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1

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

2

Increasing PCB Radiolysis Rates in Transformer Oil  

SciTech Connect

The kinetics of Aroclor 1242 radiolysis in transformer oil, using high-energy electrons, was found to be analogous to that previously measured for individual polychlorinated biphenyl (PCB) congeners irradiated with ó-rays. The plot of the pseudo-first-order rate constant for PCB decomposition versus initial PCB concentration is a power function, with high rate constants for low concentrations. The addition of alkaline isopropanol to transformer oil was found to increase the pseudo-first-order rate constant for PCB decomposition. The rate constant under these conditions is independent of concentration. This may be explained by the establishment of chain reaction dechlorination in the oil.

Mincher, Bruce Jay

2002-11-01

3

Increasing PCB radiolysis rates in transformer oil  

NASA Astrophysics Data System (ADS)

The kinetics of Aroclor 1242 radiolysis in transformer oil, using high-energy electrons, was found to be analogous to that previously measured for individual polychlorinated biphenyl (PCB) congeners irradiated with ?-rays. The plot of the pseudo-first-order rate constant for PCB decomposition versus initial PCB concentration is a power function, with high rate constants for low concentrations. The addition of alkaline isopropanol to transformer oil was found to increase the pseudo-first-order rate constant for PCB decomposition. The rate constant under these conditions is independent of concentration. This may be explained by the establishment of chain reaction dechlorination in the oil.

Mincher, Bruce J.; Brey, Richard R.; Rodriguez, René G.; Pristupa, Scott; Ruhter, Aaron

2002-11-01

4

A Substituent Effects Study Reveals the Kinetic Pathway for an Interfacial Reaction  

E-print Network

effects study to understand the mechanistic basis for an interfacial Diels-Alder reaction that does not proceed with standard second-order kinetics. Cyclopentadiene (Cp) undergoes a Diels-Alder reaction with a chemisorbed mercaptobenzoquinone to yield an immobilized Diels-Alder adduct. The pseudo-first-order rate

Mrksich, Milan

5

Absolute rate constants for the hydroxyl radical reactions with ethane, furan, and thiophene at room temperature  

SciTech Connect

Interests in the gas phase oxidation of heterocyclic organic compounds containing oxygen, nitrogen, or sulfur have become increasingly evident due to the important roles that these compounds may play in combustion processes, atmospheric chemistry, and photochemical air pollution. The rate constants for the reactions of OH with furan (C/sub 4/H/sub 4/O) and thiophene (C/sub 4/H/sub 4/S) have been determined for the first time under pseudo-first-order conditions, using the discharge flow-resonance fluorescence technique. The measured room temperature rate constants are (2.31 +- 0.40) x 10/sup -13/, (1.05 +- 0.08) x 10/sup -10/, and (4.77 +- 0.63) x 10/sup -11/ cm/sup 3//molecule/sup -1/ s/sup -1/ for ethane, furan, and thiophene, respectively. The results are discussed in terms of structural differences and probable reaction pathways.

Lee, J.H.; Tang, I.N.

1982-11-01

6

Reaction kinetics of resveratrol with tert-butoxyl radicals  

NASA Astrophysics Data System (ADS)

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

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

2012-09-01

7

Faster rates with less catalyst in template-directed reactions  

NASA Technical Reports Server (NTRS)

We have recently shown that the polycytidylic acid-directed polymerization of guanosine 5'-monophosphate 2-methylimidazolide (2-MeImpG) is amenable to kinetic study and that rate determinations as a function of 2-MeImpG concentration can reveal much mechanistic detail (Kanavarioti et al. 1993). Here we report kinetic data which show that, once the reaction has been initiated by the formation of dimers, the elongation of dimers to form longer oligomers is accelerated by decreasing polycytidylate (poly(C)) concentration from 0.05 to 0.002 M. This result is consistent with the previously proposed mechanism. The increase in the observed pseudo-first order rate constant for formation of the trimer, k3', and the corresponding constant for formation of oligomers longer than the trimer, ki' (ki' is independent of oligomer length for i > or = 4), with decreasing template concentration for a given monomer concentration is attributed to an increase in template occupancy as template concentration is reduced.

Kanavarioti, A.; Baird, E. E.

1995-01-01

8

Triphase catalysis: a rigorous mechanistic model for nucleophilic substitution reactions based on a modified Langmuir–Hinshelwood\\/Eley–Rideal approach  

Microsoft Academic Search

In the present work, a general kinetic model based on the traditional kinetic mechanisms of the Langmuir–Hinshelwood and Eley–Rideal types has been developed for nucleophilic substitution reaction systems involving a triphase catalyst in which the intrinsic reaction rate at the catalyst active site is the rate-limiting step. The present mechanistic model overcomes the limitation of the commonly used pseudo-first-order model

Justinus A. B Satrio; Holger J Glatzer; L. K Doraiswamy

2000-01-01

9

Photochemical reactions of poly(3-butoxythiophene-2,5-diyl) with chloroform  

PubMed Central

Photochemical reactions of poly(3-butoxythiophene-2,5-diyl) with chloroform under irradiation with light were studied. The reactions were separately carried out under air, oxygen, and nitrogen. The obtained results showed that this reaction belongs to the pseudo-first-order reaction with a rate constant k obs of 1.4×10?5 s?1 at room temperature. The presence or absence of air, oxygen, and nitrogen did not have obvious effects on the reaction rate under irradiation with light. PMID:16052703

Imit, Mokhtar; Yamamoto, Takakazu; Imin, Patigul

2005-01-01

10

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

PubMed

Reaction rate constants of the reaction of four large ketones with hydroxyl (OH) are investigated behind reflected shock waves using OH laser absorption. The studied ketones are isomers of hexanone and include 2-hexanone, 3-hexanone, 3-methyl-2-pentanone, and 4-methl-2-pentanone. Rate constants are measured under pseudo-first-order kinetics at temperatures ranging from 866 K to 1375 K and pressures near 1.5 atm. The reported high-temperature rate constant measurements are the first direct measurements for these ketones under combustion-relevant conditions. The effects of the position of the carbonyl group (C=O) and methyl (CH3) branching on the overall rate constant with OH are examined. Using previously published data, rate constant expressions covering, low-to-high temperatures, are developed for acetone, 2-butanone, 3-pentanone, and the hexanone isomers studied here. These Arrhenius expressions are used to devise rate rules for H-abstraction from various sites. Specifically, the current scheme is applied with good success to H-abstraction by OH from a series of n-ketones. Finally, general expressions for primary and secondary site-specific H-abstraction by OH from ketones are proposed as follows (the subscript numbers indicate the number of carbon atoms bonded to the next-nearest-neighbor carbon atom, the subscript CO indicates that the abstraction is from a site next to the carbonyl group (C=O), and the prime is used to differentiate different neighboring environments of a methylene group): PMID:24817270

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

2014-06-28

11

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

PubMed

The overall rate constant for the reaction ethanol + OH ? products was determined experimentally from 900 to 1270 K behind reflected shock waves. Ethan(18)ol was utilized for these measurements in order to avoid the recycling of OH radicals following H-atom abstraction at the ?-site of ethanol. Similar experiments were also performed with unlabeled ethan(16)ol in order to infer the rate constant that excludes reactivity at the ?-site. The two data sets were used to directly infer the branching ratio for the reaction at the ?-site. Experimental data in the current study and in previous low-temperature studies for the overall rate constant are best fit by the expression koverall = 5.07 × 10(5) T[K](2.31) exp(608/T[K]) cm(3) mol(-1) s(-1), valid from 300 to 1300 K. Measurements indicate that the branching ratio of the ?-site is between 20 and 25% at the conditions studied. Pseudo-first-order reaction conditions were generated using tert-butylhydroperoxide (TBHP) as a fast source of (16)OH with ethanol in excess. (16)OH mole fraction time-histories were measured using narrow-line width laser absorption near 307 nm. Measurements were performed at the linecenter of the R22(5.5) transition in the A-X(0,0) band of (16)OH that does not overlap with any absorption features of (18)OH, thus producing a measurement of the (16)OH mole fraction that is insensitive to the presence of (18)OH. PMID:24405356

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

2014-02-01

12

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

13

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

14

Kinetic studies of oxygen atom transfer reactions from trans-dioxoruthenium(VI) porphyrins to sulfides  

Microsoft Academic Search

The kinetics of the reactions of three trans-dioxoruthenium(VI) porphyrin derivatives with organic sulfides were measured. The dioxo systems studied were 5,10,15,20-tetramesityl porphyrin-dioxoruthenium(VI) (2a), 5,10,15,20-tetraphenylporphyrin-dioxoruthenium(VI) (2b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-dioxoruthenium(VI) (2c). Species 2 were competent oxidants and reacted rapidly with thioanisoles to generate the corresponding sulfoxides. Typical second-order rate constants determined from pseudo-first-order kinetic studies for sulfoxidation reactions are 8–60M?1s?1, which are 3

Chris Abebrese; Yan Huang; Alice Pan; Zhibo Yuan; Rui Zhang

2011-01-01

15

The gas-phase reactions of O( 3P) with perhalogenated butenes: Absolute rate constants and temperature dependences  

NASA Astrophysics Data System (ADS)

The temperature dependences of the gas-phase reactions of O( 3P) atoms with CF 3CF dbnd CFCF 3, CF 3CF dbnd CClCF 3 and CF 3CCl dbnd CClCF 3 were studied under pseudo-first-order conditions in the range 298-359 K, using the discharge flow-chemiluminescence technique. The Arrhenius expressions obtained (in units of cm 3 molecule -1 s -1) were: k1 = (0.32 ± 0.05) × 10 -12 exp[-(8.1 ± 0.5) × 10 3/ RT], k2 = (1.50 ± 0.74) × 10 -11 exp[-(17.7 ± 1.4) × 10 3/ RT] and k3 = (2.58 ± 1.10) × 10 -11 exp[-(18.0 ± 1.2) × 10 3/ RT], with Ea in J mol -1. The positive temperature dependences point to the importance of the irreversibility of the addition channel. Complete substitution by halogen atoms in the alkenes results in dramatic reduction of the rate constants. The reactivity trend with the ionization potential is also analyzed.

Cometto, Pablo M.; Taccone, Raúl A.; Lane, Silvia I.

2007-07-01

16

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

NASA Technical Reports Server (NTRS)

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

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

2004-01-01

17

Reaction Rates, Catalysis, and Pasteurization  

NSDL National Science Digital Library

In this activity, by the Concord Consortium's Molecular Literacy project, students are introduced to "key ideas of chemistry, including reaction rates, bond strength, activation energy, catalysis, and equilibrium." Upon completion of this activity students should be able to describe and manipulate models to discover how reaction rates, energy, bond strength, catalysis, and equilibrium are related; manipulate reaction rates by changing temperature and concentration; demonstrate their understanding by creating an endothermic reaction that releases a lot of heat energy, one that requires a catalyst, and one that reaches equilibrium between reactants and products. The activity itself is a java-based interactive resource built upon the free, open source Molecular Workbench software. In the activity, students are allowed to explore at their own pace in a digital environment full of demonstrations, illustrations, and models they can manipulate. In addition to the activity, visitors will find an overview of the activity, a test and rubric, central concepts, and their correlation to AAAS standards.

2008-10-23

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

One-dimensional virus transport in porous media with time-dependent inactivation rate coefficients  

Microsoft Academic Search

A model for virus transport in one-dimensional, homogeneous, saturated porous media is developed, accounting for virus sorption and inactivation of liquid phase and adsorbed viruses with different time dependent rate coefficients. The virus inactivation process is represented by a pseudo first-order rate expression. The pseudo first-order approximation is shown to simulate available experimental data from three virus inactivation batch studies

Youn Sim; Constantinos V. Chrysikopoulos

1996-01-01

20

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

Microsoft Academic Search

Over a pH range 1-4 and temperatures from 170 to 230°C, the decomposition rates of xylose, galactose, mannose, glucose, 2-furfural, and 5-hydroxymethyl-2-furfural (5-HMF) were pseudo first order. The effect of temperature and pH on the pseudo first-order decomposition rate constants was modeled using the Arrhenius equation and acid-base catalysis, respectively. Decomposition rates of the monosaccharides were minimum at a pH

Kent D. Baugh; Perry L. McCarty

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

Studies on the Reaction Kinetics and Mechanism of Iron(II) Reduction of the cis -Halogeno(cetylamine)bis(ethylenediamine)cobalt(III) Complex Ion in Aqueous Perchlorate Medium  

Microsoft Academic Search

The electron-transfer kinetics of the ionic surfactant complex cis-chloro\\/bromo(cetylamine)bis(ethylenediamine)cobalt(III) by iron(II) in aqueous perchlorate medium at ?=1.0 mol?dm?3 ionic strength have been studied at 303, 308 and 313 K by spectrophotometry under pseudo-first-order conditions using an\\u000a excess of the reductant. The effects of [H+], ionic strength and [Fe2+] on the rate were determined. The reaction was found to be second order and

N. Kumaraguru; K. Santhakumar

2009-01-01

23

Rate constant for OH,,2 ... reaction  

E-print Network

. The title reaction is exothermic by 16.7 kcal/mol and its rate constant is decidedly nonRate constant for OH,,2 ...+O,,3 P...\\H,,2 S...+O2,,3 g - ... reaction on an improved ab initio and the rate constant for the title reaction was obtained using a J-shifting method. The calculated rate

Honvault, Pascal

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

2012-04-19

25

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

26

Thermodynamic criteria of homogeneous reaction rates  

Microsoft Academic Search

A theoretical analysis of the restrictions imposed by macroscopic equilibrium thermodynamics on rate theories for homogeneous chemical reactions is described. Elementary gas-phase, constant pressure, exothermic reactions are used throughout this rate study. Reaction path, based on the experimental results from combustion processes in flames, is outlined. The sequence of the availability function is studied along this path, subject to the

F. L. Lansing

1975-01-01

27

Method of controlling fusion reaction rates  

DOEpatents

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

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

1988-01-01

28

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

SciTech Connect

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

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

1988-01-01

29

Kinetics of the reaction of iodine atoms with HO sub 2 radicals  

SciTech Connect

The rate coefficient (k{sub 7}) for the reaction of iodine atoms with HO{sub 2} radicals, I + HO{sub 2} {yields} HI + O{sub 2}, has been measured directly with use of the discharge-flow/EPR technique, and the molecular-modulation/UV-absorption spectroscopy technique. Discharge-flow measurements were made under pseudo-first-order conditions with iodine atoms in large excess over HO{sub 2}. Molecular-modulation measurements were made with iodine atoms in excess over HO{sub 2}, but the I + HO{sub 2} reaction was occurring in competition with the self-reaction of HO{sub 2}. The potential significance of this reaction as a sink for iodine in the troposphere and other aspects of tropospheric iodine chemistry are considered with a simple model of the atmospheric boundary layer.

Jenkin, M.E.; Cox, R.A. (Harwell Laboratory, Oxfordshire (England)); Mellouki, A.; Le Bras, G.; Poulet, G. (CNRS, Orleans (France))

1990-04-05

30

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 ...

31

The kinetics of phosphotriester condensation reactions on solid support.  

PubMed Central

A study has been made of the kinetics of condensation reactions in phosphotriester synthesis of oligonucleotides on polystyrene copolymer resin support, using monomers, dimers, and trimers as the coupling units. The condensations follow pseudo first order kinetics, with a rate constant that decreases from monomers to dimers to trimers. Arrhenius plots indicate a nearly constant activation energy of around 10.4 kcal mol-1. Extrapolation to find the Arrhenius preexponential factor, A, is inaccurate because of the limited temperature range studied. But if the assumption is made that Ea is the same for monomer, dimer, and trimer reactions, then the A values are inversely proportional to the surface areas of the coupling units, reinforcing the interpretation of A as a collision efficiency factor. PMID:6473109

Ikuta, S; Chattopadhyaya, R; Dickerson, R E

1984-01-01

32

Brussels nuclear reaction rate library (Aikawa+, 2005)  

NASA Astrophysics Data System (ADS)

The present data is part of the Brussels nuclear reaction rate library (BRUSLIB) for astrophysics applications and concerns nuclear reaction rate predictions calculated within the statistical Hauser-Feshbach approximation and making use of global and coherent microscopic nuclear models for the quantities (nuclear masses, nuclear structure properties, nuclear level densities, gamma-ray strength functions, optical potentials) entering the rate calculations. (4 data files).

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

2005-07-01

33

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

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

34

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

35

Analysis and improvement of rate constant determination of reactions involving charged reactants.  

PubMed

Kinetics of tris(2,2'-bipyridine)ruthenium(ii), Ru(bpy)(3)(2+), luminescence quenching by copper(ii) (in the form of chloride, nitrate, sulfate and perchlorate salt) was studied using pulse laser photolysis technique. The pseudo-first order rate constant versus quencher concentration plots obtained were found to be nonlinear, bending upward. The ionic strength effect contribution was evaluated by applying the Debye-Hückel extended law and was found to be as important as other effects such as cation-counter anion complex and ion-pairing complex formation which were all found to be dependent on the counter anion. It is shown that the slope of the tangent line to the pseudo-first order curve at zero quencher concentration is equal to the quenching rate constants at zero ionic strength. Also, this value corresponds to quenching solely by Cu(2+) and is free from contributions from other species that are present at higher concentrations. This method produced a value, (1.6 +/- 0.2) x 10(7) M(-1) s(-1), (lower than previously published ones) which is in agreement with the quenching rate constant measurement analysis presented. Comparison between Stern-Volmer plots obtained using steady-state fluorimetry data and laser photolysis data showed that in 50 mM CuCl(2) and CuSO(4) aqueous solutions about 5% of Ru(bpy)(3)(2+) is in the form of ion-pairing complexes. Our method was also applied to quenching by another divalent cation, methyl viologen, where it was found that charge transfer complexation effect contribution was about 50% of that of ionic strength effect, while ion-pairing complexation was not significant in the concentration range used. The quenching rate constant at zero ionic strength was found to be (2.3 +/- 0.2) x 10(8) M(-1) s(-1). The method proposed is also applicable to pulse radiolysis and stopped flow measurements. PMID:20532359

Krise, Keith M; Hwang, Angela A; Milosavljevic, Bratoljub H

2010-07-21

36

Decomposition of phenylarsonic acid by AOP processes: degradation rate constants and by-products.  

PubMed

The paper presents results of the studies photodegradation, photooxidation, and oxidation of phenylarsonic acid (PAA) in aquatic solution. The water solutions, which consist of 2.7 g dm(-3) phenylarsonic acid, were subjected to advance oxidation process (AOP) in UV, UV/H2O2, UV/O3, H2O2, and O3 systems under two pH conditions. Kinetic rate constants and half-life of phenylarsonic acid decomposition reaction are presented. The results from the study indicate that at pH 2 and 7, PAA degradation processes takes place in accordance with the pseudo first order kinetic reaction. The highest rate constants (10.45?×?10(-3) and 20.12?×?10(-3)) and degradation efficiencies at pH 2 and 7 were obtained at UV/O3 processes. In solution, after processes, benzene, phenol, acetophenone, o-hydroxybiphenyl, p-hydroxybiphenyl, benzoic acid, benzaldehyde, and biphenyl were identified. PMID:24824504

Jaworek, K; Czaplicka, M; Bratek, L

2014-10-01

37

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

38

Reaction coordinates and rates from transition paths  

PubMed Central

The molecular mechanism of a reaction in solution is reflected in its transition-state ensemble and transition paths. We use a Bayesian formula relating the equilibrium and transition-path ensembles to identify transition states, rank reaction coordinates, and estimate rate coefficients. We also introduce a variational procedure to optimize reaction coordinates. The theory is illustrated with applications to protein folding and the dipole reorientation of an ordered water chain inside a carbon nanotube. To describe the folding of a simple model of a three-helix bundle protein, we variationally optimize the weights of a projection onto the matrix of native and nonnative amino acid contacts. The resulting one-dimensional reaction coordinate captures the folding transition state, with formation and packing of helix 2 and 3 constituting the bottleneck for folding. PMID:15814618

Best, Robert B.; Hummer, Gerhard

2005-01-01

39

Reaction rate assessment of multiphonon relaxation  

Microsoft Academic Search

An investigation is made into whether the reaction rate formalism also applies to describing inelastic tunneling vibronic processes associated with the absorption and emission of phonons. We propose that Bardeen-Christov's quantum-mechanical approach to the transition (tunneling) probabilities originally intended for elastic tunneling alone can formally be extended to cover inelastic tunneling as well. Namely, we show that inelastic tunneling through

Mladen Georgiev; Alexander Gochev

2005-01-01

40

The reaction rate in nuclear fusion  

Microsoft Academic Search

Summary  A 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

41

Relevant energy ranges for astrophysical reaction rates  

SciTech Connect

Effective energy windows (Gamow windows) of astrophysical reaction rates for (p,gamma), (p,n), (p,alpha), (alpha,gamma), (alpha,n), (alpha,p), (n,gamma), (n,p), and (n,alpha) on targets with 10<=Z<=83 from proton to neutron dripline are calculated using theoretical cross sections. It is shown that widely used approximation formulas for the relevant energy ranges are not valid for a large number of reactions relevant to hydrostatic and explosive nucleosynthesis. The influence of the energy dependence of the averaged widths on the location of the Gamow windows is discussed and the results are presented in tabular form.

Rauscher, Thomas [Department of Physics, University of Basel, Klingelbergstr. 82, CH-4056 Basel (Switzerland)

2010-04-15

42

Cold adaptation of enzyme reaction rates.  

PubMed

A major issue for organisms living at extreme temperatures is to preserve both stability and activity of their enzymes. Cold-adapted enzymes generally have a reduced thermal stability, to counteract freezing, and show a lower enthalpy and a more negative entropy of activation compared to mesophilic and thermophilic homologues. Such a balance of thermodynamic activation parameters can make the reaction rate decrease more linearly, rather than exponentially, as the temperature is lowered, but the structural basis for rate optimization toward low working temperatures remains unclear. In order to computationally address this problem, it is clear that reaction simulations rather than standard molecular dynamics calculations are needed. We have thus carried out extensive computer simulations of the keto-enol(ate) isomerization steps in differently adapted citrate synthases to explore the structure-function relationships behind catalytic rate adaptation to different temperatures. The calculations reproduce the absolute rates of the psychrophilic and mesophilic enzymes at 300 K, as well as the lower enthalpy and more negative entropy of activation of the cold-adapted enzyme, where the latter simulation result is obtained from high-precision Arrhenius plots. The overall catalytic effect originates from electrostatic stabilization of the transition state and enolate and the reduction of reorganization free energy. The simulations, however, show psychrophilic, mesophilic, and hyperthermophilic citrate synthases to have increasingly stronger electrostatic stabilization of the transition state, while the energetic penalty in terms of internal protein interactions follows the reverse order with the cold-adapted enzyme having the most favorable energy term. The lower activation enthalpy and more negative activation entropy observed for cold-adapted enzymes are found to be associated with a decreased protein stiffness. The origin of this effect is, however, not localized to the active site but to other regions of the protein structure. PMID:18759500

Bjelic, Sinisa; Brandsdal, Bjørn O; Aqvist, Johan

2008-09-23

43

A simple reaction-rate model for turbulent diffusion flames  

NASA Technical Reports Server (NTRS)

A simple reaction rate model is proposed for turbulent diffusion flames in which the reaction rate is proportional to the turbulence mixing rate. The reaction rate is also dependent on the mean mass fraction and the mean square fluctuation of mass fraction of each reactant. Calculations are compared with experimental data and are generally successful in predicting the measured quantities.

Bangert, L. H.

1975-01-01

44

On the Rate Expressions for "Reversible" Gas-Solid Reactions  

NASA Astrophysics Data System (ADS)

It is argued that the term with the negative sign in the rate expression for an equilibrium-limited gas-solid reaction can be viewed as representing a lowering of the concentration driving force for the forward reaction due to equilibrium limitation. While the gas composition favors the forward reaction, the negative term may represent the local rate of the reverse reaction. When the gas composition changes significantly to the opposite side of equilibrium, however, this term as written for the forward reaction does not represent the rate of the reverse reaction. Even the reaction order with respect to the same gaseous species may become different for the reverse reaction.

Sohn, Hong Yong

2014-10-01

45

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

PubMed

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

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

2010-04-01

46

Kinetic studies of inverse electron demand Diels-Alder reactions (iEDDA) of norbornenes and 3,6-dipyridin-2-yl-1,2,4,5-tetrazine  

PubMed Central

Inverse electron demand Diels–Alder additions (iEDDA) between 1,2,4,5-tetrazines and olefins have recently found widespread application as a novel ‘click chemistry’ scheme and as a mild technique for the modification of materials. Norbornenes are, due to their straightforward synthetic availability, especially interesting in the latter context. Therefore, the reactivity of different norbornene-based compounds was compared with unsubstituted norbornene and other alkenes using UV-vis measurements for the determination of reaction rates under pseudo first order conditions. Thereby, exo,exo-5-norbornene-2,3-dimethanol was found to be almost as reactive as unsubstituted norbornene whereas (±)-endo,exo-dimethyl-5-norbornene-2,3-dicarboxylate reacted only insignificanty faster than unstrained alkenes. PMID:25152544

Knall, Astrid-Caroline; Hollauf, Manuel; Slugovc, Christian

2014-01-01

47

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

48

Distribution function, reaction rates and spectral line profile  

Microsoft Academic Search

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

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

2004-01-01

49

Distribution function, reaction rates and spectral line profile  

Microsoft Academic Search

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

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

2004-01-01

50

Kinetics of diffusion-assisted reactions in microheterogeneous systems.  

PubMed

This review is focused on the basic theory of diffusion-assisted reactions in microheterogeneous systems, from porous solids to self-organized colloids and biomolecules. Rich kinetic behaviors observed experimentally are explained in a unified fashion using simple concepts of competing distance and time scales of the reaction and the embedding structure. We mainly consider pseudo-first-order reactions, such as luminescence quenching, described by the Smoluchowski type of equation for the reactant pair distribution function with a sink term defined by the reaction mechanism. Microheterogeneity can affect the microscopic rate constant. It also enters the evolution equation through various spatial constraints leading to complicated boundary conditions and, possibly, to the reduction of dimensionality of the diffusion space. The reaction coordinate and diffusive motion along this coordinate are understood in a general way, depending on the problem at hand. Thus, the evolution operator can describe translational and rotational diffusion of molecules in a usual sense, it can be a discrete random walk operator when dealing with hopping of adsorbates in solids, or it can correspond to conformational fluctuations in proteins. Mathematical formulation is universal but physical consequences can be different. Understanding the principal features of reaction kinetics in microheterogeneous systems enables one to extract important structural and dynamical information about the host environments by analyzing suitably designed experiments, it helps building effective strategies for computer simulations, and ultimately opens possibilities for designing systems with controllable reactivity properties. PMID:11215811

Barzykin, A V; Seki, K; Tachiya, M

2001-01-29

51

Calculation of interchange reaction rates by a `nearest resonance' method  

Microsoft Academic Search

Thermal and suprathermal energy gas reaction rates for exothermic processes involving molecules or molecular ions are treated generally in terms of the adiabatic criterion. An absolute cross section function is derived in terms of a near-resonance energy defect which allows the determination of the dependence of reaction rate on temperature under conditions of complete or partial thermodynamic equilibrium. The reaction

D K Bohme; J B Hasted; P P Ong

1968-01-01

52

Predicting nonstatistical unimolecular reaction rates using Kramers’ theory  

Microsoft Academic Search

A method for computing unimolecular reaction rate constants in the IVR-limited regime is presented. It is based on Kramers’ energy diffusion theory, with the reaction coordinate taken as the subsystem and the rest of the vibrational modes as the bath. Applications to some bond fission reactions demonstrate that the method accurately predicts the rate constants for wide range of energies

Yin Guo; Dmitrii V. Shalashilin; Justin A. Krouse; Donald L. Thompson

1999-01-01

53

Predicting nonstatistical unimolecular reaction rates using Kramers' theory  

Microsoft Academic Search

A method for computing unimolecular reaction rate constants in the IVR-limited regime is presented. It is based on Kramers' energy diffusion theory, with the reaction coordinate taken as the subsystem and the rest of the vibrational modes as the bath. Applications to some bond fission reactions demonstrate that the method accurately predicts the rate constants for wide range of energies

Yin Guo; Dmitrii V. Shalashilin; Justin A. Krouse; Donald L. Thompson

1999-01-01

54

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

PubMed Central

Tris(2-chloroethyl)phosphates (TCEP) is a widely used flame retardant in the U.S. It has recently been identified as one of the most frequently detected contaminants in U.S. streams. This contaminant is of toxicological concern in sensitive coastal ecosystems such as estuaries and salt marshes. It is likely that reactions with reduced sulfur species such as polysulfides (Sn2?), bisulfide (HS?), and thiophenolate (PhS?) present in anoxic subregions of coastal water bodies could have a significant impact on rates of removal of such a contaminant. The kinetics of reaction of reduced sulfur species with tris(2-chloroethyl)phosphate have been determined in well-defined aqueous solutions under anoxic conditions. Reactions were monitored at varying concentrations of reduced sulfur species to obtain the second-order rate constants from the observed pseudo-first order rate constants. The determined second-order rate constant for the reaction of TCEP with polysulfide at 25°C is 5.0 (± 1.4) × 10?4 M?1 s?1, with thiophenolate at 50 °C is 34 (± 2) × 10?4 M?1 s?1 and with bisulfide at 50 °C is 0.9 × 10?4 M?1 s?1, respectively. In addition, the degradation products of hydrolysis and the reactions with polysulfides, thiophenolate, and bisulfide with TCEP were studied with GC-FID and LC-MS-MS and were quantified. PMID:21419471

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

2014-01-01

55

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

56

Reaction of NO(2) with selected conjugated alkenes.  

PubMed

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

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

2013-12-27

57

Temperature averages and rates of stratospheric reactions  

Microsoft Academic Search

Use of zonal average temperatures is common in numerical modeling of stratospheric chemistry. In most cases, this approach is accurate. The exceptions are when a heterogeneous reaction or a thermal process which is highly non-linear in temperature is involved. The differences between use of an average temperature and more realistic temperature are presented for cases where temperature fluctuations are of

D. M. Murphy; A. R. Ravishankara

1994-01-01

58

Dynamic Phase Transition, Enhanced Reaction Rate, and  

E-print Network

-reaction model that describes kinetics aspects of the gas- phase catalytic oxidation of carbon monoxide: continuous at y1; discontinuous at y2. y poisoned state y > y2 : CO poisoned state Real systems do not posses an oxygen-poisoned state. Transitions between states of low and high CO coverage have

Rikvold, Per Arne

59

Enhancement of cold fusion reaction rates  

Microsoft Academic Search

Although major controversy still remains as to the source of the excess thermal power output reported from diverse successful cold fusion calorimetry experiments, considerable independent evidence does exist that low-level, deuterium fueled, cold fusion reactions can occur based upon reported neutron and tritium measurements. Because the specific fusion power output may be very low in present cold fusion experiments, there

Gary M. Sandquist; Vern C. Rogers

1990-01-01

60

Statistical Methods for Thermonuclear Reaction Rates and Nucleosynthesis Simulations  

E-print Network

Rigorous statistical methods for estimating thermonuclear reaction rates and nucleosynthesis are becoming increasingly established in nuclear astrophysics. The main challenge being faced is that experimental reaction rates are highly complex quantities derived from a multitude of different measured nuclear parameters (e.g., astrophysical S-factors, resonance energies and strengths, particle and gamma-ray partial widths). We discuss the application of the Monte Carlo method to two distinct, but related, questions. First, given a set of measured nuclear parameters, how can one best estimate the resulting thermonuclear reaction rates and associated uncertainties? Second, given a set of appropriate reaction rates, how can one best estimate the abundances from nucleosynthesis (i.e., reaction network) calculations? The techniques described here provide probability density functions that can be used to derive statistically meaningful reaction rates and final abundances for any desired coverage probability. Examples ...

Iliadis, Christian; Coc, Alain; Timmes, F X; Champagne, Art E

2014-01-01

61

NON-ARRHENIUS RATE CONSTANTS IN HIGHLY EXOTHERMIC REACTIONS  

Microsoft Academic Search

Highly exothermic chemical reactions can produce chemical species with much kinetic energy. Since the Arrhenius expression for the rate constant is based on the assumption that the kinetic energy distribution is approximately a Maxwellian, a study has been carried out to determine if the Arrhenius expression is always valid for chemical reactions initiated by products of highly exothermic reactions. A

K. KELLY; M. D. KOSTIN

1989-01-01

62

DSMC predictions of non-equilibrium reaction rates.  

SciTech Connect

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

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

2010-04-01

63

Reactions of Three Halogenated Organophosphorus Flame Retardants with Reduced Sulfur Species  

PubMed Central

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 (Sn2?) 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 Sn2?, 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

2014-01-01

64

Workshop on Thermonuclear Reaction Rates for Astrophysics Applications  

E-print Network

Workshop on Thermonuclear Reaction Rates for Astrophysics Applications 24-25 November 2011, Athens circular for the Workshop on Thermonuclear Reaction Rates for Astrophysics Applications, to be held-of-the-art astrophysics models to the nuclear physics needs with special emphasis on the role and determination

65

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

66

Imaginary-time theory for triple-alpha reaction rate  

E-print Network

Using imaginary-time theory, it is shown that the triple-alpha reaction rate can be reliably calculated without the need to solve scattering problems involving three charged particles. The calculated reaction rate is found to agree well with the empirical NACRE rate, which is widely adopted in stellar evolution calculations. The reason for this is explained using $R$-matrix theory. Extremely slow convergence is found to occur when a coupled-channel expansion is introduced, which helps to explain the very different reaction rates obtained using different theoretical approaches.

