Propofol (PfOH, Diprivan(®)), a common anesthetic agent also characterized for its antioxidant properties. Transient UV-visible absorption spectra, kinetics at selected wavelengths and final spectra after extensive laser irradiation show that both compounds react with the m-THPC triplet state, (3)m-THPC, to ultimately restore the photosensitizer in its ground state. For PfOH, this process mainly appears as a single step obeying pseudo-first order kinetics. The bimolecular rate constant for the quenching of (3)m-THPC by PfOH is around 2 × 10(6) M(-1) s(-1), a value increased to some extent by the water content of the solution. A bimolecular reaction between (3)m-THPC and TrOH is observed with a rate constant of similar magnitude and dependence upon water. However, the reaction leads, at least partly, to intermediate species assigned to the TrO? radical and the m-THPC anion radical. Within a few ms, these species back react to yield m-THPC in its ground state. A general mechanism involving an intermediate activated complex with some charge transfer character is proposed. Depending on the redox potentials for the oxidation of the antioxidant, this complex evolves predominantly either toward the formation of radicals (TrOH) or back to the photosensitizer ground state (PfOH). Notably, the kinetics data suggest that Propofol may quench (3)m-THPC at concentrations relevant of clinical situation in PDT involving anesthesia. PMID:22307091

Friaa, O; Maillard, P; Brault, D

2012-04-01

229

Comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reaction flows  

SciTech Connect

The use of silane (SiH4) as an effective ignitor and flame stabilizing pilot fuel is well documented. A reliable chemical kinetic mechanism for prediction of its behavior at the conditions encountered in the combustor of a SCRAMJET engine was calculated. The effects of hydrogen addition on hydrocarbon ignition and flame stabilization as a means for reduction of lengthy ignition delays and reaction times were studied. The ranges of applicability of chemical kinetic models of hydrogen-air combustors were also investigated. The CHARNAL computer code was applied to the turbulent reaction rate modeling.

Magnotti, F.; Diskin, G.; Matulaitis, J.; Chinitz, W.

1984-01-01

230

Astrophysical S-Factors and Reaction Rates of Threshold (p, n)-Reactions on 99-102Ru  

NASA Astrophysics Data System (ADS)

Astrophysical S-factors of (p, n) reactions on 99Ru, 100Ru, 101Ru, and 102Ru were derived from the sum of experimental isomeric and ground states cross sections measured in the incident proton energy range of 5-9 MeV. They were compared with Hauser-Feshbach statistical model predictions of the NON-SMOKER code. Good agreement was found in the majority of cases. Reaction rates were derived up to 8.7 GK stellar temperature by combining experiment and theory.

Skakun, Ye.; Rauscher, T.

2010-08-01

231

A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reaction flows  

NASA Technical Reports Server (NTRS)

The use of silane (SiH4) as an effective ignitor and flame stabilizing pilot fuel is well documented. A reliable chemical kinetic mechanism for prediction of its behavior at the conditions encountered in the combustor of a SCRAMJET engine was calculated. The effects of hydrogen addition on hydrocarbon ignition and flame stabilization as a means for reduction of lengthy ignition delays and reaction times were studied. The ranges of applicability of chemical kinetic models of hydrogen-air combustors were also investigated. The CHARNAL computer code was applied to the turbulent reaction rate modeling.

Magnotti, F.; Diskin, G.; Matulaitis, J.; Chinitz, W.

1984-01-01

232

Reaction rate and energy-loss rate for photopair production by relativistic nuclei  

NASA Technical Reports Server (NTRS)

The process of e(+/-) pair production by relativistic nuclei on ambient photons is considered. The process is important for cosmic-ray nuclei in interstellar and intergalactic space as well as in galactic and extragalactic compact objects. The rate of this process is given by an integral of the cross section over the photon angular and energy distribution. In the case of isotropic photons, the angular integration is performed to provide an expression for the rate at given photon energy in the nucleus rest frame. The total rate then becomes a single integral of that rate over the photon energy distribution. Formulas are also given for the fractional energy loss of a relativistic nucleus colliding with a photon of a given energy in the rest frame. The nucleus energy-loss rate is integrated over the photon angular distribution in the case of isotropic photons, and simple fits are provided.

Chodorowski, Michal J.; Zdziarski, Andrzej A.; Sikora, Marek

1992-01-01

233

A kinetics study of the O({sup 3}P) + CH{sub 3}Cl reaction over the 556-1485 K range by the HTP and LP-ST techniques  

SciTech Connect

The high-temperature photochemistry (HTP) and laser photolysis-shock tube (LP-ST) techniques have been combined to study the kinetics of the reaction between ground-state oxygen atoms with CH{sub 3}Cl over the temperature range, 556--1485 K. In the HTP reactor, used for the 556--1291 K range, O atoms were generated by flash photolysis of O{sub 2}, CO{sub 2} or SO{sub 2}, and the atom concentrations were monitored by resonance fluorescence, while with the LP-ST technique, used for the 916--1485 K range, O atoms were generated by the photolysis of either SO{sub 2} or NO with the 193 nm light from a pulsed ArF excimer laser, and atomic resonance absorption spectroscopy (ARAS) was used to monitor [O]{sub t}. In both studies, rate coefficients were derived from the [O] profiles under the pseudo-first-order condition, [O]{much_lt}[CH{sub 3}Cl]. The data obtained by the two techniques are in excellent agreement and are best represented by the expression, k(T) = 2.57 {times} 10{sup {minus}11} (T/K){sup 0.31} exp({minus}5633 K/T) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} with a 2{sigma} precision varying from {plus_minus}6 {plus_minus}22% and an estimated 2{sigma} accuracy of {plus_minus}21% to {plus_minus}30%, depending on temperature. The rate coefficients for the title reaction are essentially identical to those for the O + CH{sub 4} reaction over the observed temperature range, the reasons for which are discussed.

Ko, Taeho; Fontijn, A. [Rensselaer Polytechnic Inst., Troy, NY (United States). High-Temperature Reaction Kinetics Lab.; Lim, K.P.; Michael, J.V. [Argonne National Lab., IL (United States)

1991-12-01

234

A kinetics study of the O( sup 3 P) + CH sub 3 Cl reaction over the 556-1485 K range by the HTP and LP-ST techniques  

SciTech Connect

The high-temperature photochemistry (HTP) and laser photolysis-shock tube (LP-ST) techniques have been combined to study the kinetics of the reaction between ground-state oxygen atoms with CH{sub 3}Cl over the temperature range, 556--1485 K. In the HTP reactor, used for the 556--1291 K range, O atoms were generated by flash photolysis of O{sub 2}, CO{sub 2} or SO{sub 2}, and the atom concentrations were monitored by resonance fluorescence, while with the LP-ST technique, used for the 916--1485 K range, O atoms were generated by the photolysis of either SO{sub 2} or NO with the 193 nm light from a pulsed ArF excimer laser, and atomic resonance absorption spectroscopy (ARAS) was used to monitor (O){sub t}. In both studies, rate coefficients were derived from the (O) profiles under the pseudo-first-order condition, (O){much lt}(CH{sub 3}Cl). The data obtained by the two techniques are in excellent agreement and are best represented by the expression, k(T) = 2.57 {times} 10{sup {minus}11} (T/K){sup 0.31} exp({minus}5633 K/T) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} with a 2{sigma} precision varying from {plus minus}6 {plus minus}22% and an estimated 2{sigma} accuracy of {plus minus}21% to {plus minus}30%, depending on temperature. The rate coefficients for the title reaction are essentially identical to those for the O + CH{sub 4} reaction over the observed temperature range, the reasons for which are discussed.

Ko, Taeho; Fontijn, A. (Rensselaer Polytechnic Inst., Troy, NY (United States). High-Temperature Reaction Kinetics Lab.); Lim, K.P.; Michael, J.V. (Argonne National Lab., IL (United States))

1991-12-01

235

Review of rate coefficients of ionic reactions determined from measurements made by the atmosphere explorer satellites  

NASA Technical Reports Server (NTRS)

The large data base of aeronomic parameters measured by the Atmosphere Explorer C, D, and E satellites since December 1973 has been used to determine a number of reaction rate coefficients highly relevant to our understanding of thermospheric chemistry. In this paper the results are reviewed for ionic rate coefficients for recombination of NO(+), O2(+), for reactions of O(+) + N2, N2(+) + O, and O(++) + O, and for various reactions involving O(+)(2D) and O(+)(2P) ions with O and N2.

Torr, D. G.; Torr, M. R.

1978-01-01

236

Thick target measurement of the 40Ca(alpha,gamma)44Ti reaction rate  

SciTech Connect

The thick-target yield for the {sup 40}Ca({alpha},{gamma}){sup 44}Ti reaction has been measured for E{sub beam} = 4.13, 4.54, and 5.36 MeV using both an activation measurement and online {gamma}-ray spectroscopy. The results of the two measurements agree. From the measured yield a reaction rate is deduced that is smaller than statistical model calculations. This implies a smaller {sup 44}Ti production in supernova compared to recently measured {sup 40}Ca({alpha},{gamma}){sup 44}Ti reaction rates.

Sheets, S A; Burke, J T; Scielzo, N D; Phair, L; Bleuel, D; Norman, E B; Grant, P G; Hurst, A M; Tumey, S; Brown, T A; Stoyer, M

2009-02-06

237

Proton-capture Nucleosynthesis In Low Mass Stars: Effects of New Reaction Rates  

SciTech Connect

We present computations of nucleosynthesis in low-mass asymptotic-giant-branch stars of solar metallicity experiencing deep mixing. In this framework, we discuss the effects of recent improvements in relevant reaction rates for proton captures on intermediate-mass nuclei. The calculations are then performed on the basis of a parameterized circulation, where the effects of the new nuclear inputs are best compared to previous works. We find that especially the new reaction rate for the {sup 14}N(p,{gamma}){sup 15}O reaction implies considerable modifications in the composition of low mass red giant stars.

Palmerini, S.; Busso, M. [Dipartimento di Fisica, Universita degli Studi di Perugia and INFN Sezione di Perugia (Italy); La Cognata, M. [DMFCI - Universita di Catania and Laboratori Nazionali del Sud - INFN, Catania (Italy); Cristallo, S. [Departamento de Fisica Teorica y del Cosmos, Universidad de Granada (Spain); INAF Osservatorio Astronomico di Collurania, Teramo (Italy)

2011-10-28

238

Direct measurement of intrinsic proton transfer rates in diffusion-controlled reactions  

NASA Astrophysics Data System (ADS)

Acid-base reactions are usually treated as diffusion-controlled. In this study we directly measure the intrinsic proton transfer rates at the reaction contact of several naphthols photoacids-carboxylic base pairs. Deviations from the diffusion-controlled limit result from the finite intrinsic proton transfer rates at contact and these rates correlate well with the total free-energy change following the reactions. The correction was made over 8 ?p Ka units and yielded kr0=3×10 11 s -1 and Ga0=2.9 kcal/mol for the activationless proton transfer rate and the intrinsic free-energy barrier, respectively. A similar behavior is found when the proton dissociation rates of all previously studied naphthol-like photoacids are included in the correlation. The combined correlation, over 11 ?p Ka units, yielded kr0=2×10 11 s -1 and Ga0=2.5 kcal/mol.

Pines, Ehud; Magnes, Ben-Zion; Lang, Matthew J.; Fleming, Graham R.

1997-12-01

239

The rate coefficients of unimolecular reactions in the systems with power-law distributions  

NASA Astrophysics Data System (ADS)

The rate coefficient formulae of unimolecular reactions are generalized to the systems with the power-law distributions based on nonextensive statistics, and the power-law rate coefficients are derived in the high and low pressure limits, respectively. The numerical analyses are made of the rate coefficients as functions of the ?-parameter, the threshold energy, the temperature and the number of degrees of freedom. We show that the new rate coefficients depend strongly on the ?-parameter different from one (thus from a Boltzmann-Gibbs distribution). Two unimolecular reactions, CH3CO?CH3+CO and CH3NC?CH3CN, are taken as application examples to calculate their power-law rate coefficients, which obtained with the ?-parameters slightly different from one can be exactly in agreement with all the experimental studies on these two reactions in the given temperature ranges.

Yin, Cangtao; Guo, Ran; Du, Jiulin

2014-08-01

240

STARLIB: A NEXT-GENERATION REACTION-RATE LIBRARY FOR NUCLEAR ASTROPHYSICS  

SciTech Connect

STARLIB is a next-generation, all-purpose nuclear reaction-rate library. For the first time, this library provides the rate probability density at all temperature grid points for convenient implementation in models of stellar phenomena. The recommended rate and its associated uncertainties are also included. Currently, uncertainties are absent from all other rate libraries, and, although estimates have been attempted in previous evaluations and compilations, these are generally not based on rigorous statistical definitions. A common standard for deriving uncertainties is clearly warranted. STARLIB represents a first step in addressing this deficiency by providing a tabular, up-to-date database that supplies not only the rate and its uncertainty but also its distribution. Because a majority of rates are lognormally distributed, this allows the construction of rate probability densities from the columns of STARLIB. This structure is based on a recently suggested Monte Carlo method to calculate reaction rates, where uncertainties are rigorously defined. In STARLIB, experimental rates are supplemented with: (1) theoretical TALYS rates for reactions for which no experimental input is available, and (2) laboratory and theoretical weak rates. STARLIB includes all types of reactions of astrophysical interest to Z = 83, such as (p, {gamma}), (p, {alpha}), ({alpha}, n), and corresponding reverse rates. Strong rates account for thermal target excitations. Here, we summarize our Monte Carlo formalism, introduce the library, compare methods of correcting rates for stellar environments, and discuss how to implement our library in Monte Carlo nucleosynthesis studies. We also present a method for accessing STARLIB on the Internet and outline updated Monte Carlo-based rates.

Sallaska, A. L. [National Institute of Standards and Technology, Gaithersburg, MD 20899-8462 (United States); Iliadis, C.; Champange, A. E. [University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255 (United States); Goriely, S. [Institut d'Astronomie et d'Astrophysique, Universite Libre de Bruxelles, C.P. 226, B-1050 Brussels (Belgium); Starrfield, S.; Timmes, F. X., E-mail: anne.sallaska@nist.gov [Arizona State University, Tempe, AZ 85287-1504 (United States)

2013-07-15

241

STARLIB: A Next-generation Reaction-rate Library for Nuclear Astrophysics  

NASA Astrophysics Data System (ADS)

STARLIB is a next-generation, all-purpose nuclear reaction-rate library. For the first time, this library provides the rate probability density at all temperature grid points for convenient implementation in models of stellar phenomena. The recommended rate and its associated uncertainties are also included. Currently, uncertainties are absent from all other rate libraries, and, although estimates have been attempted in previous evaluations and compilations, these are generally not based on rigorous statistical definitions. A common standard for deriving uncertainties is clearly warranted. STARLIB represents a first step in addressing this deficiency by providing a tabular, up-to-date database that supplies not only the rate and its uncertainty but also its distribution. Because a majority of rates are lognormally distributed, this allows the construction of rate probability densities from the columns of STARLIB. This structure is based on a recently suggested Monte Carlo method to calculate reaction rates, where uncertainties are rigorously defined. In STARLIB, experimental rates are supplemented with: (1) theoretical TALYS rates for reactions for which no experimental input is available, and (2) laboratory and theoretical weak rates. STARLIB includes all types of reactions of astrophysical interest to Z = 83, such as (p, ?), (p, ?), (?, n), and corresponding reverse rates. Strong rates account for thermal target excitations. Here, we summarize our Monte Carlo formalism, introduce the library, compare methods of correcting rates for stellar environments, and discuss how to implement our library in Monte Carlo nucleosynthesis studies. We also present a method for accessing STARLIB on the Internet and outline updated Monte Carlo-based rates.

Sallaska, A. L.; Iliadis, C.; Champange, A. E.; Goriely, S.; Starrfield, S.; Timmes, F. X.

2013-07-01

242

Temperature-dependent reaction-rate expression for oxygen recombination at Shuttle entry conditions  

NASA Technical Reports Server (NTRS)

A temperature-dependent oxygen surface reaction-rate coefficient has been determined from experimental STS-2 heating and wall temperature data at altitudes of 77.91 km, 74.98 km, and 71.29 km. The coefficient is presented in an Arrhenius form and is shown to be less temperature dependent than previous results. Finite-rate viscous-shock-layer heating rates based on this present expression have been compared with predicted heating rates using the previous rate coefficients and with experimental heating data obtained over an extensive range of STS-2 and STS-3 entry conditions. A substantial improvement is obtained in comparison of experimental data and predicted heating rates using the present oxygen reaction-rate expression.

Zoby, E. V.; Simmonds, A. L.; Gupta, R. N.

1984-01-01

243

Monte Carlo analysis of uncertainty propagation in a stratospheric model. 2: Uncertainties due to reaction rates  

NASA Technical Reports Server (NTRS)

A concise stratospheric model was used in a Monte-Carlo analysis of the propagation of reaction rate uncertainties through the calculation of an ozone perturbation due to the addition of chlorine. Two thousand Monte-Carlo cases were run with 55 reaction rates being varied. Excellent convergence was obtained in the output distributions because the model is sensitive to the uncertainties in only about 10 reactions. For a 1 ppby chlorine perturbation added to a 1.5 ppby chlorine background, the resultant 1 sigma uncertainty on the ozone perturbation is a factor of 1.69 on the high side and 1.80 on the low side. The corresponding 2 sigma factors are 2.86 and 3.23. Results are also given for the uncertainties, due to reaction rates, in the ambient concentrations of stratospheric species.

Stolarski, R. S.; Butler, D. M.; Rundel, R. D.

1977-01-01

244

On the curvature in logarithmic plots of rate coefficients for chemical reactions  

PubMed Central

In terms of the reduced potential energy barrier ? = ?uTS/kT, the rate coefficients for chemical reactions are usually expressed as proportional to e-?. The coupling between vibrational modes of the medium to the reaction coordinate leads to a proportionality of the regularized gamma function of Euler Q(a,?) = ?(a,?)/?(a), with a being the number of modes coupled to the reaction coordinate. In this work, the experimental rate coefficients at various temperatures for several chemical reactions were fitted to the theoretical expression in terms of Q(a,?) to determine the extent of its validity and generality. The new expression affords lower deviations from the experimental points in 29 cases out of 38 and it accounts for the curvature in the logarithmic plots of rate coefficients versus inverse temperature. In the absence of tunneling, conventional theories predict the curvature of these plots to be identically zero.

2011-01-01

245

Determination of shift in electrodic reaction rates due to the presence of stress  

NASA Astrophysics Data System (ADS)

An extension of Butler-Volmer formulation is proposed to determine the stress-induced changes in electrodic reaction rates. Gibbs-Duhem equation is used to determine the stress-dependent chemical potential and the corresponding change in the reaction rate. The scope of possible amplification or reduction in the reaction rates due to tensile and compressive stress fields is explored numerically. Though quantitative experimental validation remains to be pursued, behavioral agreement of the extended Butler-Volmer model with some observations made in the field of corrosive dissolution is established. Our numerical results also indicate that in addition to altering the speed of a reaction, a stress field can modify the shape of an anodic dissolution front. The effect of stress-generated surface patterns is also considered. It is well-established that a stress field can create surface patterns due to surface wrinkling or surface diffusion. We determine the possible significance of such patterns on the reaction rate, and identify the factors that may enhance their contribution to electrodic reaction rates.

Sarkar, Swarnavo; Aquino, Wilkins

2013-03-01

246

Rate and reaction probability of the surface reaction between ozone and dihydromyrcenol measured in a bench scale reactor and a room-sized chamber  

Microsoft Academic Search

Low volatility terpenoids emitted from consumer products can react with ozone on surfaces and may significantly alter concentrations of ozone, terpenoids and reaction products in indoor air. We measured the reaction probability and a second-order surface-specific reaction rate for the ozonation of dihydromyrcenol, a representative indoor terpenoid, adsorbed onto polyvinylchloride (PVC), glass, and latex paint coated spheres. The reaction probability

S. a b Shu; G.C.a Morrison

2012-01-01

247

High-precision (p,t) reaction to determine {sup 25}Al(p,{gamma}){sup 26}Si reaction rates  

SciTech Connect

Since the identification of ongoing {sup 26}Al production in the universe, the reaction sequence {sup 24}Mg(p,{gamma}){sup 25}Al({beta}{sup +{nu}}){sup 25}Mg(p,{gamma}){sup 26}Al has been studied intensively. At temperatures where the radiative capture on {sup 25}Al (t{sub 1/2}=7.2 s) becomes faster than the {beta}{sup +} decay, the production of {sup 26}Al can be reduced due to the depletion of {sup 25}Al. To determine the resonances relevant for the {sup 25}Al(p,{gamma}){sup 26}Si bypass reaction, we measured the {sup 28}Si(p,t){sup 26}Si reaction with high-energy precision using the Grand Raiden spectrometer at the Research Center for Nuclear Physics, Osaka. Several new energy levels were found above the p threshold and for known states excitation energies were determined with smaller uncertainties. The calculated stellar rates of the bypass reaction agree well with previous results, suggesting that these rates are well established.

Matic, A.; Berg, A. M. van den; Harakeh, M. N.; Woertche, H. J.; Berg, G. P. A.; Couder, M.; Goerres, J.; LeBlanc, P.; O'Brien, S.; Wiescher, M.; Fujita, K.; Hatanaka, K.; Sakemi, Y.; Shimizu, Y.; Tameshige, Y.; Tamii, A.; Yosoi, M.; Adachi, T.; Fujita, Y.; Shimbara, Y. [Kernfysisch Versneller Instituut, University of Groningen, Zernikelaan 25, NL-9747 AA Groningen (Netherlands); Department of Physics and the Joint Institute for Nuclear Astrophysics, University of Notre Dame, Notre Dame, Indiana 46556 (United States); Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 560-0047 (Japan); Department of Physics, Osaka University, Toyonaka, Osaka 560-0043 (Japan); School of Physics, University of the Witwatersrand, P.O. Wits, Johannesburg 2050 (South Africa); Department of Physics, Kyushu University, Fukuoka 812-8581 (Japan); Department of Physics and Astronomy, National Superconducting Cyclotron Laboratory and the Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, Michigan 48824 (United States)

2010-08-15

248

Estimation of the reaction rate constant of HOCl by SMILES observation  

NASA Astrophysics Data System (ADS)

Hypochlorous acid, HOCl plays an important role to link the odd ClOx and the odd HOx in the atmospheric chemistry with the reaction: {ClO} + {HO_{2}} \\longrightarrow {HOCl} + {O_{2}} Quantitative understanding of the rate constant of the reaction (1.1) is essential for understanding the ozone loss in the mid-latitude region because of a view point of its rate controlling role in the ozone depletion chemistry. Reassessment of the reaction rate constant was pointed out from MIPAS/Envisat observations (von Clarmann et al., 2011) and balloon-borne observations (Kovalenko et al., 2007). Several laboratory studies had been reported, although the reaction rate constants have large uncertainties, as k{_{HOCl}} = (1.75 ± 0.52) × 10^{-12} exp[(368 ± 78)/T] (Hickson et al., 2007), and large discrepancies (Hickson et al., 2007;Stimpfle et al., 1979). Moreover, theoretical ab initio studies pointed out the pressure dependence of the reaction (1.1) (Xu et al., 2003). A new high-sensitive remote sensing technology named Superconducting SubMillimeter-wave Limb-Emission Sounder (SMILES) on the International Space Station (ISS) had observed diurnal variations of HOCl in the upper stratosphere/lower mesosphere (US/LM) region for the first time. ClO and HO_{2} were slso observed simultaneously with HOCl. SMILES performed the observations between 12^{{th}} October 2009 and 21^{{th}} April 2010. The latitude coverage of SMILES observation is normally 38°S-65°N. The altitude region of HOCl observation is about 28-70 km. We estimated the time period in which the reaction (1.1) becomes dominant in the ClO_{y} diurnal chemistry in US/LM. The reaction rate constant was directly estimated by decay of [ClO] and [HO_{2}] amounts in that period. The derived reaction rate constant represented well the increase of [HOCl] amount.

