Solvent effects on chemical reaction profiles. I. Monte Carlo simulation of hydration effects on quantum chemically calculated stationary structures

Metropolis Monte Carlo simulations of hydration effects on rigid solutes characterizing the reactant, transition state (TS), and product for the rate limiting step of an acid catalized rearrangement of an a-acetylenic alcohol to α,β-unsaturated carbonyl compounds are calculated. The model compound corresponds to the protonated 3 methyl-but-1-yne-3-ol. The electronic structure and geometry of the corresponding species are determined with ab initio analytical gradient techniques; a 4-31G basis set has been used. Electrostatic and solute shape effects on samples having 125 MCY-water molecules at 300 K have been examined. Gurney's model for ion-molecule interactions has been adopted. Although the solute-water potential used is very simple, the results on hydration energetics appear to be fairly reasonable. Solute shapes are found to play a significant role in producing differential solvation effects. Electrostrictive effects have been made evident by running MC simulations with fully uncharged solutes. A solvent activation barrier is detected. From the study of the TS solvation structures a possible incidence of ionic strength, counterion presence, and structure-making or breaking solutes can be conjectured. The structural features found for the solvation sheaths of reactant, TS, and product are in excellent agreement with the postulated molecular mechanism. © 1985 American Institute of Physics.