Theoretical study of the effect of coordinating solvent on ion pair SN2 reactions: the role of unsymmetrical transition structures.

Computations are reported at the HF/6-31+g* level for ion pair SN2 reactions of methyl, ethyl, n-propyl, isopropyl, and allyl halides with LiX.E, LiX.2E, and LiX.3E (X = F, Cl, Br; E = dimethyl ether as a model for THF). Some calculations were also done at the MP2, B3LYP, and mPW1PW91 levels. In addition to normal SN2-type (type I) transition structures (TSs), novel unsymmetrical TSs were found in which the Li is coordinated to a single halide. With LiX.2E, such structures are already competitive with the type I structures, and with LiX.3E, only the type II structures were found. With incorporation of dielectric solvation, the type II structures are relatively even more stable. The results suggest that such structures are better models for ion pair displacement reactions in ethereal solvents.