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

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 phi and theta, 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 similar to 5% of that at the reactant asymptote; thus, the SO correction increases the barrier height by similar to 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.