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1

Enhanced kinetics of pseudo first-order hydrolysis in liquid phase coexistent with ice.  

PubMed

The reaction rate of the hydrolysis of fluorescein diacetate (FDA) is several times larger in the frozen state than that in the unfrozen solution of the same composition at the same temperature. The freeze concentration of reactants in the liquid phase expelled form ice crystals cannot explain the kinetic enhancement of pseudo first order reactions such as the FDA hydrolysis. However, the reaction rate increases as the freeze concentration ratio becomes larger at a constant temperature. Direct pH measurements have revealed that the basicity of the liquid phase is unchanged at any concentration ratio, suggesting that the reactivity enhancement is not caused by increased basicity. The reaction rate enhancement is clearly related to the size of the space in which the liquid phase is confined upon freezing. The ice wall itself or the water structure formed near the wall should thus be responsible for this kinetic enhancement. PMID:24063609

Anzo, Kenji; Harada, Makoto; Okada, Tetsuo

2013-10-03

2

Kinetics of the Reaction of Baker?s Yeast Glucose6-Phosphate-Dehydrogenase with 5,5?-Dithiobis(2-Nitrobenzoic Acid)  

Microsoft Academic Search

The kinetics of the reaction of baker?s yeast glucose-6-phosphate dehydrogenase with excess 5,5?-dithiobis(2-nitrobenzoic acid) (DTNB) were studied at pH 8.5 and 30C and at constant ionic strength of 0.01 and in the absence and in the presence of NADP+ or glucose 6-phosphate. The reaction follows pseudo-first-order kinetics irrespective of whether these substrates are absent or present. The observed pseudo-first-order rate

S. A. Adediran; M. R. Gbadegesin

1995-01-01

3

Reactions & Rates  

NSDL National Science Digital Library

Explore what makes a reaction happen by colliding atoms and molecules. Design experiments with different reactions, concentrations, and temperatures. When are reactions reversible? What affects the rate of a reaction?

Simulations, Phet I.; Koch, Linda; Lemaster, Ron; Loeblein, Trish; Perkins, Kathy; Gratny, Mindy

2006-12-01

4

Rate constant for the reaction of N( 4S) with CH 3 AT 298 K  

NASA Astrophysics Data System (ADS)

The absolute rate constant for the reaction of N( 4S) with CH 3 has been determined at 298 K in a discharge flow system coupled with collision-free sampling mass spectrometry at low electron energies. The rate constant was determined under pseudo-first-order conditions with [N] > [CH 3]. The result is k1 = (8.62.0) 10 -11 cm 3 s -1. This is compared to a previous estimated value as well as values of rate constants for other atom + radical reactions.

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

1988-06-01

5

Introducing total substrates simplifies theoretical analysis at non-negligible enzyme concentrations: pseudo first-order kinetics and the loss of zero-order ultrasensitivity  

Microsoft Academic Search

The BriggsHaldane standard quasi-steady state approximation and the resulting rate expressions for enzyme driven biochemical\\u000a reactions provide crucial theoretical insight compared to the full set of equations describing the reactions, mainly because\\u000a it reduces the number of variables and equations. When the enzyme is in excess of the substrate, a significant amount of substrate\\u000a can be bound in intermediate complexes,

Morten Gram Pedersen; Alberto Maria Bersani

2010-01-01

6

Flash photolysis-shock tube kinetic study of the reaction of O( sup 3 P) with ethylene: 1,052 K <= T <= 2,284 K  

SciTech Connect

Kinetic experiments on the title reaction were performed over the temperature range 1,052-2,284 K by use of the flash photolysis-shock tube apparatus. Absolute rate constants were determined directly from measured pseudo-first-order decays of O({sup 3}P) in 78 individual runs.

Klemm, R.B.; Sutherland, J.W.; Wickramaaratchi, M.A.; Yarwood, G. (Brookhaven National Lab., Upton, NY (USA))

1990-04-19

7

Isotopic exchange in mineral-fluid systems. IV. The crystal chemical controls on oxygen isotope exchange rates in carbonate-H 2O and layer silicate-H 2O systems  

Microsoft Academic Search

Oxygen isotope exchange between minerals and water in systems far from chemical equilibrium is controlled largely by surface reactions such as dissolution-precipitation. In many cases, this behavior can be modeled adequately by a simple pseudo-first order rate model that accounts for changes in surface area of the solid. Previous modeling of high temperature isotope exchange data for carbonates, sulfates, and

David R. Cole

2000-01-01

8

Rates of nitrous oxide production in the oxidation of hydroxylamine by iron(III)  

SciTech Connect

The dependence of the rate of N/sub 2/O formation on the reactants and on pH in the oxidation of NH/sub 2/OH by Fe(III) has been investigated over a range of reactant concentrations, with emphasis on low concentrations of hydroxylamine. The production rate of N/sub 2/O was pseudo first order with respect to total iron, pseudo first order or less with respect to total hydroxylamine, and inversely proportional to (H/sup +/)/sup 2.5/. The reaction is highly sensitive to temperature. Of the competing reactions producing nitrogenous products other than N/sub 2/O, at least one is catalyzed by light. An empirical rate law, based on the total concentrations of the reactants, is presented, and a more general rate law, based on the concentrations of presumed reactive species, is derived.

Butler, J.H.; Gordon, L.I.

1986-12-03

9

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

SciTech Connect

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

Stief, L.J.; Nesbitt, F.L.; Payne, W.A. (Laboratory of Extraterrestrial Laboratory, NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771 (United States)); Kuo, S.C.; Tao, W.; Klemm, R.B. (Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973 (United States))

1995-04-01

10

Investigation of the Combined Effects of External Mass Transfer and Biodegradation Rates on Phenol Removal Using Immobilized P. putida in a Packed-Bed Column Reactor  

Microsoft Academic Search

A quantitative analysis of external mass transfer combined with a biodegradation reaction and correlation of the experiment with theory was performed for phenol removal using calcium-alginate gel-immobilized P. putida in a packed-bed column reactor. Assuming first-order biodegradation kinetics, pseudo first-order biodegradation rate constants (kp) were calculated. To investigate the effect of external film diffusion on the biodegradation rate, various mass

Zmriye Aksu; Glta Blbl

1998-01-01

11

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

Cunningham, Kevin

2007-01-01

12

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

13

High-temperature photochemistry kinetics study of the reaction of O(³P) atoms with ethylene from 290 to 1510 K  

Microsoft Academic Search

The O + CH reaction has been investigated in a high-temperature photochemistry (HTP) reactor at temperatures from 290 to 1510 K and pressures from 60 to 500 Torr. Ground-state O atoms were generated by flash photolysis of CO and monitored by time-resolved atomic resonance fluorescence with pulse counting. Measurements were made under pseudo-first-order conditions (O) << (CH). Rate coefficients from

Khaled. Mahmud; Paul. Marshall; Arthur. Fontijn

1987-01-01

14

Fast liquid-liquid reactions: Role of emulsifiers  

SciTech Connect

The effect of emulsifiers such as sodium dioctyl sulfosuccinate, sorbitan monooleate, etc., on the rate of alkaline hydrolysis of higher formate esters was studied. The extraction of these formate esters is accompanied by a fast pseudo-first-order reaction in the diffusion film. Values of effective interfacial area were measured in a mechanically agitated contactor; the specific rate of extraction was measured in a stirred cell of known interfacial area. The effects of speed of agitation and the emulsifier concentration on the effective interfacial area were studied. Emulsifiers can increase the effective interfacial area markedly; the highest increase was by a factor of 14.

Smita, S.L.; Bhave, R.R.; Sharma, M.M.

1983-01-01

15

Temperature dependence of the rate constant for the HO/sub 2/ + CH/sub 3/O/sub 2/ gas-phase reaction  

SciTech Connect

The temperature dependence of the reaction between hydroperoxy and methylperoxy radicals was measured in a flash photolysis ultraviolet absorption apparatus over the temperature range 228-380 K: HO/sub 2/ + CH/sub 3/O/sub 2/ ..-->.. CH/sub 3/O/sub 2/H + O/sub 2/ (1). The data, represented by the Arrhenius expression k/sub 1/ = (3.0 +/- 1.2) x 10/sup -13/ exp((720 +/- 100)/T) cm/sup 3/ molecule/sup -1/ s/sup -1/, are compared to earlier results and discussed in terms of the reaction mechanism. Due to overlapping absorptions of the two radicals and deviations of the complex reaction system from both pseudo-first-order and pseudo-second-order behavior, the rate constants were determined from a detailed modeling of the radical decay curves. A sensitivity analysis of the rate constant determination procedure to the assumed radical absorption cross sections and correlated changes in the rate constants for the HO/sub 2/ and CH/sub 3/O/sub 2/ self-reactions was performed, and the results are reported. The present results were also used to assess the effects of secondary chemistry in the authors measurements of the temperature dependence of the rate constant of the CH/sub 3/O/sub 2/ + CH/sub 3/O/sub 2/ reaction, and the revised Arrhenius parameters are presented.

Dagut, P.; Wallington, T.J.; Kurylo, M.J.

1988-06-30

16

Measurement of Sulfur Dioxide Reaction Rates in Wintertime Orographic Clouds  

NASA Astrophysics Data System (ADS)

Releases of sulfur dioxide (SO_2 ) into the wintertime orographic clouds at Elk Mountain in southeastern Wyoming were utilized to accelerate the rate of SO_2 oxidation to cloud -water-dissolved sulfate (SO_sp{4} {2-}). Background SO_2 mixing ratios were 0.6 part-per-billion by volume (ppbv) and were consistent with the remote location of the experimental site and with supplemental cloud water, snow, and aerosol composition measurements. Background mixing ratios of hydrogen peroxide (H_2O _2) and the organohydroperoxides, expressed as methyl hydroperoxide (MHP), were 0.15 and 0.17 ppbv, respectively. The concentration of H_2O _2 in cloud water, obtained as rime, was also monitored. Observed concentrations were always lower than concentrations calculated assuming partition equilibrium with gas-phase H_2O _2. Analysis of these findings suggests that both reactive loss of H_2O _2 and volatilization during riming are mechanisms for H_2O _2 loss. The pseudo first-order SO_2 depletion rates, measured as the change in the equivalent mixing ratio of cloud-water-dissolved SO_sp {4}{2-} per unit SO _2 mixing ratio and per unit in-cloud reaction time, varied between 2 and 71%/hr (x = 32 +/- 22%/hr, n = 10). Observed depletions of H _2O_2 (x = 0.030 ppbv) were consistent with observed yields of SO_sp {4}{2-} (x = 0.027 ppbv) and with model predictions. This result is contrasted with the work of Chandler et al. (1989) who observe that field measurements of the bisulfite-H_2O _2 rate constant, in northern England, are approximately a factor of ten larger than laboratory measurements. Observed depletions of MHP were not significantly different from 0.0 ppbv. This observation is both consistent with the much smaller solubility of MHP, compared with H_2O_2, and with the results of 16 model simulations, which were initialized with cloud water and air composition measurements. With the exception of two model simulations, reactions between dissolved SO_2 and O _3, between SO_2 and O_2 (catalyzed by Mn(II) and Fe(III)), and between SO_2 and HCHO were calculated to contribute less than 40% to the total amount of SO _sp{4}{2-}. These reactions were inferred to be inhibited by the low pH (< 5) of the Elk Mountain cloud water. It is concluded that H_2O_2 is the dominant SO_2 oxidant in these clouds, and that the laboratory measurements form an adequate basis for predicting the rate of in-cloud oxidation of SO_2 by H_2O _2.

Snider, Jefferson Robert

17

Rates of Chemical Reactions  

NSDL National Science Digital Library

Science Objects are two hour on-line interactive inquiry-based content modules that help teachers better understand the science content they teach. This Science Object is the third of four Science Objects in the Chemical Reactions SciPack. It demonstrates

National Science Teachers Association (NSTA)

1900-01-01

18

QSARS FOR PREDICTING REDUCTIVE TRANSFORMATION RATE CONSTANTS OF HALOGENATED AROMATIC HYDROCARBONS IN ANOXIC SEDIMENT SYSTEMS  

EPA Science Inventory

Quantitative structure-activity relationships (QSARs) are developed relating initial and final pseudo-first-order disappearance rate constants of 45 halogenated aromatic hydrocarbons in anoxic sediments to four readily available molecular descriptors: the carbon-halogen bond stre...

19

Comparative efficiencies of the decolourisation of Methylene Blue using Fenton's and photo-Fenton's reactions.  

PubMed

The decolourisation and mineralization of Methylene Blue azo dye (MB) under Fenton's and photo-Fenton's conditions have been studied. Some parameters, such as initial concentrations of iron (Fe(2+)) and fixed hydrogen peroxide (H(2)O(2)), were evaluated to find the optimal conditions for the efficient degradation of azo dye. The experimental assays showed better efficiency for the photo-Fenton's system. Pseudo-first order degradation rate constants were obtained from batch experimental set up. It is suggested that photo-Fenton's reactions are viable processes for treatment of azo dye MB, according to the high levels of colour (UV(665)) and total organic carbon (TOC) removal. PMID:19424530

Melgoza, D; Hernndez-Ramrez, A; Peralta-Hernndez, J M

2009-01-23

20

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

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

Infrared spectroscopic studies of the heterogeneous reaction of ozone with maleic and fumaric acid aerosols  

NASA Astrophysics Data System (ADS)

Dicarboxylic acids, either directly emitted or formed in chemical processes are found to be a significant component of tropospheric aerosols. To assess any potential role of short unsaturated dicarboxylic acids in tropospheric heterogeneous chemistry, maleic acid (MaH) and fumaric acid (FuH) were selected as surrogates in this study. A new aerosol flow tube apparatus is employed to perform kinetic studies of the oxidation of organic compounds containing aerosols by gas phase ozone. The system consists of a lab-made particle generation system, a vertically oriented glass flow tube with moveable injector and a multi-pass cross beam White cell for measurement of aerosol and gas phase composition via Fourier transform infrared (FTIR) spectroscopy. A flow of single component organic aerosols with mean diameters ranging between 0.8-2.1 m are introduced in a flow tube, in which the particles are subsequently exposed to a known concentration of ozone for a controlled period of time. These studies are complemented with offline analysis on the reaction products. Data from these studies were used to determine the kinetics of the reaction under a range of conditions. The reaction exhibited pseudo first order kinetics for gas product formation, and the pseudo first order rate coefficients displayed a Langmuir-Hinshelwood dependence on gas phase ozone concentration for both materials. By assuming Langmuir-Hinshelwood behaviour, the following parameters were found: for the reaction of MaH aerosols, KO3=(1.9+0.4)x10-16cm-3 and kmaxI=0.015+0.002; for the reaction of FuH aerosols, KO3= (3.4+0.4)x10-16cm-3 and kmaxI=0.0128+0.0005, where KO3 is a parameter that describes the partitioning of ozone to the particle surface and kmaxI is the maximum pseudo-first order coefficient at high ozone concentrations. Apparent reactive uptake coefficients were extracted from the pseudo first order rate coefficient and a slight trend of decreasing uptake coefficients with increasing ozone concentrations was observed. In general, we find that the values obtained for maleic and fumaric acid fall within the range of literature values reported by other published studies for other organic or organic-coated particles, although there are some unusual dependencies on RH%.

Najera, J.; Percival, C.; Horn, A. B.

2009-04-01

23

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

24

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

2012-04-09

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

2013-06-11

26

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 ...

27

Gas-phase atom-radical kinetics of atomic hydrogen, nitrogen, and oxygen reactions with CHF reactions  

SciTech Connect

The absolute rate constants for the reactions of atomic hydrogen, nitrogen, and oxygen with CHF radicals have been measured in a gas-flow system with photoionization mass spectrometry detection. Atomic reactants were produced by dissociation of the corresponding elemental gas in a microwave discharge. The pressure was 1.7 Torr. The rate constants for CHF decay under pseudo-first-order conditions at 293 K in units of cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} are (4.9 {plus minus} 0.9) {times} 10{sup {minus}10} for CHF + H, (2.4 {plus minus} 0.4) {plus minus} {times} 10{sup {minus}11} for CHF + N, and (1.4 {plus minus} 0.2) {times} 10{sup {minus}10} for CHF + O. These reactions appear to proceed by an association-elimination mechanism.

Tsai, Cheng-ping; McFadden, D.L. (Boston College, MA (USA))

1990-04-19

28

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

29

Kinetic analysis of the heparin-enhanced plasmin-antithrombin III reaction. Apparent catalytic role of heparin  

PubMed Central

Inactivation of plasmin by a 34-fold molar excess of antithrombin III follows pseudo-first-order kinetics and the apparent rate constants are proportional to the concentration of the inhibitor. Heparin accelerates the inactivation reaction without changing its pseudo-first-order character, and the apparent rate constants are also proportional to the concentration of the polysaccharide. Heparin results in a minimum 20-fold rate enhancement of the reaction between plasmin and antithrombin III when the concentrations of heparin and plasmin are approx. 0.5?m and 1?m respectively. Heparin at a molar concentration well below that of plasmin still accelerates the reaction: one molecule of the polysaccharide is able to facilitate the inactivation of about 100 molecules of plasmin. Heparin must bind to plasmin to accelerate the plasminantithrombin III reaction, since the modification of four to five lysine residues of the enzyme inhibits the rate-enhancement effect of heparin and the dissociation of heparinplasmin complex decreases the inactivation rate of plasmin. Increasing the concentration of antithrombin III, at a constant amount of heparin, results in increase of the inactivation rate. By contrast, the effect of increasing the amount of plasmin in the presence of constant amount of heparin and antithrombin III is such that higher plasmin-to-heparin ratios are associated with lower rates of inactivation. It seems, therefore, that to obtain `optimal' conditions for fast enzyme inactivation, the amount of heparin should be matched to plasmin rather than to antithrombin III. Arrhenius plots of the plasminantithrombin III reaction are linear both in the absence and presence of heparin, at concentrations of 1 or 2?g/ml, over a range of 26K. Under these experimental conditions, heparin increases activation entropy. The findings show that heparin seems to fulfil some criteria that are characteristic for biological catalysis: binding, reaction-rate enhancement (increasing activation entropy), recycling of heparin (effectiveness of non-stoichiometric amounts of the polysaccharide) and specificity.

Machovich, Raymund; Bauer, Pal I.; Aranyi, Peter; Kecskes, Eva; Buki, Kalman G.; Horvath, Istvan

1981-01-01

30

Nuclear reaction rates in a plasma  

Microsoft Academic Search

The problem of determining the effects of the surrounding plasma on nuclear reaction rates in stars is formulated {ital ab initio}, using the techniques of quantum statistical mechanics. Subject to the condition that the nuclear reactions ensue only at very close approach of the fusing ions and the condition that the reaction be slow, the authors derive a result that

Lowell S. Brown; R. F. Sawyer

1997-01-01

31

Nuclear reaction rates in a plasma  

Microsoft Academic Search

The problem of determining the effects of the surrounding plasma on nuclear reaction rates in stars is formulated ab initio, using the techniques of quantum statistical mechanics. Subject to the condition that the nuclear reactions ensue only at very close approach of the fusing ions and the condition that the reaction be slow, the authors derive a result that expresses

Lowell S. Brown; R. F. Sawyer

1997-01-01

32

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

33

One- and two-dimensional chemical exchange nuclear magnetic resonance studies of the creatine kinase catalyzed reaction  

SciTech Connect

The equilibrium chemical exchange dynamics of the creatine kinase enzyme system were studied by one- and two-dimensional {sup 31}P NMR techniques. Pseudo-first-order reaction rate constants were measured by the saturation transfer method under an array of experimental conditions of pH and temperature. Quantitative one-dimensional spectra were collected under the same conditions in order to calculate the forward and reverse reaction rates, the K{sub eq}, the hydrogen ion stoichiometry, and the standard thermodynamic functions. The pure absorption mode in four quadrant two-dimensional chemical exchange experiment was employed so that the complete kinetic matrix showing all of the chemical exchange process could be realized.

Gober, J.R.

1988-01-01

34

Reaction rates in hydrophilic glass  

NASA Astrophysics Data System (ADS)

Sugar-based glasses are known to stabilize proteins against aggregation and chemical degradation. It has long been supposed that, due to the long timescales involved in protein aggregation and chemical degradation in the glass, relaxation processes essentially control the rate of degradation. We have shown that, although the degradation processes occur on timescales of the alpha relaxation process, secondary relaxation processes, and not alpha relaxation seems to dominate in influencing both chemical and physical degradation. In this presentation we will discuss results from time-resolved fluorescence studies in sugar-based glasses designed to help understand the physics underlying these puzzling observations.

Cicerone, Marcus; Zhong, Qin

2010-03-01

35

Rates of thermonuclear reactions in dense plasmas  

Microsoft Academic Search

The problem of plasma screening of thermonuclear reactions has attracted considerable scientific interest ever since Salpeters\\u000a seminal paper, but it is still faced with controversial statements and without any definite conclusion. It is of relevant\\u000a importance to thermonuclear reactions in dense astrophysical plasmas, for which charge screening can substantially affect\\u000a the reaction rates. Whereas Salpeter and a number of subsequent

V. N. Tsytovich; M. Bornatici

2000-01-01

36

Rate of reaction of OH with CS  

Microsoft Academic Search

The flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction OH + CS ..-->.. products (k) over the temperature range 251-363 K. Complicating secondary reactions involving CS photofragments were eliminated only when the photoflash was filtered by 10 torr cm of CS and SF was used as the buffer gas. The rate constant was found

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

1980-01-01

37

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

38

Astrophysical Reaction Rates Obtained By Indirect Techniques  

NASA Astrophysics Data System (ADS)

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.; yst, J.; Banu, A.; Burjan, V.; Carstoiu, F.; Chen, X.; Clark, H. L.; Davidson, T.; Fu, C.; Gagliardi, C. A.; Hardy, J. C.; Iacob, V. E.; Jokinen, J.; Kroha, V.; Lui, Y.-W.; McCleskey, M.; Mukhamedzhanov, A.; Nica, N.; Park, H. I.; Roeder, B.; Saastamoinen, A.; Simmons, E.; Tabacaru, G.; Tokimoto, Y.; Trache, L.; Woods, P. J.; Zhai, Y.

2010-08-01

39

The Kinetic Rate Law for Autocatalytic Reactions.  

ERIC Educational Resources Information Center

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

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

1987-01-01

40

Chemical reaction rates and solvent friction  

SciTech Connect

The role of the dynamic solvent friction in influencing the rates of chemical reactions in solution is described. Features considered include (a) the bias of the reaction coordinate toward a direction of lesser friction in the diffusive limit, (b) the importance of frequency-dependent friction in atom transfers, tunneling reactions and isomerizations, (c) the dynamic nonequilibrium solvation in charge transfers which leads to a polar solvent molecule reorientation time dependence for the rate, and (d) the importance of internal degrees of freedom in the location of the Kramers turnover for isomerizations.

Hynes, J.T.

1986-01-01

41

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

2013-05-31

42

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

43

Reaction rates from electromagnetic gauge data  

SciTech Connect

The determination of reaction rates in explosives from experimental data is a task that requires a great deal of care, patience, and curve fitting. We have found that by measuring more quantities than are mathematically necessary for a complete Lagrange analysis, curve fits to experimental data do not have to be as exacting as otherwise required. We present an experimental technique specifically designed for the determination of global reaction rates which uses both embedded electromagnetic impulse and particle velocity gauges. The methods used to efficiently analyze the data, and the results and conclusions reached from several such studies are also presented. 16 refs., 9 figs.

Vorthman, J.; Andrews, G.; Wackerle, J.

1985-01-01

44

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

45

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(297K) = (1.06 0.12) 10-10 k2(T) = (6.8 0.7) 10-12 exp[(690 20)/T]; k2(297K) = (7.12 0.73) 10-11 k3(T) = (6.8 0.8) 10-12 exp[(680 20)/T]; k3(297K) = (6.76 0.70) 10-11 k4(T) = (5.4 0.6) 10-12 exp[(690 20)/T]; k4(297K) = (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.

2011-01-01

46

Reaction rates in randomly stratified media  

NASA Astrophysics Data System (ADS)

Chemical species are advected by water and undergo mixing due to several processes including local diffusion or dispersion. In turn, mixing causes disequilibrium and reactions take place to drive the system back to local equilibrium. Depending on the characteristic times of the processes, in some cases reactions can be assumed instantaneous. In general, a multicomponent reactive transport problem is described through a set of coupled non-linear partial differential equations. Under instantaneous chemical equilibrium, a complex geochemical problem can be highly simplified by defining the system in terms of conservative quantities, termed components, and the space-time distribution of reaction rates. We investigate the parameters controlling reaction rates in a heterogeneous aquifer at short distances from the source. Hydraulic conductivity at this scale is modeled as a random process with highly anisotropic correlation structure. We derive closed-form analytical solutions for statistical moments of reaction rates for the particular case of negligible transverse dispersion. Then, we analyze numerically the effect of accounting for transverse local dispersion

Fernandez-Garcia, D.; Sanchez-Vila, X.; Guadagnini, A.

2009-04-01

47

The reaction rate in nuclear fusion  

Microsoft Academic Search

SummaryA method to evaluate the reaction rate in nuclear fusion is proposed. This method is obtained in the framework of the elastic\\u000a model for subbarrier fusion. Conditions for the validity of the method are given. Some examples are discussed.

A. Scalia

1988-01-01

48

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

2007-01-25

49

Reaction rates and reaction sequences in the rp-process  

NASA Astrophysics Data System (ADS)

Hot hydrogen burning has been analyzed for temperatures and densities typical for explosive burning on compact objects and in supernovae. Reaction flow impedances due to cyclic reaction sequences, the role of waiting point nuclei, and the transition from the rp-process to the alpha p-process are discussed as a function of increasing temperature and nuclear properties. The analysis gives a clear indication of which nuclei play a crucial role in the burning process. It is here where experimental verifications of the presented reaction rate estimates are highly desirable. The same nuclei can also be utilized to devise minimum-size energy generation networks to be employed in hydrodynamic explosion calculations. Examples for specific conditions during supernova explosions, nova outbursts, or the evolution of Thorne-Zytkow objects are given, with typical abundance patterns expected in such events.

van Wormer, L.; Goerres, J.; Iliadis, C.; Wiescher, M.; Thielemann, F.-K.

1994-09-01

50

Initial Test of the Benchmark Chemical Approach for Predicting Microbial Transformation Rates in Aquatic Environments.  

National Technical Information Service (NTIS)

Using 2,4-dichlorophenoxyacetic acid methyl ester (2,4-DME) as a benchmark chemical, the authors determined relative pseudo-first-order rate coefficients for butoxyethyl ester of 2,4-dichlorophenoxyacetic acid (2,4-DBE), methyl parathion, and methyl-3-chl...

T. D. Newton D. K. Gattie D. L. Lewis

1990-01-01

51

Thermodynamic limitations on microbially catalyzed reaction rates  

NASA Astrophysics Data System (ADS)

Quantification of global biogeochemical cycles requires knowledge of the rates at which microorganisms catalyze chemical reactions. In order for models that describe these processes to capture global patterns of change, the underlying formulations in them must account for biogeochemical transformations over seasonal and millennial time scales in environments characterized by different energy levels. Building on existing models, a new thermodynamic limiting function is introduced. With only one adjustable parameter, this function that can be used to model microbial metabolism throughout the range of conditions in which organisms are known to be active. The formulation is based on a comparison of the amount of energy available from any redox reaction to the energy required to maintain a membrane potential, a proxy for the minimum amount of energy required by an active microorganism. This function does not require species- or metabolism-specific parameters, and can be used to model metabolisms that capture any amount of energy. The utility of this new thermodynamic rate limiting term is illustrated by applying it to three low-energy processes: fermentation, methanogenesis and sulfate reduction. The model predicts that the rate of fermentation will be reduced by half once the Gibbs energy of the catalyzed reaction reaches -12 kJ (mol e-)-1, and then slowing exponentially until the energy yield approaches zero. Similarly, the new model predicts that the low energy yield of methanogenesis, -4 to -0.5 kJ (mol e-)-1, for a partial pressure of H2 between 11 and 0.6 Pa decreases the reaction rate by 95-99%. Finally, the new function's utility is illustrated through its ability to accurately model sulfate concentration data in an anoxic marine sediment.

LaRowe, Douglas E.; Dale, Andrew W.; Amend, Jan P.; Van Cappellen, Philippe

2012-08-01

52

Quantum theory of chemical reaction rates  

SciTech Connect

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

Miller, W.H. [Univ. of California, Berkeley, CA (United States). Dept. of Chemistry]|[Lawrence Berkeley Lab., CA (United States). Chemical Sciences Div.

1994-10-01

53

Conversion of waste cellulose to ethanol. Phase II. Reaction kinetics with phosphoric acid  

SciTech Connect

Waste cellulosic material can be hydrolyzed in dilute acid solution to produce fermentable sugars which can then be converted into ethanol. A laboratory investigation was made of the feasibility of using phosphoric acid as the hydrolysis catalyst. The hydrolysis reaction with phosphoric acid solutions was compared with the reaction employing the more conventional dilute sulfuric acid catalyst. The purpose of this research was to examine the hydrolysis step in a proposed process for the conversion of cellulose (from wood, newspapers, municipal solid waste, or other sources) into ethanol - by which a potentially valuable co-product, DICAL (dicalcium phosphate), might be made and sold with or without the lignin content as a fertilizer. The pertinent reaction kinetics for the acid catalyzed production of glucose from cellulose consists of consecutive, pseudo-first order reactions. The first reaction forms glucose by hydrolyzing the cellulose polymer and a subsequent reaction decomposes the glucose. The maximum theoretical yield depends on the ratio of the rate constants for these two reactions. The rate constants of both reactions were measured in a series of experiments studying temperature and concentration effects. The results suggest that the glucose decomposition reaction is similar with the two acids but that the cellulose hydrolysis reaction mechanism with phosphoric acid may be different than with sulfuric acid. The studies show phosphoric acid is unpromising and much inferior to sulfuric acid as the catalytic agent. Under the conditions studied, 0.8 wt % sulfuric acid gives a greater yield of glucose than 8.0 wt % phosphoric acid.

Moeller, M.B.; Isbell, R.E.

1982-05-01

54

Mechanisms and rates of plagioclase carbonation reactions  

NASA Astrophysics Data System (ADS)

Plagioclase is one of the most abundant sources of calcium in the earth's crust, and it may play an important role for CO 2 storage. This study address' the carbonation of anorthite-rich plagioclase (An67-An73) in a system with fluid transport, and under stagnant conditions. A combined approach of flow-through column and batch experiments has been used. Experimental conditions ranging from 100 to 250 C and 20 to 120 bar and different preparations of the starting material were applied. The overall carbonation reaction consists of plagioclase dissolution coupled to a number of precipitation reactions. The flow-through column experiments at 250 C showed stoichiometric dissolution of the plagioclase. Al-hydroxide ("proto Al-hydroxide") nucleated on the plagioclase as the first phase to precipitate. A secondary porosity developed between the shrinking plagioclase and the enclosing "proto Al-hydroxide". Calcite, as the second phase to precipitate, filled the primary pore space. A reaction front was developed separating the zone at the inlet where all the plagioclase had dissolved and the less reacted outlet of the column. Redissolution of the calcite and formation of euhedral boehmite crystals occurred when a sufficient amount of plagioclase had dissolved. Clay minerals were not precipitated in the column experiments. Between 11% and 30% of the plagioclase was dissolved within 72-168 h of reaction. A much higher extent of plagioclase dissolution was observed in the high pressure experiments compared to the low pressure. However, a smaller share of the released Ca was trapped as calcite in the high pressure experiments. Both observations are consistent with a more rapid progression of the dissolution front at high pressure. The batch experiments showed conversion of the plagioclase to a mixture of Al-hydroxide, possibly gibbsite, clays and calcite. A range in conversion from below the detection limit to 91% was observed within reaction periods of 24-72 h. Crystallinity of the feldspar was the most important factor contributing to increased reaction rates. A general positive effect of increasing temperature on the conversion is observed for all materials, whereas pressure and the addition of CaCl 2 did not have any effect. The carbonation of plagioclase at stagnant conditions is slow compared to olivine at temperatures around 200 C. However, industrial operations involving high fluid flows of CO 2-water mixtures induce gradients in pH or solute concentrations, which may lead to increased reaction rates and changes in porosity/permeability.

Munz, I. A.; Brandvoll, .; Haug, T. A.; Iden, K.; Smeets, R.; Kihle, J.; Johansen, H.

2012-01-01

55

Nuclear reaction rates and the nova outburst  

SciTech Connect

In this paper we examined the consequences of improving the nuclear reaction library on our simulations of TNRs on 1.25M, WD and 1.35M, WDS. We have found that the changes in the rates have affected the nucleosynthesis predictions of our calculations but not, to any great extent, the gross features. In addition, we have used a lower mass accretion rate than in our previous studies in order to accrete (and eject) more material. This has, as expected, caused the peak values of some important parameters to increase over our previous studies at the same WD mass. However, because some important reaction rates have declined in the new compilation this has not increased the abundances for nuclei above aluminum and, in fact, they have declined while the abundances of both {sup 26}Al and {sup 27}Al have increased at both WD masses. In contrast, the abundance of {sup 22}Na has declined at both WD masses over the values predicted in our earlier work. This has important implications with respect to predictions of the observability of novae with INTEGRAL.

Starrfield, S.G.; Iliadis, C. [and others

2000-10-01

56

Fusion Reaction Rate in an Inhomogeneous Plasma  

SciTech Connect

The local fusion rate, obtained from the assumption that the distribution is a local Maxwellian, is inaccurate if mean-free-paths of fusing particles are not sufficiently small compared with the inhomogeneity length of the plasma. We calculate the first order correction of P0 in terms of the small spatial gradient and obtain a non-local modification of P(sub)0 in a shock region when the gradient is not small. Use is made of the fact that the fusion reaction cross section has a relatively sharp peak as a function of energy.

S. Son; N.J. Fisch

2004-09-03

57

Gas-phase rate coefficients for the OH + n-, i-, s-, and t-butanol reactions measured between 220 and 380 K: non-Arrhenius behavior and site-specific reactivity.  

PubMed

Butanol (C4H9OH) is a potential biofuel alternative in fossil fuel gasoline and diesel formulations. The usage of butanol would necessarily lead to direct emissions into the atmosphere; thus, an understanding of its atmospheric processing and environmental impact is desired. Reaction with the OH radical is expected to be the predominant atmospheric removal process for the four aliphatic isomers of butanol. In this work, rate coefficients, k, for the gas-phase reaction of the n-, i-, s-, and t-butanol isomers with the OH radical were measured under pseudo-first-order conditions in OH using pulsed laser photolysis to produce OH radicals and laser induced fluorescence to monitor its temporal profile. Rate coefficients were measured over the temperature range 221-381 K at total pressures between 50 and 200 Torr (He). The reactions exhibited non-Arrhenius behavior over this temperature range and no dependence on total pressure with k(296 K) values of (9.68 0.75), (9.72 0.72), (8.88 0.69), and (1.04 0.08) (in units of 10(-12) cm(3) molecule(-1) s(-1)) for n-, i-, s-, and t-butanol, respectively. The quoted uncertainties are at the 2? level and include estimated systematic errors. The observed non-Arrhenius behavior is interpreted here to result from a competition between the available H-atom abstraction reactive sites, which have different activation energies and pre-exponential factors. The present results are compared with results from previous kinetic studies, structure-activity relationships (SARs), and theoretical calculations and the discrepancies are discussed. Results from this work were combined with available high temperature (1200-1800 K) rate coefficient data and room temperature reaction end-product yields, where available, to derive a self-consistent site-specific set of reaction rate coefficients of the form AT(n) exp(-E/RT) for use in atmospheric and combustion chemistry modeling. PMID:23627621

McGillen, Max R; Baasandorj, Munkhbayar; Burkholder, James B

2013-05-22

58

Astrophysical Reaction Rates as a Challenge for Nuclear Reaction Theory  

SciTech Connect

The relevant energy ranges for stellar nuclear reactions are introduced. Low-energy compound direct reactions are discussed. Stellar modifications of the cross sections are presented. Implications for experiments are outlined.

Rauscher, T. [Department of Physics, University of Basel, CH-4056 Basel (Switzerland)

2010-08-12

59

Temporal Scaling of Biogeochemical Reaction Rates  

NASA Astrophysics Data System (ADS)

In at least two disparate areas of organic and inorganic geochemistry---the microbial degradation of detritus and the dissolution of minerals in sediments and soils---apparent rate constants k have been observed to diminish with the "age" t of the substrate like k(t) ~eq a t-b, where a ~eq 0.2 and b ~eq 1.0. Published reports display up to ten orders of magnitude in time [1,2]. Because the accuracy of biogeochemical models typically depends crucially on the specification of such rates, an understanding of this scaling law has important implications for predicting the evolution of biogeochemical cycles, especially the cycles of carbon and oxygen. The power-law decay of rates likely derives from a combination of chemical and physical heterogeneity. In a purely chemical scenario, an intrinsically heterogeneous substrate (e.g., a mixture of organic matter ranging from "labile" to "recalcitrant") is assumed to produce the observed slowdown of k(t). In contrast, a physical model assumes a homogeneous substrate in which rates nevertheless vary microscopically due to spatially varying physical constraints. Here we consider the extreme case of a purely physical origin and test its consistency with observations [3]. We first show how a diffusion-limited reaction-diffusion system leads to a logarithmic decay of the substrate. We then show how the power-law for k(t) derives from this logarithmic decay. We obtain not only the correct exponent b=1 but also a good approximation of the prefactor a. By constructing an extensive database of previously published measurements, we show that observations compare well to predictions. The particular way in which diffusion-limitation manifests itself varies from problem to problem. In the case of detrital decay in sediments, we suggest that rates are limited by contact of substrate with extracellular enzymes [3]. Mechanisms in soils are likely similar. For mineral dissolution is sediments, we suggest that rates are limited by diffusion of reactants from the seafloor. [1] J.~J.~Middelburg, Geochim.~Cosmochim.~Acta 53, 1577 (1989). [2] K.~Maher, D.~J.~DePaolo, J.~C.-F.~Lin, Geochim.~Cosmochim.~Acta 68, 4629. [3] D.~H.~Rothman and D.~C.~Forney, Science 316, 1325 (2004).

Rothman, D. H.; Forney, D. C.

2007-12-01

60

Kinetic and product studies of the heterogeneous reactions of surface-bound polycyclic aromatic hydrocarbons with selected atmospheric oxidants  

NASA Astrophysics Data System (ADS)

The heterogeneous reaction of surface-bound polycyclic aromatic hydrocarbons (PAHs) and gas-phase ozone was used as a model system to investigate the factors that influence the heterogeneous reactions of organic compounds. The heterogeneous reactions of surface-bound anthracene and benzo[a]pyrene with ozone on different aerosol substrates were determined using an aerosol flow tube apparatus. The kinetics of these reactions displayed pseudo-first order behaviour with respect to ozone. The non-linear dependence of the pseudo-first order rate constant as a function of ozone concentration was consistent with reactions that proceed by the Langmuir---Hinshelwood mechanism for the substrates and PAHs investigated. The variability in the kinetics from substrate to substrate was influenced more by the partitioning of ozone to the aerosol surface than by the surface-phase reaction rate. The product yield of anthraquinone, one of the known products of the reaction between anthracene and ozone, as a function of ozone concentration yielded a non-linear functional dependence that was similar to a Langmuir adsorption profile thus providing the first direct evidence that ozone is involved in the rate-limiting step in the formation of this product. Investigations of the heterogeneous oxidation of anthracene, pyrene and n-hexane soot with the NO3 radical, using a suite of analytical techniques indicated the formation of nitro functional groups on the surface. Carbonyl functionalities were observed in addition to nitro functional groups on the surface of soot surfaces following exposure to a nitrating flow. Absorption experiments indicate that the nitration of PAHs alter the optical properties of the particles to which they are adsorbed, giving rise to absorption intensity in the near UV and visible portions of the spectrum. The experimental results of the kinetic studies were used in a multimedia model that was the first to assess the importance of these heterogeneous reactions in an urban environment. The kinetic, product and modeling studies illustrate that PAHs found on particle surfaces may be lost at a rate greater than by gas-phase loss mechanisms. Further, these studies suggest that the rate of formation of the toxic oxidized PAHs may also be significant in the urban environment.

