Existing rototranslational collision-induced absorption (CIA) spectra of methane pairs are analyzed with the help of spectral profiles computed from quantum mechanics. Dipoles induced by octopolar and hexadecapolar fields, hexadecapolar overlap, and the gradient of the octopolar field are considered. The spectral contributions of both bound and free pairs of molecules are accounted for. The analysis which suggests a centrifugal distortion of rotating methane molecules permits one to reproduce from theory the measured CIA spectra at all temperatures (126–300 K) and over the full range of frequencies (> 700 cm−1 at high temperatures) with rms deviations that are smaller than the experimental uncertainties. The values of the octopole and hexadecapole moments of (nonrotating) CH4 molecules needed for that purpose are consistent with state-of-the-art ab initio computations for the first time in such work.
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The collision-induced spectra of C2N2 gas and a gaseous mixture of C2Nl and Ar at 298 K have been obtained in the spectral region below 120 cm-1 using far-infrared laser and microwave techniques as well as a Fourier-transform spectrometer. In addition, the collision-induced spectra of a gaseous mixture of CO2 and Ar are reported at temperatures of 233 and 298 K in the spectral region below 230 cm-1. The theoretical values for the spectral moments α1 and γ1 for CO2 are much smaller than the experimental values, as expected for a molecule with a relatively large quadrupole moment. However, for CO2-Ar mixtures, the agreement between the theoretically and experimentally determined spectral moments is relatively good, resulting in a value of 4.6 B for the quadrupole moment of CO2 instead of the generally accepted value of 4.3 B. The quadrupole moment of C2N, is estimated to be 6.2 ± 0.4 B from our data and the theory for the spectral moments, if a correction is made for an overestimate of the quadrupole moment similar to that obtained for the CO2-Ar mixture. This value is considerably smaller than a previously reported calculated result of 9.0 B. Line-shape expressions based on information theory (IT6) do not yield good agreement with experiment, a result that is attributed to the large anisotropy of the molecules.