T. Akahori; Y. Funaki; K. Yabana

2014-01-17

67

Upscaling geochemical reaction rates using pore-scale network modeling  

NASA Astrophysics Data System (ADS)

Geochemical reaction rate laws are often measured using crushed minerals in well-mixed laboratory systems that are designed to eliminate mass transport limitations. Such rate laws are often used directly in reactive transport models to predict the reaction and transport of chemical species in consolidated porous media found in subsurface environments. Due to the inherent heterogeneities of porous media, such use of lab-measured rate laws may introduce errors, leading to a need to develop methods for upscaling reaction rates. In this work, we present a methodology for using pore-scale network modeling to investigate scaling effects in geochemical reaction rates. The reactive transport processes are simulated at the pore scale, accounting for heterogeneities of both physical and mineral properties. Mass balance principles are then used to calculate reaction rates at the continuum scale. To examine the scaling behavior of reaction kinetics, these continuum-scale rates from the network model are compared to the rates calculated by directly using laboratory-measured reaction rate laws and ignoring pore-scale heterogeneities. In this work, this methodology is demonstrated by upscaling anorthite and kaolinite reaction rates under simulation conditions relevant to geological CO 2 sequestration. Simulation results show that under conditions with CO 2 present at high concentrations, pore-scale concentrations of reactive species and reaction rates vary spatially by orders of magnitude, and the scaling effect is significant. With a much smaller CO 2 concentration, the scaling effect is relatively small. These results indicate that the increased acidity associated with geological sequestration can generate conditions for which proper scaling tools are yet to be developed. This work demonstrates the use of pore-scale network modeling as a valuable research tool for examining upscaling of geochemical kinetics. The pore-scale model allows the effects of pore-scale heterogeneities to be integrated into system behavior at multiple scales, thereby identifying important factors that contribute to the scaling effect.

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

2006-09-01

68

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

NASA Technical Reports Server (NTRS)

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

Rizvi, Farheen

2013-01-01

69

Urea degradation by electrochemically generated reactive chlorine species: products and reaction pathways.  

PubMed

This study investigated the transformation of urea by electrochemically generated reactive chlorine species (RCS). Solutions of urea with chloride ions were electrolyzed using a bismuth doped TiO2 (BiOx/TiO2) anode coupled with a stainless steel cathode at applied anodic potentials (Ea) of either +2.2 V or +3.0 V versus the normal hydrogen electrode. In NaCl solution, the current efficiency of RCS generation was near 30% at both potentials. In divided cell experiments, the pseudo-first-order rate of total nitrogen decay was an order of magnitude higher at Ea of +3.0 V than at +2.2 V, presumably because dichlorine radical (Cl2(-)·) ions facilitate the urea transformation primary driven by free chlorine. Quadrupole mass spectrometer analysis of the reactor headspace revealed that N2 and CO2 are the primary gaseous products of the oxidation of urea, whose urea-N was completely transformed into N2 (91%) and NO3(-) (9%). The higher reaction selectivity with respect to N2 production can be ascribed to a low operational ratio of free available chlorine to N. The mass-balance analysis recovered urea-C as CO2 at 77%, while CO generation most likely accounts for the residual carbon. In light of these results, we propose a reaction mechanism involving chloramines and chloramides as reaction intermediates, where the initial chlorination is the rate-determining step in the overall sequence of reactions. PMID:25219459

Cho, Kangwoo; Hoffmann, Michael R

2014-10-01

70

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

Microsoft Academic Search

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

Derek S. Liechty; Mark J. Lewis

2011-01-01

71

Rate constant for reaction of atomic hydrogen with germane  

NASA Technical Reports Server (NTRS)

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

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

1990-01-01

72

Discharge flow kinetic study of NO/sub 3/ reactions with free radicals: the reaction of NO/sub 3/ with Cl  

SciTech Connect

The discharge flow technique was used to perform a direct measurement of the rate constant for the reaction NO/sub 3/ + Cl ..-->.. ClO + NO/sub 2/ (1) under pseudo-first-order conditions with NO/sub 3/ in excess. NO/sub 3/ was produced via the reaction F + HNO/sub 3/ ..-->.. HF + NO/sub 3/ (2) for which the rate constant was also determined: k/sub 2/ = (2.7 +/- 0.5) x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/ at 298 K. Absolute titration of NO/sub 3/ was done using NO and 2,3-dimethyl-2-butene as the titrants. The titration experiments with NO were simulated, and the analysis of potential errors resulting from the secondary chemistry indicated that NO is a precise titrant of NO/sub 3/ in discharge flow studies. In the kinetic investigation of reaction 1, the obtained value of k/sub 1/ at 298 K was k/sub 1/ = (2.6 +/- 0.5) x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/.

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

1987-10-22

73

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

SciTech Connect

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

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

1992-01-01

74

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

SciTech Connect

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

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

1992-02-01

75

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

NASA Astrophysics Data System (ADS)

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

Li, Maolin; Chen, Guofang

2013-11-01

76

Nonequilibrium velocity distributions and reaction rates in fast highly exothermic reactions  

Microsoft Academic Search

The nonequilibrium velocity distributions and reaction rates in fast highly exothermic reactions with a low activation energy &egr;*f?k T0 ?slant 5, a high steric factor P ?slant 0.1, and a large heat of reaction Q*?k T0 ? O (10), are investigated by solving the semiclassical Boltzmann equation with a Monte Carlo method. The explicit time-dependent velocity distribution functions and reaction

Katsuhisa Koura

1973-01-01

77

A compilation of charged-particle induced thermonuclear reaction rates  

Microsoft Academic Search

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

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

1999-01-01

78

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

79

Thermonuclear reaction rate of 17O(p,?)18F  

NASA Astrophysics Data System (ADS)

The 17O(p,?)18F and 17O(p,?)14N reactions have a profound influence on hydrogen-burning nucleosynthesis in a number of stellar sites, including red giants, asymptotic giant branch (AGB) stars, massive stars, and classical novae. Previously evaluated thermonuclear rates for both reactions carry large uncertainties. We investigated the proton-capture reaction on 17O in the bombarding energy range of Elabp=180-540keV. We observed a previously undiscovered resonance at ElabR=193.2±0.9keV. The resonance strength amounts to (??)p?=(1.2±0.2)×10-6eV. With this value, the uncertainties of the 17O(p,?)18F reaction rates are reduced by orders of magnitude in the peak temperature range of classical novae (T=0.1-0.4GK). We also report on a reevaluation of the 17O(p,?)18F reaction rates at lower temperatures that are pertinent to red giants, AGB stars, or massive stars. The present work establishes the 17O(p,?)18F reaction rates over a temperature range of T=0.01-1.5GK with statistical uncertainties of 10 50%. The new recommended reaction rates deviate from the previously accepted values by an order of magnitude around T?0.2GK and by factors of 2 3 at T<0.1GK.

Fox, C.; Iliadis, C.; Champagne, A. E.; Fitzgerald, R. P.; Longland, R.; Newton, J.; Pollanen, J.; Runkle, R.

2005-05-01

80

The Reaction Rate Sensitivity of Nucleosynthesis in Type II Supernovae  

NASA Astrophysics Data System (ADS)

We explore the sensitivity of the nucleosynthesis of intermediate-mass elements (28<=A<~80) in supernovae derived from massive stars to the nuclear reaction rates employed in the model. Two standard sources of reaction rate data are employed in pairs of calculations that are otherwise identical. Both include as a common backbone the experimental reactions rates of Caughlan & Fowler. Two stellar models are calculated for each of two masses: 15 and 25 Msolar. Each star is evolved from core hydrogen burning to a presupernova state carrying an appropriately large reaction network and then exploded using a piston near the edge of the iron core as described by Woosley & Weaver. The final stellar yields from the models calculated with the two rate sets are compared and found to differ in most cases by less than a factor of 2 over the entire range of nuclei studied. Reasons for the major discrepancies along with the physics underlying the two reaction rate sets employed are discussed in detail. The nucleosynthesis results are relatively robust and less sensitive than might be expected to uncertainties in nuclear reaction rates, though they are sensitive to the stellar model employed.

Hoffman, R. D.; Woosley, S. E.; Weaver, T. A.; Rauscher, T.; Thielemann, F.-K.

1999-08-01

81

Rate of reaction between molecular hydrogen and molecular oxygen  

NASA Technical Reports Server (NTRS)

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

Brokaw, R. S.

1973-01-01

82

{sup 14}C({alpha},{gamma}) reaction rate  

SciTech Connect

The {sup 14}C({alpha},{gamma}) reaction rate at temperatures below 0.3 GK depends on the properties of two near threshold resonances in {sup 18}O, the 1{sup -} at 6.198 MeV and the 3{sup -} at 6.404 MeV. The {alpha}+{sup 14}C asymptotic normalization coefficients for these resonances were determined using the {alpha}-transfer reactions {sup 14}C({sup 7}Li, t) and {sup 14}C({sup 6}Li, d) at sub-Coulomb energies. The {sup 14}C({alpha},{gamma}) reaction rate at low temperatures has been evaluated. Implications of the new reaction rate on the evolution of accreting helium white dwarfs and on the nucleosynthesis of low mass stars during the asymptotic giant branch phase are discussed.

Johnson, E. D.; Rogachev, G. V.; Mitchell, J.; Miller, L.; Kemper, K. W. [Department of Physics, Florida State University, Florida 32306 (United States)

2009-10-15

83

Statistical Methods for Thermonuclear Reaction Rates and Nucleosynthesis Simulations  

E-print Network

Rigorous statistical methods for estimating thermonuclear reaction rates and nucleosynthesis are becoming increasingly established in nuclear astrophysics. The main challenge being faced is that experimental reaction rates are highly complex quantities derived from a multitude of different measured nuclear parameters (e.g., astrophysical S-factors, resonance energies and strengths, particle and gamma-ray partial widths). We discuss the application of the Monte Carlo method to two distinct, but related, questions. First, given a set of measured nuclear parameters, how can one best estimate the resulting thermonuclear reaction rates and associated uncertainties? Second, given a set of appropriate reaction rates, how can one best estimate the abundances from nucleosynthesis (i.e., reaction network) calculations? The techniques described here provide probability density functions that can be used to derive statistically meaningful reaction rates and final abundances for any desired coverage probability. Examples are given for applications to s-process neutron sources, core-collapse supernovae, classical novae, and big bang nucleosynthesis.

Christian Iliadis; Richard Longland; Alain Coc; F. X. Timmes; Art E. Champagne

2014-09-19

84

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

85

The Reaction Rate Sensitivity of Nucleosynthesis in Type II Supernovae  

E-print Network

We explore the sensitivity of the nucleosynthesis of intermediate mass elements (28 standard sources of reaction rate data (Woosley et al. 1978; and Thielemann et al. 1987) are employed in pairs of calculations that are otherwise identical. Both include as a common backbone the experimental reactions rates of Caughlan & Fowler (1988). Two stellar models are calculated for each of two main sequence masses: 15 and 25 solar masses. Each star is evolved from core hydrogen burning to a presupernova state carrying an appropriately large reaction network, then exploded using a piston near the edge of the iron core as described by Woosley & Weaver (1995). The final stellar yields from the models calculated with the two rate sets are compared and found to differ in most cases by less than a factor of two over the entire range of nuclei studied. Reasons for the major discrepancies are discussed in detail along with the physics underlying the two reaction rate sets employed. The nucleosynthesis results are relatively robust and less sensitive than might be expected to uncertainties in nuclear reaction rates, though they are sensitive to the stellar model employed.

R. D. Hoffman; S. E. Woosley; T. A. Weaver; T. Rauscher; F. -K. Thielemann; .

1998-09-18

86

SIFT and FALP determinations of ionic reactions rate coefficients  

NASA Astrophysics Data System (ADS)

A brief review is presented of some recent results of studies of ionic reactions at thermal energies using the selected ion flow tube (SIFT) and the flowing afterglow/Langmuir probe (FALP) techniques in our laboratory. From the SIFT results, we summarise the recent data on positive ion-molecule reactions, much of which relate to the synthesis of interstellar molecules. Thus, for example, data are discussed relating to the reactions of some hydrocarbon ions in the series CnHm+, and phosphorus-bearing ions in the series PHn+, which can lead to interstellar hydrocarbon and phosphorus-bearing molecules. From the FALP results, we summarise the data obtained on the rate coefficients for dissociative recombination, highlighting the recent work on the determination of the products of some dissociative recombination reactions, and discuss recent results on studies of dissociative electron attachment reactions. Reference is also made to our earlier studies of ion-ion mutual neutralization.

Smith, D.; Adams, N. G.

1990-06-01

87

Benchmark calculations of thermal reaction rates. I - Quantal scattering theory  

NASA Astrophysics Data System (ADS)

The thermal rate coefficient for the prototype reaction H + H2 yields H2 + H with zero total angular momentum is calculated by summing, averaging, and numerically integrating state-to-state reaction probabilities calculated by time-independent quantum-mechanical scattering theory. The results are very carefully converged with respect to all numerical parameters in order to provide high-precision benchmark results for confirming the accuracy of new methods and testing their efficiency.

Chatfield, David C.; Truhlar, Donald G.; Schwenke, David W.

1991-02-01

88

Reaction Rate Sensitivity of the gamma-Process Path  

E-print Network

The location of the (gamma,p)/(gamma,n) and (gamma,alpha)/(gamma,n) line at gamma-process temperatures is discussed, using recently published reaction rates based on global Hauser-Feshbach calculations. The results can directly be compared to previously published, classic gamma-process discussions. The nuclei exhibiting the largest sensitivity to uncertainties in nuclear structure and reaction parameters are specified.

T. Rauscher

2004-07-16

89

Approximate reaction rates and probabilities for a class of exothermic chemical reactions  

Microsoft Academic Search

An approximate expression for the total rate constant for certain exothermic reactions is given. The approximation can be expressed simply in terms of integrals in a region about the saddle point. More complicated procedures for computing the total probability for reaction from a given reactant state are presented. This involves solution of a set of coupled differential equations in a

Edward J. Shipsey

1975-01-01

90

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

NASA Astrophysics Data System (ADS)

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

Fegley, B.; Prinn, R. G.

1989-01-01

91

A compilation of charged-particle induced thermonuclear reaction rates  

NASA Astrophysics Data System (ADS)

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

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

1999-08-01

92

Basics of Chemical Kinetics -1 Rate of reaction = rate of disappearance of A =  

E-print Network

Kinetics - 4 Reaction Stoichiometry + Law of Conservation of Mass rA -a rB -b rC c rD d = = = (Irrespective Michaelis-Menten model of enzymatic reactions #12;Michaelis-Menten kinetics Mass conservation: Assumption/volume) ; 1 mole = 6.023x1023 molecules rA = -k [A] rA = -k [A]2 rA = -k1 [A]/(1+k2[A]) Reaction rate law

Albert, Réka

93

Photochemical Generation of Strong One-Electron Reductants via Light-Induced Electron Transfer with Reversible Donors Followed by Cross Reaction with Sacrificial Donors.  

PubMed

This work illustrates a modified approach for employing photoinduced electron transfer reactions coupled to secondary irreversible electron transfer processes for the generation of strongly reducing equivalents in solution. Through irradiation of [Ru(LL)3](2+) (LL= diimine ligands) with tritolylamine (TTA) as quencher and various alkyl amines as sacrificial electron donors, yields in excess of 50% can be achieved for generation of reductants with E(0)(2+/1+) values between -1.0 and -1.2 V vs NHE. The key to the system is the fact that the TTA cation radical, formed in high yield in reaction with the photoexcited [Ru(LL)3](2+) complex, reacts irreversibly with various sacrificial electron donating amines that are kinetically unable to directly react with the photoexcited complex. The electron transfer between the TTA(+) and the sacrificial amine is an energetically uphill process. Kinetic analysis of these parallel competing reactions, consisting of bimolecular and pseudo first-order reactions, allows determination of electron transfer rate constants for the cross electron transfer reaction between the sacrificial donor and the TTA(+). A variety of amines were examined as potential sacrificial electron donors, and it was found that tertiary 1,2-diamines are most efficient among these amines for trapping the intermediate TTA(+). This electron-donating combination is capable of supplying a persistent reducing flux of electrons to catalysts used for hydrogen production. PMID:24882233

Shan, Bing; Schmehl, Russell

2014-11-13

94

Semiclassical Calculation of Reaction Rate Constants for Homolytical Dissociations  

NASA Technical Reports Server (NTRS)

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

Cardelino, Beatriz H.

2002-01-01

95

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

96

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

97

On the reaction rates in the low pressure nitrogen discharge  

Microsoft Academic Search

We use a global (volume averaged) model to study the dissociation of the nitrogen molecule and the role of metastable species in a low pressure (1 - 100 mTorr) high density nitrogen discharge. The collisional energy loss per electron ion pair created is evaluated for the nitrogen atom and the nitrogen molecule. We explore and compare the reaction rates for

Jon Tomas Gudmundsson; Eythor Gisli Thorsteinsson

2008-01-01

98

Relative rates and kinetic isotope effects of reactions in solution  

Microsoft Academic Search

Making and breaking chemical bonds is often less important than encounter as a contributor to the overall barrier. Curvature can have an intrinsic component which is associated with changes in rate and equilibrium constants for the proton-transfer step, as well as a coupling component which arises from coupling of proton transfer with other steps in the overall reaction. It is

Joseph R. Murdoch

1980-01-01

99

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

100

Nuclear fusion in dense matter: Reaction rate and carbon burning  

E-print Network

In this paper we analyze the nuclear fusion rate between 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 which covers all cases. The parameters of this formula can be varied, taking into account current theoretical uncertainties in the reaction rate. The results are illustrated for the example of the ^{12}C+^{12}C fusion reaction. This reaction is very important for the understanding of nuclear burning in evolved stars, in exploding white dwarfs producing type Ia supernovae, 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 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 > 10^9 g/cc.

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

2005-06-16

101

Rate of reaction of OH with HNO3  

NASA Technical Reports Server (NTRS)

Measurements of the kinetics of the reaction of OH with HNO3, and mechanisms of HNO3 removal from the stratosphere, are reported. Bimolecular rate constants were determined at temperatures between 224 and 366 K by monitoring the concentrations of OH radicals produced by HNO3 photolysis and HNO3 according to their resonance fluorescence and 184.9-nm absorption, respectively. The rate constant measured at 298 K is found to be somewhat faster than previously accepted values, with a negative temperature dependence. Calculations of a one-dimensional transport-kinetic atmospheric model on the basis of the new rate constant indicate reductions in O3 depletion due to chlorofluoromethane release and NOx injection, of magnitudes dependent on the nature of the reaction products.

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

1981-01-01

102

On the Rate of Boronate Ester Formation in ortho-Aminomethyl Functionalized Phenyl Boronic Acids‡  

PubMed Central

The role of the ortho-aminomethyl functional group in phenyl boronic acids for sugar complexation is a topic of debate. To decipher its effect on the kinetics of boronate ester formation, we first performed pseudo-first order kinetics analyses at five pH values up to 4 mM in fructose, revealing a first-order kinetic dependence upon fructose. Under these conditions, the reaction is in equilibrium and does not reach completion, but at 50 mM fructose saturation is achieved revealing zero-order dependence upon fructose. This indicates rate-determining creation of an intermediate prior to reaction with fructose, which we propose involves leaving group departure of inserted solvent. Further, the region of kinetics displaying zero-order dependence has a kinetic isotope effect (KIE) of 1.42, showing involvement of a proton transfer in the leaving group departure. The ratio of forward and reverse rate constants branching from the intermediate shows that fructose is several thousand times more nucleophilic than the solvent. Overall, the data supports a mechanism where the o-aminomethyl group lowers the pKa of the proximal boronic acid and acts as a general-acid (as an ammonium) to facilitate leaving group departure. Consequently, by microscopic reversibility the resulting amine must perform general-base catalysis to deliver fructose. PMID:23441105

Collins, Byron E.; Metola, Pedro

2012-01-01

103

Construction of a microcumputer-based reaction-rate ratemeter.  

PubMed

The design and construction of a ratemeter based on a KIM-1 single-board microcomputer are discussed. The reaction monitor signal is digitized with a voltage-frequency converter and the resulting data points are stored in computer memory. Through software, the reaction-rate is calculated by the fixed-time computation method. A unique software-hardware combination is used for conversion from binary to binary-coded decimal format so that the results are printed in decimal notation. The characteristics of the ratemeter are compared with those of earlier ratemeters. PMID:18963065

Ryan, M A; Ingle, J D

1981-07-01

104

Suppression of excited-state contributions to stellar reaction rates  

NASA Astrophysics Data System (ADS)

It has been shown in previous work [Kiss , Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.101.191101 101, 191101 (2008); Rauscher , Phys. Rev. C10.1103/PhysRevC.80.035801 80, 035801 (2009)] that a suppression of the stellar enhancement factor (SEF) occurs in some endothermic reactions at and far from stability. This effect is re-evaluated using the ground-state contributions to the stellar reaction rates, which were shown to be better suited to judging the importance of excited-state contributions than the previously applied SEFs. An update of the tables shown in the latter work is given. The new evaluation finds 2350 cases (out of a full set of 57 513 reactions) for which the ground-state contribution is larger in the reaction direction with a negative reaction Q value than in the exothermic direction, thus providing exceptions to the commonly applied Q value rule. The results confirm the Coulomb suppression effect but lead to a larger number of exceptions than previously found. This is due to the fact that often a large variation in the g.s. contribution does not lead to a sizable change in the SEF. On the other hand, several previously identified cases do not appear anymore because it is found that their g.s. contribution is smaller than inferred from the SEF.

Rauscher, T.

2013-09-01

105

Quantum Low-Temperature Limit of a Chemical Reaction Rate  

Microsoft Academic Search

The radiation-induced polymerization of formaldehyde has been studied in the solid state. The time of addition of one new link to a polymer chain increases exponentially in accordance with the Arrhenius law at 140 to 80 K, but approaches a constant value (approximately 10-2 second) at temperatures below 10 K. Thus, a low-temperature limit to a chemical reaction rate has

V. I. Goldanskii; M. D. Frank-Kamenetskii; I. M. Barkalov

1973-01-01

106

Reaction rates from pressure-gauge measurements in reacting explosives  

Microsoft Academic Search

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

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

1981-01-01

107

Measurements of astrophysical reaction rates for radioactive samples  

Microsoft Academic Search

Reaction rates for both big-band and stellar nucleosynthesis can be obtained from the measurement of (n,p) and (n,?) cross sections for radioactive nuclei. In the past, large backgrounds associated with the sample activity limited these types of measurements to radioisotopes with very long half lives. The advent of the low-energy, high-intensity neutron source at the Los Alamos Neutron Scattering CEnter

P. E. Koehler; H. A. O’Brien; C. D. Bowman

1988-01-01

108

Measurements of astrophysical reaction rates for radioactive samples  

Microsoft Academic Search

Reaction rates for both big-bang and stellar nucleosynthesis can be obtained from the measurement of (n,p) and (n,..gamma..) cross sections for radioactive nuclei. In the past, large backgrounds associated with the sample activity limited these types of measurements to radioisotopes with very long half lives. The advent of the low-energy, high-intensity neutron source at the Los Alamos Neutron Scattering CEnter

P. E. Koehler; H. A. OBrien; C. D. Bowman

1987-01-01

109

Measurements of astrophysical reaction rates for radioactive samples  

Microsoft Academic Search

Reaction rates for both big-band and stellar nucleosynthesis can be obtained from the measurement of (n,p) and (n,gamma) cross sections for radioactive nuclei. In the past, large backgrounds associated with the sample activity limited these types of measurements to radioisotopes with very long half lives. The advent of the low-energy, high-intensity neutron source at the Los Alamos Neutron Scattering CEnter

P. E. Koehler; H. A. O'Brien; C. D. Bowman

1988-01-01

110

Electrostatic effects on the rates of DNA-catalyzed reactions  

NASA Astrophysics Data System (ADS)

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

Pack, George R.; Wong, Linda

1996-04-01

111

Application of semiclassical methods to reaction rate theory  

SciTech Connect

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

Hernandez, R.

1993-11-01

112

Electron reactions with thiosulphate ions at various ionic strengths  

NASA Astrophysics Data System (ADS)

The rate constant for the reaction of the hydrated electron with thiosulphate has been measured in the absence and presence of sodium sulphate. The bimolecular rate constant obtained by dividing the observed pseudo first order rate constant by the concentration of Na 2S 2O 3 did not change significantly when a fraction of the Na 2S 2O 3 was replaced by Na 2SO 4. It is concluded that both salts have the same physical effects on the rate constant. Neither salt is fully dissociated at the concentrations employed so that the solutions contained S 2O 2-3 and NaS 2O -3. Rate constants for the reaction of e -aq with each ion could not be calculated, but a value of k(e -aq+S 2O 2-3) of ca. 5 X 8 dm 3 mol -1 s -1 at zero ionic strength was estimated from a short extrapolation from 1.07 x 10 -2 mol dm -3 Na 2S 2O 3 for which k=(1.82 ± 0.02) x 10 8dm3mol-1s-1. This value is different from previously published values which vary by a factor of 24. For ca. 0.1 mol dm -3 solutions of S 2O 2-3 the end-of-pulse yield of e -aq was found to be the same as for 10 -3 mol dm -3 H +aq, indicating that S 2O 2-3 does not scavenge presolvated electrons at that concentration. Such solutions, therefore, could not inhibit muonium formation in they provide a background against which to test models of muonium formation.

Buxton, George V.; Walker, David C.

113

The reaction of nitrite with the haemocyanin of Astacus leptodactylus.  

PubMed Central

The reaction of nitrite at pH 5.7 with deoxyhaemocyanin of Astacus leptodactylus yielded methaemocyanin in two one-electron steps, as nitrite was reduced to NO. This methaemocyanin could be almost fully regenerated by an anaerobic treatment with HONH2, in contrast with the methaemocyanin prepared with H2O2. A destruction of active sites on treating oxyhaemocyanin with HONH2 explains the partial regeneration of methaemocyanin under air, as traces of H2O2 are formed in the autoxidation of HONH2. The reaction rate of nitrite with deoxyhaemocyanin is almost 15 times that with oxyhaemocyanin. The slope of -1.0 for the logarithm of the pseudo-first-order rate constants plotted against pH indicates that HNO2 is the reacting species. Methaemocyanin was e.p.r.-undetectable, but a binuclear signal was observed at g = 2 on binding nitrite to methaemocyanin. This signal disappeared with a pKa of 6.50, suggesting that a mu-aquo bridging ligand, which can be replaced by nitrite, is deprotonated to a mu-hydroxo bridging ligand, which resists substitution by nitrite. The intensity of this triplet e.p.r. signal allowed the determination of the association constant of nitrite to the active site of Astacus methaemocyanin and yielded a value of 237 M-1 at pH 5.7. The interpretation by some authors of nitrosylhaemocyanin as a nitrite derivative of semimethaemocyanin is contradicted by this rapid reaction of nitrite with copper(I) in deoxyhaemocyanin and in semi-methaemocyanin and by the low binding constant of nitrite to the active site of methaemocyanin. PMID:2833232

Tahon, J P; Van Hoof, D; Vinckier, C; Witters, R; De Ley, M; Lontie, R

1988-01-01

114

The reaction of nitrite with the haemocyanin of Astacus leptodactylus.  

PubMed

The reaction of nitrite at pH 5.7 with deoxyhaemocyanin of Astacus leptodactylus yielded methaemocyanin in two one-electron steps, as nitrite was reduced to NO. This methaemocyanin could be almost fully regenerated by an anaerobic treatment with HONH2, in contrast with the methaemocyanin prepared with H2O2. A destruction of active sites on treating oxyhaemocyanin with HONH2 explains the partial regeneration of methaemocyanin under air, as traces of H2O2 are formed in the autoxidation of HONH2. The reaction rate of nitrite with deoxyhaemocyanin is almost 15 times that with oxyhaemocyanin. The slope of -1.0 for the logarithm of the pseudo-first-order rate constants plotted against pH indicates that HNO2 is the reacting species. Methaemocyanin was e.p.r.-undetectable, but a binuclear signal was observed at g = 2 on binding nitrite to methaemocyanin. This signal disappeared with a pKa of 6.50, suggesting that a mu-aquo bridging ligand, which can be replaced by nitrite, is deprotonated to a mu-hydroxo bridging ligand, which resists substitution by nitrite. The intensity of this triplet e.p.r. signal allowed the determination of the association constant of nitrite to the active site of Astacus methaemocyanin and yielded a value of 237 M-1 at pH 5.7. The interpretation by some authors of nitrosylhaemocyanin as a nitrite derivative of semimethaemocyanin is contradicted by this rapid reaction of nitrite with copper(I) in deoxyhaemocyanin and in semi-methaemocyanin and by the low binding constant of nitrite to the active site of methaemocyanin. PMID:2833232

Tahon, J P; Van Hoof, D; Vinckier, C; Witters, R; De Ley, M; Lontie, R

1988-02-01

115

Reaction Rate of Small Diffusing Molecules on a Cylindrical Membrane  

NASA Astrophysics Data System (ADS)

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

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

2007-10-01

116

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. Catalogue identifier: AENW_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AENW_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: MIT license No. of lines in distributed program, including test data, etc.: 94512 No. of bytes in distributed program, including test data, etc.: 1395674 Distribution format: tar.gz Programming language: Fortran 90/95, Python (version 2.6.x or later, including any version of Python 3, is recommended). Computer: Not computer specific. Operating system: Any for which Python, Fortran 90/95 compiler and the required external routines are available. Has the code been vectorized or parallelized?: The program can efficiently utilize 4096+ processors, depending on problem and available computer. At low temperatures, 110 processors are reasonable for a typical umbrella integration run with an analytic potential energy function and gradients on the latest x86-64 machines.

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

2013-03-01

117

Products and kinetics of the heterogeneous reaction of suspended vinclozolin particles with ozone.  

PubMed

Vinclozolin is a widely used fungicide that can be released into the atmosphere via application and volatilization. This paper reports an experimental investigation on the heterogeneous ozonation of vinclozolin particles. The ozonation of vinclozolin adsorbed on azelaic acid particles under pseudo-first-order conditions is investigated online with a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS). The ozonation products are analyzed with a combination of VUV-ATOFMS and GC/MS. Two main ozonation products are observed. The formation of the ozonation products results from addition of O(3) on the C-C double bond of the vinyl group. The heterogeneous reactive rate constant of vinclozolin particles under room temperature is (2.4 ± 0.4) × 10(-17) cm(3) molecules(-1) s(-1), with a corresponding lifetime at 100 ppbv O(3) of 4.3 ± 0.7 h, which is almost comparable with the estimated lifetime due to the reaction with atmospheric OH radicals (?1.7 h). The reactive uptake coefficient for O(3) on vinclozolin particles is (6.1 ± 1.0) × 10(-4). PMID:21038901

Gan, Jie; Yang, Bo; Zhang, Yang; Shu, Xi; Liu, Changgeng; Shu, Jinian

2010-11-25

118

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

119

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

120

Application of Semiclassical Methods to Reaction Rate Theory  

NASA Astrophysics Data System (ADS)

This work is concerned with the development of approximate methods to describe relatively large chemical systems. This effort has been divided into two primary directions: First, we have extended and applied a semiclassical transition state theory (SCTST) originally proposed by Miller* to obtain microcanonical and canonical (thermal) rates for chemical reactions described by a nonseparable Hamiltonian, i.e., most reactions. Second, we have developed a method to describe the fluctuations of decay rates of individual energy states from the average RRKM rate in systems where the direct calculation of individual rates would be impossible. Combined with the semiclassical theory this latter effort has provided a direct comparison to the experimental results of Moore and coworkers. ^dagger. In SCTST, the Hamiltonian is expanded about the barrier and the "good" action-angle variables are obtained perturbatively; a WKB analysis of the effectively one-dimensional reactive direction then provides the transmission probabilities. ^ddagger The advantages of this local approximate treatment are that it includes tunneling effects and anharmonicity, and it systematically provides a multi-dimensional dividing surface in phase space. The SCTST thermal rate expression has been reformulated providing increased numerical efficiency (as compared to a naive Boltzmann average), an appealing link to conventional transition state theory (involving a "pre-reactive" partition function depending on the action of the reactive mode), and the ability to go beyond the perturbative approximation. ^S. In addition, the distribution of unimolecular decay rates at threshold energies to dissociation has been modeled by describing the quasi-bound states as strongly -mixed. The possible existence of globally conserved symmetries --which would break this ansatz--is included by treating each symmetry block of the Hamiltonian separably and assuming the ansatz for each symmetry manifold. Use of SCTST to describe the reaction dynamics converts the model into a predictive theory depending only on the assumed dynamical symmetries. Comparison to the experimental decay rate distributions for S_0 formaldehyde dissociation^{rm X} has revealed the presence of strong-mixing between the quasi-bound states and has further suggested that an electric field can break the C_{s} symmetry of the dissociation.^| ftn*W. H. Miller, Faraday Discussions Chem. Soc. 62, 140 (1977). ^daggerW. F. Polik, D. R. Guyer and C. B. Moore, J. Chem. Phys. 92, 3453 (1990). ^ddaggerW. H. Miller, R. Hernandez, N. C. Handy, D. Jayatilaka and A. Willetts, Chem. Phys. Lett. 172, 62 (1990). ^ SR. Hernandez and W. H. Miller, Chem. Phys. Let. 214, 129 (1993). ^{rm X}W. F. Polik, D. R. Guyer and C. B. moore, J. Chem. Phys. 92, 3453 (1990). ^|R. Hernandez, W. H. Miller, C. B. Moore and W. F. Polik, J. Chem. Phys. 99, 950 (1993).

