Hemibonding between Hydroxyl Radical and Water

The ultraviolet absorption peak commonly used to identify OH radical in liquid water is mainly due to a charge-transfer-from-solvent transition that is prominent when OH is hemibonded, rather than more stable hydrogen bonded, to H2O. This work computationally characterizes the hemibonding interaction and the extent of the geometrical region over which it is significant. Hemibonding is found to be associated with an enlarged energy separation between the two lowest-lying electronic states, which are otherwise always quite close to one another. The lower state, wherein the hemibonding occurs, retains an attractive interaction energy between OH and H2O that can be as much as one-half as strong as the optimum hydrogen-bonding interaction, while the enlarged separation between the states is mainly due to the upper state becoming repulsive over most of the hemibonding region. Hemibonding also leads to a considerable drop in the energy and a considerable increase in the oscillator strength of the characteristic charge-transfer transition. The region of significant hemibonding is found to lie within a moderate range of O-O azimuthal angles and over quite wide ranges of O-O separation distances and hydroxyl OH tilt angles. In particular, significant hemibonding interactions can extend down to surprisingly short O-O distances, where the oscillator strength for the charge-transfer-from-solvent transition becomes very large.