A theoretical study of the vibrational energy spectrum of the HOCl/HClO system on an accurate ab initio potential energy surface

A new, global analytical potential energy surface is constructed for the X 1A′ electronic ground state of HOCl that accurately includes the HClO isomer. The potential is obtained by using accurate ab initio data from a previously published surface [Skokov et al., J. Chem. Phys. 109, 2662 (1998)], as well as a significant number of new data for the HClO region of the surface at the same multireference configuration interaction, complete basis set limit level of theory. Vibrational energy levels and intensities are computed for both HOCl and HClO up to the OH+Cl dissociation limit and above the isomerization barrier. After making only minor adjustments to the ab initio surface, the errors with respect to experiment for HOCl are generally within a few cm−1 for 22 vibrational levels with the largest error being 26 cm−1. A total of 813 bound vibrational states are calculated for HOCl. The HClO potential well supports 57 localized states, of which only the first 3 are bound. The strongest dipole transitions for HClO were computed for the fundamentals—33, 2.9, and 25 km/mol for ν1, ν2, and ν3, respectively. From exact J=1 ro-vibrational calculations, state dependent rotational constants have been calculated for HClO. Lastly, resonance calculations with the new potential demonstrate that the presence of the HClO minimum has a negligible effect on the resonance states of HOCl near the dissociation threshold due to the relatively high and wide isomerization barrier.