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André Canosa - One of the best experts on this subject based on the ideXlab platform.
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Low temperature kinetics: the Association of OH radicals with O2.
Physical Chemistry Chemical Physics, 2010Co-Authors: Meryem Tizniti, Sébastien D. Le Picard, André Canosa, Ian R Sims, Ian W. M. SmithAbstract:We report the first measurements of rate constants for the Reaction in which OH radicals associate with O(2) to form HO(3). Our recent measurements (Science, 2010, 328, 1258) have shown that the HO-O(2) bond dissociation energy is only (12.3 ± 0.3) kJ mol(-1). Consequently, above ca. 90 K under attainable experimental conditions, the rate of the reverse dissociation of HO(3) becomes comparable to, and then greater than, the rate of the forward Association Reaction. We have used the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method to access low temperatures and have explored the kinetics of OH + O(2) + M → HO(3) + M in two series of experiments. At temperatures between 55.9 and 79.2 K, the OH radicals, created by pulsed laser photolysis of H(2)O(2) and observed by laser-induced fluorescence, decayed by pseudo-first-order kinetics to effectively zero concentration at longer times. The third-order rate constants derived from these experiments fit the expression: k(3rd)(o) (T) = (4.2 ± 1.9) × 10(-34) (T/298 K)(-(3.5 ± 0.3)) cm(6) molecule(-2) s(-1). At temperatures between 87.4 and 99.8 K, rate constants for the Association Reaction were determined allowing for the significant occurrence of the reverse dissociation Reaction. The values of the derived rate constants are consistent with those obtained in the lower temperature range, though the errors are larger. The experimental values of k(3rd)(o) (T) are compared with (a) those for other Association Reactions involving species of similar complexity, and (b) values of k(3rd)(o) (T) estimated according to both the energy transfer (ET) and the radical-complex (RC) mechanisms. We conclude that the RC mechanism probably makes the major contribution to the Association of OH + O(2) at the low temperatures of our experiments.
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Measurement of the rate constant for the Association Reaction CH + N2 at 53 K and its relevance to Triton's atmosphere
Geophysical Research Letters, 1998Co-Authors: Sébastien D. Le Picard, André CanosaAbstract:The pulsed laser photolysis (PLP), time-resolved laser-induced fluorescence (LIF) technique has been used to study the Association Reaction CH + N2 in a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus at 53 K and in the pressure range 0.46–6 mbar. The third order rate constant was found to be ko = (5.7±0.30) × 10−30cm6s−1. Consequences for the production of HCN in the atmosphere of Triton are discussed in light of this result.
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measurement of the rate constant for the Association Reaction ch n2 at 53 k and its relevance to triton s atmosphere
Geophysical Research Letters, 1998Co-Authors: Sebastien Le D Picard, André CanosaAbstract:The pulsed laser photolysis (PLP), time-resolved laser-induced fluorescence (LIF) technique has been used to study the Association Reaction CH + N2 in a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus at 53 K and in the pressure range 0.46–6 mbar. The third order rate constant was found to be ko = (5.7±0.30) × 10−30cm6s−1. Consequences for the production of HCN in the atmosphere of Triton are discussed in light of this result.
Sébastien D. Le Picard - One of the best experts on this subject based on the ideXlab platform.
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Low temperature kinetics: the Association of OH radicals with O2.
Physical Chemistry Chemical Physics, 2010Co-Authors: Meryem Tizniti, Sébastien D. Le Picard, André Canosa, Ian R Sims, Ian W. M. SmithAbstract:We report the first measurements of rate constants for the Reaction in which OH radicals associate with O(2) to form HO(3). Our recent measurements (Science, 2010, 328, 1258) have shown that the HO-O(2) bond dissociation energy is only (12.3 ± 0.3) kJ mol(-1). Consequently, above ca. 90 K under attainable experimental conditions, the rate of the reverse dissociation of HO(3) becomes comparable to, and then greater than, the rate of the forward Association Reaction. We have used the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method to access low temperatures and have explored the kinetics of OH + O(2) + M → HO(3) + M in two series of experiments. At temperatures between 55.9 and 79.2 K, the OH radicals, created by pulsed laser photolysis of H(2)O(2) and observed by laser-induced fluorescence, decayed by pseudo-first-order kinetics to effectively zero concentration at longer times. The third-order rate constants derived from these experiments fit the expression: k(3rd)(o) (T) = (4.2 ± 1.9) × 10(-34) (T/298 K)(-(3.5 ± 0.3)) cm(6) molecule(-2) s(-1). At temperatures between 87.4 and 99.8 K, rate constants for the Association Reaction were determined allowing for the significant occurrence of the reverse dissociation Reaction. The values of the derived rate constants are consistent with those obtained in the lower temperature range, though the errors are larger. The experimental values of k(3rd)(o) (T) are compared with (a) those for other Association Reactions involving species of similar complexity, and (b) values of k(3rd)(o) (T) estimated according to both the energy transfer (ET) and the radical-complex (RC) mechanisms. We conclude that the RC mechanism probably makes the major contribution to the Association of OH + O(2) at the low temperatures of our experiments.