Kuribayashi, Kouta; Kasai, Yasuko; Sato, Tomohiro; Sagawa, Hideo

2012-07-01

249

Theoretical rate coefficients for the exchange reaction OH + D--> OD + H.  

PubMed

In this work quasiclassical trajectory calculations were carried out to determine directly the rate coefficients for the isotopic exchange reaction, OH + D-->OD + H, using a potential-energy surface that carefully accounts for the long-range interactions. The calculated thermal rate coefficients are in good agreement with the experimental results. PMID:16497034

Wang, Wenli; Santos, Emanuel; Brandão, João

2006-02-21

250

The Effect of Conceptual Change Pedagogy on Students' Conceptions of Rate of Reaction  

ERIC Educational Resources Information Center

This paper reports on an investigation of the effect of conceptual change pedagogy on students' conceptions of "rate of reaction" concepts. The study used a pre-test/post-test non-equivalent comparison group design approach and the sample consisted of 72 Turkish grade-11 students (aged 16-18 years) selected from two intact classrooms. The "Rate of…

Calik, Muammer; Kolomuc, Ali; Karagolge, Zafer

2010-01-01

251

Method and apparatus for obtaining enhanced production rate of thermal chemical reactions  

DOEpatents

Reactors and processes are disclosed that can utilize high heat fluxes to obtain fast, steady-state reaction rates. Porous catalysts used in conjunction with microchannel reactors to obtain high rates of heat transfer are also disclosed. Reactors and processes that utilize short contact times, high heat flux and low pressure drop are described. Improved methods of steam reforming are also provided.

Tonkovich, Anna Lee Y. (Pasco, WA); Wang, Yong (Richland, WA); Wegeng, Robert S. (Richland, WA); Gao, Yufei (Kennewick, WA)

2006-05-16

252

Rate of Mixing Controls Rate and Outcome of Autocatalytic Processes: Theory and Microfluidic Experiments with Chemical Reactions and Blood Coagulation  

PubMed Central

This article demonstrates that the rate of mixing can regulate the rate and outcome of both biological and nonbiological autocatalytic reaction systems that display a threshold response to the concentration of an activator. Plug-based microfluidics was used to control the timing of reactions, the rate of mixing, and surface chemistry in blood clotting and its chemical model. Initiation of clotting of human blood plasma required addition of a critical concentration of thrombin. Clotting could be prevented by rapid mixing when thrombin was added near the critical concentration, and mixing also affected the rate of clotting when thrombin was added at concentrations far above the critical concentration in two clinical clotting assays for human plasma. This phenomenon was modeled by a simple mechanism—local and global competition between the clotting reaction, which autocatalytically produces an activator, and mixing, which removes the activator. Numerical simulations showed that the Damköhler number, which describes this competition, predicts the effects of mixing. Many biological systems are controlled by thresholds, and these results shed light on the dynamics of these systems in the presence of spatial heterogeneities and provide simple guidelines for designing and interpreting experiments with such systems.

Pompano, Rebecca R.; Li, Hung-Wing; Ismagilov, Rustem F.

2008-01-01

253

Reevaluation of the O(+)(2P) reaction rate coefficients derived from Atmosphere Explorer C observations  

NASA Technical Reports Server (NTRS)

O(+)(2P) is an important species for studies of the ionosphere and thermosphere: its emission at 7320 A can be used as a diagnostic of the thermospheric atomic oxygen density. Unfortunately, there are no laboratory measurements of the O and N2 reaction rates which are needed to determine the major sinks of (O+)(2p). We have recalculated the O and N2 reaction rates for O(+) (2P) using recent improvements in the solar EUV flux, cross sections, and photoelectron fluxes. For the standard solar EUV flux, the new N2 reaction rate of 3.4 +/- 1.5 x 10 exp -10 cu cm/s is close to the value obtained by Rusch et al. (1977), but the new O reaction rate of 4.0 +/- 1.9 x 10 exp -10 cu cm/sec is about 8 times larger. These new reaction rates are derived using neutral densities, electron density, and solar EUV fluxes measured by Atmosphere Explorer C in 1974 during solar minimum. The new theoretical emission rates are in good agreement with the data for the two orbits studied by Rusch et al.

Chang, T.; Torr, D. G.; Richards, P. G.; Solomon, S. C.

1993-01-01

254

Venus volcanism: Rate estimates from laboratory studies of sulfur gas-solid reactions  

NASA Technical Reports Server (NTRS)

Thermochemical reactions between sulfur-bearing gases in the atmosphere of Venus and calcium-, iron-, magnesium-, and sulfur-bearing minerals on the surface of Venus are an integral part of a hypothesized cycle of thermochemical and photochemical reactions responsible for the maintenance of the global sulfuric acid cloud cover on Venus. SO2 is continually removed from the Venus atmosphere by reaction with calcium bearing minerals on the planet's surface. The rate of volcanism required to balance SO2 depletion by reactions with calcium bearing minerals on the Venus surface can therefore be deduced from a knowledge of the relevant gas-solid reaction rates combined with reasonable assumptions about the sulfur content of the erupted material (gas + magma). A laboratory program was carried out to measure the rates of reaction between SO2 and possible crustal minerals on Venus. The reaction of CaCO3(calcite) + SO2 yields CaSO4 (anhydrite) + CO was studied. Brief results are given.

Ehlers, K.; Fegley, B., Jr.; Prinn, R. G.

1989-01-01

255

Venus volcanism: Rate estimates from laboratory studies of sulfur gas-solid reactions  

NASA Astrophysics Data System (ADS)

Thermochemical reactions between sulfur-bearing gases in the atmosphere of Venus and calcium-, iron-, magnesium-, and sulfur-bearing minerals on the surface of Venus are an integral part of a hypothesized cycle of thermochemical and photochemical reactions responsible for the maintenance of the global sulfuric acid cloud cover on Venus. SO2 is continually removed from the Venus atmosphere by reaction with calcium bearing minerals on the planet's surface. The rate of volcanism required to balance SO2 depletion by reactions with calcium bearing minerals on the Venus surface can therefore be deduced from a knowledge of the relevant gas-solid reaction rates combined with reasonable assumptions about the sulfur content of the erupted material (gas + magma). A laboratory program was carried out to measure the rates of reaction between SO2 and possible crustal minerals on Venus. The reaction of CaCO3(calcite) + SO2 yields CaSO4 (anhydrite) + CO was studied. Brief results are given.

Ehlers, K.; Fegley, B., Jr.; Prinn, R. G.

256

On the derivation of semiclassical expressions for quantum reaction rate constants in multidimensional systems.  

PubMed

Expressions for reaction rate constants in multidimensional chemical systems are derived by applying semiclassical approximation to the quantum path integrals of the ImF formulation of reaction rate theory. First, the transverse degrees of freedom orthogonal to the reaction coordinate are treated within the steepest descent approximation, after which the semiclassical approximation is applied to the remaining reaction coordinate. Thus derived, the semiclassical expressions account for the multidimensional nature of quantum effects and accurately incorporate nuclear quantum effects such as multidimensional tunneling and zero point energies. The obtained expressions are applicable in the broad temperature range from the deep tunneling to high-temperature regimes. The present paper provides derivation of the semiclassical instanton expressions proposed by Kryvohuz [J. Chem. Phys. 134, 114103 (2011)]. PMID:23822234

Kryvohuz, Maksym

2013-06-28

257

The rate of the reaction between CN and C2H2 at interstellar temperatures  

NASA Technical Reports Server (NTRS)

The rate coefficient for the important interstellar reaction between CN and C2H2 has been calculated as a function of temperature between 10 and 300 K. The potential surface for this reaction has been determined through ab initio quantum chemical techniques; the potential exhibits no barrier in the entrance channel but does show a small exit channel barrier, which lies below the energy of reactants. Phase-space calculations for the reaction dynamics, which take the exit channel barrier into account, show the same unusual temperature dependence as determined by experiment, in which the rate coefficient at first increases as the temperature is reduced below room temperature and then starts to decrease as the temperature drops below 50-100 K. The agreement between theory and experiment provides strong confirmation that the reaction occurs appreciably at cool interstellar temperatures.

Woon, D. E.; Herbst, E.

1997-01-01

258

On the derivation of semiclassical expressions for quantum reaction rate constants in multidimensional systems  

NASA Astrophysics Data System (ADS)

Expressions for reaction rate constants in multidimensional chemical systems are derived by applying semiclassical approximation to the quantum path integrals of the ImF formulation of reaction rate theory. First, the transverse degrees of freedom orthogonal to the reaction coordinate are treated within the steepest descent approximation, after which the semiclassical approximation is applied to the remaining reaction coordinate. Thus derived, the semiclassical expressions account for the multidimensional nature of quantum effects and accurately incorporate nuclear quantum effects such as multidimensional tunneling and zero point energies. The obtained expressions are applicable in the broad temperature range from the deep tunneling to high-temperature regimes. The present paper provides derivation of the semiclassical instanton expressions proposed by Kryvohuz [J. Chem. Phys. 134, 114103 (2011)].

Kryvohuz, Maksym

2013-06-01

259

Unbound states of 32Cl and the 31S(p,?)32Cl reaction rate  

NASA Astrophysics Data System (ADS)

The 31S(p,?)32Cl reaction is expected to provide the dominant break-out path from the SiP cycle in novae and is important for understanding enrichments of sulfur observed in some nova ejecta. We studied the 32S(3He,t)32Cl charge-exchange reaction to determine properties of proton-unbound levels in 32Cl that have previously contributed significant uncertainties to the 31S(p,?)32Cl reaction rate. Measured triton magnetic rigidities were used to determine excitation energies in 32Cl. Proton-branching ratios were obtained by detecting decay protons from unbound 32Cl states in coincidence with tritons. An improved 31S(p,?)32Cl reaction rate was calculated including robust statistical and systematic uncertainties.

Matoš, M.; Blackmon, J. C.; Linhardt, L. E.; Bardayan, D. W.; Nesaraja, C. D.; Clark, J. A.; Deibel, C. M.; O'Malley, P. D.; Parker, P. D.

2011-11-01

260

Enhancement of hydrogen generation rate in reaction of aluminum with water  

Microsoft Academic Search

The aim of this investigation is to enhance hydrogen generation rate in aluminum–water reaction by improving the activity of aluminum particles and using the heat released during the reaction. This was accomplished by developing fresh surfaces by milling aluminum particles together with salt. Salt particles not only serve as nano-millers, but also surround activated particles and prevent re-oxidation of bare

Korosh Mahmoodi; Babak Alinejad

2010-01-01

261

COMBUSTION REACTIONS OF PARAFFIN COMPONENTS IN LIQUID TRANSPORTATION FUELS USING GENERIC RATES  

Microsoft Academic Search

The approach of mechanism generation is the accepted one of assigning generic rates to reactions in the same class. The procedure has been successfully applied to higher paraffins that include detailed sub-models of n-hexane, cyclohexane, n-heptane, n-decane, n-dodecane, and n-hexadecane and semi-detailed sub-models of iso-octane and methyl cyclohexane, in addition to reactions of aromatic formation and oxidation. Comparison between predictions

HONGZHI R. ZHANG; ERIC G. EDDINGS; ADEL F. SAROFIM

2007-01-01

262

The F + HD reaction: cross sections and rate constants on an ab initio potential energy surface  

NASA Astrophysics Data System (ADS)

Reaction cross sections and rate constants for the F + HD reaction have been calculated on the recently released Stark-Werner (SW) ab initio potential energy surface (PES) using the quasiclassical trajectory (QCT) method. The results are compared to other theoretical calculations on different PESs and to experimental isotopic branching ratios. The general agreement is good. Dynamical features related to the asymmetric nature of the HD molecule are also discussed.

Aoiz, F. J.; Bañares, L.; Herrero, V. J.; Rábanos, V. Sáez; Stark, K.; Tanarro, I.; Werner, H.-J.

1996-11-01

263

The Q-K model for gas-phase chemical reaction rates  

NASA Astrophysics Data System (ADS)

The quantum-kinetic, or Q-K, model is based on the quantum vibration model that is employed in the computation of gas flows at the molecular level by the direct simulation Monte Carlo (DSMC) method. The Q-K procedure for dissociation is physically realistic within the context of the vibration model in that the reaction occurs upon the selection of the vibrational level that corresponds to dissociation. An analogous, but entirely phenomenological, procedure has been presented for endothermic exchange and chain reactions. These procedures for the endothermic reactions have been well validated, but the existing procedures for the corresponding exothermic reactions have proved to be problematic. This paper presents new procedures for the exothermic reactions that are computationally efficient and provide a near exact match with the equilibrium constant of statistical mechanics. The Q-K model does not depend on the availability of continuum rate coefficients. Instead, the simplicity of the new DSMC procedures allows analytical expressions to be written down for the corresponding rate coefficients in an equilibrium gas. These are used to validate the Q-K model for reactions in high temperature air and in hydrogen-oxygen combustion. The development of the Q-K model has been driven by the need for efficient reaction procedures in DSMC applications that often involve the computation of billions of simulated collisions. It is not intended to compete with the modern theories for gas-phase chemical reactions that employ more accurate physical representations of real reactions. At the same time, the degree of validation of the model is such that the analytical expressions for the rate coefficients that correspond to the model should be useful in their own right.

Bird, G. A.

2011-10-01

264

Matching of experimental and statistical-model thermonuclear reaction rates at high temperatures  

NASA Astrophysics Data System (ADS)

Reliable reaction rates at high stellar temperatures are necessary for the study of advanced stellar burning stages, supernovae and x-ray bursts. We suggest a new procedure for extrapolating experimental thermonuclear reaction rates to these higher temperatures (T > 1 GK) using statistical model (Hauser-Feshbach) results. Current, generally accepted, procedures involve the use of the Gamow peak, which has been shown to be unreliable for narrow resonances at high stellar temperatures [1]. Our new approach defines the effective thermonuclear energy range (ETER) by using the 8^th, 50^th and 92^nd percentiles of the cumulative distribution of fractional resonant reaction contributions. The ETER is then used to define a reliable temperature for matching experimental rates to Hauser-Feshbach rates. The resulting matching temperature is often well above the previous result using the Gamow peak concept. Our new method should provide more accurate extrapolated rates since Hauser-Feshbach rates are more reliable at higher temperatures. These ideas are applied to 21 (p,?), (p,?) and (?,?) reactions on a range of A = 20-40 target nuclei and results will be presented. [0pt] [1] J. R. Newton, C. Iliadis, A. E. Champagne, A. Coc, Y. Parpottas and R. Ugalde, Phys. Rev. C 75, 045801 (2007).

Newton, Joseph; Longland, Richard; Iliadis, Christian

2009-05-01

265

Mineral/solution reaction rates in a mixed flow reactor: Wollastonite hydrolysis  

NASA Astrophysics Data System (ADS)

A newly developed mixed flow reactor was used to measure the rate of hydrolysis of wollastonite over the pH range of 3 to 8. This design avoids abrasion of the solid sample by confining it within a nylon mesh while the reacting solution is circulated over it by a stirrer. The rate of reaction was determined from the difference of the compositions of the input and output solutions following the methods used by chemical engineers for the analysis of mixed flow reactors, also called continuously stirred tank reactors (CSTR). This apparatus, constructed from easily obtainable parts, avoids many of the problems inherent in studying mineral/solution reaction kinetics in batch reactors. The hydrolysis of wollastonite CaSiO3 + 2 H+ + H2O = Ca2+ + H4SiO4 can be fit to a rate law of the form: dnH+/ dt = kadKH+mH+/(1.0 + KH+mH+) where kad = 9.80 × 10 -8molm-2sec-1 and KH+ = 2.08 × 10 5. Over the pH range of 4 to 7, the data also may fit a simple linear form: dnH+/ dt = - Ak+( aH+) 0.40 where k+ = 3.80 × 10 -6 sec -1 at 25°C. The presence of calcium ion in the solution at concentrations up to 1.0 mol kg -1 produces only a minor reduction of the reaction rate. The activation energy for this reaction is 79.2 kJ mol -1. Examination of the surfaces of the reacted grains showed no evidence of incongruent reaction leading to a product layer but did show the extensive development of etch pits leading to a rapid increase in the specific surface area. At large extents of reaction at low pH, diffusion of ions into or from these deep etch pits may limit the reaction rate.

Rimstidt, J. Donald; Dove, Patricia M.

1986-11-01

266

Calculations on the rate of the ion-molecule reaction between NH3(+) and H2  

NASA Technical Reports Server (NTRS)

The rate coefficient for the ion-molecule reaction NH3(+) + H2 yields NH4(+) + H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50-100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH3(+) + H2 + He yields NH4(+) + H + He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.

Herbst, Eric; Defrees, D. J.; Talbi, D.; Pauzat, F.; Koch, W.

1991-01-01

267

A simple recipe for modeling reaction-rate in flows with turbulent-combustion  

NASA Technical Reports Server (NTRS)

A computationally viable scheme to account for chemical reaction in turbulent flows is presented. The multivariate beta-pdf model for multiple scalar mixing forms the basis of this scheme. Using the model scalar joint pdf and a general form of the instantaneous reaction-rate, the unclosed chemical reaction terms are expressed as simple functions of scalar means and the turbulent scalar energy. The calculation procedure requires that the mean scalar equations and only one other transport equation - for the turbulent scalar energy - be solved.

Girimaji, Sharath S.

1991-01-01

268

Rate constants for chemical reactions in high-temperature nonequilibrium air  

NASA Technical Reports Server (NTRS)

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.

Jaffe, R. L.

1986-01-01

269

Considerations Based on Reaction Rate on Char Gasification Behavior in Two-stage Gasifier for Biomass  

NASA Astrophysics Data System (ADS)

In order to develop a small-scale gasifier in which biomass can be converted to energy with high efficiency, we planned a gasification process that consists of two parts: pyrolysis part (rotary kiln) and gasification part (downdraft gasifier). We performed fundamental experiments on gasification part and discussed the appropriate conditions such as air supply location, air ratio, air temperature and hearth load. We considered the results by calculating reaction rates of representative reactions on char gasification part and found that water gas reaction is dominant in the reduction area and its behavior gives important information to decide the adequate length of the char layer.

Taniguchi, Miki; Nishiyama, Akio; Sasauchi, Kenichi; Ito, Yusuke; Akamatsu, Fumiteru

270

Effect of Liquid Composition on the Slurry Fischer-Tropsch Synthesis. 1. Rate of Reaction  

SciTech Connect

With a reduced fused magnetite catalyst, the rate of reaction in the presence of phenanthrene is nearly twice that observed in the presence of n-octacosane even though the solubility of H/sub 2/ and CO is moderately less in phenanthrene. The rate in the presence of triphenylmethane was about the same as in n-octacosane. In the presence of a perfluoropolyether, the rate of reaction was markedly reduced, even though H/sub 2/ and CO solubilities are much higher than in hydrocarbons. This was attributed to substantial mass transfer resistances through an encapsulating hydrocarbon layer around the catalyst particles. The effect of the nature of a liquid on several other reactions is also discussed.

Satterfield, C.N.; Stenger, H.G.

1984-04-01

271

Triple-alpha reaction rate studied with the Faddeev three-body formalism  

SciTech Connect

The triple-alpha (3{alpha}) reaction, {sup 4}He+{sup 4}He+{sup 4}He{yields}{sup 12}C+{gamma}, which plays a significant role in the stellar evolution scenarios, is studied in terms of a three-alpha (3-{alpha}) model. The reaction rate of the process is calculated via an inverse process, 3-{alpha} photodisintegration of a {sup 12}C nucleus. Both of 3-{alpha} bound and-continuum states are calculated by a Faddeev method with accommodating the long range Coulomb interaction. With being adjusted to the empirical E2-strength for {sup 12}C(0{sub 2}{sup +}){yields}{sup 12}C(2{sub 1}{sup +}) transition, results of the 3{alpha} reaction rate <{alpha}{alpha}{alpha}> at higher temperature (T > 10{sup 8} K), where the reaction proceeds mainly through the {sup 8}Be and {sup 12}C(0{sub 2}{sup +}) resonant states, almost agree with those of the Nuclear Astrophysics Compilation of Reaction Rates (NACRE). On the other hand, calculated values of <{alpha}{alpha}{alpha}> are about 10{sup 3} times larger than the NACRE rate at a low temperature (T= 10{sup 7} K), which means our results are remarkably smaller than recent CDCC results.

Ishikawa, Souichi [Hosei University, 2-17-1 Fujimi, Chiyoda, Tokyo 102-8160 (Japan)

2012-11-12

272

Cross-section evaluation utilizing integral reaction-rate measurements in fast neutron fields  

SciTech Connect

The role of integral reaction-rate data for cross-section evaluation is reviewed. The subset of integral data considered comprises integral reaction rates measured for dosimeter, fission-product, and actinide-type materials irradiated in reactor dosimetry fast neutron benchmark fields and in the EBR-II. Utilization of these integral data for integral testing, multigroup cross-section adjustment and pointwise cross section adjustment is treated in some detail. Examples are given that illustrate the importance of considering a priori uncertainty and correlation information for these analyses. 3 figures, 3 tables.

Anderl, R.A.

1980-01-01

273

Determination of astrophysical thermonuclear rates with a bubble chamber: The 12C(??)16O reaction case  

NASA Astrophysics Data System (ADS)

The 12C(??)O reaction rate is considered one of the most important unknown parameters in the physics of structure and evolution of massive stars. While extensive experimental campaigns have been performed trying to improve the quality of the measurements, the rate still holds very large uncertainties. Here we discuss a new experimantal scheme to measure the cross section of this reaction with a bubble chamber and a bremsstrahlung beam. The main advantage of the technique is a gain in the luminosity of several orders of magnitude when compared to other ongoing experiments.

DiGiovine, B.; Grames, J.; Henderson, D.; Holt, R. J.; Meekins, D.; Poelker, M.; Rehm, K. E.; Robinson, A.; Sonnenschein, A.; Suleiman, R.; Ugalde, C.

2013-11-01

274

Reaction rate constant of HO2+O3 measured by detecting HO2 from photofragment fluorescence  

NASA Technical Reports Server (NTRS)

A room-temperature discharge-flow system investigation of the rate constant for the reaction 'HO2 + O3 yields OH + 2O2' has detected HO2 through the OH(A-X) fluorescence produced by photodissociative excitation of HO2 at 147 nm. A reaction rate constant of 1.9 + or - 0.3 x 10 to the -15th cu cm/molecule per sec is obtained from first-order decay of HO2 in excess O3; this agrees well with published data.

Manzanares, E. R.; Suto, Masako; Lee, Long C.; Coffey, Dewitt, Jr.

