Binding Energies and Structures of NaI−(CH3CN)n=1-9 Clusters: Theoretical Study of the Contact Ion Pair versus the Solvent-Separated Ion Pair Structures in a Molecular Cluster

Abstract

We present a theoretical study of the charge separation of NaI clustered with acetonitrile molecules. A model potential specially devised for the NaI−(CH3CN)n system has been built up according to the exchange perturbation theory as developed by Claverie. The potential energy surface (PES) exploration has been carried out using the Monte Carlo growth method (MCGM) at different fixed internuclear NaI distances, to obtain a minimum energy profile for the NaI bond breaking. From four to nine solvent molecules, the NaI−(CH3CN)n PES exhibits two local minima along the NaI internuclear distance. The first one is related to the contact ion pair (CIP) structure and the second where the two ions are separated by two or three acetonitrile molecules in a solvent-separated ion pair (SSIP) structure. With less than eight solvent molecules, the CIP configurations have the highest binding energies, but for nine acetonitrile molecules, the configurations where the two ions are separated by 7 Å have a higher binding energy than those where the two ions stay in contact (2.85 Å), reflecting an evolution from the CIP to the SSIP structures with the cluster size. This can be related to a recent gas phase photoionization experiment on NaI−(CH3CN)n and leads to the same conclusion: the charge separation of NaI should be achieved within a cluster containing fewer than 10 acetonitrile molecules.