Hernandez, Rigoberto

121

Reactions of TiO2 excess electron in nanocrystallite aqueous solutions studied in pulse and gamma-radiolytic systems  

NASA Astrophysics Data System (ADS)

Reactions of excess electrons in TiO2 nanocrystallites produced by radiolysis have been studied by both steady state and pulse radiolysis techniques. The TiO2 electrons were produced by ? or pulse radiolysis of 2-propanol or t-butanol aqueous solutions in acid pH's. The effects of various materials, including such that are often present in photocatalytic systems such as hydrogen peroxide, oxygen, nitrate and perchlorate on the decay of the TiO2 electron absorption was followed, using two nanocrystallite sizes with average diameters of 1.0 and 4.7 nm. The rates of electron reactions depend on particle size. Several scavengers including Cu2+, ClO2-, ClO3-, NO2-, and NO3- show decay of the TiO2 electron predominantly by single pseudo-first order process. The rate of reaction of the above ions in the large nanocrystallites systems is 2-10 times faster than in the respective small particle systems. Hydrogen peroxide and oxygen show a multi-exponential decay, which is supposedly related to different kinds of adsorption. Large particles show reduction rates 2-3 orders faster for H2O2 and 10-20 times faster for O2, compared to the small particles. The reaction of the electron with ClO4- ions is first order in [ClO4-] but second order with respect to the TiO2 electron. In the large particle system the rate was found to depend on the number of electrons per particle. (No similar information is available for the small particles). The rates of reduction of the different scavengers, tend to increase with the driving force, although not always. Deviations from linear dependency of log k on the driving force are large. The mechanisms and role of adsorption are discussed.

Gao, Ruomei; Safrany, Agnes; Rabani, Joseph

2003-05-01

122

SHEAR LOCALIZATION AND CHEMICAL REACTION IN HIGH-STRAIN, HIGH-STRAIN-RATE DEFORMATION OF  

E-print Network

SHEAR LOCALIZATION AND CHEMICAL REACTION IN HIGH-STRAIN, HIGH-STRAIN-RATE DEFORMATION OF Ti.6±1 mm. Thermodynamic and kinetic calculations yield the reaction rate outside the shear bands a decisive role in accelerating chemical reactions, particularly, converting an exothermic reaction

Meyers, Marc A.

123

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

ERIC Educational Resources Information Center

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

Copper, Christine L.; Koubeck, Edward

1999-01-01

124

The influence of steric hindrance on kinetics and isotope effects in the reaction of 2,2-bis(4-dimethylaminophenyl)-1-nitro-1-(4-nitrophenyl)ethane with DBU base in acetonitrile  

NASA Astrophysics Data System (ADS)

The pKa value for 2,2-bis(4-dimethylaminophenyl)-1-nitro-1-(4-nitrophenyl)ethane, (dmap)2 (pKa = 25.11) has been measured spectrophotometrically using buffer solutions of a few strong amine bases: 1,8-diazabicyclo[5.4.0]undec-7-ene, (DBU); 1,1,3,3-tetramethylguanidine, (TMG); 1,5,7-triazabicyclo[4.4.0]dec-5-ene, (TBD); 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene, (MTBD) and their salts. The low energy conformers of nitrophenyl nitroalkanes have been determined using the semiempirical PM6 methods, (B3-LYP) density functional theory (DFT) together with the 6-31G(d,p) basis set. The participation of the low energy conformer in the proton transfer reaction to DBU base has been discussed. The kinetic data for proton transfer reactions between (dmap)2 and DBU in acetonitrile (MeCN) at pseudo-first order conditions have been presented. The influence of steric hindrance brought by reacting C-acid and organic base on the stability of the transition state has been discussed. The rates of second-order rate constants for series of nitrophenyl nitroalkanes, NO2PhCHRNO2 (R = Me; Et; iPr; dimethylaminophenyl = (dmap)2) are presented and discussed.

Nowak, Iwona; Jarczewski, Arnold

2014-11-01

125

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

ERIC Educational Resources Information Center

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

Cakmakci, Gultekin; Leach, John; Donnelly, James

2006-01-01

126

Effects of mineral spatial distribution on reaction rates in porous media  

E-print Network

Effects of mineral spatial distribution on reaction rates in porous media L. Li,1 C. A. Peters,2 on reaction rates in porous media. Pore-scale network models were constructed to represent sandstones. Peters, and M. A. Celia (2007), Effects of mineral spatial distribution on reaction rates in porous media

Peters, Catherine A.

127

E2C mechanism in elimination reactions. VI. Primary hydrogen isotope effects on rates of E2 reactions of alicyclics  

Microsoft Academic Search

Primany hydrogen isotope effects on the rates of bimolecular elimination ; reactions of some cyclohexyl tosylates and bromides are small (k\\/sup H\\/k\\/sup D\\/ ; approximately 2 -3) for very E2C-like reactions, pass through a maximum of 6 for ; more E2H-like reactions, and are small again (2 -- 3) for very E2H-like reactions. ; Movement from the E2C to the

David Cook; R. E. J. Hutchinson; J. K. MacLeod; A. J. Parker

1974-01-01

128

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

129

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 (1–2 s). Thus, the extent of homogenization can be monitored in a CTR using the relatively fast reaction of the reagent and p-hydroquinone. Kinetically controlled reactions of catechols, however, could be also completed in a reasonable time at increased reagent concentration. A satisfactory reactor, operating at 1.7 cm/s (2 ?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. PMID:15858975

Jung, Moon Chul; Weber, Stephen G.

2006-01-01

130

The surface reaction kinetics of salicylate on alumina  

SciTech Connect

The kinetics of reaction of salicylate with colloidal alumina in aqueous suspension and with Al(III) in homogeneous aqueous solution were studied by stopped-flow laser fluorescence spectroscopy. The emission spectra confirmed the formation of both monodentate complexes and more stable bidentate chelates. Temporal evolution of the spectra indicated that the reaction was fast (within first few minutes) for both the homogeneous and heterogeneous reactions but slowed down afterwards for the latter. Reactions completed within 10 minutes in homogeneous phase at pH 3.3 but took more than 12 hours in alumina suspension. Analysis of the fluorescence intensity within first four minutes showed that in homogeneous phase the reaction followed a single pseudo-first-order kinetics. In alumina suspension log plots were nonlinear and characteristic of multiple heterogeneous reaction paths. The kinetics are interpreted in terms of the simultaneous formation of multiple species as well as subsequent conversion between species.

Wang, Z.; Ainsworth, C.C.; Friedrich, D.M.; Joly, A.G.; Gassman, P.L. [Pacific Northwest National Lab., Richland, WA (United States)

1997-12-31

131

Reactions of hydroxyl radicals with selected hydrocarbons of importance in atmospheric chemistry  

Microsoft Academic Search

The kinetics of the reactions of OH with i-butanol and t-butanol have been studied by laser flash photolysis-laser induced fluorescence (LP-LIF), under pseudo-first-order conditions. The values, k(OH + i-butanol) equals (9.0 +\\/- 0.9) X 10-12 cm3 molecule-1 s-1 and k(OH + t-butanol) equals 1.8 +\\/- 0.2) X 10-13 cm3 molecule-1 s-1 at 292 K were obtained. As a check on

Sandra M. Saunders; K. J. Hughes; M. J. Pilling; D. L. Baulch; P. I. Smurthwaite

1993-01-01

132

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

Microsoft Academic Search

Depletion is a game that reinforces central concepts of reaction rate theory through simulation. Each player buys chemicals and guides them through a series of reactions, thereby earning money to buy more chemicals. The reactions occur when players roll a high enough value on two dice to overcome an activation barrier. The reactions may be accelerated by buying heat (which

Donald J. Olbris; Judith Herzfeld

2002-01-01

133

On the evaluation of an integral connected with the thermonuclear reaction rate in closed form  

Microsoft Academic Search

A parameter-dependent integral is obtained in connection with the theoretical investigation of the reaction rate for low-energy nonresonant thermonuclear reactions in nondegenerate nuclear plasma. Until now, in connection with nuclear physics, this integral was only approximately evaluated. Approaches for a mathematically more exact calculation of the nuclear reaction rate are considered, taking into account a more general integral. The theoretical

H. J. Haubold; R. W. John

1978-01-01

134

Big-Bang reaction rates within the R-matrix model  

NASA Astrophysics Data System (ADS)

We use the R-matrix theory to fit S-factor data on nuclear reactions involved in Big Bang nucleosynthesis. We derive the reaction rates with associated uncertainties, which are evaluated on statistical grounds. We provide S factors and reaction rates in tabular and graphical formats (available at http://pntpm3.ulb.ac.be/bigbang).

Descouvemont, P.; Adahchour, A.; Angulo, C.; Coc, A.; Vangioni-Flam, E.

2005-07-01

135

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

PubMed

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

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

2013-08-01

136

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

PubMed

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

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

2005-04-01

137

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

138

Rate constants of reactions of ?-carrageenan with hydrated electron and hydroxyl radical  

NASA Astrophysics Data System (ADS)

The rate constants for the reactions of ?-carrageenan with hydrated electron and hydroxyl radical was investigated by pulse radiolysis and laser photolysis. The kinetics of the reaction of hydrated electron indicates no seeming reaction with ?-carrageenan. On the other hand, hydroxyl radical reacts very rapidly with ?-carrageenan at a rate constant of approximately 1.2 × 10 9 M -1 s -1. This rate constant varies with pH.

Abad, L. V.; Saiki, S.; Kudo, H.; Muroya, Y.; Katsumura, Y.; de la Rosa, A. M.

2007-12-01

139

A study of rates of reaction of some substituted diphenyldiazomethanes and benzoic acid  

E-print Network

A STUDY OF RATES OF REACTION OF SOME SUBSTITUTED DIPHENYLDIAZ(METHANES AM) BENZOIC ACID A Dissertation By Ralph Fredrick Gilby, Jr. Approved as to style and content by; May 195^ A STOUT 'OF1 WffBS CF KEAC9XOI O f SOB ffliHfiiyi'f ilfl.... . . . . . . . . 11 II. Reaction Rate Constants (k) and Substituted Diphenyldiazomethanes . . . . 32 III. Determinations of the Reaction Rate of if,-Dimethoxydiphenyldiazomethane and Benzoic Acid................................. ... . %8 IV...

Gilby, Ralph Fredrick

2013-10-04

140

Kinetics and Mechanism of Formation of [Fe(CN)5DPA] by Ligand Substitution Reaction in the Presence of Surfactant Micelle in Acidic Media  

Microsoft Academic Search

The kinetics and mechanism of formation of [Fe(CN)5D-PA] by ligand substitution reaction between aquapentacyanoferrate(II) ion, and D-pencillamine (D-PA) has been investigated spectrophothmetrically in presence of anionic surfactant micelle, sodium dodecylsulphate (SDS) at 421 nm (?max of product) under pseudo-first order conditions using excess of D-PA over [Fe(CN)5H2O]. The reaction was studied as a function of [Fe(CN)5H2O], [D-PA], [SDS], pH, ionic strength,

Radhey Mohan Naik; Basant Kumar

2012-01-01

141

Kinetics and Mechanism of Formation of [Fe(CN)5DPA] by Ligand Substitution Reaction in the Presence of Surfactant Micelle in Acidic Media  

Microsoft Academic Search

The kinetics and Mechanism of Formation of [Fe(CN)5D-PA] by ligand substitution reaction between aquapentacyanoferrate(II) ion, and D-pencillamine (D-PA) has been investigated spectrophothmetrically in presence of anionic surfactant micelle, sodium dodecylsulphate (SDS) at 421 nm (?max of product) under pseudo- first order conditions using excess of D-PA over [Fe(CN)5H2O]. The reaction was studied as a function of [Fe(CN)5H2O], [D-PA], [SDS], pH, ionic

Radhey Mohan Naik; Basant kumar

2011-01-01

142

Quick and Easy Rate Equations for Multistep Reactions  

ERIC Educational Resources Information Center

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

Savage, Phillip E.

2008-01-01

143

Reaction rate oscillations during catalytic CO oxidation: A brief overview  

NASA Technical Reports Server (NTRS)

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

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

1987-01-01

144

Charged-Particle Thermonuclear Reaction Rates: I. Monte Carlo Method and Statistical Distributions  

E-print Network

A method based on Monte Carlo techniques is presented for evaluating thermonuclear reaction rates. We begin by reviewing commonly applied procedures and point out that reaction rates that have been reported up to now in the literature have no rigorous statistical meaning. Subsequently, we associate each nuclear physics quantity entering in the calculation of reaction rates with a specific probability density function, including Gaussian, lognormal and chi-squared distributions. Based on these probability density functions the total reaction rate is randomly sampled many times until the required statistical precision is achieved. This procedure results in a median (Monte Carlo) rate which agrees under certain conditions with the commonly reported recommended "classical" rate. In addition, we present at each temperature a low rate and a high rate, corresponding to the 0.16 and 0.84 quantiles of the cumulative reaction rate distribution. These quantities are in general different from the statistically meaningless "minimum" (or "lower limit") and "maximum" (or "upper limit") reaction rates which are commonly reported. Furthermore, we approximate the output reaction rate probability density function by a lognormal distribution and present, at each temperature, the lognormal parameters miu and sigma. The values of these quantities will be crucial for future Monte Carlo nucleosynthesis studies. Our new reaction rates, appropriate for bare nuclei in the laboratory, are tabulated in the second paper of this series (Paper II). The nuclear physics input used to derive our reaction rates is presented in the third paper of this series (Paper III). In the fourth paper of this series (Paper IV) we compare our new reaction rates to previous results.

Richard Longland; Christian Iliadis; Art Champagne; Joe Newton; Claudio Ugalde; Alain Coc; Ryan Fitzgerald

2010-04-23

145

Cross Sections and Reaction Rates for Comparative Planetary Aeronomy  

Microsoft Academic Search

In this chapter we describe the current knowledge of a selection of collision processes and chemical reactions of importance to planetary aeronomy. Emphasis is placed on critical evaluation of what we know and what we wish we knew about fundamental processes required for interpretation, explanation, and modeling of atmospheric observations.

David L. Huestis; Stephen W. Bougher; Jane L. Fox; Marina Galand; Robert E. Johnson; Julianne I. Moses; Juliet C. Pickering

2008-01-01

146

Cross Sections and Reaction Rates for Comparative Planetary Aeronomy  

Microsoft Academic Search

In this chapter we describe the current knowledge of a selection of collision processes and chemical reactions of importance\\u000a to planetary aeronomy. Emphasis is placed on critical evaluation of what we know and what we wish we knew about fundamental\\u000a processes required for interpretation, explanation, and modeling of atmospheric observations.

David L. Huestis; Stephen W. Bougher; Jane L. Fox; Marina Galand; Robert E. Johnson; Julianne I. Moses; Juliet C. Pickering

147

Cross Sections and Reaction Rates for Comparative Planetary Aeronomy  

Microsoft Academic Search

In this chapter we describe the current knowledge of a selection of collision processes and chemical reactions of importance\\u000a to planetary aeronomy. Emphasis is placed on critical evaluation of what we know and what we wish we knew about fundamental\\u000a processes required for interpretation, explanation, and modeling of atmospheric observations.

David L. Huestis; Stephen W. Bougher; Jane L. Fox; Marina Galand; Robert E. Johnson; Julianne I. Moses; Juliet C. Pickering

2008-01-01

148

Forward and reverse rate coefficients in equilibrating isomerization reactions  

Microsoft Academic Search

Three models for the relaxation kinetics of a reversible unimolecular isomerization reaction are formulated and analyzed: a generalization of the simple Lindemann–Hinshelwood scheme, a detailed model with the strong collision approximation, and a master equation solution. For such systems the use of a classical relaxation analysis has been questioned. In each case it is found that the relaxation analysis does

Nicholas J. B. Green; Philip J. Marchant; Michael J. Perona; Michael J. Pilling; Struan H. Robertson

1992-01-01

149

Forward and reverse rate coefficients in equilibrating isomerization reactions  

Microsoft Academic Search

Three models for the relaxation kinetics of a reversible unimolecular isomerization reaction are formulated and analyzed: a generalization of the simple Lindemann--Hinshelwood scheme, a detailed model with the strong collision approximation, and a master equation solution. For such systems the use of a classical relaxation analysis has been questioned. In each case it is found that the relaxation analysis does

Nicholas J. B. Green; Philip J. Marchant; Michael J. Perona; Michael J. Pilling; Struan H. Robertson

1992-01-01

150

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

151

Estimation of the Mutation Rate during Error-prone Polymerase Chain Reaction  

E-print Network

Estimation of the Mutation Rate during Error-prone Polymerase Chain Reaction Dai Wang1 , Cheng-prone polymerase chain reaction (PCR) is widely used to introduce point mutations during in vitro evolution step of in vitro evolution is mutagenesis. Error-prone polymerase chain reaction (PCR) (Leung et al

Sun, Fengzhu - Sun, Fengzhu

152

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

NASA Technical Reports Server (NTRS)

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

DeMore, W.; Bayes, K.

1998-01-01

153

Effect of temperature oscillation on chemical reaction rates in the atmosphere  

NASA Technical Reports Server (NTRS)

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

Eberstein, I. J.

1974-01-01

154

Modelling Thermal Runaway and Criticality in Systems with Diminishing Reaction Rates: The Uniform Temperature (Semenov) Approximation  

Microsoft Academic Search

Various complex exothermic oxidations of considerable technical importance can be represented by an empirical rate-law in which the isothermal reaction rate diminishes with the elapsed time t according to rate propto t-alpha or, more generally, rate propto (t+tpr)-alpha. Here tpr is a 'prior reaction' time and the exponent alpha lies between 0 and 1. We have computed the generalized behaviour

T. Boddington; P. Gray; S. K. Scott

1982-01-01

155

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

156

Molecular dynamics simulations of nonequilibrium rate constant in a model exothermic reaction  

Microsoft Academic Search

In this Letter, we study the nonequilibrium effects which appear in a thermally activated exothermic reaction A+A?products using the molecular dynamics for reactive hard spheres. We have found that the rate constant is reduced with respect to its equilibrium value and the relative decrease of rate constant is similar to that observed for a thermoneutral reaction with the same activation

Jerzy Gorecki; Joanna Natalia Gorecka

2000-01-01

157

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

E-print Network

1 Absolute and relative rate constants for the reactions of OH and Cl with Pentanols M. Lendar ABSTRACT 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 cm3 molecule-1 s-1 ): k(1- pentanol

Boyer, Edmond

158

Role of Fitted Reaction Rates in Predicting Thrombin Pro-Wenrui Hao1  

E-print Network

Role of Fitted Reaction Rates in Predicting Thrombin Pro- duction Wenrui Hao1 and Guang Lin2 of thrombin production. We first present algorithms based on numerical algebraic geometry to compute steady the significance of the 16 data-fitted reaction rates with respect to the total thrombin production

Sommese, Andrew J.

159

Water-Splitting Photoelectrolysis Reaction Rate via Microscopic Imaging of Evolved Oxygen Bubbles  

E-print Network

Water-Splitting Photoelectrolysis Reaction Rate via Microscopic Imaging of Evolved Oxygen Bubbles Institute of Technology, Pasadena, California 91125, USA Bubble formation and growth on a water of the gas-evolving reaction rate. Optical microscopy was used to record the bubble growth on single

Atwater, Harry

160

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

EPA Science Inventory

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

161

Survey of Reaction Rate Constants in the Reaction System of Hydrazine and NTO  

NASA Astrophysics Data System (ADS)

Hydrazine (N2H4) and NTO (dinitrogen tetroxide: N2O4) mixtures are used in spacecraft bipropellant systems, having the advantage, for sampling missions, of having no carbon composition. However, no reasonable combustion model of hydrazine and NTO has been developed, although an N/H/O kinetic reaction model has been investigated. To construct a hydrazine and NTO combustion model that is useful for bipropellant thruster simulation, we surveyed the literature concerned with elementary hydrazine and NTO mixture reactions for N/H/O systems. A total of 245 reactions found in 16 available studies are dealt with in the present paper.

Ohminami, Kaori; Ogawa, Hiroyuki

162

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

PubMed

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

Peira, Nathalie; Fredrikson, Mats; Pourtois, Gilles

2014-03-01

163

Upper atmosphere research: Reaction rate and optical measurements  

NASA Technical Reports Server (NTRS)

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

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

1990-01-01

164

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

NASA Astrophysics Data System (ADS)

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

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

2009-04-01

165

Communication: rate coefficients from quasiclassical trajectory calculations from the reverse reaction: The Mu + H2 reaction re-visited.  

PubMed

In a previous paper [P. G. Jambrina et al., J. Chem. Phys. 135, 034310 (2011)] various calculations of the rate coefficient for the Mu + H(2) ? MuH + H reaction were presented and compared to experiment. The widely used standard quasiclassical trajectory (QCT) method was shown to overestimate the rate coefficients by several orders of magnitude over the temperature range 200-1000 K. This was attributed to a major failure of that method to describe the correct threshold for the reaction owing to the large difference in zero-point energies (ZPE) of the reactant H(2) and product MuH (?0.32 eV). In this Communication we show that by performing standard QCT calculations for the reverse reaction and then applying detailed balance, the resulting rate coefficient is in very good agreement with the other computational results that respect the ZPE, (as well as with the experiment) but which are more demanding computationally. PMID:22803521

Homayoon, Zahra; Jambrina, Pablo G; Aoiz, F Javier; Bowman, Joel M

2012-07-14

166

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

NASA Technical Reports Server (NTRS)

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

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

1981-01-01

167

Chiral polymerization in open systems from chiral-selective reaction rates.  

PubMed

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. PMID:22610131

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

2012-08-01

168

Measurement of Sulfur Dioxide Reaction Rates in Wintertime Orographic Clouds  

Microsoft Academic Search

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

Jefferson Robert Snider

1989-01-01

169

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

170

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

NASA Astrophysics Data System (ADS)

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

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

2005-07-01

171

Sensitivity study of explosive nucleosynthesis in Type Ia supernovae: I. Modification of individual thermonuclear reaction rates  

E-print Network

We explore the sensitivity of the nucleosynthesis due to type Ia supernovae with respect to uncertainties in nuclear reaction rates. We have adopted a standard one-dimensional delayed detonation model of the explosion of a Chandrasekhar-mass white dwarf, and have post-processed the thermodynamic trajectories of every mass-shell with a nucleosynthetic code, with increases (decreases) by a factor of ten on the rates of 1196 nuclear reactions. We have computed as well hydrodynamic models for different rates of the fusion reactions of 12C and of 16O. For selected reactions, we have recomputed the nucleosynthesis with alternative prescriptions for their rates taken from the JINA REACLIB database, and have analyzed the temperature ranges where modifications of their rates have the strongest effect on nucleosynthesis. 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 12C nuclei. The energy of the explosion changes by less than \\sim4%. The changes in the nucleosynthesis due to the modification of the rates of fusion reactions are as well 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 two. 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 alphas. In general, the yields of Fe-group nuclei are more robust than the yields of intermediate-mass elements. Among the charged particle reactions, the most influential on supernova nucleosynthesis are 30Si + p \\rightleftarrows 31P + {\\gamma}, 20Ne + {\\alpha} \\rightleftarrows 24Mg + {\\gamma}, and 24Mg + {\\alpha} \\rightleftarrows 27Al + p. The temperatures at which a modification of their rate has a larger impact are in the range 2 < T < 4 GK. (abridged)

Eduardo Bravo; Gabriel Martínez-Pinedo

2012-04-09

172

PHYSICAL REVIEW C 72, 025806 (2005) Nuclear fusion in dense matter: Reaction rate and carbon burning  

E-print Network

PHYSICAL REVIEW C 72, 025806 (2005) Nuclear fusion in dense matter: Reaction rate and carbon August 2005) In this paper we analyze the nuclear fusion rates among equal nuclei for all five different.025806 PACS number(s): 26.50.+x, 25.60.Pj, 97.10.Cv I. INTRODUCTION We study nuclear fusion rates of identical

173

Determination of the theoretical rate constants for the reduction reactions of lanthanide ions by hydrated electrons  

Microsoft Academic Search

The mechanism of the reduction reaction of lanthanide(III) ions by hydrated electrons in polar solvents has been investigated.\\u000a The theoretical rate constants for the reaction of hydrated electrons with a number of lanthanide ions have been calculated\\u000a using the energy gap laws of the charge shift reaction(D+?A?D?A+) and compared with experimental values. With these results, we have explained the large

Kwang-Pill Lee; Keung-Shik Park; Duck-Won Kang; Yasuhiro Yamada; Shin-ichi Ohno

1998-01-01

174

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

175

Thermochemistry and accurate quantum reaction rate calculations for H2\\/HD\\/D2 + CH3  

Microsoft Academic Search

Accurate quantum-mechanical results for thermodynamic data, cumulative reaction probabilities (for J = 0), thermal rate constants, and kinetic isotope effects for the three isotopic reactions H-2 + CH3 -> CH4 + H, HD + CH3 -> CH4 + D, and D-2 + CH3 -> CH3D + D are presented. The calculations are performed using flux correlation functions and the multiconfigurational

Gunnar Nyman; Rob van Harrevelt; Uwe Manthe

2007-01-01

176

Verification of chemical reaction rate models in turbulent reacting flows at Schmidt number considerably exceeding 1  

Microsoft Academic Search

Relations widely used in numerical modeling describing the average rate of a chemical reaction have been analyzed. The spontaneous\\u000a distributions of the mixture fraction and reaction products measured simultaneously in different cross-sections of the jet\\u000a mixer under turbulent mixing of chemically reacting incompressible liquid media served as the basis for verification.

A. D. Chornyi; V. L. Zhdanov

2010-01-01

177

Magnetic field and isotope dependence of the reaction rates of micellized triplet radical pairs  

Microsoft Academic Search

Nanosecond time resolved optical absorption studies of the isotopic dependence of the magnetic field effect on geminate triplet benzylic radical pair reaction dynamics in micelles of HDTCl are reported. The isotopic susceptibilities of the T..mu..- radical pair reaction rate constants in small (<50 G) and intermediate (100-500 G) fields are substantially different. The magnitude and field dependence of the isotopic

Nicholas J. Turro; Matthew B. Zimmt; Ian R. Gould

1988-01-01

178

PHYSICAL REVIEW C 80, 014611 (2009) Nuclear fusion reaction rates for strongly coupled ionic mixtures  

E-print Network

PHYSICAL REVIEW C 80, 014611 (2009) Nuclear fusion reaction rates for strongly coupled ionic.014611 PACS number(s): 26.30.-k I. INTRODUCTION Nuclear fusion in dense stellar matter is most important the nuclear interaction. We will mostly focus on the Coulomb barrier penetration problem. Fusion reactions

179

Effects of arsenic incorporation on jarosite dissolution rates and reaction products  

E-print Network

Effects of arsenic incorporation on jarosite dissolution rates and reaction products Matthew R 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

Hu, Qinhong "Max"

180

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 Böhlke; Barbara A. Bekins; Steven P. Phillips

2010-01-01

181

Atmospheric Environment 38 (2004) 21772185 Proton transfer reaction rate constants between hydronium ion  

E-print Network

the average-dipole-orientation theory, along with the permanent dipole moment and polarizability to available literature data of the permanent dipole moment, polarizability, and ion­molecule reaction rate

182

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

183

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

DOEpatents

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

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

2003-04-01

184

The power-law TST reaction rate coefficient with tunneling correction  

NASA Astrophysics Data System (ADS)

We study the TST reaction rate for the systems with power-law distributions. We derive the expressions of the reaction rate coefficient with tunneling correction, which strongly depends on the power-law parameter. The numerical results show that a small deviation from one in the parameter can result in a significant change in the rate coefficient, but only cause a small change in the tunneling correction. Thus the tunneling correction is not sensitive to the power-law distributions. As an application example, we take the H+H2 reaction to calculate the power-law reaction rate coefficient with the tunneling correction, the results of which with the parameter slightly different from one are in good agreement with all the experimental studies in temperature range 2×102˜103 K.

Yin, Cangtao; Zhou, Yanjun; Du, Jiulin

2014-11-01

185

Rate Constants for Reactions of Phenoxyl Radicals in Solution Pedatsur Netaa...  

E-print Network

were generated by radiolysis, photolysis, thermolysis, or chemical reactions, and their rate constants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 2. Arrangement of the Tables. . . . . . . . . . . . . . . . . . . . 110 3. List of Abbreviations and Symbols. . . . . . . . . . . . . 111 4. References for the Introductory Material. . . . . . . . 112 5

Magee, Joseph W.

186

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

NASA Technical Reports Server (NTRS)

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

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

1984-01-01

187

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

SciTech Connect

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

Skakun, Ye. [Kharkiv Institute of Physics and Technology, Academicheskaja str., 1, 61108 Kharkiv (Ukraine); Rauscher, T. [Department of Physics, University of Basel, Klingelbergstr, 82, CH-4056 Basel (Switzerland)

2010-08-12

188

Reaction kinetics of plutonium with oxygen, water and humid air: moisture enhancement of the corrosion rate  

Microsoft Academic Search

Kinetic measurements for the reactions of unalloyed plutonium with water vapor and oxygen in the 35 to 400°C range show that moisture enhancement of the corrosion rate is confined to a temperature regime below 200°C. In combination with kinetic data from literature sources and measurements with air at 25°C, the data define a rate-temperature envelope in which the corrosion reaction

John M. Haschke; Thomas H. Allen; Jerry L. Stakebake

1996-01-01

189

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

Microsoft Academic Search

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

Rob van Harrevelt; Gunnar Nyman; Uwe Manthe

2007-01-01

190

THE REACTION RATE OF SODIUM SULFITE WITH DISSOLVED OXYGEN. Technical Report No. 73  

Microsoft Academic Search

The reaction rate of sodium sulfite was studied as a function of oxygen ;\\u000a concentration, NaâSOâ\\/Oâ ratio, temperature, catalyst, pH and ;\\u000a surface\\/volume ratio.The reaction rate of hydrazine with dissolved oxygen was ;\\u000a also studied. Oxygen removal is incomplete when the sulfite to oxygen ratio is ;\\u000a leas than the stoichiometric quantity. With the stoichiometric quantities of ;\\u000a oxygen and

1960-01-01

191

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

192

First Direct Measurement of the Resonant ^17F(p,?)^18Ne Reaction Rate  

NASA Astrophysics Data System (ADS)

Of significant importance in astrophysical events like novae and x-ray bursts is the rate of the ^17F(p,?)^18Ne reaction, where it contributes to element synthesis and energy generation. It is crucial for the production of radioactive ^18F in such scenarios, which is a target of gamma-ray telescopes like GLAST and INTEGRAL. However, the ^17F(p,?)^18Ne reaction rate had not been determined experimentally. A 3^+ state in ^18Ne predicted to dominate the rate was found at 599.8 keV using the ^17F(p,p)^17F reaction, but the resonance strength, which is directly related to the reaction rate, remained unknown. For the first time, the ^17F(p,?)^18Ne reaction has been measured directly with the Daresbury Recoil Separator, using a mixed beam of radioactive ^17F and stable ^17O from the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. A resonance strength and ? partial width for the 599.8 keV resonance was deduced, as well as an upper limit on the direct capture cross section at an energy of 800 keV. These results have been used to calculate a new reaction rate in the temperature range of interest, and the preliminary implications of this new, faster rate will be discussed.

Chipps, K. A.

2009-05-01

193

Kinetic study of the proton transfer reaction between 1-nitro-1-(4-nitrophenyl)alkanes and P 1- t-Bu phosphazene base in THF solvent  

NASA Astrophysics Data System (ADS)

The rates of proton transfer reactions between C-acids of the series of nitroalkanes with increasing bulk of R=H, Me, Et, i-Pr substituents as 4-nitro-phenylnitromethane ( 1), 1-(4-nitrophenyl)-1-nitroethane ( 2), 1-(4-nitrophenyl)-1-nitropropane ( 3), 2-methyl-1-(4-nitrophenyl)-1-nitropropane ( 4) and the P 1- t-Bu phosphazene have been measured in THF at pseudo-first-order conditions. The product of the proton transfer reaction with P 1- t-Bu is associated into ion pair. The equilibrium constants decrease along with growing bulk of alkyl substituent in the reacting C-acid: >100 000, 1170, 590, and 11.8 for 1, 2, 3, 4, respectively. The second order rate constants ( k2H) for this very strong base and C-acids are rapidly declining: 9357, 2.31, 0.66, 0.09 dm 3 mol -1 s -1 for 1, 2, 3, 4, respectively, and could not be accounted for the small values of the enthalpies of activation ?HH?=6.1,18.0,20.7 and 11.1 kJ mol. The appropriate values of the entropies of activation were all negative and relatively large ?SH?=-149.7,-176.5,-178.7,-227.8 J mol deg indicating an importance of steric and ordering effects in forming the transition state. The primary deuterium kinetic isotope effects are large showing tendency of reverse relation towards steric hindrance of the reacting C-acids, kH/ kD=15.8, 13.6, 13.2 for 1, 2, and 3, respectively. The explanation of the unusual slow proton transfer abstraction caused by this extremely strong base is undertaken.

Wis?ocka, ?aneta; Nowak, Iwona; Jarczewski, Arnold

2006-05-01

194

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

195

On the Presence of Limit-Cycles in a Model Exothermic Chemical Reaction: Sal'nikov's Oscillator with Two Temperature-Dependent Reaction Rates  

Microsoft Academic Search

This paper investigates a model chemical reaction in which a single substance undergoes a two-stage process of decay, first producing an intermediate species and finally giving a product chemical. Each of the two stages involves only simple first-order reaction kinetics, but the governing rate parameter for each of the two reactions is temperature dependent. The reaction vessel is assumed to

Lawrence K. Forbes; Mary R. Myerscough; Brian F. Gray

1991-01-01

196

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

NASA Technical Reports Server (NTRS)

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

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

1992-01-01

197

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

198

The nuclear fusion reaction rate based on relativistic equilibrium velocity distribution  

E-print Network

The Coulomb barrier is in general much higher than thermal energy. Nuclear fusion reactions occur only among few protons and nuclei with higher relative energies than Coulomb barrier. It is the equilibrium velocity distribution of these high-energy protons and nuclei that participates in determining the rate of nuclear fusion reactions. In the circumstance it is inappropriate to use the Maxwellian velocity distribution for calculating the nuclear fusion reaction rate. We use the relativistic equilibrium velocity distribution for this purpose. The rate based on the relativistic equilibrium velocity distribution has a reduction factor with respect to that based on the Maxwellian distribution, which factor depends on the temperature, reduced mass and atomic numbers of the studied nuclear fusion reactions. This signifies much to the solar neutrino problem.