1986-01-01

275

The Effect of Conceptual Change Pedagogy on Students’ Conceptions of Rate of Reaction  

Microsoft Academic Search

This paper reports on an investigation of the effect of conceptual change pedagogy on students’ conceptions of ‘rate of reaction’\\u000a concepts. The study used a pre-test\\/post-test non-equivalent comparison group design approach and the sample consisted of\\u000a 72 Turkish grade-11 students (aged 16–18 years) selected from two intact classrooms. The ‘Rate of Reaction’ Concept Test comprising\\u000a 9 lead and 10 sub-questions (total

Muammer Çalik; Ali Kolomuç; Zafer Karagölge

2010-01-01

276

X-ray structure and reactivity of (Ru(bpy)/sub 2/(CO)H)PF/sub 6/ x 1/2 C/sub 3/H/sub 6/O (bpy = 2,2'-bipyridyl), a possible intermediate in the water-gas shift reaction catalyzed by ruthenium polypyridyl complexes  

SciTech Connect

The structure of (Ru(C/sub 10/H/sub 8/N/sub 2/)/sub 2/(CO)H)PF/sub 6/ x 1/2 C/sub 3/H/sub 6/O is reported. The compound crystallizes in the monoclinic C2/c space group with unit cell parameters a = 26.680 (4) A, b = 7.005 (3) A, c = 25.936 (2) A, and ..beta.. = 93.96 (2)/sup 0/ (V = 4836.04 A/sup 3/, Z = 8). The cationic species has a cis geometry for CO and H. The ruthenium-hydride and ruthenium-carbon distances are 1.68 (4) and 1.804 (6) A, respectively. The average ruthenium-nitrogen distance is 2.109 A. In acidic solution the title compound acts as a hydride donor to produce dihydrogen gas in a pseudo-first-order reaction; the second-order rate constant for this reaction is 0.105 -/+ 0.01 dm/sup 3/ mol/sup -1/ s/sup -1/ at 303 K with an activation energy 56 -/+ 2 kJ mol/sup -1/. A value of -100 -/+ 8 J mol/sup -1/ deg/sup -1/ has been obtained for the entropy of activation. The consequences of these results for the possible reaction mechanism of the homogeneous water-gas shift reaction catalyzed by (Ru(bpy)/sub 2/(CO)Cl)/sup +/ are discussed. 15 references, 4 figures, 6 tables.

Haasnoot, J.G.; Hinrichs, W.; Weir, O.; Vos, J.G.

1986-11-05

277

Scale-Dependent Rates of Uranyl Surface Complexation Reaction in Sediments  

SciTech Connect

Scale-dependency of uranyl[U(VI)] surface complexation rates was investigated in stirred flow-cell and column systems using a U(VI)-contaminated sediment from the US Department of Energy, Hanford site, WA. The experimental results were used to estimate the apparent rate of U(VI) surface complexation at the grain-scale and in porous media. Numerical simulations using molecular, pore-scale, and continuum models were performed to provide insights into and to estimate the rate constants of U(VI) surface complexation at the different scales. The results showed that the grain-scale rate constant of U(VI) surface complexation was over 3 to 10 orders of magnitude smaller, dependent on the temporal scale, than the rate constant calculated using the molecular simulations. The grain-scale rate was faster initially and slower with time, showing the temporal scale-dependency. The largest rate constant at the grain-scale decreased additional 2 orders of magnitude when the rate was scaled to the porous media in the column. The scaling effect from the grain-scale to the porous media became less important for the slower sorption sites. Pore-scale simulations revealed the importance of coupled mass transport and reactions in both intragranular and inter-granular domains, which caused both spatial and temporal dependence of U(VI) surface complexation rates in the sediment. Pore-scale simulations also revealed a new rate-limiting mechanism in the intragranular porous domains that the rate of coupled diffusion and surface complexation reaction was slower than either process alone. The results provided important implications for developing models to scale geochemical/biogeochemical reactions.

Liu, Chongxuan; Shang, Jianying; Kerisit, Sebastien N.; Zachara, John M.; Zhu, Weihuang

2013-03-15

278

Quantum statistical study of O + O2 isotopic exchange reactions: cross sections and rate constants.  

PubMed

Using a wave packet based statistical model, we compute cross sections and thermal rate constants for various isotopic variants of the O + O2 exchange reaction on a recently modified ab initio potential energy surface. The calculation predicts a highly excited rotational distribution and relatively cold vibrational distribution for the diatomic product. A small but important threshold effect was identified for the (16)O + 18O2 reaction, which is suggested to contribute to the experimentally observed negative temperature dependence of the rate ratio, k(18O + 16O2)/k(16O + 18O2). Despite reasonable agreement with quasiclassical trajectory results, however, the calculated thermal rate constants are smaller than experimental measurements by a factor from 2 to 5. The experimentally observed negative temperature dependence of the rate constants is not reproduced. Possible reasons for the theory-experiment discrepancies are discussed. PMID:16623456

Lin, Shi Ying; Guo, Hua

2006-04-27

279

Acid-base chemical reaction model for nucleation rates in the polluted atmospheric boundary layer  

PubMed Central

Climate models show that particles formed by nucleation can affect cloud cover and, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmospheric boundary layer are positively correlated with concentrations of sulfuric acid vapor. However, current nucleation theories do not correctly predict either the observed nucleation rates or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach for modeling nucleation rates, based on a sequence of acid–base reactions. The model uses empirical estimates of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model predicts that nucleation rates equal the sulfuric acid vapor collision rate times a prefactor that is less than unity and that depends on the concentrations of basic gaseous compounds and preexisting particles. Predicted nucleation rates and their dependence on sulfuric acid vapor concentrations are in reasonable agreement with measurements from Mexico City and Atlanta.

Chen, Modi; Titcombe, Mari; Jiang, Jingkun; Jen, Coty; Kuang, Chongai; Fischer, Marc L.; Eisele, Fred L.; Siepmann, J. Ilja; Hanson, David R.; Zhao, Jun; McMurry, Peter H.

2012-01-01

280

A quantum statistical analysis of the rate constant for the HO 2 + NH 2 reaction  

Microsoft Academic Search

The radical-radical addition reaction of NH2 with HO2 to form energised adducts, namely, NH2OOH (A) and its isomers NH2(OH)O (B) and HN(OH)2 (C) has been analysed in the framework of quantum statistical Rice-Ramsperger-Kassel theory using calculated potential energy surfaces. The contribution of the HNO formation rate constant (KB1 and KC1) to the total rate constant (KT) for the disappearance of

R. Sumathi; S. D. Peyerimhoff

1996-01-01

281

LMFBR (LIQUID METAL FAST BREEDER REACTOR) REACTION RATE AND DOSIMETRY QUARTERLY PROGRESS REPORT JUNE JULY AUGUST 1971  

Microsoft Academic Search

This report describes progress in the USAEC-sponsored Interlaboratory LMFBR Reaction Rate (ILRR) program. This program has been established by RDT to develop a capability to accurately measure neutron-induced reaction rates for LMFBR fuels and materials development programs. The initial goal for the principal fission reactions, ²³U, ²³U, and ²³Pu, is an accuracy to within {+-} 5 at the 95% confidence

MCELROY WN

1971-01-01

282

Absolute rate coefficient and mechanism of gas phase reaction of ketenyl radical and SO2.  

PubMed

The kinetics of the gas phase reaction of the ketenyl radical with SO(2) was investigated over the temperature range 296-568 K using a laser-photofragment/laser-induced fluorescence technique (LP/LIF). The reactor pressure was 10 Torr N(2) or He. Pulsed photolysis of ketene (CH(2)CO) at 193 nm was used as the source of HCCO radicals. The rate coefficient for the title reaction was determined to be described by k(T) = (1.05 ± 0.33) × 10(-12) exp[(690 ± 98)K/T] cm(3) s(-1) molecule(-1) (2? error). We applied the coupled cluster and density functional theory to explore the mechanism of the title reaction. The dominant reaction pathway begins with a barrierless association of the C of the CH group of HCCO and the O atom of SO(2). PMID:23020066

Du, Lin; Carl, Shaun A

2012-10-18

283

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

NASA Technical Reports Server (NTRS)

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.

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

1987-01-01

284

Thermonuclear Reaction Rate Libraries and Software Tools for Nuclear Astrophysics Research  

SciTech Connect

Thermonuclear reaction rates are a crucial input for simulating a wide variety of astrophysical environments. A new collaboration has been formed to ensure that astrophysical modelers have access to reaction rates based on the most recent experimental and theoretical nuclear physics information. To reach this goal, a new version of the REACLIB library has been created by the Joint Institute for Nuclear Astrophysics (JINA), now available online at http://www.nscl.msu.edu/{approx}nero/db. A complementary effort is the development of software tools in the Computational Infrastructure for Nuclear Astrophysics, online at nucastrodata.org, to streamline, manage, and access the workflow of the reaction evaluations from their initiation to peer review to incorporation into the library. Details of these new projects will be described.

Smith, Michael S. [Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831-6354 (United States); Cyburt, Richard; Schatz, Hendrik; Smith, Karl; Warren, Scott; Ferguson, Ryan [Dept. of Physics and Astronomy, and National Superconducting Cyclotron Lab., Michigan State Univ., East Lansing, Michigan, 48824-1321 (United States); Wiescher, Michael [Dept. of Physics, Univ. of Notre Dame, Notre Dame, Indiana, 46556-5770 (United States); Lingerfelt, Eric; Buckner, Kim; Nesaraja, Caroline D. [Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831-6354 (United States); Dept. of Physics and Astronomy, Univ. of Tennessee, Knoxville, Tennessee, 37996-1200 (United States)

2008-05-21

285

Thermonuclear Reaction Rate Libraries and Software Tools for Nuclear Astrophysics Research  

SciTech Connect

Thermonuclear reaction rates are a crucial input for simulating a wide variety of astrophysical environments. A new collaboration has been formed to ensure that astrophysical modelers have access to reaction rates based on the most recent experimental and theoretical nuclear physics information. To reach this goal, a new version of the REACLIB library has been created by the Joint Institute for Nuclear Astrophysics (JINA), now available online at http://www.nscl.msu.edu/~nero/db. A complementary effort is the development of software tools in the Computational Infrastructure for Nuclear Astrophysics, online at nucastrodata.org, to streamline, manage, and access the workflow of the reaction evaluations from their initiation to peer review to incorporation into the library. Details of these new projects will be described.

Smith, Michael Scott [ORNL; Cyburt, Richard [Michigan State University, East Lansing; Schatz, Hendrik [Michigan State University, East Lansing; Wiescher, Michael [University of Notre Dame, IN; Smith, Karl [Michigan State University, East Lansing; Warren, Scott [Michigan State University, East Lansing; Ferguson, Ryan [Michigan State University, East Lansing; Lingerfelt, Eric J [ORNL; Buckner, Kim [ORNL; Nesaraja, Caroline D [ORNL

2008-01-01

286

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

NASA Technical Reports Server (NTRS)

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.

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

1986-01-01

287

A large nonequilibrium effect of decrease of the bimolecular chemical reaction rate in a dilute gas  

NASA Astrophysics Data System (ADS)

The perturbation solution of the Boltzmann equation is used to analyze the relative decrease ?( T) of the rate constant k( T) of a chemical reaction in a dilute gas in the temperature T. The Prigogine-Xhrouet model of the reactive cross-section is introduced for the reaction A+A?B+B. A large effect for ?( T) equal even nearly 75% ( for a small reduced threshold energy ?0 and a large molar fraction xB) follows from the analytical solution for k( T). A replacement of T by the Shizgal-Karplus nonequilibrium temperatures TA and TB in the equilibrium rate constants (for the forward and reverse reactions) gives ?( TA, TB) confirming this effect.

Cukrowski, A. S.; Fritzsche, S.; Cukrowski, M. J.

2003-09-01

288

Estimating rate constants in a convection-diffusion system with a boundary reaction  

NASA Astrophysics Data System (ADS)

While performing biomolecular interaction analysis (BIA), scientists often use surface plasmon resonance (SPR) to measure rate constants of the associated reactions. A mathematical model of a BIAcoreTM, a common SPR device, consists of a convection-diffusion equation in a channel with a reacting surface at the channel ceiling. Asymptotic and singular perturbation techniques are used to analyse the concentration of the reacting species in two cases: when the reaction occurs much more slowly than diffusion, and when the reaction occurs on the same time-scale as diffusion. Linear and nonlinear integral equations result from the analysis; explicit and asymptotic solutions are constructed for physically realizable cases. These expressions provide a direct way to estimate the rate constants from raw data.

Edwards, D. A.

1999-08-01

289

Biochemistry on a leash: Confinement as a regulatory mechanism for bimolecular reaction rates  

NASA Astrophysics Data System (ADS)

We describe two mechanisms by which confinement regulates diffusion-limited bimolecular reaction rates. The first mechanism, illustrated by the actin capping protein formin, uses a flexible polymer to tether ligand binding sites, which serve as intermediaries, to the reactive site. The second mechanism uses a potential (e.g. hard wall potential), to constrain the motion of a ligand receptor within a confining volume. We analyze both mechanisms theoretically, using a combination of analytic and numerical techniques, to obtain the steady state binding kinetics. We explore how the reaction rates are regulated by parameters of the model such as the length of the polymer tether, and use our findings to explain the key features of the formin system. Finally, we suggest other systems, both synthetic and biological, in which these mechanisms for regulating bimolecular reactions might be at play.

Reeves, Daniel; Cheveralls, Keith; Kondev, Jane

2009-03-01

290

Measurement of proton transfer reaction rates in a microwave cavity discharge flowing afterglow  

NASA Astrophysics Data System (ADS)

The reaction rate coefficients between the hydronium ion and the molecules ethene (C2H4), propene (C 3H6), 1-butene (C4H8) and hydrogen sulfide (H2S) were measured at 296 K. The measured reaction rates were compared to collision rates calculated using average dipole orientation (ADO) theory. Reaction efficiency depends primarily upon the proton affinity of the molecules. All the measurements were obtained using the newly developed microwave cavity discharge flowing afterglow (MCD-FA) apparatus. This device uses an Asmussen-type microwave cavity discharge ion source that is spatially separated from the flow tube, eliminating many of the problems inherent with the original FA devices. In addition to measuring reaction rate coefficients, the MCD-FA was shown to be an effective tool for measuring trace compounds in atmospheric air. This method has many advantages over current detection techniques since compounds can be detected in almost real time, large mass ranges can be scanned quickly, and repeated calibration is not required. Preliminary measurements were made of car exhaust and exhaled alveolar air. Car exhaust showed the presence of numerous hydrocarbons, such as butene, benzene and toluene while the exhaled alveolar air showed the presence of various volatile organic compounds such as methanol and acetone.

Brooke, George M., IV

291

Chlorine transfer between glycine, taurine, and histamine: reaction rates and impact on cellular reactivity  

Microsoft Academic Search

Hypochlorous acid formed by activated neutrophils reacts with amines to produce chloramines. Chloramines vary in stability, reactivity, and cell permeability. We have examined whether chloramine exchange occurs between physiologically important amines or amino acids and if this affects interactions of chloramines with cells. We have demonstrated transchlorination reactions between histamine, glycine, and taurine chloramines by measuring chloramine decay rates with

Alexander V. Peskin; Robyn G. Midwinter; David T. Harwood; Christine C. Winterbourn

2004-01-01

292

Determination of the Rate Coefficient for the N 2 + + O Reaction in the Ionosphere  

Microsoft Academic Search

Using approximately 400 simultan- eous measurements of ion and neutral densities and temperatures, and the spectrum of the solar flux measured by the Atmosphere Explorer C satel- lite, we have determined the rate constant k 1 for the reaction between N2 + and 0 in the ionosphere for ion temperatures between 600 ø and 700øK. We find that k 1

D. G. Torr; N. Orsini; M. R. Torr; W. B. Hanson; J. H. Hoffman; J. C. G. Walker

1977-01-01

293

Assessment of the expansion related to alkali-silica reaction by the Damage Rating Index method  

Microsoft Academic Search

Among all the quantitative methods that have been proposed for the quantification of the internal deterioration caused by alkali-silica reaction (ASR), the Damage Rating Index (DRI) appears to be one of the most valuable. This paper investigates the correlation between the measured expansion caused by ASR on laboratory-concrete prisms and the damage to concrete, as quantified by the DRI on

Patrice Rivard; Gérard Ballivy

2005-01-01

294

Evaluation of the Factors that Control the Time-Dependent Inactivation Rate Coefficients of Bacteriophage MS2 and PRD1  

NASA Astrophysics Data System (ADS)

Batch experiments were conducted under both static and dynamic conditions to study the effects of temperature and the presence of sand on the inactivation process of viruses. The male--specific RNA coliphage, MS2, and the Salmonella typhimurium phage, PRD1, were used as model viruses for this study. Over 100 oven--baked borosilicate glass bottles with or without Monterey sand were filled with a low--ionic--strength phosphate buffered saline solution containing both bacteriophage and incubated at temperatures of 4o, 15o, or 25oC. The results of the batch experiments indicate that the inactivation process can be represented by a pseudo first-order expression with time--dependent rate coefficients. A combination of high temperature and the presence of sand appears to produce the greatest disruption to the surrounding protein coat of MS2. However, for PRD1, the lower activation energies derived from Arrhenius plots indicate a weaker dependence of the inactivation rate on temperature. Furthermore, the presence of an air--liquid--solid interface in the dynamic batch experiment containing sand produces the greatest damage to specific viral components of PRD1 that are required for infection. These results indicate the use of thermodynamic parameters based on the pseudo first--order inactivation expression allows better prediction of the inactivation of viruses in the environment.

Anders, R.; Chrysikopoulos, C. V.

2004-12-01

295

Determination of the (24)Mg(p,gamma)(25)Al reaction rate at low stellar temperatures  

NASA Astrophysics Data System (ADS)

Recent observations of isotopic abundances in the atmospheres of M13 red giant branch stars show an anticorrelation between Al and 24Mg abundances. These correlations are thought to be the result of deep mixing of material from the surface of the star into regions where the temperature is high enough to allow proton captures to take place. Production of Al and destruction of 24Mg depend on the 24Mg( p, ?)25Al reaction rate at stellar temperatures T9 ~ 0.04 (where T9 denotes temperature in units of 109 K). However, the commonly used rate is too low to account for the observations within the present mixing models. There exists an uncertainty in the total width ? of the ER = 223-keV (Ex = 2485 keV) resonance in the 24Mg(p,?)25Al reaction which could lead to enhancements in the reaction rate up to a factor of 32 at T9 = 0.04. The main goal of this thesis is to accurately determine the properties of the ER = 223-keV resonance in order to improve the reaction rate estimate for 24Mg(p, ?) 25Al. Resonance strength ??-, ?-ray branching ratios ? ?/?, and mean lifetimes ? were determined for the E R = 223- and 419-keV resonances in 24Mg(p, ?) 25Al. These parameters allow for the determination of proton partial widths ?p, ?-ray partial widths ? ?, and total widths ? necessary for the calculation of the resonant cross section at Ep < 500 keV. The rate for the 24Mg(p, ?)25Al reaction for temperatures of T9 = 0.02 - 2 was calculated. The reaction rate deviates from the previous estimates by 18% to 45%, and therefore, the total width of the ER = 223-keV resonance does not have a significant influence on the reaction rate. In addition, branching ratios were determined for the 223-, 419-, 1616-, and 1654-keV resonances in 24Mg(p, ?) 25Al, and for the 406-keV resonance in 27Al( p, ?)28Si. Spectroscopic factors were determined from the proton partial widths for the states corresponding to the E R = 223- and 419-keV resonances. Resonance strength values were also determined for the 435-keV resonance in 25Mg(p, ?) 26 Al, the 336- and 454-keV resonances in 26Mg( p, ?)27Al, and the 406keV resonance in 27Al(p, ?)28Si.

Powell, Denise Catherine

296

First Results of Reaction Propagation Rates in HMX at High Pressure  

NASA Astrophysics Data System (ADS)

We have measured the reaction propagation rate (RPR) in octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) powder in a diamond anvil cell over the pressure range 0.7-35 GPa. In order to have a cross-comparison of our experiments, we conducted RPR experiments on nitromethane (NM) up to 15 GPa. Our results on NM are indistinguishable from previous measurements of Rice and Foltz. In comparison to high-pressure NM, the burn rates in solid HMX are 5-10 times faster at pressures above 10 GPa. Numerical simulations of the burn rate of pressurized HMX were also performed for comparison to the results obtained. The simulated burn rates closely approximate the observed rates at pressures up to 3 GPa. However, further refinement to the computational model is required for the calculated burn rates to approach those observed at higher pressures.

Farber, Daniel L.; Esposito, Anthony P.; Zaug, Joseph M.; Reaugh, John E.; Aracne, Chantel M.

2002-07-01

297

Latitudinal and voltinism compensation shape thermal reaction norms for growth rate.  

PubMed

Latitudinal variation in thermal reaction norms of key fitness traits may inform about the response of populations to climate warming, yet their adaptive nature and evolutionary potential are poorly known. We assessed the contribution of quantitative genetic, neutral genetic and environmental effects to thermal reaction norms of growth rate for populations of the damselfly Ischnura elegans. Among populations, reaction norms differed primarily in elevation, suggesting that time constraints associated with shorter growth seasons in univoltine, high-latitude as well as multivoltine, low-latitude populations selected for faster growth rates. Phenotypic divergence among populations is consistent with selection rather than drift as Q(ST) was greater than F(ST) in all cases. Q(ST) estimates increased with experimental temperature and were influenced by genotype by environment interactions. Substantial additive genetic variation for growth rate in all populations suggests that evolution of trait means in different environments is not constrained. Heritability of growth rates was higher at high temperature, driven by increased genetic rather than environmental variance. While environment-specific nonadditive effects also may contribute to heritability differences among temperatures, maternal effects did not play a significant role (where these could be accounted for). Genotype by environment interactions strongly influenced the adaptive potential of populations, and our results suggest the potential for microevolution of thermal reaction norms in each of the studied populations. In summary, the observed latitudinal pattern in growth rates is adaptive and results from a combination of latitudinal and voltinism compensation. Combined with the evolutionary potential of thermal reaction norms, this may affect populations' ability to respond to future climate warming. PMID:21689189

Shama, Lisa N S; Campero-Paz, Melina; Wegner, K Mathias; DE Block, Marjan; Stoks, Robby

2011-07-01

298

Mixing effects on apparent reaction rates and isotope fractionation during denitrification in a heterogeneous aquifer  

USGS Publications Warehouse

Gradients in contaminant concentrations and isotopic compositions commonly are used to derive reaction parameters for natural attenuation in aquifers. Differences between field-scale (apparent) estimated reaction rates and isotopic fractionations and local-scale (intrinsic) effects are poorly understood for complex natural systems. For a heterogeneous alluvial fan aquifer, numerical models and field observations were used to study the effects of physical heterogeneity on reaction parameter estimates. Field measurements included major ions, age tracers, stable isotopes, and dissolved gases. Parameters were estimated for the O2 reduction rate, denitrification rate, O 2 threshold for denitrification, and stable N isotope fractionation during denitrification. For multiple geostatistical realizations of the aquifer, inverse modeling was used to establish reactive transport simulations that were consistent with field observations and served as a basis for numerical experiments to compare sample-based estimates of "apparent" parameters with "true" (intrinsic) values. For this aquifer, non-Gaussian dispersion reduced the magnitudes of apparent reaction rates and isotope fractionations to a greater extent than Gaussian mixing alone. Apparent and true rate constants and fractionation parameters can differ by an order of magnitude or more, especially for samples subject to slow transport, long travel times, or rapid reactions. The effect of mixing on apparent N isotope fractionation potentially explains differences between previous laboratory and field estimates. Similarly, predicted effects on apparent O2 threshold values for denitrification are consistent with previous reports of higher values in aquifers than in the laboratory. These results show that hydrogeological complexity substantially influences the interpretation and prediction of reactive transport. ?? 2010 by the American Geophysical Union.

Green, C. T.; Bohlke, J. K.; Bekins, B. A.; Phillips, S. P.

