Microhydration of the selenite dianion: a theoretical study of structures, hydration energies, and electronic stabilities of SeO(3)(2-)(H(2)O)(n) (n = 0-6, 9) clusters

Abstract

In extension of the ongoing investigations of oxyanion-water clusters, we studied energetically low-lying configurations of hydrated selenite dianion (and in select cases, SeO(3)(-)) clusters using density functional theory (B3LYP, M05-2X, PBE0) and second-order Møller-Plesset perturbation theory (MP2). Water molecules doubly hydrogen bond to the selenite oxygens for n <or= 3 and increasingly form singly hydrogen bonds with selenite oxygens upon an increase of the cluster size as water-water interactions gain relative importance as compared to the selenite-water interactions. The calculated average Se-O bond length of 1.69-1.71 A and selenite tetrahedron height of 0.64-0.73 A are in accordance with recent experimental results for selenite in aqueous solution or adsorbed on calcite. Structural perturbations due to the hydration are accompanied by a considerable charge transfer (up to 0.55|e|) from the selenite substructure to the water molecules. Furthermore, the calculated electron binding energies evidence that selenite-water clusters are electronically stable only for n >or= 4 (according to M05-2X) or n >or= 5 (according to B3LYP and PBE0). The hitherto unknown hydration free energy of selenite was calculated using a cluster/continuum approach to fall into the range from -224.6 to -245.5 kcal/mol depending on the applied continuum solvation model.