The potential energy surface for the gas-phase SiH4+H→SiH3+H2 reaction and its deuterated analogs was constructed with suitable functional forms to represent the stretching and bending modes, and using as calibration criterion the reactant and product experimental properties and the ab initio saddle point properties. Using this surface, the rate constants were calculated with variational transition-state theory over the temperature range 200–1000 K, finding good agreement with experiments. We also provide a detailed analysis of the kinetic isotope effects and a comparison with the scarce experimental results.

The effects of the methane torsional (nu(2)), umbrella (nu(4)), and the combination nu(2)+nu(4) bending mode excitations on the reactivity and dynamics of the gas-phase Cl + CH(4) --> HCl + CH(3) reaction were analyzed. Quasi-classical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories, were used on an analytical potential energy surface previously developed by our group. With respect to the reactivity, we found that excitation of either bending mode independently gave similar increases in the reactivity, while the increase observed upon excitation of both modes was larger than the sum of the effect of exciting them independently. Both results agree with recent experimental measures. With respect to the dynamics (rotovibrational and angular distributions of the products), the two bending modes and their combination gave very similar pictures, reproducing the experimental behavior. The satisfactory agreement obtained with a great variety of experimental data (always qualitatively acceptable and sometimes even quantitatively) of the present QCT study lends confidence to the potential energy surface constructed by our group.

To analyze the effects of the symmetric (nu(1)) and asymmetric (nu(3)) stretch mode excitations and the role played by the "umbrella" bending (nu(4)) mode excitation in the reactivity and the dynamics of the gas-phase Cl+CH(4) reaction, an exhaustive dynamics study was performed. Quasiclassical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories, were used in this work on an analytical potential energy surface previously developed by Espinosa-Garcia et al. [J. Chem. Phys. (to be published)]. First, with respect to the reactivity, we found that the nu(1) mode excitation is more reactive than the nu(3) mode by a factor of 1.20, in agreement with the experimental tendency between these modes. The inclusion of the nu(4) bending mode practically does not affect this relative reactivity, (nu(1+)nu(4))(nu(3+)nu(4)) = 1.16. Second, with respect to the dynamics (rotovibrational and angular distributions of the products), the two stretch modes, nu(1) and nu(3), give very similar pictures, reproducing the experimental behavior, and the nu(4) "umbrella" mode does not affect the dynamics. The satisfactory reproduction (always qualitatively acceptable and sometimes even quantitatively) of a great variety of experimental data by the QCT study presented here lends confidence to the potential energy surface constructed by Espinosa-Garcia et al. [J. Chem. Phys. (to be published)].

The hydrogen abstraction reaction from H2 by the Cl atom is studied by means of the variational transition state theory with semiclassical tunneling coefficients on the BW2 potential energy surface. Vibrational anharmonicity and coupling between the bending modes are taken into account. The occurrence of trajectories that recross the transition state is estimated by means of the canonical unified statistical method and by classical trajectories calculations. Different semiclassical methods for tunneling calculations are tested. Our results show that anharmonicity has a small but nonnegligible effect on the thermal rate constants, recrossing can be neglected, and tunneling is adequately described by the least-action approximation, and less successfully by the large-curvature version 3 approximation. However, the large-curvature version 4 and small-curvature approximations lead to a severe underestimation of tunneling. Thermal rate constants calculated using transition state theory including anharmonicity and tunneling agree very well with accurate quantal thermal rate constants over a wide temperature range, although the improvement over the harmonic transition state theory with the microcanonically optimized semiclassical tunneling approximation (based on version 3 of the large-curvature tunneling method) used in a previous study of this reaction is only marginal.

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