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Measurement of the rate constant for the Association Reaction CH + N2 at 53 K and its relevance to Triton's atmosphere
Geophysical Research Letters, 1998Co-Authors: Sébastien D. Le Picard, André CanosaAbstract:The pulsed laser photolysis (PLP), time-resolved laser-induced fluorescence (LIF) technique has been used to study the Association Reaction CH + N2 in a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus at 53 K and in the pressure range 0.46–6 mbar. The third order rate constant was found to be ko = (5.7±0.30) × 10−30cm6s−1. Consequences for the production of HCN in the atmosphere of Triton are discussed in light of this result.
Neil M. Donahue - One of the best experts on this subject based on the ideXlab platform.
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The Reaction that wouldn't quit
Nature Chemistry, 2011Co-Authors: Neil M. DonahueAbstract:The radical–radical Association Reaction between hydroxyl and nitrogen dioxide plays a central role in atmospheric chemistry but has challenged experimentalists for decades. A study now measures all reactants and products and largely settles the issue.
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Atmospheric chemistry: The Reaction that wouldn't quit
Nature chemistry, 2011Co-Authors: Neil M. DonahueAbstract:The radical–radical Association Reaction between hydroxyl and nitrogen dioxide plays a central role in atmospheric chemistry but has challenged experimentalists for decades. A study now measures all reactants and products and largely settles the issue.
Abhijit K. Das - One of the best experts on this subject based on the ideXlab platform.
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Association Reaction between SiH_3 and H_2O_2: a computational study of the Reaction mechanism and kinetics
Theoretical Chemistry Accounts, 2013Co-Authors: Kaushik Sen, Bhaskar Mondal, Srimanta Pakhira, Chandan Sahu, Deepanwita Ghosh, Abhijit K. DasAbstract:The Association Reaction between silyl radical (SiH_3) and H_2O_2 has been studied in detail using high-level composite ab initio CBS-QB3 and G4MP2 methods. The global hybrid meta-GGA M06 and M06-2X density functionals in conjunction with 6-311++G( d , p ) basis set have also been applied. To understand the kinetics, variational transition-state theory calculation is performed on the first Association step, and successive unimolecular Reactions are subjected to Rice–Ramsperger–Kassel–Marcus calculations to predict the Reaction rate constants and product branching ratios. The bimolecular rate constant for SiH_3–H_2O_2 Association in the temperature range 250–600 K, k ( T ) = 6.89 × 10^−13 T ^−0.163exp(−0.22/ RT ) cm^3 molecule^−1 s^−1 agrees well with the current literature. The OH production channel, which was experimentally found to be a minor one, is confirmed by the rate constants and branching ratios. Also, the correlation between our theoretical work and experimental literature is established. The production of SiO via secondary Reactions is calculated to be one of the major Reaction channels from highly stabilized adducts. The H-loss pathway, i.e., SiH_2(OH)_2 + H, is the major decomposition channel followed by secondary dissociation leading to SiO.
