The Influence of Temperature and Pressure on the Infra-Red Absorption Spectrum of Gaseous and Liquid Hydrogen Chloride up to the Critical State

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Abstract
The absorption spectra, near 1.7μ, of liquid HCl from — 100°C to the critical temperature of 52°C, and of gaseous HCl from one atmosphere to the critical pressure of 82 atmospheres, have been measured. The spectrum of the gas retains P and R branches about an origin undisplaced from its low pressure value up to just below the critical pressure, although discreet rotational lines are not recognizable after the density of 0.1 g/cc has been reached. At all temperatures, the transition of gas to liquid is accompanied by the replacement of the P—R branched type of spectrum by a single maximum displaced to low frequencies from the gas origin by an amount increasing from 48 to 117 cm—1 as the temperature is lowered from the critical temperature to — 100°. This band is asymmetric, with a tail to the short wave side, and above 20°C shows signs of being resolved into two bands, one in the position of the gas band. It is concluded that liquid HCl contains two types of molecules, one gas‐like, the other, with the vibration frequency perturbed by intermolecular action. The variation of the perturbed frequency with temperature is partly consistent with the hypothesis that the perturbation originates in the electrostatic action of neighboring molecules, but the magnitude of the shift is smaller by a constant amount than that produced by inert solvents of the same dielectric constant as liquid HCl. The gas a few degrees above the critical temperature and above the critical pressure also contains some liquid‐like molecules.