New Infrared Spectra of the Water-CO Complex: Determination of Four Intermolecular Modes and Test of a High-Level Potential Energy Surface.

High resolution infrared spectra of water-CO dimers are further studied using tunable infrared sources to probe a pulsed slit jet supersonic expansion. The relatively weak transition of DO-CO in the DO ν fundamental region (≈2760 cm) is observed for the first time, as are various spectra of DO-CO. Combination bands involving the intermolecular in plane geared bend (disrotatory) mode are observed for HO-CO (≈1642, 2397 cm) in the HO ν and CO ν regions, for HDO-CO (≈2761 cm) in the HDO ν region, and for DO-CO (≈2386, 2705 and 2821 cm) in the CO ν, DO ν, and DO ν regions. A combination band involving the intermolecular in plane antigeared bend (conrotatory) mode is observed for DO-CO (≈2425 cm) in the CO ν region. And finally, a combination band involving the intermolecular twist (internal rotation) mode is observed for DO-CO (≈2874 cm) in the DO ν region. But this twist transition actually appears as two bands of similar intensity, separated by 2 cm, suggesting an "accidental" near-coincidence of the "real" combination state with a "dark" background state of the same symmetry. Intermolecular mode frequencies determined from the combination bands are in very good agreement with a recent theoretical calculation based on a high-level ab initio potential surface.