New Infrared Spectra of the Water–CO₂ Complex: Determination of Four Intermolecular Modes and Test of a High-Level Potential Energy Surface

High resolution infrared spectra of water-CO₂ dimers are further studied using tunable infrared sources to probe a pulsed slit jet supersonic expansion. The relatively weak transition of D₂O-CO₂ in the D₂O ν₁ fundamental region (≈2760 cm^-1) is observed for the first time, as are various spectra of D₂O-¹³CO₂. Combination bands involving the intermolecular in plane geared bend (disrotatory) mode are observed for H₂O-CO₂ (≈1642, 2397 cm^-1) in the H₂O ν₂ and CO₂ ν₃ regions, for HDO-CO₂ (≈2761 cm^-1) in the HDO ν₃ region, and for D₂O-CO₂ (≈2386, 2705 and 2821 cm^-1) in the CO₂ ν₃, D₂O ν₁, and D₂O ν₃ regions. A combination band involving the intermolecular in plane antigeared bend (conrotatory) mode is observed for D₂O-¹³CO₂ (≈2425 cm^-1) in the CO₂ ν₃ region. And finally, a combination band involving the intermolecular twist (internal rotation) mode is observed for D₂O-CO₂ (≈2874 cm^-1) in the D₂O ν₃ region. But this twist transition actually appears as two bands of similar intensity, separated by 2 cm^-1, 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.