Kwamena, Nana-Owusua Alecia

61

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

62

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

63

SENSITIVITY OF ASTROPHYSICAL REACTION RATES TO NUCLEAR UNCERTAINTIES  

SciTech Connect

Sensitivities of nuclear reaction rates to a variation of nuclear properties are studied. Target nuclei range from proton- to neutron dripline for 10 {<=} Z {<=} 83. Reactions considered are nucleon- and {alpha}-induced reactions mediated by strong interaction. The contribution of reactions occurring on the target ground state to the total stellar rate is also given. General dependencies on various input quantities are discussed. Additionally, sensitivities of laboratory cross-sections of nucleon-, {alpha}-, and {gamma}-induced reactions are shown, allowing us to estimate the impact of cross-section measurements. Finally, recommended procedures to explore and improve reaction rate uncertainties using the present sensitivity data are outlined.

Rauscher, T. [Department of Physics, University of Basel, CH-4056 Basel (Switzerland)

2012-08-01

64

Field Based Constraints on Reaction Rates in the Crust  

Microsoft Academic Search

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

E. F. Baxter

2004-01-01

65

In-cell reaction rate distributions and cell-average reaction rates in fast critical assemblies  

SciTech Connect

Measurements are described for determining average values of fission rates in /sup 235/U, /sup 238/U and /sup 239/Pu and capture rates in /sup 238/U for heterogeneous cells used to construct fast critical assemblies. The measurements are based on irradiations of foils of /sup 238/U, /sup 235/U and /sup 239/Pu with counting of fission and capture products using gamma-ray spectroscopy. Both plate and pin cells are considered. Procedures are described for inferring cell-average reaction rate values from a single foil location based on a cell using a quantity called a cell factor. Cell factors are determined from special measurements in which several foils are irradiated within a cell. Comparisons are presented between cell factors determined by measurements and by Monte Carlo calculations which lend credibility to the measurement procedures.

Brumbach, S.B.; Gasidlo, J.M.

1985-08-01

66

Reaction rates and reaction rate constant conception. One-temperature case  

NASA Astrophysics Data System (ADS)

The new method of getting a normal solution for the generalized Boltzmann equation for reacting gas mixtures was proposed. It is based on the following items: (i) slow variables are introduced via approximate summational invariants, defined within the method, (ii) kinetic equations are presented in the form of a singularly perturbed system for gas-dynamic (slow) variables and for the "fast" part of the distribution function, (iii) collisional integral is not expanded into the series over the Knudsen number (no assumption is made that the part of the collisional integral, responsible for the chemical reactions, can be treated as the perturbation of its "elastic" part). While deriving the gas-dynamic equations it is shown that the role of non-equilibrium effects is much more essential in our approach, than is generally accepted. By non-equilibrium effects we mean all kind of effects caused by deviation of the distribution function from its quasi-equilibrium value. From several examples it was shown that non-equilibrium corrections and the traditional equilibrium rate constants could be of the same order of magnitude. In this paper we derive expressions for corrections to equilibrium rate constants for arbitrary mixtures and corresponding integral equations for corrections to the quasi-equilibrium distributions. In situations where corrections to the reaction rates are not small non-equilibrium effects dramatically impacts the chemical kinetics of the reacting gas mixture. This leads to the necessity of the revision of the concept of getting information on the reaction rates from the experiments.

Kolesnichenko, Evgeniy G.; Gorbachev, Yuriy E.

2012-11-01

67

Reactions of three halogenated organophosphorus flame retardants with reduced sulfur species.  

PubMed

Tris(haloalkyl)phosphates (THAPs) are among the most widely used flame retardants in the U.S. They have been identified as one of the most frequently detected contaminants in U.S. streams. These contaminants are 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 (Sn(2-)) and bisulfide (HS(-)), present in anoxic subregions of coastal water bodies could have a significant impact on rates of removal of such contaminants, especially since no significant degradation reactions in the environment (e.g., hydrolysis, biological degradation) is reported for these compounds. The kinetics of the reaction of reduced sulfur species with three structurally related THAPs have been determined in well-defined aqueous solutions under anoxic conditions. Reactions were monitored at varying concentrations of reduced sulfur species to obtain second-order rate constants from the observed pseudo-first order rate constants. The degradation products were studied with GC-FID and LC-MS. The reactivity of Sn(2-), thiophenolate, and HS(-) were compared and steric, as well as electronic factors are used to explain the relative reactivity of the three THAPs with these three sulfur species. PMID:23948611

Saint-Hilaire, Dickens; Jans, Urs

2013-08-12

68

Neutron detectors for fusion reaction-rate measurements.  

National Technical Information Service (NTIS)

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 i...

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

1994-01-01

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

The carboxybiotin complex of chicken liver pyruvate carboxylase. A kinetic analysis of the effects of acetyl-CoA, Mg2+ ions and temperature on its stability and on its reaction with 2-oxobutyrate.  

PubMed Central

The enzyme-[14C]carboxybiotin complex of chicken liver pyruvate carboxylase has been isolated and shown to be relatively stable, with a half-life at 0 degree C of 342 min. The kinetic properties of the decay of this complex, in both the presence and the absence of the substrate analogue, 2-oxobutyrate, have been examined. The data for the reaction with 2-oxobutyrate at 0 degree C fitted a biphasic exponential decay curve, enabling the calculation of rate constants for both the fast and slow phases of the reaction at this temperature. The effect of temperature on the observed pseudo-first-order rate constant for the slow phase of the reaction with 2-oxobutyrate, and that for the decay of the enzyme-[14C]carboxybiotin complex alone, have been examined. Arrhenius plots of these data revealed that the processes being studied in each type of experiment were single reactions represented by one rate constant in each case. For the decay of the enzyme-[14C]carboxybiotin complex in the absence of 2-oxobutyrate, the rate-determining process may be the movement of carboxybiotin from the site of the first partial reaction to the site of the second. The calculated thermodynamic activation parameters indicate that this reaction is accompanied by a large change in protein conformation. With 2-oxobutyrate present, the observed process in the slow phase of the reaction was probably the dissociation of the carboxybiotin from the first subsite. Here, the activation parameters suggest that a much smaller change in protein conformation accompanies this reaction. Both sets of experiments were also performed in the presence of acetyl-CoA, but this activator had little effect on the measured thermodynamic activation parameters. However, in both cases the observed pseudo-first-order rate constants in the presence of acetyl-CoA were about 75% of those in its absence. The effects of Mg2+ on the reaction kinetics of the enzyme-[14C]carboxybiotin complex with 2-oxobutyrate were similar to those observed with the sheep enzyme by Goodall, Baldwin, Wallace & Keech [(1981) Biochem. J. 199, 603-609].

Attwood, P V; Wallace, J C

1986-01-01

71

Rate of Stannous Fluoride Reaction with Hydroxyapatite  

Microsoft Academic Search

Solutions of stannous fluoride are reacted with hydroxyapatite and the progress of the reaction is monitored by calcium analysis. The formation of Sn3F3PO4 proceeds faster with an increase in SnF2 concentration and can be further increased by adding Sn++ to the stannous fluoride solution.

Kenneth G. Nelson

1978-01-01

72

Improved predictions of nuclear reaction rates (Goriely+, 2008)  

NASA Astrophysics Data System (ADS)

The files (in "files" subdirectory) include the Maxwellian-averaged nuclear reaction rates of astrophysical interest estimated with the TALYS reaction code for 26 temperatures ranging between 1x107 to 1x1010K. The reaction rates include neutron, proton and {alpha}- captures for all 8<=Z<=105 nuclei lying between the proton and neutron drip lines. The reaction rates are filed per isotopic chain and projectile type. For example z050n corresponds to the neutron capture reaction rates of Z=50 (Sn) isotopes (in the file name n stands for neutron captures, p for protons, a for {alpha}-particles). For each projectile, the reaction rates for the three possible ejectiles differing from the projectiles are included (g stands for gamma). The target, projectile, ejectile and residual nuclei are given in the title for each set of reaction rates. The temperature T (T9) is expressed in billion degrees Kelvin and the reaction rates in mol-1.s-1.cm3. The 294 corresponding files are also available in one unique zipped tar file named zfile.tar (5.3Mb) The file "rates.dat" contains all reaction rates in a single table. (3 data files).

Goriely, S.; Hilaire, S.; Koning, A. J.

2008-06-01

73

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

74

Symmetry numbers and chemical reaction rates  

Microsoft Academic Search

This article shows how to evaluate rotational symmetry numbers for different molecular configurations and how to apply them\\u000a to transition state theory. In general, the symmetry number is given by the ratio of the reactant and transition state rotational\\u000a symmetry numbers. However, special care is advised in the evaluation of symmetry numbers in the following situations: (i)\\u000a if the reaction

Antonio Fernndez-Ramos; Benjamin A. Ellingson; Rubn Meana-Paeda; Jorge M. C. Marques; Donald G. Truhlar

2007-01-01

75

Reaction of a hydrated electron with gentamycin and collagen-a pulse radiolysis study  

NASA Astrophysics Data System (ADS)

The reactions of a hydrated electron (e-aq) with aminoglycoside antibiotic gentamycin and collagen in aqueous medium at different pH have been investigated employing a pulse radiolysis technique. The pseudo-first order equation of reaction kinetics was used to give an accurate description of the decay of e-aq in gentamycin solutions. The rate constant of the e-aq decay in collagen solution was high and reached 3.2 1010 M-1 s-1. The rate constants for the reaction of the e-aq with gentamycin were found to be influenced by pH, decreasing with the deprotonation of the -NH3 groups, while for pH > pKa which for gentamycin is equal to 7.8, the rate constant was unchanged. These observations suggest that when the amino groups are protonated, reductive deamination occurs, but for unprotonated non-reactive amino groups, a radical anion is formed on the glycoside moiety.

Pietrucha, K.; Gra, L.; Doillon, C. J.

1996-01-01

76

An age extended progress variable for conditioning reaction rates  

Microsoft Academic Search

An aging progress variable (APV) is proposed as a convenient tool for conditioning quantities used to calculate reaction rates in premixed turbulent combustion. The APV is defined to obey an advection-diffusion-reaction equation where the source term is linearly related to the fuel consumption rate when the APV is less than a threshold representative of the trailing edge of the fuel

R. W. Grout

2007-01-01

77

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

78

Interest rate reaction functions for the euro area  

Microsoft Academic Search

Estimating interest rate reaction functions for the euro area is still hampered by the short time span since the conduct of\\u000a a single monetary policy. This is why estimates of union-wide reaction functions are usually based on historical pre-EMU data.\\u000a In this paper we circumvent the common use of aggregated data before 1999 by estimating interest rate reaction functions based

Karsten Ruth

2007-01-01

79

Monte-Carlo Reaction Rate Evaluation for Astrophysics  

SciTech Connect

We present a new evaluation of thermonuclear reaction rates for astrophysics involving proton and alpha-particle induced reactions, in the target mass range between A = 14 and 40, including many radioactive targets. A method based on Monte Carlo techniques is used to evaluate thermonuclear reaction rates and their uncertainties. At variance with previous evaluations, the low, median and high rates are statistically defined and a lognormal approximation to the rate distribution is given. This provides improved input for astrophysical model calculations using also the Monte Carlo method to estimate uncertainties on isotopic abundances.

Coc, A. [Centre de Spectrometrie Nucleaire et de Spectrometrie de Masse (CSNSM), UMR 8609, CNRS/IN2P3 (France) and Universite Paris Sud 11, Batiment 104, 91405 Orsay Campus (France); Iliadis, C.; Longland, R.; Champagne, A. E. [Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255 (United States) and Triangle Universities Nuclear Laboratory, Durham, NC 27708-0308 (United States); Fitzgerald, R. [National Institute of Standards and Technology, 100 Bureau Drive, Stop 8462, Gaithersburg, MD 20899-8462 (United States)

2010-06-01

80

Charged-particle thermonuclear reaction rates: II. Tables and graphs of reaction rates and probability density functions  

NASA Astrophysics Data System (ADS)

Numerical values of charged-particle thermonuclear reaction rates for nuclei in the A=14 to 40 region are tabulated. The results are obtained using a method, based on Monte Carlo techniques, that has been described in the preceding paper of this issue (Paper I). We present a low rate, median rate and high rate which correspond to the 0.16, 0.50 and 0.84 quantiles, respectively, of the cumulative reaction rate distribution. The meaning of these quantities is in general different from the commonly reported, but statistically meaningless expressions, lower limit, nominal value and upper limit of the total reaction rate. In addition, we approximate the Monte Carlo probability density function of the total reaction rate by a lognormal distribution and tabulate the lognormal parameters ? and ? at each temperature. We also provide a quantitative measure (Anderson-Darling test statistic) for the reliability of the lognormal approximation. The user can implement the approximate lognormal reaction rate probability density functions directly in a stellar model code for studies of stellar energy generation and nucleosynthesis. For each reaction, the Monte Carlo reaction rate probability density functions, together with their lognormal approximations, are displayed graphically for selected temperatures in order to provide a visual impression. Our new reaction rates are appropriate for bare nuclei in the laboratory. The nuclear physics input used to derive our reaction rates is presented in the subsequent paper of this issue (Paper III). In the fourth paper of this issue (Paper IV) we compare our new reaction rates to previous results.

Iliadis, C.; Longland, R.; Champagne, A. E.; Coc, A.; Fitzgerald, R.

2010-10-01

81

a Global Reaction Rate for Nitric Oxide Reburning  

Microsoft Academic Search

An investigation of a global reburning-NO reaction, which is the reduction pathway of nitric oxide (NO) by reaction with hydrocarbons, was conducted. The global reburning-NO rate expression and its rate constants were determined. This global reburning-NO rate constant can be expressed as 2.72 times 10 ^6exp(-18,800\\/RT) (gmole\\/cm^3 s). This expression is applicable to atmospheric pressure, an equivalence ratio range of

Wei Chen

1994-01-01

82

Atmospheric HFEs degradation in the gas phase: reactions of HFE-7100 and HFE-7200 with Cl atoms at low temperatures.  

PubMed

Kinetic rate coefficients for the reactions of HFE-7100 (1) (C4F9OCH3) and HFE-7200 (2) (C4F9OC2H5) with Cl atoms have been measured using a discharge flow mass spectrometric technique (DFMS) at 1 Torr total pressure. The reactions have been studied under pseudo-first-order kinetic conditions in excess of HFEs over Cl atoms and the study has been extended from 333 down to 234 K to approach the tropospheric temperature profile. At room temperature the measured rate constants are k (1) = (1.43 +/- 0.28) x 10(-13) cm3molecule(-1)s(-1) and k (2) = (2.1 +/- 0.1) x 10(-12) cm3molecule(-1)s(-1). The Arrhenius expressions from our results are (units in cm3molecule(-1)s(-1)): k (1) = (2.3 +/- 1.4) x 10(-10) exp - (2254 +/- 177)/T(234-315 K) and k (2) = (3.7 +/- 0.5) x 10(-11) exp - (852 +/- 38)/T(234-333 K) (errors are sigma). The reactions proceed through the abstraction of an H atom to form HCl and the corresponding halo-alkyl radical. At 298 K and 1 Torr, yields on HCl of 0.88 +/- 0.09 and 0.95 +/- 0.10 (errors are 2sigma) were obtained for HFE-7100 and HFE-7200 reactions, respectively. PMID:17051787

Aranda, Alfonso; Daz-De-Mera, Yolanda; Bravo, Ivn; Rodrguez, Diana; Rodrguez, Ana; Martnez, Ernesto

2006-10-01

83

Mechanistic elucidation of linker and ancillary ligand substitution reactions in Pt(II) dinuclear complexes.  

PubMed

The rate of substitution of aqua ligands by three nucleophiles, thiourea (TU), N,N-dimethylthiourea (DMTU) and N,N,N,N-tetramethylthiourea (TMTU), for the complexes [cis-{PtOH2(NH3)2}2-?-pyrazine](ClO4)2 (pzn), [cis-{PtOH2(NH3)2}2-?-2,3-dimethylpyrazine](ClO4)2 (2,3pzn), [cis-{PtOH2(NH3)2}2-?-2,5-pyrazine](ClO4)2 (2,5pzn) and [cis-{PtOH2(NH3)2}2-?-2,6-dimethylpyrazine](ClO4)2 (2,6pzn) was investigated under pseudo first-order conditions as a function of concentration and temperature by stopped-flow and UV-Visible spectrophotometry. The reaction proceeded in three consecutive steps; each step follows first order kinetics with respect to each complex and nucleophile. The pseudo first-order rate constants, k(obs(1/2/3)), for sequential substitution of the aqua ligands and subsequent displacement of the linker obeyed the rate law: k(obs(1/2/3)) = k((1/2/3))[nucleophile]. The steric hindrance properties of the pyrazine-bridging ligand control the overall reaction pattern. The order of reactivity of the complexes is 2,3pzn ? 2,5pzn < 2,6pzn < pzn. The difference in reactivity attributed to the steric crowding at the Pt(II) centre imposed by the methyl groups reduces the lability of the aqua complexes. The order of reactivity of the nucleophiles decreases with the increase in steric demand TU > DMTU > TMTU. 1H and 195Pt NMR spectroscopic results confirmed the observed dissociation of the bridging ligand from the metal centre of the cis-dinuclear complexes and its derivatives in the third step. The dissociation process is accelerated by the introduction of the steric effect on the linker in conjunction with the increased ligand field strength imparted by additional thiourea ligands at each metal centre. The large negative entropy of activation ?S(?) values in all cases support an associative substitution mechanism. PMID:23223554

Ongoma, Peter O; Jaganyi, Deogratius

2013-02-28

84

From small-molecule reactions to protein folding: studying biochemical kinetics by stopped-flow electrospray mass spectrometry.  

PubMed

This work introduces stopped-flow electrospray ionization (ESI) mass spectrometry (MS) as a method for studying fast biochemical reaction kinetics. After initiating a reaction by rapid mixing of two solutions, the mixture is transferred to a reaction vessel and a steady liquid flow to the ESI source of the mass spectrometer is established. The kinetics are studied in real time by monitoring selected ion intensities as a function of time. In order to characterize the performance of this setup the acid-induced demetallation of chlorophyll a was studied. It was found that the reaction is second order in acid concentration and that pseudo-first-order rate constants of up to roughly 7 s(-1) can be measured reliably. Stopped-flow ESI MS was also applied to study the acid-induced denaturation of myoglobin. The data presented here confirm the occurrence of a short-lived unfolding intermediate during this reaction. Stopped-flow ESI MS can provide information that is not accessible by optical rapid-mixing experiments. Therefore it appears that this novel technique has the potential to become a standard tool for kinetic studies in a number of different fields. PMID:11319824

Kolakowski, B M; Konermann, L

2001-05-01

85

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.

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

2013-03-01

86

Imaginary-time method for the radiative capture reaction rate  

NASA Astrophysics Data System (ADS)

We propose a new computational method for astrophysical reaction rate of the radiative capture process. In the method, an evolution of a wave function is calculated along the imaginary-time axis, which is identified as the inverse temperature. It enables direct evaluation of reaction rate as a function of temperature without solving any scattering problem. The method is tested for two-body radiative capture reaction, 16O(?,?)20Ne, showing that it gives identical results to those calculated by the ordinary procedure. This method will be suited for calculation of triple-? radiative capture rate, for which an explicit construction of the scattering solution is difficult.

Yabana, K.; Funaki, Y.

2012-05-01

87

Generalized rate equation for single-substrate enzyme catalyzed reactions  

Microsoft Academic Search

The most widely used rate expression for single-substrate enzyme catalyzed reactions, namely the MichaelisMenten kinetics is based upon the assumption that enzyme concentration is in excess of the substrate in the medium and the rate is mainly limited by the substrate concentration according to saturation kinetics. However, this is only a special case and the actual rate expression varies depending

Fikret Kargi

2009-01-01

88

Effects of Chemical Reactions on the Bimolecular Rate Constant  

Microsoft Academic Search

The dependence of the bimolecular rate constant on the reaction rate, degree of anisotropic scattering, mass ratio, and other parameters is investigated by considering the Boltzmann equation for a dilute homogeneous reacting gas. For highly anisotropic elastic scattering, a differential form of the Boltzmann equation is derived and a solution obtained, which yields an analytical expression for the rate constant.

M. D. Kostin

1967-01-01

89

QSARS for predicting reductive transformation rate constants of halogenated aromatic hydrocarbons in anoxic sediment systems  

Microsoft Academic Search

Quantitative structure-activity relationships (QSARs) are developed relating initial and final pseudo-first-order disappearance rate constants of 45 halogenated aromatic hydrocarbons in anoxic sediments to four readily available molecular descriptors: the carbon-halogen bond strength, the summation of the Hammett sigma constants of the additional substituents, the summation of the inductive constants of these substituents, and the steric factors of the additional substituents.

W. J. G. M. Peunenburg; Maarten J.'t Hart; Henri A. den Hollander; Dik van de Meent; Hans H. Verboom; N. Lee Wolfe

1992-01-01

90

Electrostatic effects on the rates of DNA-catalyzed reactions  

Microsoft Academic Search

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

George R. Pack; Linda Wong

1996-01-01

91

Aspects and Reaction Rates for Pycnonuclear Fusion at High Densities  

Microsoft Academic Search

Pycnonuclear reactions are of great importance in the nuclear astrophysics of high density conditions such as the centers of white dwarf starts, and the deep layers of accreting neutron stars. We present here a single phenomenological expression for the calculation of pycnonuclear reaction rates which is not only valid in mixed component plasma, but can also be extended to cover

Mary Beard; Michael Wiescher; Anatoli Afanasjev; Leandro Gasques; Dima Yakovlev

2008-01-01

92

Experimental protocol for determining ozone reaction rate constants. Interim report  

Microsoft Academic Search

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 atmospheric reaction pathway (i.e. attack by ozone) of organic substances which may be emitted into the

J. N. Jr. Pitts; A. M. Winer; D. R. Fitz; S. M. Aschmann; R. Atkinson

1981-01-01

93

Recent Developments in Semiclassical Mechanics: Eigenvalues and Reaction Rate Constants.  

National Technical Information Service (NTIS)

A semiclassical treatment of eigenvalues for a multidimensional non-separable potential function and of the rate constant for a chemical reaction with an activation barrier is presented. Both phenomena are seen to be described by essentially the same semi...

W. H. Miller

1976-01-01

94

An Improved Reaction Rate Equation for Simulating the Ignition and Growth of Reaction in High Explosives  

SciTech Connect

We describe an improved reaction rate equation for simulating ignition and growth of reaction in high explosives. It has been implemented into CALE and ALE3D as an alternate to the baseline the Lee-Tarver reactive flow model. The reactive flow model treats the explosive in two phases (unreacted/reactants and reacted/products) with a reaction rate equation to determine the fraction reacted, F. The improved rate equation has fewer parameters, is continuous with continuous derivative, results in a unique set of reaction rate parameters for each explosive while providing the same functionality as the baseline rate equation. The improved rate equation uses a cosine function in the ignition term and a sine function in the growth and completion terms. The improved rate equation is simpler with fewer parameters.

Murphy, M J

2010-03-08

95

Kinetics of heterogeneous reaction of ozone with linoleic acid and its dependence on temperature, physical state, RH, and ozone concentration.  

PubMed

Heterogeneous reaction between ozone and linoleic acid (LA) thin film was investigated by a flow reactor coupled to attenuated total reflection infrared spectroscopy (FR-ATR-IR) over wide ranges of temperature, relative humidity (RH), and ozone concentration under atmospheric pressure condition. Pseudo-first-order rate constants kapp and overall reactive uptake coefficients ? were acquired on the basis of changes in absorbance from peaks located near 1743, 1710, 1172, and 1110 cm(-1), which can be assigned to C?O in ester, C?O in acid, and C-C and C-O stretching modes, respectively. Results showed that the kapp and ? increased nearly by a factor of 6 with increasing temperatures from 258 to 314 K. It was noted the temperature effect on the reaction kinetics was much more pronounced at lower temperatures. Such behavior can be explained by a change in the physical state of LA at lower temperatures. In addition, kapp and ? were enhanced by 2-fold as the RH increased from 0 to 80%. Moreover, the effect of ozone concentration on the reaction kinetics was reported for the first time. kapp was found to display a Langmuir-Hinshelwood dependence on ozone concentration with KO3 = (1.146 0.017) 10(-15) molecules cm(-3) and k[S] = 0.0522 0.0004 s(-1), where KO3 is a parameter that describes the partitioning of ozone to the thin film surface, and k[S] is the maximum pseudo-first-order coefficient at high ozone concentration. Furthermore, yields and hygroscopic properties of reaction products were also investigated by FTIR spectroscopy. The intensity ratio of two C?O stretching bands, A1743/A1710, which was utilized as an indicator of the product yields, increased sharply with increasing temperatures in the lower temperature region (258-284 K), and then remained nearly constant in the higher temperature region (284-314 K). The product yields showed no significant variation with RH, for the intensity ratio of A1743/A1710 barely changed in the wide RH range 0-80%. Water uptake studies showed that the LA thin film absorbed water with an increasing RH, and the hygroscopicity of the thin film was enhanced after ozone exposure. PMID:23347186

Zeng, Guang; Holladay, Sara; Langlois, Danielle; Zhang, Yunhong; Liu, Yong

2013-02-20

96

Catalytic reactions of formate 4. A nitrite-promoted rhodium (III) catalyst for hydrogen generation from formic acid in aqueous solution  

Microsoft Academic Search

Rhodium (III) is an active homogeneous catalyst in the presence of NO2? for the decomposition of HCO2H to H2 and CO2 in aqueous solution at 90C. Experiments at different Rh concentrations indicate that this reactions is approximately pseudo-first-order in [Rh] suggesting mononuclear nitrorhodium complexes as the catalytically active species. The is catalyst system is gradually deactivated by reducing the soluble

R. B. King; N. K. Bhattacharyya

1995-01-01

97

Immobilized enzymes: Electrokinetic effects on reaction rates under external diffusion.  

PubMed

The rates of reactions catalyzed by enzymes immobilized on a nonporous solid surface have been computed employing a Nernst film model. The Nernst-Planck equations for the transport of the charged substrate and product species in the film and the Poisson equation for the distribution of electrical potential are solved numerically with the appropriate boundary conditions. The electrical charge at the surface is assumed to arise from the dissociation equilibria of the acidic and basic surface groups of the enzyme. The pH at the surface affects both the surface charge as well as the intrinsic kinetics of the enzyme-catalyzed reaction. Factors which determine the pH at the surface include the pH in the bulk solution and the release of H(+) ions in the enzyme-catalyzed reaction. The latter causes a lowering of pH at the surface, causing the reaction rate to differ from that computed assuming an equilibrium distribution of electrical potential. Another kind of nonequilibrium contribution is caused by unequal charges or diffusivities of the substrate and products, which results in a diffusion potential being set up. Two moduli are introduced to evaluate the significance of the reaction-generated lowering of pH and the diffusion potential effect. The effect of changing various parameters, e.g., reaction rate constant, substrate concentration, enzyme concentration, pH, etc., on the overall reaction rate are studied. PMID:18546127

Kalthod, D G; Uckenstein, E

1982-10-01

98

Tables of Nuclear Cross Sections and Reaction Rates: AN Addendum to the Paper ``ASTROPHYSICAL Reaction Rates from Statistical Model Calculations'' ()  

NASA Astrophysics Data System (ADS)

In a previous publication (ATOMIC DATAAND NUCLEAR DATA TABLES75, 1 (2000)), we gave seven-parameter analytical fits to theoretical reaction rates derived from nuclear cross sections calculated in the statistical model (Hauser-Feshbach formalism) for targets with 10<=Z<=83 (Ne to Bi) and for a mass range reaching the neutron and proton driplines. Reactions considered were (n,?), (n,p), (n,?), (p,?), (p,?), (?,?), and their inverse reactions. Here, we present the theoretical nuclear cross sections and astrophysical reaction rates from which those rate fits were derived, and we provide these data as on-line electronic files. Corresponding to the fitted rates, two complete data sets are provided, one of which includes a phenomenological treatment of shell quenching for neutron-rich nuclei.

Rauscher, Thomas; Thielemann, Friedrich-Karl

2001-09-01

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  

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

100

Reaction rate constant for uranium in water and water vapor  

SciTech Connect

The literature on uranium oxidation in water and oxygen free water vapor was reviewed. Arrhenius rate equations were developed from the review data. These data and equations will be used as a baseline from which to compare reaction rates measured for K Basin fuel.

TRIMBLE, D.J.

1998-11-09

101

Fractionated reaction time and the rate of force development  

Microsoft Academic Search

The relationship between the rate of force development and components of fractionated reaction time were investigated in the present study. Subjects (N=9) were administered extensive practice before being required to produce 98N of isometric force on a hand dynamometer at a maximal rate, at 20% slower than maximal, and at 40% slower than maximal. Repeated measures analysis of variance followed

Donald Siegel

1988-01-01

102

Generalized rate equation for single-substrate enzyme catalyzed reactions.  

PubMed

The most widely used rate expression for single-substrate enzyme catalyzed reactions, namely the Michaelis-Menten kinetics is based upon the assumption that enzyme concentration is in excess of the substrate in the medium and the rate is mainly limited by the substrate concentration according to saturation kinetics. However, this is only a special case and the actual rate expression varies depending on the initial enzyme/substrate ratio (E(0)/S(0)). When the substrate concentration exceeds the enzyme concentration the limitation is due to low enzyme concentration and the rate increases with the enzyme concentration according to saturation kinetics. The maximum rate is obtained when the initial concentrations of the enzyme and the substrate are equal. A generalized rate equation was developed in this study and special cases were discussed for enzyme catalyzed reactions. PMID:19265680

Kargi, Fikret

2009-03-03

103

Improving reaction rates by confinement within biocompatible polymers  

NASA Astrophysics Data System (ADS)

The most efficient catalysts have been developed and optimized by living systems. Indeed, in vivo enzyme-catalyzed reactions are several orders of magnitude more efficient than platinum based catalyzed reactions. However, the rate of reaction and equilibrium interactions are considerably reduced when the biological systems are studied in vitro. This phenomenon is largely attributed to the effect of confinement or macromolecular crowding present in the cell. This paper will present the comprehensive characterization of amphiphilic polymeric template with hydrophobic cores inducing 1D and 2D confinement on hydrophobic reactants diffusing within the templates. The paper will show that effect of confinement allows reactions to occur without external factors essential for these reactions to occur in the bulk. The products synthesized in a very controlled environment within amphiphilic polymeric nanotubes and nanosheets are monodispersed at the nanoscale ( 2nm). The effect of confinement opens new possibilities for environmentally friendly synthesis of novel nanoscale materials.

Malardier-Jugroot, Cecile; Li, Xia; Groves, Michael N.; Jugroot, Manish

2013-03-01

104

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

NASA Astrophysics Data System (ADS)

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.

2011-05-01

105

Influence of sorption to dissolved humic substances on transformation reactions of hydrophobic organic compounds in water. Part II: hydrolysis reactions.  

PubMed

The effect of dissolved humic acid (HA) on two types of hydrolysis reactions was investigated: (I) dehydrochlorination of gamma-hexachlorocyclohexane (HCH) and 1,1,2,2-tetrachloroethane (TeCA) as a reaction involving hydroxide ions (OH(-)) and (II) hydrolysis of 1-octyl acetate (OA) which is catalyzed by H(+) at the applied pH value (pH 4.5). The rate of TeCA hydrolysis was not affected by addition of 2 g l(-1) of HA at pH 10 (k' = 0.33 h(-1)) but HCH hydrolysis was significantly inhibited (k' = 4.6 x 10(-3) h(-1) without HA and 2.8 x 10(-3)h(-1) at 2 g l(-1) HA). HCH is sorbed by 51% whereas TeCA sorption is insignificant at this HA concentration. Sorbed HCH molecules are effectively protected due to electrostatic repulsion of OH(-) by the net negative charge of the HA molecules. In contrast, OA hydrolysis at pH 4.5 (k' = 1.6 x 10(-5) h(-1)) was drastically accelerated after addition of 2 g l(-1) HA (k' = 1.1 x 10(-3) h(-1)). The ratio of the pseudo-first-order rate constants of the sorbed and the freely dissolved ester fraction is about 70. H(+) accumulation in the microenvironment of the negatively charged HA molecules was suggested to contribute to the higher reaction rate for the sorbed fraction in case of this H(+)-catalyzed reaction. Analogous effects from anionic surfactants are known as micellar catalysis. PMID:18237760

Georgi, Anett; Trommler, Ulf; Reichl, Annett; Kopinke, Frank-Dieter

2008-01-30

106

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

107

TABLES OF NUCLEAR CROSS SECTIONS AND REACTION RATES: AN ADDENDUM TO THE PAPER ASTROPHYSICAL REACTION RATES FROM STATISTICAL MODEL CALCULATIONS  

Microsoft Academic Search

In a previous publication (ATOMIC DATAAND NUCLEAR DATA TABLES75, 1 (2000)), we gave seven-parameter analytical fits to theoretical reaction rates derived from nuclear cross sections calculated in the statistical model (HauserFeshbach formalism) for targets with 10?Z?83 (Ne to Bi) and for a mass range reaching the neutron and proton driplines. Reactions considered were (n,?), (n,p), (n,?), (p,?), (p,?), (?,?), and

Thomas Rauscher; Friedrich-Karl Thielemann

2001-01-01

108

Long delay times in reaction rates increase intrinsic fluctuations.  

PubMed

In spatially distributed cellular systems, it is often convenient to represent complicated auxiliary pathways and spatial transport by time-delayed reaction rates. Furthermore, many of the reactants appear in low numbers necessitating a probabilistic description. The coupling of delayed rates with stochastic dynamics leads to a probability conservation equation characterizing a non-Markovian process. A systematic approximation is derived that incorporates the effect of delayed rates on the characterization of molecular noise valid in the limit of long delay time. By way of a simple example, we show that delayed reaction dynamics can only increase intrinsic fluctuations about the steady state. The method is general enough to accommodate nonlinear transition rates allowing characterization of fluctuations around a delay-induced limit cycle. PMID:19905084

Scott, Matthew

2009-09-22

109

Improved predictions of nuclear reaction rates with the TALYS reaction code for astrophysical applications  

NASA Astrophysics Data System (ADS)

Context: Nuclear reaction rates of astrophysical applications are traditionally determined on the basis of Hauser-Feshbach reaction codes. These codes adopt a number of approximations that have never been tested, such as a simplified width fluctuation correction, the neglect of delayed or multiple-particle emission during the electromagnetic decay cascade, or the absence of the pre-equilibrium contribution at increasing incident energies. Aims: The reaction code TALYS has been recently updated to estimate the Maxwellian-averaged reaction rates that are of astrophysical relevance. These new developments enable the reaction rates to be calculated with increased accuracy and reliability and the approximations of previous codes to be investigated. Methods: The TALYS predictions for the thermonuclear rates of relevance to astrophysics are detailed and compared with those derived by widely-used codes for the same nuclear ingredients. Results: It is shown that TALYS predictions may differ significantly from those of previous codes, in particular for nuclei for which no or little nuclear data is available. The pre-equilibrium process is shown to influence the astrophysics rates of exotic neutron-rich nuclei significantly. For the first time, the Maxwellian-averaged (n,2n) reaction rate is calculated for all nuclei and its competition with the radiative capture rate is discussed. Conclusions: The TALYS code provides a new tool to estimate all nuclear reaction rates of relevance to astrophysics with improved accuracy and reliability. Data tables are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/487/767

Goriely, S.; Hilaire, S.; Koning, A. J.

2008-08-01

110

Reaction rate uncertainties and the {nu}p-process  

SciTech Connect

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 {nu}p-process. The detailed nucleosynthesis patterns of the {nu}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 {nu}p-process nucleosynthesis.

Froehlich, C.; Rauscher, T. [Department of Physics, North Carolina State University, Raleigh, NC 27695 (United States); Dept. of Physics, University of Basel, 4056 Basel (Switzerland)

2012-11-12

111

Quantum and semiclassical theories of chemical reaction rates  

SciTech Connect

A rigorous quantum mechanical theory (and a semiclassical approximation thereto) is described for calculating chemical reaction rates ``directly``, i.e., without having to solve the complete state-to-state reactive scattering problem. The approach has many vestiges of transition state theory, for which it may be thought of as the rigorous generalization.

Miller, W.H. [Univ. of California, Berkeley, CA (United States). Dept. of Chemistry]|[Lawrence Berkeley National Lab., CA (United States). Chemical Sciences Div.

1995-09-01

112

Nuclear fusion in dense matter: Reaction rate and carbon burning  

NASA Astrophysics Data System (ADS)

In this paper we analyze the nuclear fusion rates among equal nuclei for all five different nuclear burning regimes in dense matter (two thermonuclear regimes, two pycnonuclear ones, and the intermediate regime). The rate is determined by Coulomb barrier penetration in dense environments and by the astrophysical S factor at low energies. We evaluate previous studies of the Coulomb barrier problem and propose a simple phenomenological formula for the reaction rate that covers all cases. The parameters of this formula can be varied to take into account current theoretical uncertainties in the reaction rate. The results are illustrated for the example of the 12C+12C fusion reaction. This reaction is important for the understanding of nuclear burning in evolved stars, in exploding white dwarfs producing type Ia supernovas, and in accreting neutron stars. The S factor at stellar energies depends on a reliable fit and extrapolation of the experimental data. We calculate the energy dependence of the S factor by using a recently developed parameter-free model for the nuclear interaction, taking into account the effects of the Pauli nonlocality. For illustration, we analyze the efficiency of carbon burning in a wide range of densities and temperatures of stellar matter with the emphasis on carbon ignition at densities ??109 g cm-3.

Gasques, L. R.; Afanasjev, A. V.; Aguilera, E. F.; Beard, M.; Chamon, L. C.; Ring, P.; Wiescher, M.; Yakovlev, D. G.

2005-08-01

113

ABOUT DEUTERIUM NUCLEAR REACTION RATE IN CONDENSED MATTER  

Microsoft Academic Search

The Coulomb barrier penetrability of two approaching nuclei is computed in the frame of the W.K.B approximation. A simple model for describing the screening effect of the Coulomb nuclear barrier by the high electron concentration in condensed matter is presented. The nuclear reaction rate of the hydrogen isotope nuclei, trapped in a metallic lattice is assessed, both for the unscreened

Dan Chicea

114

Design criteria for uniform reaction rates in an oxygen plasma  

Microsoft Academic Search

The physical conditions necessary to obtain uniform and reproducible chemical reaction rates in an oxygen plasma are investigated by stripping photoresist from silicon wafers. An oxygen plasma was selected due to its simplicity and the availability of a simple technique for measuring the concentration of atomic oxygen which is the reagent of importance. However, many of the considerations will apply

J. F. Battey

1977-01-01

115

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...