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The Association Reaction between C2H and 1-butyne: a computational chemical kinetics study
Physical chemistry chemical physics : PCCP, 2011Co-Authors: Debasish Mandal, Bhaskar Mondal, Abhijit K. DasAbstract:The potential energy surfaces (PES) for the Reaction of the C2H radical with 1-butyne (C4H6) have been studied using the CBS-QB3 method. Density functional B3LYP/cc-pVTZ and M06-2X/6-311++G(d,p) calculations have also been performed to analyze the Reaction energetics. For detailed theoretical calculation on the total Reaction mechanism, the initial Association Reactions on more and less substituted C atoms of 1-butyne are treated separately followed by a variational transition state theory (VTST) calculation to obtain Reaction rates. The successive unimolecular Reactions from the Association Reaction complexes are subjected to Rice–Ramsperger–Kassel–Marcus (RRKM) calculations for Reaction rate constants and product branching ratios. The calculated rate constants in the temperature range 70–295 K for both the Association Reactions are found to be highly temperature dependent at low temperatures, which is contrary to the experimental findings of temperature independent Association rates. We have explained this observation with the help of variational nature of the transition states, and we found a “loose” transition state at low temperatures. The calculated product branching ratios for the unimolecular Reactions generally agree with the available experimental data, although some channels show a significant method dependency and therefore the correlation with experiment is lost to some extent. Our detailed Reaction energetics calculations confirm that the C2H + C4H6 Reaction proceeds without an entrance barrier and leads to the important products ethynylallene + CH3, 1,3-hexadiyne + H, 3,4-hexadiene-1-yne + H, 2-ethynyl-1,3-butadiene + H, 3,4-dimethylenecyclobut-1-ene + H and fulvene + H exothermic by 25–75 kcal mol−1, with strong dependence of the product distribution on the Association mode of C2H with C4H6, making these Reactions fast under low temperature conditions of Titan's atmosphere. Therefore this study can provide a detailed picture of the complex hydrocarbon formation mechanism in the upper atmosphere.
P. H. Wine - One of the best experts on this subject based on the ideXlab platform.
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Kinetics and Thermochemistry of the Cl((sup 2)P(sub J)) + C2Cl4 Association Reaction
1997Co-Authors: J. M. Nicovich, S. Wang, M. L. Mckee, P. H. WineAbstract:A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the Cl(sup 2)P(sub j) + C2Cl4 Association Reaction as a function of temperature (231-390 K) and pressure (3-700 Torr) in nitrogen buffer gas. The Reaction is found to be in the falloff regime between third and second order over the range of conditions investigated, although the second-order limit is approached at the highest pressures and lowest temperatures. At temperatures below 300 K, the Association Reaction is found to be irreversible on the experimental time scale of approximately 20 m-s. The kinetic data at T is less than 300 K have been employed to obtain falloff parameters in a convenient format for atmospheric modeling. At temperatures above 330 K, reversible addition is observed, thus allowing equilibrium constants for C2Cl5 formation and dissociation to be determined. Second- and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the Association Reaction: Delta-H(298) = -18.1 +/- 1.3 kcal/mol, Delta-H(0) = -17.6 +/- 1.3 kcal/mol, and Delta-S(298) = -27.7 +/- 3.0 cal/mol.K. In conjunction with the well-known heats of formation of Cl((sup 2)P(sub j)) and C2Cl4 the above Delta-H values lead to the following heats of formation for C2Cl5, at 298 and 0 K: Delta-H(f,298) = 8.0 +/- 1.3 kcal/mol and Delta-H(f,0) = 8.1 +/- 1.5 kcal/mol. The kinetic and thermochemical parameters reported above are compared with other reported values, and the significance of reported Association rate coefficients for understanding tropospheric chlorine chemistry is discussed.
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Kinetics and Thermochemistry of the Cl(2PJ) + C2Cl4 Association Reaction
The Journal of Physical Chemistry, 1996Co-Authors: J. M. Nicovich And, S. Wang, M. L. Mckee, P. H. WineAbstract:A laser flash photolysis−resonance fluorescence technique has been employed to study the kinetics of the Cl(2PJ) + C2Cl4 Association Reaction as a function of temperature (231−390 K) and pressure (3−700 Torr) in nitrogen buffer gas. The Reaction is found to be in the falloff regime between third and second order over the range of conditions investigated, although the second-order limit is approached at the highest pressures and lowest temperatures. At temperatures below 300 K, the Association Reaction is found to be irreversible on the experimental time scale of ∼20 ms. The kinetic data at T < 300 K have been employed to obtain falloff parameters in a convenient format for atmospheric modeling. At temperatures above 330 K, reversible addition is observed, thus allowing equilibrium constants for C2Cl5 formation and dissociation to be determined. Second- and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the Association Reaction: ΔH°298 = −18.1 ± 1.3 kcal mol...