Jian-Miin Liu

2002-10-20

199

Estimation of the reaction rate constant of HOCl by SMILES observation  

NASA Astrophysics Data System (ADS)

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

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

2012-07-01

200

Network Modeling of Anorthite and Kaolinite Reaction Rates in Porous Media  

NASA Astrophysics Data System (ADS)

Although subsurface systems consist of porous media with a wide range of physical and chemical properties, reactive transport modeling commonly employs a continuum approach, where each grid block is characterized by spatially-averaged properties and reaction rates are calculated using uniform concentrations. Such spatial averaging can introduce significant error in the representation of geochemical reaction rates. In this work, we use network models to examine the effects of pore-scale heterogeneities on continuum-scale rates of anorthite and kaolinite reactions, and we identify conditions under which the effects of pore-scale heterogeneities are significant in reaction rate up-scaling. The network is constructed to represent consolidated sandstone, with about 10% reactive minerals (anorthite and kaolinite) distributed in reactive pore clusters according to prescribed configurations. The minerals react with acidic brine saturated with high-pressure CO2, representing conditions relevant for geological CO2 sequestration. The reaction rates computed from the network model are compared with those from a continuum model, which simulates the porous medium using spatially-averaged concentrations. Simulation results show that aqueous concentrations at the pore scale vary by orders of magnitude, and their distributions are highly skewed and, in some cases, bimodal. These concentration heterogeneities lead to significant spatial variations in pore-scale reaction rates. As a result, the continuum-scale rates from the network model are significantly different from those from the continuum model. In general, the continuum model overestimates the anorthite dissolution rates; for kaolinite, it either underestimates its precipitation rates, or predicts a different reaction direction from that of the network model. The effects of pore-scale heterogeneities are influenced by hydrodynamic conditions, as well as spatial distributions of reactive minerals. For anorthite dissolution, the continuum model overestimates its rates under all hydrodynamic conditions, with the degree of overestimation reaching a maximum in medium flow conditions. For kaolinite, the continuum model and the network model predict the same reaction direction at slow and fast flow conditions, but opposite reaction directions in medium flow conditions. With regard to the impact of reactive cluster size, larger cluster size leads to larger differences between the rates. These results provide guidelines on the conditions under which the effects of pore-scale heterogeneities are important in reactive transport modeling.

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

2004-12-01

201

Communication: Rate coefficients from quasiclassical trajectory calculations from the reverse reaction: The Mu + H2 reaction re-visited  

NASA Astrophysics Data System (ADS)

In a previous paper [P. G. Jambrina et al., J. Chem. Phys. 135, 034310 (2011), 10.1063/1.3611400] various calculations of the rate coefficient for the Mu + H2 ? MuH + H reaction were presented and compared to experiment. The widely used standard quasiclassical trajectory (QCT) method was shown to overestimate the rate coefficients by several orders of magnitude over the temperature range 200-1000 K. This was attributed to a major failure of that method to describe the correct threshold for the reaction owing to the large difference in zero-point energies (ZPE) of the reactant H2 and product MuH (˜0.32 eV). In this Communication we show that by performing standard QCT calculations for the reverse reaction and then applying detailed balance, the resulting rate coefficient is in very good agreement with the other computational results that respect the ZPE, (as well as with the experiment) but which are more demanding computationally.

Homayoon, Zahra; Jambrina, Pablo G.; Aoiz, F. Javier; Bowman, Joel M.

2012-07-01

202

Exothermic Systems with Diminishing Reaction Rates: Temperature Evolution, Criticality and Spontaneous Ignition in the Sphere  

Microsoft Academic Search

The behaviour of an exothermic, spherical mass, in which reaction rate diminishes with time t according to the law rate ? (t+t{_pr})-alpha, tpr >=slant 0, 0 <=slant alpha < 1, is studied by numerical computation. (Analytical solutions are not attainable.) This law is a useful empirical representation of various complex systems of great practical importance: coal, sawdust, wool, polypropylene and

T. Boddington; P. Gray; I. K. Walker

1980-01-01

203

Interest rate reaction functions and the Taylor rule in the Euro area  

Microsoft Academic Search

Traditional Taylor rules, which are estimated using a level specification linking the short-term interest rate to inflation and the output gap, are unstable when estimated on euro area data and forecast poorly out of sample. We present an alternative reaction function which takes the non-stationarity of the data into account. The estimated interest rate rule is stable and forecasts well.

Petra Gerlach-Kristen

2003-01-01

204

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

ERIC Educational Resources Information Center

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

Calik, Muammer; Kolomuc, Ali; Karagolge, Zafer

2010-01-01

205

Rate of the reaction of atomic hydrogen with propyne over an extended pressure and temperature range  

Microsoft Academic Search

The technique of flash photolysis coupled with time resolved detection of H via resonance fluorescence has been used to obtain rate constants for the reaction of atomic hydrogen with propyne at temperatures from 215 to 460 K and at pressures in the range 5-600 torr. The rate constants are strongly pressure dependent and the high pressure limiting values give rise

D. A. Whytock; W. A. Payne; L. J. Stief

1976-01-01

206

Rate of the reaction of atomic hydrogen with propyne over an extended pressure and temperature range  

Microsoft Academic Search

The technique of flash photolysis coupled with time resolved detection of H via resonance fluorescence has been used to obtain rate constants for the reaction of atomic hydrogen with propyne at temperatures from 215 to 460 K and at pressures in the range 5–600 torr. The rate constants are strongly pressure dependent and the high pressure limiting values give rise

D. A. Whytock; W. A. Payne; L. J. Stief

1976-01-01

207

Gas phase reactions and rate coefficients for use in astrochemistry - The UMIST ratefile  

Microsoft Academic Search

The rate coefficients of 2880 gas-phase reactions among 313 species involving 12 elements are presented for use in astrochemical models. The motivation behind this work and the caveats which attach to the data are discussed. The permanent electric dipole moments of nearly all the 112 neutral molecules contained in the data set are given, so that rate coefficients can be

T. J. Millar; A. Bennett; J. M. C. Rawlings; P. D. Brown; S. B. Charnley

1991-01-01

208

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

DOEpatents

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

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

2006-05-16

209

EFFECTS OF RING STRAIN ON GAS-PHASE RATE CONSTANTS. 2. OH RADICAL REACTIONS WITH CYCLOALKENES  

EPA Science Inventory

Relative rate constants for the gas phase reactions of OH radicals with a series of cycloalkenes have been determined at 298 + or - 2 K, using methyl nitrite photolysis in air as a source of OH radicals. The data show that the rate constants for the nonconjugated cycloalkenes stu...

210

REACLIB: A Reaction Rate Library for the Era of Collaborative Science  

NASA Astrophysics Data System (ADS)

Thermonuclear reaction rates and weak decay rates are of great importance to modern nuclear astrophysics. They are critical in the study of many topics such as Big Bang Nucleosynthesis, X-ray bursts, Supernovae, and S-process element formation, among others. The Joint Institute for Nuclear Astrophysics (JINA) has been created to increase connectivity amongst nuclear astrophysicists in our modern age of highly collaborative science. Within JINA there has been an effort to create a frequently updated and readily accessible database of thermonuclear reactions and weak decay rates. This database is the REACLIB library, which can be accessed at the web address: http://www.nscl.msu.edu/˜nero/db/. Here I will discuss the JINA REACLIB Project, including a new procedure to fit reaction rates as a function of temperature that takes full advantage of physicality. With these updated reaction rates, astrophysical modelers will no longer have to worry about the adverse effects of using obsolete reaction rate libraries lacking physical behavior.

Meisel, Zachary

2008-10-01

211

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

NASA Technical Reports Server (NTRS)

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

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

1989-01-01

212

Effects of mineral spatial distribution on reaction rates in porous media  

NASA Astrophysics Data System (ADS)

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 were compared to rates from continuum models that use uniform concentrations. With small percentages of reactive minerals the scaling effects are large, and the effect of spatial distribution is significant. Scaling effects are largest when reactive minerals are closely clustered and oriented parallel to flow. Conversely, spatial distributions that enhance mass transport to and from reactive minerals, such as small mineral clusters or elongated clusters oriented transverse to flow, result in reaction rates that are well represented by the continuum model.

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

2007-01-01

213

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

NASA Astrophysics Data System (ADS)

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. The primary goal of our study is to translate these uncertainties in reaction rate data into uncertainties in model nucleosynthesis predictions and thereby quantify the extent of disagreement between theory and observation. We also will examine the relative importance of changes in individual reaction rates and hope to provide guidance in the selection of reactions for further experimental study. Research at the Oak Ridge National Laboratory is supported by the U.S. Department of Energy under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp.

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

1999-12-01

214

Determination of rate constants for the reaction between methyldiethanolamine and carbon dioxide  

E-print Network

1985) Charles Meade Brabson, Jr. , B. S. , Virginia Polytechnic Institute and State University Chairman of Advisory Committee: Dr. J. A. Bullin The kinetics of the reaction between carbon dioxide (COz) and aqueous methyldiethanolamine (MDEA... Comparison of Observed Rate with Predicted Rates 75 8 Summary of Runs Using Three Percent Aqueous MEA Solution . 79 9 Comparison of Rate Constants for Second Order Reversible Model . . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 81 10 Summary...

Brabson, Charles Meade

2012-06-07

215

Collision limited reaction rates for arbitrarily shaped particles across the entire diffusive Knudsen number range  

Microsoft Academic Search

Aerosol particle reactions with vapor molecules and molecular clusters are often collision rate limited, hence determination of particle-vapor molecule and particle-molecular cluster collision rates are of fundamental importance. These collisions typically occur in the mass transfer transition regime, wherein the collision kernel (collision rate coefficient) is dependent upon the diffusive Knudsen number, KnD. While this alone prohibits analytical determination of

Ranganathan Gopalakrishnan; Thaseem Thajudeen; Christopher J. Hogan

2011-01-01

216

Scale-dependent rates of uranyl surface complexation reaction in sediments  

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

217

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

E-print Network

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

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

2011-11-20

218

Capture Gamma Reaction Rates at Stellar Energies by the Asymptotic Normalization Coefficient Method  

NASA Astrophysics Data System (ADS)

Peripheral transfer reactions can be used to determine asymptotic normalization coefficients (ANCs). These coefficients, which provide the normalization of the tail of the overlap function for a proton bound to a nucleus, determine S factors for direct capture reactions at astrophysical energies. A variety of proton transfer reactions have been used to measure ANCs using both stable and radioactive beams. The technique of utilizing ANCs to predict astrophysical S factors has been tested for 9Be(p, ?)10B, 16O(p, ?)17F and recently 13C(p, ?)14N using ANCs from transfer reaction measurements. Recent applications include measurements of ANCs appropriate for determining stellar reaction rates for 7Be(p, ?)8B, 11C(p, ?)12N and 14N(p, ?)15O.

Tribble, R. E.; Azhari, A.; Gagliardi, C. A.; Mukhamedzhanov, A. M.; Pirlepesov, F.; Sattarov, A.; Tang, X.; Trache, L.; Bem, P.; Burjan, V.; Kroha, V.; Novák, J.; Pisko?, Š.; Šimecková, E.; Vincour, J.; Cârstoiu, F.

2003-06-01

219

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

NASA Astrophysics Data System (ADS)

Depletion is a game that reinforces central concepts of reaction rate theory through simulation. Each player buys chemicals and guides them through a series of reactions, thereby earning money to buy more chemicals. The reactions occur when players roll a high enough value on two dice to overcome an activation barrier. The reactions may be accelerated by buying heat (which allows the player to roll three dice instead of two) or catalysts (which lower the activation barrier). The value of acceleration derives from the increasing price of fresh chemicals as resources are depleted and waste products accumulate. The player who nets the most money wins the game. The details of the game are presented, with a set of follow-up questions suitable for either a quiz or discussion. Student reaction to the game is also described.

Olbris, Donald J.; Herzfeld, Judith

2002-10-01

220

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

221

MODELS FOR ISOTOPE RATE EFFECTS IN DISPLACEMENT, DISSOCIATION, ELIMINATION, AND ADDITION REACTIONS  

Microsoft Academic Search

Detailed calculations of isotop rate factors, k.k', for representative ; reactions, e.g., dissociation, displacement, elimination, and addition are ; presented. Various approximations for three-center, reactant-like, product-; like, and complete models of the activated complex were tested. At 27 deg , some ; results are as follows: for methyl iodide in SN1 reaction, kââkââ = ; 1.05, k\\/sub H\\/k\\/sub D\\/ =

Sidney I. Miller

1962-01-01

222

Quantum corrections to the particle distribution function and reaction rates in dense media  

Microsoft Academic Search

Quantum mechanics predicts the existence of power-law tails in the momentum distribution function of particles in dense media\\u000a even under conditions of thermodynamic equilibrium. The generalized expressions allowing for the effect of the medium density\\u000a show that quantum corrections lead to a sharp increase in the reaction rates of threshold exothermic processes (such as fusion\\u000a and chemical reactions and vibrational-translational

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

2005-01-01

223

Adsorbed species and reaction rates for NO-CO-O 2 over Rh(111)  

Microsoft Academic Search

We have studied the NO-CO-O2 reaction over a Rh(111) catalyst by monitoring the reaction products (CO2, N2O, and N2) and the infrared (IR) intensity of surface CO and NO at various partial pressures of NO, CO and O2, and sample temperatures. The selectivity for N2O formation, apparent activation energy for product formation, and NO consumption rate during NO-CO-O2 are identical

H. Permana; K. Y. Simon Ng; C. H. F. Peden; S. J. Schmieg; D. K. Lambert; D. N. Belton

1997-01-01

224

Adsorbed Species and Reaction Rates for NO–CO over Rh(111)  

Microsoft Academic Search

We have studied the reaction of NO with CO over a Rh(111) catalyst by monitoring the infrared (IR) intensity of surface CO and NO at various partial pressures of NO (PNO), CO (PCO), and sample temperatures (T). Reaction rates for the products (CO2, N2O, and N2) were previously measured at the same conditions in our apparatus [J. Phys. Chem.99, 16344

H. Permana; K. Y. Simon Ng; Charles H. F. Peden; Steven J. Schmieg; David K. Lambert; David N. Belton

1996-01-01

225

Lagrangian Analysis of Velocity Gauge Data to Determine Reaction Rate Histories in EDC37  

Microsoft Academic Search

The Lagrangian analysis technique was applied to EDC37, an HMX-based explosive. The method was tested against an analytic model before being used to analyse particle-velocity-gauge data from two sustained-shock gas-gun experiments. This work provides evidence that the first stages of reaction in EDC37 are endothermic, as well as indicating that reaction-rate histories in explosives are bell-shaped.

C. A. Handley

2006-01-01

226

Lagrangian analysis of velocity-gauge data to determine reaction-rate histories in EDC37  

Microsoft Academic Search

The Lagrangian analysis technique was applied to EDC37, an HMX- based explosive. The method was tested against an analytic model before being used to analyse particle-velocity-gauge data from two sustained-shock gas-gun experiments. This work provides evidence that the first stages of reaction in EDC37 are endothermic, as well as indicating that reaction-rate histories in explosives are bell-shaped.

Caroline Handley

2005-01-01

227

Electrochemical degradation of sulfonamides at BDD electrode: Kinetics, reaction pathway and eco-toxicity evaluation.  

PubMed

The investigation dealt with electrochemical oxidation of five sulfonamides (SNs): sulfadiazine (SDZ), sulfathiazole (STZ), sulfamerazine (SMR), sulfamethazine (SMN) and sulfadimethoxine (SDM) in aqueous solution at boron-doped diamond (BDD) electrode. All studied sulfonamides were degraded according to a pseudo first order kinetics. The structure of SNs had no significant effect on the values of pseudo first order rate constants. Increased degradation efficiency was observed in higher temperature and in acidic pH. Due to the presence of chlorine and nitrate SNs were more effectively oxidized from municipal wastewater treatment plant (WWTP) effluents than from pure supporting electrolyte Na2SO4. The intermediates identified by LC-MS and GC-MS analysis suggested that the hydroxyl radicals attack mainly the SN bond, but also the aromatic ring systems (aniline, pyrimidine or triazole) of SNs. Finally, the toxicity of the SNs solutions and effluents after electrochemical treatment was assessed through the measurement of growth inhibition of green algae (Scenedesmus vacualatus) and duckweed (Lemna minor). Toxicity of SMR, STZ, SMN solutions before and after electrochemical oxidation and SDM solution after the process in L. minor test was observed. No significant toxicity of studied SNs was observed in algae test. PMID:25215656

Fabia?ska, Aleksandra; Bia?k-Bieli?ska, Anna; Stepnowski, Piotr; Stolte, Stefan; Siedlecka, Ewa Maria

2014-09-15

228

Thermonuclear reaction rate of {sup 17}O(p,{gamma}){sup 18}F  

SciTech Connect

The {sup 17}O(p,{gamma}){sup 18}F and {sup 17}O(p,{alpha}){sup 14}N reactions have a profound influence on hydrogen-burning nucleosynthesis in a number of stellar sites, including red giants, asymptotic giant branch (AGB) stars, massive stars, and classical novae. Previously evaluated thermonuclear rates for both reactions carry large uncertainties. We investigated the proton-capture reaction on {sup 17}O in the bombarding energy range of E{sub p}{sup lab} = 180-540 keV. We observed a previously undiscovered resonance at E{sub R}{sup lab} = 193.2 {+-} 0.9 keV. The resonance strength amounts to ({omega}{gamma}){sub p{gamma}}(1.2{+-}0.2)x10{sup -6} eV. With this value, the uncertainties of the {sup 17}O(p,{gamma}){sup 18}F reaction rates are reduced by orders of magnitude in the peak temperature range of classical novae (T=0.1-0.4 GK). We also report on a reevaluation of the {sup 17}O(p,{gamma}){sup 18}F reaction rates at lower temperatures that are pertinent to red giants, AGB stars, or massive stars. The present work establishes the {sup 17}O(p,{gamma}){sup 18}F reaction rates over a temperature range of T= 0.01-1.5 GK with statistical uncertainties of 10-50%. The new recommended reaction rates deviate from the previously accepted values by an order of magnitude around T{approx_equal}0.2 GK and by factors of 2-3 at T < 0.1 GK.

Fox, C.; Iliadis, C.; Champagne, A.E.; Fitzgerald, R.P.; Longland, R.; Newton, J.; Pollanen, J.; Runkle, R. [Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina, 27599-3255 (United States) and Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708-0308 (United States)

2005-05-01

229

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

NASA Astrophysics Data System (ADS)

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

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

230

Rate constants for chemical reactions in high-temperature nonequilibrium air  

NASA Technical Reports Server (NTRS)

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

Jaffe, R. L.

1986-01-01

231

Reaction Rates for the Formation of Deuterium Tritide from Deuterium and Tritium  

SciTech Connect

The rates of formation of DT in a mixture of D2 and T2 have been measured as a function of initial T2 concentration, pressure, temperature,and methane concentration in a stainless steel reaction container which had been treated to inhibit protium ingrowth. An attempt has been made to explain the experimental resuts on the basis of ion-molecule chain reactions. Some of the observations are consistent with a gas-phase ion, ground-state molecule reaction, but some of the more interesting observations require more complicated models. The addition of excited state molecules or heterogeneous catalytic effects are possibilities that will need further experiments for confirmation.

McConville, G. T.; Menke, D. A.; Ellefson, R. E.

1985-04-01

232

Determination of the rate constant of hydroperoxyl radical reaction with phenol  

NASA Astrophysics Data System (ADS)

The rate constant of HO2rad reaction with phenol (kHO2rad +phenol) was investigated. The primary radical set produced in water ? radiolysis (rad OH, eaq- and Hrad ) was transformed to HO2rad /O2rad - by using dissolved oxygen and formate anion (in the form of either formic acid or sodium formate). The concentration ratio of HO2rad /O2rad - was affected by the pH value of the solution: under acidic conditions (using HCOOH) almost all radicals were converted to HO2rad , while under alkaline conditions (using HCOONa) to O2rad -. The degradation rate of phenol was significantly higher using HCOOH. From the ratio of reaction rates under the two reaction conditions kHO2rad +phenol was estimated to be (2.7±1.2)×103 L mol-1 s-1.

Kozmér, Zsuzsanna; Arany, Eszter; Alapi, Tünde; Takács, Erzsébet; Wojnárovits, László; Dombi, András

2014-09-01

233

Direct and sequential radiative three-body reaction rates at low temperatures  

E-print Network

We investigate the low-temperature reaction rates for radiative capture processes of three particles. We compare direct and sequential capture mechanisms and rates using realistic phenomenological parametrizations of the corresponding photodissociation cross sections.Energy conservation prohibits sequential capture for energies smaller than that of the intermediate two-body structure. A finite width or a finite temperature allows this capture mechanism. We study generic effects of positions and widths of two- and three-body resonances for very low temperatures. We focus on nuclear reactions relevant for astrophysics, and we illustrate with realistic estimates for the $\\alpha$-$\\alpha$-$\\alpha$ and $\\alpha$-$\\alpha$-$n$ radiative capture processes. The direct capture mechanism leads to reaction rates which for temperatures smaller than 0.1 GK can be several orders of magnitude larger than those of the NACRE compilation.

E. Garrido; R. de Diego; D. V. Fedorov; A. S. Jensen

2011-08-24

234

Photooxidative degradation of Acid Red 27 (AR27): modeling of reaction kinetic and influence of operational parameters.  

PubMed

The decolorization and mineralization of Acid Red 27 (AR27), an anionic monoazo dye of acid class, were studied by UV/H202 process in laboratory and real samples. Effects of different process parameters such as initial H2O2 and AR27 concentrations, pH and EtOH as an electron scavenger have been studied. H2O2 and UV light have a negligible effect when they were used on their own. The decolorization rate follows pseudo-first order kinetic with respect to the dye concentration. The rate constant of the attack of *OH radicals to the AR27 has been estimated through the adoption of a simplified kinetic model (1.03 x 10(8)M(-1)s(-1)). This model allows predicting the pseudo-first order rate constant and concentration of AR27 in different illumination times for different initial concentrations of H2O2. Mineralization studies showed, the formation of sulfate ions, the decrease of pH and 85% of COD reduction occur in less than 60 min. In the real wastewater color removal could be achieved after only 75 min, whereas 45% of COD reduction occurs after 105 min of illumination. PMID:15478925

Daneshvar, Nezameddin; Rabbani, Mohammad; Modirshahla, Nasser; Behnajady, Mohammad-Ali

2004-01-01

235

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

236

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

237

[Determination of trace amounts of nitrite and its chemical reaction kinetics].  

PubMed

A catalytic kinetic spectrophotometric method for the determination of nitrite, NO2(-)-S2O8(2-)-MR, was developed. It is based on the fading reaction of methyl red (MR) oxidized by potassium persulfate which can be catalyzed by NO2- in the medium of dilute HCl. The optimum experimental conditions were gained by combining single factor experiments with orthogonal experiments. Calibration curve, detection limit, precision, and anti-interference under the optimum experimental conditions were researched. Its kinetics principles and parameters were discussed. Its quantitative principle was investigated. The results show that the optimum experimental conditions of this method should be as follows: 1.0 mL 0.3 mol x L(-1) HCl, 1.0 mL 0.01 mol x L(-1) K2S2O8, 0.6 mL 0.2 g x L(-1) MR, reaction temperature 80 degrees C and reaction time 9 min. The principles for the quantitative determination of trace nitrite is that variation of MR concentration at the maximum absorption wavelength of 518 nm, ln(A0/A), shows a good linear relationship with the concentration of NO2- under the optimum experimental conditions. Its determination range is 0.01-0.80 mg x L(-1) and its detection limit is 0.007 mg x L(-1). The kinetic characteristics are that the reaction order in NO2- is 1 and the fading reaction is a pseudo first order reaction. Its apparent activation energy is 85.04 kJ x mol(-1). Its apparent rate constant is 0.021 4 min(-1), and the half-life is 32.39 min at 80 degrees C. The kinetic principle is that the variation of MR concentration is directly proportional to the concentration of NO2-, ln(A0/A) = kc(NO2-). This new method for the determination of trace nitrite has never previously been reported in the published literature so far. It is highly sensitive and selective. Most of the common ions don't interfere with the determination of nitrite. This method has the advantages of convenient operation and the regents used are cheap and nontoxic. It was applied to the determination of trace nitrite in food and water samples with satisfactory results. PMID:25358175

Luo, Zhi-yong; Zheng, Huai-li

2014-06-01

238

Connection Between Crucial Nuclear Reaction Rates and the Modeling of Accreting Neutron Stars  

NASA Astrophysics Data System (ADS)

Effects of uncertainties of key nuclear reaction rates in the crucial nuclear processes on type I X-ray burst modeling have been reviewed/investigated. Special attention is devoted to the ignition condition of the thermonuclear flash. It is found that in the rapid proton capture process (rp-process), the ignition depends on the rates of 13N(p,?)14O, 14O(?,p)17F, and 15O(?,?)19Ne. On the other hand, for the pure helium flash suggested from a recent observation of X-ray bursts, we infer that the NCO-reaction plays a key role in triggering the flash.

Hashimoto, M.; Koike, O.; Kuromizu, R.; Fujimoto, M.; Arai, K.

2003-04-01

239

Determination of the rate constant for the reaction O + NO yields N + O2  

NASA Technical Reports Server (NTRS)

A shock-tube technique was used to determine the rate constant for the reaction O + NO yields N + O2 in the 2500-4100 K range. Mixtures of NO, N2O, and an inert gas (Ar or Kr) were shock-heated, and the decay of infrared emission from the fundamental vibration-rotation band of NO at 5.3 microns was monitored. The N2O dissociates quickly to provide a source of O atoms for the above reaction. An expression for the rate constant is derived which fits these data and previous data at higher and lower temperatures.

Hanson, R. K.; Flower, W. L.; Kruger, C. H.

1974-01-01

240

Simultaneous time-resolved measurement of the reaction rates and the refractive index of photopolymerization processes.  

PubMed

We explore the use of imaging surface plasmon resonance (iSPR) to simultaneously measure the refractive index and reaction rates of the commercially available Ormocore photosensitive resist during photopolymerization. To this end, we adapted a commercially available iSPR device. We demonstrate good accuracy in the measurement of the refractive index determined independently of the thickness of the polymerized film. Furthermore, we demonstrate that the refractive index is proportional to the degree of cure (double bond conversion) of the resist. This allows the determination of the reaction rates of the polymerization processes, which show reasonable agreement with photodifferential scanning calorimetry measurements. PMID:20539350

Bak, Tomasz M; Beusink, J Bianca; Subramaniam, Vinod; Kanger, Johannes S

2010-06-10

241

Temperature dependence of rate coefficients for reactions of ions with dipolar molecules  

NASA Astrophysics Data System (ADS)

We present a comparison of theoretical and experimental rate coefficients for the reactions of H 3+, HCO +, H -, C + and O + with HCN, and H 3+ with HCl, over the temperature range 205-540 K. The theoretical results are obtained using an adiabatic capture theory which is selective in the initial rotational states of the reacting dipolar molecules. The experiments were performed using a variable-temperature SIFT apparatus. For the H 3+ and HCO + proton transfer reactions in particular, there is excellent agreement between theory and experiment, with the rate coefficients increasing sharply with decreasing temperature.

Clary, D. C.; Smith, D.; Adams, N. G.

1985-09-01

242

Astrophysical reaction rate for Be9 formation within a three-body approach  

NASA Astrophysics Data System (ADS)

The structure of the Borromean nucleus Be9 (?+?+n) is addressed within a three-body approach using the analytical transformed harmonic oscillator method. The three-body formalism provides an accurate description of the radiative capture reaction rate for the entire temperature range relevant in astrophysics. At high temperatures, results match the calculations based on two-step sequential processes. At low temperatures, where the particles have no access to intermediate two-body resonances, the three-body direct capture leads to reaction rates larger than the sequential processes. These results support the reliability of the method for systems with several charged particles.

Casal, J.; Rodríguez-Gallardo, M.; Arias, J. M.; Thompson, I. J.

2014-10-01

243

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

244

Three dimensional mapping of the creatine kinase enzyme reaction rate in muscles of the lower leg  

PubMed Central

Phosphorous (31P) magnetization transfer (MT) techniques enable the non-invasive measurement of metabolic turnover rates of important enzyme catalyzed reactions, such as the creatine kinase reaction (CK), a major transducing reaction involving adenosine triphosphate and phosphocreatine. Alteration in the kinetics of the CK reaction rate appears to play a central role in many disease states. In this study, we developed and implemented at ultra-high field (7T), a novel three-dimensional 31P-MT imaging sequence that maps the kinetics of CK in the entire volume of the lower leg at relatively high resolution (0.52 mL voxel size), and within acquisition times that can be tolerated by patients (below 60 min). We tested the sequence on five healthy and two clinically diagnosed type 2-diabetic patients. Overall, we obtained measurements that are in close agreement with measurements reported previously using spectroscopic methods. Importantly, our spatially resolved method allowed us to measure local CK reaction rate constants and metabolic fluxes in individual muscles in healthy subjects. Furthermore, in the case of patients with diabetes, it allowed us to detect variations of the CK rate of different muscles, which would not have been possible using unlocalized MRS methods. The results of this work suggest that 3D-mapping of the CK reaction rates and metabolic fluxes can be achieved in the skeletal muscle in vivo at relatively high spatial resolution and with acquisition times well tolerated by patients. The ability to measure bioenergetics simultaneously in large areas of muscles will bring new insights into possible heterogeneous patterns of muscle metabolism associated with several diseases and serve as a valuable tool for monitoring the efficacy of interventions. PMID:23436474

Parasoglou, Prodromos; Xia, Ding; Chang, Gregory; Convit, Antonio; Regatte, Ravinder R.

2013-01-01

245

Quasiclassical trajectory scattering calculations for the OH + O ? H + O2 reaction: Cross sections and rate constants  

E-print Network

explanation is that the rate constant of the title reaction used in the models was too high. Unfortunately of hydrogen and most hydrocarbons. The OH + O ? H + O2 reaction is exothermic by 0.71 eV. Its reaction pathQuasiclassical trajectory scattering calculations for the OH + O ? H + O2 reaction: Cross sections

Honvault, Pascal

246

Model Proton-Coupled Electron Transfer Reactions in Solution: Predictions of Rates, Mechanisms, and Kinetic Isotope Effects  

E-print Network

Model Proton-Coupled Electron Transfer Reactions in Solution: Predictions of Rates, Mechanisms) the proton transfer reaction becomes more exothermic, (4) the electron transfer reaction becomes more as the temperature increases and as the electron transfer reaction becomes more exothermic in the normal Marcus

Hammes-Schiffer, Sharon

247

Scale-Dependent Rates of Uranyl Surface Complexation Reaction in Sediments  

SciTech Connect

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

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

2013-03-15

248

Vibrational and total energy dependence of the reaction rate of Na with CO2 laser-excited SF6  

Microsoft Academic Search

The rate of reaction between sodium and sulfur hexafluoride has been studied as a function of the vibrational energy and total thermal energy of the SF6 in the temperature range 380–440 °K. Vibrational energy was increased by absorption of radiation from a pulsed CO2 laser, and total energy was increased by heating the reaction system. The reaction rate was determined

Mark Eyal; Frederick R. Grabiner; Uri Agam; Leonard A. Gamss

1981-01-01

249

Rate constant calculations of H-atom abstraction reactions from ethers by H?2 radicals.  

PubMed

In this work, we detail hydrogen atom abstraction reactions from six ethers by the hydroperoxyl radical, including dimethyl ether, ethyl methyl ether, propyl methyl ether, isopropyl methyl ether, butyl methyl ether, and isobutyl methyl ether, in order to test the effect of the functional group on the rate constant calculations. The Møller-Plesset (MP2) method with the 6-311G(d,p) basis set has been employed in the geometry optimizations and frequency calculations of all of the species involved in the above reaction systems. The connections between each transition state and the corresponding local minima have been determined by intrinsic reaction coordinate calculations. Energies are reported at the CCSD(T)/cc-pVTZ level of theory and include the zero-point energy corrections. As a benchmark in the electronic energy calculations, the CCSD(T)/CBS extrapolation was used for the reactions of dimethyl ether + H?2 radicals. A systematic calculation of the high-pressure limit rate constants has been performed using conventional transition-state theory, including asymmetric Eckart tunneling corrections, in the temperature range of 500-2000 K. The one dimensional hindrance potentials obtained at MP2/6-311G(d,p) for the reactants and transition states have been used to describe the low frequency torsional modes. Herein, we report the calculated individual, average, and total rate constants. A branching ratio analysis for every reaction site has also been performed. PMID:24483837

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

2014-02-27

250

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

PubMed

Climate models show that particles formed by nucleation can affect cloud cover and, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmospheric boundary layer are positively correlated with concentrations of sulfuric acid vapor. However, current nucleation theories do not correctly predict either the observed nucleation rates or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach for modeling nucleation rates, based on a sequence of acid-base reactions. The model uses empirical estimates of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model predicts that nucleation rates equal the sulfuric acid vapor collision rate times a prefactor that is less than unity and that depends on the concentrations of basic gaseous compounds and preexisting particles. Predicted nucleation rates and their dependence on sulfuric acid vapor concentrations are in reasonable agreement with measurements from Mexico City and Atlanta. PMID:23091030

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

2012-11-13

251

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

PubMed Central

Climate models show that particles formed by nucleation can affect cloud cover and, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmospheric boundary layer are positively correlated with concentrations of sulfuric acid vapor. However, current nucleation theories do not correctly predict either the observed nucleation rates or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach for modeling nucleation rates, based on a sequence of acid–base reactions. The model uses empirical estimates of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model predicts that nucleation rates equal the sulfuric acid vapor collision rate times a prefactor that is less than unity and that depends on the concentrations of basic gaseous compounds and preexisting particles. Predicted nucleation rates and their dependence on sulfuric acid vapor concentrations are in reasonable agreement with measurements from Mexico City and Atlanta. PMID:23091030

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

252

Electron transfer rates in highly exothermic reactions on semiconductor electrolyte interfaces, and the deuterium isotope effect  

Microsoft Academic Search

Highly exothermic electron transfer rates were measured by semiconductor electrochemical techniques. Experiments demonstrate that the decreases in the electron transfer rates with the exothermicity of the reactions in so-called abnormal regions are much more moderate than the classical predictions, and that this is probably due to the effects of intramolecular vibrations such as C-H stretching and\\/or bending. 3 figures.