116

Nuclear fusion in dense matter: Reaction rate and carbon burning  

SciTech Connect

In this paper we analyze the nuclear fusion rates among equal nuclei for all five different nuclear burning regimes in dense matter (two thermonuclear regimes, two pycnonuclear ones, and the intermediate regime). The rate is determined by Coulomb barrier penetration in dense environments and by the astrophysical S factor at low energies. We evaluate previous studies of the Coulomb barrier problem and propose a simple phenomenological formula for the reaction rate that covers all cases. The parameters of this formula can be varied to take into account current theoretical uncertainties in the reaction rate. The results are illustrated for the example of the {sup 12}C+{sup 12}C fusion reaction. This reaction is important for the understanding of nuclear burning in evolved stars, in exploding white dwarfs producing type Ia supernovas, and in accreting neutron stars. The S factor at stellar energies depends on a reliable fit and extrapolation of the experimental data. We calculate the energy dependence of the S factor by using a recently developed parameter-free model for the nuclear interaction, taking into account the effects of the Pauli nonlocality. For illustration, we analyze the efficiency of carbon burning in a wide range of densities and temperatures of stellar matter with the emphasis on carbon ignition at densities {rho} > or approx. 10{sup 9} g cm{sup -3}.

Gasques, L.R.; Afanasjev, A.V.; Beard, M.; Wiescher, M. [Department of Physics and Joint Institute for Nuclear Astrophysics, University of Notre Dame, Notre Dame, Indiana 46556 (United States); Aguilera, E.F. [Departamento del Accelerador, Instituto Nacional de Investigaciones Nucleares, A.P. 18-1027, C.P. 11801, Destrito Federal (Mexico); Chamon, L.C. [Departamento de Fisica Nuclear, Instituto de Fisica da Universidade de Sao Paulo, Caixa Postal 66318, 05315-970, Sao Paulo, SP (Brazil); Ring, P. [Physik-Department, Technische Universitat Muenchen, D-85747, Garching (Germany); Yakovlev, D.G. [Ioffe Physical Technical Institute, Poliekhnicheskaya 26, RU-194021 St. Petersburg (Russian Federation)

2005-08-01

117

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

118

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

119

STELLAR EVOLUTION CONSTRAINTS ON THE TRIPLE-{alpha} REACTION RATE  

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 sun} {<=} 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 {approx}> 10 M{sub sun}) is minimal. We find that employing the revised rate suppresses helium shell flashes on asymptotic giant branch phase for stars in the initial mass range 0.8 {<=} M/M{sub sun} {<=} 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. In an effort to compromise with the revised rates, we calculate and analyze models with enhanced CNO cycle reaction rates to increase the maximum luminosity of the first giant branch. However, it is impossible to reach the typical red giant branch tip luminosity even if all the reaction rates related to CNO cycles are enhanced by more than 10 orders of magnitude.

Suda, Takuma; Fujimoto, Masayuki Y. [Department of Cosmosciences, Graduate School of Science, Hokkaido University, Kita 10 Nishi 8, Kita-ku, Sapporo 060-0810 (Japan); Hirschi, Raphael, E-mail: suda@astro1.sci.hokudai.ac.jp [Astrophysics Group, EPSAM, Keele University, Keele, Staffordshire ST5 5BG (United Kingdom)

2011-11-01

120

Measurement and analysis of rate data: The rate of reaction of ferric iron with pyrite  

Microsoft Academic Search

Geochemists use several types of experiments to measure the rates of geochemical processes and analyze the results of these experiments by a variety of methods. In order to assess the efficacy of these experiments and methods of analysis, we measured the rate of reaction of ferric iron with pyrite using batch, mixed flow, and plug flow reactors and analyzed the

J. Donald Rimstidt; William D. Newcomb

1993-01-01

121

Neutron-induced astrophysical reaction rates (Panov+, 2010)  

NASA Astrophysics Data System (ADS)

The organization of Tables 3-6 with calculated rates for 24 different values of T9 is extremely simple - all data are in 8 columns. The columns give, in this order, the target element, the atomic mass number A of the target, the temperature in T9, partition function of the target, neutron-induced fission rate for the ground state, neutron-induced fission rate with thermally populated target states, neutron capture rate for the ground state, neutron capture rate with thermally populated target states. The rates in Tables 3-6 were calculated on the basis of different mass and fission barrier predictions: ETFSI, TF and FRDM(masses)+TF(barriers), respectively (see main text of the paper for details). The fitting coefficients for the (n,g), (g,n), and neutron-induced fission rates (n,f) with different mass and fission-barrier predictions are placed in the Tables 7-18 (see example on how to use them in Appendix A of the main paper). The columns in Tables 7-18 are organized as follows: target element, atomic mass number A of the target, target charge number Z, the number of fitting curves i_{fit}, seven coefficients of the forward reaction a_i, and the mean square error for direct reactions. A value i{fit}=0 means that there is only one seven-parameter set to fit the rate. Values i_{fit}>1 give the number of parameter sets which have to be added up to yield the final rate, i.e. the rate r is calculated as r={sum.on.i}ri, with each ri computed from the i-th parameter set and using Eq. ri=exp(a0+a1/T9+a2/T91/3+a3*T91/3+a4*T9+a5*T95/3^+a6*ln(T9)) (4). (16 data files).

Panov, I. V.; Korneev, I. Yu.; Rauscher, T.; Martinez-Pinedo, G.; Kelic-Heil, A.; Zinner, N. T.; Thielemann, F.-K.

2010-01-01

122

Reaction rates from pressure-gauge measurements in reacting explosives  

SciTech Connect

The proper hydrodynamic data and an equation of state are sufficient to describe quantitatively the reaction rates of explosives during the shock-to-detonation transition. Manganin pressure gauges embedded in the reacting explosive have provided these data for the explosives PETN, PBX 9404, TATB, and TNT. Once a pressure-field history has been assembled from individual pressure histories at different depths in the explosive, the conservation equations can be applied in a Lagrangian analysis of the data. The combination of a reactant-product equation of state with this analysis then allows the calculation of the extent of reaction and reaction rate. Successful correlation of the calculated reaction rate values with other thermodynamic variables, such as pressure or temperature, allows formulation of a rate law and the prediction of initiation behavior under circumstances quite different from the experiments that led to the rate law. The best dynamic piezoresistive pressure gauge for most applications would have a substantial output voltage and present negligible disturbance to the flow. In explosives, however, requirements for survival in the extreme temperature and pressure environment encountered by the gauge dictate compromise. Low electrical resistance (approx. 20 m..cap omega..) helps to minimize shunt conductivity failures, but this drastically reduces output and demands that much attention be given to reducingnoise. Although relatively thick insulation perturbs the flow to some extent, survivability requirements dictate its use. Pressure measurements in reactive flow can now be made routinely with gauges that successfully produce data leading to a description of the flow and a powerful predictive capability.

Ginsberg, M.J.; Anderson, A.B.; Wackerle, J.

1981-01-01

123

High-valent intermediates in the reaction of N alpha-acetyl microperoxidase-8 with hydrogen peroxide: models for compounds 0, I and II of horseradish peroxidase.  

PubMed

N-acetyl microperoxidase-8 (Ac-MP-8) is a water soluble, ferric heme model for the peroxidases. The reaction of Ac-MP-8 with H2O2 in 10 mM potassium phosphate over the pH range of 7-11 gives rise sequentially to relatively stable green and red species with properties that closely mimic those of HRP compounds I and II, respectively. Low-temperature stopped-flow studies of this reaction carried out in 50% v/v methanol/10 mM potassium phosphate, pH* 9.1 at -25.8 degrees C indicate that the pseudo-first-order rate constant, kobs, that describes the formation of the green intermediate exhibits saturation kinetics as a function of [H2O2] with kmaxobs = 95 s-1 and KM = 87 mM. Rapid-scan studies carried out with [H2O2] = 200 mM at -38.0 degrees C show that a compound 0 species with a characteristic band near 340 nm is formed whose conversion to the green species is rate limiting. Thus, Ac-MP-8 has high-valent forms that are models for all three known intermediates in the peroxidase cycle of horseradish peroxidase. PMID:1656947

Wang, J S; Baek, H K; Van Wart, H E

1991-09-30

124

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

125

The astrophysical reaction rate for the {sup 18}F(p,{alpha}){sup 15}O reaction  

SciTech Connect

Proton and alpha widths for a 3/2{sup +} ({ell}{sub p} = 0) state in {sup 19}Ne at E{sub x} = 7.1 MeV have been extracted using the results of recent measurements of the {sup 18}F(p,{alpha}){sup 15}O reaction. This {ell}{sub p} = 0 resonance dominates the astrophysical reaction rates at temperatures T{sub 9} > 0.5.

Rehm, K.E. [Argonne National Lab., IL (United States); Paul, M. [Hebrew Univ., Jerusalem (Israel); Roberts, A.D. [Wisconsin Univ., Madison, WI (United States)] [and others

1996-03-01

126

Cross sections and reaction rates of d + 8Li reactions involved in Big Bang nucleosynthesis  

Microsoft Academic Search

We have measured angular distributions of the 2H(8Li, 7Li)3H and 2H(8Li, 9Be)n reactions at Ec.m. = 1.5 to 2.8 MeV using an 8Li- radioactive-beam technique. Astrophysical S-factors and reaction rates were calculated form the measured cross sections. Although the 2H(8Li, 9Be)n cross section is small, it can contribute to 9Be synthesis. The 2H(8Li, 7Li)3H reaction has a sufficiently large cross

M. J. Balbes; M. M. Farrell; R. N. Boyd; X. Gu; M. Hencheck; J. D. Kalen; C. A. Mitchell; J. J. Kolata; K. Lamkin; R. Smith; R. Tighe; K. Ashktorab; F. D. Becchetti; J. Brown; D. Roberts; T.-F. Wang; D. Humphrey; G. Vourvopoulos; M. S. Islam

1995-01-01

127

Microenvironment effects on the kinetics of electron-transfer reactions involving dithionite ions and viologens. 1. A comparison between two types of polyelectrolytes  

SciTech Connect

The retardation of the rate of electron-transfer reactions constitutes a major goal in the development of energy-producing photochemical processes. The stopped-flow technique was used in this work to investigate the retarding effects of two polyelectrolytes on the rate of reduction of a series of dialkylviologens (N,N{prime}-dialkyl-4,4{prime}-bipyridinium, with alkyl chains varying from methyl to hexadecyl) by sodium dithionite. The apparent rate constants for reduction have been measured in the absence and in the presence of polyelectrolytes having widely different shapes: poly(styrenesulfonic acid) (PSSH), which is essentially linear, and maleic acid-cetyl vinyl ether (MA-CVE), which is a polysoap. The latter one was shown to be much more efficient than the former. The larger retardation effect (about 50-fold) was obtained with MA-CVE in combination with di-C{sub 16}-viologen. The effect of polyelectrolyte concentration on the observed pseudo-first-order rate constant could be theoretically predicted for both polyelectrolytes.

Kim, Hag Seong; Claude, B.; Tondre, C. (Universite de Nancy I, Vandoeuvre-les-Nancy (France))

1990-09-20

128

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

129

Reaction rate calculation with time-dependent invariant manifolds  

NASA Astrophysics Data System (ADS)

The identification of trajectories that contribute to the reaction rate is the crucial dynamical ingredient in any classical chemical reactivity calculation. This problem often requires a full scale numerical simulation of the dynamics, in particular if the reactive system is exposed to the influence of a heat bath. As an efficient alternative, we propose here to compute invariant surfaces in the phase space of the reactive system that separate reactive from nonreactive trajectories. The location of these invariant manifolds depends both on time and on the realization of the driving force exerted by the bath. These manifolds allow the identification of reactive trajectories simply from their initial conditions, without the need of any further simulation. In this paper, we show how these invariant manifolds can be calculated, and used in a formally exact reaction rate calculation based on perturbation theory for any multidimensional potential coupled to a noisy environment.

Bartsch, Thomas; Revuelta, F.; Benito, R. M.; Borondo, F.

2012-06-01

130

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

131

Triple-? reaction rate constrained by stellar evolution models  

NASA Astrophysics Data System (ADS)

We investigate the quantitative constraint on the triple-? 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/Msolar<=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 > 10Msolar) 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/Msolar<=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-? 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 ? > 10 at T = 1-1.2108K where the cross section is proportional to T?. 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 ~ 10-29 cm6 s-1 mole-2 at ~ 107.8 K, which corresponds to about three orders of magnitude larger than that of the NACRE compilation.

Suda, Takuma; Hirschi, Raphael; Fujimoto, Masayuki Y.

2012-11-01

132

Spatial rock-paper-scissors models with inhomogeneous reaction rates  

Microsoft Academic Search

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

Qian He; Mauro Mobilia; Uwe C. Tuber

2010-01-01

133

Micellar effects on reaction rates and acid-base equilibria  

Microsoft Academic Search

The effects of ionic surfactant micelles on reaction rates in solution on acid-base equilibria often conform quite well to a theory known as the pseudophase ion-exchange model (PPIEM). Despite its success the theory suffers from several limitations. These are outlined and an alternative approach which overcomes them is forwarded. This new theory is based on the equilibrium formulation of transition-state

Denver G. Hall

1987-01-01

134

A Study of the ^30S(,)^33Cl Reaction Rate  

NASA Astrophysics Data System (ADS)

The ^30S(,)^33Cl reaction rate has major implications for x-ray bursts (XRBs). No experimental information exists for this reaction rate, though XRB models have shown that it affects final isotopic abundances and the total energy output.footnotetextA. Parikh et al., ApJ SS 178, 110 (2008). This rate may also influence XRB observables such as the structure of double-peaked luminosity curvesfootnotetextJ.L. Fisker et al., ApJ 608, L61 (2004). and the composition of the neutron star crust.footnotetextH. Schatz and K.E. Rehm, NPA 777, 601 (2006). We have studied the time-inverse reaction p(^33Cl,^30S)? at ATLAS using a radioactive ^33Cl beam. The residual ^30S nuclei were detected at the focal plane of the split-pole spectrograph, which was used in gas-filled mode, in coincidence with the ? particles, which were detected in a double-sided Si detector. The experimental results and conclusions about the impact on XRB nucleosynthesis will be discussed.

Deibel, C. M.; Jiang, C. L.; Kay, B. P.; Lee, H. Y.; Pardo, R. C.; Rehm, K. E.; Ugalde, C.; Woodard, A.; Figueira, J. M.; Marley, S. T.; Patel, N. R.; Paul, M.; Wuosmaa, A.

2009-10-01

135

Omphacite breakdown reactions and relation to eclogite exhumation rates  

NASA Astrophysics Data System (ADS)

Clinopyroxene + plagioclase (Hbl Qtz) symplectites after omphacite are widely cited as evidence for prior eclogite-facies or high-pressure (HP) metamorphism. Precursor omphacite compositions of retrograde eclogites, used for reconstructing retrograde P- T paths, are commonly estimated by reintegrating symplectite phases with the assumption that the symplectite-forming reactions were isochemical. Comparisons of broadbeam symplectite compositions to adjacent unreacted pyroxene from various symplectites after clinopyroxene from the Appalachian Blue Ridge (ABR) and Western Gneiss Region (WGR) suggest that the symplectite forming reactions are largely isochemical. Endmember calculations based on reintegrated symplectite compositions from the ABR and WGR suggest that a minor Ca-Eskola (CaEs) component (XCaEs = 0.04-0.15) was present in precursor HP clinopyroxene. WGR symplectites consist of fine-grained (1 ?m-scale), vermicular intergrowths of Pl + Cpx II Hbl that occur at grain boundaries or internally. ABR symplectites contain coarser (10 ?m-scale) planar lamellae and rods of Pl + Cpx II + Qtz + Hbl within clinopyroxene cores. The contrasting textures correlate with decompression and cooling rate, and degree of overstepping of the retrograde reaction (lamellar: slow, erosionally controlled exhumation with slow/low overstepping; fine-grained, grainboundary symplectite: rapid, tectonic exhumation with rapid/high overstepping). Variations in XCaEs, Xjd, and XCaTs of precursor HP omphacite are related to the symplectic mineral assemblages that result from decompression. Quartz-normative symplectities indicate quartz-producing retrograde reactions (e.g., breakdown of precursor CaEs); quartz-free symplectities (e.g., diopside + plagioclase after omphacite) indicate quartz-consuming reactions (jd, CaTs breakdown) outpaced quartz-producing reactions.

Anderson, Eric D.; Moecher, David P.

2007-09-01

136

Effect of confinement on reaction rates within polymer nanotemplates  

NASA Astrophysics Data System (ADS)

The most efficient catalysts have been developed and optimized by living systems. Indeed, in vivo enzyme-catalyzed reactions are several orders of magnitude more efficient than platinum based catalyzed reactions. However, the rate of reaction and equilibrium interactions are considerably reduced when the biological systems are studied in vitro. This phenomenon is largely attributed to the effect of confinement or macromolecular crowding present in the cell. Confinement can also be observed in an aqueous solution containing surfactants (amphiphilic copolymers). For example, copolymers can self-assemble into well defined ordered structures such as micelles, nanotubes, vesicles; and the geometries and shapes of a given copolymer can be controlled by their solvent affinity. The hollow nanoarchitectures obtained by self-assembly can be used as a model template to study confinement within a soft shell system to mimic biosystems. These systems provide a very controlled environment for the study of confinement. In this paper we will present the effect of confinement on polymerisation reactions combining both simulation and experimental characterisation for a comprehensive study of the effect of confinement on the interactions among confined molecules.

Malardier-Jugroot, Cecile; Li, Xia

2012-02-01

137

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

138

Kinetics of Imidazole Catalyzed Ester Hydrolysis: Use of Buffer Dilutions to Determine Spontaneous Rate, Catalyzed Rate, and Reaction Order.  

ERIC Educational Resources Information Center

|Described is an advanced undergraduate kinetics experiment using buffer dilutions to determine spontaneous rate, catalyzed rate, and reaction order. The reaction utilized is hydrolysis of p-nitro-phenyl acetate in presence of imidazole, which has been shown to enhance rate of the reaction. (Author/JN)|

Lombardo, Anthony

1982-01-01

139

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

140

"Virtual injector" flow tube method for measuring relative rates kinetics of gas-phase and aerosol species.  

PubMed

A new method for measuring gas-phase and aerosol reaction kinetics is described in which the gas flow, itself, acts as a "virtual injector" continuously increasing the contact time in analogy to conventional movable-injector kinetics techniques. In this method a laser is directed down the length of a flow tube, instantly initiating reaction by photodissociation of a precursor species at every point throughout the flow tube. Key tropospheric reactants such as OH, Cl, NO(3), and O(3) can be generated with nearly uniform concentrations along the length of the flow tube in this manner using 355 nm radiation from the third harmonic of a Nd:YAG laser. As the flow travels down the flow tube, both the gas-phase and particle-phase species react with the photogenerated radicals or O(3) for increasingly longer time before exiting and being detected. The advantages of this method are that (1) any wall loss of gas-phase and particle species is automatically accounted for, (2) the reactions are conducted under nearly pseudo-first-order conditions, (3) the progress of the reaction is followed as a continuous function of reaction time instead of reactant concentration, (4) data collection is quick with an entire decay trace being collected in as little as 1 min, (5) relative rates of several species can be measured simultaneously, and (6) bimolecular rate constants at least as small as k = 10(-17) (cm(3)/molecule)/s, or aerosol uptake coefficients at least as small as ? = 10(-4), can be measured. Using the virtual injector technique with an aerosol chemical ionization mass spectrometer (CIMS) as a detector, examples of gas-phase relative rates and uptake by oleic acid particles are given for OH, Cl, NO(3), and O(3) reactions with most agreeing to within 20% of published values, where available. PMID:22702447

Renbaum-Wolff, Lindsay; Smith, Geoffrey D

2012-06-15

141

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)

1999-04-29

142

Investigation of the reaction rates between uranium and liquid aluminum  

SciTech Connect

A limited study consisting of scoping experiments was carried out to determine if a uranium-niobium alloy was weakly or strongly resistant to penetration by liquid aluminum alloy 6061. Our investigation was limited to temperatures between 700{degrees}and 900{degrees}C and carried out using small cylindrical coupons of U-6wt.%Nb and unalloyed U in unsaturated molten aluminum and aluminum alloy 6061 baths. The results indicate that indeed, significant dissolution of uranium into molten aluminum occurs and in relatively short times. The diameters of U-6wt.%Nb test cylinders immersed in unsaturated Al-6061 decreased linearly with time at 700{degrees}C, 800{degrees}C, and 900{degrees}C at rates of 1.14 mm/h, 3.0 mm/h, and 3.5 mm/h, respectively. However, we have found that the reaction rates were significantly reduced by the alloying elements niobium and magnesium. These results suggest that a more detailed investigation could lead to a predictive capability for control of these reaction rates. In unalloyed U, the rate of U dissolution increased by up to a factor of ten relative to that in U- 6wt.%Nb. In both materials, the dissolution of the U core was found to occur by advance of a continuous intermetallic layer, which redissolves into the unsaturated liquid Al bath. The Nb additions resulted in the formation of a thick transition layer composed of intermetallic and U-saturated liquid. In unalloyed U, however, the transition layer was largely reduced in thickness, allowing unsaturated liquid adjacent to the continuous intermetallic layer. Another important variable identified was the influence of surface condition on interface reactivity. At 900{degrees}C, liquid attack was completely absent over large regions of U-6wt.% Nb samples immersed in Al-6061. Surface analysis by EDX identified the presence of up to 24 at.% Mg in unattacked areas.

Strum, M.J.; Sedillo, E. [Lawrence Livermore National Lab., CA (United States); Smugeresky, J. [Sandia National Labs., Albuquerque, NM (United States)

1995-05-05

143

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; Tuber, Uwe C

2010-11-04

144

Spatial rock-paper-scissors models with inhomogeneous reaction rates  

NASA Astrophysics Data System (ADS)

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 systems 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.

He, Qian; Mobilia, Mauro; Tuber, Uwe C.

2010-11-01

145

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 (1.530.23)10 -13 and (1.390.19)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 (5.02.8)10 -19 cm 3 molecule -1 s -1 for ( Z)-pent-2-en-1-ol, and (9.15.8)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.

146

Solvated electron reaction rates in alcohols and water. Solvent effect. [Reactions with dissolved gases  

Microsoft Academic Search

In alcohols electron capture by sulfur hexafluoride is less efficient than that by oxygen. This is the reverse of the relative efficiencies in the gas phase. Solvation affects the electron attachment efficiency by altering the energetics of the reaction. The ratio of electron capture rate constants for a given solute in methanol and ethanol, k\\/sub 4,M\\/\\/k\\/sub 4,E\\/, decreases with decreasing

G. L. Bolton; G. R. Freeman

1976-01-01

147

Kinetics and Mechanism of the Reactions of Picolinic Acid withDichloro-{1-alkyl-2-(arylazo)imidazole}palladium(II) Complexes  

Microsoft Academic Search

The reaction between Pd(N,N?)Cl2 [N,N? ? 1-alkyl-2-(arylazo)imidazole (N,N?) and picolinic acid (picH) have been studied spectrophotometrically at ? = 463nm in MeCN at 298K. The product is [Pd(pic)2] which has been verified by the synthesis of the pure compound from Na2[PdCl4] and picH. The kinetics of the nucleophilic substitution reaction have been studied under pseudo-first-order conditions.\\u000a The reaction proceeds in

Sushanta Saha; Prodyut Kumar Sarkar; Ambikesh Mahapatra

2006-01-01

148

Neutron-induced astrophysical reaction rates for translead nuclei  

NASA Astrophysics Data System (ADS)

Neutron-induced reaction rates, including fission and neutron capture, are calculated in the temperature range 108 ? T(K) ? 1010 within the framework of the statistical model for targets with the atomic number 84 ? Z ? 118 (from Po to Uuo) from the neutron to the proton drip-line. Four sets of rates have been calculated, utilizing - where possible - consistent nuclear data for neutron separation energies and fission barriers from Thomas-Fermi (TF), Extended Thomas-Fermi plus Strutinsky Integral (ETFSI), Finite-Range Droplet Model (FRDM) and Hartree-Fock-Bogolyubov (HFB) predictions. Tables of calculated values as well as analytic seven parameter fits in the standard REACLIB format are supplied. We also discuss the sensitivity of the rates to the input, aiming at a better understanding of the variations introduced by the nuclear input. Tables 3-18 with these data are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/513/A61

Panov, I. V.; Korneev, I. Yu.; Rauscher, T.; Martnez-Pinedo, G.; Keli?-Heil, A.; Zinner, N. T.; Thielemann, F.-K.

2010-04-01

149

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 (?=510-3) of HNO3 on deliquesced NaCl particles agrees with data reported previously.

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

2006-12-01

150

Effect of Substrate Character on Heterogeneous Ozone Reaction Rate with Individual PAHs and Their Reaction Mixtures  

NASA Astrophysics Data System (ADS)

Vehicle exhaust contains many unregulated chemical compounds that are harmful to human health and the natural environment, including polycyclic aromatic hydrocarbons (PAH), a class of organic compounds derived from fuel combustion that can be carcinogenic and mutagenic. PAHs have been quantified in vehicle-derived ultrafine particles (Dp<100nm), which are more toxic than larger particles and are linked to adverse health problems, including respiratory and cardiac disease. Once emitted into the atmosphere, particle-bound PAHs can undergo aging reactions with oxidants, such as ozone, to form more polar species. These polar reaction products include species such as quinones that can be more toxic than the parent PAH compounds. Here, 0.4ppm ozone was reacted over a 24-hour period with the 16 EPA priority PAHs plus coronene adsorbed to (i) a quartz fiber filter and (ii) NIST diesel PM. The difference in the PAH/O3 heterogeneous reaction rate resulting from the two substrates will be discussed. The experiments were completed by spiking a known PAH mixture to the solid, reacting the samples with gas-phase ozone, and determining both PAH loss over time and products formed, using thermal-desorption gas chromatography / mass spectrometry (TD-GC/MS). The individual PAHs anthracene, phenanthrene, and fluorene, adsorbed to a QFF were also separately reacted with 0.4 ppm ozone. A volatilization control and the collection of volatilized PAHs using a Tenax-packed thermal desorption vial completed the mass balance and aided determination parent-product relationships. Heterogeneous reaction products analyzed directly without derivatization indicate the formation of 9,10-anthracenedione, 9H-fluoren-9-one, and (1,1-biphenyl)-2,2-dicarboxaldehyde from the reaction of ozone with the PAH mix on a QFF, but only 9,10-anthracenedione was detected for the diesel PM reaction. The implications of these results for aging of diesel particulate in urban environments will be discussed.

Holmen, B. A.; Stevens, T.

2009-12-01

151

Determination of the Astrophysical S(E) Factors or Rates for Radiative Capture Reaction with One Nucleon Transfer Reaction  

NASA Astrophysics Data System (ADS)

The radiative capture reaction plays an important role in nuclear astrophysics. We have indirectly measured the astrophysical S(E) factors for some proton capture reactions and reaction rates for several neutron capture reactions with one nucleon transfer reactions at HI-13 tandem accelerator in recent years. Some of them are compiled into IAEA EXFOR database and JINA REACLIB project, and used in the network calculations of Big Bang nucleosynthesis and type-I X-ray bursts.

Li, Zhihong; Guo, Bing; Li, Yunju; Su, Jun; Li, Ertao; Bai, Xixiang; Wang, Youbao; Zeng, Sheng; Wang, Baoxiang; Yan, Shengquan; Li, Zhichang; Liu, Jiancheng; Lian, Gang; Jin, Sunjun; Liu, Xin; Liu, Weiping

2012-06-01

152

Rate constants for the reactions between OH and perfluorinated alkenes.  

PubMed

The rate constants for the reactions of OH radicals with fully fluorinated alkenes containing different numbers of -CF(3) groups next to olefinic carbon, CF(2)?CF(2), CF(2)?CFCF(3), CF(3)CF?CFCF(3), and (CF(3))(2)C?CFC(2)F(5), were measured between 230 and 480 K using the flash photolysis resonance fluorescence technique to give the following expressions: k(C(2)F(4))(250-480 K) = 1.32 10(-12) (T/298 K)(0.9) exp(+600 K/T) cm(3) molecule(-1) s(-1), k(C(3)F(6))(230-480 K) = 9.75 10(-14) (T/298 K)(1.94) exp(+922 K/T) cm(3) molecule(-1) s(-1), k(trans-C(4)F(8))(230-370 K) = 7.50 10(-14) (T/298 K)(1.68) exp(+612 K/T) cm(3) molecule(-1) s(-1), k(cis-C(4)F(8))(230-370 K) = 2.99 10(-14) (T/298 K)(2.61) exp(+760 K/T) cm(3) molecule(-1) s(-1), and k(C(6)F(12))(250-480 K) = 2.17 10(-15) (T/298 K)(3.90) exp(+1044 K/T) cm(3) molecule(-1) s(-1). The kinetics of the OH reaction in an industrial sample of octofluoro-2-propene (a mixture of the cis- and trans-isomers of CF(3)CF?CFCF(3)) was studied to determine the "effective" reaction rate constant for the typically industrial mixture: k()(230-480 K) = 7.89 10(-14) (T/298 K)(1.71) exp(+557 K/T) cm(3) molecule(-1) s(-1). On the basis of these results, the atmospheric lifetimes were estimated to be 1.2, 5.3, 21, 34, and 182 days for CF(2)?CF(2), CF(3)CF?CF(2), trans-CF(3)CF?CFCF(3), cis-CF(3)CF?CFCF(3), and (CF(3))(2)C?CFC(2)F(5), respectively. The general pattern of halolalkene reactivity toward OH is discussed. PMID:21615161

Orkin, Vladimir L; Poskrebyshev, Gregory A; Kurylo, Michael J

2011-05-26

153

Free Energy and the Rate of Chemical Reactions The Relation Between the Rate and Oxidation-Reduction Potentials  

Microsoft Academic Search

The theories of the rates of chemical reactions previously developed by Eyring, Rice and the author give a relationship between the rate of reaction and the free energy of formation of the activated state. With the aid of this relation it is possible to explain the experimental data which have shown that there is a connection between the rates of

Harold Gershinowitz

1936-01-01

154

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 410(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

2013-04-19

155

Survey of Photochemical and Rate Data for Twenty-eight Reactions of Interest in Atmospheric Chemistry  

Microsoft Academic Search

Photochemical and rate data have been evaluated for 28 gas phase reactions of interest for the chemistry of the stratosphere. The results are presented on data sheets, one per reaction. For each reaction, the available data are summarized. Where possible there is given a preferred value for the rate constant or, for the photochemical reactions, preferred values for primary quantum

R. F. Hampson; W. Braun; R. L. Brown; D. Garvin; J. T. Herron; R. E. Huie; M. J. Kurylo; A. H. Laufer; J. D. McKinley; H. Okabe; M. D. Scheer; W. Tsang; D. H. Stedman

1973-01-01

156

Influence of Chemical Kinetics on Postcolumn Reaction in a Capillary Taylor Reactor with Catechol Analytes and Photoluminescence Following Electron Transfer  

PubMed Central

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-?m-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)33+ by analyte to the photoluminescent Os(bpy)32+) 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 (12 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 ?L/min) velocity with solutes having diffusion coefficients in the 5 10?6 cm2/s range, can be constructed from 8.0 cm of 25-?m-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-?m-capillary chromatography column by 10% if the pseudo-first-order rate constant is larger than 0.1 s?1.

Jung, Moon Chul; Weber, Stephen G.

2006-01-01

157

Similarity Solution of Two Coupled Reaction - Diffusion Rate Equations.  

National Technical Information Service (NTIS)

A coupled system of nonlinear evolution equations describing the combined effects of diffusion and reactions, e.g. fusion reactions or electron-ion recombinations, is transformed into a system of coupled similarity equations in one, two and three dimensio...

H. Wilhelmsson

1986-01-01

158

Lignin peroxidase compounds II and III. Spectral and kinetic characterization of reactions with peroxides.  

PubMed

Stopped-flow techniques were used to investigate the kinetics of the reaction of lignin peroxidase compounds II and III (LiPII and LiPIII) with peroxides. Rate data were obtained from single-turnover experiments under pseudo-first-order conditions. LiPII reacts with H2O2 or peracetic acid (AcOOH) to form a modified LiPIII, designated as LiPIII*, via a biphasic reaction. During the first phase, LiPIII is formed as an intermediate. Kinetic analysis also indicates a LiPII-peroxide complex. The first-order dissociation rate constants for the reaction of LiPII with H2O2 and AcOOH are 7.9 +/- 0.5 and 4.9 +/- 0.6 s-1, respectively. The rate of the H2O2 reaction is approximately 500 times the rate of the comparable reaction with horseradish peroxidase, suggesting it is physiologically significant. The activation energy for the formation of LiPIII is 23 kJ mol-1. During the second phase, the intermediate LiPIII is converted to LiPIII*, confirmed by analyzing the reaction of exogenously prepared LiPIII with peroxides. The second-order rate constants for the reaction of LiPIII with H2O2 and AcOOH are (3.7 +/- 0.2) x 10(2) M-1 s-1 and (2.9 +/- 0.2) x 10(2) M-1 s-1, respectively. The conversion of LiPIII to LiPIII* is reversible; the first-order rate constant for the reverse reaction is approximately (6.6 +/- 0.6) x 10(-2) s-1. The rates of both LiPIII and LiPIII* formation decrease markedly above pH 4.0. The pH dependence of these reactions is controlled by a heme-linked ionizable group of pK alpha congruent to 4.2. PMID:2162833

Wariishi, H; Marquez, L; Dunford, H B; Gold, M H

1990-07-01

159

Relativistic equilibrium velocity distribution, nuclear fusion reaction rate and the solar neutrino problem  

Microsoft Academic Search

In solar interior, it is the equilibrium velocity distribution of few high-energy protons and nuclei that participates in determining nuclear fusion reaction rates. So, it is inappropriate to use the Maxwellian velocity distribution to calculate the rates of solar nuclear fusion reactions. We have to use the relativistic equilibrium velocity distribution for the purpose. The nuclear fusion reaction rate based

Jian-Miin Liu

2003-01-01

160

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

161

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)22H2O or Cu(Ac)22H2O 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

2013-06-17

162

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

163

Hydrotropic Enhancement of Rate of the Browning Reaction; A Kinetic Study  

Microsoft Academic Search

The rate of a nonenzymatic browning reaction, that is, the reaction between reducing sugar and amino acid(commonly known as Millard reaction) is strongly enhanced in the presence of a common hydrotrope viz. sodium benzoate or sodium salicylate even below its minimal hydrotropic concentration (MHC). The effect of hydrotrope on the rate has been studied in four systems taking combination of

Neelu Kambo; Shalini Pandey; Santosh K. Upadhyay

2006-01-01

164

On the use temperature parameterized rate coefficients in the estimation of non-equilibrium reaction rates  

NASA Astrophysics Data System (ADS)

The present paper considers a detailed analysis of the nonequilibrium effects for a model reactive system with the Chapman Eskog (CE) solution of the Boltzmann equation as well as an explicit time dependent solution. The elastic cross sections employed are a hard sphere cross section and the Maxwell molecule cross section. Reactive cross sections which model reactions with and without activation energy are used. A detailed comparison is carried out with these solutions of the Boltzmann equation and the approximation introduced by Cukrowski and coworkers [J. Chem. Phys. 97 (1992) 9086; Chem. Phys. 89 (1992) 159; Physica A 188 (1992) 344; Chem. Phys. Lett. A 297 (1998) 402; Physica A 275 (2000) 134; Chem. Phys. Lett. 341 (2001) 585; Acta Phys. Polonica B 334 (2003) 3607.] based on the temperature of the reactive particles. We show that the Cukrowski approximation has limited applicability for the large class of reactive systems studied in this paper. The explicit time dependent solutions of the Boltzmann equation demonstrate that the CE approach is valid only for very slow reactions for which the corrections to the equilibrium rate coefficient are very small.

Shizgal, Bernie D.; Chikhaoui, Aziz

2006-06-01

165

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

166

Manganese peroxidase from the lignin-degrading basidiomycete Phanerochaete chrysosporium: Transient state kinetics and reaction mechanism  

SciTech Connect

Stopped-flow techniques were used to investigate the kinetics of the formation of manganese peroxidase compound I (MnPI) and of the reactions of MnPI and manganese peroxidase compound II (MnPII) with p-cresol and Mn{sup II}. All of the rate data were obtained from single turnover experiments under pseudo-first order conditions. In the presence of H{sub 2}O{sub 2} the formation of MnPI is independent of pH over the range 3.12-8.29 with a second-order rate constant of (2.0{+-}0.1) {times} 10{sup 6} M{sup {minus}1} s{sup {minus}1}. The activation energy for MnPI formation is 20 kJ mol{sup {minus}1}. MnPI formation also occurs with organic peroxides such as peracetic acid, m-chloroperoxybenzoic acid, and p-nitroperoxybenzoic acid with second-order rate constants of 9.7 x 10{sup 5}, 9.5 {times} 10{sup 4}, and 5.9 {times} 10{sup 4} M{sup {minus}1} s{sup {minus}1}, respectively. The reactions of MnPI and MnPII with p-cresol strictly obeyed second-order kinetics. The second-order rate constant for the reaction of MnPII with p-cresol is extremely low, (9.5{+-}0.5) m{sup {minus}1} s{sup {minus}1}. Kinetic analysis of the reaction of Mn{sup II} with MnPI and MnPII showed a binding interaction with the oxidized enzymes which led to saturation kinetics. The first-order dissociation rate constants for the reaction of Mn{sup II} with MnPI and MnPII are (0.7{+-}0.1) and (0.14{+-}0.01) s{sup {minus}1}, respectively, when the reaction is conducted in lactate buffer. Rate constants are considerably lower when the reactions are conducted in succinate buffer. Single turnover experiments confirmed that Mn{sup II} serves as an obligatory substrate for MnPII and that both oxidized forms of the enzyme form productive complexes with Mn{sup II}. Finally, these results suggest the {alpha}-hydroxy acids such as lactate facilitate the dissociation of Mn{sup II} from the enzyme.

Wariishi, Hiroyuki; Gold, M.H. (Oregon Graduate Center, Beaverton (United States)); Dunford, H.B.; MacDonald, I.D. (Univ. of Alberta, Edmonton (Canada))

1989-02-25

167

Kinetics of solute adsorption at solid/solution interfaces: a theoretical development of the empirical pseudo-first and pseudo-second order kinetic rate equations, based on applying the statistical rate theory of interfacial transport.  