2010-01-01

299

Quasiequilibrium Treatment of Gas-Solid Reactions. III. Rate of Volatilization of Tungsten by High-Temperature Oxidation.  

National Technical Information Service (NTIS)

Theoretical predictions computed on the basis of the quasiequilibrium treatment of gas-solid reactions are compared with existing experimental data on the rate of volatilization (erosion) of solid tungsten by reaction with gaseous O2 at high temperature (...

J. C. Batty R. E. Stickney

1971-01-01

300

Rates of grain boundary diffusion through enstatite and forsterite reaction rims  

Microsoft Academic Search

The growth rates of enstatite rims produced by reaction of Fo92 and SiO2 were determined at 250–1500 MPa and 900–1100°C for a wide range of water contents. Growth rates were also determined for\\u000a forsterite rims between MgO and Mg2Si2O6 and between MgO and SiO2. Rim growth rates are parabolic indicating diffusion-controlled growth of the polycrystalline rims which are composed of?˜?2

Richard A. Yund

1997-01-01

301

A refined ring polymer molecular dynamics theory of chemical reaction rates  

NASA Astrophysics Data System (ADS)

We further develop the ring polymer molecular dynamics (RPMD) method for calculating chemical reaction rates [I. R. Craig and D. E. Manolopoulos, J. Chem. Phys. 122, 084106 (2005)]. We begin by showing how the rate coefficient we obtained before can be calculated in a more efficient way by considering the side functions of the ring-polymer centroids, rather than averaging over the side functions of the individual ring-polymer beads. This has two distinct advantages. First, the statistics of the phase-space average over the ring-polymer coordinates and momenta are greatly improved. Second, the resulting flux-side correlation function converges to its long-time limit much more rapidly. Indeed the short-time limit of this flux-side correlation function already provides a ``quantum transition state theory'' approximation to the final rate coefficient. In cases where transition state recrossing effects are negligible, and the transition state dividing surface is put in the right place, the RPMD rate is therefore obtained almost instantly. We then go on to show that the long-time limit of the new flux-side correlation function, and hence the fully converged RPMD reaction rate, is rigorously independent of the choice of the transition state dividing surface. This is especially significant because the optimum dividing surface can often be very difficult to determine for reactions in complex chemical systems.

Craig, Ian R.; Manolopoulos, David E.

2005-07-01

302

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

NASA Technical Reports Server (NTRS)

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.

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

1979-01-01

303

Absolute and relative rate constants for the reactions of OH and Cl with pentanols  

NASA Astrophysics Data System (ADS)

The rate constants for the reactions of OH with three pentanols have been determined in the range 267-373 K and P = 100 Torr. The data obtained were (in units of cm3molecule-1s-1): k(1-pentanol) = (6.7 ± 3.8) × 10-12 exp [(132 ± 176)/T], k(2-pentanol) = (5.2 ± 1.8) × 10-12 exp [(218 ± 116)/T], k(3-pentanol) = (5.8 ± 2.3) × 10-12 exp [(164 ± 118)/T]. The present work provides the first temperature dependence data on these reactions. In addition, using the relative rate method, the rate constants for OH and Cl with these pentanols have been measured. The results are compared with the literature data and discussed with respect to atmospheric chemistry.

Lendar, M.; Aissat, A.; Cazaunau, M.; Daële, V.; Mellouki, A.

2013-09-01

304

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

PubMed

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. Copyright © 2014 John Wiley & Sons, Ltd. PMID:24639024

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

2014-06-01

305

Reaction rate kinetics for in situ combustion retorting of Michigan Antrim oil shale  

USGS Publications Warehouse

The intrinsic reaction rate kinetics for the pyrolysis of Michigan Antrim oil shale and the oxidation of the carbonaceous residue of this shale have been determined using a thermogravimetric analysis method. The kinetics of the pyrolysis reaction were evaluated from both isothermal and nonisothermal rate data. The reaction was found to be second-order with an activation energy of 252.2 kJ/mole, and with a frequency factor of 9.25 ?? 1015 sec-1. Pyrolysis kinetics were not affected by heating rates between 0.01 to 0.67??K/s. No evidence of any reactions among the oil shale mineral constituents was observed at temperatures below 1173??K. However, it was found that the presence of pyrite in oil shale reduces the primary devolatilization rate of kerogen and increases the amount of residual char in the spent shale. Carbonaceous residues which were prepared by heating the oil shale at a rate of 0.166??K/s to temperatures between 923??K and 1073??K, had the highest reactivities when oxidized at 0.166??K/s in a gas having 21 volume percent oxygen. Oxygen chemisorption was found to be the initial precursor to the oxidation process. The kinetics governing oxygen chemisorption is (Equation Presented) where X is the fractional coverage. The oxidation of the carbonaceous residue was found also to be second-order. The activation energy and the frequency factor determined from isothermal experiments were 147 kJ/mole and 9.18??107 sec-1 respectively, while the values of these parameters obtained from a nonisothermal experiment were 212 kJ/mole and 1.5??1013 sec-1. The variation in the rate constants is attributed to the fact that isothermal and nonisothermal analyses represent two different aspects of the combustion process.

Rostam-Abadi, M.; Mickelson, R. W.

1984-01-01

306

Low-temperature rate coefficients for the reaction of ethynyl radical (C2H) with benzene.  

PubMed

The reaction of the C2H radical with benzene is studied at low temperature using a pulsed Laval nozzle apparatus. The C2H radical is prepared by 193-nm photolysis of acetylene, and the C2H concentration is monitored using CH(A2Delta) chemiluminescence from the C2H + O2 reaction. Measurements at very low photolysis energy are performed using CF3C2H as the C2H precursor to study the influence of benzene photodissociation on the rate coefficient. Rate coefficients are obtained over a temperature range between 105 and 298 K. The average rate coefficient is found to be five times greater than the estimated value presently used in the photochemical modeling of Titan's atmosphere. The reaction exhibits a slight negative temperature dependence which can be fitted to the expression k(cm3 molecule(-1) s(-1)) = 3.28(+/-1.0) x 10(-10) (T/298)(-0.18(+/-0.18)). The results show that this reaction has no barrier and may play an important role in the formation of large molecules and aerosols at low temperature. Our results are consistent with the formation of a short lifetime intermediate that decomposes to give the final products. PMID:16451020

Goulay, Fabien; Leone, Stephen R

2006-02-01

307

Aqueous Complexation Reactions Governing the Rate and Extent of Biogeochemical U(VI) Reduction  

SciTech Connect

The proposed research will elucidate the principal biogeochemical reactions that govern the concentration, chemical speciation, and reactivity of the redox-sensitive contaminant uranium. The results will provide an improved understanding and predictive capability of the mechanisms that govern the biogeochemical reduction of uranium in subsurface environments. In addition, the work plan is designed to: (1) Generate fundamental scientific understanding on the relationship between U(VI) chemical speciation and its susceptibility to biogeochemical reduction reactions. ? Elucidate the controls on the rate and extent of contaminant reactivity. (2) Provide new insights into the aqueous and solid speciation of U(VI)/U(IV) under representative groundwater conditions.

Scott C. Brooks; Wenming Dong; Sue Carroll; Jim Fredrickson; Ken Kemner; Shelly Kelly

2006-06-01

308

Aqueous Complexation Reactions Governing the Rate and Extent of Biogeochemical U(VI) Reduction  

SciTech Connect

The proposed research will elucidate the principal biogeochemical reactions that govern the concentration, chemical speciation, and reactivity of the redox-sensitive contaminant uranium. The results will provide an improved understanding and predictive capability of the mechanisms that govern the biogeochemical reduction of uranium in subsurface environments. In addition, the work plan is designed to: (1) Generate fundamental scientific understanding on the relationship between U(VI) chemical speciation and its susceptibility to biogeochemical reduction reactions. (2) Elucidate the controls on the rate and extent of contaminant reactivity. (3) Provide new insights into the aqueous and solid speciation of U(VI)/U(IV) under representative groundwater conditions.

Scott C. Brooks; Wenming Dong; Sue Carroll; James K. Fredrickson; Kenneth M. Kemner; Shelly D. Kelly

2006-06-01

309

Determination of the rate coefficient for the N2/+/ + O reaction in the ionosphere  

NASA Technical Reports Server (NTRS)

Using approximately 400 simultaneous measurements of ion and neutral densities and temperatures, and the spectrum of the solar flux measured by the Atmosphere Explorer C satellite, we have determined the rate constant k1 for the reaction between N2(+) and O in the ionosphere for ion temperatures between 600 and 700 K. We find that k1 = 1.1 x 10 to the minus 10th power cu cm per sec, with a standard deviation of + or - 15%. If we use the temperature dependence for this reaction determined in the laboratory then at 300 K we find excellent agreement with the recommended laboratory value.

Torr, D. G.; Torr, M. R.; Orsini, N.; Hanson, W. B.; Hoffman, J. H.; Walker, J. C. G.

1977-01-01

310

Algorithm to evaluate rate constants for polyatomic chemical reactions. I. Theory and computational details.  

PubMed

A classical reaction path, Liouville algorithm, as previously developed by the authors, has been reformulated in a more efficient form, based on the time-dependent first integral method from the theory of partial differential equations. Both the theory and computational details are presented. The formulation is based on solving the complete rate constant problem, concerning both the density distribution as well as the reactive flux, by means of solving the classical Liouville equation. The numerical implementation is discussed in detail, including some specifics concerning the reaction path calculation. The formulation is tested with the well-known Müller-Brown bidimensional potential energy surface. PMID:17450549

González, Javier; Giménez, Xavier; Bofill, Josep Maria

2007-10-01

311

A new theoretical approach to thermonuclear radiative-capture reaction rate  

SciTech Connect

We propose a new computational method for astrophysical reaction rate of radiative capture process, which does not require any solution of scattering problem. It is tested for twobody radiative caputure reaction {sup 16}O({alpha},{gamma}){sup 20}Ne and a comparison is made with an ordinary method solving two-body scattering problem. The method is shown to work well in practice and thus will be useful for problems in which an explicit construction of scattering solution is difficult such as the triple-alpha capture process.

Funaki, Yasuro; Yabana, Kazuhiro; Akahori, Takahiko [Nishina Center for Accelerator-Based Science, Institute of Physical and Chemical Research (RIKEN), Wako 351-0198 (Japan); Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8571 (Japan) and Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571 (Japan); Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571 (Japan)

2012-11-12

312

Rates for neutron-capture reactions on tungsten isotopes in iron meteorites. [Abstract only  

NASA Technical Reports Server (NTRS)

High-precision W isotopic analyses by Harper and Jacobsen indicate the W-182/W-183 ratio in the Toluca iron meteorite is shifted by -(3.0 +/- 0.9) x 10(exp -4) relative to a terrestrial standard. Possible causes of this shift are neutron-capture reactions on W during Toluca's approximately 600-Ma exposure to cosmic ray particles or radiogenic growth of W-182 from 9-Ma Hf-182 in the silicate portion of the Earth after removal of W to the Earth's core. Calculations for the rates of neutron-capture reactions on W isotopes were done to study the first possibility. The LAHET Code System (LCS) which consists of the Los Alamos High Energy Transport (LAHET) code and the Monte Carlo N-Particle(MCNP) transport code was used to numerically simulate the irradiation of the Toluca iron meteorite by galactic-cosmic-ray (GCR) particles and to calculate the rates of W(n, gamma) reactions. Toluca was modeled as a 3.9-m-radius sphere with the composition of a typical IA iron meteorite. The incident GCR protons and their interactions were modeled with LAHET, which also handled the interactions of neutrons with energies above 20 MeV. The rates for the capture of neutrons by W-182, W-183, and W-186 were calculated using the detailed library of (n, gamma) cross sections in MCNP. For this study of the possible effect of W(n, gamma) reactions on W isotope systematics, we consider the peak rates. The calculated maximum change in the normalized W-182/W-183 ratio due to neutron-capture reactions cannot account for more than 25% of the mass 182 deficit observed in Toluca W.

Masarik, J.; Reedy, R. C.

1994-01-01

313

Kinetics and mechanism of molecular A-frame formation in reactions of (Pt/sub 2/(PPh/sub 3/)/sub 2/(. mu. -dppm)/sub 2/)(PF/sub 6/)/sub 2/ with diazomethane, carbon monoxide, sulfur dioxide, sulfur, and hydrogen chloride  

SciTech Connect

The mechanism of the formation of A frames of the type (Pt/sub 2/(PPh/sub 3/ triphenylphosphine)/sub 2/(..mu..-Y)(..mu..-dppm)/sub 2/)(PF/sub 6/)/sub 2/ (y = CH/sub 2/, CO, S, SO/sub 2/) from (Pt/sub 2/(PPh/sub 3/)/sub 2/(..mu..-dppm)/sub 2/)(PF/sub 6/)/sub 2/ by reaction with CH/sub 2/N/sub 2/, CO, S, and SO/sub 2/ (R) in CH/sub 2/Cl/sub 2/ and other organic solvents has been investigated by using kinetic studies and product analysis. Pseudo-first-order kinetic data (excess (R)/sub 0/) are consistent with either preequilibrium association of R with the complex prior to rate-limiting product formation or unimolecular formation of a common intermediate which is then attacked by R to form the A-frame product. Kinetic studies with (R)/sub 0/ less than or equal to (complex)/sub 0/ substantiate the latter. The most likely intermediate leading to the products is one with a dangling dppm ligand formed by Pt-P bond heterolysis. When (R)/sub 0/ is very large, formation of this intermediate becomes rate limiting and is independent of the nature of R, with k = 2.73 +/- 0.05 x 10/sup -2/ s/sup -1/ (at 25/sup 0/C; ..delta..H = 25.0 kcal mol/sup -1/ and ..delta..S = 18.5 cal mol/sup -1/ K/sup -1/). The same process and mechanism is observed for R = HCl, although with minor numerical differences; similar salt and solvent polarity effects occur in all of the reactions (dppm-bis(diphenylphosphino)methane). 23 references, 1 figure, 3 tables.

Muralidharan, S.; Espenson, J.H.

1984-12-26

314

Enzymatic reaction of silent substrates: kinetic theory and application to the serine protease chymotrypsin.  

PubMed

Investigating the selectivity that an enzyme expresses toward its substrates can be technically challenging if reaction of these substrates is not accompanied by a conveniently monitored change in some physicochemical property. In this paper, we describe a simple method for determining steady-state kinetic parameters for enzymatic turnover of such "silent" substrates. According to this method, silent substrate S is allowed to compete for enzymic reaction with signal-generating substrate S*, whose conversion to product can be conveniently monitored. Full reaction progress curves are collected under conditions of [S*](o) < K(m)* and [S](o) >or= 3K(m). Progress curves collected under these conditions are characterized by an initial lag phase of duration tau that is followed by the pseudo-first-order reaction of S. Steady-state kinetic parameters for the silent substrate can be obtained by one of two methods. One method combines least-squares fitting with numerical integration of appropriate rate equations to analyze the progress curves, while the other method relies on direct graphical analysis in which K(m) is the value of [S](o) that reduces the control velocity by a factor of 2 and V(max) is shown to simply equal the ratio [S](o)/tau. We use these methods to analyze the alpha-chymotrypsin-catalyzed hydrolysis of silent substrate Suc-Ala-Phe-AlaNH(2) with signal generator Suc-Ala-Phe-pNA. From the curve-fitting method, k(c) = 0.9 +/- 0.2 s(-1) and K(m) = 0.4 +/- 0.1 mM, while by direct graphical analysis, k(c) = 1.1 +/- 0.1 s(-1) and K(m) = 0.51 +/- 0.03 mM. As validation of this new method, we show agreement of these values with those determined independently by HPLC analysis of the hydrolysis of Suc-Ala-Phe-AlaNH(2) by alpha-CT, where k(c) = 1.1 +/- 0.1 s(-1) and K(m) = 0.5 +/- 0.1 mM. PMID:12705836

Case, April; Huskey, W Phillip; Stein, Ross L

2003-04-29

315

On OH and O3 Reaction Rates with Isoprene-Derived Hydroxy Nitrates  

NASA Astrophysics Data System (ADS)

3 of the 8 primary isoprene hydroxy nitrate isomers ((E & Z)-2-methyl-4-nitrooxybut-2-ene-1-ol and 2-methyl-1-nitrooxybut-3-ene-2-ol) were synthesized. The purity of these isomers was confirmed with proton NMR spectroscopy. Oxidation of these unsaturated nitrates by OH and O3 were studied in an environmental chamber equipped with both Chemical Ionization Mass Spectrometer (CIMS) and Thermo-Dissociation Laser Induced Fluorescence (TD-LIF) instrument for continuous monitoring of reactants and products. The OH reaction rates of the individual isomers were measured relative to propene resulting in rate constants for each isomer, in the same order as above, of: 1.1×10-10, 1.1×10-10 and 4.2×10-11 cm3/s. For the O3 reaction rate constant of 2-methyl-1-nitrooxybut-3-ene-2-ol, we report an interval of (2.5~5)×10-19 cm3/s. The precision of our estimate on this very slow process was limited by the chamber wall loss. For (E) and (Z)-2-methyl-4-nitrooxybut-2-ene-1-ol the O3 rate constants are 2.7×10-17 and 2.9×10-17 cm3/s, respectively. Reaction pathways, product yields and implications for atmospheric chemistry are discussed.

Lee, L.; Teng, A.; Crounse, J.; Wennberg, P. O.; Cohen, R. C.

2013-12-01

316

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

PubMed

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. PMID:22421957

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

2012-04-28

317

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)

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.

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

1986-01-01

318

Rate coefficients of hydroxyl radical reactions with pesticide molecules and related compounds: A review  

NASA Astrophysics Data System (ADS)

Rate coefficients published in the literature on hydroxyl radical reactions with pesticides and related compounds are discussed together with the experimental methods and the basic reaction mechanisms. Recommendations are made for the most probable values. Most of the molecules whose rate coefficients are discussed have aromatic ring: their rate coefficients are in the range of 2×109-1×1010 mol-1 dm3 s-1. The rate coefficients show some variation with the electron withdrawing-donating nature of the substituent on the ring. The rate coefficients for triazine pesticides (simazine, atrazine, prometon) are all around 2.5×109 mol-1 dm3 s-1. The values do not show variation with the substituent on the s-triazine ring. The rate coefficients for the non-aromatic molecules which have C=C double bonds or several C-H bonds may also be above 1×109 mol-1 dm3 s-1. However, the values for molecules without C=C double bonds or several C-H bonds are in the 1×107-1×109 mol-1 dm3 s-1 range.

Wojnárovits, László; Takács, Erzsébet

2014-03-01

319

A transfer function approach to reaction rate analysis with applications to phase-change materials and devices  

NASA Astrophysics Data System (ADS)

A very good approximate, closed-form solution to the reaction rate equation with Arrhenius temperature dependence is derived, valid for activation energies E >> kBT0 (kB is Boltzmann constant and T0 is room temperature) and monotonically decreasing temperatures. This solution is then used to develop a transfer function description of the reaction rate equation, enabling the bandwidth of the reaction rate to be determined and related to the kinetic and thermophysical parameters of the medium. Applications of the transfer function approach to understanding and predicting reaction (i.e., crystallization) rates in phase-change materials and devices are discussed.

Aziz, M. M.; Wright, C. D.

2013-09-01

320

Rate coefficients for the reaction of iodine oxide with methyl peroxy radicals.  

PubMed

Pulsed laser photolysis radical generation is used to study the title reaction IO+CH(3)O(2)?products. Sensitive and selective laser-induced fluorescence detection of IO allows excess CH(3)O(2) conditions to be maintained throughout, ensuring minimal interference from other fast IO reactions. The rate coefficients, k(5)(296 K)=(3.4±1.4)×10(-12) cm(3) molecule(-1) s(-1), are obtained relative to a well-characterised reference value (k(3) for IO+HO(2)). This result agrees well with a previous determination from this laboratory and demonstrates that the above reaction proceeds an order of magnitude slower than suggested in other recent experimental and theoretical studies. Implications for HOx production/O(3) destruction within the marine boundary layer are briefly discussed. PMID:20963744

Dillon, Terry J; Tucceri, María E; Crowley, John N

2010-12-17

321

Irreversible bimolecular reactions with inertia: from the trapping to the target setting at finite densities.  

PubMed

We investigate numerically pseudo-first-order irreversible bimolecular reactions of the type A + B ? B between hard spheres undergoing event-driven Brownian dynamics. We study the encounter rate and the survival probability of A particles as functions of the packing fraction ? in the trapping (a single particle diffusing among static non-overlapping traps) and target (many traps diffusing in the presence of a single static target particle) settings, as well as in the case of diffusing traps and particles (full mobility). We show that, since inertial effects are accounted for in our simulation protocol, the standard Smoluchowski theory of coagulation of non-interacting colloids is recovered only at times greater than a characteristic time ?t, marking the transition from the under-damped to the over-damped regime. We show that the survival probability S(t) decays exponentially during this first stage, with a rate 1/?0 is proportional to ?. Furthermore, we work out a simple analytical expression that is able to capture to an excellent extent the numerical results for t < ?t at low and intermediate densities. Moreover, we demonstrate that the time constant of the asymptotic exponential decay of S(t) for diffusing traps and particles is k(S)(-1), where kS = 4?(DA + DB)R? is the Smoluchowski rate. Detailed analyses of the effective decay exponent ? = d [log(-logS(t))]/d (logt) and of the steady-state encounter rate reveal that the full mobility and trapping problem are characterized by very similar kinetics, rather different from the target problem. Our results do not allow one to ascertain whether the prediction S(t) is proportional to exp(-at(3/2)) (a = const.) as t ? ? for the trapping problem in 3D is indeed recovered. In fact, at high density, S(t) is dominated by short encounter times, which makes it exceedingly hard to record the events corresponding to the exploration of large, trap-free regions. As a consequence, at high densities the steady-state rate simply tends to 1/?0. Finally, we work out an analytical formula for the rate that shows a remarkable agreement with the numerics up ? = 0.4. PMID:23670209

Piazza, Francesco; Foffi, Giuseppe; De Michele, Cristiano

2013-06-19

322

Beyond transition state theory: Rigorous quantum approaches for determining chemical reaction rates  

SciTech Connect

Transition state theory (TST) has historically been the most important and widely used theoretical approach for describing the rates of chemical reactions, and for qualitative pictures and order-of-magnitude estimates one does not expect this situation to change. However a rigorous, quantitative treatment of chemical reaction rates must go beyond TST. A rigorous description, for example, must be based on a quantum mechanical description of the molecular system, but the fundamental assumption on which TST is based - namely that the molecular dynamics is {open_quotes}direct,{close_quotes} i.e., that no trajectories re-cross a dividing surface which separates reactants and products (vide infra) - is couched inherently in the language of classical mechanics. There is no unambiguous way to quantize TST, for the various ways of trying to do so invariably require one to introduce additional assumptions about the reaction dynamics. As one tries to eliminate these {open_quotes}additional assumptions{close_quotes} one is driven ultimately to an exact quantum treatment of the reaction dynamics which is then no longer a transition state theory (i.e., approximation) but simply an exact formulation. It is such exact approaches, those without inherent approximations, that are the subject of this chapter.

Miller, W.H.

