Theoretical study of the microsolvation of the bromide anion in water, methanol, and acetonitrile:: Ion-solvent vs solvent-solvent interactions

In this paper a theoretical study of the bromide solvation in three different polar solvents is presented: water, methanol, and acetonitrile. DFT (B3LYP) calculations on the structure and energetics of [Br(Solv)(n)](-) clusters, for n = 1-9 and Solv = H2O, CH3OH, and CH3CN, have been carried out. Different structures where the anion is placed inside or on the surface of the cluster have been explored. The relative importance of solvent-solvent vs ion-solvent interactions determines the geometrical distribution of the microsolvation. Aggregates of solvent molecules within the bromide clusters are more defined in the case of water. Methanolated bromide clusters show a defined trend to place some solvent molecule at the second solvation shell. The bigger acetonitrile complexes (n > 5) are the more representative cases of interior complexes where the solvent molecules surround quite symmetrically the bromide anion whereas, in water and methanol, the microsolvation is more compromised between bromide-solvent and solvent-solvent interactions, then favoring arrangements with the ion on the surface of the cluster, particularly for n < 5. To rationalize the key components of the microsolvation, ion-solvent and solvent-solvent interaction energies have been decomposed in terms of two-body, three-body, and four-body contributions. Three-body terms are important for methanol and acetonitrile clusters due to the bromide-solvent contribution, whereas for aqueous clusters a significant cancellation between bromide-water and water-water interactions largely reduces the total three-body component.