PubMed

For practical applications of solid/solution adsorption processes, the kinetics of these processes is at least as much essential as their features at equilibrium. Meanwhile, the general understanding of this kinetics and its corresponding theoretical description are far behind the understanding and the level of theoretical interpretation of adsorption equilibria in these systems. The Lagergren empirical equation proposed at the end of 19th century to describe the kinetics of solute sorption at the solid/solution interfaces has been the most widely used kinetic equation until now. This equation has also been called the pseudo-first order kinetic equation because it was intuitively associated with the model of one-site occupancy adsorption kinetics governed by the rate of surface reaction. More recently, its generalization for the two-sites-occupancy adsorption was proposed and called the pseudo-second-order kinetic equation. However, the general use and the wide applicability of these empirical equations during more than one century have not resulted in a corresponding fundamental search for their theoretical origin. Here the first theoretical development of these equations is proposed, based on applying the new fundamental approach to kinetics of interfacial transport called the Statistical Rate Theory. It is shown that these empirical equations are simplified forms of a more general equation developed here, for the case when the adsorption kinetics is governed by the rate of surface reactions. The features of that general equation are shown by presenting exhaustive model investigations, and the applicability of that equation is tested by presenting a quantitative analysis of some experimental data reported in the literature. PMID:16913785

Rudzinski, Wladyslaw; Plazinski, Wojciech

2006-08-24

168

Rates of primary electron transfer reactions in the photosystem I reaction center reconstituted with different quinones as the secondary acceptor  

SciTech Connect

Rates of sequential electron transfer reactions from the primary electron donor chlorophyll dimer (P700) to the electron acceptor chlorophyll a-686 (A[sub 0]) and to the secondary acceptor quinone (Q[sub [phi

Kumazaki, Shigeichi; Kandori, Hideki; Yoshihara, Keitaro (Inst. for Molecular Science, Okazaki (Japan)); Iwaki, Masayo; Itoh, Shigeru (National Inst. for Basic Biology, Okazaki (Japan)); Ikegamu, Isamu (Teikyo Univ., Kanagawa (Japan))

1994-10-27

169

The expectations hypothesis of the term structure of interest rates, open interest rate parity and central bank policy reaction  

Microsoft Academic Search

A rational expectations model with endogenous monetary policy reacting to the exchange rate and the term spread shows that the empirical performance of the expectations hypothesis of the term structure and the uncovered interest rate parity hypothesis improves with the strength of the policy reaction to the exchange rate and the term spread, respectively.

Peter Kugler

2000-01-01

170

Development of the new approach to the diffusion-limited reaction rate theory  

SciTech Connect

The new approach to the diffusion-limited reaction rate theory, recently proposed by the author, is further developed on the base of a similar approach to Brownian coagulation. The traditional diffusion approach to calculation of the reaction rate is critically analyzed. In particular, it is shown that the traditional approach is applicable only in the special case of reactions with a large reaction radius and the mean inter-particle distances, and become inappropriate in calculating the reaction rate in the case of a relatively small reaction radius. In the latter case, most important for chemical reactions, particle collisions occur not in the diffusion regime but mainly in the kinetic regime characterized by homogeneous (random) spatial distribution of particles on the length scale of the mean inter-particle distance. The calculated reaction rate for a small reaction radius in three dimensions formally (and fortuitously) coincides with the expression derived in the traditional approach for reactions with a large reaction radius, but notably deviates at large times from the traditional result in the planar two-dimensional geometry. In application to reactions on discrete lattice sites, new relations for the reaction rate constants are derived for both three-dimensional and two-dimensional lattices.

Veshchunov, M. S., E-mail: vms@ibrae.ac.ru [Russian Academy of Sciences, Nuclear Safety Institute (IBRAE) (Russian Federation)

2012-04-15

171

Acceleration of the reaction of carbon dioxide into aqueous 2-amino-2-hydroxymethyl-1,3-propanediol solutions by piperazine addition  

Microsoft Academic Search

In this work, the kinetics of the reaction between CO2 and piperazine-activated aqueous solutions of a sterically hindered alkanolamine, 2-amino-2-hydroxymethyl-1,3-propanediol (AHPD) was studied in a wetted wall column contactor at 303.15, 313.15 and 323.15K. The AHPD concentration in the aqueous solutions was kept at 1kmolm-3 while the piperazine (PZ) concentration varied in the range 0.10.4kmolm-3. Under pseudo-first-order CO2 absorption conditions,

Francis Bougie; Julien Lauzon-Gauthier; Maria C. Iliuta

2009-01-01

172

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

173

A Rate-Governing Reaction of Protein Synthesis  

Microsoft Academic Search

RABBIT reticulocytes synthesize hmoglobin in vitro. In order to maintain a high rate for at least four hours there needs to be added to the saline solution in which the cells are suspended a mixture of amino-acids, glucose, iron and an iron-chelating agent. The rate then (in four hours) is about five times that in saline alone. Glucose, iron and

Henry Borsook

1958-01-01

174

Reaction of hydroquinine with hematite II. Calculated electron-transfer rates and comparison to the reductive dissolution rate  

Microsoft Academic Search

The rate of reaction of hematite with quinones and the quinone moieties of larger molecules may be an important factor in limiting the rate of reductive dissolution of hematite, especially by iron-reducing bacteria. It is possible that the rate of reductive dissolution of hematite in the presence of excess hydroquinone at pH 2.5 may be limited by the electron-transfer rate.

Andrew G Stack; Kevin M Rosso; Dayle Smith; Carrick M Eggleston

2004-01-01

175

H 2O 2\\/UV degradation kinetics of isoprene in aqueous solution  

Microsoft Academic Search

Hydrogen peroxide and UV radiation have been used in the photochemical degradation of isoprene in aqueous solutions. A kinetic study is carried out taking into account the contribution of the UV radiation reaction and the combined reaction with hydrogen peroxide. An empirical reaction rate expression, which considers the two reactions taking place in parallel, is suggested. Pseudo-first order rate constants

E. M. Elkanzi; Goh Bee Kheng

2000-01-01

176

Method and Apparatus for Obtaining Enhanced Production Rate of Thermal Chemical Reactions.  

National Technical Information Service (NTIS)

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 proc...

A. L. Y. Tonkovich R. S. Wegeng Y. Gao Y. Wang

2005-01-01

177

Effect of reaction heat on Maxwellian distribution functions and rate of reactions  

NASA Astrophysics Data System (ADS)

A binary gaseous mixture undergoing a reversible reaction of type A+A \\rightleftharpoons B+B is modeled with the chemical kinetic Boltzmann equation, assuming hard sphere cross sections for elastic collisions, and two different models with activation energy for reactive interactions, namely the line-of-centers and step cross-section models. The Chapman Enskog method and Sonine polynomial representation of the distribution functions are used to obtain the solution of the Boltzmann equation in a chemical regime for which the reactive interactions are less frequent than the elastic collisions, i.e. in the early stage of the reaction when the constituent A is in a large amount with respect to B and the affinity of the reaction tends to infinity. The aim of this paper is twofold: (i) to evaluate the effect of the reaction heat on the Maxwellian distribution functions and on the production terms of both particle number densities and mixture energy density; (ii) to analyze spatially homogeneous solutions for the particle number density and temperature of the reactants when the chemical reaction advances. It is shown that the reaction heat changes the Maxwellian distribution functions, the production terms and hence the trend to equilibrium of the particle number density and temperature of the reactants. Moreover, these changes differ for exothermic and endothermic reactions.

Kremer, Gilberto M.; Jacinta Soares, Ana

2007-12-01

178

THE {sup 8}Li({alpha}, n){sup 11}B REACTION RATE AT ASTROPHYSICAL TEMPERATURES  

SciTech Connect

At temperatures (0.5-1.2) x 10{sup 9} K, the {sup 8}Li + {sup 4}He {yields} {sup 11}B+n reaction can allow for {sup 12}C and heavier element production in the framework of the inhomogeneous big bang nucleosynthesis. At temperatures (2.5-5) x 10{sup 9} K, it can influence the production of seed nuclei, later burnt to heavier elements by means of rapid neutron capture reactions, during Type II supernova explosions. Previous determinations of the reaction rate show an untenable disagreement. In this work, a new reaction rate calculation is proposed for the intervals of astrophysical interest. This new recommendation turns out to be up to a factor of five larger than the most recent rate in the literature, thus enforcing the role of {sup 8}Li + {sup 4}He {yields} {sup 11}B+n as a candidate for key astrophysical reactions. The analytical expression of the recommended reaction rate is given.

La Cognata, Marco; Del Zoppo, Antonio, E-mail: delzoppo@lns.infn.it [INFN-Laboratori Nazionali del Sud, Via S. Sofia 62, I95123 Catania (Italy)

2011-08-01

179

Effects of mineral spatial distribution on reaction rates in porous media  

Microsoft Academic Search

This study examined the effects of variations in mineral spatial distributions on reaction rates in porous media. Pore-scale network models were constructed to represent sandstones with anorthite and kaolinite present in various amounts and in different spatial patterns. Simulation conditions corresponded to geological sequestration of carbon dioxide. To examine scaling effects arising from pore-scale heterogeneities, reaction rates from network models

L. Li; C. A. Peters; M. A. Celia

2007-01-01

180

Compilation and R-matrix analysis of Big Bang nuclear reaction rates  

Microsoft Academic Search

We use the R-matrix theory to fit low-energy data on nuclear reactions involved in Big Bang nucleosynthesis. Special attention is paid to the rate uncertainties which are evaluated on statistical grounds. We provide S factors and reaction rates in tabular and graphical formats.

Pierre Descouvemont; Abderrahim Adahchour; Carmen Angulo; Alain Coc; Elisabeth Vangioni-Flam

2004-01-01

181

BRUSLIB and NETGEN: the Brussels nuclear reaction rate library and nuclear network generator for astrophysics  

Microsoft Academic Search

Nuclear reaction rates are quantities of fundamental importance in astrophysics. Substantial efforts have been devoted in the last decades to measuring or calculating them. This paper presents a detailed description of the Brussels nuclear reaction rate library BRUSLIB and of the nuclear network generator NETGEN. BRUSLIB is made of two parts. The first one contains the 1999 NACRE compilation based

M. Aikawa; M. Arnould; S. Goriely; A. Jorissen; K. Takahashi

2005-01-01

182

Review and Evaluation of Rate Data for Gas Phase Reactions of the N-H System.  

National Technical Information Service (NTIS)

While a number of recent reviews of rate data have been undertaken, there is not a current review of data relating to the N-H system. Reaction rate data for gas phase reactions between species containing nitrogen and hydrogen have been compiled and review...

M. C. Branch R. F. Sawyer

1971-01-01

183

Temperature Dependence of the Rate Constant and Product Studies for BrO with CH3O2 using a Turbulent Flow Tube coupled to Chemical Ionisation Mass Spectrometry  

NASA Astrophysics Data System (ADS)

The importance of halogen chemistry in the atmosphere and its contribution to ozone depletion is widely recognised (Farman et al. 1985; Lary and Toumi, 1997). Bromine plays a major role in catalytic cycles leading to the destruction of ozone (Salawitch, 2009; Yang, 2005) not only in the stratosphere (Anderson et al., 1991) but also parts of the troposphere, in particular the marine boundary layer (Saiz-Lopez et al., (2006) as exemplified by the catalytic cycle below. Br + O 3 ? BrO + O2(1) BrO + CH3O2 ? HOBr + CH2O2(2) HOBr + hv ? Br + OH (3) OH + CH4 ? CH3 + H2O (4) CH3 + O2 ? CH3O2(5) Net: CH4 + O3 ? CH2O2 + H2O (6) Satellite measurements of BrO have been taken in northern and southern hemispheres (Simpson et al., 2007 and references therein). Gas-phase kinetic data exist for reaction of XO (BrO, ClO, IO) with O3, NOx and HOxthough there are limited data concerning the reaction of XO with CH3O2. In order to improve modelling of halogen related chemical processes leading to ozone depletion, the determination of reaction rates and product branching ratios is essential. Since the fates of BrO are not yet fully understood, a turbulent flow chemical ionisation mass spectrometer (CIMS) shall be employed to study the kinetics and product yields of the BrO + CH3O2 pathway. Experiments will be conducted for reaction (2) under pseudo first order conditions using very low radical concentrations, typically (0.5 - 20) - 1010 molecule cm-3. Rate determinations will be retrieved as a function of atmospherically relevant temperatures (T = 180 - 300 K).

Leather, K.; Bacak, A.; Shallcross, D.; Percival, C.

2012-04-01

184

Rate constants for reactions in iodine laser active medium:a review  

NASA Astrophysics Data System (ADS)

With the use of kinetic models for a photodissociation iodine laser the calculation of the effective rate constants for chemical reactions in a gaseous active mixture is carried out in the case of a strong influence of molecular collisions. The analysis uses a detailed reaction scheme and Lindemann's collisional mechanism. A list of measured values of rate constants for all reactions occurring in the photodissociation iodine laser active volume for the most important perfluorocarbon radicals is attached.

Skorobogatov, G. A.

1996-02-01

185

Reaction of hydroquinine with hematite II. Calculated electron-transfer rates and comparison to the reductive dissolution rate.  

SciTech Connect

The rate of reaction of hematite with quinones and the quinone moieties of larger molecules may be an important factor in limiting the rate of reductive dissolution of hematite, especially by iron-reducing bacteria. It is possible that the rate of reductive dissolution of hematite in the presence of excess hydroquinone at pH 2.5 may be limited by the electron-transfer rate. Here, a reductive dissolution rate was measured and compared to electron-transfer rates calculated using Marcus theory. An experimental rate constant was measured at 9.5 x 10{sup -6} s{sup -1} and the reaction order with respect to the hematite concentration was found to be 1.1. Both the dissolution rate and the reaction order of hematite concentration compare well with previous measurements. Of the Marcus theory calculations, the inner-sphere part of the reorganization energy and the electronic coupling matrix element for hydroquinone self-exchange electron transfer are calculated using ab initio methods. The second order self-exchange rate constant was calculated to be 1.3 x 10{sup 7} M{sup -1} s{sup -1}, which compares well with experimental measurements. Using previously published data calculated for hexaquairon(III)/(II), the calculated electron-transfer rate for the cross reaction with hydroquinone also compares well to experimental measurements. A hypothetical reductive dissolution rate is calculated using the first-order electron-transfer rate constant and the concentration of total adsorbed quinone. Three different models of the hematite surface are used as well as multiple estimates for the reduction potential, the surface charge, and the adsorption density of hydroquinone. No calculated dissolution rate is less than five orders of magnitude faster than the experimentally measured one.

Stack, Andrew G [ORNL; Rosso, Kevin M [ORNL; Smith, Dayle [Whitman College; Eggleston, Carrick M [ORNL

2004-01-01

186

Effect of coal minerals on reaction rates during coal liquefaction  

Microsoft Academic Search

Four different types of experiments are reported, including catalyst screening, recycle of mineral residue, hydrogenation and hydrodesulfurization of demineralized coal, and hydrogenation and hydrodesulfurization using prehydrogenated solvent. It is found that certain coal minerals, particularly pyrite, catalyze hydrogenation of coal-derived solvents such as creosote oil and solvent refined coal recycle solvent. The rate limiting step in liquefaction of coal is

A. R. Tarrer; J. A. Guin; W. S. Pitts; J. P. Henley; J. W. Prather; G. A. Styles

1977-01-01

187

Kinetic Concepts for Measuring Microbial Rate Constants: Effects of Nutrients on Rate Constants  

PubMed Central

We investigated the effect of preincubation of environmental waters amended with inorganic nutrients (nitrogen, phosphorus, and traces of iron and magnesium) on the kinetics of the microbial transformation of phenol, propanil, propyl ester of (2,4-dichlorophenoxy)acetic acid, methyl parathion, Ronnel, and methoxychlor in pond and river waters. No effect on the second-order rate constants for these compounds was observed, although there was an increase in the bacterial populations and the pseudo-first-order rate constants. The use of nutrient-amended waters could be a useful tool for estimating second-order rate constants for an expanded number of compounds. This technique would provide a larger data base for predicting the behavior of xenobiotic compounds in the environment by using currently available mathematical models.

Paris, D. F.; Rogers, J. E.

1986-01-01

188

Stellar reaction rate of 20Ne(?, ?)24Mg  

NASA Astrophysics Data System (ADS)

The reaction 20Ne(?, ?)24Mg has been investigated at E?(lab) = 0.55-3.20 MeV. Neon gas enriched to 99.95% in 20Ne was recirculated in differentially pumped gas target systems of the extended and quasipoint jet types. New resonances were found at E?(lab) = 958, 1226, 1260, 1704 and 2277 keV, which correspond to known states in 24Mg. Excitation energies, ?-ray decay schemes, ?-ray angular distributions, resonance widths and strengths as well as J? and T-assignments are reported for all the resonances. Information on low-lying states in 24Mg is also obtained. The nuclear and astrophysical aspects of the results are discussed. Travel and other support has been given by the Natural Sciences and Engineering Research Council of Canada.

Schmalbrock, P.; Becker, H. W.; Buchmann, L.; Grres, J.; Kettner, K. U.; Kieser, W. E.; Krwinkel, H.; Rolfs, C.; Trautvetter, H. P.; Hammer, J. W.; Azuma, R. E.

1983-04-01

189

Reaction Rates in Deformation and Hydrostatic Experiments in the Anhydrous System Anorthite - Forsterite  

NASA Astrophysics Data System (ADS)

The reaction anorthite + forsterite --> cpx + opx + spinel gnt proceeds at high temperatures and elevated pressures in the lower crust and upper mantle. This solid-solid reaction was studied experimentally at 900 C in the pressure range of 1000 to 1600 MPa in both shearing deformation and hydrostatic experiments. Powder mixtures (1:1 by vol) of anorthite (An92) and forsterite (Fo93) are hot pressed at 970 C, 750 MPa for 48 hrs in a Griggs apparatus and deformed (? ? = 5 105 sec-1) after adjustment of P and T to run conditions. H2O content of the samples has been measured by FTIR and is < 30 ppm. At small pressure overstepping (ca. 200 to 300 MPa) undeformed samples show only 10 % reaction progress after 168 hrs, whereas reaction progress in deformed samples after 72 hrs is 60 %. At greater pressure overstepping (700 to 800 MPa) the difference between deformed and undeformed samples is less pronounced (95 % after 60 hrs deformed, 75 % after 168 hrs undeformed) but still present. At greater pressure overstepping, undeformed samples show an exponential reaction rate, whereas that of deformed samples is always linear. Samples initially deformed and then kept hydrostatically show a fast initial reaction rate (85 % of total reaction progress after 0.25 of total run time), followed by a slower reaction progress (15 % reaction after 0.75 of total time) under hydrostatic conditions. The difference in reaction progress is mainly attributed to different nucleation rates. In all experiments, enstatite rims form around olivine grains separating those from other reaction products. Such coronas are indicative of diffusion-controlled reactions. Plots of rim thickness vs time indicate a relative increase of the bulk diffusion coefficient by a factor 5 in the deformed samples compared to undeformed. However, as the grain size of reaction products of deformed samples is 10 times smaller than in undeformed ones, the nucleation rate in deformed samples is 5000 times higher. The increased nucleation rate is interpreted to result from higher defect densities and greater driving potential for the reaction in deformed samples. The increased nucleation rate enhances reaction rate and thus rim growth rate, so that a straightforward interpretation of rim growth in terms of diffusion parameters is impossible. Thus, nucleation and length scales of reaction processes in deforming rocks may be very different from those in hydrostatic cases, and for the purpose of extrapolation to nature rates of reaction from hydrostatic experiments provide only minimum values.

Stunitz, H.; de Ronde, A.; Tullis, J.

2004-12-01

190

Reaction rate theory perspectives on some problems in materials science  

NASA Astrophysics Data System (ADS)

Early applications of rate theory to atomic processes in solids were largely concerned with the postirradiation annealing of native defects or the growth of precipitates from a supersaturated solution. However, the materials problems spawned by the demands of fast reactor and fusion reactor technology necessitated generalizations of the existing formalism to account for the possibilities of both time-dependent point defect production and the simultaneous presence of defect sinks of various distinct types. Ensuing developments led to theories of void swelling and irradiation creep. Attention was also redirected to essentially the same problem area in the fields of physical chemistry and statistical physics. Concomitantly, there arose the realization that many of the concepts emerging from these advances in rate theory were readily adaptable to further topics in materials science. A historical synopsis of these developments is presented, and three examples of the latter topics, specifically, the volume fraction dependence of Ostwald ripening, the internal friction effects arising from the association and dissociation of solutedefect complexes in doped materials, and the effect of catalytic particles upon the response of one type of solid-state gas sensor, are discussed at length. The possible further adaptation of rate theory to some new problem areas is also addressed.

Brailsford, A. D.

1989-12-01

191

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

Microsoft Academic Search

The mechanism of the formation of A frames of the type (Pt(PPh triphenylphosphine)(..mu..-Y)(..mu..-dppm))(PF) (y = CH, CO, S, SO) from (Pt(PPh)(..mu..-dppm))(PF) by reaction with CHN, CO, S, and SO (R) in CHCl and other organic solvents has been investigated by using kinetic studies and product analysis. Pseudo-first-order kinetic data (excess (R)) are consistent with either preequilibrium association of R with

S. Muralidharan; James H. Espenson

1984-01-01

192

Rate constants for reactions of alkyl radicals with water and methanol complexes of triethylborane.  

PubMed

Reactions of secondary alkyl radicals with triethylborane and several of its complexes were studied. The H-atom transfer reactions from Et3B-OH2 and Et3B-OD2 were suppressed by addition of pyridine to the reaction mixture. Rate constants for reactions of secondary alkyl radicals with triethylborane and its complexes with water, deuterium oxide, methanol, and THF at ambient temperature were determined by radical clock methods. Cyclization of the 1-undecyl-5-hexenyl radical and ring opening of the 1-cyclobutyldodecyl radical were evaluated as clock reactions. The cyclobutylcarbinyl radical ring opening had the appropriate velocity for relatively precise determinations of the ratios of rate constants for H-atom transfer trapping and rearrangement, and these ratios combined with an estimated rate constant for the cyclobutylcarbinyl radical ring opening gave absolute values for the rate constants for the H-atom transfer reactions. For example, the triethylborane-water complex reacts with a secondary alkyl radical in benzene at 20 degrees C with a rate constant of 2 x 10(4) M(-1) s(-1). Variable temperature studies with the Et3B-CH3OH complex in toluene indicate that the hydrogen atom transfer reaction has unusually high entropic demand, which results in substantially more efficient hydrogen atom transfer trapping reactions in competition with radical ring opening and cyclization reactions at reduced temperatures. PMID:17579450

Jin, Jing; Newcomb, Martin

2007-06-19

193

Chiral Polymerization in Open Systems From Chiral-Selective Reaction Rates  

NASA Astrophysics Data System (ADS)

We investigate the possibility that prebiotic homochirality can be achieved exclusively through chiral-selective reaction rate parameters without any other explicit mechanism for chiral bias. Specifically, we examine an open network of polymerization reactions, where the reaction rates can have chiral-selective values. The reactions are neither autocatalytic nor do they contain explicit enantiomeric cross-inhibition terms. We are thus investigating how rare a set of chiral-selective reaction rates needs to be in order to generate a reasonable amount of chiral bias. We quantify our results adopting a statistical approach: varying both the mean value and the rms dispersion of the relevant reaction rates, we show that moderate to high levels of chiral excess can be achieved with fairly small chiral bias, below 10%. Considering the various unknowns related to prebiotic chemical networks in early Earth and the dependence of reaction rates to environmental properties such as temperature and pressure variations, we argue that homochirality could have been achieved from moderate amounts of chiral selectivity in the reaction rates.

Gleiser, Marcelo; Nelson, Bradley J.; Walker, Sara Imari

2012-08-01

194

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

NASA Astrophysics Data System (ADS)

The production of 26Al in novae is uncertain, in part, because of the uncertain rate of the 25 Al(p,?)26Si reaction at novae temperatures. This reaction is thought to be dominated by a long- sought 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,?)26Si reaction rate in a nova nucleosynthesis calculation and have addressed the effects of the remaining uncertainties in the rate on 26Al production.

Bardayan, Dan; Blackmon, J. C.; Hix, W. R.; Liang, J. F.; Smith, M. S.; Howard, J. A.; Kozub, R. L.; Brune, C. R.; Chae, K. Y.; Lingerfelt, E. J.; Scott, J. P.; Johnson, M. S.; Jones, K. L.; Pain, S. D.; Thomas, J. S.; Livesay, R. J.; Wisser, D. W.

195

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

196

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

197

Improving the rate of the triple alpha reaction  

NASA Astrophysics Data System (ADS)

The rate of the triple alpha process, which plays a central role in the production of 12C in stars, is known with an accuracy of about 12%. Variations within the 12% errors in this rate can cause significant changes in the determination of the mass of the iron core in core-collapse supernovae and the composition of the material later ejected in the interstellar medium, as well as a factor of two change in the surface abundance of 12C in light ABG stars. The triple alpha experiment presented here is a collaborative effort between the Western Michigan University (WMU) and the National Superconducting Cyclotron Laboratory (NSCL) and aims at reducing the uncertainty on the knowledge of this rate to about 6% by measuring more accurately than has been done in the past the pair branch for the 7.654 MeV state in 12C. This state is excited by inelastic proton scattering ( 12C(p, p )12C(7.654MeV ) ), taking advantage of a strong resonance at a bombarding energy of about 10.6 MeV and a scattering angle of 135 degrees in the laboratory. The decay pairs are detected in an almost 4? scintillator system surrounding the target. The protons are produced using the Tandem accelerator at Western Michigan University and detected by silicon detectors at 135 degrees in the lab. The pair branch is given by the ratio of the number of electron-positron pairs detected in the plastic scintillators in coincidence with the protons in the 7.654 MeV silicon spectra to the number of those protons. A reduction in the gamma ray background, mainly due to the cascade gamma decay of the 7.654 MeV state through the 4.44 MeV 2+ state, is achieved by a coincidence requirement between a thin scintillator tube and the large block of plastic scintillator surrounding it. The measurement is expected to take place in the summer of 2006.

Tur, Clarisse; Austin, S. M.; Wuosmaa, A.; Lighthall, J.; Marley, S.; Goodman, N.; Bos, J. J.

198

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

199

Simulation of Transport and Reaction Using Random Walks: Reactions Without Concentrations and the Automatic Simulation of Drastically Different Thermodynamic--- Versus Diffusion---Limited Reaction Rates  

NASA Astrophysics Data System (ADS)

We extend the advantages of Lagrangian random walk particle tracking (RWPT) methods that have long been used to simulate advection and dispersion in highly heterogeneous media. By formulating dissolution as a random, independent decay process, the classical continuum rate law is recovered. A novel formulation of the random precipitation process requires a consideration of the probability that two nearby particles will occupy the same differential volume in a given time period. This depends on local mixing (as by diffusion) and the total domain particle number density, which are fixed and therefore easy to calculate. The result is that the effective reaction rate follows two regimes. First, for high thermodynamic reaction probability and/or fast mixing, the classical continuum rate laws are reproduced. These are coded in the Gillespie method. This implies an exponentially fast approach to equilibrium. Second, for diffusion (mixing) limited reaction rates, equilibrium is approached much more slowly, following a power law that differs for 1-, 2-, or 3-d. At long enough times, the classical law of mass action for equilibrium reactions is reproduced, in an ensemble sense, for either rate regime. The same number of parameters for A+B ? C are needed in a probabilistic versus continuum reaction simulation---one each for forward and backward probabilities that correspond to continuum thermodynamic rates. The random nature of the simulations allows for significant disequilibrium in any given region at any time that is independent of the numerical details such as time stepping or particle density. This is exemplified by nearby or intermingled groups of reactants and little or no product---a result that is often noted in the field that is difficult to reconcile with continuum methods or coarse-grained Eulerian models. Our results support both the recent experiments that show mixing-limited reactions and the results of perturbed advection-dispersion-reaction continuum models. We show that many different kinds of reactions can be easily added to existing RWPT codes.

Benson, D. A.; Meerschaert, M. M.

2008-12-01

200

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

SciTech Connect

Rate constants for the reactions of OH radicals with (CH{sub 3}){sub 4}Pb and (C{sub 2}H{sub 5}){sub 4}Pb 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 the analogous reactions with (CH{sub 3}){sub 4}C and (C{sub 2}H{sub 5}){sub 4}C for comparison purposes. The results are discussed in terms of structure-reactivity relationships.

Nielsen, O.J. (Risoe National Lab., Roskilde (Denmark)); O'Farrell, D.J.; Treacy, J.J. (Dublin Inst. of Tech. (Ireland)); Sidebottom, H.W. (University College Dublin (Ireland))

1991-06-01

201

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

202

One-dimensional reaction coordinates for diffusive activated rate processes in many dimensions.  

PubMed

For multidimensional activated rate processes controlled by diffusive crossing of a saddle point region, we show that a one-dimensional reaction coordinate can be constructed even when the diffusion anisotropy is arbitrary. The rate constant, found using the potential of mean force along this coordinate, is identical to that predicted by the multidimensional Kramers-Langer theory. This reaction coordinate minimizes the one-dimensional rate constant obtained using a trial reaction coordinate and is orthogonal to the stochastic separatrix, the transition state that separates reactants from products. PMID:15638670

Berezhkovskii, Alexander; Szabo, Attila

2005-01-01

203

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

204

Sensitivity study of explosive nucleosynthesis in type Ia supernovae: Modification of individual thermonuclear reaction rates  

NASA Astrophysics Data System (ADS)

Background: Type Ia supernovae contribute significantly to the nucleosynthesis of many Fe-group and intermediate-mass elements. However, the robustness of nucleosynthesis obtained via models of this class of explosions has not been studied in depth until now.Purpose: We explore the sensitivity of the nucleosynthesis resulting from thermonuclear explosions of massive white dwarfs with respect to uncertainties in nuclear reaction rates. We put particular emphasis on indentifying the individual reactions rates that most strongly affect the isotopic products of these supernovae.Method: We have adopted a standard one-dimensional delayed detonation model of the explosion of a Chandrasekhar-mass white dwarf and have postprocessed the thermodynamic trajectories of every mass shell with a nucleosynthetic code to obtain the chemical composition of the ejected matter. We have considered increases (decreases) by a factor of 10 on the rates of 1196 nuclear reactions (simultaneously with their inverse reactions), repeating the nucleosynthesis calculations after modification of each reaction rate pair. We have computed as well hydrodynamic models for different rates of the fusion reactions of 12C and of 16O. From the calculations we have selected the reactions that have the largest impact on the supernova yields, and we have computed again the nucleosynthesis using two or three alternative prescriptions for their rates, taken from the JINA REACLIB database. For the three reactions with the largest sensitivity we have analyzed as well the temperature ranges where a modification of their rates has the strongest effect on nucleosynthesis.Results: The nucleosynthesis resulting from the type Ia supernova models is quite robust with respect to variations of nuclear reaction rates, with the exception of the reaction of fusion of two 12C nuclei. The energy of the explosion changes by less than 4% when the rates of the reactions 12C+12C or 16O+16O are multiplied by a factor of 10 or 0.1. The changes in the nucleosynthesis owing to the modification of the rates of these fusion reactions are also quite modest; for instance, no species with a mass fraction larger than 0.02 experiences a variation of its yield larger than a factor of 2. We provide the sensitivity of the yields of the most abundant species with respect to the rates of the most intense reactions with protons, neutrons, and ?. In general, the yields of Fe-group nuclei are more robust than the yields of intermediate-mass elements. Among the species with yields larger than 10-8M?, 35S has the largest sensitivity to the nuclear reaction rates. It is remarkable that the reactions involving elements with Z>22 have a tiny influence on the supernova nucleosynthesis. Among the charged-particle reactions, the most influential on supernova nucleosynthesis are 30Si+p?31P+?, 20Ne+??24Mg+?, and 24Mg+??27Al+p. The temperatures at which a modification of their rate has a larger impact are in the range 2?T?4 GK.Conclusions: The explosion model (i.e., the assumed conditions and propagation of the flame) chiefly determines the element production of type Ia supernovae and derived quantities such as their luminosity, while the nuclear reaction rates used in the simulations have a small influence on the kinetic energy and final chemical composition of the ejecta. Our results show that the uncertainty in individual thermonuclear reaction rates cannot account for discrepancies of a factor of 2 between isotopic ratios in type Ia supernovae and those in the solar system, especially within the Fe group.

Bravo, Eduardo; Martnez-Pinedo, Gabriel

2012-05-01

205

Flow Rate and Fluence Rate Dependences of the Triplet-State Lifetime of Talaporfin Sodium Photochemical Reaction in Flowing Solution  

NASA Astrophysics Data System (ADS)

The flow rate and fluence rate dependences of the triplet-state lifetime (?T) and singlet oxygen (1O2) luminescence intensity of a Talaporfin sodium (mono-l-aspartyl chlorin e6, Laserphyrin\\textregistered) photochemical reaction in a flowing solution were studied. The oxygen consumption rate constant (k) did not proportionally increase as the fluence rate increased. The relationship between the static oxygen concentration in the flowing solution and ?T was estimated. ?T at the fluence rate of 0.8 104 W/m2 was 16% longer than that at 0.2 104 W/m2 at the same oxygen concentration. The ?T increase at the high-fluence rate is attributed to local and temporal oxygen depletion. The local and temporal oxygen depletion might cause the fluence-rate dependence nonproportionality of the measured k.

Ohmori, Sayaka; Hakomori, Shiho; Arai, Tsunenori

2007-03-01

206

The reaction of exchange rates and interest rates to news releases  

Microsoft Academic Search

This paper examines the response of exchange rates and interest rates--U.S. and foreign--to economic news. The news is associated with the surprise component of the monthly release of six U.S. macroeconomic variables. The results suggest that dollar exchange rates systematically react to news about real economic activity--a surprise of 100,000 on nonfarm payroll employment leads to a 0.2 percent appreciation

Hali J. Edison

1996-01-01

207

The Reaction of Exchange Rates and Interest Rates to News Releases  

Microsoft Academic Search

This paper examines the response of exchange rates and interest rates--US and foreign--to economic news. The news is associated with the surprise component of the monthly release of six US macroeconomic variables. The results suggest that dollar exchange rates systematically react to news about real economic activity--a surprise of 100,000 on non-farm payroll employment leads to a 0.2% appreciation of

Hali J. Edison

1997-01-01

208

Some Nuclear Reaction Rates of Importance for Nucleosynthesis around Mass 45  

Microsoft Academic Search

This thesis describes the measurement of absolute cross sections and the determination of thermonuclear reaction rates for a series of reactions which are of importance in stellar nucleosynthesis. The yield of (gamma)-rays from the reaction ('42)Ca(p,(gamma))('43)Sc has been measured as a function of bombarding energy over the range 0.63 - 3.01 MeV, from ('44)Ca(p,(gamma))('45)Sc over the range 0.775 - 4.00

Leon William Mitchell

1985-01-01

209

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

SciTech Connect

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,{gamma}), (p,{alpha}), and ({alpha},{gamma}) 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. [Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina, 27599-3255, USA and (United States); Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708-0308 (United States)

2008-08-15

210

Determination of the Rate Constants from Phase Delay Effect in Chemical Reactions  

Microsoft Academic Search

A new method for the determination of rate constant k of a reaction of the type A + X ? P1 + P2 is presented. The method is based on the phase delay between the oscillatory inflow of the parent substance X into the reaction medium and the concentration of the parent substance in the field changing as a result

A. Krupska; J. Konarski; R. Fiedorow; J. Adamiec

2002-01-01

211

Effect of mass transfer on the reaction rate in a monolithic catalyst with porous walls  

Microsoft Academic Search

The influence of reacting gas flow on the heterogeneous reaction in catalyst volume of honeycomb porous monolith with triangular channels was studied. The spatial distributions of the reacting gas flow, the rates of local mass transfer between channel walls and gas flow, as well as the interaction of transfer processes and a catalytic reaction were determined on the basis of

O. P. Klenov; S. A. Pokrovskaya; N. A. Chumakova; S. N. Pavlova; V. A. Sadykov; A. S. Noskov

2009-01-01

212

Integrated rate equations for irreversible enzyme-catalysed first-order and second-order reactions.  

PubMed Central

Integrated rate equations are presented that describe irreversible enzyme-catalysed first-order and second-order reactions. The equations are independent of the detailed mechanism of the reaction, requiring only that it be hyperbolic and unbranched. The results should be directly applicable in the laboratory.

Boeker, E A

1985-01-01

213

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 ...

214

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

Microsoft Academic Search

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

Christopher T. Green; John Karl Bhlke; Barbara A. Bekins; Steven P. Phillips

2010-01-01

215

Critical Review of Aqueous Solution Reaction Rate Constants for Hydrogen Atoms  

NASA Astrophysics Data System (ADS)

Kinetic data published in the peer-reviewed literature over the period of 1988-2007 for H. radical reactions with molecules and ions derived from inorganic and organic solutes in aqueous solution have been critically reviewed. Rate constants for over 250 reactions, as studied by pulse radiolysis, end-product analysis, and other methods, have been tabulated.

Madden, Keith P.; Mezyk, Stephen P.

2011-06-01

216

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

217

Reaction rate constant of SO sub 3 + NH sub 3 in the gas phase  

SciTech Connect

The reaction rate constant of SO{sub 3} + NH{sub 3} in the gas phase was measured using a photofragment emission flow tube technique. The reaction rate constant at room temperature is (6.9 {plus minus} 1.5) {times} 10{sup {minus}11} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} at He pressures of 1-2 torr. This measured reaction rate is new information for studying the formation process of ammonium sulfates in the atmosphere. The reaction rate constant of SO{sub 3} + CH{sub 3}NH{sub 2}, which is (1.1 {plus minus} 0.2) {times} 10{sup {minus}11} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}, is also reported.

Shen, Gaunlin; Suto, Masako; Lee, L.C. (San Diego State Univ., CA (USA))

1990-08-20

218

Modified landfill gas generation rate model of first-order kinetics and two-stage reaction  

Microsoft Academic Search

This investigation was carried out to establish a new domestic landfill gas (LFG) generation rate model that takes into account\\u000a the impact of leachate recirculation. The first-order kinetics and two-stage reaction (FKTSR) model of the LFG generation\\u000a rate includes mechanisms of the nutrient balance for biochemical reaction in two main stages. In this study, the FKTSR model\\u000a was modified by

Jiajun Chen; Hao Wang; Na Zhang

2009-01-01

219

A Unified Equation for the Reaction Rate in Dense Matter Stars  

SciTech Connect

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 Sao 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. [Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200 (Australia); Wiescher, M. [Department of Physics and The Joint Institute for Nuclear Astrophysics, University of Notre Dame, Notre Dame, IN 46556 (United States); Yakovlev, D. G. [Ioffe Physico-Technical Institute, Poliekhnicheskaya 26, 194021 Saint-Petersburg (Russian Federation)

2007-10-26

220

Three-body reaction rate in spin-polarized atomic hydrogen  

Microsoft Academic Search

An order-of-magnitude estimate of the three-body reaction of spin-polarized atomic hydrogen is obtained by expressing this rate as a product of the gas-kinetic reaction rate and the probability of a spin-flip transition. The former is estimated from experimental results on atomic hydrogen and the latter by using a Landau-Zener type approach. We find k3 (H?) ~ 5 10-40 cm6

J. M. Yuan; T. K. Lim; L. H. Nosanow

1981-01-01

221

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

222

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

223

Up-Scaling Geochemical Reaction Rates for Carbon Dioxide (CO2) in Deep Saline Aquifers  

SciTech Connect

The overall goal of the project was to bridge the gap between our knowledge of small-scale geochemical reaction rates and reaction rates meaningful for modeling transport at core scales. The working hypothesis was that reaction rates, determined from laboratory measurements based upon reactions typically conducted in well mixed batch reactors using pulverized reactive media may be significantly changed in in situ porous media flow due to rock microstructure heterogeneity. Specifically we hypothesized that, generally, reactive mineral surfaces are not uniformly accessible to reactive fluids due to the random deposition of mineral grains and to the variation in flow rates within a pore network. Expected bulk reaction rates would therefore have to be correctly up-scaled to reflect such heterogeneity. The specific objective was to develop a computational tool that integrates existing measurement capabilities with pore-scale network models of fluid flow and reactive transport. The existing measurement capabilities to be integrated consisted of (a) pore space morphology, (b) rock mineralogy, and (c) geochemical reaction rates. The objective was accomplished by: (1) characterizing sedimentary sandstone rock morphology using X-ray computed microtomography, (2) mapping rock mineralogy using back-scattered electron microscopy (BSE), X-ray dispersive spectroscopy (EDX) and CMT, (3) characterizing pore-accessible reactive mineral surface area, and (4) creating network models to model acidic CO{sub 2} saturated brine injection into the sandstone rock samples.

