Ab initio study of solvent kinetic isotope effects in microhydrated SN2 reactions

The solvent kinetic isotope effects in the prototype SN2 reaction of methyl chloride with a chloride ion in the presence of each of one, two, three or four water molecule(s) were examined within transition-state theory formalism on the basis of ab initio calculations. The rate ratios kHkD were found to be less than 1 for the monohydrated system and greater than 1 for the system with two waters and to increase with an increase in the number of the microsolvated waters due to the breaking of the hydrogen bonds at the transition state.