Seiichiro Nakabayashi; Kiminori Itoh; Akira Fujishima; Kenichi Honda

1983-01-01

253

Investigating rare events with nonequilibrium work measurements. II. Transition and reaction rates  

NASA Astrophysics Data System (ADS)

We present a formalism for investigating transition pathways and transition probabilities for rare events in biomolecular systems. The formalism is based on combining Transition Path Theory with the results of nonequilibrium work relations, and shows that the equilibrium and nonequilibrium transition rates are in fact related. Aside from its fundamental importance, this allows for the calculation of relative equilibrium reaction rates with driven nonequilibrium simulations such as Steered Molecular Dynamics. The workings of the formalism are illustrated with a few typical numerical examples.

Moradi, Mahmoud; Sagui, Celeste; Roland, Christopher

2014-01-01

254

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

E-print Network

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 present preliminary results of systematic analyses of the impact of reaction rate uncertainties on nova nucleosynthesis.

W. Raphael Hix; Michael S. Smith; Anthony Mezzacappa; Sumner Starrfield; Donald L. Smith

2000-01-27

255

Graph theoretical analysis on the kinetic rate equations of linear chain and cyclic reaction networks.  

PubMed

Graph theoretical solutions for kinetic rate equations of some reaction networks involving linear chains and cycles have been derived; condensation polymerization and long chain of radioactive decay come under the purview of the former whereas the interconversion of the species in cycles under the later. The reactions for the linear chains considered here proceed monotonically to the steady states with time whereas the cycle with all irreversible steps has been found to have either periodic or monotonic time evaluation of concentrations depending on the values of rate constants of the involved paths. In case of a cyclic reaction having all reversible paths, the condition for the microscopic reversibility has been derived on the basis of the assumption that the decay constants obtained for this case are all real. PMID:25120236

Karmakar, Somnath; Mandal, Bholanath

2014-09-11

256

Influence of new reaction rates on 18F production in novae  

NASA Astrophysics Data System (ADS)

Gamma-ray emission from classical novae is dominated, during the first hours, by positron annihilation resulting from the beta decay of radioactive nuclei. The main contribution comes from the decay of 18F and hence is directly related to 18F formation during the outburst. A good knowledge of the nuclear reaction rates of production and destruction of 18F is required to study 18F synthesis in novae and the resulting gamma-ray emission. The rates relevant for the main mode of 18F destruction (i.e, through proton captures) have been the object of many recent experiments. However, subsequent analyses were focused on providing rates for X-ray burst nucleosynthesis not valid at nova temperatures (lower than 3.5 x 108 K). Accordingly, it is crucial to propose and discuss new reaction rates, incorporating all new experimental results, down to the domain of nova nucleosynthesis. We show that in this temperature regime, the 18F(p,gamma )19Ne and 18F(p,alpha )15O reaction rates remain uncertain and deserve further experimental and theoretical efforts. Our hydrodynamic calculations including the new nuclear rates demonstrate that their impact on 18F synthesis in nova explosions is quite large and, consequently, the early gamma-ray emission from classical novae is also affected.

Coc, A.; Hernanz, M.; José, J.; Thibaud, J.-P.

2000-05-01

257

The efficient computation of transition state resonances and reaction rates from a quantum normal form  

E-print Network

A quantum version of a recent formulation of transition state theory in {\\em phase space} is presented. The theory developed provides an algorithm to compute quantum reaction rates and the associated Gamov-Siegert resonances with very high accuracy. The algorithm is especially efficient for multi-degree-of-freedom systems where other approaches are no longer feasible.

Roman Schubert; Holger Waalkens; Stephen Wiggins

2005-10-14

258

Application of Ab Initio Molecular Orbital and Reaction Rate Theories to Nucleation Kinetics  

Microsoft Academic Search

This paper describes the application of ab initio molecular orbital (MO) theories in conjunction with reaction rate theory to obtain thermochemistry and energetics of nucleation processes. The specific example used for the illustration of this approach is the nucleation of SiO. MO computations on the equilibrium structure have shown the polymers up to the tetramer to be planar rings and

M. R. Zachariah; W. Tsang

1993-01-01

259

Effect of Finite-rate Chemical Reactions on Turbulence in Hypersonic Turbulent Boundary Layers  

E-print Network

Effect of Finite-rate Chemical Reactions on Turbulence in Hypersonic Turbulent Boundary Layers Lian on future air-breathing hypersonic cruise vehicles will be turbulent and chemically reacting. To aid the design of such vehicles, a greater understanding of turbulent hypersonic flows is needed. Although

Martín, Pino

260

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

NASA Astrophysics Data System (ADS)

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

Brooke, George M., IV

261

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

262

Fundamental Kinetic Modeling: ab initio rate constant calculations of elementary reactions  

E-print Network

Fundamental Kinetic Model' provides an alternative method for collecting rate constant data of elementary reactions when experimental observation is unavailable. A standard procedure of PMP4/6-3 IG*//UHF/6-3 IG* ab initio calculations of reactants...

Jenks, Richard Lee

2012-06-07

263

Effect of Conceptual Change Approach on Students' Understanding of Reaction Rate Concepts  

ERIC Educational Resources Information Center

The purpose of the present study was to investigate the effect of conceptual change text oriented instruction compared to traditional instruction on 10th grade students' understanding of reaction rate concepts. 45 students from two classes of the same teacher in a public high school participated in this study. Students in the experimental group…

Kingir, Sevgi; Geban, Omer

2012-01-01

264

Efficient ab initio sampling methods in rate constant calculations for proton-transfer reactions  

E-print Network

Efficient ab initio sampling methods in rate constant calculations for proton-transfer reactions, the classical potential based importance Monte Carlo sampling method of Iftimie et al. J. Chem. Phys. 113, 4852 the statistics of the calculation by three orders of magnitude. The sampling method is extended to simulations

Schofield, Jeremy

265

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

266

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

267

Laboratory-Measured Sulfuric Acid and Water Homogeneous Nucleation Rates from the SO2 + OH Reaction  

Microsoft Academic Search

Sulfuric acid and water binary homogeneous nucleation is the most important atmospheric nucleation system, but the nucleation mechanisms are poorly understood. Here, we report laboratory-measured sulfuric acid and water binary homogeneous nucleation rates at the atmospheric pressure, 288 K and 10 - 55 % relative humidity. The SO2 + OH reaction was used to produce sulfuric acid vapor. Residual sulfuric

D. Benson; S. Lee

2007-01-01

268

Rate constants from the reaction path Hamiltonian. II. Nonseparable semiclassical transition state theory  

E-print Network

Rate constants from the reaction path Hamiltonian. II. Nonseparable semiclassical transition state theory Baron Peters and Alexis T. Bell Department of Chemical Engineering, University of California semiclassical transition state theory TST of Hernandez and Miller Chem. Phys. Lett. 214, 129 1993 accounts

Bell, Alexis T.

269

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 + BeF 2) 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 T 2 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 = BeF 2 + H 2 in Flibe were experimentally determined. Sufficiently fast rates of the Be dissolution and the reduction reaction of HF to H 2 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

270

Laser studies of methyl radical reactions with Cl2 and Br2: Absolute rate constants, product vibrational excitation, and hot radical reactions  

Microsoft Academic Search

Reactions of methyl radicals with Cl2 and Br2 are studied by pulsed laser dissociation of CH3I followed by time-resolved detection of infrared vibrational fluorescence from the C–H stretch modes of the methyl halide product. This method provides a determination of the absolute rate constants for the methyl radical reactions. The rate constants for thermal reactions of CH3 with Cl2 and

Laurie J. Kovalenko; Stephen R. Leone

1984-01-01

271

Atmospheric degradation of saturated alcohols: Room temperature rate coefficients for NO3 radical reactions  

NASA Astrophysics Data System (ADS)

Rate coefficients for the reactions of NO3 radicals with a series of saturated alcohols are reported here using the relative rate technique. Experiments were performed using air as bath gas in a 50 L glass-pyrex reaction chamber at room temperature (298 ± 2) K with long-path FTIR spectroscopy used to monitor the reaction at atmospheric pressure (708 ± 8) Torr. The reference compounds used and their rate coefficients are: propanal kNO3 = (6.0 ± 0.6) × 10-15, methyl methacrylate kNO3 = (3.55 ± 0.62) × 10-15, acetaldehyde kNO3 = (2.62 ± 0.29) × 10-15 and propene kNO3 = (9.50 ± 1.9) × 10-15, in cm3 molecule-1 s-1. Rate coefficients obtained were (in units cm3 molecule-1 s-1): (1.87 ± 0.14) × 10-15, (2.39 ± 0.20) × 10-15, (2.28 ± 0.17) × 10-15, (1.80 ± 0.13) × 10-15 and (3.52 ± 0.19) × 10-15 for 1-butanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 3,3-dimethyl-1-butanol and 3,3-dimethyl-2-butanol respectively. Reactivity trend can be explained in terms of the different types of hydrogen inside the hydrocarbon chain. The reaction occurs by an initial H-atom abstraction mainly from C-H groups of the alcohols by the NO3 radical being NO3 more reactive towards an H atom attached to a tertiary carbon than that attached to a secondary or primary carbon. Reactivity trend is compared with their similar structural 2-butanol and with the corresponding alkanes. Atmospheric implications are also discussed calculating lifetimes of the saturated alcohols studied here due to the reaction with NO3 radicals in comparison to their reactions with the other major atmospheric oxidants.

Moreno, Alberto; Salgado, Sagrario; Taccone, Raul; Martín, Pilar; Cabañas, Beatriz

2014-10-01

272

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

273

Dienophile Twisting and Substituent Effects Influence Reaction Rates of Intramolecular Diels-Alder Cycloadditions: A DFT Study  

E-print Network

Dienophile Twisting and Substituent Effects Influence Reaction Rates of Intramolecular Diels-Alder reactivities in intra- molecular Diels-Alder reactions.1 Whereas 1 cannot be isolated at 30 °C because

Trauner, Dirk

274

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 Russell’s 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]:20–25 min) versus 120 min (IQR:75–120 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 PMID:22719932

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

2012-01-01

275

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

276

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

277

30P(p,?)31S Reaction Rate for Classical Novae: An Indirect Approach  

NASA Astrophysics Data System (ADS)

Isotopic abundance ratios of 30Si/28Si found in presolar SiC grains of suspected nova origin agree qualitatively with proposed oxygen-neon (ONe) nova composition but fail to agree quantitatively with ejecta predictions made by hydrodynamic ONe nova models. The Astrophysical 30P(p,?)31S reaction rate is a key quantity used in nova models that predict isotopic abundances produced during nucleosynthesis leading up to the outburst. Currently, there is a large uncertainty in the rate at nova temperatures (0.1 < T < 0.4 GK) causing the predicted 30Si abundance ratio to vary by a factor of 4. The 30P(p,?)31S reaction rate can be determined indirectly by measuring triton momenta from 32S(d,t)31S reactions. 31S Resonant states measured up to 600 keV above the proton threshold of 6131 keV and within the Gamow window which contribute most significantly to the rate can then be used to re-evaluate the rate for nova temperatures and reduce the uncertainty.

Irvine, D.

2012-09-01

278

Thermal release rate studies of nuclear reaction products from polycrystalline metal matrices  

NASA Astrophysics Data System (ADS)

The thermal release rate of nuclear reaction products was investigated in offline annealing experiments. This work was motivated by the search for a high melting catcher material for recoiling products from heavy ion induced nuclear fusion reactions. Polycrystalline refractory metal foils of Ni, Y, Zr, Nb, Mo, Hf, W, and Re were investigated as catcher metals. Diffusion data for various tracer/host combinations were deduced from the measured release rates. This work focuses on the diffusion and the release rate of volatile p-elements from row 5 and 6 of the periodic table as lighter homologues of the superheavy elements with Z ? 113 to be studied in future experiments. A massive radiation damage enhancement of the diffusion velocity was observed. Diffusion trends have been established along the groups and rows of the periodic table based on the dependence of diffusion velocity on atomic sizes.

Wittwer, D.; Dressler, R.; Eichler, R.; Gäggeler, H. W.; Piguet, D.; Türler, A.

2013-02-01

279

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

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

280

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

281

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

USGS Publications Warehouse

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

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

1984-01-01

282

?-Terpineol reactions with the nitrate radical: Rate constant and gas-phase products  

NASA Astrophysics Data System (ADS)

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

Jones, Brian T.; Ham, Jason E.

283

Benchmark experiments for validation of reaction rates determination in reactor dosimetry  

NASA Astrophysics Data System (ADS)

The precision of Monte Carlo calculations of quantities of neutron dosimetry strongly depends on precision of reaction rates prediction. Research reactor represents a very useful tool for validation of the ability of a code to calculate such quantities as it can provide environments with various types of neutron energy spectra. Especially, a zero power research reactor with well-defined core geometry and neutronic properties enables precise comparison between experimental and calculated data. Thus, at the VR-1 zero power research reactor, a set of benchmark experiments were proposed and carried out to verify the MCNP Monte Carlo code ability to predict correctly the reaction rates. For that purpose two frequently used reactions were chosen: He-3(n,p)H-3 and Au-197(n,?)Au-198. The benchmark consists of response measurement of small He-3 gas filled detector in various positions of reactor core and of activated gold wires placed inside the core or to its vicinity. The reaction rates were calculated in MCNP5 code utilizing a detailed model of VR-1 reactor which was validated for neutronic calculations at the reactor. The paper describes in detail the experimental set-up of the benchmark, the MCNP model of the VR-1 reactor and provides a comparison between experimental and calculated data.

Rataj, J.; Huml, O.; Heraltova, L.; Bily, T.

2014-11-01

284

Rapid heartbeat, but dry palms: reactions of heart rate and skin conductance levels to social rejection  

PubMed Central

Background: Social rejection elicits negative mood, emotional distress, and neural activity in networks that are associated with physical pain. However, studies assessing physiological reactions to social rejection are rare and results of these studies were found to be ambiguous. Therefore, the present study aimed to examine and specify physiological effects of social rejection. Methods: Participants (n = 50) were assigned to either a social exclusion or inclusion condition of a virtual ball-tossing game (Cyberball). Immediate and delayed physiological [skin conductance level (SCL) and heart rate] reactions were recorded. In addition, subjects reported levels of affect, emotional states, and fundamental needs. Results: Subjects who were socially rejected showed increased heart rates. However, social rejection had no effect on subjects' SCLs. Both conditions showed heightened arousal on this measurement. Furthermore, psychological consequences of social rejection indicated the validity of the paradigm. Conclusions: Our results reveal that social rejection evokes an immediate physiological reaction. Accelerated heart rates indicate that behavior activation rather than inhibition is associated with socially threatening events. In addition, results revealed gender-specific response patterns suggesting that sample characteristics such as differences in gender may account for ambiguous findings of physiological reactions to social rejection. PMID:25221535

Iffland, Benjamin; Sansen, Lisa M.; Catani, Claudia; Neuner, Frank

2014-01-01

285

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

E-print Network

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 on the relativistic equilibrium velocity distribution has a reduction factor with respect to that based on the Maxwellian distribution. The reduction factor depends on the temperature, reduced mass and atomic numbers of the studied nuclear fusion reactions, in other words, it varies with the sort of neutrinos. Substituting the relativistic equilibrium velocity distribution for the Maxwellian distribution is not important for the calculation of solar sound speeds. The relativistic equilibrium velocity distribution, if adopted in standard solar models, will lower solar neutrino fluxes and change solar neutrino energy spectra but maintain solar sound speeds. This velocity distribution is possibly a solution to the solar neutrino problem.

Jian-Miin Liu

2003-07-07

286

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

287

Gas-phase reaction rate of sodium superoxide with hydrochloric acid  

SciTech Connect

Metal compounds originating meteor ablation may provide an additional mechanism for the release of free chlorine from HCl in the stratosphere. For the alkali metals, and sodium in particular, catalytic chemical pathways have been postulated that describe these processes. A critical step in this mechanism is the reaction of NaO/sub 2/ with HCl. The rate constant for this reaction has been measured in a fast-flow reactor at 295 K and found to be (2.3 +/- 0.4) x 10/sup -10/ cm/sup 3/ molecule/sup -1/ s/sup -1/. The implication of this result on stratospheric ozone chemistry is discussed.

Silver, J.A.; Kolb, C.E.

1986-07-03

288

The thermal reaction rate of muonium with methane (and ethane) in the gas phase  

Microsoft Academic Search

Rates for the gas-phase thermal reaction Mu+CH4?MuH+CH3 (Mu=?+e?), have been measured using the ?SR (muon spin rotation) technique, over the temperature range 625–820 K. A good fit is obtained to the usual Arrhenius expression, k=A exp(?Ea\\/RT), giving an activation energy Ea=24.6±0.9 kcal\\/mol, ?12 kcal\\/mol higher than that of the H-atom isotopic variant of this reaction, H+CH4?H2+CH3. This Ea difference is

Rodney Snooks; Donald J. Arseneau; Donald G. Fleming; Masayoshi Senba; James J. Pan; Mee Shelley; Susan Baer

1995-01-01

289

Heterogeneous photocatalysis of real textile wastewater: evaluation of reaction kinetics and characterization.  

PubMed

Real textile wastewater collected from the cotton dyeing bath of a fabric dyeing and finishing plant was subjected to heterogeneous photocatalysis using Ag(+) doped TiO(2) under UV irradiation in a batch reactor. The photocatalysts were characterized by FESEM, XRD, EDS, FTIR, DRS and BET analyses. The kinetics of the reaction was also evaluated. Colour removal was more than 88%, 94% and 99%, respectively for undiluted, 2 times diluted and 5 times diluted wastewater with Ag(+) doped TiO(2) (2.5 g/L) after UV irradiation for 360 minutes. The COD removal for undiluted, 2 times diluted and 5 times diluted wastewater was 47%, 70% and 92%, respectively under similar conditions. The reaction followed Langmuir-Hinshelwood pseudo first order kinetic model and the data fitted well to polynomial regression analysis. PMID:22871009

Sahoo, Chittaranjan; Gupta, Ashok K; Pillai, Indu M Sasidharan

2012-01-01

290

Expressions for the 14O(?,p)17F and 17F(p,?)18Ne astrophysical reaction rates  

NASA Astrophysics Data System (ADS)

The 14O(?,p)17F and 17F(p,?)18Ne astrophysical reaction rates are needed to determine the breakout conditions from the hot CNO cycle into the rapid proton capture process in hot stellar environments. We have examined recent indirect experimental measurements of these reactions and generated analytical expressions for the reaction rates as a function of temperature. We also present a correction to the previously reported calculation of the 17F(p,?) rate.

Bardayan, D. W.; Smith, M. S.

1997-09-01

291

Reaction of atomic bromine with acetylene and loss rate of atmospheric acetylene due to reaction with OH, Cl, O, and Br  

NASA Technical Reports Server (NTRS)

The first-order, diffusion, and bimolecular rate constants for the reaction Br + C2H2 yields C2H3Br are evaluated. The rate constants are measured at 210, 248, 298, and 393 K and at pressures between 15-100 torr Ar using flash photolysis combined with time-resolved detection of atomic bromine via Br resonance radiation. It is observed that the reaction is not affected by pressure or temperature and the bimolecular constant = (4.0 + or - 0.8) x 10 to the -15th cu cm/sec with an error of two standard deviations. The C2H2 + Br reaction rates are compared with reactions of C2H2 with Cl, OH, NH2, and H. The loss rates for atmospheric C2H2 for reactions with OH, Cl, O, and Br are calculated as a function of altitude.

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

1986-01-01

292

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

293

Relationships between enzymatic flux capacities and metabolic flux rates: Nonequilibrium reactions in muscle glycolysis  

PubMed Central

The rules that govern the relationships between enzymatic flux capacities (Vmax) and maximum physiological flux rates (v) at enzyme-catalyzed steps in pathways are poorly understood. We relate in vitro Vmax values with in vivo flux rates for glycogen phosphorylase, hexokinase, and phosphofructokinase, enzymes catalyzing nonequilibrium reactions, from a variety of muscle types in fishes, insects, birds, and mammals. Flux capacities are in large excess over physiological flux rates in low-flux muscles, resulting in low fractional velocities (%Vmax = v/Vmax × 100) in vivo. In high-flux muscles, close matches between flux capacities and flux rates (resulting in fractional velocities approaching 100% in vivo) are observed. These empirical observations are reconciled with current concepts concerning enzyme function and regulation. We suggest that in high-flux muscles, close matches between enzymatic flux capacities and metabolic flux rates (i.e., the lack of excess capacities) may result from space constraints in the sarcoplasm. PMID:9192692

Suarez, R. K.; Staples, J. F.; Lighton, J. R. B.; West, T. G.

1997-01-01

294

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

NASA Technical Reports Server (NTRS)

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

Brunning, J.; Stief, L.

1986-01-01

295

Determination of diffusion controlled reaction rates at a solid/liquid interface using scanning electron microscopy.  

PubMed

A high-resolution method has been developed for the determination of localized values of interfacial reaction rate and mass transfer coefficient in aqueous solution. Scanning electron microscopy has been successfully applied to this problem through the measurement of electroplated film thickness formed under limiting current conditions. The method involves the calculation of local values of reaction rate via Faraday's laws and subsequent conversion of the data to absolute values of mass transfer coefficient. The technique has been verified in an undisturbed, turbulent flow regime (rotating cylinder electrode) through the use of Sherwood group dimensionless analysis. The resulting relationship shows comparable accuracy relative to electrochemical measurements. Favourable comparison has also been made with the generally accepted rotating cylinder correlation of Eisenberg, Tobias and Wilke. Differential rates of mass transfer to a single surface under conditions of disturbed flow have also been examined at a high spatial resolution using the stepped rotating cylinder electrode geometry. In this case, reaction rates have been measured as a function of circumferential distance within a recirculation zone situated immediately downstream of a backward-facing step. PMID:17535261

Kear, Gareth; Huang, Sheng-Huei; Bremhorst, Klaus; Purchase, Andrew

2007-06-01

296

Substituent effects on the reaction rates of hydrogen abstraction in the pyrolysis of phenethyl phenyl ethers  

SciTech Connect

We report reaction profiles and forward rate constants for hydrogen abstraction reactions occurring in the pyrolysis of methoxy-substituted derivatives of phenethyl phenyl ether (PhCH{sub 2}CH{sub 2}OPh, PPE), where the substituents are located on the aryl ether ring (PhCH{sub 2}CH{sub 2}OPh-X). We use density functional theory in combination with transition-state theory, and anharmonic corrections are included within the independent mode approximation. PPE is the simplest model of the abundant {beta}-O-4 linkage in lignin. The mechanism of PPE pyrolysis and overall product selectivities have been studied experimentally by one of us, which was followed by computational analysis of key individual hydrogen-transfer reaction steps. In the previous work, we have been able to use a simplified kinetic model based on quasi-steady-state conditions to reproduce experimental {alpha}/{beta} selectivities for PPE and PPEs with substituents on the phenethyl ring (X-PhCH{sub 2}CH{sub 2}OPh). This model is not applicable to PPE derivatives where methoxy substituents are located on the phenyl ring adjacent to the ether oxygen because of the strongly endothermic character of the hydrogen abstraction by substituted phenoxy radicals as well as the decreased kinetic chain lengths resulting from enhanced rates of the initial C?O homolysis step. Substituents decelerate the hydrogen abstraction by the phenoxy radical, while the influence on the benzyl abstraction is less homogeneous. The calculations provide insight into the contributions of steric and polar effects in these important hydrogen-transfer steps. We emphasize the importance of an exhaustive conformational space search to calculate rate constants and product selectivities. The computed rate constants will be used in future work to numerically simulate the pyrolysis mechanism, pending the calculation of the rate constants of all participating reactions.

Beste, Ariana [ORNL; Buchanan III, A C [ORNL

2010-01-01

297

Continuum Diffusion Reaction Rate Calculations of Wild-Type and Mutant Mouse Acetylcholinesterase: Adaptive Finite Element Analysis  

E-print Network

Continuum Diffusion Reaction Rate Calculations of Wild-Type and Mutant Mouse Acetylcholinesterase acetylcholinesterase. The results show good agreement with experiment and help to define optimal reactive boundary

Texas at Austin, University of

298

Ab initio variational transition-state-theory reaction-rate calculations for the gas-phase reaction H+HNO --> H2+NO  

Microsoft Academic Search

We report abinitio calculations of the potential-energy surface and variational transition-state-theory calculations of the temperature dependence of the reaction rate for the chemical reaction H+HNO?H2+NO. This reaction is very exothermic and proceeds nearly without activation. Steepest-descent reaction pathways on the potential-energy surface for the abstraction reaction are presented both at the five-electron, five-orbital complete-active-space self-consistent-field level, and at the single-

Maribel R. Soto; Michael Page

1992-01-01

299

Rate coefficients of the reactions of ions with polar molecules at interstellar temperatures  

NASA Astrophysics Data System (ADS)

A theory has been developed recently which predicts that the rate coefficients, k, for the reactions of ions with polar molecules at low temperatures will be much greater than the canonical value of 10 to the -9th cu cm/s. The new theory indicates that k is greatest for low-lying rotational states and increases rapidly with decreasing temperature. Recent laboratory measurements which validate the theory, are examined, calculated values of k for the reactions of H3(+) ions with several polar molecules are presented, and their significance to interstellar chemistry is discussed. For the reactions of ions with molecules having large dipole moments, it is recommended that k values as large as 10 to the -7th cu cm/s should be used in ion-chemical models of low-temperature instellar clouds.

Adams, N. G.; Smith, D.; Clary, D. C.

1985-09-01

300

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

301

Rates of sensitivity reactions to aspirin: problems in interpreting the data.  

PubMed

This work was done to determine the reasons for variation in the reported rates--ranging from less than 1% to greater than 50%--of sensitivity to aspirin and cross-reactivity to acetaminophen and ibuprofen. In 47 studies that reported rates of sensitivity and in 23 reports that contained series of sensitive patients, we examined the research setting, source of patients, clinical attributes of the study group, admission process, and selection, operational definition, and method of determining sensitivity reactions. In five studies with reasonably well-specified methods, the reported sensitivity rates to aspirin were lowest (0.3% to 0.9%) for patients without allergic tendencies, higher in asthmatics, and highest if patients had nasal polyps or severe atopy. Although not determined in any of these studies, the rate of sensitivity in a general (nonclinical) population would doubtlessly be substantially lower than the rate of three per 1000 reported for nonallergic patients. The admixture of different clinical groups, varying definitions, and ascertainment of a sensitivity reaction seem to be responsible for the variations in the reported rates of sensitivity and cross-reactivity. PMID:3769380

Kwoh, C K; Feinstein, A R

1986-11-01

302

The effects of physical and geochemical heterogeneities on hydro-geochemical transport and effective reaction rates.  

PubMed

The role of coupled physical and geochemical heterogeneities in hydro-geochemical transport is investigated by simulating three-dimensional transport in a heterogeneous system with kinetic mineral reactions. Ensembles of 100 physically heterogeneous realizations were simulated for three geochemical conditions: 1) spatially homogeneous reactive mineral surface area, 2) reactive surface area positively correlated to hydraulic heterogeneity, and 3) reactive surface area negatively correlated to hydraulic heterogeneity. Groundwater chemistry and the corresponding effective reaction rates were calculated at three transverse planes to quantify differences in plume evolution due to heterogeneity in mineral reaction rates and solute residence time (?). The model is based on a hypothetical CO2 intrusion into groundwater from a carbon capture utilization and storage (CCUS) operation where CO2 dissolution and formation of carbonic acid created geochemical dis-equilibrium between fluids and the mineral galena that resulted in increased aqueous lead (Pb(2+)) concentrations. Calcite dissolution buffered the pH change and created conditions of galena oversaturation, which then reduced lead concentrations along the flow path. Near the leak kinetic geochemical reactions control the release of solutes into the fluid, but further along the flow path mineral solubility controls solute concentrations. Simulation results demonstrate the impact of heterogeneous distribution of geochemical reactive surface area in coordination with physical heterogeneity on the effective reaction rate (Krxn,eff) and Pb(2+) concentrations within the plume. Dissimilarities between ensemble Pb(2+) concentration and Krxn,eff are attributed to how geochemical heterogeneity affects the time (?eq) and therefore advection distance (Leq) required for the system to re-establish geochemical equilibrium. Only after geochemical equilibrium is re-established, Krxn,eff and Pb(2+) concentrations are the same for all three geochemical conditions. Correlation between reactive surface area and hydraulic conductivity, either positive or negative, results in variation in ?eq and Leq. PMID:25113426

Atchley, Adam L; Navarre-Sitchler, Alexis K; Maxwell, Reed M

2014-09-01

303

Relative rate and product studies of the OH-acetone reaction.  

PubMed

The rate constants for the reaction of acetone (kH) and d6-acetone (kD) with OH radicals have been measured at atmospheric pressure over a range of temperatures by a relative rate method by using on-line mass spectrometry. The following Arrhenius expressions have been determined for these reactions (in units of cm(3) molecule(-1) s(-1)): k(H)(T) = (9.8 x 10(-13)) exp[-(484 +/- 44)/T] between 253 and 373 K, and kD(T) = (4.0 x 10(-13)) exp[-(755 +/- 89)/T] between 293 and 373 K. This is the first study to investigate the temperature dependence of kH and kD by using a relative rate method and confirms previous rate constants determined by absolute methods. Agreement of our rate constants with those determined in the absence of water suggests that the presence of water vapor has a minimal effect on the kinetics of this reaction under the conditions of our study. The observed kinetic isotope effect (kH/kD = 5.6 +/- 0.4 at 293 K) is evidence that H-atom abstraction occurs in the mechanism. The acetic acid yields of the reaction of OH with acetone and d6-acetone were also investigated by on-line mass spectrometry. Acetic acid yields show a negative temperature dependence that decreases from 0.12 at 273 K to 0.05 at 353 K. The yields of d3-acetic acid decrease from 0.20 at 283 K to 0.13 at 323 K. Kinetic modeling of our data suggests that 50-70% of the observed acetic acid in our system may be due to secondary reactions involving acetonoxy and HOx radical reactions. However, secondary chemistry cannot easily explain the observed formation of d3-acetic acid in the deuterated system, where about 90% of the observed d3-acetic acid is likely due to an OH-addition mechanism. PMID:16833814

Raff, Jonathan D; Stevens, Philip S; Hites, Ronald A

2005-06-01

304

Temporal and spatial variation of surface reaction rates in porous media: Applications to silicate weathering  

NASA Astrophysics Data System (ADS)

Percolation theory provides a promising framework for modeling transport in heterogeneous porous media, including hydraulic and electrical conductivity, air permeability, gas diffusivity, and solute transport. Using percolation concepts (e.g., critical path analysis, fractal scaling of percolation clusters, and cluster statistics), we developed a physically-based model for predicting solute transport. Our model predicted spatial solute distributions as a function of time, and arrival time distributions as a function of system size. Our solute transport predictions gave good matches to a wide range of experiments. We now apply our solute transport model to silicate weathering. We assume that surface chemical reactions are at equilibrium at the scale of a single pore, but that at larger length scales, reactions are limited by transport of reactants or products. Using results from published field experiments, we find that the temporal and spatial dependence derived from solute velocity successfully predicts the measured time- and length-dependence of reaction rates and weathering of silicate minerals over a wide range of time and length scale. A similar analysis of lab experiments (uranium breakthrough curves measured in two short and long columns from the Hanford site) indicates that normalized reaction rate versus normalized time follow 2D invasion and 3D random percolation.

Ghanbarian, B.; Hunt, A. G.; Skinner, T. E.; Ewing, R. P.

2013-12-01

305

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

306

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

NASA Technical Reports Server (NTRS)

The rate constant for the reaction Br + O3 yields BrO + O2 has been measured from 200 to 360 K by the technique of flash photolysis coupled to time resolved detection of bromine atoms by resonance fluorescence (FP-RF). Br atoms were produced by the flash photolysis of CH3Br at a wavelength of 165 nm. O3 concentration was monitored continuously under reaction conditions by absorption at 253.7 nm. At each of five temperatures the results were independent of substantial variations in O3 concentration, total pressure (Ar), and limited variations in flash intensity (i.e., initial Br concentration). The measured rate constants obey the Arrhenius expression, k = (7.74 plus or minus 0.50) x 10 to the -12th exp(-603 plus or minus 16/T) cu cm/molecule/sec, where the error quoted is two standard deviations.

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

1978-01-01

307

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

NASA Technical Reports Server (NTRS)

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

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

1978-01-01

308

Modelling of silicon oxynitridation by nitrous oxide using the reaction rate approach  

SciTech Connect

Large technological progress in oxynitridation processing leads to the introduction of silicon oxynitride as ultra-thin gate oxide. On the theoretical side, few studies have been dedicated to the process modelling of oxynitridation. Such an objective is a considerable challenge regarding the various atomistic mechanisms occurring during this fabrication step. In this article, some progress performed to adapt the reaction rate approach for the modelling of oxynitride growth by a nitrous ambient are reported. The Ellis and Buhrman's approach is used for the gas phase decomposition modelling. Taking into account the mass balance of the species at the interface between the oxynitride and silicon, a minimal kinetic model describing the oxide growth has been calibrated and implemented. The influence of nitrogen on the reaction rate has been introduced in an empirical way. The oxidation kinetics predicted with this minimal model compares well with several experiments.