Lindquist, W Brent

2009-03-03

224

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

225

Recent Augmentations of the Functionality of the Thermonuclear Reaction Rate Calculator (TReRaC)  

NASA Astrophysics Data System (ADS)

The chemical variety of our universe can be explained by stellar nucleosynthesis. Many thermonuclear reactions are studied by reproducing them in accelerator experiments and determining their rates. Using the codes available through the Computational Infrastructure for Nuclear Astrophysics (CINA), researchers can process the results of these experiments. One such program is the Thermonuclear Reaction Rate Calculator (TReRaC), which uses various experimental inputs including resonant energies, strengths, channel widths, and information on non-resonant contributions to calculate reaction rates. Presently, TReRaC is capable of quickly generating accurate rates which closely match those given in a number of publications. This adds to CINA capabilities by enabling a wider variety of nuclear information to generate rates. The next step in TReRaC's evolution is integration into the existing CINA complex so that it can be used by researchers worldwide.

Thomsen, Kyle; Smith, Michael

2011-10-01

226

On the accurate determination of reaction rate constants in batch-type solar photocatalytic oxidation facilities  

SciTech Connect

A rigorous analysis is presented which allows for the accurate determination of reaction rate constants in batch-type photocatalytic oxidation facilities as well as any other batch-type chemical process where the reaction kinetics are known. Experimental verification of the model is accomplished using an indoor facility which utilizes an ultraviolet (UV) lamp source for the destruction of 4-chlorophenol (4CP). It has been observed that the first-order reaction rate constant increases by about 50 percent with an order of magnitude increase in flow rate. Using the present analysis, it is demonstrated that the approximate method of determining rate constants (the product of the apparent rate constant and the ratio of the reactor volume to the total system volume) is useful over a surprisingly wide range of operating conditions.

Klausner, J.F.; Martin, A.R.; Goswami, D.Y.; Schanze, K.S. (Univ. of Florida, Gainesville, FL (United States))

1994-02-01

227

Analysis and modelling of the effective reaction rate in a developing mixing layer  

NASA Astrophysics Data System (ADS)

Simulations of the chemically reacting mixing layer were performed. Especially the early stage of the flow transition, when the coherent Kelvin-Helmholtz vortices can be recognized, was studied. This is a physical problem of great importance in many industrial and environmental systems. Simple passive reaction of the second order is considered. Its effective rate is analysed for different values of the Damkhler number. The results are compared to the case of the steady, laminar shear flow. It is found that slow and fast reactions respond in different ways to the fluctuations of the reactant concentration. Faster reactions appeared to be much more sensitive to the character of mixing. Moreover a buffer layer of product of fast reactions forms in the system and separates the substrates what influence the effective reaction rate.

W?do?owski, Karol; Bajer, Konrad; Kwiatkowski, Kamil

2011-12-01

228

BRUSLIB and NETGEN: the Brussels nuclear reaction rate library and nuclear network generator for astrophysics  

NASA Astrophysics Data System (ADS)

Nuclear reaction rates are quantities of fundamental importance in astrophysics. Substantial efforts have been devoted in the last decades to measuring or calculating them. This paper presents a detailed description of the Brussels nuclear reaction rate library BRUSLIB and of the nuclear network generator NETGEN. BRUSLIB is made of two parts. The first one contains the 1999 NACRE compilation based on experimental data for 86 reactions with (mainly) stable targets up to Si. BRUSLIB provides an electronic link to the published, as well as to a large body of unpublished, NACRE data containing adopted rates, as well as lower and upper limits. The second part of BRUSLIB concerns nuclear reaction rate predictions to complement the experimentally-based rates. An electronic access is provided to tables of rates calculated within a statistical Hauser-Feshbach approximation, which limits the reliability of the rates to reactions producing compound nuclei with a high enough level density. These calculations make use of global and coherent microscopic nuclear models for the quantities entering the rate calculations. The use of such models makes the BRUSLIB rate library unique. A description of the Nuclear Network Generator NETGEN that complements the BRUSLIB package is also presented. NETGEN is a tool to generate nuclear reaction rates for temperature grids specified by the user. The information it provides can be used for a large variety of applications, including Big Bang nucleosynthesis, the energy generation and nucleosynthesis associated with the non-explosive and explosive hydrogen to silicon burning stages, or the synthesis of the heavy nuclides through the s-, ?- and r-, rp- or p-processes.

Aikawa, M.; Arnould, M.; Goriely, S.; Jorissen, A.; Takahashi, K.

2005-10-01

229

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

230

Isotopic exchange in mineral-fluid systems. 4. The crystal chemical controls on oxygen isotope exchange rates in carbonate-H{sub 2}O and layer silicate-H{sub 2}O systems  

SciTech Connect

Oxygen isotope exchange between minerals and water in systems far from chemical equilibrium is controlled largely by surface reactions such as dissolution-precipitation. In many cases, this behavior can be modeled adequately by a simple pseudo-first order rate model that accounts for changes in surface area of the solid. Previous modeling of high temperature isotope exchange data for carbonates, sulfates, and silicates indicated that within a given mineral group there appears to be a systematic relationship between rate and mineral chemistry. The author tested this idea by conducting oxygen isotope exchange experiments in the systems, carbonate-H{sub 2}O and layer silicate-H{sub 2}O at 300 and 350 C, respectively. Witherite (BaCO{sub 3}), strontianite (SrCO{sub 3}) and calcite (CaCO{sub 3}) were reacted with pure H{sub 2}O for different lengths of time (271--1,390 H) at 300 C and 100 bars. The layer silicates, chlorite, biotite and muscovite were reacted with H{sub 2}O for durations ranging from 132 to 3,282 h at 350 C and 250 bars. A detailed survey of grain sizes and grain habits using scanning electron microscopy (SEM) indicated that grain regrowth occurred in all experiments to varying extents. The isotopic rates (ln r) for the carbonate-H{sub 2}O system are {minus}20.75 {+-} 0.44, {minus}18.95 {+-} 0.62 and {minus}18.51 {+-} 0.48 mol O/m{sup 2} s for calcite, strontianite and witherite, respectively. The oxygen isotope exchange rates for layer silicate-H{sub 2}O systems are {minus}23.99 {+-} 0.89, {minus}23.14 {+-} 0.74 and {minus}22.40 {+-} 0.66 mol O/m{sup 2} s for muscovite, biotite and chlorite, respectively.

Cole, D.R.

2000-03-01

231

Reinterpretation of rate data for the reaction of ClF and F(sub 2).  

National Technical Information Service (NTIS)

Experimental data reported by Reiner and Simmons on the rate of recombination of ClF + F(sub 2) have been reinterpreted to yield a single rate equation. This reinterpretation assumes that the recombination reaction is surface area dependent and is first o...

L. D. Trowbridge

1994-01-01

232

The u+ d u+ s reaction rate and bulk viscosity of strange matter  

Microsoft Academic Search

The influence of the strong interactions in strange matter on its nonleptonic reaction rate and bulk viscosity is studied. We find the rate is strongly suppressed by these interactions. The bulk viscosity is also influenced by them. At low temperatures, the viscosity is strongly suppressed; but at high temperatures, it is slightly enhanced. The astrophysical implications of these results are

Zigao Dai; Tan Lu

1996-01-01

233

The $u+d \\\\leftrightarrow u+s$ reaction rate and bulk viscosity of strange matter  

Microsoft Academic Search

The influence of the strong interactions in strange matter on its nonleptonic reaction rate and bulk viscosity is studied. We find the rate is strongly suppressed by these interactions. The bulk viscosity is also influenced by them. At low temperatures, the viscosity is strongly suppressed; but at high temperatures, it is slightly enhanced. The astrophysical implications of these results are

Zigao Dai; Tan Lu

1996-01-01

234

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

235

STARLIB: A Next-Generation Reaction-Rate Library for Nuclear Astrophysics  

NASA Astrophysics Data System (ADS)

One of the major inadequacies of current reaction-rate libraries is the absence of information on uncertainties. Although estimates have been attempted, these uncertainties are generally not based on rigorous statistical definitions. Clearly, a common standard for deriving uncertainties is warranted. STARLIB is a new, next-generation reaction-rate library that addresses this deficiency by providing a tabular, up-to-date database that supplies not only the recommended rate but also its factor uncertainty. The foundation of this library rests on an entirely new method for calculating reaction rates: Monte-Carlo simulation, which utilizes experimental nuclear physics quantities as inputs, yields a probability-density function for the reaction rate at a given temperature [1]. From the cumulative distribution of rate probability densities, the low, median, and high rates are naturally defined. In addition, quantities with upper limits are seamlessly included. This library attempts to bridge the gap between experimental nuclear physics data and stellar modelers by providing a convenient tabular format with reliable uncertainties for use in the simulation of astrophysical phenomena. We expect to submit STARLIB for publication by year's end, which will coincide with the unveiling of a webpage for ease of dissemination and updating.[0pt] [1] C. Iliadis et al., Nucl. Phys., A841, 31 (2010).

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

2011-10-01

236

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

237

Unbound states of (32)Cl andthe (31)S(p,gamma)(32)Cl reaction rate  

SciTech Connect

The {sup 31}S(p,{gamma}){sup 32}Cl 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 {sup 32}S(3He,t){sup 32}Cl charge-exchange reaction to determine properties of proton-unbound levels in {sup 32}Cl that have previously contributed significant uncertainties to the {sup 31}S(p,{gamma}){sup 32}Cl reaction rate. Measured triton magnetic rigidities were used to determine excitation energies in {sup 32}Cl. Proton-branching ratios were obtained by detecting decay protons from unbound {sup 32}Cl states in coincidence with tritons. An improved {sup 31}S(p,{gamma}){sup 32}Cl reaction rate was calculated including robust statistical and systematic uncertainties.

Matos, M. [Louisiana State University; Blackmon, Jeff C [ORNL; Linhardt, Laura [Louisiana State University; Bardayan, Daniel W [ORNL; Nesaraja, Caroline D [ORNL; Clark, Jason [Argonne National Laboratory (ANL); Diebel, C. [Yale University; O'Malley, Patrick [Rutgers University; Parker, P.D. [Yale University

2011-01-01

238

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

239

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

240

Systematic analysis of astrophysical S-factors and thermonuclear reaction rates  

NASA Astrophysics Data System (ADS)

The astrophysical S-factors of the 13C(?,n)16O, 17O(?,n)20Ne, 21Ne(?,n)24Mg and 25Mg(?,n)28Si reactions are analyzed with DWBA. The gross structures of the experimental data are reproduced by the DWBA calculations. The resulting reaction rates are compared with those in the CF88 and NACRE compilations.

Katsuma, M.

2008-05-01

241

RATE OF INFORMATION PROCESSING AND REACTION TIME OF AIRCRAFT PILOTS AND NON-PILOTS  

Microsoft Academic Search

Reaction time and rate of information processing are cited as critical components in the make-up of pilots. A need was identified to establish the validity of various chronometric measures in the selection of pilots. Fifty-eight military and commercial pilots and twenty non-pilots were subjected to Schepers' Computerised Information Processing Test Battery, which measures reaction time, form discrimination time, colour discrimination

WERNER BARKHUIZEN; JOHANN SCHEPERS; JOHAN COETZEE

242

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

243

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

PubMed

The Henry-Michaelis-Menten (HMM) mechanism of enzymatic reaction is studied by means of perturbation theory in the reaction rate constant k (2) of product formation. We present analytical solutions that provide the concentrations of the enzyme (E), the substrate (S), as well as those of the enzyme-substrate complex (C), and the product (P) as functions of time. For k (2) small compared to k (-1), we properly describe the entire enzymatic activity from the beginning of the reaction up to longer times without imposing extra conditions on the initial concentrations E ( o ) andS ( o ), which can be comparable or much different. PMID:22926529

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

2012-08-28

244

Aggregate behavior and lumped kinetics of many reactions in backmixed and plug-flow reactors  

SciTech Connect

The long-time behavior of a reaction mixture containing infinitely many species in isothermal continuous stirred tank reactors (CSTRs) is analyzed and contrasted with that in a plug-flow reactor (PFR). The reaction kinetics considered are irreversible first-order, irreversible coupled pseudo first-order, and reversible first-order. Asymptotic lumped kinetics for the mixture as a whole are developed as a function of feed properties and reactor type. Such lumped kinetics in many cases are of the power law form, with the exponent for a CSTR being lower than that for a PFR. Conditions under which non-power-law kinetics arise are given. Due to the wide spread of reactor residence times, the mixture in CSTRs admits a much richer variety of asymptotic possibilities than in the PFR. The results uncover some behavior differences and relationships between CSTRs and a PFR for converting complex mixtures such as petroleum distillates or coal liquids.

Ho, T.C. [Exxon Research and Engineering Co., Annandale, NJ (United States). Corporate Research Labs.

1996-01-01

245

Impact of strange quark matter nuggets on pycnonuclear reaction rates in the crusts of neutron stars  

NASA Astrophysics Data System (ADS)

This paper presents an investigation into the pycnonuclear reaction rates in dense crustal matter of neutron stars contaminated with strange quark matter nuggets. The presence of such nuggets in the crustal matter of neutron stars would be a natural consequence if Witten's strange quark matter hypothesis is correct. The methodology presented in this paper is a recreation of a recent representation of nuclear force interactions embedded within pycnonuclear reaction processes. The study then extends the methodology to incorporate distinctive theoretical characteristics of strange quark matter nuggets, like their low charge-per-baryon ratio, and then assesses their effects on the pycnonuclear reaction rates. Particular emphasis is put on the impact of color superconductivity on the reaction rates. Depending on whether or not quark nuggets are in this novel state of matter, their electric charge properties vary drastically which turns out to have a dramatic effect on the pycnonuclear reaction rates. Future nuclear fusion network calculations may thus have the potential to shed light on the existence of strange quark matter nuggets and on whether or not they are in a color superconducting state, as suggested by QCD.

Weber, Fridolin; Golf, Barbara; Hellmers, Joe

2009-10-01

246

Impact of strange quark matter nuggets on pycnonuclear reaction rates in the crusts of neutron stars  

NASA Astrophysics Data System (ADS)

This article presents an investigation into the pycnonuclear reaction rates in dense crustal matter of neutron stars contaminated with strange quark matter nuggets. The presence of such nuggets in the crustal matter of neutron stars would be a natural consequence if Witten's strange quark matter hypothesis is correct. The methodology presented in this article is a recreation of a recent representation of nuclear force interactions embedded within pycnonuclear reaction processes. The study then extends the methodology to incorporate distinctive theoretical characteristics of strange quark matter nuggets, like their low charge-per-baryon ratio, and then assesses their effects on the pycnonuclear reaction rates. Particular emphasis is put on the impact of color superconductivity on the reaction rates. Depending on whether quark nuggets are in this novel state of matter, their electric charge properties vary drastically, which turns out to have a dramatic effect on the pycnonuclear reaction rates. Future nuclear fusion network calculations may thus have the potential to shed light on the existence of strange quark matter nuggets and on whether they are in a color superconducting state, as suggested by QCD.

Golf, B.; Hellmers, J.; Weber, F.

2009-07-01

247

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

NASA Astrophysics Data System (ADS)

The triple-alpha (3?) reaction, 4He+4He+4He-->12C+?, which plays a significant role in the stellar evolution scenarios, is studied in terms of a three-alpha (3-?) model. The reaction rate of the process is calculated via an inverse process, 3-? photodisintegration of a 12C nucleus. Both of 3-? 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 12C(02+)-->12C(21+) transition, results of the 3? reaction rate at higher temperature (T > 108 K), where the reaction proceeds mainly through the 8Be and 12C(02+) resonant states, almost agree with those of the Nuclear Astrophysics Compilation of Reaction Rates (NACRE). On the other hand, calculated values of are about 103 times larger than the NACRE rate at a low temperature (T = 107 K), which means our results are remarkably smaller than recent CDCC results.

Ishikawa, Souichi

2012-11-01

248

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

249

Impact of strange quark matter nuggets on pycnonuclear reaction rates in the crusts of neutron stars  

SciTech Connect

This article presents an investigation into the pycnonuclear reaction rates in dense crustal matter of neutron stars contaminated with strange quark matter nuggets. The presence of such nuggets in the crustal matter of neutron stars would be a natural consequence if Witten's strange quark matter hypothesis is correct. The methodology presented in this article is a recreation of a recent representation of nuclear force interactions embedded within pycnonuclear reaction processes. The study then extends the methodology to incorporate distinctive theoretical characteristics of strange quark matter nuggets, like their low charge-per-baryon ratio, and then assesses their effects on the pycnonuclear reaction rates. Particular emphasis is put on the impact of color superconductivity on the reaction rates. Depending on whether quark nuggets are in this novel state of matter, their electric charge properties vary drastically, which turns out to have a dramatic effect on the pycnonuclear reaction rates. Future nuclear fusion network calculations may thus have the potential to shed light on the existence of strange quark matter nuggets and on whether they are in a color superconducting state, as suggested by QCD.

Golf, B.; Hellmers, J.; Weber, F. [Department of Physics, San Diego State University, 5500 Campanile Drive, San Diego, California 92182 (United States)

2009-07-15

250

Simplified method of ``push-pull`` test data analysis for determining in situ reaction rate coefficients  

SciTech Connect

The single-well, ``push-pull`` test method is useful for obtaining information on a wide variety of aquifer physical, chemical, and microbiological characteristics. A push-pull test consists of the pulse-type injection of a prepared test solution into a single monitoring well followed by the extraction of the test solution/ground water mixture from the same well. The test solution contains a conservative tracer and one or more reactants selected to investigate a particular process. During the extraction phase, the concentrations of tracer, reactants, and possible reaction products are measured to obtain breakthrough curves for all solutes. This paper presents a simplified method of data analysis that can be used to estimate a first-order reaction rate coefficient from these breakthrough curves. Rate coefficients are obtained by fitting a regression line to a plot of normalized concentrations versus elapsed time, requiring no knowledge of aquifer porosity, dispersivity, or hydraulic conductivity. A semi-analytical solution to the advective-dispersion equation is derived and used in a sensitivity analysis to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push-pull tests in a homogeneous, confined aquifer with a fully-penetrating injection/extraction well and varying porosity, dispersivity, test duration, and reaction rate. A numerical flow and transport code (SUTRA) is used to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push-pull tests in a heterogeneous, unconfined aquifer with a partially penetrating well. In all cases the simplified method provides accurate estimates of reaction rate coefficients; estimation errors ranged from 0.1 to 8.9% with most errors less than 5%.

Haggerty, R.; Schroth, M.H.; Istok, J.D. [Oregon State Univ., Corvallis, OR (United States)

1998-03-01

251

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

252

Kinetics of the benzyl + O(3P) reaction: a quantum chemical/statistical reaction rate theory study.  

PubMed

The resonance stabilized benzyl radical is an important intermediate in the combustion of aromatic hydrocarbons and in polycyclic aromatic hydrocarbon (PAH) formation in flames. Despite being a free radical, benzyl is relatively stable in thermal, oxidizing environments, and is predominantly removed through bimolecular reactions with open-shell species other than O(2). In this study the reaction of benzyl with ground-state atomic oxygen, O((3)P), is examined using quantum chemistry and statistical reaction rate theory. C(7)H(7)O energy surfaces are generated at the G3SX level, and include several novel pathways. Transition state theory is used to describe elementary reaction kinetics, with canonical variational transition state theory applied for barrierless O atom association with benzyl. Apparent rate constants and branching ratios to different product sets are obtained as a function of temperature and pressure from solving the time-dependent master equation, with RRKM theory for microcanonical k(E). These simulations indicate that the benzyl + O reaction predominantly forms the phenyl radical (C(6)H(5)) plus formaldehyde (HCHO), with lesser quantities of the C(7)H(6)O products benzaldehyde, ortho-quinone methide, and para-quinone methide (+H), along with minor amounts of the formyl radical (HCO) + benzene. Addition of O((3)P) to the methylene site in benzyl produces a highly vibrationally excited C(7)H(7)O* adduct, the benzoxyl radical, which can ?-scission to benzaldehyde + H and phenyl + HCHO. In order to account for the experimental observation of benzene as the major reaction product, a roaming radical mechanism is proposed that converts the nascent products phenyl and HCHO to benzene + HCO. Oxygen atom addition at the ortho and para ring sites in benzyl, which has not been previously considered, is shown to lead to the quinone methides + H; these species are less-stable isomers of benzaldehyde that are proposed as important combustion intermediates, but are yet to be identified experimentally. Franck-Condon simulations of the benzaldehyde, o-quinone methide, and p-quinone methide photoelectron spectra suggest that these C(7)H(6)O isomers could be distinguished using tunable VUV photoionization mass spectrometry. PMID:23108328

da Silva, Gabriel; Bozzelli, Joseph W

2012-12-14

253

Accurate quantum calculations of the reaction rates for H/D+CH4  

NASA Astrophysics Data System (ADS)

In previous work [T. Wu, H. J. Werner, and U. Manthe, Science 306, 2227 (2004)], accurate quantum reaction rate calculations of the rate constant for the H+CH4-->CH3+H2 reaction have been presented. Both the electronic structure calculations and the nuclear dynamics calculations are converged with respect to the basis sets employed. In this paper, the authors apply the same methodology to an isotopic variant of this reaction: D+CH4-->CH3+HD. Accurate rate constants are presented for temperatures between 250 and 400 K. For temperatures between 400 and 800 K, they use a harmonic extrapolation to obtain approximate rate constants for H/D+CH4. The calculations suggest that the experimentally reported rate constants for D+CH4 are about a factor of 10-20 too high. For H+CH4, more accurate experiments are available and agreement is much better: the difference is less than a factor of 2.6. The kinetic isotope effect for the H/D+CH4 reactions is studied and compared with experiment and transition state theory (TST) calculations. Harmonic TST was found to provide a good description of the kinetic isotope effect.

van Harrevelt, Rob; Nyman, Gunnar; Manthe, Uwe

2007-02-01

254

ARTICLE Predicting Rate Constants for Nucleophilic Reactions of Amines with Diarylcarbenium Ions Using an ONIOM Method  

NASA Astrophysics Data System (ADS)

The rate constants of the nucleophilic reactions between amines and benzhydrylium ions were calculated using first-principles theoretical methods. Solvation models including PCM, CPCM, and COSMORS, as well as different types of atomic radii including UA0, UAKS, UAHF, Bondi, and UFF, and several single-point energy calculation methods (B3LYP, B3P86, B3PW91, BHANDH, PBEPBE, BMK, M06, MP2, and ONIOM method) were examined. By comparing the correlation between experimental rate constants and the calculated values, the ONIOM(CCSD(T)/6-311++G(2df,2p):B3LYP/6-311++G(2df,2p))// B3LYP/6-31G(d)/PCM/UFF) method was found to perform the best. This method was then employed to calculate the rate constants of the reactions between diverse amines and diarylcarbenium ions. The calculated rate constants for 65 reactions of amines with diarylcarbenium ions are in agreement with the experimental values, indicating that it is feasible to predict the rate constant of a reaction between an amine and a diarylcarbenium ion through ab initio calculation.

Zhang, Zhi-ping; Wang, Chen; Fu, Yao; Guo, Qing-xiang

2010-12-01

255

Observation of oscillating reaction rates during the isothermal oxidation of ferritic steels  

SciTech Connect

Oscillating reaction rates have been observed in the steam oxidation of 2 1/4Cr-1MoNb and 9Cr-1Mo ferritic steels at 500-550C. Changes in reaction rate are associated with the formation of a laminated, inner-oxide layer, made up of bands of fine and coarse-grain spinel oxide. The lowest reaction rates occur during growth of the fine-grain oxide. Coarse-grain oxide generally contains the same levels of Cr, Mo, and Si as the steel (after allowing for loss of Fe to the outer layer), while the fine-grain material contains three times these levels. Ni builds up in the metal and is present in the oxide as metallic particles (mostly associated with fine-grain oxide). A mechanism is proposed in which the highest reaction rates are controlled by diffusion of Fe ions through the oxide layer (as in normal parabolic oxidation) and the lowest rates by diffusion of Fe through the Ni-rich layer in the metal.

Hurdus, M.H.; Tomlinson, L.; Titchmarsh, J.M. (AEA Technology, Oxfordshire (England))

1990-12-01

256

Studying exothermic reactions in the Ni-Al system at rapid heating rates using a nanocalorimeter  

NASA Astrophysics Data System (ADS)

Heats of reaction and heat capacity changes were measured using scanning nanocalorimetry for a nickel and aluminum bilayer where initial heating rates of 104 K/s were achieved. Multiple exotherms were observed on the initial heating, but the number of intermediate exotherms decreased with increasing heating rate. The final phase was the B2 NiAl intermetallic. Results from the nanocalorimeter were compared with a conventional differential scanning calorimeter (operating at 0.7 K/s) to understand the effect of significant (10 000) increases in heating rate on the phase transformation sequence. The high heating rate in the nanocalorimeter delays reaction initiation, causes the exothermic peaks to shift to higher temperatures, and appears to suppress the formation of intermediate, metastable phases. Potential explanations for this apparent suppression are discussed.

Swaminathan, P.; Grapes, M. D.; Woll, K.; Barron, S. C.; LaVan, D. A.; Weihs, T. P.

2013-04-01

257

Effects of a New Triple-? Reaction Rate on the Helium Ignition of Accreting White Dwarfs  

NASA Astrophysics Data System (ADS)

The effects of a new triple-? reaction rate on the ignition of carbon-oxygen white dwarfs accreting helium in a binary system have been investigated. The ignition points determine the properties of a thermonuclear explosion of a Type Ia supernova. We examine the cases of different accretion rates of helium and different initial masses of the white dwarf, which was studied in detail by Nomoto [K. Nomoto, Astrophys. J. 253 (1982), 798]. We find that for all cases from slow to intermediate accretion rates, nuclear burnings are ignited at the helium layers. As a consequence, carbon deflagration would be triggered for lower accretion rate than dM/dt ? 4 10^{-8} M_{?} yr^{-1} which has been believed to be the lower limit of the accretion rate for the deflagration supernova. Furthermore, off-center helium detonation should result for intermediate and slow accretion rates, and the region of carbon deflagration for slow accretion rates disappears.

Saruwatari, M.; Hashimoto, M.

2010-11-01

258

Impacts of a Newly Calculated 3? Reaction Rate on the Helium Accreting White Dwarfs  

NASA Astrophysics Data System (ADS)

Effects of a new triple-? reaction rate on the ignition of carbon-oxygen white dwarfs accreting helium in a binary systems have been investigated. The ignition points determine the properties of a thermonuclear explosion of a Type la supernova. We examine the cases of different accretion rates of helium and different initial masses of the white dwarf, which was studied in detail by Nomoto[4]. We find that for all cases from slow to intermediate accretion rates, nuclear burnings are ignited at the helium layers. As a consequence, carbon deflagration would be triggered for the lower accretion rate compared to that of dM/dt~=41O-8 Msolar yr-1 which has been believed to the lower limit of the accretion rate for the deflagration supernova. Furthermore, off-center helium detonation should result for intermediate and slow accretion rates and the region of carbon deflagration for slow accretion rate is disappeared.

Saruwatari, Motoaki; Hashimoto, Masa-Aki

2010-08-01

259

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 acidbase 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

260

Suppression of the stellar enhancement factor and reaction rates far from stability  

NASA Astrophysics Data System (ADS)

Nuclei in astrophysical plasmas occur in excited states because they are in thermal equilibrium with the stellar plasma. This modifies the reaction cross sections and has important consequences for the determination of stellar reaction rates. The application of detailed balance implies that stellar effects are less pronounced in the direction of positive Q-value and that measurements should preferably be performed in this reaction direction. However, we show that the general Q-value rule does not apply for a number of cases due to the suppression of low-energy transitions in the exit channel by an additional barrier (Coulomb or centrifugal). Additionally, it has to be realized that the validity of detailed balance cannot be taken for granted in all cases. This is well known for nuclei at stability having isomeric states but may also become problematic in nuclei with low level densities. Another complication with low level densities is that the Hauser-Feshbach model cannot be applied anymore to predict reaction cross sections. Resonant and direct reactions also become important. Preliminary results of a new large-scale prediction of astrophysical reaction rates across the nuclear chart including both the compound and the direct mechanism will be shown.

Rauscher, Thomas

2009-10-01

261

Level structure of 21Mg and the 20Na(p, ?)21Mg stellar reaction rate  

NASA Astrophysics Data System (ADS)

The nuclear level structure of 21Mg has been studied by the 24Mg(3He, 6He)21Mg reaction at 74 MeV. Angular distributions of the three-nucleon transfer reaction (3He, 6He) are measured for the first time, and successfully analyzed with exact finite-range distorted-wave-Born-approximation calculations, giving clear transferred angular momentum assignments. More than 20 states have been identified with excitation energy and spin-parity determinations, including a possible s-wave resonance just above the proton threshold. One of the s-wave resonances assumed in the previous stellar reaction rate estimates is found to be a bound state. The stellar reaction rate of the 20Na(p, ?)21Mg process is estimated using the experimental data. The results predict the ignition of the proton radiative-capture process at T = 1108K under typical nova conditions (hydrogen density, XH? = 5105 g/cm3). This temperature happens to be in agreement with the previous theoretical estimates, but the new reaction rate is a few orders of magnitude smaller than the previous predictions for T ? 2-10108K. The results also suggest that the nucleosynthesis flow of the rapid-proton process will run up to 21Mg immediately after breakout from the hot-CNO cycle.

Kubono, S.; Funatsu, Y.; Ikeda, N.; Yasue, M.; Nomura, T.; Fuchi, Y.; Kawashima, H.; Kato, S.; Miyatake, H.; Orihara, H.; Kajino, T.

1992-02-01

262

The Impact of Nuclear Reaction Rate Uncertainties on Evolutionary Studies of the Nova Outburst  

NASA Astrophysics Data System (ADS)

Viewed originally as the sudden appearance of a new star, nova stella, novae have had a long history of studies with a variety of techniques. Observational studies have revealed an elemental composition that differs markedly from solar. Theoretical studies show that the differences are caused by the combination of convection with explosive hydrogen burning which results in a unique nucleosynthesis that is rich in odd numbered nuclei such as (15) N, (17) O and (13) C. These nuclei are difficult to form in other astrophysical events. The observable consequences of a nova outburst depend sensitively on the details of the thermonuclear runaway which initiates the outburst. One of the more important sources of uncertainty is the nuclear reaction data used as input for the evolutionary calculations. A recent paper by Starrfield, Truran, Wiescher & Sparks (1998) has demonstrated that changes in the reaction rate library used within a nova simulation have significant effects, not just on the production of individual isotopes (which can change by an order of magnitude), but on global observables such as the peak luminosity and the amount of mass ejected. We will present preliminary results of systematic analyses of the impact of reaction rate uncertainties on nova nucleosynthesis. While the primary goal of our studies is to discern the extent to which theory is discrepant from observations, we will also determine the relative importance of changes in individual reaction rates and provide guidance in the selection of reactions for further experimental study.

Hix, W. R.; Smith, M. S.; Mezzacappa, A.; Starrfield, S.

1998-12-01

263

Implications of a new triple-? nuclear reaction rate. Consequences for Cepheids  

NASA Astrophysics Data System (ADS)

Context. Recently the triple-? reaction rate has been re-evaluated. In the temperature range, 107-108 K, with respect to the NACRE rate, the new rate is enhanced by up to 20 orders-magnitude. Aims: To validate this new rate, we investigate its consequences for the evolution of Cepheid models. Methods: The stellar evolutionary tracks are calculated with the CESAM code and displayed in the domain 4-10 M_?. Results: With the new rate, the first dredge-up does not occur. For masses larger than gse 4.5 M_? each evolutionary track crosses the instability strip only once. The luminosities are higher than with the previous rate, then leading to smaller theoretical masses that better agree with the pulsational mass. Conversely, and inconsistently with one century of observations of more than two hundred Cepheids, the temporal derivative of the period keeps a positive sign. Moreover the observed depletions of atmospheric lithium and C/N ratio do not occur. A slight modification of only a few percents of the new nuclear rate allows us however to restore the loops inside the instability strip and the changes of sign of the temporal derivative of periods. Conclusions: This preliminary work indicates that the new rate may solve some of the long-lasting unsolved theoretical problems of Cepheids. Yet indisputable observations argue against its pertinence. Nonetheless, with regard to its theoretical importance, the triple-? new reaction rate still needs to be confirmed or revisited.

Morel, P.; Provost, J.; Pichon, B.; Lebreton, Y.; Thvenin, F.

2010-09-01

264

Role of heavy-ion nuclear reactions in determining on-orbit single event error rates  

Microsoft Academic Search

Simulations show that neglecting ion-ion interaction processes (both particles having Z>1) results in an underestimation of the total on-orbit single event upset error rate by more than two orders of magnitude for certain technologies. The inclusion of ion-ion nuclear reactions leads to dramatically different SEU error rates for CMOS devices containing high Z materials compared with direct ionization by the

Christina L. Howe; Robert A. Weller; Robert A. Reed; Marcus H. Mendenhall; Ronald D. Schrimpf; Kevin M. Warren; Dennis R. Ball; Lloyd W. Massengill; Kenneth A. LaBel; Nadim F. Haddad

2005-01-01

265

Reaction and Evaporation Rates of High Temperature Candidate Fluids from a TFOST Test  

Microsoft Academic Search

Five high temperature fluids have been evaluated in a thin-film oxidation sealed tube (TFOST) test at 300C to assess their capability for high temperature application. Degradation maps were constructed for each individual lubricant based on the amount of fresh lubricant remaining. Liquid-phase and gas-phase reaction rates and evaporation rate were then calculated and plotted against test time to illustrate their

Kenneth K. Chao; Costandy S. Saba

1997-01-01

266

Influence of a New 17-F(p, gamma)18Ne Reaction Rate on Nova Nucleosynthesis  

Microsoft Academic Search

Proton capture by 17F plays an important role in nova explosions. A revised rate for this reaction has been determined based on measurements of the 1H (17F,p)17F excitation function using a radioactive 17F beam at ORNL's Holifield Radioactive Ion Beam Facility. We have used this new rate in calculations of the nucleosynthesis resulting from nova outbursts on the surfaces of

S. Parete-Koon; W. Hix; M. Smith; S. Starrfield; D. Bardayan; M. Guidry; A. Mezzacappa

2002-01-01

267

Effects of Relative Humidity on Ozone-Initiated Oxidation Reaction Rate of Gaseous Elemental Mercury  

Microsoft Academic Search

Ozone is assumed to be the predominant tropospheric oxidant of gaseous elemental mercury [Hg0(g)], defining mercury global atmospheric lifetime. In this study, for the first time, we have examined the effect of relative humidity (RH) has on the apparent O3-initiated oxidation of rate of Hg0(g). Kinetics of these reactions was studied by absolute rate technique using gas chromatography coupled to

G. A. Snider; F. Raofie; P. Ariya

2006-01-01

268

Survey of Two-Body and Three-Body Reaction-Rate Coefficients for the Ionized Stratosphere and Mesosphere.  

National Technical Information Service (NTIS)

Rate coefficients for two-body and three-body reactions applicable to the ionized stratosphere and mesosphere have been surveyed. The reactions are divided into five classes, viz., (1) positive-ion reactions, (2) electron reactions, (3) negative-ion react...

F. E. Niles

1974-01-01

269

Relationship of the geometry of the observed steady state chemical conversion rate to the basic surface reaction process in electrochemistry  

Microsoft Academic Search

The steady state surface reaction rate for electrochemical reactions is frequently measured by the polarization curve. In the anodic region of this curve the dynamics of the reaction process are generally described by a semiflow map. The fixed points of this map describe the steady state dynamics of the surface reactions. These generic singularities are classified according to their bifurcation

D. G. Retzloff; B. DeFacio; J. E. Bauman; P. H. Ragatz

1979-01-01

270

Relation between State-Selected or State-Averaged Cross Sections of Endothermic Reactions and Rate Constants of Exothermic Reactions. Application of Bimolecular Microcanonical Activated Complex Theory  

Microsoft Academic Search

An expression is derived relating state-selected or state-averaged molecular beam reaction cross sections of endothermic reactions to observed rate constants for the inverse (exothermic) reactions. An approximation of Anlauf, Maylotte, Polanyi, and Bernstein is used, together with bimolecular microcanonical activated complex theory [R. A. Marcus, J. Chem. Phys. 45, 2138 (1966)].

R. A. Marcus

1970-01-01

271

Rate-promoting vibrations and coupled hydrogen-electron transfer reactions in the condensed phase: A model for enzymatic catalysis  

Microsoft Academic Search

A model is presented for coupled hydrogen-electron transfer reactions in condensed phase in the presence of a rate promoting vibration. Large kinetic isotope effects (KIEs) are found when the hydrogen is substituted with deuterium. While these KIEs are essentially temperature independent, reaction rates do exhibit temperature dependence. These findings agree with recent experimental data for various enzyme-catalyzed reactions, such as

Joshua S. Mincer; Steven D. Schwartz

2004-01-01

272

Rate-promoting vibrations and coupled hydrogenelectron transfer reactions in the condensed phase: A model for enzymatic catalysis  

Microsoft Academic Search

A model is presented for coupled hydrogenelectron transfer reactions in condensed phase in the presence of a rate promoting vibration. Large kinetic isotope effects (KIEs) are found when the hydrogen is substituted with deuterium. While these KIEs are essentially temperature independent, reaction rates do exhibit temperature dependence. These findings agree with recent experimental data for various enzyme-catalyzed reactions, such as

Joshua S. Mincer; Steven D. Schwartz

2004-01-01

273

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

2012-10-10

274

A kinetic-theory approach for computing chemical-reaction rates in upper-atmosphere hypersonic flows  

Microsoft Academic Search

Recently proposed 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 investigated for chemical reactions occurring in upper-atmosphere hypersonic flows. The new models are in good agreement with the measured Arrhenius rates for near-equilibrium conditions and with both measured rates and other theoretical models for

Michael A. Gallis; Ryan B. Bond; John R. Torczynski

2009-01-01

275

Integral reaction rate measurements in /sup 252/Cf and /sup 235/U fission spectra. Final report  

SciTech Connect

In support of the light water reactor pressure vessel (LWR-PV) surveillance dosimetry program established by the U.S. Nuclear Regulatory Commission, the National Bureau of Standards is undertaking a series of measurements to provide a physical basis for neutron dosimetry standards. Reaction rate measurements have been made with both 252 Cf and 235U fission neutron fields. The following reactions have been measured through an activation technique: 115In(n,n') 58Ni(n,p) and 54Fe(n,p). The neutron emission rate of the 252Cf source has been measured with the MnSO4 bath technique, thus permitting a direct measure of the spectrum averaged integral cross section for Cf. The source strength of the 235U fission neutron field has been measured relative to the 252Cf neutron field using the 115In(n,n') reaction. All measurement of cross sections in the 235U fission field are therefore relative measurements.

Lamaze, G.P.; McGarry, E.D.; Schima, F.

