Infra-Red Absorption of Solid Ammonium Chloride and Ammonium Bromide

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Abstract
According to theoretical considerations based on specific heat, dielectric constant, coefficient of expansion, and other properties, the molecules of the solid ammonium halides, at room temperature, are thought to possess rotatory motion. This rotation is expected not to exist at temperatures below the transition temperature; namely, −30.4°C for the chloride and −37.9°C for the bromide. Molecular rotation, if present, should give rise to rotational fine structure superposed on the infra‐red vibrational bands of the salts. Previous workers found evidence of this, but have not resolved the fine structure. The present investigation was undertaken in the hope of resolving this fine structure, if any, and studying the changes in it resulting from lowering the temperature below the transition point. Fine structure in the 5.6‐μ vibrational band of ammonium chloride and ammonium bromide was resolved. This structure was of the order of magnitude expected if the NH4 group rotating in the molecule were responsible for the fine structure. The fine structure existed practically unaltered to temperatures down to −20°C. There was considerable change between −20°C and −45°C, but then the altered fine structure persisted down to at least −60°C with little additional change.

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