| Abstract
| - 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 symmetricwith respect to the permutation of the four methane hydrogen atoms, and it is calibrated to reproduce thetopology of the reaction and the experimental thermal rate constants. For the kinetics, the thermal rate constantswere calculated using variational transition-state theory with semiclassical transmission coefficients over awide temperature range, 180−500 K. The theoretical results reproduce the experimental variation withtemperature. The influence of the tunneling factor is negligible, due to the flattening of the surface in theentrance valley, and we found a direct dependence on temperature, and therefore positive and small activationenergies, in agreement with experiment. Two sets of kinetic isotope effects were calculated, and they showgood agreement with the sparse experimental data. The coupling between the reaction coordinate and thevibrational modes shows qualitatively that the FH stretching and the CH3 umbrella bending modes in theproducts appear vibrationally excited. The dynamics study was performed using quasi-classical trajectorycalculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we foundthat the FH(ν‘,j‘) rovibrational distributions agree with experiment. Second, the excitation function presentsan oscillatory pattern, reminiscent of a reactive resonance. Third, the state specific scattering distributionspresent reasonable agreement with experiment, and as the FH(ν‘) vibrational state increases the scatteringangle becomes more forward. These kinetics and dynamics results seem to indicate that a single, adiabaticpotential 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.
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