Hydrogen bonding between the water molecule and the hydroxyl radical (H2O⋅OH): The 2A″ and 2A′ minima

Abstract

The two degenerate components of the OH radical ²Π 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 (∼2.1 Å for ²A‘, ∼2.2 Å for ²A’) are significantly longer than observed for the water dimer. Preliminary estimates of the H₂O⋅OH vibrational frequencies are made.