Vibronically and spin-orbit coupled diabatic potentials for X(P) + CH₄ → HX + CH₃ reactions: General theory and...

An approach to systematically construct vibronically and spin-orbit coupled diabatic potential energy surfaces (PESs) for X(P) + CH₄ → HX + CH₃ reactions is proposed. Permutational symmetry and permutational invariants of the S₄ group and its S₃ and S₂ × S₂ subgroups are used to construct a diabatic model which properly describes the reaction starting from reactants to products. As a first example, the approach is applied to the construction of diabatic potentials for the F(²P) + CH₄ → HF + CH₃ reaction. The description of the entrance channel relies on a set of vibronically and spin-orbit coupled diabatic PESs previously developed by Westermann et al. [Angew. Chem., Int. Ed. 53, 1122 (2014)]. The same set of diabatic electronic states is also used in the transition state region and all four exit channels. There the lowest adiabatic PES derived from the diabatic model reproduces the CSBB-PES of Czakó et al. [J. Chem. Phys. 130, 084301 (2009)]. Interesting aspects of the newly developed diabatic potential matrix and the corresponding adiabatic PESs are discussed.