1995-01-01

323

Absolute rate of the reaction of bromine atoms with ozone from 200-360 K  

NASA Technical Reports Server (NTRS)

The rate constant for the reaction Br + O3 yields BrO + O2 was measured from 200 to 360 K by the technique of flash photolysis coupled to time resolved detection of bromine atoms by resonance fluorescence (FP-RF). Br atoms were produced by the flash photolysis of CH3Br at lambda 165nm.O3 was monitored continuously under reaction conditions by absorption at 253.7 nm. At each of five temperatures the results were independent of substantial variations in O3, total pressure and limited variations in flash intensity. The measured rate constants obeyed the Arrhenius expression, where the error quoted is two standard deviations. Results are compared with previous determinations which employed the discharge flow-mass spectrometric technique.

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

1978-01-01

324

High-pressure rate rules for alkyl + O2 reactions. 2. The isomerization, cyclic ether formation, and ?-scission reactions of hydroperoxy alkyl radicals.  

PubMed

The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O(2) molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the ?-, ?-, and ?-QOOH radicals: isomerization to RO(2), cyclic ether formation, and selected ?-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO(2)), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radical with O(2) by comparison of concentration versus time profiles that were obtained using two mechanisms at 10 atm: one that contains pressure-dependent rate constants that are obtained from a QRRK/MSC analysis and another that only contains high-pressure rate expressions. These simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2) and QOOH. For the same conditions, we also address whether the various isomers equilibrate during reaction. These results indicate that equilibrium is established between the alkyl, RO(2), and ?- and ?-QOOH radicals. PMID:22548467

Villano, Stephanie M; Huynh, Lam K; Carstensen, Hans-Heinrich; Dean, Anthony M

2012-05-31

325

Turnover rate, reaction order, and elementary steps for the hydrodechlorination of chlorofluorocarbon compounds on palladium catalysts  

SciTech Connect

The rates of hydrodechlorination catalyzed by Pd supported on carbon for four chlorofluorocarbons spanned a range of 7 orders of magnitude. The rates scaled up to the bond strength of the carbon-chlorine bond for the gas-phase reactant. This finding demonstrates that the rate-determining step involves the scission of the C-Cl bond and suggests, through Polanyi and linear free-energy relationships, that rates for other compounds can be estimated if the C-Cl bond strength is known. The reaction orders for the most abundant products are approximately first-order for the chlorine-containing compound, half-order in H{sub 2}, and inverse first-order in HCl. The reaction steps consistent with these orders include a rate-determining step involving the adsorption of the chlorofluorocarbon to a single site (which could be a single surface palladium atom) and equilibrated steps between gas-phase H{sub 2}, gas-phase HCl, and adsorbed hydrogen and chlorine atoms. The rates on the supported catalysts are comparable to the ones reported before on a Pd foil, indicating that the support does not play a role in the reaction. The product distribution is independent of conversion, implying that the various products are formed from a single visit of the reactant on the surface and not from readsorption of gas-phase products. The four compounds studied were chloropentafluoroethane (CF{sub 3}-CF{sub 2}Cl), 2-chloro-1,1,1,2-tetrafluoroethane (CF{sub 3}-CFClH), 1,1-dichlorotetrafluoroethane (CF{sub 3}-CFCl{sub 2}), and 1,1,1-trichloro-2,2,2-trifluoroethane (CF{sub 3}-CCl{sub 3}).

Thompson, C.D.; Rioux, R.M.; Chen, N.; Ribeiro, F.H.

2000-04-13

326

COAL CHAR GASIFICATION KINETICS IN A JET-FLUIDIZED BED (COMBUSTION, REACTION, RATE PHENOMENA)  

Microsoft Academic Search

The kinetics of gasification of -10 + 65 mesh coke and coal char with O(,2) and O(,2)-CO(,2) mixtures was studied in a continuous, bench scale, jet-fluidized reactor over a wide range of temperatures (900(DEGREES)C to > 1400(DEGREES)C) and bed depths under atmospheric pressure. The measured specific reaction rates of carbon ranged from 0.01 to 0.1 gram carbon reacting per gram

MING-YUAN KAO

1984-01-01

327

First Results of Reaction Propagation Rates in HMX at High Pressure  

Microsoft Academic Search

We have measured the reaction propagation rate (RPR) in octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) powder in a diamond anvil cell over the pressure range 0.7–35 GPa. In order to have a cross-comparison of our experiments, we conducted RPR experiments on nitromethane (NM) up to 15 GPa. Our results on NM are indistinguishable from previous measurements of Rice and Foltz. In comparison to high-pressure

Daniel L. Farber; Anthony P. Esposito; Joseph M. Zaug; John E. Reaugh; Chantel M. Aracne

2002-01-01

328

First Results of Reaction Propagation Rates in HMX at High Pressure  

Microsoft Academic Search

We have measured the reaction propagation rate (RPR) in octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) powder in a diamond anvil cell over the pressure range 0.7-35 GPa. In order to have a cross-comparison of our experiments, we conducted RPR experiments on nitromethane (NM) up to 15 GPa. Our results on NM are indistinguishable from previous measurements of Rice and Foltz. In comparison to high-pressure

Daniel L. Farber; Anthony P. Esposito; Joseph M. Zaug; John E. Reaugh; Chantel M. Aracne

2002-01-01

329

Absolute rate of the reaction of hydrogen atoms with ozone from 219-360 K  

NASA Technical Reports Server (NTRS)

Absolute rate constants for the reaction of atomic hydrogen with ozone were obtained over the temperature range 219-360 K by the flash photolysis-resonance fluorescence technique. The results can be expressed in Arrhenius form by K = (1.33 plus or minus 0.32)x10 to the minus 10 power exp (-449 plus or minus 58/T) cu cm/molecule/s (two standard deviations). The present work is compared to two previous determinations and is discussed theoretically.

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

1978-01-01

330

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

NASA Technical Reports Server (NTRS)

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.

Watson, R. T.

1977-01-01

331

19F? widths and the 18F+p reaction rates  

NASA Astrophysics Data System (ADS)

Understanding the properties of 19F levels in the range Ex=6.4-7.5MeV is important for constraining the contributions of 19Ne levels to the 18F(p,?)15O and 18F(p,?)19Ne thermonuclear reaction rates. We have reanalyzed 15N(?,?)15N data from H. Smotrichet al., Phys. Rev. to determine properties of 19F levels in this energy range. We find the energies and widths of broad levels to be different than previously reported and have set upper limits on the widths of postulated 3/2+ resonances, analogs of which are important for the 18F+p rates.

Bardayan, D. W.; Kozub, R. L.; Smith, M. S.

2005-01-01

332

The Rate of the Phosphorous Reaction Between Liquid Iron and Slag  

NASA Astrophysics Data System (ADS)

In the current study, the rates of dephosphorization and rephosphorization of liquid iron with simulated steelmaking slags were investigated at 1873 K (1600° C). The experiments were conducted in an induction furnace with supplemental heating to maintain a consistent temperature within both the metal and slag phases. An integrated form of the rate equation was used to evaluate the results, assuming mass transfer in both the slag and metal was rate controlling. The results of the current and previous studies indicate that the mass transfer parameter, the slag-metal surface area, and the overall mass transfer coefficient ( A* k 0), decreased as the reaction proceeded. It is proposed that initially when the rate and oxygen flux are high, the interfacial energy decreases, and the interfacial fluid velocity increases causing disruption of the slag metal interface. The consequent increases in interfacial area and interfacial fluid flow cause A* k 0 to be high initially and then decrease as the oxygen flux decreases.

Manning, Christopher P.; Fruehan, Richard J.

2013-02-01

333

Reaction rate and collisional efficiency of the rhodopsin-transducin system in intact retinal rods.  

PubMed Central

A model of transducin activation is constructed from its partial reactions (formation of metarhodopsin II, association, and dissociation of the rhodopsin-transducin complex). The kinetic equations of the model are solved both numerically and, for small photoactivation, analytically. From data on the partial reactions in vitro, rate and activation energy profile of amplified transducin turnover are modeled and compared with measured light-scattering signals of transducin activation in intact retinal rods. The data leave one free parameter, the rate of association between transducin and rhodopsin. Best fit is achieved for an activation energy of 35 kJ/mol, indicating lateral membrane diffusion of the proteins as its main determinant. The absolute value of the association rate is discussed in terms of the success of collisions to form the catalytic complex. It is greater than 30% for the intact retina and 10 times lower after permeabilization with staphylococcus aureus alpha-toxin. Dissociation rates for micromolar guanosinetriphosphale (GTP) (Kohl, B., and K. P. Hofmann, 1987. Biophys. J. 52:271-277) must be extrapolated linearly up to the millimolar range to explain the rapid transducin turnover in situ. This is interpreted by an unstable rhodopsin-transducin-GTP transient state. At the time of maximal turnover after a flash, the rate of activation is determined as 30, 120, 800, 2,500, and 4,000 activated transducins per photoactivated rhodopsin and second at 5, 10, 20, 30, 37 degrees C, respectively.

Kahlert, M; Hofmann, K P

1991-01-01

334

Foaming and the Rate of the Carbon-Iron Oxide Reaction in Slag  

NASA Astrophysics Data System (ADS)

Foaming in the electric arc furnace is achieved by injecting carbon into slag, where the resulting reaction of the carbon with FeO dissolved in the slag generates gas (CO) that causes the slag to foam. In this research, the rate of the reaction of FeO in slag with carbon and the resulting foam height were measured. In these experiments, the FeO content of the slag ranged from 15 to 45 mass pct, and several different types of carbon were used including graphite, coals, and chars. The rate of the slag-carbon reaction and the consequent CO generation increased with FeO content of the slag from 15 to 45 mass pct. However, the slag foam height reached a maximum at about 25 mass pct FeO and decreased at higher FeO contents. The decrease in foaming is apparently due to a decrease in the foam index or foamability caused by a decrease in viscosity and an increase in density of the slag with FeO content. The results of this work indicate that the foam height is influenced more significantly by the decrease in the foam index compared to the increase in the CO gas generation rate at higher FeO contents. The decrease in the foam index with FeO agrees with that predicted from the slag properties.

Corbari, Rodrigo; Matsuura, Hiroyuki; Halder, Sabuj; Walker, Matthew; Fruehan, Richard J.

2009-12-01

335

Rates and mechanisms of the atomic oxygen reaction with nickel at elevated temperatures  

NASA Technical Reports Server (NTRS)

The oxidation of nickel by atomic oxygen at pressure from 1 to 45 N/sq m between 1050 and 1250 K was investigated. In these ranges, the oxidation was found to follow the parobolic rate law, viz., K sub p = 0.0000114 exp(-13410/T) g squared/cm4/sec for films of greater than 1 micron thickness and was pressure independent. The activation enthalpy for the oxidation reaction was 112 + or - 11 kj/mole (27 + or - 3 kcal/mole). Of a number of possible mechanisms and defect structures considered, it was shown that the most likely was a saturated surface defect model for atomic oxidation, based on reaction activation enthalpies, impurity effects, pressure independence, and magnitudes of rates. A model judged somewhat less likely was one having doubly ionized cationic defects rate controlling in both atomic and molecular oxygen. From comparisons of the appropriate processes, the following enthalpy values were derived: enthalpy of activation (Ni diffusion in Ni0) = 110 + or - 30 kj/mole and standard enthalpy change for reaction formation (doubly ionized cation vacancies in Ni0 from atomic oxygen)= -9 + or - 25 kj/mole.

Christian, J. D.; Gilbreath, W. P.

1973-01-01

336

Material interactions with the low earth orbital environment Accurate reaction rate measurements  

NASA Technical Reports Server (NTRS)

Interactions between spacecraft surfaces and atomic oxygen within the low earth orbital (LEO) environment have been observed and measured during Space Shuttle flights over the past 3 yr. The results of these experiments have demonstrated that interaction rates for many materials proposed for spacecraft applications are high and that protective coatings must be developed to enable long-lived operation of spacecraft structures in the LEO environment. A flight experiment discussed herein uses the Space Shuttle as an orbiting exposure laboratory to obtain accurate reaction rate measurements for materials typically used in spacecraft construction. An ion-neutral mass spectrometer, installed in the Orbiter cargo bay, will measure diurnal ambient oxygen densities while material samples are exposed at low altitude (222 km) to the orbital environment. From in situ atomic oxygen density information and postflight material recession measurements, accurate reaction rates can be derived to update the Space Station materials interaction data base. Additionally, gases evolved from a limited number of material surfaces subjected to direct oxygen impingement will be identified using the mass spectrometer. These measurements will aid in mechanistic definitions of chemical reactions which cause atom-surface interactions and in validating results of upcoming degradation studies conducted in ground-based neutral beam laboratories.

Visentine, J. T.; Leger, L. J.

1985-01-01

337

Experimental and theoretical rate coefficients for the gas phase reaction of ?-Pinene with OH radical  

NASA Astrophysics Data System (ADS)

Rate coefficient for the reaction of ?-pinene with OH radicals was determined at 298 K and 800 Torr of N2 using the relative rate technique. Isobutylene was used as a reference compound and the concentrations of the organics were followed by gas chromatographic analysis. The rate coefficient for the reaction of ?-pinene with OH radical was measured to be (9.35 ± 2.79) × 10-11 cm3 molecule-1 s-1. Theoretical kinetic calculations were also performed for the title reaction using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT). The kinetics data obtained over the temperature range of 200-400 K were used to derive the Arrhenius expression: k(T) = 8.24 × 10-23T3.41 exp[2421/T] cm3 molecule-1 s-1. The OH-driven atmospheric lifetime (?) and global warming potential (GWP) for ?-pinene were computed and concluded that ?-pinene is very short lived (2.5 h) in the Earth's atmosphere with a GWP of 1.6 × 10-2 at 20 years horizon of time and which is negligible. The ozone formation potential of ?-pinene was also calculated and reported in this present work.

Dash, Manas Ranjan; Rajakumar, B.

2013-11-01

338

Gas-phase reaction of ( E)-?-farnesene with ozone: Rate coefficient and carbonyl products  

NASA Astrophysics Data System (ADS)

The gas-phase ozonolysis of ( E)-?-farnesene was investigated in a 3.91 m 3 atmospheric simulation chamber at 296 ± 2 K and relative humidity of around 0.1%. The relative rate method was used to determine the reaction rate coefficient of (4.01 ± 0.17) × 10 -16 cm 3 molecule -1 s -1, where the indicated errors are two least-squares standard deviations and do not include uncertainties in the rate coefficients for the reference compounds (?-terpinene, cis-cyclooctene and 1,5-cyclooctadiene). Gas phase carbonyl products were collected using a denuder sampling technique and analyzed with GC/MS following derivatization with O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA). The reaction products detected were acetone, 4-oxopentanal, methylglyoxal, 4-methylenehex-5-enal, 6-methylhept-5-en-2-one, and ( E)-4-methyl-8-methylenedeca-4,9-dienal. A detailed mechanism for the gas-phase ozonolysis of ( E)-?-farnesene is proposed, which accounts for all of the products observed in this study. The results of this work indicate that the atmospheric reaction of ( E)-?-farnesene with ozone has a lifetime of around 1 h and is another possible source of the ubiquitous carbonyls, acetone, 4-oxopentanal and 6-methylhept-5-en-2-one in the atmosphere.

Kourtchev, Ivan; Bejan, Iustinian; Sodeau, John R.; Wenger, John C.

339

Role of Conformational Structures and Torsional Anharmonicity in Controlling Chemical Reaction Rates and Relative Yields: Butanal + HO2 Reactions  

SciTech Connect

Aldehyde–radical reactions are important in atmospheric and combustion chemistry, and the reactions studied here also serve more generally to illustrate a fundamental aspect of chemical kinetics that has been relatively unexplored from a quantitative point of view, in particular the roles of multiple structures and torsional anharmonicity in determining the rate constants and branching ratios (product yields). We consider hydrogen abstraction from four carbon sites of butanal (carbonyl-C, a-C, b-C and g-C) by hydroperoxyl radical. We employed multi-structural variational transition state theory for studying the first three channels; this uses a multi-faceted dividing surface and allows us to include the contributions of multiple structures of both reacting species and transition states. Multiconfigurational Shepard interpolation (MCSI) was used to obtain the geometries and energies of the potential energy surface along the minimum-energy paths, with gradients and Hessians calculated by the M08-HX/maug-cc-pVTZ method. We find the numbers of structures obtained for the transition states are 46, 60, 72 and 76respectively for the H abstraction at the carbonyl C, the a position, the b position and the g position. Our results show that neglecting the factors arising from multiple structures and torsional anharmonicity would lead to errors at 300, 1000 and 2400 K of factors of 8, 11 and 10 for abstraction at the carbonyl-O, 2, 11 and 25 at the a-C position, 2, 23 and 47 at the b-C position, and 0.6, 8 and 18 at the g-C position. The errors would be even larger at high temperature for the reverse of the H abstraction at the b-C. Relative yields are changed as much as a factor of 7.0 at 200 K, a factor of 5.0 at 298 K, and a factor of 3.7 in the other direction at 2400 K. The strong dependence of the product ratios on the multi-structural anharmonicity factors shows that such factors play an important role in controlling branching ratios in reaction mechanism networks.

Zheng, Jingjing; Seal, Prasenjit; Truhlar, Donald G.

2012-09-24

340

Quantum functional sensitivity analysis for the collinear H+H sub 2 reaction rate coefficient  

SciTech Connect

The effects of features in the potential energy surface on the collinear H+H{sub 2} reaction rate coefficient are investigated by the method of quantum functional sensitivity analysis (QFSA). The calculations use QFSA to connect features in the microscopic realm, with their response upon macroscopic quantities of chemical interest, via the intermediary sensitivities of the reactive transition probabilities. While the sensitivities of the individual transition probabilities show considerable structure, there is an attendant loss of structure in the rate coefficient sensitivities because of the thermal averaging. For the range of temperatures used in our study (200--2400 K), the most important region of the potential energy surface is found to be not at the top of the barrier, but rather at the lower energy shoulders of the barrier. There are also regions near the barrier where an increase in the potential surface actually increases the reaction rate The effects of using different underlying potentials (the Porter--Karplus (PK2), Liu--Siegbahn--Truhlar--Horowitz (LSTH), and double many-body expansion (DMBE) surfaces) on the nature of the results were also compared. The absolute sensitivity magnitudes on the PK2 surface vary considerably from the other two, but the relative change in the rate coefficient is about the same on all three surfaces. Furthermore, the identified regions of importance on the potential surfaces remain essentially the same. The reactive scattering calculations were performed with the log-derivative version of the Kohn variational principle.

Chang, J.; Brown, N.J. (Applied Science Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 (United States)); D'Mello, M.; Wyatt, R.E. (Department of Chemistry and Institute for Theoretical Chemistry, University of Texas, Austin, Texas 78712 (United States)); Rabitz, H. (Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (United States))

1992-03-01

341

A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows  

NASA Technical Reports Server (NTRS)

A computationally-viable model describing the interaction between fluid-mechanical turbulence and finite-rate combustion reactions, principally in high-speed flows was developed. Chemical kinetic mechanisms, complete and global, were developed describing the finite rate reaction of fuels of interest to NASA. These fuels included principally hydrogen and silane, although a limited amount of work involved hydrocarbon fuels as well.

Chinitz, W.

1986-01-01

342

Absolute rate of the reaction of Cl(p-2) with molecular hydrogen from 200 - 500 K  

NASA Technical Reports Server (NTRS)

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.

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

1976-01-01

343

A study of the photocatalytic effects of aqueous suspensions of platinized semiconductor materials on the reaction rates of candidate redox reactions  

NASA Technical Reports Server (NTRS)

The effectiveness of powdered semiconductor materials in photocatalyzing candidate redox reactions was investigated. The rate of the photocatalyzed oxidation of cyanide at platinized TiO2 was studied. The extent of the cyanide reaction was followed directly using an electroanalytical method (i.e. differential pulse polarography). Experiments were performed in natural or artificial light. A comparison was made of kinetic data obtained for photocatalysis at platinized powders with rate data for nonplatinized powders.

Miles, A. M.

1982-01-01

344

Nuclear halo effect on nucleon capture reaction rates at stellar energies  

NASA Astrophysics Data System (ADS)

The capture cross sections at stellar energies are very difficult to measure directly. Hence, data are usually evaluated by using indirect methods or extrapolations from the experimental data obtained at the lowest possible energies. The asymptotic normalization coefficient (ANC) approach of the transfer reactions provides a reliable way for the determination of the capture cross sections at stellar energies. By virtue of its reliability, we have calculated the capture cross sections of the 10Be(n,?)11Be reaction by using nuclear ANC method. 11Be is a well-known neutron halo nucleus with two weakly bound states. As a typical example, we have shown that the radiative cross sections for a nucleon captured into a halo state are obviously enhanced. The enormous enhancement of the capture cross section is just due to the large overlap of the incident neutron wave with the extended tail of the halo. The 10Be(n,?)11Be capture reaction is involved in the inhomogeneous big-bang nucleosynthesis. We have evaluated its reaction rates at stellar energies with the nuclear ANC method.

Liu, Zu-Hua; Zhou, Hong-Yu

2005-08-01

345

Reactions of OH with butene isomers: measurements of the overall rates and a theoretical study.  

PubMed

Reactions of hydroxyl (OH) radicals with 1-butene (k(1)), trans-2-butene (k(2)), and cis-2-butene (k(3)) were studied behind reflected shock waves over the temperature range 880-1341 K and at pressures near 2.2 atm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH(3))(3)-CO-OH, and monitored by narrow-line width ring dye laser absorption of the well-characterized R(1)(5) line of the OH A-X (0, 0) band near 306.7 nm. OH time histories were modeled using a comprehensive C(5) oxidation mechanism, and rate constants for the reaction of OH with butene isomers were extracted by matching modeled and measured OH concentration time histories. We present the first high-temperature measurement of OH + cis-2-butene and extend the temperature range of the only previous high-temperature study for both 1-butene and trans-2-butene. With the potential energy surface calculated using CCSD(T)/6-311++G(d,p)//QCISD/6-31G(d), the rate constants and branching fractions for the H-abstraction channels of the reaction of OH with 1-butene were calculated in the temperature range 300-1500 K. Corrections for variational and tunneling effects as well as hindered-rotation treatments were included. The calculations are in good agreement with current and previous experimental data and with a recent theoretical study. PMID:21388160

Vasu, Subith S; Huynh, Lam K; Davidson, David F; Hanson, Ronald K; Golden, David M

2011-03-31

346

A Study of the ^13C(?,n) Reaction Rate Through the ANC Technique  

NASA Astrophysics Data System (ADS)

The ^13C(?,n) reaction is the main source of neutrons for the s-process. Currently the adopted rate has an uncertainty of ˜300%[C. Angulo et al., Nucl. Phys. A656, 3 (1999)] at the relevant stellar temperatures(˜10^8 K). This leads to a large uncertainty in the modeling of AGB stars, which is where the s-process occurs. Recently, we measured the ANC of the .5ex1-.1em/ -.15em.25ex2 ^+, 6.356 MeV, near threshold state in ^17O. This was done via the ?-transfer reaction ^13C(^6Li,d)^17O(.5ex1-.1em/ -.15em.25ex2 ^+, 6.356) at sub-Coulomb energies. Using this information we were able to calculate the contribution of the .5ex1-.1em/ -.15em.25ex2 ^+ state to the astrophysical S-factor. From our S-factor curve we calculated that the ^13C(?,n) reaction rate is reduced by a factor of 3, also the associated uncertainty is improved to ˜15%[E.D. Johnson et al., currently under review with PRL].

