MOLECULAR-DYNAMICS STUDY OF SOLVATION EFFECTS ON ACID DISSOCIATION INAPROTIC MEDIA

Acid ionization in aprotic media is studied using molecular dynamics t echniques. In particular, models for HCl ionization in acetonitrile an d dimethylsulfoxide are investigated. The proton is treated quantum me chanically using Feynman path integral methods and the remaining molec ules are treated classically. Quantum effects are shown to be essentia l for the proper treatment of the ionization, The potential of mean fo rce is computed as a function of the ion pair separation and the local solvent structure is examined. The computed dissociation constants in both solvents differ by several orders of magnitude which are in reas onable agreement with experimental results. Solvent separated ion pair s are found to exist in dimethylsulfoxide but not in acetonitrile. Dis sociation mechanisms in small clusters are also investigated. Solvent separated ion pairs persist even in aggregates composed of rather few molecules, for instance, as few as 30 molecules, For smaller clusters or for large ion pair separations cluster finite-size effects come int o play in a significant fashion. (C) 1996 American Institute of Physic s.