Toward an Accurate Black-Box Tool for the Kinetics of Gas-Phase Reactions Involving Barrier-less Elementary Steps

An enhanced computational protocol has been devised for the accurate characterization of gas-phase barrier-less reactions in the framework of the reaction-path (RP) and variable reaction coordinate variational transition-state theory. In particular, the synergistic combination of density functional theory and Monte Carlo sampling to optimize reactive fluxes led to a reliable yet effective computational workflow. A black-box strategy has been developed for selecting the most suited density functional with reference to a high-level one-dimensional reference potential. At the same time, different descriptions of hindered rotations are automatically selected, depending on the corresponding harmonic frequencies along the RP. The performance of the new tool is investigated by means of two prototypical reactions involving different degrees of static and dynamic correlation, namely, H2S + Cl and CH3 + CH3. The remarkable agreement of the computed kinetic parameters with the available experimental data confirms the accuracy and robustness of the proposed approach. Together with their intrinsic interest, these results also pave the way toward systematic investigations of gas-phase reactions involving barrier-less elementary steps by a reliable, user-friendly tool, which can be confidently used also by nonspecialists.

Studies on the Kinetics of the CH + H2 Reaction and Implications for the Reverse Reaction, 3CH2 + H

The reaction of CH radicals with H₂ has been studied by the use of laser flash photolysis, probing CH decays under pseudo-first-order conditions using laser-induced fluorescence (LIF) over the temperature range 298-748 K at pressures of ∼5-100 Torr. Careful data analysis was required to separate the CH LIF signal at ∼428 nm from broad background fluorescence, and this interference increased with temperature. We believe that this interference may have been the source of anomalous pressure behavior reported previously in the literature (Brownsword, R. A.; J. Chem. Phys. 1997, 106, 7662-7677). The rate coefficient k₁ shows complex behavior: at low pressures, the main route for the CH₃* formed from the insertion of CH into H₂ is the formation of ³CH₂ + H, and as the pressure is increased, CH₃* is increasingly stabilized to CH₃. The kinetic data on CH + H₂ have been combined with experimental shock tube data on methyl decomposition and literature thermochemistry within a master equation program to precisely determine the rate coefficient of the reverse reaction, ³CH₂ + H → CH + H₂. The resulting parametrization is k_CH2+H(T) = (1.69 ± 0.11) × 10^-10 × (T/298 K)^{(0.05±0.010)} cm³ molecule^-1 s^-1, where the errors are 1σ.

Reaction of Methylidyne with Ethane: The C-C Insertion Is Unimportant

High-accuracy coupled-cluster calculations in combination with the E,J-resolved master-equation analysis are used to study the reaction mechanism and kinetics of methylidyne with ethane. This reaction plays an important role in the combustion of hydrocarbon fuels and in interstellar chemistry. Two distinct mechanisms, the C-C and the C-H insertions of CH in C₂H₆, are characterized. The C-C insertion pathway is identified to have a large barrier of 34.5 kcal mol^-1 and hence plays no significant role in kinetics. The C-H insertion pathway is found to have no barrier, leading to a highly vibrationally excited n-C₃H₇ radical, which rapidly dissociates (within 50 ps) to yield CH₃ + C₂H₄ and H + C₃H₆ in a roughly 7:3 ratio. These findings are in good agreement with an experimental result that indicates that about 20% of the reaction goes to H + C₃H₆. The reaction of the electronically excited quartet state of the CH radical with C₂H₆ is examined for the first time and found to proceed as a direct H-abstraction via a small barrier of 0.4 kcal mol^-1 to yield triplet CH₂ and C₂H₅. The reaction on the quartet state surface is negligibly slow at low temperatures characteristic of interstellar environments but becomes important at high combustion temperatures.