Johnson, Eric; Rogachev, Grigory; Baby, Lagy; Cluff, Warren; Crisp, Amy; Diffenderfer, Eric; Green, Bert; Hinners, Trisha; Hoffman, Calem; Kemper, Kirby; Momotyuk, Olexander; Peplowski, Patrick; Pipidis, Akis; Reynolds, Rob; Roeder, Brian; Mukhamedzhanov, Akram; Gol'Dberg, V.; Brown, Simon

2006-10-01

347

Contribution of 19F resonances on 18O(p,?)15N reaction rate  

NASA Astrophysics Data System (ADS)

The 18O(p,?)15N reaction influences the isotopes production such as 19F, 18O, and 15N which can be used to test the models of stellar evolution. 19F is synthesized in both asymptotic giant branch (AGB) and metal-rich Wolf-Rayet (WR) stars. Using R-matrix theory we allow new values of resonances parameters in 19F. We show that the most important contribution to the differential and total cross section at low energies, comes from the levels in 19F situated at resonances energies E R =151, 680 and 840 keV with spin and parity 1/2+. The total width of the 680 keV resonance is badly known. So, we have focused on this broad resonance corresponding to the 8.65 MeV level in 19F. We delimit the temperature range in which each resonance contribution to the total reaction rate occurs by analyzing the ratio (N A i /N A ). This allowed us to show that the 680 and 840 keV broad resonances strongly dominate the reaction rate over the stellar temperature range T 9=0.02-0.06 and T 9=0.5-5. Finally, these results were compared to NACRE and Iliadis astrophysical compilations.

Benmeslem, Meriem; Chafa, Azzedine; Barhoumi, Slimane; Tribeche, Mouloud

2014-05-01

348

A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows  

NASA Technical Reports Server (NTRS)

A principal element to be derived from modeling turbulent reacting flows is an expression for the reaction rates of the various species involved in any particular combustion process under consideration. A temperature-derived most-likely probability density function (pdf) was used to describe the effects of temperature fluctuations on the Arrhenius reaction rate constant. A most-likely bivariate pdf described the effects of temperature and species concentrations fluctuations on the reaction rate. A criterion is developed for the use of an "appropriate" temperature pdf. The formulation of models to calculate the mean turbulent Arrhenius reaction rate constant and the mean turbulent reaction rate is considered and the results of calculations using these models are presented.

Foy, E.; Ronan, G.; Chinitz, W.

1982-01-01

349

A Reaction-Diffusion-Based Coding Rate Control Mechanism for Camera Sensor Networks  

PubMed Central

A wireless camera sensor network is useful for surveillance and monitoring for its visibility and easy deployment. However, it suffers from the limited capacity of wireless communication and a network is easily overflown with a considerable amount of video traffic. In this paper, we propose an autonomous video coding rate control mechanism where each camera sensor node can autonomously determine its coding rate in accordance with the location and velocity of target objects. For this purpose, we adopted a biological model, i.e., reaction-diffusion model, inspired by the similarity of biological spatial patterns and the spatial distribution of video coding rate. Through simulation and practical experiments, we verify the effectiveness of our proposal.

Yamamoto, Hiroshi; Hyodo, Katsuya; Wakamiya, Naoki; Murata, Masayuki

2010-01-01

350

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

NASA Technical Reports Server (NTRS)

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.

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

1986-01-01

351

Quantifying metabolic rates in submarine hydrothermal vent chimneys: A reaction transport model  

NASA Astrophysics Data System (ADS)

The fluids emanating from active submarine hydrothermal vent chimneys provide a window into subseafloor processes and, through mixing with seawater, are responsible for steep thermal and compositional gradients that provide the energetic basis for diverse biological communities. Although several models have been developed to better understand the dynamic interplay of seawater, hydrothermal fluid, minerals and microorganisms inside chimney walls, none provide a fully integrated approach to quantifying the biogeochemistry of these hydrothermal systems. In an effort to remedy this, a fully coupled biogeochemical reaction transport model of a hydrothermal vent chimney has been developed that explicitly quantifies the rate of microbial catalysis while taking into account geochemical processes such as fluid flow, solute transport and oxidation-reduction reactions associated with fluid mixing as a function of temperature. Methanogenesis, hydrogen oxidation by oxygen and sulfate, sulfide oxidation by oxygen and methane oxidation by oxygen and sulfate are the metabolisms included in the reaction network. Model results indicate that microbial catalysis is fastest in the hottest habitable portion of the vent chimney except for methane oxidation by oxygen, which peaks near the seawater-side of the chimney at 20 nmol /cm^3 yr. The dominant metabolisms in the chimney are hydrogen oxidation by sulfate and oxygen and sulfide oxidation at peak rates 3200 , 300 and 900 nmol /cm^3 yr, respectively. The maximum rate of hydrogenotrophic methanogensis is just under 0.07 nmol /cm^3 yr, the slowest of the metabolisms considered. Due to thermodynamic inhibition, there is no anaerobic oxidation of methane by sulfate (AOM). The model developed here provides a quantitative approach to understanding the rates of biogeochemical transformations in hydrothermal systems and can be used to better understand the role of microbial activity in the deep subsurface.

LaRowe, D.; Dale, A.; Aguilera, D.; Amend, J. P.; Regnier, P.

2012-12-01

352

Reaction rates for sulfur fixation with iron at 1100 to 1275 K  

NASA Astrophysics Data System (ADS)

The rates of sulfidizing iron in a simulated coal gasification atmosphere were studied. Mixtures of H2S and CO were passed through fixed beds of coal char and prereduced iron ore, and effluent gas compositions were measured as a function of time. These mixtures ranged from 2.5 pct to 10 pct H2S at various flow rates, with temperatures from 1100 K to 1275 K and iron ore sizes from 10 mesh down to 100 mesh. Experimental conditions were established to form a steady state reaction profile in the fixed bed. Analysis of the exit gas provided a measurement of the profile. The slope of the profile was used directly as a measure of the reactivity of the solids in the bed. The development of this experimental technique and its experimental design requirements are discussed. The observed sulfidization rate of the in situ reduced iron ore is characterized by a single rate constant m (minutes-1), which varies primarily with temperature and particle size and is substantially independent of gas flow rate, bed configuration, and H2S content of the incoming gas. Accordingly, the rate constant m can be applied in the design of a combined sulfur fixation, coal gasification reactor to estimate the solids retention time, and the minimum mass of iron required per cross sectional area of reactor.

Shumaker, Craig B.; Schuhmann, R.

1983-06-01

353

Process for the oxidation of materials in water at supercritical temperatures utilizing reaction rate enhancers  

SciTech Connect

A method is described for substantially completely oxidizing combustible materials in which an aqueous stream bearing the combustible materials is reacted in the presence of an oxidant comprising diatomic oxygen and at a temperature greater than the critical temperature of water and at a pressure greater than about 25 bar, within a reactor for a period of less than about 5 minutes to produce a reaction product stream, wherein the reaction is initiated in the presence of a rate enhancer comprising at least one oxidizing agent in addition to said oxidant selected from the group consisting of ozone, hydrogen peroxide, salts containing persulfate, salts containing permanganate, nitric acid, salts containing nitrate, oxyacids of chlorine and their corresponding salts, hypochlorous acid, salts containing hypochlorite, chlorous acid, salts containing chlorite, chloric acid, salts containing chlorate, perchloric acid, and salts containing perchlorate.

Swallow, K.C.; Killilea, W.R.; Hong, G.T.; Bourhis, A.L.

1993-08-03

354

Effects of arsenic incorporation on jarosite dissolution rates and reaction products  

NASA Astrophysics Data System (ADS)

Batch dissolution experiments were undertaken on synthetic arsenojarosites at pH 2, pH 8, and in ultra-pure water to better understand the influence of As incorporation on the kinetics and reaction products of jarosite dissolution. Incongruent jarosite dissolution was observed in all experiments. Arsenojarosite lacks the pH dependency observed in K-jarosite dissolution, likely the result of surface arsenate-iron complexes preventing protonation at low pH and repelling hydroxyls at high pH. The stronger bonding of arsenate to iron, compared to sulfate to iron, leads to an enrichment of surface layer arsenate-iron complex sites, inhibiting the dissolution of jarosite with time. The secondary reaction products formed during the dissolution of arsenojarosite include maghemite, goethite, and hematite in ultra-pure water, and ferrihydrite in pH 8 Tris buffered solution. Maghemite initially forms and transitions to hematite with time in ultra-pure water, but increasing arsenic concentrations slow this transition. At pH >3.5, arsenic from the dissolution of arsenojarosite adsorbs onto newly formed reaction products. Arsenic also inhibits the formation of goethite and reduces the crystallinity of the observed maghemite reaction products. The coprecipitation of iron oxides with increasing amounts of arsenic results in a change from spherical to "worm-like" aggregate morphology and provides a sink for arsenic released during arsenojarosite dissolution. This study shows that in open systems with a flush of fresh solution, arsenic incorporation in jarosite results in an increase in dissolution rates. In closed systems, however, increasing surface arsenate-iron complexes inhibit further dissolution of the underlying bulk material, causing a reduction in dissolution rates as arsenic incorporation increases.

Kendall, Matthew R.; Madden, Andrew S.; Elwood Madden, Megan E.; Hu, Qinhong

2013-07-01

355

Reaction-rate approach to the dipolar relaxation in alkali halides  

NASA Astrophysics Data System (ADS)

An alternative reaction-rate approach is suggested for dealing with the relaxation times of dipolar defects in alkali halides. It rests on an occurrence-probability definition of the transition rate, accounting for both classical and quantal effects. A simple model Hamiltonian is postulated to describe the reorientational motion of a dipole within the framework of the adiabatic approximation. Two infamous examples are considered from this point of view. In one of these, the OH- dipole is assumed to couple only electronically to the neighboring cations, while all its remaining reorientational characteristics are, to a first approximation, independent of the host lattice. Assuming further that a single local oscillator drives the reorientational motion, this model agrees well with the experimental temperature dependence of the OH- relaxation time in four host halides up to 10 K, as measured by Kapphan and Lüty. The other example is the off-center Ag+ ion in RbBr and RbCl where a formal application of the reaction-rate method has been found to fit closely, over the entire range, the experimental temperature dependence of the relaxation time for 90° reorientation, measured by the same authors.

Georgiev, M.; Gochev, A.

1985-04-01

356

Rate Equations and Kinetic Parameters of the Reactions Involved in Pyrite Oxidation by Thiobacillus ferrooxidans  

PubMed Central

Rate equations and kinetic parameters were obtained for various reactions involved in the bacterial oxidation of pyrite. The rate constants were 3.5 ?M Fe2+ per min per FeS2 percent pulp density for the spontaneous pyrite dissolution, 10 ?M Fe2+ per min per mM Fe3+ for the indirect leaching with Fe3+, 90 ?M O2 per min per mg of wet cells per ml for the Thiobacillus ferrooxidans oxidation of washed pyrite, and 250 ?M O2 per min per mg of wet cells per ml for the T. ferrooxidans oxidation of unwashed pyrite. The Km values for pyrite concentration were similar and were 1.9, 2.5, and 2.75% pulp density for indirect leaching, washed pyrite oxidation by T. ferrooxidans, and unwashed pyrite oxidation by T. ferrooxidans, respectively. The last reaction was competitively inhibited by increasing concentrations of cells, with a Ki value of 0.13 mg of wet cells per ml. T. ferrooxidans cells also increased the rate of Fe2+ production from Fe3+ plus pyrite.

Lizama, Hector M.; Suzuki, Isamu

1989-01-01

357

Evolutionary sequences for Nova V1974 Cygni using new nuclear reaction rates and opacities  

NASA Astrophysics Data System (ADS)

The outburst of Nova V1974 Cyg 1992 is arguably the best observed of this century, with realistic estimates now available for the amount of mass ejected, the composition of the ejecta and the total energy budget. These data strongly support the conclusion that this was indeed a `neon' nova that occurred on an oxygen, neon, magnesium white dwarf. In addition, X-ray studies of its outburst imply that the mass of the white dwarf is about 1.25Msolar. We, therefore, report on the results of new calculations of thermonuclear runaways on 1.25-Msolar oxygen, neon, magnesium white dwarfs, using our one-dimensional, fully implicit, hydrodynamic stellar evolution code that includes a large nuclear reaction network. We have updated the nuclear reaction network, with the inclusion of new and improved experimental and theoretical determinations of the nuclear reaction rates. We have also incorporated the OPAL carbon rich tables and have investigated the effects of changes in convective efficiency on the evolution. Our results show that the changes in the reaction rates and opacities that we have introduced produce important changes with respect to our previous studies. For example, relevant to nucleosynthesis considerations, a smaller amount of ^26Al is produced, while the abundances of ^31P and ^32S increase by factors of more than two. This change is attributed to the increased proton-capture reaction rates for some of the intermediate mass nuclei near ^26Al and beyond, such that nuclear fusion to higher mass nuclei is enhanced. The characteristics of our models are then compared to observations of the outburst of V1974 Cyg 1992 and we find that the predicted amount of mass ejected is at least a factor of 10 less than observed. The low values for the amount of ejected mass are a consequence of the fact that the OPAL opacities are larger than those we previously used, which results in less mass being accreted on to the white dwarf. This is a general problem with respect to the comparison of observations and theory for ONeMg novae and we suggest a possible resolution of this discrepancy.

Starrfield, S.; Truran, J. W.; Wiescher, M. C.; Sparks, W. M.

1998-05-01

358

Theory and simulation of the time-dependent rate coefficients of diffusion-influenced reactions.  

PubMed Central

A general formalism is developed for calculating the time-dependent rate coefficient k(t) of an irreversible diffusion-influenced reaction. This formalism allows one to treat most factors that affect k(t), including rotational Brownian motion and conformational gating of reactant molecules and orientation constraint for product formation. At long times k(t) is shown to have the asymptotic expansion k(infinity)[1 + k(infinity) (pie Dt)-1/2 /4 pie D + ...], where D is the relative translational diffusion constant. An approximate analytical method for calculating k(t) is presented. This is based on the approximation that the probability density of the reactant pair in the reactive region keeps the equilibrium distribution but with a decreasing amplitude. The rate coefficient then is determined by the Green function in the absence of chemical reaction. Within the framework of this approximation, two general relations are obtained. The first relation allows the rate coefficient for an arbitrary amplitude of the reactivity to be found if the rate coefficient for one amplitude of the reactivity is known. The second relation allows the rate coefficient in the presence of conformational gating to be found from that in the absence of conformational gating. The ratio k(t)/k(0) is shown to be the survival probability of the reactant pair at time t starting from an initial distribution that is localized in the reactive region. This relation forms the basis of the calculation of k(t) through Brownian dynamics simulations. Two simulation procedures involving the propagation of nonreactive trajectories initiated only from the reactive region are described and illustrated on a model system. Both analytical and simulation results demonstrate the accuracy of the equilibrium-distribution approximation method.

Zhou, H X; Szabo, A

1996-01-01

359

Accurate quantum thermal rate constants for the three-dimensional H+H/sub 2/ reaction  

SciTech Connect

The rate constants for the three-dimensional H+H/sub 2/ reaction on the Liu--Siegbahn--Truhlar--Horowitz (LSTH) surface are calculated using Pack--Parker hyperspherical (APH) coordinates and a /ital C//sub 2/ital v// symmetry adapted direct product discrete variable representation (DVR). The /ital C//sub 2/ital v// symmetry decomposition and the parity decoupling on the basis are performed for the internal coordinate xi. The symmetry decomposition results in a block diagonal representation of the flux and Hamiltonian operators. The multisurface flux is introduced to represent the multichannel reactive flux. The eigenvalues and eigenvectors of the /ital J/=0 internal Hamiltonian are obtained by sequential diagonalization and truncation. The individual symmetry blocks of the flux operator are propagated by the corresponding blocks of the Hamiltonian, and the /ital J/=0 rate constant /ital k//sup 0/(/ital T/) is obtained as a sum of the rate constants calculated for each block. /ital k//sup 0/(/ital T/) is compared with the exact /ital k//sup 0/(/ital T/) obtained from thermal averaging of the /ital J/=0 reaction probabilities; the errors are within 5%--20% up to /ital T/=1500 K. The sequential diagonalization--truncation method reduces the size of the Hamiltonian greatly, but the resulting Hamiltonian matrix still describes the time evolution very accurately. For the /ital J//ne/0 rate constant calculations, the truncated internal Hamiltonian eigenvector basis is used to construct reduced (/ital JK//sub /ital J//) blocks of the Hamiltonian. The individual (/ital JK//sub /ital J//) blocks are diagonalized neglecting Coriolis coupling and treating the off-diagonal /ital K//sub /ital J//+-2 couplings by second order perturbation theory. The full wave function is parity decoupled. The rate constant is obtained as a sum over /ital J/ of (2/ital J/+1)/ital k//sup /ital J//(/ital T/).

Park, T. J.; Light, J. C.

1989-07-15

360

Stress-associated cardiovascular reaction masks heart rate dependence on physical load in mice.  

PubMed

When tested on the treadmill mice do not display a graded increase of heart rate (HR), but rather a sharp shift of cardiovascular indices to high levels at the onset of locomotion. We hypothesized that under test conditions cardiovascular reaction to physical load in mice is masked with stress-associated HR increase. To test this hypothesis we monitored mean arterial pressure (MAP) and heart rate in C57BL/6 mice after exposure to stressful stimuli, during spontaneous locomotion in the open-field test, treadmill running or running in a wheel installed in the home cage. Mice were treated with ?1-adrenoblocker atenolol (2mg/kg ip, A), cholinolytic ipratropium bromide (2mg/kg ip, I), combination of blockers (A+I), anxiolytic diazepam (5mg/kg ip, D) or saline (control trials, SAL). MAP and HR in mice increased sharply after handling, despite 3weeks of habituation to the procedure. Under stressful conditions of open field test cardiovascular parameters in mice were elevated and did not depend on movement speed. HR values did not differ in I and SAL groups and were reduced with A or A+I. HR was lower at rest in D pretreated mice. In the treadmill test HR increase over speeds of 6, 12 and 18m/min was roughly 1/7-1/10 of HR increase observed after placing the mice on the treadmill. HR could not be increased with cholinolytic (I), but was reduced after sympatholytic (A) or A+I treatment. Anxiolytic (D) reduced heart rate at lower speeds of movement and its overall effect was to unmask the dependency of HR on running speed. During voluntary running in non-stressful conditions of the home cage HR in mice linearly increased with increasing running speeds. We conclude that in test situations cardiovascular reactions in mice are governed predominantly by stress-associated sympathetic activation, rendering efforts to evaluate HR and MAP reactions to workload unreliable. PMID:24802359

Andreev-Andrievskiy, A A; Popova, A S; Borovik, A S; Dolgov, O N; Tsvirkun, D V; Custaud, M; Vinogradova, O L

2014-06-10

361

Re-evaluation of the rate constant for the H atom reaction with tert-butanol in aqueous solution  

NASA Astrophysics Data System (ADS)

The rate constant ( k) of the H+ tert-butanol reaction at pH˜1-2 was measured by the time profile of the absorbance build-up of the tert-butanol radical and by competitive reactions for H atoms between maleic acid and tert-butanol. The determined rate constant, k=(1.15±0.2)×10 6 mol -1 dm 3 s -1, is nearly an order of magnitude higher than the previously published rate constants.

Wojnárovits, László; Takács, Erzsébet; Dajka, Katalin; Emmi, Salvatore S.; Russo, Marialuisa; D'Angelantonio, Mila

2004-02-01

362

Rates of reaction and process design data for the Hydrocarb Process  

SciTech Connect

In support of studies for developing the coprocessing of fossil fuels with biomass by the Hydrocarb Process, experimental and process design data are reported. The experimental work includes the hydropryolysis of biomass and the thermal decomposition of methane in a tubular reactor. The rates of reaction and conversion were obtained at temperature and pressure conditions pertaining to a Hydrocarb Process design. A Process Simulation Computer Model was used to design the process and obtain complete energy and mass balances. Multiple feedstocks including biomass with natural gas and biomass with coal were evaluated. Additional feedstocks including green waste, sewage sludge and digester gas were also evaluated for a pilot plant unit.

Steinberg, M.; Kobayashi, Atsushi [Brookhaven National Lab., Upton, NY (United States); Tung, Yuanki [Hydrocarb Corp., New York, NY (United States)

1992-08-01

363

Redox reaction rates in shallow aquifers: Implications for nitrate transport in groundwater and streams  

USGS Publications Warehouse

Groundwater age and water chemistry data along flow paths from recharge areas to streams were used to evaluate the trends and transformations of agricultural chemicals. Results from this analysis indicate that median nitrate recharge concentrations in these agricultural areas have increased markedly over the last 50 years from 4 mg N/L in samples collected prior to 1983 to 7.5 mg N/L in samples collected since 1983. The effect that nitrate accumulation in shallow aquifers will have on drinking water quality and stream ecosystems is dependent on the rate of redox reactions along flow paths and on the age distribution of nitrate discharging to supply wells and streams.

Tesoriero, Anthony J.

2012-01-01

364

Study on reaction rates for methanol synthesis from carbon monoxide, carbon dioxide, and hydrogen  

Microsoft Academic Search

The reaction rates for methanol synthesis from carbon monoxide, carbon dioxide, and hydrogen over a copper-zinc-based catalyst were measured under the following conditions: temperature, 210-300°C; pressure, 4-10 MPa, feed gas composition Hâ\\/CO 2.7-720, Hâ\\/COâ, 4.3-120, COâ\\/CO, 0.05-180 (in moles). It was concluded that direct methanol production from COâ takes place under the above conditions in addition to the two well-known

M. Takagawa; M. Ohsugi

1987-01-01

365

Reduction in TFTR (Tokamak Fusion Test Reactor) fusion reaction rate by unbalanced beam injection and rotation  

SciTech Connect

In TFTR plasmas at low to moderate density, the highest fusion energy gain Q/sub dd/ (D-D fusion power/injected power P/sub b/) is obtained with nearly balanced co- and counter-injection of neutral beams. For a given beam power, significantly unbalanced injection reduces Q/sub dd/ because the accompanying plasma rotation reduces the beam-target fusion reactivity, the fast-ion slowing-down time, and the beam-beam reaction rate, while and decrease from their maximum values. 9 refs., 3 figs., 1 tab.

Hendel, H.W.; Jassby, D.L.; Bitter, M.L.; Taylor, G.

1987-06-01

366

On the theory of the reaction rate of vibrationally excited CO molecules with OH radicals.  

PubMed

The dependence of the rate of the reaction CO+OH-->H+CO2 on the CO-vibrational excitation is treated here theoretically. Both the Rice-Ramsperger-Kassel-Marcus (RRKM) rate constant kRRKM and a nonstatistical modification knon [W.-C. Chen and R. A. Marcus, J. Chem. Phys. 123, 094307 (2005).] are used in the analysis. The experimentally measured rate constant shows an apparent (large error bars) decrease with increasing CO-vibrational temperature Tv over the range of Tv's studied, 298-1800 K. Both kRRKM(Tv) and knon(Tv) show the same trend over the Tv-range studied, but the knon(Tv) vs Tv plot shows a larger effect. The various trends can be understood in simple terms. The calculated rate constant kv decreases with increasing CO vibrational quantum number v, on going from v=0 to v=1, by factors of 1.5 and 3 in the RRKM and nonstatistical calculations, respectively. It then increases when v is increased further. These results can be regarded as a prediction when v state-selected rate constants become available. PMID:16422582

Chen, Wei-Chen; Marcus, R A

2006-01-14

367

On the theory of the reaction rate of vibrationally excited CO molecules with OH radicals  

NASA Astrophysics Data System (ADS)

The dependence of the rate of the reaction CO+OH-->H+CO2 on the CO-vibrational excitation is treated here theoretically. Both the Rice-Ramsperger-Kassel-Marcus (RRKM) rate constant kRRKM and a nonstatistical modification knon [W.-C. Chen and R. A. Marcus, J. Chem. Phys. 123, 094307 (2005).] are used in the analysis. The experimentally measured rate constant shows an apparent (large error bars) decrease with increasing CO-vibrational temperature Tv over the range of Tv's studied, 298-1800 K. Both kRRKM(Tv) and knon(Tv) show the same trend over the Tv-range studied, but the knon(Tv) vs Tv plot shows a larger effect. The various trends can be understood in simple terms. The calculated rate constant kv decreases with increasing CO vibrational quantum number v, on going from v=0 to v=1, by factors of 1.5 and 3 in the RRKM and nonstatistical calculations, respectively. It then increases when v is increased further. These results can be regarded as a prediction when v state-selected rate constants become available.

