Free energetics of NaI contact and solvent-separated ion pairs in water clusters

The thermodynamic stability of NaI salt ion pairs in water clusters has bee n investigated by means of ion pair potential of mean force calculations em ploying Monte Carlo simulations with model potentials and free energy pertu rbation theory. In the simulations the ion pair is described by semiempiric al valence-bond theory, while the water model potentials employed include t he standard liquid-phase TIP4P/OPLS and a polarizable five-site water model that we have developed for cluster simulations. The latter model is parame terized in order to reproduce small cluster experimental data supplemented by ab initio MP2 calculations with a modified 6-31+G** basis (and pseudopot entials for iodine). Simulations with both models yield similar qualitative features for the cluster ion pair potentials of mean force and resulting c luster equilibrium constants, even though they exhibit some quantitative di fferences. A major finding of our theoretical study is that the ion pair is quite stable with respect to dissociation into free ions, even in very lar ge clusters, and an analysis of cluster solvation energies with a simple di electric model suggests that the stability of the ion pairs is in fact rela ted to the very slow convergence of cluster ion solvation energies with inc reasing cluster size, which makes separated cluster ions thermodynamically unlikely. Rather, the ion pairs tend to exist as "contact" ion pairs and so lvent-separated ion pairs in the larger clusters, a feature which is likely to be overemphasized in simulations with the TIP4P/OPLS model potentials, which illustrates the importance of solvent-solvent and solute-solvent pola rization in model potentials. Preliminary ab initio characterization of mod el cluster excited states suggests that NaI(H2O)(n) cluster "contact" ion p airs have optically accessible excited states akin to that of gas-phase NaI , hence making photodissociation experiments feasible, but that electronic transition oscillator strengths significantly decrease for model solvent-se parated ion pairs. As a result, the larger cluster ion pairs, which are mai nly solvent-separated, will not be involved in cluster photodissociation re actions via a mechanism akin to gas-phase NaI photodissociation, in agreeme nt with recent experimental findings.