Infrared Spectra of Ammonium Carbamate and Deuteroammonium Carbamate

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Abstract
Infrared spectra of thin solid films formed by the condensation of gaseous mixtures of NH3 or ND3 and CO2 at low temperatures were obtained in the region from 4000 to 400 cm—1 at —25°, —78°, and —190°C. Two types of spectra were obtained, depending upon the temperature of formation of the solid. Type I, formed at —78°C, was assumed to be a supercooled glassy solid, but the possibility of a molecular complex exists and is discussed briefly. Type II solidspectra were satisfactorily interpreted on the basis of ammonium and carbamate ions being present in the crystal. Vibrational assignments are made for the carbamate ion assuming over‐all symmetry of Cs for the ion. The heavy atom skeletal vibrations of the carbamate ion are discussed in terms of a plane symmetric XY3 model distorted to C 2v symmetry. Approximate force constants are calculated for the two ends of the ion assuming the NH2 group behaves as a triatomic molecule and the NCO2 skeleton vibrates as a ZXY2 molecule with C 2v symmetry. To aid in the assignments of the carbamate ion the Teller—Redlich product ratios are calculated. A complete vibrational assignment for the ammonium ion is not attempted, but the position and number of bands associated with this species indicate the possibility of several strong hydrogen bonds of different strength existing in the solid. The instability of ammonium carbamate is attributed to a low potential barrier for the transfer of a proton from the NH4 + to the NH2CO2 — ion, rather than a weak bond in either species.