Ab initio computational insight into the ion-pair S(N)2 reaction of lithium isothiocyanate and methyl fluoride in the gas phase and in acetone solution.

The ion-pair S(N)2 reaction LiNCS + CH3F with two mechanisms, inversion and retention, was investigated at the MP2(full)/6-311+G**//HF/6-311+G** level in the gas phase and in acetone solution. All HF-optimized structures were confirmed by vibrational frequency analysis. Based on IRC analyses, eight possible reaction pathways in the title reaction are proposed. The inversion mechanism through a six-membered-ring transition-state structure is the most favorable. Methyl thiocyanate should form preferentially in the gas phase and the more stable methyl isothiocyanate will be the main product in CH3COCH3. The retardation of the reaction in CH3COCH3 solution was attributed to the differences in the solvation free energies in the separated reactants and transition structures. All of the theoretical results are consistent with the experiment.