The structure and internal dynamics of CO–CO–H2O determined by microwave spectroscopy

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Abstract
The rotational spectra of CO–CO–H2O, CO–CO–HDO, 13CO–CO–H2O, and 13CO–13CO–H2O have been measured using a pulsed‐molecular‐beam Fabry–Perot Fourier‐transform microwave spectrometer. The complex exhibits internal motion involving an exchange of the CO subunits as well as an hydrogen exchange. In the normal species this is indicated in the spectrum by transition doublets separated by a few hundred kHz and an effective shift of alternating transitions which prevents a good semirigid rotor fit. The other isotopically substituted complexes have spectra in which the transitions are either singlet, doublet or quartets depending on the appropriate spin weights or because of dampening of the internal motion. All the spectra are mutually consistent with a tunneling path with four isoenergetic states. By treating the tunneling frequency of the CO interchange as a vibrational frequency, the rotational constants of two internal rotor states and a tunneling frequency could be determined. The tunneling frequency in CO–CO–H2O is 372 kHz and the ground state rotational constants are A=4294.683(70) MHz, B=1685.399(35) MHz, C=1205.532(35) MHz. The tunneling frequency corresponding to the hydrogen exchange is not determined but the observed transition splittings are comparable to those found for other van der Waals complexes containing a water subunit. The dipole moments determined for CO–CO–HDO are μa=4.790(87)×10−30 C m [1.436(26) D], μb=1.79(12)×10−30 C m [0.533(35) D], and μc=1.10(37)×10−30 C m [0.33(11) D]. The general structure of the complex is found to be cyclic. The CO–CO configuration is approximately T‐shaped with the carbon atom of one subunit directed toward the molecular axis of the other subunit. The H2O subunit has a hydrogen atom directed toward the CO subunits but not in the expected linear hydrogen bonded configuration. The uncertainties given in parentheses are one standard deviation.