Kinetics and product branching ratios of the reaction of (1)CH2 with H2 and D2

The reactions of singlet methylene (a(1)A1 (1)CH2) with hydrogen and deuterium have been studied by experimental and theoretical techniques. The rate coefficients for the removal of singlet methylene with H2 (k1) and D2 (k2) have been measured from 195 to 798 K and are essentially temperature-independent with values of k1 = (10.48 +/- 0.32) x 10(-11) cm(3) molecule(-1) s(-1) and k2 = (5.98 +/- 0.34) x 10(-11) cm(3) molecule(-1) s(-1), where the errors represent 2sigma, giving a ratio of k1/k2 = 1.75 +/- 0.11. In the reaction with H2, singlet methylene can be removed by reaction giving CH3 + H or deactivated to ground-state triplet methylene. Direct measurement of the H atom product showed that the fraction of relaxation decreased from 0.3 at 195 K to essentially zero at 398 K. For the reaction with deuterium, either H or D may be eliminated. Experimentally, the H:D ratio was determined to be 1.8 +/- 0.5 over the range 195-398 K. Theoretically, the reaction kinetics has been predicted with variable reaction coordinate transition state theory and with rigid-body trajectory simulations employing various high-level, ab initio-determined potential energy surfaces. The magnitudes of the calculated rate coefficients are in agreement with experiment, but the calculations show a significant negative temperature dependence that is not observed in the experimental results. The calculated and experimental H to D ratios from the reaction of singlet methylene with D2 are in good agreement, suggesting that the reaction proceeds entirely through the formation of a long-lived methane intermediate with a statistical distribution of energy.

Rate Constants and H Atom Branching Ratios of the Gas-Phase Reactions of Methylidyne CH(X2Π) Radical with a Series of Alkanes

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

Combined Experimental and Master Equation Investigation of the Multiwell Reaction H + SO2

The temperature and pressure dependence of the rate coefficient for the reaction H + SO2 has been measured using a laser flash photolysis/laser-induced fluorescence technique, for 295 <or= T/K <or= 423 and for 3 <or= [He]/10(18) molecules cm(-3) <or= 23. Under these conditions, the reaction occurs exclusively to form HSO2. These data have been fitted to, and used to refine, a master equation (ME) model of the H + SO2 reaction system that includes the isomers HSO2 and HOSO and the high-temperature products OH + SO. The potential energy for the system is based on literature ab initio calculations. The three eigenvalues of lowest magnitude, obtained from solution of the ME, are the chemically significant eigenvalues that are related to the pressure and temperature dependent rate coefficients for the chemical system. The eigenvalues are generally well separated from each other and from the eigenvalues relating to collisional relaxation, except at high temperatures. The rate coefficients were extracted from the eigenpairs of the ME solution, using an analysis developed by Klippenstein and Miller (J. Phys. Chem. A 2002, 106, 9267) and an adaptation of an earlier analysis by Bartis and Widom (J. Chem. Phys. 1974, 60, 3474). The relationship between the temperature dependence of the chemically significant eigenvalues and combinations of rate coefficients was elucidated via an approximate analysis of the chemical rate equations and it was demonstrated that each eigenvalue relates primarily to passage over a single transition state. Some problems were encountered in equating ratios of rate coefficients for forward and reverse reactions to the equilibrium constant. This problem arises from the nonconservative nature of the reaction system; it was resolved by imposing orthogonality on the coupled rate equations of the chemical system, giving good agreement between rate coefficient ratios and equilibrium constants. The temperature and pressure dependences of the rate coefficients were parametrized according to Troe and modified Troe expressions. The key processes, from a combustion perspective, are the formation and dissociation of HOSO and the formation of OH + SO. Except at pressures >10(3) atm, the latter proceeds directly from H + SO2, via the energized states of HOSO. The derived rate coefficients rely heavily on measurements of the reverse reaction, OH + SO, which has only been determined at temperatures up to 700 K.

Kinetic measurements on methylidyne radical reactions with several hydrocarbons at low temperatures

The temperature dependences of the methylidyne radical reactions with methane, allene, methylacetylene and propene were studied. This work was carried out in a supersonic flow reactor coupled with pulsed laser photolysis (PLP) and laser-induced fluorescence (LIF) techniques. Three Laval nozzles were designed to provide uniform supersonic expansions of nitrogen at Mach 2 and of argon at Mach 2 and 3 to reach low temperatures, e.g. 170, 128 and 77 K, respectively. CH radicals were produced by PLP of CHBr3 at 266 nm and probed by LIF. The exponential decays of the CH fluorescence were acquired, hydrocarbons being introduced in excess. The rate constants for the CH+CH4 reaction are in good agreement with the temperature dependence proposed by Canosa et al. (A. Canosa, I. R. Sims, D. Travers, I. W. M. Smith and B. R. Rowe, Astron. Astrophys., 1997, 323, 644-651, ) i.e. 3.96x10(-8)(T/K)(-1.04) exp(-36.1 K/T) in the range 23-298 K. The rate constants of the CH+C3H4(allene), CH+C3H4(methylacetylene) and CH+C3H6(propene) reactions exhibit a small temperature dependence between 77 and 170 K, with a maximum rate around 100 K close to (4.3-4.6)x10(-10) cm3 molecule-1 s-1.