THE INFRARED ABSORPTION SPECTRA OF MATRIX ISOLATED HCl AND HBr.

The infrared spectra of gaseous HCl isolated in neon, argon, krypton, and xenon matrices have observed in the region of the HCl fundamental frequency at approximately 4, 14, and $20^{\\circ}K$ with a small grating instrument. At high dilution (1:1000) the spectra at $20^{\\circ} K$ for all matrices are nearly the same. They consist of three closely spaced narrow bands in the vicinity of the HCl fundamental and some additional bands at lower frequencies. The three closely spaced bands consist of central strong feature with two weaker ones on either side. On cooling, the matrix from 20 to $4^{\\circ} K$., the intensity of the strong band increases and the two week ones completely disappear. The effect is completely reversible on temperature cycling. There is a progressive shift of the observed bands to the red in going from neon to xenon matrices. The results are very similar for matrix isolated HBr. The experimental results have been interpreted on the basis of a hindered molecular rotation. The matrix isolated molecules are assumed to be confined by a potential barrier arising from the crystal field ion which equilibrium orientation of the axis of the molecule with respect to this field is nearly the same for all cavities. The one strong and weak, temperature dependent, bands are assigned to transitions R(0), P(1) and $R^{\\prime}(1)$ of the hindered rotator with rotational spacing smaller than those for gaseous HCl. The observed red shifts of the R(0) frequency, in different matrices, are related to the magnitude of the intermolecular interactions between the matrix material and the hydrogen halide.