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(N)2 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 center dot center dot HCH2 center dot center dot I](-) connecting the hydrogen-bonded pre-reaction complex F-center dot center dot center dot HCH2I and C-3v post-reaction complex FCH3 center dot center dot center dot 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 center dot center dot CH3 center dot center dot I](-) connecting a C-3v pre-reaction complex F-center dot center dot center dot CH3I with the C-3v post-reaction complex FCH3 center dot center dot center dot I-. Though CCSD(T) gives a CH3F center dot center dot center dot I- binding energy and CH3F and CH3I geometries in almost exact agreement with experiment, it gives a heat of reaction similar to 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. (C) 2014 Elsevier B.V. All rights reserved.