HYDROGEN-BONDING BETWEEN THE WATER MOLECULE AND THE HYDROXYL RADICAL (H2O.OH) - THE 2A'' AND 2A' MINIMA

The two degenerate components of the OH radical 2-PI ground electronic state give rise to independent minima (of 2A" and 2A' symmetries) upon interaction with the water molecule. These two minima have been investigated here for the first time using ab initio quantum mechanical methods. Minimum, double zeta, double zeta plus polarization, and triple zeta plus double polarization basis sets have been employed in conjunction with self-consistent-field, second-order perturbation, and configuration interaction methods. At all levels of theory, the 2A" state is predicted to be the global minimum, with a dissociation energy D(e) of about 3.5 kcal/mol. The 2A' state is predicted to lie about 1 kcal higher in energy. Both minima occur for structures with OH...O linkages close to linear and are reminiscent of the water dimer. However, the H...O distances (approximately 2.1 angstrom for 2A", approximately 2.2 angstrom for 2A') are significantly longer than observed for the water dimer. Preliminary estimates of the H2O.OH vibrational frequencies are made.