High-level ab initio studies of the electronic excited states of the hydroxyl radical and water–hydroxyl complex

The lowest-energy electronic transitions in the hydroxyl radical and the hydrogen bound complex $H2O⋅HO$ have been studied using ab initio methods. We have used the complete active-space self-consistent field and multireference configuration interaction (MRCI) methods to calculate vertical excitation energies and oscillator strengths. At the MRCI level the lowest-lying $Σ+2←2Π$ electronic transition is redshifted by about 2500 cm⁻¹ upon formation of the $H2O⋅HO$ complex. We propose that this transition could be used to identify the complex in the gas phase, which in turn could be used to examine the role of $H2O⋅HO$ in atmospheric reactions.