Infrared Spectrum and the Structure of the Low-Temperature Phases of Crystalline HCl, HBr, and HI

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Abstract
The infrared spectra of crystalline HCl, HBr, and HI have been studied at —205°C. The fundamental vibrations were observed in HCl at 2704 and 2746 cm—1, in HBr at 2404 and 2438 cm—1, and in HI at 2120 cm—1. In addition, low‐frequency bands associated with torsional lattice vibrations were observed directly and in combination with the fundamental vibrations of the molecules. It is concluded that in the low‐temperature crystalline phases both HCl and HBr form zigzag hydrogen‐bonded chains, the angle between adjacent molecules being about 107° in HCl and 97° in HBr. The crystalline potential function is investigated for all three molecules.