Far-Infrared Spectra of HCl Trapped in the β-Quinol Clathrate

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Abstract
To investigate the rotational and translational states of HCl trapped in the β‐quinol clathrate, the clathrate dielectric response has been measured in the spectral region between 10 and 100 cm−1, at liquid‐helium temperatures. The attenuation coefficient of dilute mixtures of 40μ clathrate powder in paraffin was measured with a slowly modulated lamellar grating interferometer. By making suitable assumptions about the particle geometry, it is possible to extract, by an analysis based on the Hilbert transform, both real and imaginary parts of an effective dielectric constant for the suspended powder. Inherent in the technique are corrections for dielectric mismatch between the powder and paraffin. To help interpret the spectra, clathrates of SO2, HBr, N2, and a mixture of HCl, DCl have also been measured. At 1.2°K, the HCl clathrate has three salient features: an absorption due to the β‐quinol lattice at 66 cm−1, a translational vibration of the HCl dipole near 52 cm−1, and in the vicinity of 20 cm−1 a resonance absorption due to the rotational motion of the trapped dipole. The translational vibration is consistent with cage‐wall interactions described by van der Waals forces. Both the intensity of this absorption and the shift of the rotational resonance 3 cm−1 below the gas‐phase value of 21 cm−1, at the lowest concentration of HCl can be consistently explained by a coupling between the rotation and translational vibration. At high concentrations of HCl, the rotational resonance increases by nearly 50% and is commensurate with shifts predicted by dipole—dipole interactions between solute molecules.

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