Infrared Spectra and Vapor Pressures of Crystalline C2N2, with Comparisons to Crystalline HCN

In a continuation of our work on nitriles, we have examined cyanogen (C _2 N _2 ) as a crystalline solid, comparing the results to our recent work on crystalline hydrogen cyanide (HCN). A density and refractive index for C _2 N _2 ice were measured and used to prepare solid samples from which infrared (IR) spectra, band strengths, and optical constants were measured. The vapor pressures (sublimation pressures) of both C _2 N _2 and HCN ices were determined with a quartz-crystal microbalance at temperatures relevant to Titan's atmosphere and much lower than those in the literature. Comparisons of the newly measured low-temperature vapor pressures to values extrapolated from higher temperatures revealed differences on the order of 45 and 130%. The enthalpies of sublimation of C _2 N _2 and HCN in the 120 K region were measured and found to be similar to those at higher temperatures. Comparisons were made to previous spectroscopic work, but hindered in the case of IR intensities by the lack of published details.