The quadrupole moment of cyanogen: a comparative study of collision-induced absorption in gaseous C2N2, CO2, and mixtures with argon

Abstract

The collision-induced spectra of C2N2 gas and a gaseous mixture of C2N2 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 C2N2 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.