Infrared spectrum of the water–carbon monoxide complex in the CO stretching region

The CO-stretching vibration–rotation spectra of water–carbon monoxide complexes (${\mathrm{H}}_{\mathrm{2}}\mathrm{O–CO},$ ${\mathrm{D}}_{\mathrm{2}}\mathrm{O–CO},$ and HDO–CO) in the 2150 cm⁻¹ region have been studied in a supersonic slit jet expansion using a rapid-scan tunable diode laser spectrometer. Accurate band origins and rotational parameters were determined in order to characterize the CO stretching dependence of the bonding in this system. The symmetric isotopes, ${\mathrm{H}}_{\mathrm{2}}\mathrm{O–CO}$ and ${\mathrm{D}}_{\mathrm{2}}\mathrm{O–CO},$ exhibited two distinct nuclear spin components arising from proton- (deuteron-) exchange tunneling, as observed previously in the microwave spectrum. The tunneling splittings were found to increase by about 10% upon excitation of the CO stretching vibration, indicating a lowering of the tunneling barrier in the excited state. The D-bonded isomer, HOD–CO, of the unsymmetrical isotope was observed prominently, and the higher-energy H-bonded isomer, DOH–CO, was also detected for the first time. From the intensities of their spectra, the energy difference between these two forms of the complex was estimated to be $\text{12.4±2.5\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}.$