Structure of NaI ion pairs in water clusters

Abstract

The structural properties of the thermodynamically stable NaI salt ion pairs in water clusters have been investigated by means of Monte Carlo simulations with model potentials. Attention was also paid to the structure of single ion–water Na +(H 2O) n and I −(H 2O) n clusters, which are found to be non-spherically symmetric at room temperature. In agreement with earlier studies, I −(H 2O) n clusters exhibit surface structures, with the “hydrophobic” iodide ion sitting at the surface of a water network, while room-temperature Na +(H 2O) n clusters exhibit a solvation shell structure, where solvent molecules beyond the first solvation shell tend to accumulate on one side of the cluster instead of forming a spherical droplet. Both “contact” ion pairs (CIP) and solvent-separated ion pairs (SSIP) are found to have surface structures for the smaller clusters while both interior and surface structures may exist at room temperature for cluster size of 32. A remarkable feature of the ion pair cluster structural properties is that they are very much akin to those for individual ion–water clusters, especially for SSIPs, and some insight into the ion pair cluster structures can thus be gained from single ion–water cluster structures. We propose that the (small) extent of solvent–solvent hydrogen bonding and the magnitude of the (large) solvent dipole moments in the clusters can be used to illustrate the extent of the perturbation introduced by the ions or the ion pairs in the solvent environments. In contrast to the ion pair free energetics investigated in previous work, ion–water and salt-water cluster structural properties are rather insensitive to the choice of model potentials, whether one employs non-polarizable optimized potentials for liquid simulations (OPLS) such as TIP4P/OPLS or polarizable optimized potentials for cluster simulations. The structure of NaI(H 2O) n CIP and SSIP clusters have implications for the NaI(H 2O) n cluster photodissociation dynamics. The large solvent dipole moments obtained for NaI(H 2O) n clusters are indicative of increasingly larger local solvent dipoles in the clusters, which may then grow large enough to dipole-bind an electron upon cluster photoexcitation. Photoexcitation of the larger clusters might then proceed via a different route than it does for the small clusters and isolated NaI possibly involving a charge-transfer-to-solvent excited state akin to that of I −(H 2O) n clusters. For the NaI photodissociation pathway, the surface structure of the small NaI(H 2O) n clusters may imply a slow change in the reaction dynamics with cluster size.