Dynamics of the Cl− + CH3I reaction on a high-level ab initio analytical potential energy surface

Abstract

We have developed a full-dimensional analytical ab initio potential energy surface (PES) for the Cl− + CH3I reaction using the Robosurfer program system. The energy points have been computed using a robust composite method defined as CCSD-F12b + BCCD(T) – BCCD with the aug-cc-pVTZ(-PP) basis set and have been fitted by the permutationally invariant polynomial approach. Quasi-classical trajectory simulations on the new PES reveal that two product channels are open in the collision energy (Ecoll) range of 1–80 kcal/mol, i.e., SN2 leading to I− + CH3Cl and iodine abstraction (above ∼45 kcal/mol) resulting in ICl− + CH3. Scattering angle, initial attack angle, product translational energy, and product internal energy distributions show that the SN2 reaction is indirect at low Ecoll and becomes direct-rebound–back-side (CH3-side) attack-type, as Ecoll increases. Iodine abstraction mainly proceeds with direct stripping mechanism with side-on/back-side attack preference. Comparison with crossed-beam experiments and previous direct dynamics simulations shows quantitative or qualitative agreement and also highlights possible theoretical and/or experimental issues motivating further research.