1983-01-01

276

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

277

Ozonation of naphthalene sulfonic acids in aqueous solutions: Part IIRelationships of their COD, TOC removal and the frontier orbital energies  

Microsoft Academic Search

This paper evaluated the COD, TOC removal and molecular frontier energies in the ozonation of naphthalene sulfonic acids (NSAs). It was found that both COD and TOC degradation of the 11 compounds followed the pseudo-first-order kinetic and the reaction rate constants with ozone varied widely. A linear relationship between logarithm of global COD degradation rate constant and the energy of

Zhu Shiyun; Zheng Xuesong; Li Daotang; Cai Weimin

2003-01-01

278

RATE CONSTANTS FOR THE REACTIONS OF THE HYDROXYL RADICAL WITH SEVERAL PARTIALLY FLUORINATED ETHERS  

EPA Science Inventory

The paper gives results of experimental measurements on the rate constants for the reactions of the hydroxyl radical, OH, with a number of fluorinated ethers. hese ethers, and their atmospheric lifetimes (in years), estimated relative to CH3CC13, are: CF3 - O -CH3. 3.3; CF3 - O -...

279

Nuclear reaction rate uncertainties and astrophysical modeling: Carbon yields from low-mass giants  

SciTech Connect

Calculations that demonstrate the influence of three key nuclear reaction rates on the evolution of asymptotic giant branch stars have been carried out. We study the case of a star with an initial mass of 2 M{sub {center_dot}} and a metallicity of Z=0.01, somewhat less than the solar metallicity. The dredge-up of nuclear processed material from the interior of the star and the yield predictions for carbon are sensitive to the rate of the {sup 14}N(p,{gamma}){sup 15}O and triple-{alpha} reactions. These reactions dominate the H- and He-burning shells of stars in this late evolutionary phase. Published uncertainty estimates for each of these two rates propagated through stellar evolution calculations cause uncertainties in carbon enrichment and yield predictions of about a factor of 2. The other important He-burning reaction, {sup 12}C({alpha},{gamma}){sup 16}O, although associated with the largest uncertainty in our study, does not have a significant influence on the abundance evolution compared with other modeling uncertainties. This finding remains valid when the entire evolution from the main sequence to the tip of the asymptotic giant branch is considered. We discuss the experimental sources of the rate uncertainties addressed here and give some outlooks for future work.

Herwig, Falk [Los Alamos National Laboratory, Theoretical Astrophysics Group in T-Division, MS B227, Los Alamos, New Mexico 87545 (United States); Austin, Sam M. [National Superconducting Cyclotron Laboratory and Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, Michigan 48824 (United States); Lattanzio, John C. [School of Mathematical Sciences, Monash University, Wellington Road, Clayton, Vic 3800 (Australia)

2006-02-15

280

Rate acceleration of S N2 reactions through selective solvation of the transition state  

NASA Astrophysics Data System (ADS)

High level ab initio calculations show that the S N2 reaction of the cyanide ion with ethyl chloride is catalysed by 1,4-benzenedimethanol in dipolar aprotic solvents through selective two hydrogen bonds. In apolar solvents, combined with phase transfer catalysis, the 1,4-benzenedimethanol could replace some water molecules hydrating the cyanide ion and induce a substantial rate acceleration effect.

Almerindo, Gizelle I.; Pliego, Josefredo R.

2006-06-01

281

Investigate the Effect of Internal Heat Losses on Aluminum Dust Cloud Combustion with Continues Reaction rate  

Microsoft Academic Search

In this paper, a m athematical model for i nvestigation the effects of internal heat losses on the flame speed and temperature profile in Aluminum Particle Clouds combustion with continues reaction rate have been studied. The p resent work ext ended previous re sults by bridging t he t heories of the no n- adiabatic stationary d ust fl ame

M. Bidabadi; A. Jebreyli

2009-01-01

282

Reconsideration of the rate constant for the reaction of hydroxyl radicals with nitric acid  

Microsoft Academic Search

The authors report rate coefficients for the reaction of OH with HNO, k, between 10 and 500 Torr of He, SF, N, and O and at 10 different temperatures between 200 and 375 K. They generated OH via pulsed photolysis of HNO and monitored the [OH] temporal profile via pulsed laser induced fluorescence. Below 300 K the value of k

Steven S. Brown; Ranajit K. Talukdar; A. R. Ravishankara

1999-01-01

283

Isotope Exchange Rates. Iv. The Homogeneous Reaction Between Methane and Deuterium.  

National Technical Information Service (NTIS)

The rate of exchange of deuterium for hydrogen in the homogeneous gas-phase reaction between D2 and CH4 (highly diluted in argon) was measured behind reflected shocks in a single-pulse shock tube of the 'magic hole' design. The studies covered the tempera...

W. S. Watt P. Borrell D. Lewis S. H. Bauer

1965-01-01

284

Impact of strange quark matter nuggets on pycnonuclear reaction rates in the crusts of neutron stars  

Microsoft Academic Search

This article presents an investigation into the pycnonuclear reaction rates in dense crustal matter of neutron stars contaminated with strange quark matter nuggets. The presence of such nuggets in the crustal matter of neutron stars would be a natural consequence if Witten's strange quark matter hypothesis is correct. The methodology presented in this article is a recreation of a recent

B. Golf; J. Hellmers; F. Weber

2009-01-01

285

Impact of strange quark matter nuggets on pycnonuclear reaction rates in the crusts of neutron stars  

Microsoft Academic Search

This paper presents an investigation into the pycnonuclear reaction rates in dense crustal matter of neutron stars contaminated with strange quark matter nuggets. The presence of such nuggets in the crustal matter of neutron stars would be a natural consequence if Witten's strange quark matter hypothesis is correct. The methodology presented in this paper is a recreation of a recent

Fridolin Weber; Barbara Golf; Joe Hellmers

2009-01-01

286

Approximation for the enhancement factor applicable to reversible reactions of finite rate in chemically loaded solutions  

Microsoft Academic Search

A new explicit relation is proposed for the prediction of the enhancement factor for reversible reactions of finite rate in chemically loaded solutions which also allows for unequal diffusivities. The relation for the enhancement factor is not based on an approximation of the absorption process, but is derived from a similarity which can be observed between the results of the

J. A. Hogendoorn; R. D. Vas Bhat; J. A. M. Kuipers; W. P. M. van Swaaij; G. F. Versteeg

1997-01-01

287

Laminar Flamelet Modelling of the Mean Reaction Rate in a Premixed Turbulent Flame,  

National Technical Information Service (NTIS)

The modelling of the mean reaction rate in a premixed turbulent flame is approached by means of a library of laminar flame solutions. A formulation of such a library is presented and the detailed data requirements are investigated. Laminar flame calculati...

R. S. Cant B. Rogg K. N. C. Bray

1989-01-01

288

On Laminar Flamelet Modelling of the Mean Reaction Rate in a Premixed Turbulent Flame  

Microsoft Academic Search

The modelling of the mean reaction rate in a premixed turbulent flame is approached by means of a library of laminar flame solutions. A formulation of such a library is presented and the detailed data requirements are investigated. Laminar flame calculations are performed in an asymmetrical counterflow configuration with a cold, reactive methane-air mixture flowing against the corresponding hot equilibrium

R. S. CANT; B. ROGG; K. N. C. BRAY

1990-01-01

289

Determination of the reaction and adsorption rates of a catalyst in a transient state  

Microsoft Academic Search

A gas mixture is passed through a bed of granular catalyst. The component of low concentration is lost by chemisorption and parallel catalytic reactions, the former reducing the activity of the catalyst. We consider these processes when they have first-order kinetic equations and the adsorption occurs on a homogeneous surface. This gives analytic expressions for the rate constant of each

V. M. Belousov; A. M. Aizen; V. A. Vozovik

1972-01-01

290

Probing the Rate-Determining Step of the Claisen-Schmidt Condensation by Competition Reactions  

ERIC Educational Resources Information Center

Competition experiments are a useful tool for preliminary study of the linear free energy relationship of organic reactions. This article describes a physical organic experiment for upper-level undergraduates to identify the rate-determining step of the Claisen-Schmidt condensation of benzaldehyde and acetophenone by studying the linear free

Mak, Kendrew K. W.; Chan, Wing-Fat; Lung, Ka-Ying; Lam, Wai-Yee; Ng, Weng-Cheong; Lee, Siu-Fung

2007-01-01

291

Relative Reaction Rates of Sulfamic Acid and Hydroxylamine with Nitric Acid  

SciTech Connect

This report describes a study of comparative reaction rates where the reductant is in excess, as in the 1B bank in the Purex process. The results of this work apply to planned plant tests to partially substitute HAN for the ferrous sulfamate reductant in the Purex 1B bank.

Karraker, D.G.

2001-03-28

292

Probing the Rate-Determining Step of the Claisen-Schmidt Condensation by Competition Reactions  

ERIC Educational Resources Information Center

|Competition experiments are a useful tool for preliminary study of the linear free energy relationship of organic reactions. This article describes a physical organic experiment for upper-level undergraduates to identify the rate-determining step of the Claisen-Schmidt condensation of benzaldehyde and acetophenone by studying the linear free

Mak, Kendrew K. W.; Chan, Wing-Fat; Lung, Ka-Ying; Lam, Wai-Yee; Ng, Weng-Cheong; Lee, Siu-Fung

2007-01-01

293

Reaction rate of beryllium with fluorine ion for Flibe redox control  

NASA Astrophysics Data System (ADS)

An experimental effort to apply Flibe (a mixed molten salt of 2LiF + BeF2) to a self-cooled liquid blanket of a fusion reactor was carried out under a Japan US collaboration called JUPITER-II. Maintaining Flibe under a reducing atmosphere is a key issue to transform TF to T2 with a faster reaction rate compared with the residence time in blanket. One of the tasks was to clarify whether or not the redox control of Flibe can be achieved with Be. The dissolution rate of a Be rod and the reaction rate of Be + 2HF = BeF2 + H2 in Flibe were experimentally determined. Sufficiently fast rates of the Be dissolution and the reduction reaction of HF to H2 were clarified by our redox control experiment. Close agreement was obtained between experiments and our simplified complete-mixing model. In particular, the reaction between Be and a F- ion immediately after their contact was found to be limited by diffusion of the F- ion.

Fukada, S.; Simpson, M. F.; Anderl, R. A.; Sharpe, J. P.; Katayama, K.; Smolik, G. R.; Oya, Y.; Terai, T.; Okuno, K.; Hara, M.; Petti, D. A.; Tanaka, S.; Sze, D.-K.; Sagara, A.

2007-08-01

294

The Temperature Dependence of the Rate Constant for the Reaction of Hydroxyl Radicals With Nitric Acid  

Microsoft Academic Search

The flash photolysis resonance fluorescence technique has been used to measure the rate constant for the reaction OH + HONO2 - products over the temperature range 225-443 K. The data below room temperature can be fit to the expression k = (1.05 _+ 0.40) x 10 TM exp [(759 -+ 100)\\/T] cm 3 molec - s -. This equation, however,

Michael J. Kurylo; Kenneth D. Cornett; Jennifer L. Murphy

1982-01-01

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

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

297

Correcting reaction rates measured by saturation-transfer magnetic resonance spectroscopy  

NASA Astrophysics Data System (ADS)

Off-resonance or spillover irradiation and incomplete saturation can introduce significant errors in the estimates of chemical rate constants measured by saturation-transfer magnetic resonance spectroscopy (MRS). Existing methods of correction are effective only over a limited parameter range. Here, a general approach of numerically solving the Bloch McConnell equations to calculate exchange rates, relaxation times and concentrations for the saturation-transfer experiment is investigated, but found to require more measurements and higher signal-to-noise ratios than in vivo studies can practically afford. As an alternative, correction formulae for the reaction rate are provided which account for the expected parameter ranges and limited measurements available in vivo. The correction term is a quadratic function of experimental measurements. In computer simulations, the new formulae showed negligible bias and reduced the maximum error in the rate constants by about 3-fold compared to traditional formulae, and the error scatter by about 4-fold, over a wide range of parameters for conventional saturation transfer employing progressive saturation, and for the four-angle saturation-transfer method applied to the creatine kinase (CK) reaction in the human heart at 1.5 T. In normal in vivo spectra affected by spillover, the correction increases the mean calculated forward CK reaction rate by 6 16% over traditional and prior correction formulae.

Gabr, Refaat E.; Weiss, Robert G.; Bottomley, Paul A.

2008-04-01

298

Equilibrium density fluctuations and the rate of a Langmuir-Hinshelwood type reaction on microcrystal particles  

NASA Astrophysics Data System (ADS)

We investigate the effect of restricting the area of planes of microcrystals and equilibrium density fluctuations in components of binary mixtures on partial isotherms of the adsorption of binary mixtures of molecules and the rate of a surface reaction of the Langmuir-Hinshelwood type. Adsorption of components of mixture is considered in a large canonical assembly, and the rate of an elementary step is calculated in kinetic regime. The value of a section of the surface on a plane contains a number of adsorption centers in the range of 10 to 105. The effect of the structure of a heterogeneous surface on the rate of the considered reaction is studied. The effect of the density fluctuations of adsorbed molecules on partial adsorption isotherms and fluctuations in the rate of reaction on heterogeneous surfaces is discussed. It is shown that the greatest effect of density fluctuations on the rate of a step is observed at low fillings of each plane of a particle and at the almost complete filling of a plane.

Tovbin, Yu. K.; Titov, S. V.

2013-01-01

299

A New 17F(p, ?)18Ne Reaction Rate and Its Implications for Nova Nucleosynthesis  

NASA Astrophysics Data System (ADS)

Proton capture by 17F plays an important role in the synthesis of nuclei in nova explosions. A revised rate for this reaction, based on a measurement of the 1H(17F, p)17F excitation function using a radioactive 17F beam at Oak Ridge National Laboratory's Holifield Radioactive Ion Beam Facility, is used to calculate the nucleosynthesis in nova outbursts on the surfaces of 1.25 and 1.35 Msolar ONeMg white dwarfs and a 1.00 Msolar CO white dwarf. We find that the new 17F (p, ?)18Ne reaction rate changes the abundances of some nuclides (e.g., 17O) synthesized in the hottest zones of an explosion on a 1.35 Msolar white dwarf by more than a factor of 104 compared to calculations using some previous estimates for this reaction rate, and by more than a factor of 3 when the entire exploding envelope is considered. In a 1.25 Msolar white dwarf nova explosion, this new rate changes the abundances of some nuclides synthesized in the hottest zones by more than a factor of 600, and by more than a factor of 2 when the entire exploding envelope is considered. Calculations for the 1.00 Msolar white dwarf nova show that this new rate changes the abundance of 18Ne by 21% but has negligible effect on all other nuclides. Comparison of model predictions with observations is also discussed.

Parete-Koon, S.; Hix, W. R.; Smith, M. S.; Starrfield, S.; Bardayan, D. W.; Guidry, M. W.; Mezzacappa, A.

2003-12-01

300

A Randomised Controlled Trial of Two Infusion Rates to Decrease Reactions to Antivenom  

PubMed Central

Background Snake envenoming is a major clinical problem in Sri Lanka, with an estimated 40,000 bites annually. Antivenom is only available from India and there is a high rate of systemic hypersensitivity reactions. This study aimed to investigate whether the rate of infusion of antivenom reduced the frequency of severe systemic hypersensitivity reactions. Methods and Findings This was a randomized comparison trial of two infusion rates of antivenom for treatment of non-pregnant adult patients (>14 y) with snake envenoming in Sri Lanka. Snake identification was by patient or hospital examination of dead snakes when available and confirmed by enzyme-immunoassay for Russells viper envenoming. Patients were blindly allocated in a 11 randomisation schedule to receive antivenom either as a 20 minute infusion (rapid) or a two hour infusion (slow). The primary outcome was the proportion with severe systemic hypersensitivity reactions (grade 3 by Brown grading system) within 4 hours of commencement of antivenom. Secondary outcomes included the proportion with mild/moderate hypersensitivity reactions and repeat antivenom doses. Of 1004 patients with suspected snakebites, 247 patients received antivenom. 49 patients were excluded or not recruited leaving 104 patients allocated to the rapid antivenom infusion and 94 to the slow antivenom infusion. The median actual duration of antivenom infusion in the rapid group was 20 min (Interquartile range[IQR]:2025 min) versus 120 min (IQR:75120 min) in the slow group. There was no difference in severe systemic hypersensitivity reactions between those given rapid and slow infusions (32% vs. 35%; difference 3%; 95%CI:?10% to +17%;p?=?0.65). The frequency of mild/moderate reactions was also similar. Similar numbers of patients in each arm received further doses of antivenom (30/104 vs. 23/94). Conclusions A slower infusion rate would not reduce the rate of severe systemic hypersensitivity reactions from current high rates. More effort should be put into developing better quality antivenoms. Trial Registration www.slctr.lk SLCTR/2007/005

Isbister, Geoffrey K.; Shahmy, Seyed; Mohamed, Fahim; Abeysinghe, Chandana; Karunathilake, Harendra; Ariaratnam, Ariaranee

2012-01-01

301

New empirical rate expression for reactions without a barrier: Analysis of the reaction of CN with O2  

NASA Astrophysics Data System (ADS)

The rate coefficients of reactions that occur on potential energy surfaces without a barrier often exhibit a negative temperature dependence at low temperatures. Generally, this behavior is modeled with either the Harcourt-Essen equation, k(T)=AT-m, or a ``negative'' activation energy, k(T)=ATm exp{?E/kBT}. Neither of these expressions is consistent with the Wigner threshold law. The general expression k(T)=(1+T/TW)-m?l=0?Al(1+T/TW)-l(T/TW)l is proposed where the relative angular momentum of the reacting species is l, TW and m are independent parameters to be extracted from the data, and the amplitude of each partial wave is Al. This expression may be approximated by k(T)=A0(1+T/TW)-m exp[(T/TW)/(1+T/TW)]. For CN+O2--> NCO+O and CO+NO the above expression reproduces the rate data, the branching ratio to the CO+NO channel, and the reactive cross section for the NCO+O channel. The rate coefficient for the NCO+O channel is given by k(cm3 s-1)=1.7910-10(+T/21.7)-1.38{exp[(T/21.7)/(1+T/21.7)]-1}+4.6210-12 exp[(T/21.7)/(1+T/21.7)] while for CO+NO we obtain k(cm3 s-1)=1.7910-10(1+T/21.7)-1.38. An analytic form of the C-O bonding potential and the electric dipole-quadrupole interaction is used to show that the quantum threshold region extends up to 7 K. These results demonstrate the need of a complete quantum treatment for reactions that proceed on potential surfaces without a barrier.

Hessler, Jan P.

1999-09-01

302

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.; Dale, V.; Mellouki, A.

2013-09-01

303

Estimating Reaction Rate Coefficients Within a Travel-Time Modeling Framework  

SciTech Connect

A generalized, efficient, and practical approach based on the travel-time modeling framework is developed to estimate in situ reaction rate coefficients for groundwater remediation in heterogeneous aquifers. The required information for this approach can be obtained by conducting tracer tests with injection of a mixture of conservative and reactive tracers and measurements of both breakthrough curves (BTCs). The conservative BTC is used to infer the travel-time distribution from the injection point to the observation point. For advection-dominant reactive transport with well-mixed reactive species and a constant travel-time distribution, the reactive BTC is obtained by integrating the solutions to advective-reactive transport over the entire travel-time distribution, and then is used in optimization to determine the in situ reaction rate coefficients. By directly working on the conservative and reactive BTCs, this approach avoids costly aquifer characterization and improves the estimation for transport in heterogeneous aquifers which may not be sufficiently described by traditional mechanistic transport models with constant transport parameters. Simplified schemes are proposed for reactive transport with zero-, first-, nth-order, and Michaelis-Menten reactions. The proposed approach is validated by a reactive transport case in a two-dimensional synthetic heterogeneous aquifer and a field-scale bioremediation experiment conducted at Oak Ridge, Tennessee. The field application indicates that ethanol degradation for U(VI)-bioremediation is better approximated by zero-order reaction kinetics than first-order reaction kinetics.

Gong, R [Georgia Institute of Technology; Lu, C [Georgia Institute of Technology; Luo, Jian [Georgia Institute of Technology; Wu, Wei-min [Stanford University; Cheng, H. [Stanford University; Criddle, Craig [Stanford University; Kitanidis, Peter K. [Stanford University; Gu, Baohua [ORNL; Watson, David B [ORNL; Jardine, Philip M [ORNL; Brooks, Scott C [ORNL

2011-03-01

304

Atomic hydrogen reaction rates in aqueous solution via free-induction decay attenuation  

SciTech Connect

This new pulsed EPR method is providing data on H-atom reaction rates in water, which are important in radiation chemistry of aqueous solutions, eg, corrosion in reactor cooling systems, H2 production in high-level radioactive waste, and spread of radioactive iodine following the Chernobyl accident. This AECL-Argonne collaborative effort was undertaken to investigate fundamental aqueous H-atom chemistry for the purpose of developing predictive models of iodine transport that will be used by the nuclear power industry to enhance plant safety. Reactions of (H){sub aq} with many representative organic and inorganicmolecules have already been investigated. Radiolysis of aqueous solns with 3-MeV electrons produces H atoms in high yield; in the free induction decay method, a powerful 30 ns microwave pulse is applied immediately following pulse radiolysis, creating a phase coherence of the H-atom electron spins that is detected in the pulsed EPR spectrometer as a damped cosine free induction decay (FID). Natural decay time of the spin coherence is many microseconds, but if a reaction partner for the H atoms is introduced, the FID becomes shorter, revealing the H reaction rate. Results of H atom reaction with benzene, methanol, periodate and periodic acid are discussed briefly.

Bartels, D.M.

1996-02-01

305

An Improved Reaction Rate Formulation for Charged-Particle Induced Thermonuclear Reaction of {sup 2}H(d,{gamma}){sup 4}He  

SciTech Connect

The reaction rate formula utilized in compilations such as the Nuclear Astrophysics Compilation of Reaction Rates (NACRE) uses low energy approximation due to temperatures in stars are in the region of a few keVs. Most nuclear reaction experiments were done in MeV range and the interior temperatures of massive stars are {approx}10{sup 9} K. Hence an improved formulation for calculating the nuclear reaction rate that is applicable to high temperatures is discussed in this work. The exact tunneling probability that is applicable for all energies is obtained by solving the Schroedinger equation. This yields an enhanced expression for the astrophysical S-factor for calculating the thermonuclear reaction rate at high temperature. The thermonuclear reaction rate from this work is applied to the {sup 2}H(d,{gamma}){sup 4}He reaction and is compared with the NACRE compilation. This improved reaction rate can be included in the nuclear reaction network in a Big Bang nucleosynthesis (BBN) code or a stellar nuclear network code.

Aziz, Azni Abdul; Yusof, Norhasliza; Idris, Mahirah; Kassim, Hasan Abu [Department of Physics, University of Malaya, 50603 Kuala Lumpur (Malaysia)

2011-03-30

306

Pore-scale heterogeneous reaction rates on a dissolving limestone surface  

NASA Astrophysics Data System (ADS)

The rate at which limestone dissolves determines the resistance of buildings and monuments to weathering, the efficiency of carbon capture in deep geological reservoirs, and the processes by which soils, rocks, and landscapes form and evolve. However, the normalized rates of mineral dissolution measured in laboratory experiments are often found to be far greater than those measured in field settings. Here, we use atomic force microscopy (AFM) measurements to demonstrate experimentally that the rate of calcite dissolution within micron-size pores at the surface of a limestone sample is much lower than the rate of dissolution in the surrounding calcite surface. In addition, we use numerical simulations to show that this difference cannot be explained using a simple diffusion-surface reaction model. We suggest that the observed heterogeneous reaction rates could instead be due to the elevated density of reactive high curvature features on the polished surface surrounding the pore. These high curvature features can strongly affect local interfacial free energy, making such surfaces more prone to dissolution. As a result, polished surfaces should be more reactive than pore surfaces that have effectively been smoothed during prolonged contact with natural fluids. As rate experiments routinely use polished and powdered samples, the results may help to explain the widely reported discrepancy between lab and field-based dissolution rates.

Levenson, Yael; Emmanuel, Simon

2013-10-01

307

Effective medium theory for reaction rates and diffusion coefficients of heterogeneous systems.  

PubMed

A simple effective medium theory is derived for spatially heterogeneous nonlinear reaction-diffusion media. Its validity is tested through comparisons with simulations of front and pulse propagation in systems with spatially varying diffusion coefficients and reaction rates. The theory is able to predict wave speeds if the characteristic front width is much larger than the length scale of the heterogeneities. This condition is violated in media with isolated or weakly connected sites. However, the theory nevertheless provides good results in cases where it correctly predicts the percolation threshold of the medium. PMID:19658977

Alonso, Sergio; Kapral, Raymond; Br, Markus

2009-06-12

308

Effective Medium Theory for Reaction Rates and Diffusion Coefficients of Heterogeneous Systems  

NASA Astrophysics Data System (ADS)

A simple effective medium theory is derived for spatially heterogeneous nonlinear reaction-diffusion media. Its validity is tested through comparisons with simulations of front and pulse propagation in systems with spatially varying diffusion coefficients and reaction rates. The theory is able to predict wave speeds if the characteristic front width is much larger than the length scale of the heterogeneities. This condition is violated in media with isolated or weakly connected sites. However, the theory nevertheless provides good results in cases where it correctly predicts the percolation threshold of the medium.

Alonso, Sergio; Kapral, Raymond; Br, Markus

2009-06-01

309

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

310

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

311

Quantum Corrections to the Particle Distribution Function and Reaction Rates in Dense Media  

SciTech Connect

Quantum mechanics predicts the existence of power-law tails in the momentum distribution function of particles in dense media even under conditions of thermodynamic equilibrium. The generalized expressions allowing for the effect of the medium density show that quantum corrections lead to a sharp increase in the reaction rates of threshold exothermic processes (such as fusion and chemical reactions and vibrational-translational relaxation). The accompanying modification of the distribution function changes the wings of the emission and absorption lines. The profiles of the absorption lines in dense gaseous media are shown to be asymmetric with respect to the line center.

Starostin, A.N.; Petrushevich, Yu.V.; Rerikh, Vl.K. [Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow oblast, 142190 (Russian Federation); Leonov, A.G. [Moscow Institute of Physics and Technology, Institutskii pr. 9, Dolgoprudnyi, Moscow oblast, 141700 (Russian Federation)

2005-02-15

312

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

313

A new theoretical approach to thermonuclear radiative-capture reaction rate  

NASA Astrophysics Data System (ADS)

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 16O(?,?)20Ne 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

2012-11-01

314

Learned cardiac control with heart rate biofeedback transfers to emotional reactions.  

PubMed

Emotions involve subjective feelings, action tendencies and physiological reactions. Earlier findings suggest that biofeedback might provide a way to regulate the physiological components of emotions. The present study investigates if learned heart rate regulation with biofeedback transfers to emotional situations without biofeedback. First, participants learned to decrease heart rate using biofeedback. Then, inter-individual differences in the acquired skill predicted how well they could decrease heart rate reactivity when later exposed to negative arousing pictures without biofeedback. These findings suggest that (i) short lasting biofeedback training improves heart rate regulation and (ii) the learned ability transfers to emotion challenging situations without biofeedback. Thus, heart rate biofeedback training may enable regulation of bodily aspects of emotion also when feedback is not available. PMID:23894574

Peira, Nathalie; Pourtois, Gilles; Fredrikson, Mats

2013-07-23

315

A study of the impact of reaction rates on equation of state  

SciTech Connect

We present experimental evidence on high explosives pushing metal plates that shows the tendency of the equation of state behaving more energetically in the low-pressure region. We believe the deficiency in predicting low- and also high-pressure performance has a theoretical origin, and it is related to reaction rate. The reaction process with a slow stage generates a higher effective Chapman-Jouguet pressure. The development of an equation of state based on this information, using the classical detonation theory but without the consideration of the slow component in reaction, would produce a more energetic condition in the low-pressure region and a less powerful one in high pressure. We demonstrate how a polytropic gas equation of state is constructed and the properties is possesses as mentioned. Finally, we show how the effective Chapman-Jouguet condition comes about and define the property of a pseudo Hugoniot associated with it. 3 refs., 5 figs.

Tang, P.K.

1991-01-01

316

From the Sun to supernovae: experimental determinations of stellar reaction rates at TRIUMF  

NASA Astrophysics Data System (ADS)

The ongoing goal of the TRIUMF astrophysics group is to measure resonance strengths and cross sections for reactions of astrophysical importance. This is done using accelerated radioactive and stable beams from ISAC I and II delivered to the DRAGON and TUDA facilities. The wide range of available beams at the energies provided by the ISAC I and II accelerators enable us to measure reaction rates of interest in many different astrophysical environments. Experiments in the past year have studied reactions important for understanding the production of solar neutrinos as well as nucleosynthesis in the Big Bang, Asymptotic Giant Branch (AGB) stars, novae, and supernovae. These experiments and the various techniques required for these measurements will be discussed.

Fallis, Jennifer

2013-03-01

317

Calculation of the reaction cross section from a rate constant by the method of steepest-descent.  

PubMed

The reaction cross section of a bimolecular reaction has been calculated from the corresponding rate constant by the inverse Laplace transform. In carrying out the inverse Laplace transform, the method of steepest-descent has been used. As applications, the reaction cross sections for the rate constant of k(T) = k' T(n)e(-E0/kT) and for the rate constant of the absolute reaction rate theory have been calculated. The computation of the energy density function associated with the definition of the weighted-average cross section has also been discussed. PMID:16591906

Lin, S H; Eyring, H

1971-02-01

318

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

319

Laboratory Experiments and Investigations on the Reaction Rates of Mg-sulfates Under Mars Relevant Conditions  

NASA Astrophysics Data System (ADS)

Large deposits of hydrous Mg-sulfates was identified on Mars by orbital remote sensing (OMEGA on Mars Express and CRISM on Mars Reconnaissance Orbiter). Kieserite (MgSO4.H2O) and a non-specific polyhydrated sulfates are among the most observed and widely distributed sulfates (Bibring et al., 2005, Murchie et al., 2007). They frequently co-exist (Gendrin et al., 2005) and sometimes occur in alternative stratigraphic layers (Roach et al., 2008). Mg-sulfates were suggested, by compositional correlations and mineral models, to exist in Meridiani outcrops (Clark et al., 2005) and in rocks and regolith at Gusev (Squyres et al., 2006, Haskin et al., 2005, Wang et al., 2006, 2008); but no information on the hydration state of these sulfates can be extracted. We have conducted 188 experiments to investigate the stability fields and phase transition pathways of hydrous Mg-sulfates (Wang et al., 2009). In addition, we can extract the information on the reaction rates of five important dehydration and rehydration processes involved in these experiments. Our experiments were done at four temperatures (50C, 21C, 5C, and -10C) and ten relative humidity levels, with five hydrous Mg-sulfate species as starting phases. The rate information was extracted from the mineral identifications of the intermediate reaction products, measured by non-invasive Raman spectroscopy at regular time intervals during the entire duration of experiments (tens thousands hours). The rates for five processes are all strongly controlled by temperatures. We found that the experimental results match Arrhenius equation very well, thus the rate constants for dehydration and rehydration processes of Mg-sulfates at lower temperatures (down to 180K) can be approximately estimated by using the experimentally derived pre-exponential factor(s) and activation energy(s). In this study, only the orders of magnitudes for reaction rate ratios at different temperatures were considered. The estimated reaction rate ratios at different temperatures for five important processes helped us to understand the stable, especially the metastable, Mg-sulfate species that could be seen at Mars surface in non-polar regions during a moderate obliquity period. Therefore in addition to exam the spectral similarity, we now can use the knowledge gained through the laboratory experiments on stability field, phase transition pathway, and reaction rate of Mg-sulfates to evaluate the realistic mineral candidates for polyhydrated sulfates, that were so widely observed on Mars by OMEGA and CRISM. Furthermore, we will be able to investigate the formation mechanism of alternative stratigraphic layers of sulfates on Mars and the paleo-climatic conditions that they may imply.

Wang, A.; Freeman, J. J.

2009-12-01

320

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

321

Probing the interplay between factors determining reaction rates on silica gel using termolecular systems.  

PubMed

In this study we have compared energy and electron transfer reactions in termolecular systems using a nanosecond diffuse reflectance laser flash photolysis technique. We have previously investigated these processes on silica gel surfaces for bimolecular systems and electron transfer in termolecular systems. The latter systems involved electron transfer between three arene molecules with azulene acting as a molecular shuttle. In this study we present an alternative electron transfer system using trans ?-carotene as an electron donor in order to effectively immobilise all species except the shuttle, providing the first unambiguous evidence for radical ion mobility. In the energy transfer system we use naphthalene, a structural isomer of azulene, as the shuttle, facilitating energy transfer from a selectively excited benzophenone sensitiser to 9-cyanoanthracene. Bimolecular rate constants for all of these processes have been measured and new insights into the factors determining the rates of these reactions on silica gel have been obtained. PMID:22898878

Kirkpatrick, Iain; Worrall, David R; Williams, Sin L; Buck, Craig J T; Meseguer, Rafael G

2012-10-01

322

Persistence of instanton connections in chemical reactions with time-dependent rates.  

PubMed

The evolution of a system of chemical reactions can be studied, in the eikonal approximation, by means of a Hamiltonian dynamical system. The fixed points of this dynamical system represent the different states in which the chemical system can be found, and the connections among them represent instantons or optimal paths linking these states. We study the relation between the phase portrait of the Hamiltonian system representing a set of chemical reactions with constant rates and the corresponding system when these rates vary in time. We show that the topology of the phase space is robust for small time-dependent perturbations in concrete examples and state general results when possible. This robustness allows us to apply some of the conclusions on the qualitative behavior of the autonomous system to the time-dependent situation. PMID:18351841

Escudero, Carlos; Rodrguez, Jos Angel

2008-01-30

323

Oxygen consumption rates in subseafloor basaltic crust derived from a reaction transport model.  

PubMed

Oceanic crust is the largest potential habitat for life on Earth and may contain a significant fraction of Earth's total microbial biomass; yet, empirical analysis of reaction rates in basaltic crust is lacking. Here we report the first assessment of oxygen consumption in young (~8?Ma) and cool (<25?C) basaltic crust, which we calculate from modelling dissolved oxygen and strontium pore water gradients in basal sediments collected during Integrated Ocean Drilling Program Expedition 336 to 'North Pond' on the western flank of the Mid-Atlantic Ridge. Dissolved oxygen is completely consumed within the upper to middle section of the sediment column, with an increase in concentration towards the sediment-basement interface, indicating an upward supply from oxic fluids circulating within the crust. A parametric reaction transport model of oxygen behaviour in upper basement suggests oxygen consumption rates of 1?nmol?cm(-3)ROCK d(-1) or less in young and cool basaltic crust. PMID:24071791

Orcutt, Beth N; Wheat, C Geoffrey; Rouxel, Olivier; Hulme, Samuel; Edwards, Katrina J; Bach, Wolfgang

2013-01-01

324

Absolute rate constants for some intermolecular reactions of alpha-aminoalkylperoxyl radicals. Comparison with alkylperoxyls.  

PubMed

Seven alpha-aminoalkylperoxyl radicals have been generated by 355 nm laser flash photolysis (LFP) of oxygen-saturated di-tert-butyl peroxide containing mono-, di-, and trialkylamines and a dialkylarylamine. All these peroxyls possess absorptions in the near-UV (strongest for the trialkylamine-derived peroxyls) which permits direct monitoring of the kinetics of their reactions with many substrates. The measured rate constants for hydrogen atom abstraction from some phenols and oxygen atom transfer to triphenylphosphine demonstrated that all seven alpha-aminoalkylperoxyls have similar reactivities toward each specific substrate. More importantly, a comparison with literature data for alkylperoxyls shows that alpha-aminoalkylperoxyls and these alkylperoxyls have essentially the same reactivities. The combination of LFP and alkylamines provides a quick, reliable method for determining absolute rate constants for alkylperoxyl radical reactions, an otherwise laborious task. PMID:18665644

Laleve, Jacques; Allonas, Xavier; Fouassier, Jean-Pierre; Ingold, K U

2008-07-30

325

Two effects relevant for the study of astrophysical reaction rates: {gamma} transitions in capture reactions and Coulomb suppression of the stellar enhancement  

SciTech Connect

Nucleosynthesis processes involve reactions on several thousand nuclei, both close to and far off stability. The preparation of reaction rates to be used in astrophysical investigations requires experimental and theoretical input. In this context, two interesting aspects are discussed: (i) the relevant {gamma} transition energies in astrophysical capture reactions, and (ii) the newly discovered Coulomb suppression of the stellar enhancement factor. The latter makes a number of reactions with negative Q value more favorable for experimental investigation than their inverse reactions, contrary to common belief.

Rauscher, Thomas [Departement Physik, Universitaet Basel, CH-4056 Basel (Switzerland)

2009-01-28

326

On quantifying multisensory interaction effects in reaction time and detection rate  

Microsoft Academic Search

Both mean reaction time (RT) and detection rate (DR) are important measures for assessing the amount of multisensory interaction\\u000a occurring in crossmodal experiments, but they are often applied separately. Here we demonstrate that measuring multisensory\\u000a performance using either RT or DR alone misses out on important information. We suggest an integration of RT and DR into a\\u000a single measure of

Stefan Rach; Adele Diederich; Hans Colonius

2011-01-01

327

Reaction Zone Structure and Scalar Dissipation Rates in Turbulent Diffusion Flames  

Microsoft Academic Search

Images of mixture fraction, temperature, scalar dissipation rates and OH concentrations in turbulent diffusion flames of methane-air and hydrogen-carbon dioxide are presented. The images are derived from Rayleigh scattering, fuel Raman scattering and OH-LIF. The images reveal that the reaction zones in these flames are strongly affected by the turbulence within them and that they become broadly distributed as the

J. B. KELMAN; A. R. MASRI

1997-01-01

328

Reaction Zone Structure and Scalar Dissipation Rates in Turbulent Diffusion Flames  

Microsoft Academic Search

Images of mixture fraction, temperature, scalar dissipation rates and OH concentrations in turbulent diffusion flames of methane-air and hydrogen-carbon dioxide are presented. The images are derived from Rayleigh scattering, fuel Raman scattering and OH-LIF. The images reveal that the reaction zones in these flames are strongly affected by the turbulence within them and that they become broadly distributed as the

J. B. KELMAN; A. R. MASRI

1998-01-01

329

Indirect photolysis of acetochlor: Rate constant of a nitrate-mediated hydroxyl radical reaction  

Microsoft Academic Search

The second order rate constant, k2, for reaction of the corn herbicide acetochlor with OH radicals was found to be 7.5 (2.0) 109M?1 s?1, in close agreement with the value reported for a similar herbicide, alachlor, based on structure-activity relationships. Summer-time half-lives of 120 days for acetochlor were estimated for the Blue Earth River (southern Minnesota), an agriculturally impacted

Jennifer Fulkerson Brekken; Patrick L. Brezonik

1998-01-01

330

Four-angle saturation transfer (FAST) method for measuring creatine kinase reaction rates in vivo  

Microsoft Academic Search

A new fast method of measuring kinetic reaction rates for two-site chemical exchange is described. The method employs saturation transfer magnetic resonance spectroscopy (MRS) and acquisition of only four spectra under partially saturated, high signal-to-noise ratio (SNR) conditions. In two acquisitions one of the exchanging species is saturated; the other two em- ploy a control saturation. Each pair of acquisitions

Paul A. Bottomley; Ronald Ouwerkerk; Ray F. Lee; Robert G. Weiss

2002-01-01

331

Actinide complexation kinetics: rate and mechanism of dioxoneptunium (V) reaction with chlorophosphonazo III  

SciTech Connect

Rates of complex formation and dissociation in NpO{sub 2}{sup +}- Chlorophosphonazo III (2,7-bis(4-chloro-2-phosphonobenzeneazo)-1,8- dihydroxynapthalene-3,6-disulfonic acid)(CLIII) were investigated by stopped-flow spectrophotometry. Also, limited studies were made of the rates of reaction of La{sup 3+}, Eu{sup 3+}, Dy{sup 3+}, and Fe{sup 3+} with CLIII. Rate determining step in each system is an intramolecular process, the NpO{sub 2}{sup +}-CLIII reaction proceeding by a first order approach to equilibrium in the acid range from 0.1 to 1.0 M. Complex formation occurs independent of acidity, while both acid dependent and independent dissociation pathways are observed. Activation parameters for the complex formation reaction are {Delta}H=46.2{+-}0.3 kJ/m and {Delta}S=7{+-} J/mK (I=1.0 M); these for the acid dependent and independent dissociation pathways are {Delta}H=38.8{+-}0.6 kJ/m, {Delta}S=-96{+-}18 J/mK, {Delta}H=70.0{+-} kJ/m, and {Delta}S=17{+-}1 J/mK, respectively. An isokinetic relationship is observed between the activation parameters for CLIII complex formation with NpO{sub 2}{sup +}, UO{sub 2}{sup 2+}, Th{sup 4+}, and Zr{sup 4+}. Rates of CLIII complex formation reactions for Fe{sup 3+}, Zr{sup 4+}, NpO{sub 2}{sup +}, UO{sub 2}{sup 2+}, Th{sup 4+}, La{sup 3+}, Eu{sup 3+}, and Dy{sup 3+} correlate with cation radius rather than charge/radius ratio.

