The F^-+CH₃I → FCH₃ +I^- entrance channel potential energy surface Comparison of electronic structure methods

The potential energy surface (PES) of the F^- + CH₃I → FCH₃ +I^- S_n2 nucleophilic substitution reaction has been studied previously using MP2 and DFT levels of theory (J. Phys. Chem. A 2010,114,9635-9643). This work indicated that DFT gives a better representation of the PES which has only an hydrogen-bonded entrance channel reaction path, with a hydrogen-bonded transition state [F··HCH₂··I]^- connecting the hydrogen-bonded pre-reaction complex F^-··· HCH₂I and C_3v post-reaction complex FCH₃··· I^-. For the work presented here, CCSD(T) with three different basis set and two effective core potentials (i.e. PP/d, PP/t and ECP/d) was employed to investigate stationary point properties for this reaction. Besides the hydrogen-bonded entrance channel stationary points, CCSD(T) also predicts a traditional C_3v transition state [F··CH₃·· I]^- connecting a C_3v pre-reaction complex F^-·· CH₃I with the C_3v post-reaction complex FCH₃ · · ·I^-. Though CCSD(T) gives a CH₃F···I^- binding energy and CH₃ F and CH₃I geometries in almost exact agreement with experiment, it gives a heat of reaction ∼20 kJ/mol less exothermic than experiment. The MP2 PES for this reaction, determined in the previous study, is very similar to the CCSD(T), but obtained with a much smaller computational cost. Direct dynamics simulations for the F^- + CH₃I→ FCH₃ +I^- reaction are feasible with MP2.