Calculations pair/methyl fluoride

The, S n2 reaction between lithium isothiocyanate ion pair and methyl fluoride has been calculated at the MP2(full)/6-31H-G //HF/6-311- -G level of theory in the gas phase and in acetone using the PCM model.108 Both the gas phase and acetone reactions occur with inversion, rather than retention, of configuration. However, the transition states and products are different in the gas phase and in solution methyl thiocyanate is formed in the gas phase by transition state (72) whereas methyl isothiocyanate is formed much more slowly in acetone via the looser transition state (73). 

In a related study, MP2(full)/6-31+G //HF/6-31+G calculations of the 5n reaction between methyl fluoride and the free isothiocyanate ion or the lithium isothiocyanate ion pair108 indicate that the free-ion reaction occurs at the sulfur atom via a normal Sn2 mechanism with inversion of configuration. In the ion-pair reaction, sulfur is again the nucleophilic atom and the reaction occurs with an inversion of configuration but via a six-membered ring transition structure (23). As one might expect, the ion-pair reaction in the gas phase is faster than the free ion gas-phase reaction. 

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