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Thermochemistry and kinetics of the Cl+O2 Association Reaction
Chemical Physics Letters, 1991Co-Authors: J. M. Nicovich, K. D. Kreutter, C.j. Shackelford, P. H. WineAbstract:Abstract Laser flash photolysis of Cl 2 /O 2 mixtures has been employed in conjunction with Cl( 2 P 3 2 ) detection by time-resolved resonance fluorescence spectroscopy to investigate equilibration kinetics for the Reactions Cl + O 2 + O 2 ⇌ ClOO + O 2 at temperatures of 181–200 K and O 2 pressures of 15–40 Torr. The third-order rate coefficient for the Association Reaction at 186.5 ± 5.5 K is (8.9 ± 2.9) × 10 −33 cm 6 molecule −2 s −1 and the equilibrium constant ( K p ) at 185.4 K is 18.9 atm −1 (factor of 1.7 uncertainty). A third law analysis of our data leads to a value for the ClOO bond dissociation energy of 4.76 ± 0.49 kcal mol −1 .
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Kinetics and Thermochemistry of the Br((sup 2)P3/2) + NO2 Association Reaction
The Journal of Physical Chemistry, 1991Co-Authors: K. D. Kreutter, J. M. Nicovich, P. H. WineAbstract:A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the Br((sup 2)P3/2) + NO2 Association Reaction as a function of temperature (259-432 K) pressure (12.5 - 700 Torr), and buffer gas identity (He, Ar, H2, N2, CO2, CF4, SF6). The Reaction is found to be in the falloff regime between third and second order over the entire range of conditions investigated. At temperatures below 350 K, the Association Reaction is found to be irreversible on the time scale of the experiment (approximately 30 ms). At higher temperatures reversible addition is observed, allowing equilibrium constants for BrNO2 formation and dissociation to be determined. Second- and third-law analyses of the equilibrium data are in only fair agreement and lead to the following thermochemical parameters for the Association Reaction: Delta-H(298) = 19.6 +/- 1.7 kcal/mol, Delta-H(0) = -18.6 +/- 2.0 kcal/mol, Delta-S(298) = 29.3 +/- 4.2 cal/mol/K, Delta-H(sub f)(sub 298)(BrNO2) = 17.0 +/-1.8 kcal/mol(uncertainties are 2 sigma estimates of absolute accuracy). The value for Delta-H(0) determined in this study has been employed to calculate k(sub 0)(sup SC), the low-pressure third-order rate coefficient in the strong collision limit, by using the method of Troe; calculated values of k(sub 0)(sup SC) are inconsistent with experimental results unless Delta-H(0) is assigned a value near the lower limit derived from analysis of the high-temperature approach to equilibrium data, i.e. delta-H(0) approximately equals -16.6 kcal/mol. A potential source of systematic error in the calculation of both k(sub 0)(sup SC) and the absolute entropy of BrNO2 results from the complete lack of knowledge of the energies and degeneracies of the electronic states of BrNO3. The procedure developed by Troe and co-workers has been employed to extrapolate experimental falloff curves to the low- and high-pressure limits. Derived values for k(sub 0)(M,298K) in units of 10(exp -31) cm(exp 6)/sq molecule/s range from 2.75 for M = He to 6.54 for M = CO2; 2 sigma uncertainties are estimated to be +/- 20%. Values for k(sub 0)(N2,T) in units of 10(exp -31) cm(exp 6)/sq molecule/s are 5.73 at 259 K, 4.61 at 298 K, and 3.21 at 346 K; the observed temperature dependence for k(sub 0)(N2,T) is consistent with the theoretical temperature dependence for Beta(sub c)k(sub 0)(sup SC). Values for k(sub infinity)(T) in units of 10(exp -11) cu cm/molecule/s are 2.86 at 259 K, 3.22 at 298 K, and 3.73 at 346 K; 2 sigma uncertainties are estimated to be a factor of 2. Approximate falloff parameters in a convenient format for atmospheric modeling are also derived.