Chen, Wei-Chen; Marcus, R. A.

2006-01-01

368

Kinetics and Mechanism of the Reaction of Hydoxyl Radicals with Acetonitrile under Atmospheric Conditions  

NASA Technical Reports Server (NTRS)

The pulsed laser photolysis-pulsed laser induced fluorescence technique has been employed to determine absolute rate coefficients for the reaction OH + CH3CN (1) and its isotopic variants, OH + CD3CN (2), OD + CH3CN (3), and OD + CD3CN (4). Reactions 1 and 2 were studied as a function of pressure and temperature in N2, N2/O2, and He buffer gases. In the absence of O2 all four reactions displayed well-behaved kinetics with exponential OH decays and pseudo-first rate constants which were proportional to substrate concentration. Data obtained in N2 over the range 50-700 Torr at 298 K are consistent with k(sub 1), showing a small pressure dependence. The Arrhenius expression obtained by averaging data at all pressures in k(sub 1)(T) = (1.1(sup +0.5)/(sub -0.3)) x 10(exp -12) exp[(-1130 +/- 90)/T] cu cm /(molecule s). The kinetics of reaction 2 are found to be pressure dependent with k(sub 2) (298 K) increasing from (1.21 +/- 0.12) x 10(exp -14) to (2.16 +/- 0.11) x 10(exp -14) cm(exp 3)/ (molecule s) over the pressure range 50-700 Torr of N2 at 298 K. Data at pressures greater than 600 Torr give k(sub 2)(T) = (9.4((sup +13.4)(sub -5.0))) x 10(exp -13) exp[(-1180 +/- 250)/T] cu cm/(molecule s). The rates of reactions 3 and 4 are found to be independent of pressure over the range 50-700 Torr of N2 with 298 K rate coefficient given by k(sub 3) =(3.18 +/- 0.40) x 10(exp -14) cu cm/(molecule s) and k(sub 4) = (2.25 +/-0.28) x 10(exp -14) cu cm/(molecule s). In the presence of O2 each reaction shows complex (non-pseudo-first-order) kinetic behavior and/or an apparent decrease in the observed rate constant with increasing [O2], indicating the presence of significant OH or OD regeneration. Observation of regeneration of OH in (2) and OD in (3) is indicative of a reaction channel which proceeds via addition followed by reaction of the adduct, or one of its decomposition products, with O2. The observed OH and OD decay profiles have been modeled by using a simple mechanistic scheme to extract kinetic information about the adduct reations with O2 and branching ratios for OH regeneration. A plausible mechanism for OH regeneration in (2) involves OH addition to the nitrogen atom followed by O2 addition to the cyano carbon atom, isomeriazation and decomposition to D2CO + DOCN + OH. Our results suggest that the OH + CH3CN reaction occurs via a complex mechanism involving both bimolecular and termolecular pathways, analogous to the mechanisms for the the important atmospheric reactions of OH with CO and HNO3.

Hynes, A. J.; Wine, P. H.

1997-01-01

369

The Nickel Ion Catalysed Iodination of 2-Amino-2-Hydroxymethyl-1,3-Propanediol.  

National Technical Information Service (NTIS)

Nickel (II) catalyses the iodination of 2-amino-2-hydroxymethyl-1,3-propanediol. The product is the N-iodo compound, and equilibrium exists between the reactants and products. The rate of the reaction followed under pseudo first order kinetics, is increas...

J. W. Hosking M. M. Jones

1967-01-01

370

Shock tube measurements of the rate constant for the reaction cyclohexene ? ethylene + 1,3-butadiene  

NASA Astrophysics Data System (ADS)

The rate constant for the reaction cyclohexene ? ethylene + 1,3-butadiene was determined by monitoring the formation of ethylene during the pyrolysis of cyclohexene behind reflected shock waves. Ethylene mole-fraction time-histories were measured using direct laser absorption at 10.532 ?m. Experiments were performed between 950-1300 K and 0.8-3.7 atm. Measurements do not indicate any pressure dependence at these conditions. The data are best-fit by an Arrhenius expression k [s-1] = 4.84 × 1014 exp (-31 900 [K]/T) with uncertainties of ±19-36%, depending on the temperature. This appears to be the most accurate determination to date of the rate constant for cyclohexene decomposition.

Stranic, Ivo; Davidson, David F.; Hanson, Ronald K.

2013-10-01

371

Rate coefficient for the termolecular channel of the self-reaction of ClO  

SciTech Connect

The rate coefficient for the reaction ClO+ClO{sup M}{yields}Cl{sub 2}O{sub 2} has been measured by using a flash photolysis/UV absorption technique at temperatures from 200 to 263 K and pressures from 25 to 600 Torr in N{sub 2}, O{sub 2}, He, and SF{sub 6}. Conventional single-wavelength detection and diode array spectrometry have both been employed in monitoring the reactant and product species. The UV absorption cross section of ClO has been measured in the same temperature range, from 220 to 350 nm, by using the diode array, and at 282.65 nm (the maximum of the ClO (9,0) band) by using the single-wavelength detection. The reactions of chlorine atoms with O{sub 3} and Cl{sub 2}O have been used to produce ClO. The title reaction is believed to play a role in the loss of ozone from the Antarctic stratosphere during the austral spring. At the temperatures and pressures relevant to the Antarctic stratosphere, our measured values agree with the recent work of Sander et al. and are considerably smaller than the values obtained by Hayman et al.

Trolier, M.; Mauldin, R.L. III; Ravishankara, A.R. (National Oceanic and Atmospheric Administration, Boulder, CO (USA) Univ. of Colorado, Boulder (USA))

1990-06-14

372

Methanesulfinic acid reaction with OH: mechanism, rate constants, and atmospheric implications.  

PubMed

The mechanism for the atmospheric oxidation of methanesulfinic acid (MSIA) has been studied. This is the first theoretical study of the reaction between MSIA and the OH radical. All the possible channels in this reaction have been studied theoretically, and their corresponding rate constants have been evaluated under the variational transition-state theory (VTST) formalism. Two different products can be formed: the CH3S(O)2 radical (which had been experimentally proposed as the only one), and sulfurous acid (H2SO3). The CH3S(O)2 radical can be formed directly or can form via an intermediate adduct, which yields to the radical through the elimination of a water molecule. For the first time, it is theoretically demonstrated that SO2 is formed in the addition channel of the DMS + OH reaction. The consequences of this result in the interpretation of the T-dependence of the SO4(2-)/MSA (methanesulfonic acid) quocient are analyzed. The competition between the unimolecular dissociation of the CH3S(O)2 radical and OH-addition to yield MSA is proposed as one of the possible multiple branching points (along the DMS + OH degradation scheme) influencing the T-dependence of the SO4(2-)/MSA relation. PMID:17636968

Gonzalez-García, Núria; Gonzalez-Lafont, Angels; Lluch, José M

2007-08-16

373

Solutions to a reduced Poisson-Nernst-Planck system and determination of reaction rates  

PubMed Central

We study a reduced Poisson–Nernst–Planck (PNP) system for a charged spherical solute immersed in a solvent with multiple ionic or molecular species that are electrostatically neutralized in the far field. Some of these species are assumed to be in equilibrium. The concentrations of such species are described by the Boltzmann distributions that are further linearized. Others are assumed to be reactive, meaning that their concentrations vanish when in contact with the charged solute. We present both semi-analytical solutions and numerical iterative solutions to the underlying reduced PNP system, and calculate the reaction rate for the reactive species. We give a rigorous analysis on the convergence of our simple iteration algorithm. Our numerical results show the strong dependence of the reaction rates of the reactive species on the magnitude of its far field concentration as well as on the ionic strength of all the chemical species. We also find non-monotonicity of electrostatic potential in certain parameter regimes. The results for the reactive system and those for the non-reactive system are compared to show the significant differences between the two cases. Our approach provides a means of solving a PNP system which in general does not have a closed-form solution even with a special geometrical symmetry. Our findings can also be used to test other numerical methods in large-scale computational modeling of electro-diffusion in biological systems.

Li, Bo; Lu, Benzhuo; Wang, Zhongming; McCammon, J. Andrew

2010-01-01

374

Absolute rate constants for the reactions of primary alkyl radicals with aromatic amines  

SciTech Connect

Hydrogen abstraction from diarylamines (4-X-C{sub 6}H{sub 4}) [X = H, CH{sub 3}, C{sub 8}H{sub 17}, CH{sub 3}O, and Br] by the 2-methyl-2-phenylpropyl radical in n-dodecane solution was investigated by thermolysis of 3-methyl-3-phenylbutanoyl peroxide in the presence of various concentrations of the amines. The reaction is a non-chain process in which the 2-methyl-2-phenylpropyl radical and its rearrangement product, the 2-benzylpropan-2-yl radical, abstract hydrogen from both the solvent and the amine. Cross-disproportionation reactions of the rearranged radical led to the formation of significant amounts of {beta}, {beta}-dimethylstyrene. Rate constants for hydrogen abstraction by the unrearranged, primary alkyl radical form n-dodecane (k{sup 373K}= 3.5 x 10{sup 3} M{sup {minus}1} s{sup {minus}1}), diphenylamine (k{sup 373K} = 1.3 x 10{sup 6} M{sup {minus}1} s{sup {minus}1}), and the substituted diarylamines were determined from the product yields and the known rate constant for the radical rearrangement. From kinetic experiments with N-deuteriodiphenylamine and deuterium kinetic isotope effect, k{sub NH}/k{sub ND}, was found to be 2.3 at 373 K. 28 refs., 1 fig., 1 tab.

Burton, A.; Ingold, K.U.; Walton, J.C. [Univ. of St. Andrews, Fife (United Kingdom)] [Univ. of St. Andrews, Fife (United Kingdom)

1996-05-31

375

Advanced methods comparisons of reaction rates in the Purdue Fast Breeder Blanket Facility  

SciTech Connect

A review of worldwide results revealed that reaction rates in the blanket region are generally underpredicted with the discrepancy increasing with penetration; however, these results vary widely. Experiments in the large uniform Purdue Fast Breeder Blanket Facility (FBBF) blanket yield an accurate quantification of this discrepancy. Using standard production code methods (diffusion theory with 50 group cross sections), a consistent Calculated/Experimental (C/E) drop-off was observed for various reaction rates. A 50% increase in the calculated results at the outer edge of the blanket is necessary for agreement with experiments. The usefulness of refined group constant generation utilizing specialized weighting spectra and transport theory methods in correcting this discrepancy was analyzed. Refined group constants reduce the discrepancy to half that observed using the standard method. The surprising result was that transport methods had no effect on the blanket deviations; thus, transport theory considerations do not constitute or even contribute to an explanation of the blanket discrepancies. The residual blanket C/E drop-off (about half the standard drop-off) using advanced methods must be caused by some approximations which are applied in all current methods. 27 refs., 3 figs., 1 tab.

Hill, R.N.; Ott, K.O.

1988-01-01

376

Kinetic sonication effects in aqueous acetonitrile solutions. Reaction rate levelling by ultrasound.  

PubMed

The kinetics of the pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate were investigated with and without ultrasonic irradiation in acetonitrile-water binary mixtures containing 0.008 to 35 wt.% of acetonitrile and the kinetic sonication effects (kson/knon) were calculated. Molecular dynamics (MD) simulations of the structure of the solutions were performed with ethyl acetate as the model ester. The ester is preferentially solvated by acetonitrile. The excess of acetonitrile over water in the solvation shell grows fast with an increase in the co-solvent content in the bulk solution. In parallel, the formation of a second solvation shell rich in acetonitrile takes place. Significant kinetic sonication effects for the hydrolysis were explained with facile destruction of the diffuse second solvation shell followed by a rearrangement of the remaining solvent layer under sonication. The rate levelling effect of ultrasound was discussed. In an aqueous-organic binary solvent, independent of the solvent composition, the ultrasonic irradiation evokes changes in the reaction medium which result in an almost identical solvation state of the reagent thus leading to the reaction rate levelling. PMID:23643415

Piiskop, Sander; Salmar, Siim; Tuulmets, Ants; Kuznetsov, Aleksei; Järv, Jaak

2013-11-01

377

Rate Constants and Products of the OH Reaction with Isoprene-Derived Epoxides  

NASA Astrophysics Data System (ADS)

Recent laboratory and field work has suggested that isoprene-derived epoxides (IEPOX) are crucial intermediates that can explain the existence of a variety of compounds found in ambient secondary organic aerosol (SOA). However, IEPOX species are also able to undergo gas phase oxidation, which competes with the aerosol phase processing of IEPOX. In order to better quantify the atmospheric fate of IEPOX, the gas phase OH reaction rate constants and product formation mechanisms have been determined using a flow tube chemical ionization mass spectrometry technique. The new OH rate constants are generally larger than previous estimations and some features of the product mechanism are well predicted by the Master Chemical Mechanism Version 3.2 (MCM v3.2), while other features are at odds with MCM v3.2. Some observed products will likely impact gas phase ozone and NOx cycling while other products will likely partition to the SOA phase. Using a previously proposed kinetic model for the quantitative prediction of the atmospheric fate of IEPOX, it is found that gas phase OH reaction is an even more dominant fate for chemical processing of IEPOX than previously suggested. The present results suggest that aerosol phase processing of IEPOX will be competitive with gas phase OH oxidation only under conditions of high liquid water content and low pH SOA conditions.

Elrod, M. J.

2013-12-01

378

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

NASA Technical Reports Server (NTRS)

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.

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

1973-01-01

379

RATE CONSTANTS FOR THE REACTION OF OH RADICALS WITH A SERIES OF ALKENES AND DIALKENES AT 295 + OR - 1 K  

EPA Science Inventory

Using a relative rate technique, rate constants for the gas phase reactions of the OH radical with n-butane, n-hexane and a series of alkenes and dialkenes, relative to that for propene, have been determined in one atmosphere of air at 295 + or - 1K. The resulting rate constant d...

380

Chromo-fluorogenic detection of nerve-agent mimics using triggered cyclization reactions in push-pull dyes.  

PubMed

A family of azo and stilbene derivatives (1-9) are synthesized, and their chromo-fluorogenic behavior in the presence of nerve-agent simulants, diethylchlorophosphate (DCP), diisopropylfluorophosphate (DFP), and diethylcyanophosphate (DCNP) in acetonitrile and mixed solution of water/acetonitrile (3:1 v/v) buffered at pH 5.6 with MES, is investigated. The prepared compounds contain 2-(2-N,N-dimethylaminophenyl)ethanol or 2-[(2-N,N-dimethylamino)phenoxy]ethanol reactive groups, which are part of the conjugated pi-system of the dyes and are able to give acylation reactions with phosphonate substrates followed by a rapid intramolecular N-alkylation. The nerve-agent mimic-triggered cyclization reaction transforms a dimethylamino group into a quaternary ammonium, inducing a change of the electronic properties of the delocalized systems that results in a hypsochromic shift of the absorption band of the dyes. Similar reactivity studies are also carried out with other "non-toxic" organophosphorus compounds, but no changes in the UV/Vis spectra were observed. The emission behaviour of the reagents in acetonitrile and water-acetonitrile 3:1 v/v mixtures is also studied in the presence of nerve-agent simulants and other organophosphorous derivatives. The reactivity between 1-9 and DCP, DCNP, or DFP in buffered water-acetonitrile 3:1 v/v solutions under pseudo first-order kinetic conditions, using an excess of the corresponding simulant, are studied in order to determine the rate constants (k) and the half-life times (t(1/2)=ln2/k) for the reaction. The detection limits in water/acetonitrile 3:1 v/v are also determined for 1-9 and DCP, DCNP, and DFP. Finally, the chromogenic detection of nerve agent simulants both in solution and in gas phase are tested using silica gel containing adsorbed compounds 1, 2, 3, 4, or 5 with fine results. PMID:20512798

Costero, Ana M; Parra, Margarita; Gil, Salvador; Gotor, Raúl; Mancini, Pedro M E; Martínez-Máñez, Ramón; Sancenón, Félix; Royo, Santiago

2010-07-01

381

Much improved upper limit for the rate constant for the reaction of O2+ with N2.  

PubMed

The rate constant for the reaction of O2+ with N2 to produce NO+ plus NO has been measured at 423, 523, and 623 K in a turbulent ion flow tube. Much improved upper limits for this reaction at the three temperatures are 2, 4, and 10x10(-21) cm3 s-1, respectively. These results should render this reaction irrelevant when modeling all plasmas involving atmospheric gases. PMID:17034151

Viggiano, A A

2006-10-19

382

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

Microsoft Academic Search

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

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

2012-01-01

383

Evaluation of thermal rates for reactions with intermediate wells: Removal of bound state contributions to quantum flux correlation functions  

Microsoft Academic Search

We consider the calculation of rates of chemical reactions that have bound intermediate states (i.e., wells along the reaction path) using flux correlation function methods. When time-dependent wave packets are used to evaluate the propagator matrix elements, and the dividing surface is located at a point where bound states have nonzero probability density, the standard expression for the flux correlation

Mark Thachuk; Howard R. Mayne; George C. Schatz

1993-01-01

384

Mixed quantum classical calculation of proton transfer reaction rates: From deep tunneling to over the barrier regimes.  

PubMed

We present mixed quantum classical calculations of the proton transfer (PT) reaction rates represented by a double well system coupled to a dissipative bath. The rate constants are calculated within the so called nontraditional view of the PT reaction, where the proton motion is quantized and the solvent polarization is used as the reaction coordinate. Quantization of the proton degree of freedom results in a problem of non-adiabatic dynamics. By employing the reactive flux formulation of the rate constant, the initial sampling starts from the transition state defined using the collective reaction coordinate. Dynamics of the collective reaction coordinate is treated classically as over damped diffusive motion, for which the equation of motion can be derived using the path integral, or the mixed quantum classical Liouville equation methods. The calculated mixed quantum classical rate constants agree well with the results from the numerically exact hierarchical equation of motion approach for a broad range of model parameters. Moreover, we are able to obtain contributions from each vibrational state to the total reaction rate, which helps to understand the reaction mechanism from the deep tunneling to over the barrier regimes. The numerical results are also compared with those from existing approximate theories based on calculations of the non-adiabatic transmission coefficients. It is found that the two-surface Landau-Zener formula works well in calculating the transmission coefficients in the deep tunneling regime, where the crossing point between the two lowest vibrational states dominates the total reaction rate. When multiple vibrational levels are involved, including additional crossing points on the free energy surfaces is important to obtain the correct reaction rate using the Landau-Zener formula. PMID:24811623

Xie, Weiwei; Xu, Yang; Zhu, Lili; Shi, Qiang

2014-05-01

385

Mixed quantum classical calculation of proton transfer reaction rates: From deep tunneling to over the barrier regimes  

NASA Astrophysics Data System (ADS)

We present mixed quantum classical calculations of the proton transfer (PT) reaction rates represented by a double well system coupled to a dissipative bath. The rate constants are calculated within the so called nontraditional view of the PT reaction, where the proton motion is quantized and the solvent polarization is used as the reaction coordinate. Quantization of the proton degree of freedom results in a problem of non-adiabatic dynamics. By employing the reactive flux formulation of the rate constant, the initial sampling starts from the transition state defined using the collective reaction coordinate. Dynamics of the collective reaction coordinate is treated classically as over damped diffusive motion, for which the equation of motion can be derived using the path integral, or the mixed quantum classical Liouville equation methods. The calculated mixed quantum classical rate constants agree well with the results from the numerically exact hierarchical equation of motion approach for a broad range of model parameters. Moreover, we are able to obtain contributions from each vibrational state to the total reaction rate, which helps to understand the reaction mechanism from the deep tunneling to over the barrier regimes. The numerical results are also compared with those from existing approximate theories based on calculations of the non-adiabatic transmission coefficients. It is found that the two-surface Landau-Zener formula works well in calculating the transmission coefficients in the deep tunneling regime, where the crossing point between the two lowest vibrational states dominates the total reaction rate. When multiple vibrational levels are involved, including additional crossing points on the free energy surfaces is important to obtain the correct reaction rate using the Landau-Zener formula.

Xie, Weiwei; Xu, Yang; Zhu, Lili; Shi, Qiang

2014-05-01

386

Determination of the thermal rate coefficient, products, and branching ratios for the reaction of O/+/ /D-2/ with N2  

NASA Technical Reports Server (NTRS)

Atmosphere Explorer-C satellite measurements are used to determine rate coefficients (RCs) for the following reactions: O(+)(D-2) + N2 yields N2(+) + O (reaction 1), O(+)(D-2) + N2 yields O(+)(S-4) + N2 (reaction 2), and O(+)(D-2) + N2 yields NO(+) + N (reaction 3). Results show the RC for reaction 1 to be 1 (plus 1 or minus 0.5) x 10 to the -10th cu cm per sec, for reaction 2 to be 3 (plus 1 or minus 2) x 10 to the -11th cu cm per sec, and 3 to be less than 5.5 x 10 to the -11th cu cm per sec. It is also found that the reaction of O(+)(D-2) with N2 does not constitute a detectable source of NO(+) ions in the thermosphere.

Torr, D. G.; Torr, M. R.

1980-01-01

387

A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows  

NASA Technical Reports Server (NTRS)

The use of probability theory to determine the effects of turbulent fluctuations on reaction rates in turbulent combustion systems is briefly reviewed. Results are presented for the effect of species fluctuations in particular. It is found that turbulent fluctuations of species act to reduce the reaction rates, in contrast with the temperature fluctuations previously determined to increase Arrhenius reaction rate constants. For the temperature fluctuations, a criterion is set forth for determining if, in a given region of a turbulent flow field, the temperature can be expected to exhibit ramp like fluctuations. Using the above results, along with results previously obtained, a model is described for testing the effects of turbulent fluctuations of temperature and species on reaction rates in computer programs dealing with turbulent reacting flows. An alternative model which employs three variable probability density functions (temperature and two species) and is currently being formulated is discussed as well.

Chinitz, W.; Foy, E.; Rowan, G.; Goldstein, D.

1982-01-01

388

Preliminary Comparison of Reaction Rate Theory and Object Kinetic Monte Carlo Simulations of Defect Cluster Dynamics Under Irradiation.  

National Technical Information Service (NTIS)

The multiscale modeling scheme encompasses models from the atomistic to the continuum scale. Phenomena at the mesoscale are typically simulated using reaction rate theory (RT), Monte Carlo (MC), or phase field models. These mesoscale models are appropriat...

