Exploring the Energy Landscape of Hydroxide–Water Complexes

The structures and energies of complexes of hydroxide ions with two to five water molecules are explored at the DLPNO-CCSD(T)(-F12) level of theory. For complexes with two or three water molecules, the most stable structures have all of the water molecules forming hydrogen bonds to the hydroxide ion, oriented so as to form weak water–water hydrogen bonds. For the complex with four water molecules, the lowest energy structures with three and four water molecules bound to the hydroxide ion are nearly isoenergetic, with the complex with three water molecules bound to the hydroxide ion roughly 10 cm–1 lower in energy. This result is consistent with photoelectron and infrared spectroscopy for this ion. For the larger complexes, the structures with four water molecules in the first solvation shell are the lowest in energy, while structures with three water molecules bound to the hydroxide ion and two in the second solvation shell resemble low-energy isomers of (H2O)6. The effect of the introduction of harmonic zero-point energy on the relative energies of these isomers is explored, and the sizes of the shifts in the relative energies with the addition of zero-point energy are attributed to a balance between increases in the frequencies of the intermolecular vibrations and decreases in the frequencies of the OH stretching vibrations with increased hydrogen-bond strength. Finally, we find that DLPNO-CCSD(T)/aug-cc-pVTZ calculations reproduce the relative energies of the isomers of these hydroxide–water complexes evaluated at the complete basis set limit.