Potential energy surface for the CH 3 + HBr → CH 4 + Br hydrogen abstraction reaction: Thermal and stateselected rate constants, and kinetic isotope effects Analytical potential energy surface for the GeH 4 +H→GeH 3 +H 2 reaction: Thermal and vibrational-state selected rate constants and kinetic isotope effects A new potential energy surface is reported for the gas-phase reaction ClϩCH 4 →HClϩCH 3 . It is based on the analytical function of Jordan and Gilbert for the analog reaction HϩCH 4 →H 2 ϩCH 3 , and it is calibrated by using the experimental thermal rate coefficients and kinetic isotope effects. The forward and reverse thermal rate coefficients were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range, 200-2500 K. This surface is also used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate coefficients. We find that excitation of C-H stretching modes and Cl-H stretching modes enhances the rate of both the forward and the reverse reactions, and excitation of the lowest frequency bending mode in the CH 4 reactant also enhances the rate coefficient for the forward reaction. However, the vibrational excitation of the CH 3 umbrella mode ͑lowest frequency mode in products͒ slows the reaction at temperatures below 1000 K, while above 1000 K it also accelerates the reaction.

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We report calculations of the reaction rates of O( 3 P) + CH 4 f OH + CH 3 and O( 3 P) + CD 4 f OD + CD 3 over the temperature range 300-2500 K. The calculations are based on variational transition state theory in curvilinear coordinates with transmission coefficients calculated by the microcanonical optimized multidimensional tunneling approximation. A dual-level algorithm is used for the dynamical calculations. The higher level is UMP2/cc-pVTZ, and two lower levels are employed: PM3-SRP and an analytical potential energy surface. Using the canonical unified statistical model with microcanonical optimized multidimensional tunneling contributions, we obtain good agreement with experimental rate constants.

A new potential energy surface for the gas-phase F(2P)+CH4 reaction and its deuterated analogues is reported, and its kinetics and dynamics are studied exhaustively. This semiempirical surface is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and it is calibrated to reproduce the topology of the reaction and the experimental thermal rate constants. For the kinetics, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 180-500 K. The theoretical results reproduce the experimental variation with temperature. The influence of the tunneling factor is negligible, due to the flattening of the surface in the entrance valley, and we found a direct dependence on temperature, and therefore positive and small activation energies, in agreement with experiment. Two sets of kinetic isotope effects were calculated, and they show good agreement with the sparse experimental data. The coupling between the reaction coordinate and the vibrational modes shows qualitatively that the FH stretching and the CH3 umbrella bending modes in the products appear vibrationally excited. The dynamics study was performed using quasi-classical trajectory calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found that the FH(nu',j') rovibrational distributions agree with experiment. Second, the excitation function presents an oscillatory pattern, reminiscent of a reactive resonance. Third, the state specific scattering distributions present reasonable agreement with experiment, and as the FH(nu') vibrational state increases the scattering angle becomes more forward. These kinetics and dynamics results seem to indicate that a single, adiabatic potential energy surface is adequate to describe this reaction, and the reasonable agreement with experiment (always qualitative and sometimes quantitative) lends confidence to the new surface.