Infrared Spectra of the Water-CO 2 Complex in the 4.3-3.6 μm Region and Determination of the Ground State Tunneling Splitting for HDO-CO 2 .

Spectra of water─CO 2 dimers are studied using a tunable mid-infrared source to probe a pulsed slit jet supersonic expansion. H 2 O-CO 2 and D 2 O-CO 2 are observed in the CO 2 ν 3 fundamental region (≈2350 cm -1 ), D 2 O-CO 2 is also observed in the D 2 O ν 3 fundamental region (≈2790 cm -1 ), and HDO-CO 2 is observed in the HDO O-D stretch fundamental region (≈2720 cm -1 ), all for the first time in these regions. Analysis of the spectra yields excited state rotational parameters and vibrational shifts. They also yield the first experimental values of the ground state internal rotation tunneling splittings for D 2 O-CO 2 (0.003 cm -1 ) and HDO-CO 2 (0.0234 cm -1 ). The latter value is a direct determination made possible by the reduced symmetry of HDO-CO 2 . These results provide stringent and easily interpreted tests for theoretical water-CO 2 potential energy surface calculations.