Internal rotation effects and hyperfine structure in the rotational spectrum of a water–carbon dioxide complex

The microwave spectrum of a T-shaped water-carbon dioxide complex has been recorded and analysed between 4 and 37 GHz using pulsed-nozzle Fourier-transform microwave spectrometers. Apart from the parent H2O–CO2 complex, the isotopomers HDO–CO2, D2O–CO2 and H2O–C16O18O have been investigated. The rotational spectra show a splitting of the 2 energy levels from internal rotation between the water and carbon dioxide molecules around the van der Waals bond of the complex. As a consequence of the 16O spin statistics in carbon dioxide only half of the energy levels are populated except for the H2O–C16O18O species. Hyperfine splittings of the rotational transitions due to the hydrogen and deuterium nuclei have been resolved. Rotational and centrifugal distortion constants have been fitted to the measured transition frequencies for the different internal rotation states. Quadrupole coupling constants have been determined from the deuterium hyperfine splittings. The hydrogen hyperfine splittings for the parent species have been attributed to the spin–spin interaction of the hydrogen nuclei. A barrier equivalent to 285.6(14)cm−1 has been adjusted with the help of a flexible model for the internal rotation in the complex of water about its C2 symmetry axis. The van der Waals bond length has been allowed to vary during the least-squares fit of internal rotation parameters. Structural parameters of the complex have been estimated from the moments of inertia and their differences between isotopomers.