Microwave investigation of the CO-CH4 van der Waals complex

Rotational spectra of eight isotopomers of the weakly bound van der Waals complex CO-CH4 were recorded in the frequency range from 4 to 19 GHz using a pulsed molecular beam Fourier transform microwave spectrometer. For the isotopomers containing methane monomers of T-d symmetry, namely, (CO)-C-12-O-16-(CH4)-C-12, (CO)-C-12-O-16-(CH4)-C-13, (CO)-C-12-O-16-(CD4)-C-12, (CO)-C-13-O-16-(CH4)-C-12, and (CO)-C-13-O-18-(CH4)-C-12, three rotational progressions were observed that correlate to the j(m)=0, 1, and 2 rotational levels of free methane. For those containing partially deuterated methane monomers with C-3V symmetry, namely, (CO)-C-12-O-16-(CH3D)-C-12 and (CO)-C-12-O-16-(CHD3)-C-12, only two progressions were recorded, correlating to the j(k)=0(0) and 1(1) rotational levels of free CH3D and CHD3, respectively. The van der Waals bond distance R, intermolecular stretching frequency nu(s), and the corresponding stretching force constant k(s) were derived from the obtained spectroscopic results. The results obtained for the j(m)=0 ground state are compared to the previous infrared and millimeter wave data. A O-17 nuclear quadrupole coupling constant was determined from the resolved hyperfine structure of (CO)-C-13-O-17-(CH4)-C-12 and was used to obtain angular information about the carbon monoxide subunit. A Coriolis interaction was deduced from the irregular spectral pattern involving levels with j(m)=1. Qualitative information about the extent of the perturbation was obtained from a comparison of spectroscopic constants of different isotopomers. (C) 2004 American Institute of Physics.