Steric effects in SN2 reactions.: The influence of microsolvation

Ab initio molecular orbital calculations at the B3LYP/6-31+G* and MP2/6-31+G* levels have been performed to study the effect of microsolvation on the S(N)2 reaction profile for Cl- + RCl (R = methyl, ethyl, i-propyl, and tert-butyl). Microsolvation corresponding to 0-4 water molecules, and 0-2 molecules of methanol, acetonitrile, acetone, dimethyl ether and propane has been investigated. The polarizable continuum solvent model has been used to investigate the effect of bulk solvation. The calculated barrier heights increase with the number of solvent molecules and the size of the R group. Microsolvation causes only small changes in the TS geometries for the methyl, ethyl, and i-propyl systems, whereas the tert-butyl TS becomes significantly looser. Microsolvation decreases the steric effect, with the modulation depending on the dielectric constant of the solvent. For water, the decrease in steric effect is shown to be due to an increased solvation of the TS, mediated by the electron donating effect of the methyl group. The polarizable continuum solvent model in general underestimates changes in steric effects due to solvation.