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 = H 2 O, CH 3 OH, and CH 3 CN, 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.