Fugate, G.; Feil-Jenkins, J.F.; Sullivan, J.C.; Nash, K.L.

1996-12-01

332

Reaction rate of beryllium with fluorine ion for Flibe redox control  

Microsoft Academic Search

An experimental effort to apply Flibe (a mixed molten salt of 2LiF+BeF2) to a self-cooled liquid blanket of a fusion reactor was carried out under a JapanUS collaboration called JUPITER-II. Maintaining Flibe under a reducing atmosphere is a key issue to transform TF to T2 with a faster reaction rate compared with the residence time in blanket. One of the

S. Fukada; M. F. Simpson; R. A. Anderl; J. P. Sharpe; K. Katayama; G. R. Smolik; Y. Oya; T. Terai; K. Okuno; M. Hara; D. A. Petti; S. Tanaka; D.-K. Sze; A. Sagara

2007-01-01

333

Kinetics of the CH3 + HCl/DCl ? CH4/CH3D + Cl and CD3 + HCl/DCl ? CD3H/CD4 + Cl reactions: An experimental H atom tunneling investigation  

NASA Astrophysics Data System (ADS)

The kinetics of the radical reactions of CH3 with HCl or DCl and CD3 with HCl or DCl have been investigated in a temperature controlled tubular reactor coupled to a photoionization mass spectrometer. The CH3 (or CD3) radical, R, was produced homogeneously in the reactor by a pulsed 193 nm exciplex laser photolysis of CH3COCH3 (or CD3COCD3). The decay of CH3/CD3 was monitored as a function of HCl/DCl concentration under pseudo-first-order conditions to determine the rate constants as a function of temperature, typically from 188 to 500 K. The rate constants of the CH3 and CD3 reactions with HCl had strong non-Arrhenius behavior at low temperatures. The rate constants were fitted to a modified Arrhenius expression k = QA exp (-Ea/RT) (error limits stated are 1? + Students t values, units in cm3 molecule-1 s-1): k(CH3 + HCl) = [1.004 + 85.64 exp (-0.02438 T/K)] (3.3 1.3) 10-13 exp [-(4.8 0.6) kJ mol-1/RT] and k(CD3 + HCl) = [1.002 + 73.31 exp (-0.02505 T/K)] (2.7 1.2) 10-13 exp [-(3.5 0.5) kJ mol-1/RT]. The radical reactions with DCl were studied separately over a wide ranges of temperatures and in these temperature ranges the rate constants determined were fitted to a conventional Arrhenius expression k = A exp (-Ea/RT) (error limits stated are 1? + Students t values, units in cm3 molecule-1 s-1): k(CH3 + DCl) = (2.4 1.6) 10-13 exp [-(7.8 1.4) kJ mol-1/RT] and k(CD3 + DCl) = (1.2 0.4) 10-13 exp [-(5.2 0.2) kJ mol-1/RT] cm3 molecule-1 s-1. Curvature in the Arrhenius plots of the H-atom abstraction reactions at low temperatures was analyzed by considering H-atom tunneling through the reaction barrier and primary kinetic isotope effect. Contribution of tunneling in it was concluded to be negligible. In addition, secondary isotope effect was measured.

Eskola, Arkke J.; Seetula, Jorma A.; Timonen, Raimo S.

2006-12-01

334

Nuclear reaction rates and energy in stellar plasmas: The effect of highly damped modes  

NASA Astrophysics Data System (ADS)

The effects of the highly damped modes in the energy and reaction rates in a plasma are discussed. These modes, with wave numbers k>>kD, even being only weakly excited, with less than kBT per mode, make a significant contribution to the energy and screening in a plasma. When the de Broglie wavelength is much less than the distance of closest approach of thermal electrons, a classical analysis of the plasma can be made. It is assumed, in the classical analysis, with ?-->0, that the energy of the fluctuations ??<Reaction rates are enhanced in a plasma due to the screening of the reacting ions. This is taken into account by the Salpeter factor, which assumes slow motion for the ions. The implication of including the highly damped modes (with ??0) in the nuclear reaction rates in a plasma is discussed. Finally, the investigations presently done on these effects in particle simulations with the sheet model and the multiparticle quantum simulation code are described.

Opher, Merav; Silva, Luis O.; Dauger, Dean E.; Decyk, Viktor K.; Dawson, John M.

2001-05-01

335

Rate coefficients for reaction of OH with acetone between 202 and 395 K  

SciTech Connect

The kinetics of the title reaction were investigated between 202 and 395 K and at 20, 50, and 100 Torr of Ar or N{sub 2} bath gas using pulsed laser photolysis (PLP) generation of OH combined with both resonance fluorescence (RF) and laser-induced fluorescence (LIF) detection. OH was generated either by the sequential 439 nm, two-photon dissociation of NO{sub 2} in the presence of H{sub 2}, or by HONO photolysis at 351 nm. The accuracy of the rate constants obtained was enhanced by optical absorption measurements of acetone concentrations both before and after the photolysis reactor. The temperature dependence is not describe by a simple Arrhenius expression but by k{sub 1} (202--395 K) = 8.8 x 10{sup {minus}12} exp({minus}1,320/T) + 1.7 x 10{sup {minus}14} exp(423/T) cm{sup 3} s{sup {minus}1}, indicating that a simple H atom abstraction may not be the only reaction mechanism. The estimated total error (95% confidence) associated wit the rate coefficient derived from this expression is estimated as 5% and is independent of temperature. The curvature in the Arrhenius plot results in a significantly larger rate coefficient at low temperatures than obtained by extrapolation of the previous measurement and implies greater significance for the reaction with OH as a sink for acetone in the upper troposphere than presently assumed.

Wollenhaupt, M.; Carl, S.A.; Horowitz, A.; Crowley, J.N.

2000-03-30

336

Transformation of acetaminophen using manganese dioxide-mediated oxidative processes: reaction rates and pathways.  

PubMed

This study investigates the oxidative transformation kinetics of acetaminophen (APAP) by ?-MnO2 under different conditions. APAP was rapidly oxidized by ?-MnO2 with the generation of Mn(2+). The measured APAP reaction rate considerably increased with an increase in initial ?-MnO2 and APAP concentration, but decreased as pH increased. The APAP reaction rate also increased with an increase in temperature. The addition of inorganic ions (Mn(2+), Ca(2+), and Fe(3+)) and substituted phenols (guaiacol, caffeic acid, and p-coumaric acid) as co-solutes remarkably decreased the transformation rate of APAP. The UV-Vis absorption spectra exhibited the ? ? ?* transition, typical for aromatic rings. In addition, the intensity of the absorption peak gradually improved with increasing reaction time, suggesting that APAP can polymerize to form oligomers. Moreover, the secondary mass spectra of the dimers elucidated that the dimers were formed by the covalent bonding of phenol aromatic rings. Moreover, the higher-degree oligomers were formed by the coupling polymerization of phenolic and anilidic groups of dimers. These results are useful in understanding the fate of APAP in natural systems. PMID:23434488

Xiao, Hong; Song, Haiyan; Xie, Hongqin; Huang, Wei; Tan, Jun; Wu, Jianzhong

2013-02-04

337

The CH + CO reaction: Rate coefficient for carbon atom exchange at 294 K  

SciTech Connect

A fast-flow reactor equipped with isotope-specific laser-excited fluorescence detection of CH radicals has been used to study carbon atom exchange in the reaction between CH and CO at 294 K and 2 Torr of total pressure. The rate coefficient for exchange, k{sub 3} = (2.1 {times} 0.3) {times} 10{sup {minus}12} cm{sup 3} s{sup {minus}1}, is about an order of magnitude larger than the bimolecular rate for the addition reaction, k{sub 2} = (2.7 {plus minus} 0.4) {times} 10{sup {minus}13}. High-pressure limiting bimolecular and low-pressure termolecular recombination rate coefficients of 1.1 {times} 10{sup {minus}10} cm{sup 3} s{sup {minus}1} and 4.9 {times} 10{sup {minus}30} cm{sup 6} s{sup {minus}1} are derived. The results are discussed in the context of previous work on the title reaction and on the chemistry of singlet CH{sub 2}.

Anderson, S.M.; McCurdy, K.E.; Kolb, C.E. (Aerodyne Research, Inc., Billerica, MA (USA))

1989-02-09

338

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

339

Relative Rates of Reaction of Olefins with the Ground and the First Excited Electronic States of Methylene.  

National Technical Information Service (NTIS)

Relative rates of reaction of a number of olefins with the ground state triplet and the first excited electronic state (the lowest singlet) of methylene were determined at room temperature. The rates depend relatively little on olefin structure. Triplet m...

S. Krzyzanowski R. J. Cvetanovic

1966-01-01

340

Incidence of multi-particle events on soft error rates caused by n-Si nuclear reactions  

Microsoft Academic Search

Neutron reactions with silicon nuclei can be responsible for much of the soft errors rate (SER) observed, for instance, in high density memories. The nuclear reactions create ionizing particles that then can induce charge collection at sensitive nodes. In many cases, the nuclear reaction produces a shower of ions. Models for the prediction of SER are much more complicated if

F. Wrobel; J.-M. Palau; M. C. Calvet; O. Bersillon; H. Duarte

2000-01-01

341

Relationship of the Geometry of the Observed Steady State Chemical Conversion Rate to the Basic Surface Reaction Process in Electrochemistry.  

National Technical Information Service (NTIS)

The steady state surface reaction rate for electrochemical reactions is frequently measured by the polarization curve. In the anodic region of this curve the dynamics of the reaction process are generally described by a semiflow map. The fixed points of t...

D. G. Retzloff B. DeFacio J. E. Bauman P. H. Ragatz

1979-01-01

342

High-precision 28Si(p,t)26Si reaction to determine 22Mg(?,p)25Al reaction rates  

NASA Astrophysics Data System (ADS)

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 (?,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 (?,p) reaction to natural parity states. We measured the 28Si(p,t)26Si reaction with a high-resolution spectrometer to identify resonance levels in 26Si above the ?-emission threshold at 9.164 MeV excitation energy. These resonance levels are used to calculate the stellar reaction rate of the 22Mg(?,p)25Al reaction and to test the validity of the statistical assumption.

Matic, A.; van den Berg, A. M.; Harakeh, M. N.; Wrtche, H. J.; Beard, M.; Berg, G. P. A.; Grres, 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.; Fujita, H.; Wakasa, T.; Greene, J. P.; Crowter, R.; Schatz, H.

2011-08-01

343

Groundwater ages and reaction rates during seawater circulation in the Dead Sea aquifer  

NASA Astrophysics Data System (ADS)

The Dead Sea system provides a unique opportunity to study flow velocities and reaction rates during seawater circulation in the aquifer. We present here a novel application of calculating groundwater age and velocity along the flow path of the hypersaline water from the Dead Sea into the aquifer using the buildup rate of 228Ra in this water. The calculated circulation velocities are 1-10 m/y, which is in agreement with estimates based on the Na/Cl ratios in this water (1.5-4 m/y). The latter is unique to the Dead Sea-aquifer system, where the Na/Cl ratio has been decreasing during the past 50 years due to the precipitation of halite in the lake. The velocity estimates facilitated the calculation of the rates of water-rock interactions in the Dead Sea aquifer. SO4 is removed relatively fast (k=0.8 y-1) due to gypsum precipitation while barite or celestine precipitation removes 226Ra and Ba in a time scale of years (k=0.22 y-1). Similar rates were found for redox-driven reactions, such as U removal (k=0.4 y-1) and Fe and Mn contribution due to the dissolution of oxides (k=0.15 y-1). In the fresh-saline groundwater transition zone, gypsum which precipitated from hypersaline water in higher lake stands, is now being dissolved and enrich the water with SO4.

Kiro, Yael; Weinstein, Yishai; Starinsky, Abraham; Yechieli, Yoseph

344

Characterization of the reaction rate coefficient of DNA with the hydroxyl radical  

SciTech Connect

Using agarose gel electrophoresis, we have measured the yield of single-strand breaks (SSBs) induced by {sup 137}Cs {gamma} irradiation in a variety of plasmid DNA substrates ranging in size from 2.7 kb to 38 kb irradiated in aerobic aqueous solution in the presence of the hydroxyl radical scavenger dimethyl sulfoxide (DMSO). Under these conditions DNA SSBs are caused mainly by the hydroxyl radical. Using the competition between DMSO and DNA for the hydroxyl radical, we have estimated the rate coefficient for the reaction of the hydroxyl radical with DNA. The results cannot be characterized by conventional steady-state competition kinetics. However, it is possible to describe the second-order rate constant for the reaction as a function of the scavenging capacity of the solution. The second-order rate constant increases with increasing scavenging capacity, rising from about 5x10{sup 8} dm{sup 3} mol{sup -1} s{sup -1} at 10{sup 5} s{sup -1} to about 10{sup 10} dm{sup 3} mol{sup -1} s{sup -1} at 10{sup 10} s{sup -1}. This dependence of the second-order rate constant on the scavenging capacity appears to be more pronounced for larger plasmids. 17 refs., 4 figs.

Milligan, J.R.; Ward, J.F.; Aguilera, J.A. [Univ. of California, La Jolla, CA (United States)] [and others

1996-11-01

345

Nova Nucleosynthesis with a New 18F(p,alpha) Reaction Rate  

NASA Astrophysics Data System (ADS)

The long-lived radionuclide ^18F is synthesized in nova outbursts and its decay may serve as an observational tracer of the explosion mechanism. Because the ^18F(p,?)^15O reaction is the dominant destruction mechanism for ^18F, the flux of gamma rays from ^18F decay is very sensitive to the rate of this reaction. A revised ^18F(p,?)^15O rate was determined from recent ORNL measurements of ^1H(^18F,p)^18F and ^2H(^18F,p)^19F, combined with a reanalysis of archival ^15N(?,?)^15N data. We used this new rate in nova element synthesis calculations and compared new predictions of the synthesized abundance ^18F (and other nuclides) to that obtained using the two most recent (p,?) rates. We used a post- processing approach with temperature and density histories of 28 zones of ejected material determined from separate hydrodynamics calculations. The implications for satellite observations of novae will be discussed. These calculations were performed and visualized with the Computational Infrastructure for Nuclear Astrophysics, an online suite of codes available at nucastrodata.org.

Smith, Michael; Kozub, Raymond; Brune, Carl; Starrfield, Sumner

2005-04-01

346

Using Variable and Fixed Topological Indices for the Prediction of Reaction Rate Constants of Volatile Unsaturated Hydrocarbons with OH Radicals  

Microsoft Academic Search

Volatile organic compounds (VOCs) play an important role in different photochemical processes in the troposphere. In order to predict their impact on ozone formation processes a detailed knowledge about their abundance in the atmosphere as well as their reaction rate constants is required. The QSPR models were developed for the prediction of reaction rate constants of volatile unsaturated hydrocarbons. The

Matev Pompe; Marjan Veber; Alexandru T. Balaban

2004-01-01

347

THEORETICAL REACTION RATES OF {sup 12}C({alpha}, {gamma}){sup 16}O BELOW T{sub 9} = 3  

SciTech Connect

The astrophysical reaction rates of {sup 12}C({alpha},{gamma}){sup 16}O below T{sub 9} = 3, calculated from the potential model, are provided in tabular form and as analytic expressions. The uncertainties of the reaction rates are estimated by using variations of the model parameters.

Katsuma, M. [Advanced Mathematical Institute, Osaka City University, Osaka 558-8585 (Japan)

2012-02-01

348

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

349

Calculation of astrophysical reaction rate of 82Ge(n,?)83Ge  

NASA Astrophysics Data System (ADS)

The neutron capture reaction on a neutron-rich near closed-shell nucleus 82Ge may play an important role in the r-process following the fallout from nuclear statistical equilibrium in core-collapse supernovae. By carrying out a DWBA analysis for the experimental angular distribution of 82Ge(d, p)83Ge reaction we obtain the single particle spectroscopic factors, S2,5/2 and S0,1/2 for the ground and first excited states of 83Ge = 82Geotimesn, respectively. And then these spectroscopic factors are used to calculate the direct capture cross sections for the 82Ge(n, ?)83Ge reaction at energies of astrophysical interest. The optical potential for neutron scattering on unstable nucleus 82Ge is not known experimentally. We employed a real folding potential which was calculated by using the proper 82Ge density distribution and an effective nucleon-nucleon force DDM3Y. The neutron capture reactions on neutron-rich closed-shell nuclei are expected to be dominated by the direct capture to bound states. We will show that the direct capture rates on these nuclei are sensitive to the structure of the low-lying states.

Wang, Mian; Chen, Yong-Shou; Li, Zhi-Hong; Liu, Wei-Ping; Shu, Neng-Chuan

2009-03-01

350

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

SciTech Connect

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

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

1987-04-23

351

Phosphatase-coupled universal kinase assay and kinetics for first-order-rate coupling reaction.  

PubMed

Kinases use adenosine-5'-triphosphate (ATP) as the donor substrate and generate adenosine-5'-diphosphate (ADP) as a product. An ADP-based phosphatase-coupled kinase assay is described here. In this assay, CD39L2, a nucleotidase, is added into a kinase reaction to hydrolyze ADP to AMP and phosphate. The phosphate is subsequently detected using malachite green phosphate-detection reagents. As ADP hydrolysis by CD39L2 displays a first-order rate constant, relatively simple equations are derived to calculate the coupling rate and the lagging time of the coupling reaction, allowing one to obtain kinase kinetic parameters without the completion of the coupling reaction. ATP inhibition of CD39L2-catalyzed ADP hydrolysis is also determined for correction of the kinetic data. As examples, human glucokinase, P. chrysogenum APS kinase and human ERK1, kinases specific for sugar, nucleotide and protein respectively, are assayed. To assess the compatibility of the method for high-throughput assays, Z' factors >0.5 are also obtained for the three kinases. PMID:21853082

Wu, Zhengliang L

2011-08-11

352

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

2011-03-09

353

Multi-Dimensional Simulations of Fluorocarbon Plasmas with Ion Energy Resolved Surface Reaction Rates  

NASA Astrophysics Data System (ADS)

The commercial CFD-ACE+ software has been extended to account for ion energy dependent surface reactions. The ion energy distribution function and the mean ion energy at a biased wafer were obtained using the Riley sheath model extended by the NASA group (Bose et al., J. Appl. Phys. v.87, 7176(2000)). The plasma chemistry model (by P. Ho et al., SAND2001-1292) consisting of 132-step gas-phase reactions and 55-step ion energy dependent surface reactions, was implemented to simulate the C2F6 plasma etching of silicon dioxide in an Inductively Coupled Plasma. Validation studies have been performed against the experimental data by Anderson et al. of UNM for a lab-scale GEC reactor. For a wide range of operating conditions (pressure: 5-25 mTorr; plasma power: 205-495 Watts; bias power: 22-148 Watts), the average etch rate calculated by CFD-ACE+ 2-D simulations agrees very well with those by 0-D AURORA predictions and the experimental data. The CFD-ACE+ simulations allow one to study the radial uniformity of the etch rate depending on discharge conditions.

Zhou, Ning; Kolobov, Vladimir; Kudriavtsev, Vladimir

2001-10-01

354

Phosphatase-Coupled Universal Kinase Assay and Kinetics for First-Order-Rate Coupling Reaction  

PubMed Central

Kinases use adenosine-5?-triphosphate (ATP) as the donor substrate and generate adenosine-5?-diphosphate (ADP) as a product. An ADP-based phosphatase-coupled kinase assay is described here. In this assay, CD39L2, a nucleotidase, is added into a kinase reaction to hydrolyze ADP to AMP and phosphate. The phosphate is subsequently detected using malachite green phosphate-detection reagents. As ADP hydrolysis by CD39L2 displays a first-order rate constant, relatively simple equations are derived to calculate the coupling rate and the lagging time of the coupling reaction, allowing one to obtain kinase kinetic parameters without the completion of the coupling reaction. ATP inhibition of CD39L2-catalyzed ADP hydrolysis is also determined for correction of the kinetic data. As examples, human glucokinase, P. chrysogenum APS kinase and human ERK1, kinases specific for sugar, nucleotide and protein respectively, are assayed. To assess the compatibility of the method for high-throughput assays, Z? factors >0.5 are also obtained for the three kinases.

Wu, Zhengliang L.

2011-01-01

355

ABCRATE: A program for the calculation of atom-diatom reaction rates  

NASA Astrophysics Data System (ADS)

ABCRATE is a computer program for the calculation of atom-diatom chemical reaction rates for systems with collinear-dominated dynamics. The dynamical methods used are conventional or generalized transition state theory (GTST) and multidimensional semiclassical approximations for tunneling and nonclassical reflection. The GTST methods included in this version of the program are the canonical and improved canonical variational transition state theory (VTST) and the canonical unified statistical (CUS) method. Rate constants may be calculated for canonical ensembles or for specific vibrational states of selected modes with translational, rotational, and other vibrational modes treated thermally. The potential energy surface required by the program may be a global or semiglobal analytic function. The reaction path is calculated as the path of steepest descent in mass-scaled coordinates from a collinear saddle point, and vibrations transverse to the reaction path are treated by curvilinear internal coordinates. The vibrational modes are quantized, and anharmonicity may be included by various options, including the WKB approximation for bond stretches and the centrifugal oscillator approximation through quartic terms for the curvilinear bend coordinate. Tunneling probabilities are calculated by a variety of semiclassical methods, in particular zero-curvature tunneling (ZCT), small-curvature tunneling (SCT), large-curvature tunneling (LCT), least-action tunneling (LAT), and the microcanonical optimized multidimensional tunneling ( OMT) methods.

Garrett, Bruce C.; Lynch, Gillian C.; Allison, Thomas C.; Truhlar, Donald G.

1998-03-01

356

Evaluation of electron capture reaction rates in Ni isotopes in stellar environments  

SciTech Connect

Electron capture rates in Ni isotopes are studied in stellar environments, that is, at high densities and high temperatures during the core-collapse and postbounce explosive nucleosynthesis in supernovae. Reaction rates in {sup 58}Ni and {sup 60}Ni, as well as in {sup 56}Ni, {sup 62}Ni, and {sup 64}Ni, are evaluated by shell-model calculations with the use of a new shell-model Hamiltonian in the fp shell, GXPF1J. While the previous shell-model calculations failed to reproduce the measured peaks of Gamow-Teller strength in {sup 58}Ni and {sup 60}Ni, the present new Hamiltonian is found to reproduce them very well, as well as the capture rates obtained from the observed strengths. Strengths and energies of the Gamow-Teller transitions in {sup 56}Ni, {sup 62}Ni, and {sup 64}Ni are also found to be consistent with the observations.

Suzuki, Toshio [Department of Physics and Graduate School of Integrated Basic Sciences, College of Humanities and Sciences, Nihon University Sakurajosui 3-25-40, Setagaya-ku, Tokyo 156-8550 (Japan); Center for Nuclear Study, University of Tokyo, Hirosawa, Wako-shi, Saitama 351-0198 (Japan); National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588 (Japan); Honma, Michio [Center for Mathematical Sciences, University of Aizu, Aizu-Wakamatsu, Fukushima 965-8580 (Japan); Mao, Helene [ENSPS, Pole API-Parc d'Innovation, Boulevard Sebastien Brant, BP 10413, F-67412 Illkirch Cedex (France); Otsuka, Takaharu [Department of Physics and Center for Nuclear Study, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan); National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824 (United States); Kajino, Toshitaka [National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588 (Japan); Deaprtment of Astronomy, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033 (Japan)

2011-04-15

357

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

358

Development of the GEM-MSTPC for studies of astrophysical nuclear reaction rates  

SciTech Connect

We have developed an active-target type gas-detector, a Gas Electron Multiplier Multiple-Sampling and Tracking Proportional Chamber (GEM-MSTPC) operating with low-pressure He-base mixed gas, where He is used as a target for studies of astrophysical nuclear reaction rates. Different kinds of 400 {mu}m thick GEMs were examined. The gain stability was examined with a configuration of GEMs of 400 {mu}m in thickness fabricated in different ways, against the injection rate of low-energy heavy ions of 10{sup 5} particles per second. The gain of GEM with Cu electrodes coated by Au was observed to be stable up to the injection rate of 10{sup 5} particles per second.

Yamaguchi, K.; Arai, I. [Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tenodai, Tsukuba, Ibaraki 305-8571 (Japan); Ishiyama, H.; Watanabe, Y. X.; Tanaka, M. H.; Miyatake, H.; Hirayama, Y.; Imai, N.; Fuchi, Y.; Jeong, S. C.; Nomura, T. [Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801 (Japan); Mizoi, Y.; Das, S. K.; Fukuda, T. [Research Center for Physics and Mathematics, Faculty of Engineering, Osaka Electro-Communication University, 18-8 Hatsucho, Neyagawa, Osaka 572-8530 (Japan); Hashimoto, T.; Yamaguchi, H.; Kubono, S.; Hayakawa, S. [Center for Nuclear Study, University of Tokyo, 2-1 Hirosawa, Wako, Saitama 351-0198 (Japan); Makii, H.; Mitsuoka, S. [Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195 (Japan)

2010-08-12

359

Linear free energy relationships between aqueous phase hydroxyl radical reaction rate constants and free energy of activation.  

PubMed

The hydroxyl radical (HO()) is a strong oxidant that reacts with electron-rich sites on organic compounds and initiates complex radical chain reactions in aqueous phase advanced oxidation processes (AOPs). Computer based kinetic modeling requires a reaction pathway generator and predictions of associated reaction rate constants. Previously, we reported a reaction pathway generator that can enumerate the most important elementary reactions for aliphatic compounds. For the reaction rate constant predictor, we develop linear free energy relationships (LFERs) between aqueous phase literature-reported HO() reaction rate constants and theoretically calculated free energies of activation for H-atom abstraction from a C-H bond and HO() addition to alkenes. The theoretical method uses ab initio quantum mechanical calculations, Gaussian 1-3, for gas phase reactions and a solvation method, COSMO-RS theory, to estimate the impact of water. Theoretically calculated free energies of activation are found to be within approximately 3 kcal/mol of experimental values. Considering errors that arise from quantum mechanical calculations and experiments, this should be within the acceptable errors. The established LFERs are used to predict the HO() reaction rate constants within a factor of 5 from the experimental values. This approach may be applied to other reaction mechanisms to establish a library of rate constant predictions for kinetic modeling of AOPs. PMID:21410278

Minakata, Daisuke; Crittenden, John

2011-03-16

360

A kinetic-theory approach for computing chemical-reaction rates in upper-atmosphere hypersonic flows.  

PubMed

Recently proposed 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 investigated for chemical reactions occurring in upper-atmosphere hypersonic flows. The new models are in good agreement with the measured Arrhenius rates for near-equilibrium conditions and with both measured rates and other theoretical models for far-from-equilibrium conditions. Additionally, the new models are applied to representative combustion and ionization reactions and are in good agreement with available measurements and theoretical models. Thus, molecular-level chemistry modeling provides an accurate method for predicting equilibrium and nonequilibrium chemical-reaction rates in gases. PMID:19791885

Gallis, Michael A; Bond, Ryan B; Torczynski, John R

2009-09-28

361

Quantitative genetics of continuous reaction norms: thermal sensitivity of caterpillar growth rates.  

PubMed

A continuous reaction norm or performance curve represents a phenotypic trait of an individual or genotype in which the trait value may vary with some continuous environmental variable. We explore patterns of genetic variation in thermal performance curves of short-term caterpillar growth rate in a population of Pieris rapae. We compare multivariate methods, which treat performance at each test temperature as a distinct trait, with function-valued methods that treat a performance curve as a continuous function. Mean growth rate increased with increasing temperatures from 8 to 35 degrees C, was highest at 35 degrees C, and declined at 40 degrees C. There was substantial and significant variation among full-sib families in their thermal performance curves. Estimates of broad-sense genetic variances and covariances showed that genetic variance in growth rate increased more than 30-fold from low (8-11 degrees C) to high (35-40 degrees C) temperatures, even after differences in mean growth rate across temperatures were removed. Growth rate at 35 and 40 degrees C was negatively correlated genetically, suggesting a genetic trade-off in growth rate at these temperatures; this trade-off may represent either a generalist-specialist trade-off and/or variation in the optimal temperature for growth. The estimated genetic variance-covariance function (G function), the function-valued analog of the variance-covariance matrix (G matrix), was quite bumpy compared with the estimated G matrix; and results of principal component analyses of the G function were difficult to interpret. The use of orthogonal polynomials as the basis functions in current function-valued estimation methods may generate artifacts when the true G function has prominent local features, such as strong negative covariances at nearby temperatures (e.g. at 35 and 40 degrees C); this may be a particular issue for thermal performance curves and other highly nonlinear reaction norms. PMID:15341154

Kingsolver, Joel G; Ragland, Gregory J; Shlichta, J Gwen

2004-07-01

362

Contribution of the photo-Fenton reaction to hydroxyl radical formation rates in river and rain water samples.  

PubMed

The hydroxyl radical (OH radical) formation rates from the photo-Fenton reaction in river and rain water samples were determined by using deferoxamine mesylate (DFOM), which makes a stable and strong complex with Fe(III), resulting in a suppression of the photo-Fenton reaction. The difference between the OH radical formation rates with and without added DFOM denotes the rate from the photo-Fenton reaction. The photoformation rates from the photo-Fenton reaction were in the range of 0.7 - 45.8 x 10(-12) and 2.7 - 32.3 x 10(-12) M s(-1) in river and rain water samples, respectively. A strong positive correlation between the OH radical formation rate from the photo-Fenton reaction and the amount of fluorescent matter in river water suggests that fluorescent matter, such as humic substances, plays an important role in the photo-Fenton reaction. In rain water, direct photolysis of hydrogen peroxide was an important source of OH radicals as well as the photo-Fenton reaction. The contributions of the photo-Fenton reaction to the OH radical photoformation rates in river and rain water samples were in the ranges of 2 - 29 and 5 - 38%, respectively. Taking into account the photo-Fenton reaction, 33 - 110 (mean: 80) and 42 - 110 (mean: 84)% of OH radical sources in river and rain water samples, respectively, collected in Hiroshima prefecture were elucidated. PMID:17878592

Nakatani, Nobutake; Ueda, Marina; Shindo, Hirotaka; Takeda, Kazuhiko; Sakugawa, Hiroshi

2007-09-01

363

Temperature-dependent, relative-rate study of the reactions of 1- and 2-butoxyl radicals  

NASA Astrophysics Data System (ADS)

Alkoxyl radicals (RO\\cdot) are important intermediates in the chains of free radical reactions that constitute the gas-phase oxidation of volatile organic compounds (VOCs). Generally there are a number of different reaction pathways available to alkoxyl radicals and, depending on conditions of temperature and the structure of RO\\cdot, these may be in competition. The major reactions of RO\\cdot are (1) reaction with O_2 to yield a carbonyl product and a hydroperoxy, HO_2, radical, (2) decomposition to yield a carbonyl product and a radical fragment, and (3) isomerisation via a six-membered transition state to yield a d-hydroxylated radical species. Thus the chemistry of alkoxyl radicals determines the atmospheric impact of the oxidation of a given VOC, in terms of the immediate effects of closed-shell products, and as a result of the further chemistry of free radical products. HO_2 can react with NO to yield photolabile NO_2 (and hence contribute to photochemical ozone formation), and organic radical fragments act to propagate the oxidation chain of reactions. As ozone has been identified to be an important greenhouse gas in the upper troposphere/lower stratosphere (UTLS) region it is important to understand how temperature affects the relative rates of reactions (1) to (3), and thus how the presence of VOCs in the UTLS region affects the coupled chemistries of HO_x and O_3. In the present study, we have looked at the reactions of 1- and 2-butoxyl radicals (formed in the reaction of OH with butane) in terms of the relative rates of their bimolecular reactions with O_2 (1) and unimolecular processes (2,3). The two butoxyl radicals were studied separately and were formed directly from the photolysis of 1- or 2-butylnitrite. Experiments were carried out using a metre-long photochemical flow cell made of quartz. The temperature of the cell could be controlled and for the experiments carried out in the present study was operated between about 250 and 340 K. Reactant gases comprised N_2 and O_2 (from cylinders) and a dilute mixture of the relevant alkyl nitrite precursor in nitrogen (prepared in a 250 L, collapsible Tedlar bag), and were pre-mixed in a thermostatted manifold before entering the cell. By altering the relative flow rates of N_2 and O_2 (but keeping the sum of their flows constant) oxygen mixing ratios of between 0 and 80% were obtained. Reactant and end-product concentrations were measured by gas chromatography with flame ionisation detection (GC-FID). For 1-butoxyl, the competition between bimolecular reaction with O_2 (forming butanal, CH_3CH_2CH_2CHO) and isomerisation (a H atom shift to give CH_2CH_2CH_2CH_2OH) was assessed by measuring the yield of butanal as a function of the oxygen concentration. Experiments were carried out at six different temperatures and hence the temperature-dependence of the relative rate of iosmerisation to bimolecular reaction was obtained. For 2-butoxyl, the competition between bimolecular reaction with O_2 (forming butanone, CH_3CH(O)CH_2CH_3) and decomposition (forming ethanal, CH_3CHO, and an ethyl radical which is then expected to yield another molecule of ethanal) was assessed by measuring the yields of butanone and ethanal as a function of oxygen concentration. Experiments were again carried out at six different temperatures. By using temperature-dependent rate data, for the reactions of butoxyl radicals with O_2, existant in the literature, the Arrhenius parameters for the isomerisation of 1-butoxyl and for the decomposition of 2-butoxyl have been estimated. These data agree excellently with the limited data that has been reported in literature to date for these processes. The next immediate step in the study is to look at the chemistries of 1- and 2-butoxyl radicals formed from the reaction of OH with butane, in the presence of O_2 and NO.

Johnson, D.; Cassanelli, P.; Cox, R. A.

2003-04-01

364

Absolute rate constant for the reaction of diphenylcarbene with C[sub 60  

SciTech Connect

In the present work, laser flash photolysis (LFP) technique has been used to measure the absolute rate constant for the reaction of Ph[sub 2]C: with C[sub 60]. This reaction represents the overall process of inflation of buckminsterfullerene C[sub 60] to fulleroid Ph[sub 2]C[sub 61]n. Results of the study show that it is possible that the absorption signal observed following LFP of Ph[sub 2]CN[sub 2] in the presence of C[sub 60], is due to the initial cyclopropane adduct formed by carbene addition; however, the typical instability of norcaradienes and the spectral similarities with the thermally generated product of Ph[sub 2]CN[sub 2] and C[sub 60] strongly suggest that the absorption signal observed following LFP is that of the fulleroid Ph[sub 2]C[sub 61]. 18 refs., 2 figs.

Chateauneuf, J.E. (Univ. of Notre Dame, IN (United States))

1995-03-08

365

Estimates of neutron reaction rates in three portable He-3 proportional counters  

SciTech Connect

The goal of this study is to obtain Monte Carlo estimates of neutron reaction rates for the {sup 3}He(n,p){sup 3}H reaction in two portable He-3 proportional counters in several configurations to quantify contributions from the environment, and optimize the tube characteristics. The smallest tube (0.5-inch diameter, 2-inch long, P = 10 atm) will not meet requirements. The largest tube (1-inch diameter, 4-inch long, P = 6 or 10 atm) will meet requirements and the tube length could be decreased to 2-inch at 6 atm and 1-inch at 10 atm. The 'medium' tube (3/4-inch diameter, 2-inch long, P = 10 atm) will meet requirements for the parallelepiped body, but will not for the cylindrical body.

Descalle, M; Labov, S

2007-03-01

366

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

367

Structural and dynamical control of the reaction rate in protein electron transfer  

NASA Astrophysics Data System (ADS)

Electron transfer (ET) reactions in proteins are key steps in many vital bioenergetic processes, and the reaction rate is known to be highly sensitive to the protein structure in some cases. For most bioenergetic reactions, as described by the Fermi Golden rule, the rate is proportional to a product of the average square of the effective electronic donor to acceptor coupling and a Franck-Condon factor, which accounts for the nuclear control of the energy gap. The nuclear factor is reasonably well described in Marcus theory and its modifications, and this work is focused on the mechanisms that control the effective coupling. About ten years ago, the Pathways model described for the first time how protein environment may control the effective coupling. In this work, a novel theoretical approach is developed to explore the mechanisms of structural and dynamical control beyond the qualitative level of the Pathways model. In Chapter 1, the assumptions of the Pathways model, its limitations and effects of the structure and the electronic Hamiltonian are investigated for model chain-like bridges using the Dyson's equations. In Chapter II, the framework to explore the sensitivity of the effective coupling to quality of the electronic Hamiltonian, the interference among the dominant pathways and the bridge dynamics is presented. This analysis employs the Green's function technique and includes combined molecular dynamics and electronic structure calculations. Finally, in Chapter III, this framework is tested on the bacterial photosynthetic reaction center, and the mechanisms of the structural and dynamical control for different ET steps are discussed.

Balabin, Ilya A.

368

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

369

Improving the 33S(p,?)34Cl Reaction Rate for Models of Classical Nova Explosions  

NASA Astrophysics Data System (ADS)

Reduced uncertainty in the thermonuclear rate of the 33S(p,?)34Cl reaction would help to improve our understanding of nucleosynthesis in classical nova explosions. At present, models are generally in concordance with observations that nuclei up to roughly the calcium region may be produced in these explosive phenomena; better knowledge of this rate would help with the quantitative interpretation of nova observations over the S-Ca mass region, and contribute towards the firm establishment of a nucleosynthetic endpoint. As well, models find that the ejecta of nova explosions on massive oxygen-neon white dwarfs may contain as much as 150 times the solar abundance of 33S. This characteristic isotopic signature of a nova explosion could possibly be observed through the analysis of microscopic grains formed in the environment surrounding a nova and later embedded within primitive meteorites. An improved 33S(p,?)34Cl rate (the principal destruction mechanism for 33S in novae) would help to ensure a robust model prediction for the amount of 33S that may be produced. Finally, constraining this rate could confirm or rule out the decay of an isomeric state of 34Cl (Ex = 146 keV, t1/2 = 32 m) as a source for observable gamma-rays from novae. We have performed several complementary experiments dedicated to improving our knowledge of the 33S(p,?)34Cl rate, using both indirect methods (measurement of the 34S(3He,t)34Cl and 33S(3He,d)34Cl reactions with the Munich Q3D spectrograph) and direct methods (in normal kinematics at CENPA, University of Washington, and in inverse kinematics with the DRAGON recoil mass separator at TRIUMF). Our results will be used with nova models to facilitate comparisons of model predictions with present and future nova observables.

