Computer Simulations of Organic Reactions in Solution^<i>a</i>

Quantum and statistical mechanics have been used to determine energy profiles for the SN2 reaction of Cl- + CH3Cl in the gas phase, in aqueous solution, and in liquid DMF. The energy profile in the gas phase has the characteristic double-well form featuring unsymmetrical ion-dipole complexes as minima and a symmetrical transition state. Hydration causes the reaction surface to become almost unimodal and increases the barrier significantly. The reaction profile in DMF is intermediate between those for the gas phase and aqueous solution. The ion-dipole complexes are still free energy minima in DMF. Thus, the reaction in DMF involves initial formation of the complex before the rate-determining step. The computed results are shown to be in good accord with experimental free energies of activation. The same technique has been applied to the addition reaction of OH- + H2C = O in the gas phase and aqueous solution. Ab initio 6-31 + G* calculations indicate that the reaction proceeds essentially without activation in the gas phase. Hydration introduces a substantial energy barrier. The transition state in water has been located at a C-O separation of roughly 2 A. A key finding for both reactions is that the activation barriers induced by hydration result primarily from change in strengths rather than in numbers of solute-water hydrogen bonds along the reaction paths.