Theoretical Characterization of Oxoanion, XO_m^n-, Solvation

We describe a new cavity definition protocol that yields accurate solvation energies and electrode potentials for selected oxoanions, XO_m^n-, including some for which other cavity protocols do not perform well enough. In this new definition scheme with cavities made of interlocked atomic spheres, the radii are given by simple empirically based expressions involving effective atomic charges of the solute atoms that fit the solute molecular electrostatic potential and a bond length-dependent factor to account for atomic size and hybridization. The scheme shows substantial qualitative differences from other previously proposed schemes, for example, by assigning a large radius to the central atom of the oxoanions. This difference is put on a firm theoretical basis in the case of NO₃^- through an analysis of the molecular electrostatic potential of the nitrate ion and an analysis of its interaction with a “solvent” water molecule. Despite a large positive partial charge assigned to nitrogen in the nitrate ion, the water solvent molecule continues to act as an H-bond donor in the region of the central N atom as a result of the electrostatic potential of the anion, although the water−nitrate interaction in that region is weaker than near the terminal O atoms. From these results, we surmise that the solvent molecules remain further away from the nitrogen atom, a finding that is consistent with the large radius assigned by the new scheme for nitrogen. The same qualitative feature holds true for all of the oxoanions considered here.