Dominique Krzeminski, Christophe, E-mail: christophe.krzeminski@isen.fr [Départment ISEN, IEMN-UMR-8520, 41 Boulevard Vauban, 59046 Lille Cedex (France)

2013-12-14

309

Monte Carlo uncertainty of the He3(?,?)Be7 reaction rate  

NASA Astrophysics Data System (ADS)

Background: The He3(?,?)Be7 reaction is of critical importance in determining the flux of solar neutrinos through the pp-II and pp-III chains. For this reason and others, the description of the cross section and its extrapolation towards low-energy has always been a matter of intense debate. While large systematic differences have been present in the past, several recent measurements are all in excellent statistical agreement. Purpose: The convergence of the recent individual experimental measurements of the He3(? ,?)Be7 reaction prompts a global analysis of the reaction data. From the combined data, a more precise estimate of the low-energy cross section can be determined. Results: A global R-matrix fit is used to describe the He3(?,?)Be7 data as well as scattering data over a similar energy range. The R-matrix fit is then subjected to a Monte Carlo analysis to extract the uncertainties on the cross section and corresponding reaction rate. Conclusion: By combining several recent measurements of the He3(?,?)Be7 reaction, the combined data yield a zero energy S factor of S(0)=0.542±0.011(MCfit)±0.006(model)-0.011+0.019(phaseshifts) keV b. This gives a total uncertainty in S (0) of +0.023/-0.017 keV b.

deBoer, R. J.; Görres, J.; Smith, K.; Uberseder, E.; Wiescher, M.; Kontos, A.; Imbriani, G.; Di Leva, A.; Strieder, F.

2014-09-01

310

Shock tube/laser absorption measurements of the reaction rates of OH with ethylene and propene.  

PubMed

Reaction rates of hydroxyl (OH) radicals with ethylene (C?H?) and propene (C?H?) were studied behind reflected shock waves. OH + ethylene ? products (rxn 1) rate measurements were conducted in the temperature range 973-1438 K, for pressures from 2 to 10 atm, and for initial concentrations of ethylene of 500, 751, and 1000 ppm. OH + propene ? products (rxn 2) rate measurements spanned temperatures of 890-1366 K, pressures near 2.3 atm, and initial propene concentrations near 300 ppm. OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH?)?-CO-OH, and monitored by laser absorption near 306.7 nm. Rate constants for the reactions of OH with ethylene and propene were extracted by matching modeled and measured OH concentration time-histories in the reflected shock region. Current data are in excellent agreement with previous studies and extend the temperature range of OH + propene data. Transition state theory calculations using recent ab initio results give excellent agreement with our measurements and other data outside our temperature range. Fits (in units of cm³/mol/s) to the abstraction channels of OH + ethylene and OH + propene are k? = 2.23 × 10? (T)(2.745) exp(-1115 K/T) for 600-2000 K and k? = 1.94 × 10? (T)(2.229) exp(-540 K/T) for 700-1500 K, respectively. A rate constant determination for the reaction TBHP ? products (rxn 3) was also obtained in the range 745-1014 K using OH data from behind both incident and reflected shock waves. These high-temperature measurements were fit with previous low-temperature data, and the following rate expression (0.6-2.6 atm), applicable over the temperature range 400-1050 K, was obtained: k? (1/s) = 8.13 × 10?¹² (T)(7.83) exp(-14598 K/T). PMID:20923208

Vasu, Subith S; Hong, Zekai; Davidson, David F; Hanson, Ronald K; Golden, David M

2010-11-01

311

Enzyme reaction rate studies in electromotor neurons of the weakly electric fish Apteronotus leptorhynchus  

Microsoft Academic Search

Summary  A histochemical analysis of reaction rates of a series of enzymes was performed in electromotor neurons of the weakly electric fishApteronotus leptorhynchus. These neurons were selected because of their functional homogeneity. The high metabolic activity of these cells as well as their large size facilitate cytophotometric analysis in cryostat sections. Sections were incubated for the activity of hexokinase, glucose-6-phosphate dehydrogenase,

Irene H. Straatsburg; Frank De Graaf; CORNELIS J. F. VAN NOORDENI; Wim Van Raamsdonk

1989-01-01

312

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

NASA Technical Reports Server (NTRS)

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

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

1978-01-01

313

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

Microsoft Academic Search

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

MING-YUAN KAO

1984-01-01

314

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

NASA Technical Reports Server (NTRS)

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

Watson, R. T.

1977-01-01

315

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

E-print Network

The effects of the highly damped modes in the energy and reaction rates in a plasma are discussed. These modes, with wavenumbers $k \\gg k_{D}$, even being only weakly excited, with less than $k_{B}T$ 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

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

2001-05-09

316

A Review of the Thermodynamic, Transport, and Chemical Reaction Rate Properties of High-temperature Air  

NASA Technical Reports Server (NTRS)

Thermodynamic and transport properties of high temperature air, and the reaction rates for the important chemical processes which occur in air, are reviewed. Semiempirical, analytic expressions are presented for thermodynamic and transport properties of air. Examples are given illustrating the use of these properties to evaluate (1) equilibrium conditions following shock waves, (2) stagnation region heat flux to a blunt high-speed body, and (3) some chemical relaxation lengths in stagnation region flow.

Hansen, C Frederick; Heims, Steve P

1958-01-01

317

Diffusion control of the Diels—Alder reaction rate at elevated pressures  

Microsoft Academic Search

The influence of the temperature and external pressure on the rate of the Diels—Alder reaction between 9,10-dimethylanthracene and maleic anhydride was studied in the series of solvents with wide intervals of viscosity (0.3—43.2 mPa s), dielectric constant (2—38), and internal pressure (3—8.8 kbar). At a standard pressure these properties of the solvent exert a weak and irregular effect on the

V. D. Kiselev; E. A. Kashaeva; M. S. Shihab; L. N. Potapova; G. G. Iskhakova

2004-01-01

318

Redetermination of the rate coefficient for the reaction of O(1D) with N2  

Microsoft Academic Search

The rate coefficient for the reaction of O(1D) with N2 at 295 K has been measured in three laboratories to be 3.1 × 10?11 cm3 molecule?1 s?1, with an uncertainty of 10% at the 95% confidence level, and to be 2.1 × 10?11 exp {(115 ± 10) \\/T} cm3 molecule?1 s?1 as a function of temperature. (The quoted uncertainty in

A. R. Ravishankara; E. J. Dunlea; M. A. Blitz; T. J. Dillon; D. E. Heard; M. J. Pilling; R. S. Strekowski; J. M. Nicovich; P. H. Wine

2002-01-01

319

Redetermination of the rate coefficient for the reaction of O(1D) with N2  

Microsoft Academic Search

The rate coefficient for the reaction of O(1D) with N2 at 295 K has been measured in three laboratories to be 3.1 × 10-11 cm3 molecule-1 s-1, with an uncertainty of 10% at the 95% confidence level, and to be 2.1 × 10-11 exp {(115 +\\/- 10) \\/T} cm3 molecule-1 s-1 as a function of temperature. (The quoted uncertainty in

A. R. Ravishankara; E. J. Dunlea; M. A. Blitz; T. J. Dillon; D. E. Heard; M. J. Pilling; R. S. Strekowski; J. M. Nicovich; P. H. Wine

2002-01-01

320

Theoretical study of rate coefficients and branching fractions in the propene + OH reaction  

Microsoft Academic Search

High-level ab initio calculation of the C3H6 + OH potential energy surface was coupled with master equation methods to compute rate coefficients and product branching ratios for temperatures of 300-2500 K. Our model reproduces the available experimental results well. We find a surprisingly wide range of bimolecular product channels for this reaction, including vinyl alcohol, acetaldehyde, allyl radical, acetone, ethene,

Judit Zádor; Ahren W. Jasper; James A. Miller

321

Potential resonant screening effects on stellar C12+C12 reaction rates  

E-print Network

The C12+C12 fusion cross sections show resonant behavior down to the lowest energies accessible so far in the laboratory. If this tendency continues into the astrophysical energy range, the stellar C12+C12 reaction rates have to be corrected for resonant screening effects, in addition to the conventional screening corrections. We estimate the resonant screening effects in the weak electron screening limit for hydrostatic burning and white dwarf environments.

R. Cussons; K. Langanke; T. Liolios

2002-10-11

322

Relative rate constants for the reactions of OH with methane and methyl chloroform  

NASA Technical Reports Server (NTRS)

Atmospheric lifetimes of methane and methyl chloroform are largely determined by the rates of their reactions with hydroxyl radical. The relative lifetimes for this loss path are inversely proportional to the ratio of the corresponding rate coefficients. The relative rate constants were measured in a slow-flow, temperature-controlled photochemical reactor, and were based on rates of disappearance of the parent compounds as measured by FTIR spectroscopy. The temperature range was 277-356 K. Hydroxyl radicals were generated by 254 nm photolysis of O3 in the presence of water vapor. The preferred Arrhenius expression for the results is k(CH3CCl3)/k(CH4) = 0.62 exp (291/T), corresponding to a value of 1.65 at 298 K and 1.77 at 277 K. The respective uncertainties are 5 and 7 percent.

Demore, W. B.

1992-01-01

323

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

PubMed Central

A model of transducin activation is constructed from its partial reactions (formation of metarhodopsin II, association, and dissociation of the rhodopsin-transducin complex). The kinetic equations of the model are solved both numerically and, for small photoactivation, analytically. From data on the partial reactions in vitro, rate and activation energy profile of amplified transducin turnover are modeled and compared with measured light-scattering signals of transducin activation in intact retinal rods. The data leave one free parameter, the rate of association between transducin and rhodopsin. Best fit is achieved for an activation energy of 35 kJ/mol, indicating lateral membrane diffusion of the proteins as its main determinant. The absolute value of the association rate is discussed in terms of the success of collisions to form the catalytic complex. It is greater than 30% for the intact retina and 10 times lower after permeabilization with staphylococcus aureus alpha-toxin. Dissociation rates for micromolar guanosinetriphosphale (GTP) (Kohl, B., and K. P. Hofmann, 1987. Biophys. J. 52:271-277) must be extrapolated linearly up to the millimolar range to explain the rapid transducin turnover in situ. This is interpreted by an unstable rhodopsin-transducin-GTP transient state. At the time of maximal turnover after a flash, the rate of activation is determined as 30, 120, 800, 2,500, and 4,000 activated transducins per photoactivated rhodopsin and second at 5, 10, 20, 30, 37 degrees C, respectively. PMID:1901231

Kahlert, M; Hofmann, K P

1991-01-01

324

Dependence of the Rate of an Interfacial Diels-Alder Reaction on the Steric Environment of the Immobilized Dienophile  

E-print Network

Dependence of the Rate of an Interfacial Diels-Alder Reaction on the Steric Environment for an interfacial Diels-Alder reaction and the steric environment around the reacting molecules. The study used catalysis,8 and combinatorial libraries.9 In this paper we use as a model reaction the Diels-Alder

Mrksich, Milan

325

Upscaling of reaction rates in reactive transport using pore-scale reactive transport model  

NASA Astrophysics Data System (ADS)

Dissolved CO2 during geological CO2 storage may react with minerals in fractured rocks, confined aquifers, or faults, resulting in mineral precipitation and dissolution. The overall rate of reaction can be affected by coupled processes among hydrodynamics, transport, and reactions at the (sub) pore-scale. In this research pore-scale modeling of coupled fluid flow, reactive transport, and heterogeneous reaction at the mineral surface is applied to account for permeability alterations caused by precipitation-induced pore-blocking. This work is motivated by the observed CO2 seeps from a natural analog to geologic CO2 sequestration at Crystal Geyser, Utah. A key observation is the lateral migration of CO2 seep sites at a scale of ~ 100 meters over time. A pore-scale model provides fundamental mechanistic explanations of how calcite precipitation alters flow paths by pore plugging under different geochemical compositions and pore configurations. In addition, response function of reaction rates will be constructed from pore-scale simulations which account for a range of reaction regimes characterized by the Damkohler and Peclet numbers. Newly developed response functions will be used in a continuum scale model that may account for large-scale phenomena mimicking lateral migration of surface CO2 seeps. Comparison of field observations and simulations results will provide mechanistic explanations of the lateral migration and enhance our understanding of subsurface processes associated with the CO2 injection. This work is supported as part of the Center for Frontiers of Subsurface Energy Security, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001114. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Yoon, H.; Dewers, T. A.; Arnold, B. W.; Major, J. R.; Eichhubl, P.; Srinivasan, S.

2013-12-01

326

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

NASA Technical Reports Server (NTRS)

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

Chinitz, W.

1986-01-01

327

A combined quantum-classical dynamics method for calculating thermal rate constants of chemical reactions in solution  

E-print Network

-flux correlation function for calculating the thermal rate constants of chemical reactions in solutionA combined quantum-classical dynamics method for calculating thermal rate constants of chemical reactions in solution Thanh N. Truong,") J. Andrew McCammon, and Donald J. Kouri Department of Chemistry

Truong, Thanh N.

328

Measurement of Screening Enhancement to Nuclear Reaction Rates using a Strongly Magnetized and Strongly Correlated Non-neutral Plasma  

E-print Network

with typical collisions (here b e2 =T is the distance of closest ap- proach and v T=m p is the thermal speedMeasurement of Screening Enhancement to Nuclear Reaction Rates using a Strongly Magnetized) An analogy is uncovered between the nuclear reaction rate in a dense neutral plasma and the energy

California at San Diego, University of

329

Journal of Molecular Catalysis A: Chemical 163 (2000) 189204 Catalytic reaction rates in thermodynamically non-ideal systems  

E-print Network

Journal of Molecular Catalysis A: Chemical 163 (2000) 189­204 Catalytic reaction rates in the defining equations for all relevant thermodynamic properties. Catalytic reactions in gas of Molecular Catalysis A: Chemical 163 (2000) 189­204 cracking rates on intrazeolite paraffin activities

Iglesia, Enrique

330

Laser flash photolysis studies of some halogen monoxide reactions of atmospheric interest  

NASA Astrophysics Data System (ADS)

The kinetics of the atmospherically important reactions HO2 + XO --> products (where X = Br, I) have been investigated using 308nm laser flash photolysis of Cl2/CH3OH/O2/Br 2/O3/N2 mixtures (for X = Br) or Cl2/CH 3OH/O2/I2/NO2/N2 (for X = I) coupled with simultaneous time-resolved detection of HO2 (by infrared tunable diode laser absorption spectroscopy) and XO (by UV/visible absorption spectroscopy). HO2+ BrO was studied at 298 +/- 3 K; HO2 + IO was studied over the temperature range 274-373 K. All experiments were performed under near pseudo-first-order conditions with HO2 in excess over XO. The HO2 + BrO rate coefficient at 298 K was found to be k = (2.0 +/- 0.6) × 10 -11 cm3 molecule-1s- 1. The HO2 + IO kinetics were found to be described by the Arrhenius expression K2 = (9.3 +/- 3.3) × 10-12 exp(680 +/- 110/T). The results reported in this study are compared with other recent studies of HO2 + XO kinetics, and their implications for our understanding of atmospheric chemistry are discussed. Laser flash photolysis of RBr/O3/SF6/He mixtures at 248 nm has been coupled with BrO detection by time-resolved UV absorption spectroscopy to measure BrO product yields from O(1D) reactions with HBr, CF3Br, CH3Br, CF2ClBr, and CF 2HBr at 298 +/- 3 K. The measured yields are: HBr, 0.20 +/- 0.04; CF3Br, 0.49 +/- 0.07; CH3Br, 0.44 + 0.05; CF2ClBr, 0.31 +/- 0.06; CF2HBr, 0.39 +/- 0.07 (uncertainties are 2 s and include estimates of both random and systematic errors). The results are discussed in light of other available information on O(1D) + RBr reactions.

Cronkhite, Jeffrey M.

1998-12-01

331

Absolute rate constant for the reaction of atomic hydrogen with acetylene over an extended pressure and temperature range  

Microsoft Academic Search

The technique of flash photolysis coupled with time resolved detection of H via resonance fluorescence has been used to obtain absolute rate parameters for the reaction of atomic hydrogen with acetylene, i.e., H+C2H2?C2H3* (1); C2H3*+M-->C2H3+M (2). The rate constant for the reaction is strongly pressure dependent and was measured over the pressure range 10 to 700 torr. The reaction was

W. A. Payne; L. J. Stief

1976-01-01

332

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

333

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

334

A correlation between slightly endothermic reactivity and ion-molecule association reaction rates  

NASA Astrophysics Data System (ADS)

Primary binary reaction processes which are slightly endothermic can create relatively long-lived complexes in ion-neutral molecule collisions. The endothermic process absorbs translational energy, inhibiting separation to reactants, and in addition may facilitate access to an overall reaction potential well. The endothermicity must be less than the available collision energy, i.e., the electrostatic attractive energy and possibly some chemical interaction energy. An enhanced complex lifetime can result in enhanced rate constants for three-body ion-neutral association, radiative association, vibrational relaxation or complicated binary reactions. Application is made to the three-body association of hydrocarbon ions with H2, a selective H/D isotope effect in three-body association, the three-body association of Si+ with O2, the efficient vibrational relaxation of O+2 by CH4 and the fast low temperature reaction of O+2 with CH4 to produce protonated formic acid which involves the making and breaking of four chemical bonds.

Ferguson, E. E.; Smith, D.; Adams, N. G.

1984-07-01

335

Theoretical calculations of the thermal rate constants for the gas-phase chemical reactions H + NH  

SciTech Connect

Rate constants for the title reactions are computed by using variational transition-state theory with semiclassical ground-state adiabatic transmission coefficients for the temperature range from 200 to 2,400 K. The rates are computed from selected information about the potential energy surface along the minimum energy path as parameters of the reaction path Hamiltonian. The potential information is obtained from ab initio electronic structure calculations with an empirical bond additivity correction. The accuracy of this semiempirical technique for obtaining the potential information is tested by comparing the results of the underlying ab initio calculations with higher quality multiconfiguration SCF and multireference CI calculations and by using increasingly higher quality ab initio electronic structure calculations before applying the bond additivity correction. For the reactions H + NH{sub 3} {yields} H{sub 2} + NH{sub 2}, H{sub 2} + NH{sub 2} {yields} H + NH{sub 3}, and D + ND{sub 3} {yields} D{sub 2} + ND{sub 2}, the ultimate test is given by comparison with recent experimental results. Although the agreement is good in general, the comparisons of experiment and theory indicate that the computed barrier height is overestimated by about 1 kcal/mol.

Garrett, B.C. (Chemical Dynamics Corp., Marlboro, MD (USA)); Koszykowski, M.L.; Melius, C.F. (Sandia National Lab., Livermore, CA (USA)); Page, M. (Naval Research Lab., Washington, DC (USA))

1990-09-06

336

THE IMPACT OF HELIUM-BURNING REACTION RATES ON MASSIVE STAR EVOLUTION AND NUCLEOSYNTHESIS  

SciTech Connect

We study the sensitivity of presupernova evolution and supernova nucleosynthesis yields of massive stars to variations of the helium-burning reaction rates within the range of their uncertainties. The current solar abundances from Lodders are used for the initial stellar composition. We compute a grid of 12 initial stellar masses and 176 models per stellar mass to explore the effects of independently varying the {sup 12}C({alpha}, {gamma}){sup 16}O and 3{alpha} reaction rates, denoted R{sub {alpha},12} and R{sub 3{alpha}}, respectively. The production factors of both the intermediate-mass elements (A = 16-40) and the s-only isotopes along the weak s-process path ({sup 70}Ge, {sup 76}Se, {sup 80}Kr, {sup 82}Kr, {sup 86}Sr, and {sup 87}Sr) were found to be in reasonable agreement with predictions for variations of R{sub 3{alpha}} and R{sub {alpha},12} of {+-}25%; the s-only isotopes, however, tend to favor higher values of R{sub 3{alpha}} than the intermediate-mass isotopes. The experimental uncertainty (one standard deviation) in R{sub 3{alpha}}(R{sub {alpha},12}) is approximately {+-}10%({+-}25%). The results show that a more accurate measurement of one of these rates would decrease the uncertainty in the other as inferred from the present calculations. We also observe sharp changes in production factors and standard deviations for small changes in the reaction rates, due to differences in the convection structure of the star. The compactness parameter was used to assess which models would likely explode as successful supernovae, and hence contribute explosive nucleosynthesis yields. We also provide the approximate remnant masses for each model and the carbon mass fractions at the end of core-helium burning as a key parameter for later evolution stages.

West, Christopher; Heger, Alexander [Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 (United States); Austin, Sam M., E-mail: west0482@umn.edu, E-mail: alexander.heger@monash.edu, E-mail: austin@nscl.msu.edu [Joint Institute for Nuclear Astrophysics, Notre Dame, IN 46556 (United States)

2013-05-20

337

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

338

Modeling microbial reaction rates in a submarine hydrothermal vent chimney wall  

NASA Astrophysics Data System (ADS)

The fluids emanating from active submarine hydrothermal vent chimneys provide a window into subseafloor processes and, through mixing with seawater, are responsible for steep thermal and compositional gradients that provide the energetic basis for diverse biological communities. Although several models have been developed to better understand the dynamic interplay of seawater, hydrothermal fluid, minerals and microorganisms inside chimney walls, none provide a fully integrated approach to quantifying the biogeochemistry of these hydrothermal systems. In an effort to remedy this, a fully coupled biogeochemical reaction-transport model of a hydrothermal vent chimney has been developed that explicitly quantifies the rates of microbial catalysis while taking into account geochemical processes such as fluid flow, solute transport and oxidation-reduction reactions associated with fluid mixing as a function of temperature. The metabolisms included in the reaction network are methanogenesis, aerobic oxidation of hydrogen, sulfide and methane and sulfate reduction by hydrogen and methane. Model results indicate that microbial catalysis is generally fastest in the hottest habitable portion of the vent chimney (77-102 °C), and methane and sulfide oxidation peak near the seawater-side of the chimney. The fastest metabolisms are aerobic oxidation of H2 and sulfide and reduction of sulfate by H2 with maximum rates of 140, 900 and 800 pmol cm-3 d-1, respectively. The maximum rate of hydrogenotrophic methanogenesis is just under 0.03 pmol cm-3 d-1, the slowest of the metabolisms considered. Due to thermodynamic inhibition, there is no anaerobic oxidation of methane by sulfate (AOM). These simulations are consistent with vent chimney metabolic activity inferred from phylogenetic data reported in the literature. The model developed here provides a quantitative approach to describing the rates of biogeochemical transformations in hydrothermal systems and can be used to constrain the role of microbial activity in the deep subsurface.

LaRowe, Douglas E.; Dale, Andrew W.; Aguilera, David R.; L'Heureux, Ivan; Amend, Jan P.; Regnier, Pierre

2014-01-01

339

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

NASA Technical Reports Server (NTRS)

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

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

1986-01-01

340

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

341

Non-explosive hydrogen and helium burnings: abundance predictions from the NACRE reaction rate compilation  

NASA Astrophysics Data System (ADS)

The abundances of the isotopes of the elements from C to Al produced by the non-explosive CNO, NeNa and MgAl modes of hydrogen burning, as well as by helium burning, are calculated with the thermonuclear rates recommended by the European compilation of reaction rates for astrophysics (NACRE). The impact of nuclear physics uncertainties on the derived abundances is discussed in the framework of a simple parametric astrophysical model. These calculations have the virtue of being a guide in the selection of the nuclear uncertainties that have to be duly analyzed in detailed model stars, particularly in order to perform meaningful confrontations between abundance observations and predictions. They are also hoped to help nuclear astrophysicists pinpointing the rate uncertainties that have to be reduced most urgently. An electronic version of this paper, with colour figures, is available at {\\it http://astro.ulb.ac.be}

Arnould, M.; Goriely, S.; Jorissen, A.

1999-07-01

342

Studies on self-sustained reaction-rate oscillations. III. The carbon model  

NASA Astrophysics Data System (ADS)

A simple model based upon the combination of the Langmuir-Hinshelwood surface reaction, slow catalyst deactivation by carbon, and reactive regeneration of these deactivated sites by oxygen is presented and analyzed by a singular perturbation approach. It is shown that this model is capable of describing the major features of the slow, relaxation oscillations observed experimentally during CO oxidation over Pt. These features can also be described equally well by a model based on slow catalyst oxidation/reduction cycles proposed by Sales, Turner, and Maple. However, the carbon model and the oxide model predict vastly different behavior in the steady, high reaction-rate regime oberved at low {P CO}/{P O 2} ratios, suggesting an experimental study in this nonoscillatory regime which permits discrimination between these two models.

Collins, N. A.; Sundaresan, S.; Chabal, Y. J.

1987-02-01

343

Quantum chemical and conventional TST calculations of rate constants for the OH + alkane reaction  

NASA Astrophysics Data System (ADS)

Reactions of OH with methane, ethane, propane, i-butane, and n-butane have been modeled using ab initio (MP2) and hybrid DFT (BHandHLYP) methods, and the 6-311G(d,p) basis set. Furthermore, single-point calculations at the CCSD(T) level were carried out at the optimized geometries. The rate constants have been calculated using the conventional transition-state theory (CTST). Arrhenius equations are proposed in the temperature range of 250-650 K. Hindered Internal Rotation partition functions calculations were explicitly carried out and included in the total partition functions. These corrections showed to be relevant in the determination of the pre-exponential parameters, although not so important as in the NO 3 + alkane reactions [G. Bravo-Pérez, J.R. Alvarez-Idaboy, A. Cruz-Torres, M.E. Ruíz, J. Phys. Chem. A 106 (2002) 4645]. The explicit participation of the tunnel effect has been taken into account. The calculated rate coefficients provide a very good agreement with the experimental data. The best agreement for the overall alkane + OH reactions seemed to occur when the BHandHLYP geometries and partition functions are used. For propane and i-butane, in addition to the respective secondary and tertiary H-abstraction channels, the primary one has been considered. These pathways are confirmed to be significant in spite of the large differences in activation energies between primary and secondary or primary and tertiary channels, respectively of propane and i-butane reactions and should not be disregarded.

Bravo-Pérez, Graciela; Alvarez-Idaboy, J. Raúl; Jiménez, Annia Galano; Cruz-Torres, Armando

2005-04-01

344

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

345

Eight-dimensional quantum reaction rate calculations for the H+CH4 and H2+CH3 reactions on recent potential energy surfaces.  

PubMed

Eight-dimensional (8D) transition-state wave packet simulations have been performed on two latest potential energy surfaces (PES), the Zhou-Fu-Wang-Collins-Zhang (ZFWCZ) PES [Y. Zhou, B. Fu, C. Wang, M. A. Collins, and D. H. Zhang, J. Chem. Phys. 134, 064323 (2011)] and the Xu-Chen-Zhang (XCZ)-neural networks (NN) PES [X. Xu, J. Chen, and D. H. Zhang, Chin. J. Chem. Phys. 27, 373 (2014)]. Reaction rate constants for both the H+CH4 reaction and the H2+CH3 reaction are calculated. Simulations of the H+CH4 reaction based on the XCZ-NN PES show that the ZFWCZ PES predicts rate constants with reasonable high accuracy for low temperatures while leads to slightly lower results for high temperatures, in line with the distribution of interpolation error associated with the ZFWCZ PES. The 8D H+CH4 rate constants derived on the ZFWCZ PES compare well with full-dimensional 12D results based on the equivalent m-ZFWCZ PES, with a maximum relative difference of no more than 20%. Additionally, very good agreement is shown by comparing the 8D XCZ-NN rate constants with the 12D results obtained on the ZFWCZ-WM PES, after considering the difference in static barrier height between these two PESs. The reaction rate constants calculated for the H2+CH3 reaction are found to be in good consistency with experimental observations. PMID:25416891

Zhou, Yong; Zhang, Dong H

2014-11-21

346

A review of the rates of reaction of uranium with oxygen and water vapour at temperatures up to 300° c  

NASA Astrophysics Data System (ADS)

The literature on the uranium-oxygen (and dry air), uranium-water vapour and uranium-oxygen-water vapour reactions has been reviewed in order to establish the rates of these reactions. The following linear reaction rates were found: Uranium-dry air reaction: 6.9 × 10 8exp(- 18300/ RT) mgU/ cn2· h (40 ? T(° C)?300) Uranium-water vapour reaction: 3.2 × 10 8exp(- 13800/ RT) mgU/ cm2· h (100% RH) (20 ? T(° C)?100) Uranium-oxygen-water vapour reaction: 4.6 × 10 9exp(- 17800/ RT) mgU/ cm2· h (100% RH) (25 ? T(° C)?100) Uranium-oxygen-water vapour reaction: 4.8 × 10 13exp(- 25000/ RT) mgU/ cm2· h (2-90% RH) (40 ? T(° C)?130) The units used are milligrams of uranium reacted per square centimetre of uranium per hour.

Ritchie, A. G.

1981-11-01

347

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

PubMed

When tested on the treadmill mice do not display a graded increase of heart rate (HR), but rather a sharp shift of cardiovascular indices to high levels at the onset of locomotion. We hypothesized that under test conditions cardiovascular reaction to physical load in mice is masked with stress-associated HR increase. To test this hypothesis we monitored mean arterial pressure (MAP) and heart rate in C57BL/6 mice after exposure to stressful stimuli, during spontaneous locomotion in the open-field test, treadmill running or running in a wheel installed in the home cage. Mice were treated with ?1-adrenoblocker atenolol (2mg/kg ip, A), cholinolytic ipratropium bromide (2mg/kg ip, I), combination of blockers (A+I), anxiolytic diazepam (5mg/kg ip, D) or saline (control trials, SAL). MAP and HR in mice increased sharply after handling, despite 3weeks of habituation to the procedure. Under stressful conditions of open field test cardiovascular parameters in mice were elevated and did not depend on movement speed. HR values did not differ in I and SAL groups and were reduced with A or A+I. HR was lower at rest in D pretreated mice. In the treadmill test HR increase over speeds of 6, 12 and 18m/min was roughly 1/7-1/10 of HR increase observed after placing the mice on the treadmill. HR could not be increased with cholinolytic (I), but was reduced after sympatholytic (A) or A+I treatment. Anxiolytic (D) reduced heart rate at lower speeds of movement and its overall effect was to unmask the dependency of HR on running speed. During voluntary running in non-stressful conditions of the home cage HR in mice linearly increased with increasing running speeds. We conclude that in test situations cardiovascular reactions in mice are governed predominantly by stress-associated sympathetic activation, rendering efforts to evaluate HR and MAP reactions to workload unreliable. PMID:24802359

Andreev-Andrievskiy, A A; Popova, A S; Borovik, A S; Dolgov, O N; Tsvirkun, D V; Custaud, M; Vinogradova, O L

2014-06-10

348

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

349

Rate constants for the reaction of ground state atomic oxygen with methanol  

NASA Astrophysics Data System (ADS)

The reaction of O(3P) with methanol has been studied using the complementary discharge flow and flash photolysis techniques. In both cases, resonance fluorescence detection of atomic oxygen was employed. The discharge flow (DF) apparatus was used in a temperature range of 298-998 K while the flash photolysis (FP) apparatus was used in the overlapping range of 329-527 K. The apparent bimolecular rate constants for the O-atom/methanol reaction obtained from DF experiments at low temperatures (T?450 K) were independent of both the initial O-atom concentration and the mode of O-atom production. In addition, large excesses of O2 were added to the flow to intercept the primary reaction product (CH2OH), but had no apparent effect on the measured rate constant. Results from the two methods were in good agreement within this limited temperature range (˜300-500 K). At temperatures above ˜450 K, the apparent rate constants obtained from DF experiments were increasingly sensitive to the O2 concentration, with the rate constants being smaller when determined in the presence of large [O2]. Since the initial O-atom concentrations were on the order of 1011 or less, a simple stoichiometry effect can be ruled out. However, the results of the present kinetic experiments indicated that heterogeneous pyrolysis of CH3OH may have occurred in the flow system. This observation is consistent with studies of the adsorption of methanol on silica surfaces. This problem was apparently overcome by adding small amounts of O2 and the rate constants obtained in this way were seen to agree well with values extrapolated from the lower temperature DF and FP experiments. The rate data from DF and FP experiments were thus combined to obtain the following Arrhenius expression (298-998 K): k1 (T) = (2.70±0.50)×10-11 exp(-5030±160/RT cm3 molecule-1 s-1. This result is compared with those of previous studies, and kinetic complications (stoichiometry) and heterogeneous effects are further discussed.

Keil, David G.; Tanzawa, Tsuneo; Skolni, Edward G.; Klemm, R. Bruce; Michael, Joe V.

1981-09-01

350

Enhancement of Resonant Thermonuclear Reaction Rates in Extremely Dense Stellar Plasmas  

E-print Network

The enhancement factor of the resonant thermonuclear reaction rates is calculated for the extremely dense stellar plasmas in the liquid phase. In order to calculate the enhancement factor we use the screening potential which is deduced from the numerical experiment of the classical one-component plasma. It is found that the enhancement is tremendous for white dwarf densities if the ^{12}C + ^{12}C fusion cross sections show resonant behavior in the astrophysical energy range. We summarize our numerical results by accurate analytic fitting formulae.

