Steric effects in S(N)2 reactions. The influence of microsolvation

Ab initio molecular orbital calculations at the B3LYP/6-31+G* and MP2/6-31G* 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 molec ules of methanol, acetonitrile, acetone, dimethyl ether and propane has bee n investigated. The polarizable continuum solvent model has been used to in vestigate the effect of bulk solvation. The calculated barrier heights incr ease with the number of solvent molecules and the size of the R group. Micr osolvation causes only small changes in the TS geometries for the methyl, e thyl, and i-propyl systems, whereas the tert-butyl TS becomes significantly looser. Microsolvation decreases the steric effect, with the modulation de pending 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, me diated by the electron donating effect of the methyl group. The polarizable continuum solvent model in general underestimates changes in steric effect s due to solvation.