  Acid ionization in aprotic media is studied using Molecular Dynamics
techniques. In particular, models for HCl ionization in acetonitrile and
dimethylsulfoxide are investigated. The proton is treated quantum mechanically
using Feynman path integral methods and the remaining molecules are treated
classically. Quantum effects are shown to be essential for the proper treatment
of the ionization. The potential of mean force 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 reasonable agreement with experimental results. Solvent separated
ion pairs are found to exist in dimethylsulfoxide but not in acetonitrile.
Dissociation 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 thirty molecules. For smaller clusters or
for large ion pair separations cluster finite-size effects come into play in a
significant fashion.
