Investigations on the vibration spectra of certain condensed gases at the temperature of liquid nitrogen II. The infra-red and Raman spectra of hydroand deuterochloric acid at liquid nitrogen temperatures

The infra-red absorption spectrum of solid hydrochloric acid has been obtained at the temperature of liquid nitrogen (i.e. approximately 20° below its transition point), in the region of the fundamental frequency. The results obtained agree in general with the earlier observations of Hettner and disagree with those of Shearin who reported discrete rotation lines. The doublet band observed by Hettner has been resolved into two separate bands with maxima at 2701 and 2744 cm.-1 and indications of a third band were obtained. Some fine structure very different from that reported by Shearin was observed on each of the two bands which were unsymmetrical in shape, being steeper on the low frequency side. The Raman spectrum was obtained at the same temperature and also showed a doublet structure in contrast to earlier observations at higher temperatures which had given only a single line. The maxima of the Raman lines lay at 2709 and 2759 cm.-1. The explanation of this doubling is to be sought in the manner in which the hydrochloric acid molecules associate in the solid state, since the alteration in the fundamental frequency from that found in the gaseous state shows that strong interaction must exist. The absorption due to the fundamental frequency of deuterochloric acid was observed under the same conditions. Here three maxima were obtained at 1965, 1982 and 1992 cm.-1 and indications of more structure. These results show that the association of HCl and DCl molecules in the crystal below the transition temperature can probably take place in several different ways. Such a conclusion is in agreement with the X-ray investigations which revealed a symmetry so low that the crystalline structure could not be determined.