Naoki Itoh; Nami Tomizawa; Shinya Wanajo; Satoshi Nozawa

2002-08-03

351

Kinetics and Mechanism of the Reaction of Hydoxyl Radicals with Acetonitrile under Atmospheric Conditions  

NASA Technical Reports Server (NTRS)

The pulsed laser photolysis-pulsed laser induced fluorescence technique has been employed to determine absolute rate coefficients for the reaction OH + CH3CN (1) and its isotopic variants, OH + CD3CN (2), OD + CH3CN (3), and OD + CD3CN (4). Reactions 1 and 2 were studied as a function of pressure and temperature in N2, N2/O2, and He buffer gases. In the absence of O2 all four reactions displayed well-behaved kinetics with exponential OH decays and pseudo-first rate constants which were proportional to substrate concentration. Data obtained in N2 over the range 50-700 Torr at 298 K are consistent with k(sub 1), showing a small pressure dependence. The Arrhenius expression obtained by averaging data at all pressures in k(sub 1)(T) = (1.1(sup +0.5)/(sub -0.3)) x 10(exp -12) exp[(-1130 +/- 90)/T] cu cm /(molecule s). The kinetics of reaction 2 are found to be pressure dependent with k(sub 2) (298 K) increasing from (1.21 +/- 0.12) x 10(exp -14) to (2.16 +/- 0.11) x 10(exp -14) cm(exp 3)/ (molecule s) over the pressure range 50-700 Torr of N2 at 298 K. Data at pressures greater than 600 Torr give k(sub 2)(T) = (9.4((sup +13.4)(sub -5.0))) x 10(exp -13) exp[(-1180 +/- 250)/T] cu cm/(molecule s). The rates of reactions 3 and 4 are found to be independent of pressure over the range 50-700 Torr of N2 with 298 K rate coefficient given by k(sub 3) =(3.18 +/- 0.40) x 10(exp -14) cu cm/(molecule s) and k(sub 4) = (2.25 +/-0.28) x 10(exp -14) cu cm/(molecule s). In the presence of O2 each reaction shows complex (non-pseudo-first-order) kinetic behavior and/or an apparent decrease in the observed rate constant with increasing [O2], indicating the presence of significant OH or OD regeneration. Observation of regeneration of OH in (2) and OD in (3) is indicative of a reaction channel which proceeds via addition followed by reaction of the adduct, or one of its decomposition products, with O2. The observed OH and OD decay profiles have been modeled by using a simple mechanistic scheme to extract kinetic information about the adduct reations with O2 and branching ratios for OH regeneration. A plausible mechanism for OH regeneration in (2) involves OH addition to the nitrogen atom followed by O2 addition to the cyano carbon atom, isomeriazation and decomposition to D2CO + DOCN + OH. Our results suggest that the OH + CH3CN reaction occurs via a complex mechanism involving both bimolecular and termolecular pathways, analogous to the mechanisms for the the important atmospheric reactions of OH with CO and HNO3.

Hynes, A. J.; Wine, P. H.

1997-01-01

352

Immediate effects of Suryanamaskar on reaction time and heart rate in female volunteers.  

PubMed

Suryanamaskar (SN), a yogic technique is composed of dynamic muscular movements synchronised with deep rhythmic breathing. As it may have influence on CNS, this study planned to investigate immediate effects of SN on reaction time (RT) and heart rate (HR). 21 female volunteers attending yoga classes were recruited for study group and 19 female volunteers not participating in yoga were recruited as external-controls. HR, auditory reaction time (ART) and visual reaction time (VRT) were recorded before and after three rounds of SN in study group as well as 5 minutes of quiet sitting in both groups. Performance of SN produced immediate decrease in both VRT and ART (P < 0.001). This was significant when compared to self-control period (P < 0.001) and compared to external-control group, it decreased significantly in ART (p = 0.02). This was pronounced when delta% was compared between groups (P < 0.001). HR increased significantly following SN compared with both self-control (p = 0.025) and external-control group (p = 0.032). Faster reactivity may be due to intermediate level of arousal by conscious synchronisation of dynamic movements with breathing. Rise in HR is attributed to sympathetic arousal and muscular exertion. We suggest that SN may be used as an effective training means to improve neuro-muscular abilities. PMID:24617172

Bhavanani, Ananda Balayogi; Ramanathan, Meena; Balaji, R; Pushpa, D

2013-01-01

353

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

354

Estimating biofilm reaction kinetics using hybrid mechanistic-neural network rate function model.  

PubMed

This work describes an alternative method for estimation of reaction rate of a biofilm process without using a model equation. A first principles model of the biofilm process is integrated with artificial neural networks to derive a hybrid mechanistic-neural network rate function model (HMNNRFM), and this combined model structure is used to estimate the complex kinetics of the biofilm process as a consequence of the validation of its steady state solution. The performance of the proposed methodology is studied with the aid of the experimental data of an anaerobic fixed bed biofilm reactor. The statistical significance of the method is also analyzed by means of the coefficient of determination (R2) and model efficiency (ME). The results demonstrate the effectiveness of HMNNRFM for estimating the complex kinetics of the biofilm process involved in the treatment of industry wastewater. PMID:22055105

Kumar, B Shiva; Venkateswarlu, Ch

2012-01-01

355

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

NASA Astrophysics Data System (ADS)

The major objective of this program is to measure thermal rate coefficients and branching ratios of elementary reactions which involve atomic and diatomic reactants or products. To perform these measurements, we have constructed an ultrahigh-purity shock tube to generate temperatures between 1500 and 5500 K. A high-resolution, laser light source is being used to measure the rate of change of the concentration of species which absorb below 50,000 per cm e.g.: OH, CH, CH3. State-of-the-art laser techniques are being used to extend the technique 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.: O2 and OH.

Hessler, J. P.

1991-03-01

356

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, the authors 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 one 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. [Argonne National Laboratory, IL (United States)

1993-12-01

357

Noise-induced convergence of the low flow rate chaos in the Belousov-Zhabotinsky reaction  

NASA Astrophysics Data System (ADS)

The effect of noise on the low flow-rate chaos in the Belousov-Zhabotinsky (BZ) reaction was studied. The chaos was simulated using the three-variable model of Györgyi and Field. Gaussian white noise was imposed on the flow-rate of the reactant solutions fed into CSTR to simulate the so-called type P noise. The range of average noise amplitudes was chosen between 0.01% and 1% related to the inverse residence time. The calculated time series were analyzed on the basis of their Fourier spectra, maximum Lyapunov exponent, Kolmogorov entropies, return maps and invariant density. We found that the noise induces partial order of the period-3-like oscillations in the low flowrate chaos.

Yoshimoto, Minoru; Nakaiwa, Masaru; Akiya, Takaji; Ohmori, Takao; Yamaguchi, Tomohiko

358

Reaction Rate Measurement at the Californium User Facility (CUF) for unfolding the neutron spectrum  

NASA Astrophysics Data System (ADS)

Neutron Activation Analysis was used to determine Reaction Rate measurement of several activation detectors at the ORNL Californium User Facility (CUF). The irradiations were performed with 34 mg Cf^252 neutron source strength.. Ten source capsules > 34 mg were positioned concentrically around a sample cavity. We have determined absolute activity per atom of 9 detectors: Au^197 (n,?) Au^198, Al^27(n,?)Na^24, Al^27(n,p) Mg^27, Fe^56(n,p) Mn^5, Fe^54(n,p) Mn^54, In^115 (n,?)In^116, Ti^46(n,p)Sc^46, Ni^60 (n,p) Co^60, Fe^58(n,?) Fe^59. The errors are within 1.5-8% except Ni^60 and Fe^58 have errors of 46% and 32 %. These high errors may be attributed to the counting statistics. These reaction rate values will be used to unfold the neutron spectrum of the CUF using the MAXED 2000, a computer code for the de convolution of multi sphere neutron spectrometer data and the results are discussed.

Hannan, Mohammad; Ortega, Ruben

2011-03-01

359

The Effects of Changes in Reaction Rates on Simulations of Nova Explosions  

SciTech Connect

Classical novae participate in the cycle of Galactic chemical evolution in which grains and metal enriched gas in their ejecta, supplementing those of supernovae, AGB stars, and Wolf-Rayet stars, are a source of heavy elements for the ISM. Once in the diffuse gas, this material is mixed with the existing gases and then incorporated into young stars and planetary systems during star formation. Infrared observations have confirmed the presence of carbon, SiC, hydrocarbons, and oxygen-rich silicate grains in nova ejecta, suggesting that some fraction of the pre-solar grains identified in meteoritic material come from novae. The mean mass returned by a nova outburst to the ISM probably exceeds {approx}2 x 10{sup -4} Solar Masses. Using the observed nova rate of 35 per year in our Galaxy, it follows that novae introduce more than {approx}7 x 10{sup -3} Solar Masses per year of processed matter into the ISM. Novae are expected to be the major source of 15N and 17O in the Galaxy and to contribute to the abundances of other isotopes in this atomic mass range. Here, we report on how changes in the nuclear reaction rates affect the properties of the outburst and alter the predictions of the contributions of novae to Galactic chemical evolution. We also discuss the necessity of including the pep reaction in studies of thermonuclear runaways in material accreted onto white dwarfs.

Starrfield, Sumner [Arizona State University; Iliadis, Christian [University of North Carolina; Hix, William Raphael [ORNL; Timmes, Francis X. [Los Alamos National Laboratory (LANL); Sparks, Warren M. [Los Alamos National Laboratory (LANL)

2007-01-01

360

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

PubMed

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 propellant. The first two mass-loss stages occur in succession and the temperature ranges are near apart, and the decomposition peaks of the two stages overlap each other, inducing only one visible exothermic peak appear in DSC curve during 350-550 K. The reaction mechanisms of the main exothermal decomposition processes of RB0601 and RB0602 are all classified as chemical reaction, the mechanism functions are f(alpha)=(1-alpha)(2), and the kinetic equations are dalpha/dt = 10(19.24)(1-alpha)(2)e(-2.32x10(4)/T) and dalpha/dt = 10(20.32)(1-alpha)(2)e(-2.32x10(4)/T). The thermal safety evaluation on the BTATz-CMDB propellants was obtained. With the substitution of 26% RDX by BTATz and with the help of the ballistic modifier in the CMDB propellant formulation, the burning rate can be improved by 89.0% at 8 MPa and 47.1% at 22 MPa, the pressure exponent can be reduced to 0.353 at 14-20 MPa. PMID:20542638

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

2010-09-15

361

RATE CONSTANTS FOR THE REACTION OF OH RADICALS WITH A SERIES OF ALKENES AND DIALKENES AT 295 + OR - 1 K  

EPA Science Inventory

Using a relative rate technique, rate constants for the gas phase reactions of the OH radical with n-butane, n-hexane and a series of alkenes and dialkenes, relative to that for propene, have been determined in one atmosphere of air at 295 + or - 1K. The resulting rate constant d...

362

An Analogy Using Pennies and Dimes to Explain Chemical Kinetics Concepts  

ERIC Educational Resources Information Center

In this article, the authors present an analogy that uses coins and graphical analysis to teach kinetics concepts and resolve pseudo-first-order rate constants related to transition-metal complexes ligand-solvent exchange reactions. They describe an activity that is directed to upper-division undergraduate and graduate students. The activity…

Cortes-Figueroa, Jose E.; Perez, Wanda I.; Lopez, Jose R.; Moore-Russo, Deborah A.

2011-01-01

363

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. PMID:24149735

Puddle, Damien L.; Maulder, Peter S.

2013-01-01

364

Ground reaction forces and loading rates associated with parkour and traditional drop landing techniques.  

PubMed

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 pointsParkour 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. PMID:24149735

Puddle, Damien L; Maulder, Peter S

2013-01-01

365

Mixed quantum classical calculation of proton transfer reaction rates: From deep tunneling to over the barrier regimes  

NASA Astrophysics Data System (ADS)

We present mixed quantum classical calculations of the proton transfer (PT) reaction rates represented by a double well system coupled to a dissipative bath. The rate constants are calculated within the so called nontraditional view of the PT reaction, where the proton motion is quantized and the solvent polarization is used as the reaction coordinate. Quantization of the proton degree of freedom results in a problem of non-adiabatic dynamics. By employing the reactive flux formulation of the rate constant, the initial sampling starts from the transition state defined using the collective reaction coordinate. Dynamics of the collective reaction coordinate is treated classically as over damped diffusive motion, for which the equation of motion can be derived using the path integral, or the mixed quantum classical Liouville equation methods. The calculated mixed quantum classical rate constants agree well with the results from the numerically exact hierarchical equation of motion approach for a broad range of model parameters. Moreover, we are able to obtain contributions from each vibrational state to the total reaction rate, which helps to understand the reaction mechanism from the deep tunneling to over the barrier regimes. The numerical results are also compared with those from existing approximate theories based on calculations of the non-adiabatic transmission coefficients. It is found that the two-surface Landau-Zener formula works well in calculating the transmission coefficients in the deep tunneling regime, where the crossing point between the two lowest vibrational states dominates the total reaction rate. When multiple vibrational levels are involved, including additional crossing points on the free energy surfaces is important to obtain the correct reaction rate using the Landau-Zener formula.

Xie, Weiwei; Xu, Yang; Zhu, Lili; Shi, Qiang

2014-05-01

366

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

NASA Technical Reports Server (NTRS)

The use of probability theory to determine the effects of turbulent fluctuations on reaction rates in turbulent combustion systems is briefly reviewed. Results are presented for the effect of species fluctuations in particular. It is found that turbulent fluctuations of species act to reduce the reaction rates, in contrast with the temperature fluctuations previously determined to increase Arrhenius reaction rate constants. For the temperature fluctuations, a criterion is set forth for determining if, in a given region of a turbulent flow field, the temperature can be expected to exhibit ramp like fluctuations. Using the above results, along with results previously obtained, a model is described for testing the effects of turbulent fluctuations of temperature and species on reaction rates in computer programs dealing with turbulent reacting flows. An alternative model which employs three variable probability density functions (temperature and two species) and is currently being formulated is discussed as well.

Chinitz, W.; Foy, E.; Rowan, G.; Goldstein, D.

1982-01-01

367

Shock tube measurements of specific reaction rates in branched chain CH4-CO-O2 system  

NASA Technical Reports Server (NTRS)

Rate constants of two elementary bimolecular reactions involved in the oxidation of methane were determined by monitoring the exponential growth of CO flame band emission behind incident shocks in three suitably chosen gas mixtures.

Brabbs, T. A.; Brokaw, R. S.

1974-01-01

368

Revised Production Rates for Na-22 and Mn-54 in Meteorites Using Cross Sections Measured for Neutron-induced Reactions  

NASA Technical Reports Server (NTRS)

The interactions of galactic cosmic rays (GCR) with extraterrestrial bodies produce small amounts of radionuclides and stable isotopes. The production rates of many relatively short-lived radionuclides, including 2.6-year Na-22 and 312-day Mn-54, have been measured in several meteorites collected very soon after they fell. Theoretical models used to calculate production rates for comparison with the measured values rely on input data containing good cross section measurements for all relevant reactions. Most GCR particles are protons, but secondary neutrons make most cosmogenic nuclides. Calculated production rates using only cross sections for proton-induced reactions do not agree well with measurements. One possible explanation is that the contribution to the production rate from reactions initiated by secondary neutrons produced in primary GCR interactions should be included explicitly. This, however, is difficult to do because so few of the relevant cross sections for neutron-induced reactions have been measured.

Sisterson, J. M.; Kim, K. J.; Reedy, R. C.

2004-01-01

369

Effect of high pressure on the rate constant of the Diels-Alder reaction between furan and maleic anhydride  

Microsoft Academic Search

The effect of high pressure (up to 700 MPa) on the rate of the Diels-Alder reaction between furan and maleic anhydride has been investigated in deuterated acetone at 20‡C. The volume effect of the reaction at atmospheric pressure is found to be dV0=-28.0 cm3\\/mole. The rate constants for the formation, decomposition and isomerization of the products have been found by

V. M. Zhulin; M. V. Kel'tseva; V. S. Bogdanov; Yu. D. Koreshkov; E. B. Kabotyanskaya

1990-01-01

370

Reaction rate kinetics for the non-catalytic hydrogenation of Texas lignite with tetralin and hydrogen gas  

E-print Network

REACTION RATE KINETICS FOR HE NON-CATALYTIC HYDROGENATION OF TEXAS LIGNITE WITH TETRALIN AND HYDROGEN GAS A Thesis by DAVID ALLEN SHUMBERA Submitted to the Graduate College of Texas A&M University in partial fulfillment of the requirement... for the degree of MASTER OF SCIENCE May 1980 Major Subject: Chemical Engineering REACTION RATE KINETICS FOR THE NON-CATALYTIC HYDROGENATION OF TEXAS LIGNITE WITH TETRALIN AND HYDROGEN GAS A Thesis by DAVID ALLEN SHUMBERA Approved as to style and content...

Shumbera, David Allen

2012-06-07

371

LWR-PROTEUS Verification of Reaction Rate Distributions in Modern 10 x 10 Boiling Water Reactor Fuel  

Microsoft Academic Search

HELIOS, CASMO-4, and MCNP4B calculations of reaction rate distributions in a modern, fresh 10 x 10 boiling water reactor fuel element have been validated using the experimental results of the LWR-PROTEUS Phase I project corresponding to full-density water moderation conditions (core 1B). The reaction rate distributions measured with a special gamma-scanning machine employing twin germanium detectors consisted of total fission

F. Jatuff; P. Grimm; O. Joneja; M. Murphy; A. Luethi; R. Seiler; R. Brogli; R. Jacot-Guillarmod; T. Williams; S. Helmersson; R. Chawla

2001-01-01

372

Zero energy resonances in reactive scattering: anomalous temperature dependence of atom--molecule reaction rates  

NASA Astrophysics Data System (ADS)

We show that rate coefficients for inelastic processes---reactive, or nonreactive---in the (ultra)cold regime can be greatly affected by the presence of a resonance pole near E=0 in the entrance channel. This problem has been investigated previously [E. Bodo et al., J. Phys. B 37 (2004) 3641] but their analysis was restricted to the energy dependence of the reaction cross section for a particular case. Here, we present the general case, and we emphasize the possibility of a wide intermediate regime of temperatures where the rate coefficient has an anomalous temperature dependence; namely it increases as 1/T when T decreases. Eventually, the temperature dependence reverts back to the standard behavior given by Wigner's law, i.e., the rate coefficient becomes constant, but this may only be recovered at extremely low T (very deep into the ultracold regime). Thus, at least in some exceptional cases, most of the (ultra)cold regime could be dominated by this anomalous behavior.

Simbotin, I.; Ghosal, S.; Côté, R.

2010-03-01

373

Analysis of turbulent free jet hydrogen-air diffusion flames with finite chemical reaction rates  

NASA Technical Reports Server (NTRS)

The nonequilibrium flow field resulting from the turbulent mixing and combustion of a supersonic axisymmetric hydrogen jet in a supersonic parallel coflowing air stream is analyzed. Effective turbulent transport properties are determined using the (K-epsilon) model. The finite-rate chemistry model considers eight reactions between six chemical species, H, O, H2O, OH, O2, and H2. The governing set of nonlinear partial differential equations is solved by an implicit finite-difference procedure. Radial distributions are obtained at two downstream locations of variables such as turbulent kinetic energy, turbulent dissipation rate, turbulent scale length, and viscosity. The results show that these variables attain peak values at the axis of symmetry. Computed distributions of velocity, temperature, and mass fraction are also given. A direct analytical approach to account for the effect of species concentration fluctuations on the mean production rate of species (the phenomenon of unmixedness) is also presented. However, the use of the method does not seem justified in view of the excessive computer time required to solve the resulting system of equations.

Sislian, J. P.

1978-01-01

374

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

375

Analytical Solution of Steady State Equations for Chemical Reaction Networks with Bilinear Rate Laws  

PubMed Central

True steady states are a rare occurrence in living organisms, yet their knowledge is essential for quasi-steady state approximations, multistability analysis, and other important tools in the investigation of chemical reaction networks (CRN) used to describe molecular processes on the cellular level. Here we present an approach that can provide closed form steady-state solutions to complex systems, resulting from CRN with binary reactions and mass-action rate laws. We map the nonlinear algebraic problem of finding steady states onto a linear problem in a higher dimensional space. We show that the linearized version of the steady state equations obeys the linear conservation laws of the original CRN. We identify two classes of problems for which complete, minimally parameterized solutions may be obtained using only the machinery of linear systems and a judicious choice of the variables used as free parameters. We exemplify our method, providing explicit formulae, on CRN describing signal initiation of two important types of RTK receptor-ligand systems, VEGF and EGF-ErbB1. PMID:24334389

Halasz, Adam M.; Lai, Hong-Jian; McCabe, Meghan M.; Radhakrishnan, Krishnan; Edwards, Jeremy S.

2014-01-01

376

A mm/submm Wave Spectrometer to Quantify Astrochemical Reaction Rates  

NASA Astrophysics Data System (ADS)

Complex organic molecules (COMs) are being routinely detected at millimeter and submillimeter wavelengths toward a variety of interstellar environments. There is a growing consensus that their formation is dominated by barrierless, diffusion-limited addition reactions of radicals on icy grain surfaces. While astrochemical models have predicted the presence of many of these COMs, discrepancies have arisen between their observed and predicted relative abundances. It is likely that these discrepancies arise from uncertainties in the rates for the dissociation reactions that form the precursor radicals. More complete laboratory information is needed to improve the predictive power of astrochemical models of organic chemistry. We have developed a laboratory experiment that utilizes mm/submm wave direct absorption spectroscopy to probe photodissociation branching ratios that are relevant to astrochemical models of complex organic chemistry. We have benchmarked the performance of this spectrometer by examining the dissociation of methanol using a high-voltage discharge source. We will report on these results, and the progress of utilizing this spectrometer for photodissociation studies of COMs and COM precursors.

Laas, Jacob C.; Weaver, Susanna L. Widicus

2013-06-01

377

Rate constant and branching ratio for the reaction of OH with ClO  

SciTech Connect

The rate constant and branching ratio for the reaction OH + ClO ..-->.. products (HO/sub 2/ + Cl (1a) and HCl + O/sub 2/ (1b)) have been measured in a discharge flow system at 298 K. OH was monitored via the laser-induced fluorescence technique. ClO and HCl were analyzed by means of a modulated molecular beam mass spectrometer. The results are k/sub 1/ = (1.94 +/- 0.38) x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/ and k/sub 1a//k/sub 1/ = 0.98 +/- 0.12. This implies that channel 1b cannot be a significant process for converting ClO into inactive HCl in stratospheric ozone-chlorine chemistry. 17 references, 6 figures, 4 tables.

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

1986-01-02

378

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

2013-04-01

379

Relative rate study of the kinetic isotope effect in the 13CH3D + Cl reaction  

NASA Astrophysics Data System (ADS)

The 13CH3D/12CH4 kinetic isotope effect, ?13CH3D, of CH4 + Cl is determined for the first time, using the relative rate technique and Fourier transform infrared (FTIR) spectroscopy. ?13CH3D is found to be 1.60 ± 0.04. In addition, a quantum chemistry/transition state theory model with tunneling correction is constructed and the primary cause for ?13CH3D is found to be the substantially reduced reactivity of the D atom, which, in turn, can be explained by a significant increase in the reaction barrier due to changes in the vibrational zero point energy and to a lesser extent tunneling.

Joelsson, L. M. T.; Forecast, R.; Schmidt, J. A.; Meusinger, C.; Nilsson, E. J. K.; Ono, S.; Johnson, M. S.

2014-06-01

380

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

381

Reaction rates for 8Li(p, ?) and 8Li(p, n, ?) and their effect on primordial nucleosynthesis  

NASA Astrophysics Data System (ADS)

Differential-cross-section data are presented for the 8Li(p, ?)5He reaction at 1.5 MeV center-of-mass energy, measured with a (radioactive) 8Li beam. The data are used to calculate one of the terms of the thermonuclear reaction rate for destruction of 8Li. In addition, other data are used to estimate the total reaction rate for p+8Li, which is found to be comparable to that used in previous calculations of abundances from primordial nucleosynthesis.

Becchetti, F. D.; Brown, J. A.; Liu, W. Z.; Jänecke, J. W.; Roberts, D. A.; Kolata, J. J.; Smith, R. J.; Lamkin, K.; Morsad, A.; Warner, R. E.; Boyd, R. N.; Kalen, J. D.

1992-12-01

382

Rate and mechanism of the reactions of OH and Cl with 2-methyl-3-buten-2-ol  

Microsoft Academic Search

An environmental chamber\\/Fourier transform infrared system was used to determine the rate coefficient k1 for the gas-phase reaction of OH with 2-methyl-3-buten-2-ol (MBO, (CH3)2C(OH)CH=CH2), relative to the rate of its reaction with ethylene (k2) and propylene (k3). Experiments performed at 295+\\/-1K, in 700 torr total pressure of air, gave k1=(6.9+\\/-1.0)×10-11cm3molecule-1s-1. At 295+\\/-1K, the reaction of OH with MBO yielded, on

C. Ferronato; J. J. Orlando; G. S. Tyndall

1998-01-01

383

Absolute rate constant for the reaction of atomic hydrogen with acetylene over an extended pressure and temperature range  

Microsoft Academic Search

The technique of flash photolysis coupled with time resolved detection of H via resonance fluorescence has been used to obtain absolute rate parameters for the reaction of atomic hydrogen with acetylene, i.e., H+C2H2?C2H3* (1); C2H3*+M?C2H3+M (2). The rate constant for the reaction is strongly pressure dependent and was measured over the pressure range 10 to 700 torr. The reaction was

W. A. Payne; L. J. Stief

1976-01-01

384

Comparison of Nernst-Planck and reaction rate models for multiply occupied channels.  

PubMed Central

The Nernst-Planck continuum equation for a channel that can be occupied by at most two ions is solved for two different physical cases. The first case is for the assumption that the water and ion cannot get around each other anywhere in the channel, so that if there are two ions in the channel the distance between them is fixed by the number of water molecules between them. The second case is for the assumption that there are regions at he ends of the channel where the ions and water can get around each other. For these two cases, the validity of the simple two-site reaction-rate approximation when there is a continuously varying central energy barrier was evaluated by comparing it with the exact Nernst-Planck solution. For the first continuum case, the kinetics for the continuum and reaction-rate models are nearly identical. For the second case, the agreement depends on the strength of the ion-ion interaction energy. For a low interaction energy (large channel diameter) a high ion concentrations, there is a large difference in the flux as a function of voltage for the two models-with the continuum flux becoming more than four times larger at 250 mV. Simple analytical expressions are derived for the two-ion continuum channel for the case where the ends are in equilibrium with the bulk solution and for the case where ion mobility becomes zero when there are two ions in the channel. The implications of these results for biological channels are discussed. PMID:6280783

Levitt, D G

1982-01-01

385

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

386

Oxygen isotopic ratios in first dredge-up red giant stars and nuclear reaction rate uncertainties revisited  

E-print Network

We describe a general yet simple method to analyse the propagation of nuclear reaction rate uncertainties in a stellar nucleosynthesis and mixing context. The method combines post-processing nucleosynthesis and mixing calculations with a Monte Carlo scheme. With this approach we reanalyze the dependence of theoretical oxygen isotopic ratio predictions in first dredge-up red giant branch stars in a systematic way. Such predictions are important to the interpretation of pre-solar Al_2 O_3 grains from meteorites. The reaction rates with uncertainties were taken from the NACRE compilation (Angulo etal., 1999). We include seven reaction rates in our systematic analysis of stellar models with initial masses from 1 to 3 M_sun. We find that the uncertainty of reaction rate for reaction O18(p,alpha)N15 typically causes an error in the theoretical O16/O18 ratio of about +20/-5 per cent. The error of the O16/O17 prediction is +-10 to 40 per cent depending on the stellar mass, and is persistently dominated by the comparatively small uncertainty of the O16(p,gamma)F17 reaction. With the new estimates on reaction rate uncertainties by the NACRE compilation, the p-capture reactions O17(p,alpha)N14 and O17(p,gamma)F18 have virtually no impact on theoretical predictions for stellar mass 1.5 M_sun, where core mixing and subsequent envelope mixing interact. In these cases where core mixing complicates post-dredge-up surface abundances, uncertainty in other reactions have a secondary but noticeable effect on surface abundances.

J. A. Stoesz; F. Herwig

2002-12-05

387

Evaporation Rate Study and NDMA Formation from UDMH/NO2 Reaction Products  

NASA Technical Reports Server (NTRS)

Laboratory samples of uns-dimethylhydrazine (UDMH) fuel/oxidizer (nitrogen dioxide) non-combustion reaction products (UFORP) were prepared using a unique permeation tube technology. Also, a synthetic UFORP was prepared from UDMH, N-nitrosodimethylamine (NDMA), dimethylammonium nitrate, sodium nitrite and purified water. The evaporation rate of UFORP and synthetic UFORP was determined under space vacuum (approx 10(exp -3) Torr) at -40 ?C and 0 ?C. The material remaining was analyzed and showed that the UFORP weight and NDMA concentration decreased over time; however, NDMA had not completely evaporated. Over 85% of the weight was removed by subjecting the UFORP to 10(-3) Torr for 7 hours at -40 ?C and 4 hours at 0 ?C. A mixture of dimethylammonium nitrate and sodium nitrite formed NDMA at a rapid rate in a moist air environment. A sample of UFORP residue was analyzed for formation of NDMA under various conditions. It was found that NDMA was not formed unless nitrite was added.

Buchanan, Vanessa D.; Dee, Louis A.; Baker, David L.

2003-01-01

388

The Effects of Changes in Reaction Rates on Simulations of Nova Explosions  

SciTech Connect

Classical novae participate in the cycle of Galactic chemical evolution in which grains and metal enriched gas in their ejecta, supplementing those of supernovae, AGB stars, and Wolf-Rayet stars, are a source of heavy elements for the ISM. Once in the diffuse gas, this material is mixed with the existing gases and then incorporated into young stars and planetary systems during star formation. Infrared observations have confirmed the presence of carbon, SiC, hydrocarbons, and oxygen-rich silicate grains in nova ejecta, suggesting that some fraction of the pre-solar grains identified in meteoritic material come from novae. The mean mass returned by a nova outburst to the ISM probably exceeds {approx} 2 x 10-4 M{center_dot}. Using the observed nova rate of 35{+-}11 per year in our Galaxy, it follows that novae introduce more than {approx} 7 x 10-3 M{center_dot} yr-1 of processed matter into the ISM. Novae are expected to be the major source of 15N and 17O in the Galaxy and to contribute to the abundances of other isotopes in this atomic mass range. Here, we report on how changes in the nuclear reaction rates affect the properties of the outburst and alter the predictions of the contributions of novae to Galactic chemical evolution.

Starrfield, S. [School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404 (United States); Iliadis, C. [Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC27599-3255 (United States); Hix, W. R. [Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6354 (United States); Timmes, F. X. [X-2, Los Alamos National Laboratory, Los Alamos, NM, 87545 (United States); Sparks, W. M. [Science Applications International Corporation, San Diego CA, 92121 (United States); X-4, Los Alamos National Laboratory, Los Alamos, NM, 87545 (United States)

2007-02-26

389

Analysis of turbulent free-jet hydrogen-air diffusion flames with finite chemical reaction rates  

NASA Technical Reports Server (NTRS)

A numerical analysis is presented of the nonequilibrium flow field resulting from the turbulent mixing and combustion of an axisymmetric hydrogen jet in a supersonic parallel ambient air stream. The effective turbulent transport properties are determined by means of a two-equation model of turbulence. The finite-rate chemistry model considers eight elementary reactions among six chemical species: H, O, H2O, OH, O2 and H2. The governing set of nonlinear partial differential equations was solved by using an implicit finite-difference procedure. Radial distributions were obtained at two downstream locations for some important variables affecting the flow development, such as the turbulent kinetic energy and its dissipation rate. The results show that these variables attain their peak values on the axis of symmetry. The computed distribution of velocity, temperature, and mass fractions of the chemical species gives a complete description of the flow field. The numerical predictions were compared with two sets of experimental data. Good qualitative agreement was obtained.

Sislian, J. P.; Glass, I. I.; Evans, J. S.

1979-01-01

390

Assembling gas-phase reaction mechanisms for high temperature inorganic systems based on quantum chemistry calculations and reaction rate theories  

NASA Astrophysics Data System (ADS)

Detailed chemical kinetic modeling based on computational quantum chemistry has been quite successful in making quantitative predictions about some systems, particularly the combustion of small hydrocarbons and certain areas of atmospheric chemistry. The gas phase chemistry of many processes in high-temperature inorganic systems, from materials synthesis to propulsion to waste incineration, could in principle be modeled with equal or greater success using detailed chemical kinetic modeling. This contribution provides examples from our own work of how computational quantum chemistry can be used in developing gas phase reaction mechanisms for modeling of high temperature materials processing. In the context of CVD of silicon from dichlorosilane, CVD of alumina from AlCl3/H2/CO2 mixtures, and particle nucleation from silane, this detailed chemical kinetic modeling has given us insight into gas phase reaction pathways that we would not likely have gained by other means.

Swihart, Mark T.

2005-02-01

391

Rate Constants for the Reactions of ATP-and ADP-Actin with the Ends of Actin Filaments  

E-print Network

and dissociation of ATP-actin are constant. This shows that the nucleotide composition at or near the end nine different rate constants: the association and dissociation rate constants for ATP-actin and ADPRate Constants for the Reactions of ATP- and ADP-Actin with the Ends of Actin Filaments Thomas D

392

Influence of changing particle structure on the rate of gas-solid gasification reactions. Final report, July 1981-March 1984  

SciTech Connect

The objetive of this work is to determine the changes in the particle structure of coal as it undergoes the carbon/carbon dioxide reaction (C + CO/sub 2/ ..-->.. 2CO). Char was produced by heating the coal at a rate of 25/sup 0/C/min to the reaction temperatures of 800/sup 0/C, 900/sup 0/C, 1000/sup 0/C and 1100/sup 0/C. The changes in surface area and effective diffusivity as a result of devolitization were determined. Changes in effective diffusivity and surface area as a function of conversion have been measured for reactions conducted at 800, 900, 1000 and 1100/sup 0/C for Wyodak coal char. The surface areas exhibit a maximum as a function of conversion in all cases. For the reaction at 1000/sup 0/C the maximum in surface area is greater than the maxima determined at all other reaction temperatures. Thermogravimetric rate data were obtained for five coal chars; Wyodak, Wilcox, Cimmeron, Illinois number 6 and Pittsburgh number 6 over the temperature range 800-1100/sup 0/C. All coal chars exhibit a maximum in reaction rate. Five different models for gas-solid reactions were evaluated. The Bhatia/Perlmutter model seems to best represent the data. 129 references, 67 figures, 37 tables.

