Quasiclassical Trajectory Studies of the O(³P) + CX₄(<i>v</i>_<i>k</i> = 0, 1) → OX(<i>v</i>) + CX₃(<i>n</i>₁<i>n</i>₂<i>n</i>₃<i>n</i>₄) [X = H and D] Reactions on an Ab Initio Potential Energy Surface

Abstract

We report quasiclassical trajectory calculations of the integral and differential cross sections and the mode-specific product state distributions for the “central-barrier” O­(³P) + CH₄/CD₄(<i>v</i>_<i>k</i> = 0, 1) [<i>k</i> = 1, 2, 3, 4] reactions using a full-dimensional ab initio potential energy surface. The mode-specific vibrational distributions for the polyatomic methyl products are obtained by doing a normal-mode analysis in the Eckart frame, followed by standard histogram binning (HB) and energy-based Gaussian binning (1GB). The reactant bending excitations slightly enhance the reactivity, whereas stretching excitations activate the reaction more efficiently. None of the reactant vibrational excitations is as efficient as an equivalent amount of translational energy to promote the reactions. The excitation functions without product zero-point energy (ZPE) constraint are in good agreement with previous 8-dimensional quantum mechanical (QM) results for the ground-state and stretching-excited O + CH₄ reactions, whereas for the bending-excited reactions the soft ZPE constraint, which is applied to the sum of the product vibrational energies, provides better agreement with the QM cross sections. All angular distributions show the dominance of backward scattering indicating a direct rebound mechanism, in agreement with experiment. The title reactions produce mainly OH/OD­(<i>v</i> = 0) products for all the initial states. HB significantly overestimates the populations of OH/OD­(<i>v</i> = 1), especially in the energetic threshold regions, whereas 1GB provides physically correct results. The CH₃/CD₃ vibrational distributions show dominant populations for ground (<i>v</i> = 0), umbrella-excited (<i>v</i>₂ = 1, 2), in-plane-bending-excited (<i>v</i>₄ = 1), and <i>v</i>₂ + <i>v</i>₄ methyl product states. Neither translational energy nor reactant vibrational excitation transfers significantly into product vibrations