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 Fo urier-transform microwave spectrometers. Apart from the parent H2O-CO2 complex, the isotopomers HDO-CO2, D2O-CO2 and H2O-(COO)-O-16-O-18 hav e been investigated. The rotational spectra show a splitting of the en ergy levels from internal rotation between the water and carbon dioxid e molecules around the van der Waals bond of the complex. As a consequ ence of the O-16 spin statistics in carbon dioxide only half of the en ergy levels are populated except for the H2O-(COO)-O-16-O-18 species. Hyperfine splittings of the rotational transitions due to the hydrogen and deuterium nuclei have been resolved. Rotational and centrifugal d istortion constants have been fitted to the measured transition freque ncies 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 a ttributed to the spin-spin interaction of the hydrogen nuclei. A barri er 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 abou t its C-2 symmetry axis. The van der Waals bond length has been allowe d to vary during the least-squares fit of internal rotation parameters . Structural parameters of the complex have been estimated from the mo ments of inertia and their differences between isotopomers.