C. Domain C. S. Becquart R. E. Stoller S. I. Golubov

2006-01-01

389

Rate and reaction probability of the surface reaction between ozone and dihydromyrcenol measured in a bench scale reactor and a room-sized chamber  

NASA Astrophysics Data System (ADS)

Low volatility terpenoids emitted from consumer products can react with ozone on surfaces and may significantly alter concentrations of ozone, terpenoids and reaction products in indoor air. We measured the reaction probability and a second-order surface-specific reaction rate for the ozonation of dihydromyrcenol, a representative indoor terpenoid, adsorbed onto polyvinylchloride (PVC), glass, and latex paint coated spheres. The reaction probability ranged from (0.06-8.97) × 10 -5 and was very sensitive to humidity, substrate and mass adsorbed. The average surface reaction probability is about 10 times greater than that for the gas-phase reaction. The second-order surface-specific rate coefficient ranged from (0.32-7.05) × 10 -15 cm 4 s -1 molecule -1and was much less sensitive to humidity, substrate, or mass adsorbed. We also measured the ozone deposition velocity due to adsorbed dihydromyrcenol on painted drywall in a room-sized chamber, Based on that, we calculated the rate coefficient ((0.42-1.6) × 10 -15 cm 4 molecule -1 s -1), which was consistent with that derived from bench-scale experiments for the latex paint under similar conditions. We predict that more than 95% of dihydromyrcenol oxidation takes place on indoor surfaces, rather than in building air.

Shu, Shi; Morrison, Glenn C.

2012-02-01

390

Bimolecular reaction rates from ring polymer molecular dynamics: application to H + CH4 ? H2 + CH3.  

PubMed

In a recent paper, we have developed an efficient implementation of the ring polymer molecular dynamics (RPMD) method for calculating bimolecular chemical reaction rates in the gas phase, and illustrated it with applications to some benchmark atom-diatom reactions. In this paper, we show that the same methodology can readily be used to treat more complex polyatomic reactions in their full dimensionality, such as the hydrogen abstraction reaction from methane, H + CH(4) ? H(2) + CH(3). The present calculations were carried out using a modified and recalibrated version of the Jordan-Gilbert potential energy surface. The thermal rate coefficients obtained between 200 and 2000 K are presented and compared with previous results for the same potential energy surface. Throughout the temperature range that is available for comparison, the RPMD approximation gives better agreement with accurate quantum mechanical (multiconfigurational time-dependent Hartree) calculations than do either the centroid density version of quantum transition state theory (QTST) or the quantum instanton (QI) model. The RPMD rate coefficients are within a factor of 2 of the exact quantum mechanical rate coefficients at temperatures in the deep tunneling regime. These results indicate that our previous assessment of the accuracy of the RPMD approximation for atom-diatom reactions remains valid for more complex polyatomic reactions. They also suggest that the sensitivity of the QTST and QI rate coefficients to the choice of the transition state dividing surface becomes more of an issue as the dimensionality of the reaction increases. PMID:21280711

Suleimanov, Yury V; Collepardo-Guevara, Rosana; Manolopoulos, David E

2011-01-28

391

Effect of pretreatment on the initial reaction rates of PGX and H451 graphites with HO and O  

Microsoft Academic Search

Initial reaction rates were examined for PGX and H451 graphites exposed to different atmospheres prior to oxidation in 2% HO\\/He or 2% O\\/N. Exposure to ambient air or 2% HO\\/He at room temperature caused high initial reaction rates of PGX and H451 graphites oxidized in 2% HO\\/He at 750°C or 800°C, which may be attributed to oxidant adsorption prior to

M. Eto; F. B. Growcock

1981-01-01

392

Actinometric measurement of solar ultraviolet and development of a weighted solar UV integral. [photochemical reaction rate determination  

NASA Technical Reports Server (NTRS)

An actinometer has been developed to measure outdoor irradiance in the range 295-400 nm. Actinometric measurements of radiation are based on determination of photochemical reaction rates for reactions of known quantum efficiency. Actinometers have the advantage of providing irradiance data over surfaces of difficult accessibility; in addition, actinometrically determined irradiance data are wavelength weighted and therefore provide a useful means of assessing the degradation rates of polymers employed in solar energy systems.

Gupta, A.; Coulbert, C.

1978-01-01

393

Kinetics for reaction of low-carbon steel melts with CO?CO 2 gas mixtures using nonlinear rate equations  

Microsoft Academic Search

The transfer reactions of carbon and oxygen from or to molten steel have been analyzed using nonlinear rate equations. The\\u000a nonlinear rate equations successfully reproduced the change of carbon and oxygen concentrations in molten steel. It was shown\\u000a that the coupling of chemical reaction and diffusion of carbon and oxygen should be taken into account within the diffusion\\u000a layer in

Masahiro Susa; Kazuhiro Nagata

1995-01-01

394

Analysis of turbulent free jet hydrogen-air diffusion flames with finite chemical reaction rates  

NASA Technical Reports Server (NTRS)

The nonequilibrium flow field resulting from the turbulent mixing and combustion of a supersonic axisymmetric hydrogen jet in a supersonic parallel coflowing air stream is analyzed. Effective turbulent transport properties are determined using the (K-epsilon) model. The finite-rate chemistry model considers eight reactions between six chemical species, H, O, H2O, OH, O2, and H2. The governing set of nonlinear partial differential equations is solved by an implicit finite-difference procedure. Radial distributions are obtained at two downstream locations of variables such as turbulent kinetic energy, turbulent dissipation rate, turbulent scale length, and viscosity. The results show that these variables attain peak values at the axis of symmetry. Computed distributions of velocity, temperature, and mass fraction are also given. A direct analytical approach to account for the effect of species concentration fluctuations on the mean production rate of species (the phenomenon of unmixedness) is also presented. However, the use of the method does not seem justified in view of the excessive computer time required to solve the resulting system of equations.

Sislian, J. P.

1978-01-01

395

Effective Rates in Dilute Reaction-Advection Systems for the Annihilation Process A+ A??  

NASA Astrophysics Data System (ADS)

A dilute system of reacting particles transported by fluid flows is considered. The particles react as A+ A?? with a given rate when they are within a finite radius of interaction. The system is described in terms of the joint n-point number spatial density that it is shown to obey a hierarchy of transport equations. An analytic solution is obtained in the dilute or, which is equivalent, the long-time limit by using a Lagrangian approach where statistical averages are performed along non-reacting trajectories. In this limit, it is shown that the moments of the number of particles have an exponential decay rather than the algebraic prediction of standard mean-field approaches. The effective reaction rate is then related to Lagrangian pair statistics by a large-deviation principle. A phenomenological model is introduced to study the qualitative behavior of the effective rate as a function of the interaction length, the degree of chaoticity of the dynamics and the compressibility of the carrier flow. Exact computations, obtained via a Feynman-Kac approach, in a smooth, compressible, random delta-correlated-in-time Gaussian velocity field support the proposed heuristic approach.

Krstulovic, G.; Cencini, M.; Bec, J.

2013-11-01

396

The Reaction Mechanism of Decomposing Chloroform by Bi-Metal Nano-Metallic Particles of Fe/Ni  

SciTech Connect

By adding Ni into the production of Fe/Ni nano-metallic particles, the acceleration of reduction ability of particles to decompose Chloroform is observed. The addition also could inhibit the shielding effect of pure iron compounds. This research studied the production and properties of the nano-particle metallic compounds of Fe and Ni, the decomposition of Chloroform by the particles and the mechanism of the decomposition processes. The experimental results indicated effective and rapid decomposition of chloroform by the Fe/Ni nano-particles on aluminum oxides, comparing to nano particles of iron in other researches. The reaction mechanism of Fe/Ni particles was pseudo first order with the half life about 0.7 hour, which was much shorter than the nano-Fe particles.

Hsieh, Su-Hwei; Horng, Jao-Jia

2004-03-31

397

Low vs high dose-rate effects on the acute reaction of pig skin to Cobalt-60 gamma rays  

SciTech Connect

The acute moist reaction of pig skin to regimens of low and high dose-rate irradiation with plaques containing a single, small /sup 60/Co source was studied in order to determine the application time for a continuous, low dose-rate exposure which is equivalent to a fractionated, high dose-rate course. The dose distributions for the high and low dose-rate sources were identical. The visual threshold for moist desquamation, i.e., the margin of the initial peak reaction, served as the biological endpoint. Ten young Chester White female pigs were irradiated-eight areas on the high dose-rate (control) side, and eight corresponding areas on the low dose-rate (experimental) side. During the subsequent 60-day observation period the longitudinal and transverse diameters of the acute moist reactions were measured. For the purpose of analysis the average initial peak diameter of the moist reaction was plotted against application time for the low dose-rate (experimental) side of each pig. It was found that a continuous, low dose-rate application of 63.8 hours duration at a dose-rate of 88.6 rad per hour was equivalent to four 1100 rad hgh dose-rate fractions separated by 24-hour intervals.

Morrison, R.A.; Zellmer, D.L.; Dean, R.D.

1981-03-01

398

Early diagenetic processes, fluxes, and reaction rates in sediments of the South Atlantic  

SciTech Connect

Porewaters recovered from sediment cores (gravity corers, box corers, and multicorers) from various subregions of the South Atlantic (Amazon River mouth, equatorial upwelling, Congo River mouth, Benguela coastal upwelling area, and Angola Basin) were investigated geochemically. Objectives included determination of Eh, pH, oxygen, nitrate, sulfate, alkalinity, phosphate, ammonium, fluoride, sulfide, Ca, Mg, Sr, Fe, Mn, and Si, in order to quantify organic matter diagenesis and related mineral precipitation and dissolution processes. Porewater profiles from the eastern upwelling areas of the South Atlantic suggest that sulfate reduction in the deeper parts of the sediment may be attributed mainly to methane oxidation, whereas organic matter degradation by sulfate reduction is restricted to the near-surface sediments. Further, a prominent concentration gradient change of sulfate and related mineralization products occurred typically in the upwelling sediments at a depth of 4 to 8 m, far below the zone of bioturbation or bioirrigation. Because other sedimentological reasons seem to fail as explanations, an early diagenetic sulfide oxidation to sulfate within the anoxic environment is discussed. Porewater profiles from the sediments of the Amazon fan area are mainly influenced by reactions with Fe(III)-phases. The remarkable linearity of the concentration gradients of sulfate supports the idea of distinct reaction layers in these sediments. In contrast to the upwelling sediments, the sulfate gradient develops from the sediment surface to a sulfate reduction zone at a depth of 5.3 m, probably because a reoxidation of sulfide is prevented by the reaction with iron oxides and the formation of iron sulfide minerals. A comparison of organic matter degradation rates from the different areas of the South Atlantic show the expected relationship to primary productivity.

Schulz, H.D.; Dahmke, A.; Schinzel, U.; Wallmann, K.; Zabel, M. (Univ. of Bremen (Germany))

1994-05-01

399

Analytical Solution of Steady State Equations for Chemical Reaction Networks with Bilinear Rate Laws  

PubMed Central

True steady states are a rare occurrence in living organisms, yet their knowledge is essential for quasi-steady state approximations, multistability analysis, and other important tools in the investigation of chemical reaction networks (CRN) used to describe molecular processes on the cellular level. Here we present an approach that can provide closed form steady-state solutions to complex systems, resulting from CRN with binary reactions and mass-action rate laws. We map the nonlinear algebraic problem of finding steady states onto a linear problem in a higher dimensional space. We show that the linearized version of the steady state equations obeys the linear conservation laws of the original CRN. We identify two classes of problems for which complete, minimally parameterized solutions may be obtained using only the machinery of linear systems and a judicious choice of the variables used as free parameters. We exemplify our method, providing explicit formulae, on CRN describing signal initiation of two important types of RTK receptor-ligand systems, VEGF and EGF-ErbB1.

Halasz, Adam M.; Lai, Hong-Jian; McCabe, Meghan M.; Radhakrishnan, Krishnan; Edwards, Jeremy S.

2014-01-01

400

Partial reactions of the Na,K-ATPase: determination of rate constants  

PubMed Central

Experiments were designed to characterize several partial reactions of the Na,K-ATPase and to demonstrate that a model can be defined that reproduces most of the transport features of the pump with a single set of kientic parameters. We used the fluorescence label 5- iodoacetamidofluorescein, which is thought to be sensitive to conformational changes, and the styryl dye RH 421, which can be applied to detect ion-binding and -release reactions. In addition transient electric currents were measured, which are associated mainly with the E1-->E2 conformational transition. Numerical simulations were performed on the basis of a reaction model, that has been developed from the Post- Albers cycle. Analysis of the experimental data allows the determination of several rate constants of the pump cycle. Our conclusions may be summarized as follows: (a) binding of one Na+ ion at the cytoplasmic face is electrogenic. This Na+ ion is specifically bound to a neutral binding site with an affinity of 8 mM in the presence of 10 mM Mg2+. In the absence of divalent cations, the intrinsic binding affinity was found to be 0.7 mM. (b) The analysis of fluorescence experiments with the cardiotonic steroid strophanthidin indicates that the 5-iodoacetamidofluorescein label monitors the conformational transition (Na3)E1-P-->P-E2(Na2), which is accompanied by the release of one Na+ ion. 5-IAF does not respond to the release of the subsequent two Na+ ions, which can be monitored by the RH 421 dye. These experiments indicate further that the conformational transition E1P-->P-E2 is the rate limiting process of the Na+ translocation. The corresponding rate constant was determined to be 22 s-1 at 20 degrees C. From competition experiments with cardiotonic steroids, we estimated that the remaining 2 Na+ ions are released subsequently with a rate constant of at least 5,000 s-1 from their negatively charged binding sites. (c) Comparing the fluorescence experiments with electric current transients, which were performed at various Na concentrations in the absence and presence of strophanthidin, we found that the transition (Na3).E1-P-->P-E2.(Na2) is the major charge translocating step in the reaction sequence Na3.E1-->(Na3).E1-P-->P-E2.(Na2)-->P-E2. The subsequent release of 2 Na+ ions contributed less than 25% to the total electric current transient. (d) The well known antagonism between cardiotonic steroids and K+ binding can be explained by a kinetic model. A quantitative description has been obtained under the assumption that these inhibitors bind only to the states P-E2(Na2) and P-E2(K2).(ABSTRACT TRUNCATED AT 400 WORDS)

1994-01-01

401

Relative rate study of the kinetic isotope effect in the 13CH3D + Cl reaction  

NASA Astrophysics Data System (ADS)

The 13CH3D/12CH4 kinetic isotope effect, ?13CH3D, of CH4 + Cl is determined for the first time, using the relative rate technique and Fourier transform infrared (FTIR) spectroscopy. ?13CH3D is found to be 1.60 ± 0.04. In addition, a quantum chemistry/transition state theory model with tunneling correction is constructed and the primary cause for ?13CH3D is found to be the substantially reduced reactivity of the D atom, which, in turn, can be explained by a significant increase in the reaction barrier due to changes in the vibrational zero point energy and to a lesser extent tunneling.

Joelsson, L. M. T.; Forecast, R.; Schmidt, J. A.; Meusinger, C.; Nilsson, E. J. K.; Ono, S.; Johnson, M. S.

2014-06-01

402

Temperature dependence of the NO + O3 reaction rate from 195 to 369 K  

NASA Technical Reports Server (NTRS)

The temperature dependence of the NO + O3 reaction rate was examined by means of the fast flow technique. Several different experimental conditions and detection schemes were employed. With excess NO or excess O3, NO2 chemiluminescence was monitored. In addition, with excess O3, NO was followed by fluorescence induced by an NO microwave discharge lamp. The results of the three independent sets of data are compared and found to agree within experimental error, indicating the absence of secondary chemistry which might complicate the kinetics. The data exhibit curvature on an Arrhenius plot; however, the simple Arrhenius expression k = (2.6 + or - 0.8) x 10 to the -12th exp(-1435 + or - 64/T) cu cm/molecule s is an adequate description for T between 195 and 369 K. This result is compared to earlier determinations.

Michael, J. V.; Allen, J. E., Jr.; Brobst, W. D.

1981-01-01

403

Pressure dependence on the reaction propagation rate of PETN at high pressure  

SciTech Connect

The reaction propagation rate (RPR) of the sensitive high explosive pentaerythritol tetranitrate (PETN) was measured in a diamond anvil cell (DAC) over the pressure range of 2--20 GPa. The experimental technique used is the same as that previously reported. The RPR data shows that it burns one to two orders of magnitude faster in the DAC than 1,3,5,-triamino-2,4,6-trinitrobenzene (TATB) and nitromethane (CH{sub 3}NO{sub 2}) respectively. The PETN RPR curve did not show sample pressure-dependent behavior like that of nitromethane, but instead varied abruptly like the RPR curve of TATB. In order to interpret these changes, static-pressure DAC mid-IR FTIR spectra were taken of micro-pellets of PETN embedded in KBr. The relationship between changes in the spectra, the RPR curve, and published single crystal PETN wedge test data are discussed.

Foltz, M.F.

1993-04-01

404

Solubility and Reaction Rates of Aluminum Solid Phases Under Hydrothermal Conditions  

SciTech Connect

Experimental studies involving equilibrium solubility and dissolution/precipitation rates were initiated on boehmite (AIOOH) using a hydrogen-electrode concentration cell (HECC). This cell provides continuous, accurate in situ pH measurements of solid/solution mixtures to 295 C with provision for either removing solution samples for analysis of the metal content, or adding either of two titrants. This cell has been recently used to measure the solubility of minerals such as brucite; boehmite, zincite, arid magnetite. The ability to perturb pH, isothermally by addition of acidic or basic titrant opens the door for studies of the kinetics of dissolution/precipitation, even for relatively fast reactions. By monitoring the change in pH, with time, detailed kinetic information can be obtained without the need for sampling.

Benezeth, P.; Palmer, D.A.; Wesolowski, D.

1999-11-14

405

A simplified perturbation model for calculation of few-group cross sections and reaction rates  

SciTech Connect

A fast efficient perturbation model is developed for generating four-group cross sections and flux spectrum and performing fuel depletion in a light water reactor unit cell configuration. In the thermal energy region, an approximate solution of the neutron spectrum is determined based on the Wigner-Wilkins free-gas hydrogen scattering model. In the fast energy region, a combination of analytical and empirical techniques is used to determine resonance cross sections. These models are combined in a perturbation scheme and incorporated as the SIFAS code. Reaction rates of important nuclides in reactor cores can be estimated by the code to within 0.5% for fuel depletion studies with fuel burnup of up to 30,000 MWd/tonne.

Sinclair, A.N.; Lee, J.C.

1983-10-01

406

Dual effect of adsorbed polymeric hydroxide films on the rates of different electrode reactions  

SciTech Connect

During the cathodic polarization of a dropping mercury electrode in the presence of M/sup 3 +/ ions of the lanthanide series, H/sub 2/ evolution occurs which starts at E -1.7 V (sce) and is provoked by a catalytically active surface film (CF) produced by hydrolysis of these ions and consisting of polymeric oxohydroxo rare-earth compounds. The authors see how the CF mentioned above will affect the rate of electrode reactions involving metal ions; to this end they studied the influence of Er/sup 3 +/ ion hydrolysis products on the electro-chemical behavior of Zn and of Na amalgam. Results of the study are discussed in detail.

Zakharkina, P.S.; Korshunov, V.N.

1986-02-01

407

Thermochemical sulphate reduction (TSR): experimental determination of reaction kinetics and implications of the observed reaction rates for petroleum reservoirs  

Microsoft Academic Search

Thermochemical Sulphate Reduction (TSR), responsible for substantial quantities (10–80%) of H2S in some high temperature petroleum accumulations, has been simulated in gold-titanium reaction vessels at 280–350 °C and 250–500 bars and in-situ pH values between 5.2 and 6.8 using on-line fluid sampling and analysis to monitor continuous reaction progress. The calculated activation energy of TSR is 142 kJ\\/mol with a half

Martin M. Cross; David A. C. Manning; Simon H. Bottrell; Richard H. Worden

2004-01-01

408

Rate coefficient for the reaction H + O{sub 2} {yields} OH + O: Results from 2000 to 5300 K  

SciTech Connect

Tunable-laser flash-absorption has recently provided an unprecedented extension of the temperature range over which the rate of the reaction H + O{sub 2} {yields} OH + O has been measured, resolving an 18-year-old controversy over the high-temperature behavior of the rate of the most important reaction in combustion. Studies have shown that the rate of the above reaction follows a simple Arrhenius expression, k(T) = 1.62 {times} 10{sup {minus}10} exp[{minus}7474/T(K)] cm{sup 3} s{sup {minus}1}, from 960 to 5300 K. Previously, measurements by one group extended only up to 3300 K. At 2600 K, the highest temperature studied by three other groups, the measured rates differed by 220%. To obtain the new results, a shock wave was used to quickly heat a sample and the rate of production of OH directly behind the shock wave was measured by tuning the wavelength of a pulsed dye laser to a region of strong absorption by the OH molecule. The strength of this absorption was measured as a function of distance behind the shock wave by directing the light from the laser beam onto a linear-array detector. This technique is capable of measuring the rates of very fast reactions, and is ideal for precise studies at very high temperatures where the rates of chemical reactions are the fastest.

Hessler, J.P.; Du, Hong

1994-03-01

409

Reaction rates, depositional history and sources of indium in sediments from Appalachian and Canadian Shield lakes  

NASA Astrophysics Data System (ADS)

Sediment cores were collected at the deepest site of twelve headwater lakes from the Province of Québec, Canada that receive contaminants only from atmospheric deposition, either directly to the lake surface or indirectly from the watershed. Several of the lakes are located within relatively short distance (<40 km) and others at more than 200 km from potential sources of contamination. The sediments were dated and analyzed for In and other elements including Fe, Mn, Al and organic C. Fe-rich authigenic material was collected on Teflon sheets inserted vertically into the sediments at the only study site whose hypolimnion remains perennially oxic. Porewater samples collected at the coring site of four of the lakes were also analyzed for In and other solutes including sulfide, sulfate, Fe, Mn, inorganic and organic C and major ions. The porewater In profiles display concentration gradients at or below the sediment-water interface. Modeling these profiles with a one-dimensional transport-reaction equation assuming steady state allows definition of depth intervals (zones) where In is either released to or removed from porewater and quantification of net In reactions rates in each zone. The position of the In consumption zones, the shape of the vertical profiles of dissolved In, sulfide and iron, as well as thermodynamic calculations of saturation states collectively suggest that In(OH)3(s) and In2S3(s) do not precipitate in the sediments and that adsorption of In onto sedimentary FeS(s) does not occur. However, similarities in the In and Fe porewater profiles, and the presence of In in the authigenic Fe-rich solids, reveal that part of the In becomes associated with authigenic Fe oxyhydroxides in the perennially oxic lake and is coupled to the Fe redox cycling. Comparison of the In/Corg and In/Fe molar ratios in the authigenic Fe-rich material and in surface sediments (0-0.5 cm) of this lake suggests that most non-lithogenic In was bound to humic substances. From the magnitude of the net In reaction rates, we infer that the post-depositional redistribution of this element is quantitatively not important and that the In sedimentary record represents accurately In deposition at the sampling sites. Reconstructed chronologies of the anthropogenic In deposition and comparison of In inventories among lakes point to non-ferrous metal smelters as a past significant source of atmospheric In contamination and to a significant reduction of industrial In emissions into the North American atmosphere in recent decades.

Tessier, André; Gobeil, Charles; Laforte, Lucie

2014-07-01

410

High-precision {sup 28}Si(p,t){sup 26}Si reaction to determine {sup 22}Mg({alpha},p){sup 25}Al reaction rates  

SciTech Connect

The rise time of stellar x-ray bursts is a signature of thermonuclear runaway processes in the atmosphere of neutron stars and is highly sensitive to a series of ({alpha},p) reactions via high-lying resonances in sd-shell nuclei. Lacking data for the relevant resonance levels, the stellar reaction rates have been calculated using statistical, Hauser-Feshbach models, assuming a high-level density. This assumption may not be correct in view of the selectivity of the ({alpha},p) reaction to natural parity s