Interpretation of the Infrared Spectra of the Solid Hydroxylammonium Halides

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Abstract
Infrared spectra of sublimed films of hydroxylammonium and deuterohydroxylammonium chlorides, bromides, and iodides have been obtained at 25°, —78°, and —180°C from 4000 to 400 cm—1. Satisfactory vibrational assignments for the hydroxylammonium ion can be made on the basis of an ionic model in which one end of the ion (H3NO) is axially summetric (C3v) and the other end (OH) is of Cs symmetry. A normal coordinate treatment based on a simplified valence force potential with ten constants reproduces 13 of the 18 frequencies of the hydroxylammonium and deuterohydroxylammonium ions to better than 2% and all 18 frequencies to better than 5%. The low NH and OH frequencies are believed to be due to the formal positive charge being distributed over all the atoms in the ion thereby weakening the covalently bonded H atoms and increasing the strengths of the electrostatic hydrogen bonds.