Parikh, A.; Faestermann, Th.; Krcken, R.; Bildstein, V.; Bishop, S.; Eppinger, K.; Herlitzius, C.; Lepyoshkina, O.; Maierbeck, P.; Seiler, D.; Wimmer, K.; Hertenberger, R.; Wirth, H.-F.; Fallis, J.; Hager, U.; Hutcheon, D.; Ruiz, Ch.; Buchmann, L.; Ottewell, D.; Freeman, B.; Wrede, Ch.; Garca, A.; Delbridge, B.; Knecht, A.; Sallaska, A.; Chen, A. A.; Clark, J. A.; Deibel, C. M.; Fulton, B.; Laird, A.; Greife, U.; Guo, B.; Li, E.; Li, Z.; Lian, G.; Wang, Y.; Liu, W.; Parker, P. D.; Setoodehnia, K.

2011-10-01

370

Influence of a New 17-F(p, gamma)18-Ne Reaction Rate on Nova Nucleosynthesis  

NASA Astrophysics Data System (ADS)

Proton capture by 17F plays an important role in nova explosions. A revised rate for this reaction has been determined based on measurements of the 1H (17F,p)17F excitation function using a radioactive 17F beam at ORNL's Holifield Radioactive Ion Beam Facility. We have used this new rate in calculations of the nucleosynthesis resulting from nova outbursts on the surfaces of 1.25 M and 1.35 M solar mass ONeMg white dwarfs (WD) and a 1.00 solar mass CO WD. We find that the new reaction rate changes the abundances of some nuclides (e.g. 17O) synthesized in the innermost regions of the explosion on the 1.35 solar mass WD by more than a factor of 10,000 compared to some previous estimates, and by more than factor of 3 when the entire exploding envelope is considered. The calculations for the 1.25 solar mass WD nova show that this new rate changes the abundances of some nuclides synthesized in the innermost regions by more than a factor of 600 compared to some previous estimates, and by more than a factor of 2 when the entire exploding envelope is considered. Calculations for the 1.00 solar mass WD nova show that this new rate changes the abundance of 18Ne by 20.6 all other nuclides. A comparison of model predictions with observation will also be discussed. ORNL Physics Divisioni is Managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725.

Parete-Koon, S.; Hix, W.; Smith, M.; Starrfield, S.; Bardayan, D.; Guidry, M.; Mezzacappa, A.

2002-12-01

371

Rate constants for the reactions of the hydroxyl radical with several partially fluorinated ethers  

Microsoft Academic Search

The authors have measured rate constants for the reactions of the hydroxyl radical, OH, with a number of fluorinated ethers. These ethers and their calculated atmospheric lifetimes for removal by OH (in years), estimated relative to CHCCl, are as follows: CF-O-CH, 3.0; CF-O-CHF, 19; CHF-O-CHF, 2.6; CFCH-O-CH, 0.1; CFCH-O-CHF, 5.2; cyclo-CFCHFCF-O, 26; cyclo-(CF)-O, >330.

Z. Zhang; R. D. Saini; M. J. Kurylo; R. E. Huie

1992-01-01

372

Thermal behaviors, nonisothermal decomposition reaction kinetics, thermal safety and burning rates of BTATz-CMDB propellant  

Microsoft Academic Search

The composite modified double base (CMDB) propellants (nos. RB0601 and RB0602) containing 3,6-bis (1H-1,2,3,4-tetrazol-5-yl-amino)-1,2,4,5-tetrazine (BTATz) without and with the ballistic modifier were prepared and their thermal behaviors, nonisothermal decomposition reaction kinetics, thermal safety and burning rates were investigated. The results show that there are three mass-loss stages in TG curve and two exothermic peaks in DSC curve for the BTATz-CMDB

Jian-Hua Yi; Feng-Qi Zhao; Bo-Zhou Wang; Qian Liu; Cheng Zhou; Rong-Zu Hu; Ying-Hui Ren; Si-Yu Xu; Kang-Zhen Xu; Xiao-Ning Ren

2010-01-01

373

Chemical reaction rates using the semiclassical Van-Vleck initialvalue representation  

SciTech Connect

A semiclassical IVR formulation using the Van-Vleck propagator has been used to calculate the flux correlation function and thereby reaction rate constants. This Van-Vleck formulation of the flux-flux correlation function is computationally as simple as the classical Wigner model. However unlike the latter, it has the ability to capture quantum interference/coherence effects. Classical trajectories are evolved starting from the dividing surface that separates reactants and products, and are evolved negatively in time. This formulation has been tested on model problems ranging from the Eckart barrier, double well to the collinear H + H{sub 2}.

Venkataraman, Charulatha; Miller, William H.

2006-11-29

374

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

375

RATE CONSTANT FOR THE REACTION NO2 + O->NO + O2  

SciTech Connect

By use of tunable laser flash photolysis and resonance fluorescence to detect primary products, the rate constant for the reaction 0({sup 3}P{sub J}) + NO{sub 2} {yields} O{sub 2} + NO has been measured. A value of k = 1.03 {+-} .01 x 10{sup -11} cm{sup 3} molecules{sup -1} sec{sup -1} was obtained at 296 K, which is a factor of 1.1 faster than the currently recommended value. The present work utilized in situ measurement of NO{sub 2} via laser induced fluorescence and generated oxygen atoms from five sources.

Magnotta, Frank; Johnston, Harold S.

1980-01-01

376

Elementary reaction rate measurements at high temperatures by tunable-laser flash-absorption  

SciTech Connect

The major objective of this program is to measure thermal rate coefficients and branching ratios of elementary reactions. To perform these measurements, we have constructed an ultrahigh-purity shock tube to generate temperatures between 1000 and 5500 K. The tunable-laser flash-absorption technique is used to measure the rate of change of the concentration of species which absorb below 50,000 cm{sup {minus}1} e.g.: OH, CH, and CH{sub 3}. This technique is being extended into the vacuum-ultraviolet spectral region where we can measure atomic species e.g.: H, D, C, O, and N; and diatomic species e.g.: O{sub 2}, CO, and OH.

Hessler, J.P.

1992-03-20

377

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

378

Solvent effects on the rate constants for reaction of trichloromethylperoxyl radicals with organic reductants  

SciTech Connect

Absolute rate constants for the reactions of trichloromethylperoxyl radicals with chloropromazine and trolox have been determined by pulse radiolysis in 16 different solvents. The rate constants were found to vary over two orders of magnitude (10[sup 7]-10[sup 9] L mol[sup [minus]1] s[sup [minus]1]) and to correlate with the Hildebrand solubility parameter (cohesive energy density) of the solvent better than with any other single solvent parameter. The correlation was satisfactory for ClPz and did not improve significantly by including additional parameters. For trolox, however, the correlation was relatively poor but was improved considerably by taking into account the basicity of the solvent. This effect is due to the transfer of a proton upon the oxidation of trolox. 21 refs., 3 figs., 2 tabs.

Alfassi, Z.B.; Huie, R.E.; Neta, P. (National Inst. of Standards and Technology, Gaithersburg, MD (United States))

1993-07-15

379

Fluorometric reaction rate method for determination of hydrogen perioxide at the nanomolar level  

SciTech Connect

A kinetic nonenzymatic method is proposed for determination of hydrogen peroxide at the nanomolar level and applied to the analysis of several types of coffee, tea, and milk samples. The method is based on the Mn(II)-catalyzed oxidation of 2-hydroxynaphthaldehyde thiosemicarbazone (HNTS) by H/sub 2/O/sub 2/. The reaction is monitored spectrofluorometrically by measuring the initial rate at the excitation and emission wavelength of the oxidation product. The kinetic study involved obtaining the rate law and evaluating and optimizing the chemical variables. Sallent features of the method are (a) a linear calibration graph is obtained from 50 to 2000 nM; (b) no enzymes are required; and (c) it tolerates moderate amounts of various metal ions.

Peinado, J.; Torbio, F.; Perez-Bendito, D.

1986-07-01

380

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

381

Rate constant for the reaction C2H5 + HBr ? C2H6 + Br.  

PubMed

RRKM theory has been employed to analyze the kinetics of the title reaction, in particular, the once-controversial negative activation energy. Stationary points along the reaction coordinate were characterized with coupled cluster theory combined with basis set extrapolation to the complete basis set limit. A shallow minimum, bound by 9.7 kJ?mol(-1) relative to C(2)H(5) + HBr, was located, with a very small energy barrier to dissociation to Br + C(2)H(6). The transition state is tight compared to the adduct. The influence of vibrational anharmonicity on the kinetics and thermochemistry of the title reaction were explored quantitatively. With adjustment of the adduct binding energy by ?4 kJ?mol(-1), the computed rate constants may be brought into agreement with most experimental data in the literature, including new room-temperature results described here. There are indications that at temperatures above those studied experimentally, the activation energy may switch from negative to positive. PMID:22268593

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

2012-02-17

382

Influence of oxygen flow rate on reaction rate of organic matter in leachate from aerated waste layer containing mainly incineration ash.  

PubMed

It is known that aeration reduces rapidly the concentration of organic matter in leachate. However, the oxygen flow rate required to attain a certain reaction rate of organic matter should be carefully estimated. In this study, using the oxygen ratio (the ratio of oxygen flow rate by aeration to oxygen consumption rate of waste layer) as a parameter, the reaction rate of organic matter in leachate from landfilled incineration ash and incombustible waste upon aeration was evaluated. Total organic carbon (TOC) in the leachate was reduced rapidly when the oxygen ratio was high. The decomposition rate exceeded the elution rate of TOC in the leachate from the waste layer for several days when the oxygen ratio was above 10(2). The results indicate that the oxygen ratio can be used as a parameter for the aeration operation in actual landfill sites, to rapidly stabilize organic matter in leachate. PMID:20591643

Asakura, Hiroshi; Endo, Kazuto; Yamada, Masato; Inoue, Yuzo; Ono, Yusaku; Ono, Yoshiro

2010-06-29

383

Temperature-induced gene expression associated with different thermal reaction norms for growth rate.  

PubMed

Although nearly all organisms are subject to fluctuating temperature regimes in their natural habitat, little is known about the genetics underlying the response to thermal conditions, and even less about the genetic differences that cause individual variation in thermal response. Here, we aim to elucidate possible pathways involved in temperature-induced phenotypic plasticity of growth rate. Our model organism is the collembolan Orchesella cincta that occurs in a wide variety of habitats and is known to be adapted to local thermal conditions. Because sequence information is lacking in O. cincta, we constructed cDNA libraries enriched for temperature-responsive genes using suppression subtractive hybridization. We compared gene expression of O. cincta with steep thermal reaction norms (high plasticity) to those with flat thermal reaction norms (low plasticity) for juvenile growth after exposure to a temperature switch composed of a cooling or a warming treatment. Using suppression subtractive hybridization, we found differential expression of ten nuclear genes, including several genes involved in energy metabolism, such as pantothenate kinase and carbonic anhydrase. In addition, seven mitochondrial genes were found in the cloned subtracted library, but further analysis showed this was caused by allelic variation in mitochondrial genes in our founder population, and that a specific haplotype was associated with high thermal responsiveness. Future work will focus on candidate genes from pathways such as the oxidative phosphorylation and biosynthesis of coenzyme A which are possibly involved in thermal responsiveness of juvenile growth rate. PMID:17886827

Ellers, Jacintha; Marin, Janine; Driessen, Gerard; van Straalen, Nico M

2008-03-15

384

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

385

Measurement of rate constants of elementary gas reactions of importance to upper atmosphere and combustion systems  

NASA Astrophysics Data System (ADS)

The vibrational energy transfer of highly excited HCl and HF was studied by the infrared chemiluminescence method in a series of five papers. HCl(v < or = 7) and HF(v < or = 7) were produced by fast generating reations, e.g., H + IC1 yields (v < or = 7) + I or H +F2 yields (< or = 7) + F in large excess He which relaxes the rotational but not the vibrational excitation. With various added quencher gases, about 200 rate constants for stepwise vibrational energy transfer were measured. They tend to increase rapidly with increasing v, often independent of the vibrational energy defect. For HF (v) + HF (O), self-relaxation, the V-V channel decreases from 55% at v=2 to zero at v > 5 even though the relaxation rate constant rises as v to the 2.7 power. A versatile flow reactor system was built that features three detection methods (laser-induced fluorescence, vacuum u.v. resonance fluorescence, and modulated molecular beam mass spectrometry) plus upstream radical production by IR laser multiphoton decomposition. The NH2 + NO and CH3O + NO2 reactions were studied successfully. The latter was found to have a recombination (CH3ONO2) and disproportionation (CH2O + HNO2) channel and both were measured indirectly. The 18O + NO or O2 isotope exchange reactions were measured, and the results compared with measured and calculated high-pressure limits of the respective recombinations and with NO vibrational relaxation.

Kaufman, F.

1985-05-01

386

Observation of l-dependent rate constants for ion production in Rydberg electron transfer reactions  

SciTech Connect

Relative rate constants for negative ion production have been measured for the Rydberg electron transfer reactions Cs**(ns,np,nd) +SF/sub 6/..-->..Cs/sup +/+SF/sup -//sub 6/ and Cs**(ns,np,nd) +CCl/sub 4/..-->..Cs/sup +/+Cl/sup -/+CCl/sub 3/. We find that rate constant for production of Cl/sup -/ from CCl/sub 4/ are dependent upon the angular momentum of the Rydberg electron, with k/sub n//sub d/>k/sub n//sub s/ for values of n as large as 40. Preliminary measurements for potassium (ns,nd) Rydberg atoms show that the l dependence for K is much smaller than that for Cs. These results are inconsistent with the ''free electron model'' used to describe these reactions. It is suggested that the observed l dependence is related to postattachment interactions with the Rydberg atom core which affect the survival of the Cl/sup -/ ions.

Carman H.S. Jr.; Klots, C.E.; Compton, R.N.

1989-03-01

387

Absolute Rate Constants for the Reactions of Primary Alkyl Radicals with Aromatic Amines(1).  

PubMed

Hydrogen abstraction from diarylamines (4-X-C(6)H(4))(2)NH [X = H, CH(3), C(8)H(17), CH(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 from n-dodecane (k(373K) = 3.5 x 10(3) M(-)(1) s(-)(1)), diphenylamine (k(373K) = 1.3 x 10(6) M(-)(1) s(-)(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 the deuterium kinetic isotope effect,k(NH)/k(ND), was found to be 2.3 at 373 K. PMID:11667229

Burton, Alan; Ingold, K. U.; Walton, J. C.

1996-05-31

388

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)

1996-05-31

389

High accuracy measurements of OH reaction rate constants and IR absorption spectra: substituted 2-propanols.  

PubMed

Rate constants for the gas phase reactions of OH radicals with 2-propanol and three fluorine substituted 2-propanols, (CH(3))(2)CHOH (k(0)), (CF(3))(2)CHOH (k(1)), (CF(3))(2)C(OH)CH(3) (k(2)), and (CF(3))(3)COH (k(3)), were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions: k(0)(T) = 1.46 10(-11) exp{-883/T} + 1.30 10(-12) exp{+371/T} cm(3) molecule(-1) s(-1); k(1)(T) = 1.19 10(-12) exp{-1207/T} + 7.85 10(-16) exp{+502/T } cm(3) molecule(-1) s(-1); k(2)(T) = 1.68 10(-12) exp{-1718/T} + 7.32 10(-16) exp{+371/T} cm(3) molecule(-1) s(-1); k(3)(T) = 3.0 10(-20) (T/298)(11.3) exp{+3060/T} cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 2.4 days and 1.9, 6.3, and 46 years, respectively. UV absorption cross sections were measured between 160 and 200 nm. The IR absorption cross sections of the three fluorinated compounds were measured between 450 and 1900 cm(-1), and their global warming potentials were estimated. PMID:22372969

Orkin, Vladimir L; Khamaganov, Victor G; Kurylo, Michael J

2012-04-02

390

Reaction rate theory: what it was, where is it today, and where is it going?  

PubMed

A brief history is presented, outlining the development of rate theory during the past century. Starting from Arrhenius [Z. Phys. Chem. 4, 226 (1889)], we follow especially the formulation of transition state theory by Wigner [Z. Phys. Chem. Abt. B 19, 203 (1932)] and Eyring [J. Chem. Phys. 3, 107 (1935)]. Transition state theory (TST) made it possible to obtain quick estimates for reaction rates for a broad variety of processes even during the days when sophisticated computers were not available. Arrhenius' suggestion that a transition state exists which is intermediate between reactants and products was central to the development of rate theory. Although Wigner gave an abstract definition of the transition state as a surface of minimal unidirectional flux, it took almost half of a century until the transition state was precisely defined by Pechukas [Dynamics of Molecular Collisions B, edited by W. H. Miller (Plenum, New York, 1976)], but even this only in the realm of classical mechanics. Eyring, considered by many to be the father of TST, never resolved the question as to the definition of the activation energy for which Arrhenius became famous. In 1978, Chandler [J. Chem. Phys. 68, 2959 (1978)] finally showed that especially when considering condensed phases, the activation energy is a free energy, it is the barrier height in the potential of mean force felt by the reacting system. Parallel to the development of rate theory in the chemistry community, Kramers published in 1940 [Physica (Amsterdam) 7, 284 (1940)] a seminal paper on the relation between Einstein's theory of Brownian motion [Einstein, Ann. Phys. 17, 549 (1905)] and rate theory. Kramers' paper provided a solution for the effect of friction on reaction rates but left us also with some challenges. He could not derive a uniform expression for the rate, valid for all values of the friction coefficient, known as the Kramers turnover problem. He also did not establish the connection between his approach and the TST developed by the chemistry community. For many years, Kramers' theory was considered as providing a dynamic correction to the thermodynamic TST. Both of these questions were resolved in the 1980s when Pollak [J. Chem. Phys. 85, 865 (1986)] showed that Kramers' expression in the moderate to strong friction regime could be derived from TST, provided that the bath, which is the source of the friction, is handled at the same level as the system which is observed. This then led to the Mel'nikov-Pollak-Grabert-Hanggi [Mel'nikov and Meshkov, J. Chem. Phys. 85, 1018 (1986); Pollak, Grabert, and Hanggi, ibid. 91, 4073 (1989)] solution of the turnover problem posed by Kramers. Although classical rate theory reached a high level of maturity, its quantum analog leaves the theorist with serious challenges to this very day. As noted by Wigner [Trans. Faraday Soc. 34, 29 (1938)], TST is an inherently classical theory. A definite quantum TST has not been formulated to date although some very useful approximate quantum rate theories have been invented. The successes and challenges facing quantum rate theory are outlined. An open problem which is being investigated intensively is rate theory away from equilibrium. TST is no longer valid and cannot even serve as a conceptual guide for understanding the critical factors which determine rates away from equilibrium. The nonequilibrium quantum theory is even less well developed than the classical, and suffers from the fact that even today, we do not know how to solve the real time quantum dynamics for systems with "many" degrees of freedom. PMID:16035918

Pollak, Eli; Talkner, Peter

2005-06-01

391

Ground Reaction Forces and Loading Rates Associated with Parkour and Traditional Drop Landing Techniques  

PubMed Central

Due to the relative infancy of Parkour there is currently a lack of empirical evidence on which to base specific technique instruction upon. The purpose of this study was to compare the ground reaction forces and loading rates involved in two Parkour landing techniques encouraged by local Parkour instructors and a traditional landing technique recommended in the literature. Ten male participants performed three different drop landing techniques (Parkour precision, Parkour roll, and traditional) onto a force plate. Compared to the traditional technique the Parkour precision technique demonstrated significantly less maximal vertical landing force (38%, p < 0.01, ES = 1.76) and landing loading rate (54%, p < 0.01, ES = 1.22). Similarly, less maximal vertical landing force (43%, p < 0.01, ES = 2.04) and landing loading rate (63%, p < 0.01, ES = 1.54) were observed in the Parkour roll technique compared to the traditional technique. It is unclear whether or not the Parkour precision technique produced lower landing forces and loading rates than the Parkour roll technique as no significant differences were found. The landing techniques encouraged by local Parkour instructors such as the precision and roll appear to be more appropriate for Parkour practitioners to perform than a traditional landing technique due to the lower landing forces and loading rates experienced. Key points Parkour precision and Parkour roll landings were found to be safer than a traditional landing technique, resulting in lower maximal vertical forces, slower times to maximal vertical force and ultimately lesser loading rates. Parkour roll may be more appropriate (safer) to utilize than the Parkour precision during Parkour landing scenarios. The Parkour landing techniques investigated n this study may be beneficial for landing by non-Parkour practitioners in everyday life.

Puddle, Damien L.; Maulder, Peter S.

2013-01-01

392

The effect of internal energy on chemical reaction rates as predicted by Bird's quantum-kinetic model  

NASA Astrophysics Data System (ADS)

The effect of non-equilibrium internal energy excitation on the reaction rates predicted by Bird's Quantum-Kinetic (Q-K) model for dissociation and exchange reactions is analyzed. The effect of vibrational non-equilibrium is treated explicitly by the Q-K model. The effect of rotational non-equilibrium is introduced as a perturbation to the effect of vibrational non-equilibrium in chemical reactions. For dissociation reactions, a small but measurable improvement in the rates is observed. For exchange reactions, the change is negligible. These findings are in agreement with experimental observations and theoretical predictions. The results from one-dimensional stagnation-streamline and two-dimensional axi-symmetric DSMC code implementations of the original and modified Q-K models are compared for a typical re-entry flow. The influence of rotational non-equilibrium in promoting chemical reactions is seen to be small for this type of flow.

Gallis, Michael A.; Torczynski, John R.

2012-11-01

393

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

394

Reaction rate vs. transport limited dissolution during carbonate acidizing: Application of network model  

SciTech Connect

Coreflood experiments have been performed and a two dimensional network model has been developed to study the flow, dissolution, and permeability increase in carbonates during acid stimulation treatments. The goal is to be able to predict the behaviour of the random flow channels, called wormholes, that form in the rock as a result of acid attack. Understanding this channeling phenomenon is essential in predicting acid penetration distances and the resulting injectivity or productivity increases during matrix acidizing, or in understanding acid leakoff behaviour during fracture acidizing. Until now, relatively little has been known about the wormholing phenomenon because of the stochastic nature of channel formation and the inherent difficulty in modelling the random behaviour. Model predictions and experimental coreflood results are compared for linear systems. The model predicts wormholing and the effects of acidizing parameters such as fluid velocity, mineral dissolution rate, and rock properties on channel structure and the corresponding rate of channel propagation. Experimental data and model prediction support the conclusion that wormholing is necessary for matrix treatment effectiveness, but does not occur for the extremes of high and low values of the Damkohler number for flow and reaction, which controls the process. The model thus accounts for the ineffectiveness of aqueous hydrochloric acid for matrix treatments in limestone due to rapid acid spending, and for the possible ineffectiveness of treatments in dolomite due to mineral dissolution rates that are too slow to promote channel formation.

Hoefner, M.L.; Fogler, H.S.

1986-01-01

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.

2013-09-01

396

Field measurement of high temperature bulk reaction rates II: Interpretation of results from a field site near Simplon Pass, Switzerland  

Microsoft Academic Search

Bulk metamorphic reaction rates have been measured using Sr iso- topes as tracers of reactive transport processes at a field site near Simplon Pass, Switzerland, employing the technique described in Part I of this study (Baxter and DePaolo, 2002a). The reaction rate, R, inferred, similar to the value first reported in Baxter and DePaolo (2000), is 1.40.4 1.1 107 yr1

ETHAN F. BAXTER; DONALD J. DePAOLO

2002-01-01

397

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

398

Path-integral virial estimator for reaction-rate calculation based on the quantum instanton approximation.  

PubMed

The quantum instanton approximation is a type of quantum transition-state theory that calculates the chemical reaction rate using the reactive flux correlation function and its low-order derivatives at time zero. Here we present several path-integral estimators for the latter quantities, which characterize the initial decay profile of the flux correlation function. As with the internal energy or heat-capacity calculation, different estimators yield different variances (and therefore different convergence properties) in a Monte Carlo calculation. Here we obtain a virial (-type) estimator by using a coordinate scaling procedure rather than integration by parts, which allows more computational benefits. We also consider two different methods for treating the flux operator, i.e., local-path and global-path approaches, in which the latter achieves a smaller variance at the cost of using second-order potential derivatives. Numerical tests are performed for a one-dimensional Eckart barrier and a model proton transfer reaction in a polar solvent, which illustrates the reduced variance of the virial estimator over the corresponding thermodynamic estimator. PMID:16512703

Yang, Sandy; Yamamoto, Takeshi; Miller, William H

2006-02-28

399

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

400

Rate constants for the reactions of the hydroxyl radical with several partially fluorinated ethers  

SciTech Connect

The authors have measured rate constants for the reactions of the hydroxyl radical, OH, with a number of fluorinated ethers. These ethers and their calculated atmospheric lifetimes for removal by OH (in years), estimated relative to CH{sub 3}CCl{sub 3}, are as follows: CF{sub 3}-O-CH{sub 3}, 3.0; CF{sub 3}-O-CHF{sub 2}, 19; CHF{sub 2}-O-CHF{sub 2}, 2.6; CF{sub 3}CH{sub 2}-O-CH{sub 3}, 0.1; CF{sub 3}CH{sub 2}-O-CHF{sub 2}, 5.2; cyclo-CF{sub 2}CHFCF{sub 2}-O, 26; cyclo-(CF{sub 2}){sub 3}-O, >330.

Zhang, Z.; Saini, R.D.; Kurylo, M.J.; Huie, R.E. [National Institute of Standards and Technology, Gaithersburg, MD (United States)

1992-11-12

401

Elementary reaction rate measurements at high temperatures by tunable-laser flash-absorption  

SciTech Connect

The major objective of this program is to measure thermal rate coefficients and branching ratios of elementary reactions. An ultrahigh-purity shock tube produces experimental temperatures between 1000 and 5500 K. The tunable-laser flash-absorption technique is used to measure the time dependence of the concentration of species which absorb light in the ultraviolet spectral region (e.g., OH, and CH{sub 3}). In addition, calculational techniques for designing experiments and analyzing data have been developed. In the immediate future our technique will be extended into the vacuum-ultraviolet spectral region where we can measure atomic species (e.g., H, D, C, O, and N) and diatomic species (e.g., O{sub 2}, CO, and OH).

Hessler, J.P.

1996-05-01

402

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

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

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

405

Solvent effects on the rates and mechanisms of reaction of phenols with free radicals.  

PubMed

The rates of formal abstraction of phenolic hydrogen atoms by free radicals, Y* + ArOH --> YH + ArO*, are profoundly influenced by the hydrogen-bond-accepting and anion-solvation abilities of solvents, by the electron affinities and reactivities (Y-H bond dissociation enthalpies) of radicals, and by the phenol's ring substituents. These apparently simple reactions can occur by at least three different, nonexclusive mechanisms: hydrogen atom transfer, proton-coupled electron transfer, and sequential proton-loss electron transfer. The delicate balance among these mechanisms depends on both the environment and the reactants. The main features of these mechanisms are described, together with some interesting kinetic consequences. PMID:17370994

Litwinienko, Grzegorz; Ingold, K U

2007-03-01

406

Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption.  

PubMed

The development of hydrogen-based energy sources as viable alternatives to fossil-fuel technologies has revolutionized clean energy production using fuel cells. However, to date, the slow rate of the hydrogen oxidation reaction (HOR) in alkaline environments has hindered advances in alkaline fuel cell systems. Here, we address this by studying the trends in the activity of the HOR in alkaline environments. We demonstrate that it can be enhanced more than fivefold compared to state-of-the-art platinum catalysts. The maximum activity is found for materials (Ir and Pt?.?Ru?.?) with an optimal balance between the active sites that are required for the adsorption/dissociation of H? and for the adsorption of hydroxyl species (OHad). We propose that the more oxophilic sites on Ir (defects) and PtRu material (Ru atoms) electrodes facilitate the adsorption of OHad species. Those then react with the hydrogen intermediates (Had) that are adsorbed on more noble surface sites. PMID:23511418

Strmcnik, Dusan; Uchimura, Masanobu; Wang, Chao; Subbaraman, Ram; Danilovic, Nemanja; van der Vliet, Dennis; Paulikas, Arvydas P; Stamenkovic, Vojislav R; Markovic, Nenad M

2013-02-24

407

Temperature dependence of the rate coefficients for the reactions of atomic bromine with toluene, tetrahydrofuran, and tetrahydropyran.  

PubMed

The rate coefficients for the reactions of atomic bromine with toluene, tetrahydrofuran, and tetrahydropyran were measured from approximately 295 to 362 K using the relative rate method. Iso-octane was used as the reference compound for the reaction with toluene, and iso-octane and toluene were used as the reference compounds for the reaction with tetrahydrofuran; tetrahydrofuran was used as the reference compound for the reaction with tetrahydropyran. The rate coefficients were found to be unaffected by changes in pressure and oxygen concentration. The rate coefficient ratios were converted to absolute values using the absolute rate coefficient for the reaction of Br with the reference compound. The absolute rate coefficients, in the units cm(3) molecule(-1) s(-1), for the reaction of Br with toluene are given by k(T) = (3.7 +/- 1.7) x 10(-12) exp(-(1.63 +/- 0.15) x 10(3)/T), for the reaction of Br with tetrahydrofuran by k(T) = (3.7 +/- 2.7) x 10(-10) exp(-(2.20 +/- 0.22) x 10(3)/T), and for the reaction of Br with tetrahydropyran by k(T) = (3.6 +/- 1.8) x 10(-10) exp(-(2.35 +/- 0.16) x 10(3)/T). The uncertainties represent one standard deviation. The Arrhenius parameters for these reactions are compared with results in the literature for dimethyl ether, diethyl ether, and a series of saturated hydrocarbons, and the effects of structural variation on these parameters are identified. PMID:19555110

Giri, Binod Raj; Roscoe, John M

2009-07-16

408

Reinterpretation of rate data for the reaction of ClF and F{sub 2}  

SciTech Connect

Experimental data reported by Reiner and Simmons on the rate of recombination of ClF + F{sub 2} have been reinterpreted to yield a single rate equation. This reinterpretation assumes that the recombination reaction is surface area dependent and is first order in partial pressure of each of F{sub 2} and ClF. The recombination rate equation used fits quite well three data sets which varied significantly in available NiF{sub 2} surface area. Resulting equation is dn(ClF{sub 3})/dt = A S P(F{sub 2})P(ClF)(1 {minus} {Theta})e-{sup {minus}E/RT}, where dn(ClF{sub 3})/dt is the rate of formation of ClF{sub 3}in mol/s, S is the surface area of NiF{sub 2} catalyst in cm{sup 2}, P(X) is the partial pressure of species X in Torr, R is the gas constant (1.987 cal/mol-K), and T is the temperature in Kelvin. The pre-exponential A, and activation energy E are given by: A = 2.853 {times} 10{sup {minus}8} mole/s-cm{sup 2} {minus} Torr{sup 2}, and E = 11665 cal/mole, and the term (1 {minus} {Theta}), a product surface-coverage related inhibition factor, is given by: (1 {minus} {Theta}) = (1 + 0.184 P(ClF{sub 3})){sup {minus}1} (but (1 {minus} {Theta}) is never allowed to be less than 16%)

Trowbridge, L.D.

1994-01-01

409

Experimental Determination of Olivine Reaction Rim Growth Rates in the Lassen Peak, CA Magma Chamber  

NASA Astrophysics Data System (ADS)

A series of hydrothermal (high-pressure and -temperature) experiments have been performed on starting materials consisting of ~5 wt% of Fo85 olivine separated from a spinel lherzolite xenolith (UM-5) from Kilbourne Hole, NM combined with powdered natural dacite pumice from the 1915 eruption of Lassen Peak, CA. The dacite represents the silicic end member (67.8 wt% SiO2) of mixed dacite and andesite magmas present in the 1915 Lassen Peak eruptive products. The olivine represents xenocrystic grains from the admixed andesite that combined with the dacite magma prior to the eruption. In the natural dacitic eruptive products, xenocrystic olivine grains display morphologically complex reaction rims of orthopyroxene. These rims are interpreted to have grown during magma mixing/mingling prior to the eruption. The growth rate of these rims may be used to constrain the length of time between magma recharge and eruption. Time series experiments were performed to reproduce olivine reaction rims within the dacite. Experiments were performed with run durations of 50, 100, 200, 400, and 600 hours at 50 MPa and 825C and 875C. Water-saturated run conditions at the Ni-NiO buffer represent plausible dacite equilibration conditions based on earlier phase equilibria experiments. Preliminary measurements of orthopyroxene rim widths measured on two olivine grains from experiments run at 50 MPa and 825C for 50 and 600 hours show an increase in rim width from 2.21.8 ?m to 3.32.5 ?m, respectively (average and 1? standard deviation of 490 and 651 linear measurements). In the 50-hour experiment, rim widths range from 0 to 13.4 ?m, whereas in the 600 hour run, widths are 0.63-23.1 ?m. The 600 hour run also has a larger percentage of width measurements above 5 ?m (24.8% of measured widths, compared to 2.8% of measurements in the 50-hour experiment.) In the 50-hour experiment, 21.0% of the rim width measurements are 1 ?m or less, whereas only 6.4% of measured olivine reaction rim widths are <1?m in the 600-hour run. Additional measurements on these and other experiments are under way using the linear measurement technique applied here, as well as other methods to estimate rim width and area. Experimentally determined rim growth rates will be compared with rims on the natural samples in order to constrain the timing and duration of magma mixing prior to the eruption.

Schwab, B. E.; Castro, J. M.

2007-12-01

410

THE LAMINAR FLOW TUBULAR REACTOR WITH HOMOGENEOUS AND HETEROGENEOUS REACTIONS I. Integral Equations for Diverse Reaction Rate Regimes  

Microsoft Academic Search

Design equations for non-isothermal Laminar Flow Tubular Reactors (LFTRs) with homogeneous and heterogeneous - at the reactor wall - reactions with arbitrary kinetic equations have been satisfactorily treated transforming the original P.D.E. problem into a system of integral equations. The kernels of the integral operators are related to an eigenvalue problem which does not depend on the kinetic equations; this

RICARDO J. GRAU; MARA I. CABRERA; ALBERTO E. CASSANO

2001-01-01

411

Influence of salts, including amino acids, on the rate and outcome of the in-water prebiotic reactions  

NASA Astrophysics Data System (ADS)

Most prebiotic reactions are hypothesized to have occurred in water. However, organic compounds in general are not water-soluble. This has created a great problem for prebiotic chemistry. However, it has been shown recently that many organic materials which are not soluble in water are still capable of reacting in water, often at faster rates than in the organic solvents. This has provided a new era in the study of the prebiotic reactions. Simulated prebiotic reactions are typically performed in water. However, presence of inorganic salts and the amino acids was ubiquitous in the aqueous media on the prebiotic Earth. We thus address the influence of common salts and selected water-soluble amino acids on the rate and outcome of selected prebiotic reactions in water. We focus on Diels-Alder (DA) reaction and multicomponent Passerini reaction (PR), as models for other prebiotic reactions. Some of the results came from our laboratory, and others are from the literature. DA reaction is influenced by salts and by selected water-soluble amino acids, but generally not to a large extent. The PR is also influenced by salts, but not dramatically. However, concentrations of salts and amino acids could have been extremely high in various local niches on the early Earth, and the influence of such concentrations on these prebiotic reactions awaits further study.

Kolb, Vera M.

2013-09-01

412

Estimating rate constants of heterogeneous catalytic reactions without supposition of rate determining surface steps an application of a genetic algorithm  

Microsoft Academic Search

A method providing initial estimates for rate constants of nonlinear chemical kinetics is proposed. This method is based on a genetic algorithm. The particular structure of the genetic algorithm adapted to the problem of estimating rate constants is introduced. The rate and certainty of convergence of the estimation procedure is analysed illustrating the proposed strategy by example of its application

Dorit Wolf; Ralf Moros

1997-01-01

413

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 states. We measured the {sup 28}Si(p,t){sup 26}Si reaction with a high-resolution spectrometer to identify resonance levels in {sup 26}Si above the {alpha}-emission threshold at 9.164 MeV excitation energy. These resonance levels are used to calculate the stellar reaction rate of the {sup 22}Mg({alpha},p){sup 25}Al reaction and to test the validity of the statistical assumption.

Matic, A.; Berg, A. M. van den; Harakeh, M. N.; Woertche, H. J.; Beard, M.; Berg, G. P. A.; 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, PO Wits, Johannesburg 2050 (South Africa); Department of Physics, Kyushu University, Fukuoka 812-8581 (Japan); Physics Division, Argonne National Laboratory, 9700 S. Cass. Avenue, Argonne, Illinois 60439 (United States); Department of Physics, University of Surrey (United Kingdom); National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824 (United States)

2011-08-15

414

Rate of reaction of hydrogen sulfide-carbonyl sulfide mixtures with fully calcined dolomite  

SciTech Connect

Kinetic data are obtained by a gravimetric technique for rates of reaction of calcium oxide in fully calcined dolomite with hydrogen sulfide and hydrogen sulfide-carbonyl sulfide mixtures. The data are presented as values for a factor k defined by d(CAO) = -k (CAO) dt. At 600, 700, and 800 degrees C with (H/sub 2/S) from 0.5% to 5.0% by volume and (H/sub 2/S)/(COS) = 20 for mixtures, expressions for k show apparent rate constants and the dependence on sulfurous gas concentration. For example, at 700 degrees C, k = 1.43 x 10/sup -4/ (h2s) 1.06/S and k = 1.70 x 10/sup -4/ (H/sub 2/S + COS) 1.00/s. Since the date show first-order dependence on calcium oxide, k's for H/sub 2/S alone as the sulfurous gas and h2s-cos mixtures can be obtained for the same sample, free from scatter due to variations from sample to sample. Addition of values for k from runs with H/sub 2/S as the only sulfurous gas and runs with COS as the only sulfurous gas are compared to measurements with actual mixtures. K's for the mixtures are approximately 30% higher than the sum of the appropriate separate values.

Kamath, V.S.; Petrie, T.W.

1981-01-01

415

The influence of the reaction rate of explosives on blast effects  

NASA Astrophysics Data System (ADS)

Many of the tools developed for the study of shock physics for weapons related work have direct application in industry. For example, instrumentation developed in the study of shock physics has been used to good advantage in the commercial explosives industry. The development of additional sensors is ongoing, though the manner in which these sensors are used and the data analyzed are slightly different because of the application. On the other hand, some of the tools, procedures, and methodologies need some changes in order to provide the best utility to the industrial community. Take reactive hydrocode modeling as an example. The use of reaction rate laws which are very good at modeling the ideal explosives used in shock physics are not universally applicable to industry uses of explosives because of the need to use the energy release rates to tailor the explosive to the job at hand. For example, warheads typically use ideal explosives which release their energy very rapidly and under shock conditions while the commercial composite explosives needed for open pit mining must release the major part of their energy very slowly in order to allow time to over come the inertia of the overburden in heaving it off the ore body. These, and other applications of shock physics techniques will be discussed.