Theoretical Study of the Microsolvation of the Bromide Anion in Water, Methanol, and Acetonitrile:  Ion−Solvent vs Solvent−Solvent Interactions

Abstract

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.

Development of first-principles interaction model potentials. An application to the study of the bromide hydration

“…The geometries of trimers and tetramers were taken from a previous quantum-mechanical work 68 concerning the microsolvation of the bromide anion. They present water molecular geometries distorted by the presence of the ion.…”

“…87 Nevertheless, the influence of these small variations on the results of the Monte Carlo simulations and numerical minimizations is not clear, so we will proceed to check it. In a previous work 68 dealing with the microsolvation of the bromide ion in different solvents, a…”

“…31,32,[41][42][43]68 Moreover, the number of possible structures increases with the size of the cluster, 30,68,91 due to the significant role played by the water-water interactions. In order to guarantee an adequate prospecting, numerical energy minimizations using different strategies were carried out.…”

“…On one hand, random geometries generated by heating the system were considered. On the other hand, starting points close to the quantum-mechanical minima 68 were also considered. Comparison between results calculated using the interaction potentials developed and ab initio methods.…”

Formation of Large Molecular Cluster Anions and Elucidation of Their Electronic Structures

“…104 Furthermore, the previous experimental and theoretical studies have shown that 9-11 CH 3 CN molecules can nearly complete the first solvation layer for halide ions. 31,116 In the first solvation layer, the strong direct ion (or excess electron)-solvent interaction results in efficient anion (or excess electron) stabilization. In contrast, in the second (and larger) layer(s), solvent molecules do not directly interact with the core charge and hence provide no further substantial stabilization for the anion or excess electron.…”