An ab initio spin–orbit-corrected potential energy surface and dynamics for the F + CH4 and F + CHD3 reactions

Abstract

We report an analytical ab initio three degrees of freedom (3D) spin–orbit-correction surface for the entrance channel of the F + methane reaction obtained by fitting the differences between the spin–orbit (SO) and non-relativistic electronic ground state energies computed at the MRCI+Q/aug-cc-pVTZ level of theory. The 3D model surface is given in terms of the distance, R(C–F), and relative orientation, Euler angles ϕ and θ, of the reactants treating CH₄ as a rigid rotor. The full-dimensional (12D) “hybrid” SO-corrected potential energy surface (PES) is obtained from the 3D SO-correction surface and a 12D non-SO PES. The SO interaction has a significant effect in the entrance-channel van der Waals region, whereas the effect on the energy at the early saddle point is only ∼5% of that at the reactant asymptote; thus, the SO correction increases the barrier height by ∼122 cm⁻¹. The 12D quasiclassical trajectory calculations for the F + CH₄ and F + CHD₃ reactions show that the SO effects decrease the cross sections by a factor of 2–4 at low collision energies and the effects are less significant as the collision energy increases. The inclusion of the SO correction in the PES does not change the product state distributions.