Not Available

1984-04-04

393

Electrostatic control of the rate-determining step of the copper, zinc superoxide dismutase catalytic reaction  

SciTech Connect

The dependence of the activity of bovine Cu,Zn superoxide dismutase on pH and ionic strength was extensively investigated in the ranges of pH 7.4-pH 12.3 and of ionic strength of 0.02-0.25 M. The results obtained indicate that two positively charged groups having pK values of approximately 10.1 and 10.8 are involved in the control of the activity. On the basis of previous work on the three-dimensional structure and on the chemically modified enzyme, these groups are likely to be lysine side chains, in particular Lys-120 and Lys-134. The oxidation state of the enzyme-bound copper ion at the steady state was found to be the same at either pH 7.4 or pH 11.5. The diffusion of superoxide ion into the active site, which is controlled by the positive charges around the active site itself, appears to be the rate-determining step of the dismutation reaction. NMR measurements of the relaxation rates of F/sup -/ showed that this control also applies to the access of F/sup -/ to the active site. Comparison of the nuclear relaxation rates of F/sup -/ with the enzyme activity indicates that F/sup -/ relaxation is controlled by the deprotonation of the group with pK approx. 10.8, which appears to be responsible for about 50% of the total activity measured at neutral pH.

Argese, E.; Viglino, P.; Rotilio, G.; Scarpa, M.; Rigo, A.

1987-06-02

394

Collision limited reaction rates for arbitrarily shaped particles across the entire diffusive Knudsen number range  

NASA Astrophysics Data System (ADS)

Aerosol particle reactions with vapor molecules and molecular clusters are often collision rate limited, hence determination of particle-vapor molecule and particle-molecular cluster collision rates are of fundamental importance. These collisions typically occur in the mass transfer transition regime, wherein the collision kernel (collision rate coefficient) is dependent upon the diffusive Knudsen number, KnD. While this alone prohibits analytical determination of the collision kernel, aerosol particle- vapor molecule collisions are further complicated when particles are non-spherical, as is often the case for particles formed in high temperature processes (combustion). Recently, through a combination of mean first passage time simulations and dimensional analysis, it was shown that the collision kernel for spherical particles and vapor molecules could be expressed as a dimensionless number, H, which is solely a function of KnD. In this work, it is shown through similar mean first passage times and redefinitions of H and KnD that the H(KnD) relationship found for spherical particles applies for particles of arbitrary shape, including commonly encountered agglomerate particles. Specifically, it is shown that to appropriately define H and KnD, two geometric descriptors for a particle are necessary: its Smoluchowski radius, which defines the collision kernel in the continuum regime (KnD?0) and its orientationally averaged projected area, which defines the collision kernel in the free molecular regime (KnD??). With these two parameters, as well as the properties of the colliding vapor molecule (mass and diffusion coefficient), the particle-vapor molecule collision kernel in the continuum, transition, and free molecular regimes can be simply calculated using the H(KnD) relationship.

Gopalakrishnan, Ranganathan; Thajudeen, Thaseem; Hogan, Christopher J.

2011-08-01

395

Variational RRKM theory calculation of thermal rate constant for carbon—hydrogen bond fission reaction of nitro benzene  

NASA Astrophysics Data System (ADS)

The present work provides quantitative results for the rate of unimolecular carbon-hydrogen bond fission reaction of benzene and nitro benzene at elevated temperatures up to 2000 K. The potential energy surface for each C-H (in the ortho, meta, and para sites) bond fission reaction of nitro benzene was investigated by ab initio calculations. The geometry and vibrational frequencies of the species involved in this process were optimized at the MP2 level of theory, using the cc-pvdz basis set. Since C-H bond fission channel is barrier less reaction, we have used variational RRKM theory to predict rate constants. By means of calculated rate constant at the different temperatures, the activation energy and exponential factor were determined. The Arrhenius expression for C-H bond fission reaction of nitro benzene on the ortho, meta and para sites are k( T) = 2.1 × 1017exp(-56575.98/ T), k( T) = 2.1 × 1017exp(-57587.45/ T), and k( T) = 3.3 × 1016exp(-57594.79/ T) respectively. The Arrhenius expression for C-H bond fission reaction of benzene is k( T) = 2 × 1018exp(-59343.48.18/ T). The effect of NO2 group, location of hydrogen atoms on the substituted benzene ring, reaction degeneracy, benzene ring resonance and tunneling effect on the rate expression have been discussed.

Manesh, Afshin Taghva; Heidarnezhad, Zabi alah; Masnabadi, Nasrin

2013-07-01

396

Quantum mechanical reaction rate constants by vibrational configuration interaction: The OH + H2->H2O + H reaction as a function of temperature  

PubMed Central

The thermal rate constant of the 3D OH + H2?H2O + H reaction was computed by using the flux autocorrelation function, with a time-independent square-integrable basis set. Two modes that actively participate in bond making and bond breaking were treated by using 2D distributed Gaussian functions, and the remaining (nonreactive) modes were treated by using harmonic oscillator functions. The finite-basis eigenvalues and eigenvectors of the Hamiltonian were obtained by solving the resulting generalized eigenvalue equation, and the flux autocorrelation function for a dividing surface optimized in reduced-dimensionality calculations was represented in the basis formed by the eigenvectors of the Hamiltonian. The rate constant was obtained by integrating the flux autocorrelation function. The choice of the final time to which the integration is carried was determined by a plateau criterion. The potential energy surface was from Wu, Schatz, Lendvay, Fang, and Harding (WSLFH). We also studied the collinear H + H2 reaction by using the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential energy surface. The calculated thermal rate constant results were compared with reported values on the same surfaces. The success of these calculations demonstrates that time-independent vibrational configuration interaction can be a very convenient way to calculate converged quantum mechanical rate constants, and it opens the possibility of calculating converged rate constants for much larger reactions than have been treated until now. PMID:15774583

Chakraborty, Arindam; Truhlar, Donald G.

2005-01-01

397

Ambiguities in the rate of oxygen formation during stellar helium burning in the 12C(?,?) reaction  

NASA Astrophysics Data System (ADS)

The rate of oxygen formation determines the C/O ratio during stellar helium burning. It is the single most important nuclear input in stellar evolution theory, including the evolution of type II and type Ia supernova. However, the low-energy cross section of the fusion of 4He +12C, denoted as the 12C(?,?)16O reaction, still remains uncertain. I analyze and critically review the most recent measurements of complete angular distributions of the outgoing ? rays at very low energies (Ec.m.?1.0 MeV). My analysis of the angular distributions measured with the EUROGAM/GANDI arrays leads to considerably larger error bars than have been published, which excludes them from the current sample of “world data.” I show that the current sample of “world data” of the measured E2 cross-section factors below 1.7 MeV cluster into two distinct groups that lead to two distinct extrapolations: SE2(300)?60 or SE2(300)?154 keVb. There is a discrepancy between the measured E1-E2 phase difference (?12) and unitarity as required by the Watson theorem, which suggests systematic problem(s) in some of the measured ?-ray angular distributions. The ambiguity of the extrapolated SE2(300) together with the previously observed ambiguity of SE1(300) (approximately 80 or 10 keVb) must be resolved by future measurements of complete and detailed angular distributions of the 12C(?,?) reaction at very low energies (Ec.m.?1.0 MeV).

Gai, Moshe

2013-12-01

398

Correlation Between in-situ Redox Reaction Rates and Microbial Biomass Distribution in Porous Media Influenced by Different Transport Regimes  

NASA Astrophysics Data System (ADS)

Microbially mediated redox transformations of organic carbon play an important role for the fate of reactive species in porous media. The terminal electron acceptors (TEAs) involved in such reactions depend on the amount and degradability of the organic carbon species and lead to a succession of redox reactions where the TEAs are used-up in a temporal or, in case transport is considered, spatial sequence of decreasing energy yields. A direct characterization of redox stratified systems is challenged by our ability to measure reaction rates in-situ. One novel approach consists in quantifying and characterizing microorganisms in aquifers and sediments and to use such results to predict in-situ redox reaction rates. However, the existence of a spatial correlation between microbial abundance and associated in-situ redox reaction rates should be questioned. Here, we investigate this correlation for porous media having different transport regimes. In the environment, these regimes vary between systems such as aquifers, where advective transport in the water phase is the dominant transport mechanism, and aquatic sediments, where close to the sediment water interface the mixing activity of benthic macrofauna contributes significantly to transport. Results from estuarine sediments show that for such systems, the spatial distributions of redox reaction rates and the associated microorganisms are not correlated. This observation is supported by reactive transport simulations, which show that the ratio of the time scale of the mixing processes to the time scale of microbial growth is controlling the spatial correlation between redox reaction rates and the distribution of microorganisms. For sediments highly affected by mixing, the correlation is missing or weak, while in advection controlled systems such as aquifers, a good correlation between redox rates and microbial biomass distribution can be expected.

Thullner, M.; Pallud, C.; van Cappellen, P.; Regnier, P.

2004-12-01

399

Investigations of solvent properties and solvent effects on chemical equilibria and reaction rates  

NASA Astrophysics Data System (ADS)

Thermodynamic and structural properties computed via simulations of pure liquids and dilute solutions are routinely used as a metric of accuracy for condensed-phase force fields and in the development and testing of new methodology. Additionally, reliable modeling of solvent systems is critical to investigations of physical phenomena, such as the elucidation of solvent effects on chemical equilibria and reaction rates. This dissertation highlights a series of studies that span these topics. The Lennard-Jones 12-6 functional form, often invoked to model van der Waals interactions, has been argued to be too repulsive at short internuclear separations. Monte Carlo simulations of organic liquids at various temperatures and pressures show that this function, in conjunction with OPLS parameters, is capable of reproducing experimental densities. In order to address potential cumulative deviations of computed properties and conformational differences between the gas phase and pure liquids, Monte Carlo simulations have been carried out for the homologous n-alkane series C4H10-C12H26 using the OPLS-AA force field. Favorable structural motifs of longer alkanes were also investigated to study self solvation in the gas phase. Next, an overview of the computation of free energy changes in solution using perturbation theory, overlap sampling, and related approximate methods is presented. Results are provided for free energies of hydration of OPLS-AA substituted benzenes in TIP4P water. For comparable amounts of computer time, the double-wide and overlap sampling methods yield very similar results. QM/MM simulations of the Diels-Alder reactions of cyclopentadiene with 1,4-naphthoquinone, methyl vinyl ketone, and acrylonitrile have been carried out at the water-vacuum interface and in the gas phase. The relative free energies of activation and transition structure geometries at the interface were intermediate between those calculated in the gas phase and in bulk water, consistent with estimated experimental rate constants. Energy pair distributions reveal a loss of slightly favorable solute-solvent pair contacts but retention of stronger interactions upon transition from bulk to surface hydration. These strong interactions cause the methyl vinyl ketone transition structure to preferentially orient its carbonyl toward the surface, while the other transition structures prefer orientations parallel to the surface.

Defeo, Laura Lynn Thomas

400

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

NASA Astrophysics Data System (ADS)

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

Hedgecock, I. M.; Pirrone, N.

2003-04-01

401

Pressure effect on the rate and equilibrium constants of the Diels-Alder reaction 9-chloroanthracene with tetracyanoethylene  

Microsoft Academic Search

The pressure effect on the forward and backward rate constants and equilibrium constants of the Diels-Alder reaction of 9-chloroanthracene with tetracyanoethylene has been investigated in 1,2-dichloroethane up to 1000 kgcm?2 at 298.15 K. The reaction volume obtained from the pressure effect on equilibrium (?20.6 ±1.5 cm3mol?1) and as difference of activation volumes of forward (?28.5±1.5) and backward (?6.5 ± 0.5)

Vladimir D. Kiselev; Elena A. Kashaeva; Alexander I. Konovalov

1999-01-01

402

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

SciTech Connect

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

Allen, J.M.; Allen, S.K. [Indiana State Univ., Terre Haute, IN (United States)

1995-12-31

403

Dynamics calculations for the Cl+C2H6 abstraction reaction: Thermal rate constants and kinetic isotope effects  

Microsoft Academic Search

Thermal rate constants and H\\/D kinetic isotope effects for the Cl+C2H6-->HCl+C2H5 reaction were computed by microcanonical variational transition state theory on a high-level ab initio potential energy surface (PES). The calculations show that the reaction proceeds through a ``loose'' transition state, and so the consideration of variational effects is important. The PES presents a van der Waals minimum in the

A. Fernández-Ramos; E. Martínez-Núñez; J. M. C. Marques; S. A. Vázquez

2003-01-01

404

Dynamics calculations for the Cl+C2H6 abstraction reaction: Thermal rate constants and kinetic isotope effects  

Microsoft Academic Search

Thermal rate constants and H\\/D kinetic isotope effects for the Cl+C2H6?HCl+C2H5 reaction were computed by microcanonical variational transition state theory on a high-level ab initio potential energy surface (PES). The calculations show that the reaction proceeds through a “loose” transition state, and so the consideration of variational effects is important. The PES presents a van der Waals minimum in the

A. Ferna´ndez-Ramos; J. M. C. Marques; S. A. Va´zquez

2003-01-01

405

The kinetic ion mobility mass spectrometer: Measurements of ion-molecule reaction rate constants at atmospheric pressure  

SciTech Connect

This paper discusses how the kinetic ion mobility mass spectrometer is used to measure the rate constants of ion-molecule reactions in an atmospheric pressure buffer gas. One operational mode of the instrument avoids measurement errors associated with aperture sampling of a high-pressure ionized gas. This instrument is used to study the S{sub N}2 nucleophilic displacement reactions of chloride anions with a series of alkyl bromides in nitrogen. 40 refs., 13 figs., 2 tabs.

Giles, K.; Grimsrud, E.P. [Montana State Univ., Bozeman, MT (United States)

1992-08-06

406

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

NASA Astrophysics Data System (ADS)

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

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

1986-04-01

407

NMR-based screening method for transglutaminases: rapid analysis of their substrate specificities and reaction rates.  

PubMed

Incorporation of inter- or intramolecular covalent cross-links into food proteins with microbial transglutaminase (MTG) improves the physical and textural properties of many food proteins such as tofu, boiled fish paste, and sausage. Other transglutaminases (TGases) are expected to be used in the same way, and also to extend the scope of industrial applications to materials, drugs, and so on. The TGases have great diversity, not only in amino acid sequence and size, but also in their substrate specificities and catalytic activities, and therefore, it is quite difficult to estimate their reactivity. We have developed an NMR-based method using the enzymatic labeling technique (ELT) for simultaneous analysis of the substrate specificities and reaction rates of TGases. It is quite useful for comparing the existing TGases and for screening new TGases or TGases variants. This method has shown that MTG is superior for industrial use because of its lower substrate specificity compared with those of guinea pig liver transglutaminase (GTG) and red sea bream liver transglutaminase (FTG). We have also found that an MTG variant lacking an N-terminal aspartic acid residue has higher activity than that of the native enzyme. PMID:11878998

Shimba, Nobuhisa; Yokoyama, Kei-ichi; Suzuki, Ei-ichiro

2002-03-13

408

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

PubMed Central

Reaction and activation energy barriers are calculated for the H abstraction reactions (C6H5SH + X• ? C6H5S + XH, X = H, OH and HO2) at the BB1K/GTLarge level of theory. The corresponding reactions with H2S and CH3SH are also investigated using the G3B3 and CBS-QB3 methods in order to demonstrate the accuracy of BB1K functional in finding activation barriers for hydrogen atom transfer reactions. Arrhenius parameters for the title reactions are fitted in the temperature range of 300 K–2000 K. The calculated reaction enthalpies are in good agreement with their corresponding experimental reaction enthalpies. It is found that H abstraction by OH radicals from the thiophenol molecule proceed in a much slower rate in reference to the analogous phenol molecule. ?fH298o of thiophenoxy radical is calculated to be 63.3 kcal/mol. Kinetic parameters presented herein should be useful in describing the decomposition rate of thiophenol; i.e., one of the major aromatic sulfur carriers, at high temperatures. PMID:22485200

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

2011-01-01

409

Phosphorus-31 NMR magnetization transfer measurements of metabolic reaction rates in the rat heart and kidney in vivo  

SciTech Connect

/sup 31/P NMR is a unique tool to study bioenergetics in living cells. The application of magnetization transfer techniques to the measurement of steady-state enzyme reaction rates provides a new approach to understanding the regulation of high energy phosphate metabolism. This dissertation is concerned with the measurement of the rates of ATP synthesis in the rat kidney and of the creatine kinase catalyzed reaction in the rat heart in situ. The theoretical considerations of applying magnetization transfer techniques to intact organs are discussed with emphasis on the problems associated with multiple exchange reactions and compartmentation of reactants. Experimental measurements of the ATP synthesis rate were compared to whole kidney oxygen consumption and Na/sup +/ reabsorption rates to derive ATP/O values. The problems associated with ATP synthesis rate measurements in kidney, e.g. the heterogeneity of the inorganic phosphate resonance, are discussed and experiments to overcome these problems proposed. In heart, the forward rate through creatine kinase was measured to be larger than the reverse rate. To account for the difference in forward and reverse rates a model is proposed based on the compartmentation of a small pool of ATP.

Koretsky, A.P.

1984-01-01

410

EFFECTS OF PORE STRUCTURE CHANGE AND MULTI-SCALE HETEROGENEITY ON CONTAMINANT TRANSPORT AND REACTION RATE UPSCALING  

SciTech Connect

This project addressed the scaling of geochemical reactions to core and field scales, and the interrelationship between reaction rates and flow in porous media. We targeted reactive transport problems relevant to the Hanford site specifically the reaction of highly caustic, radioactive waste solutions with subsurface sediments, and the immobilization of 90Sr and 129I through mineral incorporation and passive flow blockage, respectively. We addressed the correlation of results for pore-scale fluid-soil interaction with field-scale fluid flow, with the specific goals of (i) predicting attenuation of radionuclide concentration; (ii) estimating changes in flow rates through changes of soil permeabilities; and (iii) estimating effective reaction rates. In supplemental work, we also simulated reactive transport systems relevant to geologic carbon sequestration. As a whole, this research generated a better understanding of reactive transport in porous media, and resulted in more accurate methods for reaction rate upscaling and improved prediction of permeability evolution. These scientific advancements will ultimately lead to better tools for management and remediation of DOE legacy waste problems.

Peters, Catherine A [Princeton University] [Princeton University

2013-05-15

411

An effective rate equation approach to reaction kinetics in small volumes: theory and application to biochemical reactions in nonequilibrium steady-state conditions.  

PubMed

Chemical master equations provide a mathematical description of stochastic reaction kinetics in well-mixed conditions. They are a valid description over length scales that are larger than the reactive mean free path and thus describe kinetics in compartments of mesoscopic and macroscopic dimensions. The trajectories of the stochastic chemical processes described by the master equation can be ensemble-averaged to obtain the average number density of chemical species, i.e., the true concentration, at any spatial scale of interest. For macroscopic volumes, the true concentration is very well approximated by the solution of the corresponding deterministic and macroscopic rate equations, i.e., the macroscopic concentration. However, this equivalence breaks down for mesoscopic volumes. These deviations are particularly significant for open systems and cannot be calculated via the Fokker-Planck or linear-noise approximations of the master equation. We utilize the system-size expansion including terms of the order of Omega(-1/2) to derive a set of differential equations whose solution approximates the true concentration as given by the master equation. These equations are valid in any open or closed chemical reaction network and at both the mesoscopic and macroscopic scales. In the limit of large volumes, the effective mesoscopic rate equations become precisely equal to the conventional macroscopic rate equations. We compare the three formalisms of effective mesoscopic rate equations, conventional rate equations, and chemical master equations by applying them to several biochemical reaction systems (homodimeric and heterodimeric protein-protein interactions, series of sequential enzyme reactions, and positive feedback loops) in nonequilibrium steady-state conditions. In all cases, we find that the effective mesoscopic rate equations can predict very well the true concentration of a chemical species. This provides a useful method by which one can quickly determine the regions of parameter space in which there are maximum differences between the solutions of the master equation and the corresponding rate equations. We show that these differences depend sensitively on the Fano factors and on the inherent structure and topology of the chemical network. The theory of effective mesoscopic rate equations generalizes the conventional rate equations of physical chemistry to describe kinetics in systems of mesoscopic size such as biological cells. PMID:20649359

Grima, R

2010-07-21

412

Determination of the astrophysical 12N(p,?)13O reaction rate from the 2H(12N,13O)n reaction and its astrophysical implications  

NASA Astrophysics Data System (ADS)

The evolution of massive stars with very low-metallicities depends critically on the amount of CNO nuclides which they produce. The 12N(p,?)13O reaction is an important branching point in the rap processes, which are believed to be alternative paths to the slow 3? process for producing CNO seed nuclei and thus could change the fate of massive stars. In the present work, the angular distribution of the 2H(12N, 13O)n proton transfer reaction at Ec.m.=8.4 MeV has been measured for the first time. Based on the Johnson-Soper approach, the square of the asymptotic normalization coefficient (ANC) for the virtual decay of 13Og.s. ? 12N+p was extracted to be 3.92±1.47 fm-1 from the measured angular distribution and utilized to compute the direct component in the 12N(p,?)13O reaction. The direct astrophysical S factor at zero energy was then found to be 0.39±0.15 keV b. By considering the direct capture into the ground state of 13O, the resonant capture via the first excited state of 13O and their interference, we determined the total astrophysical S factors and rates of the 12N(p,?)13O reaction. The new rate is two orders of magnitude slower than that from the REACLIB compilation. Our reaction network calculations with the present rate imply that 12N(p,?)13O will only compete successfully with the ?+ decay of 12N at higher (˜2 orders of magnitude) densities than initially predicted.

Guo, B.; Su, J.; Li, Z. H.; Wang, Y. B.; Yan, S. Q.; Li, Y. J.; Shu, N. C.; Han, Y. L.; Bai, X. X.; Chen, Y. S.; Liu, W. P.; Yamaguchi, H.; Binh, D. N.; Hashimoto, T.; Hayakawa, S.; Kahl, D.; Kubono, S.; He, J. J.; Hu, J.; Xu, S. W.; Iwasa, N.; Kume, N.; Li, Z. H.

2013-01-01

413

Calcite dissolution and Ca/Na ion-exchange reactions in columns with different flow rates through high ESR soil  

E-print Network

CALCITE DISSOLUTION AND Ca/Na ION-EXCHANGE REACTIONS IN COLUMNS WITH DIFFERENT FLOW RATES THROUGH HIGH ESR SOIL A Thesis by AUDREY NAVARRE Submitted to the Office of Graduate Studies of Texas A&:M University in partial fulfillment... of the requirements for the degree of MASTER OF SCIENCE August 1999 Major Subject: Soil Science CALCITE DISSOLUTION AND Ca/Na ION-EXCHANGE REACTIONS IN COLUMNS WITH DIFFERENT FLOW RATES THROUGH HIGH ESR SOIL A Thesis by AUDREY NAVARRE Submitted to Texas A...

Navarre, Audrey

2012-06-07

414

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

NASA Technical Reports Server (NTRS)

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

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

1979-01-01

415

Voluntary Control of Human Heart Rate: Effect on Reaction to Aversive Stimulation: A Replication and Extension  

ERIC Educational Resources Information Center

Hypothesized that biofeedback training for fear-relevant physiological changes might provide a useful therapeutic strategy to influence directly a person's reaction to anxiety-inducing situations. (Author)

Sirota, Alan D.; And Others

1976-01-01

416

NONRANDOM DISTRIBUTION OF ADSORBATES ON CATALYTIC SURFACES: THE ROLE OF ADSORBATE MOBILITIES ON REACTION RATES  

Microsoft Academic Search

An important step in any Langmuir-Hinshelwood process is the surface migration of reactants to a critical distance at which a reaction can occur with reasonable probability. It is usually assumed that once the reactants reach catalytic surface, they diffuse rapidly and the main barrier for the product formation on the surface is in the reaction step. In highly exothermic surface

SANKARAN SUNDARESAN; KESAVA RAO KAZA

1985-01-01

417

Method for Determining the Faradaic Impedance of an Electrode Reaction: Application to Metal Corrosion Rate Measurements  

Microsoft Academic Search

A method was developed to determine the faradaic impedance (Z[sub t]) of an electrode reaction on the basis of a measurement of the electrode impedance (Z). Neither the reaction mechanism nor the equivalent electric circuit needed to be known. Z[sub f] was determined by a method based on determination of the minimum of the length of the electrode admittance diagram

F. Berthier; C. Montella; J.-P. Diard; B. Le Gorrec

1995-01-01

418

An Interactive Classroom Activity Demonstrating Reaction Mechanisms and Rate-Determining Steps  

ERIC Educational Resources Information Center

An interactive classroom activity that includes two-step reaction of unwrapping and eating chocolate candies is described which brings not only the reaction intermediate, but also the reactants and products into macroscopic view. The qualitative activation barriers of both steps can be adjusted independently.

Jennings, Laura D.; Keller, Steven W.

2005-01-01

419

A note on the quantum-tail effect on fusion reaction rate  

E-print Network

A study is made of the power-law tail effect in the quantum particle distribution over momentum on the nuclear fusion reactions. Our results do not support the idea of averaging the fusion reaction cross-section over the momentum distribution postulated and used in many publications.

Alexander L. Zubarev

2008-05-28

420

Stochastic theory of large-scale enzyme-reaction networks: finite copy number corrections to rate equation models.  

PubMed

Chemical reactions inside cells occur in compartment volumes in the range of atto- to femtoliters. Physiological concentrations realized in such small volumes imply low copy numbers of interacting molecules with the consequence of considerable fluctuations in the concentrations. In contrast, rate equation models are based on the implicit assumption of infinitely large numbers of interacting molecules, or equivalently, that reactions occur in infinite volumes at constant macroscopic concentrations. In this article we compute the finite-volume corrections (or equivalently the finite copy number corrections) to the solutions of the rate equations for chemical reaction networks composed of arbitrarily large numbers of enzyme-catalyzed reactions which are confined inside a small subcellular compartment. This is achieved by applying a mesoscopic version of the quasisteady-state assumption to the exact Fokker-Planck equation associated with the Poisson representation of the chemical master equation. The procedure yields impressively simple and compact expressions for the finite-volume corrections. We prove that the predictions of the rate equations will always underestimate the actual steady-state substrate concentrations for an enzyme-reaction network confined in a small volume. In particular we show that the finite-volume corrections increase with decreasing subcellular volume, decreasing Michaelis-Menten constants, and increasing enzyme saturation. The magnitude of the corrections depends sensitively on the topology of the network. The predictions of the theory are shown to be in excellent agreement with stochastic simulations for two types of networks typically associated with protein methylation and metabolism. PMID:21090871

Thomas, Philipp; Straube, Arthur V; Grima, Ramon

2010-11-21

421

The Effects of Improvements in the Nuclear Reaction Rates on Hydrodynamic Simulations of the Classical Nova Outburst  

NASA Astrophysics Data System (ADS)

We report on a new series of calculations of the accretion of hydrogen-rich material onto 1.25M? and 1.35M? ONeMg white dwarfs (WD) where we have varied the nuclear reaction library from that used in Politano et al. (ApJ, 448, 807, 1995), through intermediate libraries, to the latest compilation of Iliadis. In addition, we have replaced the nuclear reaction network from that of Weiss & Truran (WT: A&A, 238, 178, 1990) to that of Hix & Thielemann (J. Comp. & Appl. Math., 109, 321 1999). As part of the verification process for the new reaction network, we discovered that the pep reaction (p + e- +p -> d + ? ) had been omitted from previous studies. While the energy production from this reaction is unimportant in the Sun, because the density is ˜ 104 gm cm-3 in our Classical Nova simulations, it increases the rate of energy generation in the pp-chain by 40%. This increase, in turn, has the effect of producing more heating per unit accreted mass so that the thermonuclear runaway occurs earlier with a smaller amount of accreted mass. As a direct result, peak temperatures are lower and the resulting nucleosynthesis differs from previous published studies (c.f., Starrfield et al. MNRAS, 296, 502, 1998) even with no change in the reaction library. Nevertheless, improving the reaction library (and including the pep reaction) does change the quantitative predictions on the amount of 22Na and 26Al produced by WDs of various mass. We will also show how the peak temperature, peak nuclear luminosity, ejected mass, and light curve are affected by the reaction library. The influence of the addition of the pep reaction to our studies of the evolution of the hot, accreting, white dwarfs in the Super Soft X-ray Binaries will also be presented. We acknowledge partial support by the NSF, NASA, and DOE under various grants to our respective institutions.

Starrfield, S.; Hix, W. R.; Iliadis, C.; Timmes, F. X.; Sparks, W. M.

2005-12-01

422

Reactions of yeast thioredoxin peroxidases I and II with hydrogen peroxide and peroxynitrite: Rate constants by competitive kinetics  

Microsoft Academic Search

Peroxiredoxins are receiving increasing attention as defenders against oxidative damage and sensors of hydrogen peroxide-mediated signaling events. Likely to be critical for both functions is a rapid reaction with hydrogen peroxide, typically with second-order rate constants higher than 105 M?1 s?1. Until recently, however, the values reported for these rate constants have been in the range of 104?105 M?1 s?1, including those

Renata Ogusucu; Daniel Rettori; Daniela Cristina Munhoz; Luis Eduardo Soares Netto; Ohara Augusto

2007-01-01

423

Measurements of Reaction Rates in Zone-Type Cores of Fast Critical Assembly Simulating High Conversion Light Water Reactor  

Microsoft Academic Search

Measurements of reaction rates have been performed in three uranium-fueled zone-type cores of the FCA constructed for a series of experiments on a high conversion light water reactor (HCLWR). These cores possess central test zones of different fuel enrichments and moderator to fuel volume ratios. Radial and axial fission rates of 236U, 239Pu, 238U and 23,Np were measured in each

Makoto ?BU; Tatsuo NEMOTO; Susumu IIJIMA; Takeshi SAKURAI; Yoshihisa TAHARA

1989-01-01

424

Solvent refined coal (SRC) process. Effect of mixing energy on hydrogen reaction rates in SRC-II reactors  

SciTech Connect

This study attempted to analyze theoretically the conditions which may lead to hydrogen starvation in SRC-II reactors. Literature correlations for mass transfer coefficients in bubble columns were combined with experimental data from a stirred vessel (CSTR) on the basis of specific power consumption. Specific power consumption, i.e., consumption of mixing energy per unit volume per unit time, is proportional to N/sup 3/, where N is the stirred speed, and approximately proportional to superficial gas velocity (Ug) in a bubble column. For operating conditions selected to give maximum practicable reaction rate, i.e., under conditions of highest possibility of hydrogen starvation, results indicate that the fractional decrease in the rate of reaction would be insignificant (<4%) for superficial gas velocity, Ug, above 1 cm/s. Based on this result it can be claimed that in the SRC-II process, hydrogen starvation is unlikely. The rate of reaction was shown to be sensitive to the value of the mass transfer coefficient, K/sub L/a. A decrease in the rate of reaction due to lower value of mass transfer coefficient (K/sub L/a) can be compensated by increasing the gas rate (superficial gas velocity). This shows that the possibility of hydrogen starvation would decrease with increase in the length of SRC-II reactors, since the superficial velocity in a taller reactor will have to be higher. Based on the present study, it is believed that the higher the gas velocity (the longer the reactor) in the large-scale reactor situation, hydrogen reaction rates will not be significantly influenced by mass transfer in normal operation of the reactor.

Singh, C.P.P.; Shah, Y.T.; Carr, N.L.

1982-01-01

425

Mechanism and kinetics of synthesizing dibutoxymethane by phase transfer catalysis  

Microsoft Academic Search

The synthesis of CH2(OC4H9)2 by phase transfer catalysis (PTC) has been studied in C6H5Cl\\/KOH solution. A pseudo-first-order rate law was applied for describing the reaction at a rather high [KOH]w. The reaction system was limited by the reaction equilibrium in the aqueous phase at low [KOH]w and the reaction in the organic phase at high [KOH]w.

Maw-Ling Wang; Shahng-Wern Chang

1993-01-01

426

Direct and inverse reactions of LiH+ with He(1S) from quantum calculations: mechanisms and rates.  

PubMed

The gas-phase reaction of LiH(+) (X(2)?) with He((1)S) atoms, yielding Li(+)He with a small endothermicity for the rotovibrational ground state of the reagents, is analysed using the quantum reactive approach that employs the Negative Imaginary Potential (NIP) scheme discussed earlier in the literature. The dependence of low-T rates on the initial vibrational state of LiH(+) is analysed and the role of low-energy Feshbach resonances is also discussed. The inverse destruction reaction of LiHe(+), a markedly exothermic process, is also investigated and the rates are computed in the same range of temperatures. The possible roles of these reactions in early universe astrophysical networks, in He droplets environments or in cold traps are briefly discussed. PMID:22086258

Tacconi, M; Bovino, S; Gianturco, F A

2012-01-14

427

Parasitic reaction and its effect on the growth rate of AlN by metalorganic chemical vapor deposition  

NASA Astrophysics Data System (ADS)

The Al composition of metalorganic chemical vapor deposition (MOCVD)-grown AlGaN alloy layers is found to be greatly influenced by the parasitic reaction between ammonia (NH 3) and trimethylaluminum (TMAl). The growth process of AlN is carefully investigated by monitoring the in situ optical reflection. The abnormal dependencies of growth rate on growth temperature, reactor pressure, and flux of NH 3 are observed and can be well explained by the effect of parasitic reaction. The increase of growth rate with increasing flux of TMAl is found to depend on the growth temperature and reactor pressure due to the presence of parasitic effect. A relatively low growth temperature and a reduced reactor pressure are suggested for the effective decrease of parasitic reaction during the MOCVD growth of AlN and probably lead to a more effective incorporation of Al into the AlGaN layers.

Zhao, D. G.; Zhu, J. J.; Jiang, D. S.; Yang, Hui; Liang, J. W.; Li, X. Y.; Gong, H. M.

2006-03-01