The coordination of Sr2+ by hydroxide: a density functional theoretical study

The molecular structures of gas-phase strontium hydroxide complexes are quantum chemically calculated using density functional theory, and the effects of hydroxyl groups on strontium coordination are studied. It is found that the presence of a single hydroxyl group results in the near-degeneracy of complexes with a coordination number (CN) of 5, 6 and 7. The presence of a second hydroxyl group destabilises the heptacoordinated complexes, and marks the onset of a weakening of the Sr-O(H(2)O) bonds, as evidenced by analysis via the quantum theory of atoms in molecules (QTAIM) and measurements of the average angle between the Sr-O(H(2)O) bond and the H(2)O dipole moment. A third hydroxyl group strongly destabilises both CN = 6 and 7 complexes through significant weakening of the Sr-O(H(2)O) interaction; here, hydrogen bonding interactions between hydroxyl groups and water molecules begin to dominate. The tetrahydroxide complex is found to be electronically unstable in the gas phase, but can be stabilised by coordination of explicit water molecules. Replacement of the explicit water molecules by a continuum solvation model poorly reproduces the polarisation of the wavefunction by the explicit solvent, suggesting that a combined approach incorporating both explicit solvation and a continuum model is required for the accurate modelling of this dianionic complex.