The equilibrium geometries and corresponding binding energies of the different electronic states (ground, excited and ionic) of p-difluorobenzene-(H2O)(n=1-3) clusters have been determined using a semiempirical perturbation exchange theory model for intermolecular interactions. The theoretical results correctly reproduce the spectroscopic properties of these clusters and allow interpretation of experimental observations. The calculated energetics can then be compared to experimental results in order to understand the role of the cluster size in the ionic nucleophilic substitution reaction.