Infrared spectra of thin solid films formed by the condensation of gaseous mixtures of NH3or ND3and CO2at low temperatures were obtained in the region from 4000 to 400 cmmdash;1at mdash;25deg;, mdash;78deg;, and mdash;190deg;C. Two types of spectra were obtained, depending upon the temperature of formation of the solid. Type I, formed at mdash;78deg;C, was assumed to be a supercooled glassy solid, but the possibility of a molecular complex exists and is discussed briefly. Type II solid spectra 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 overhyphen;all symmetry ofCsfor the ion. The heavy atom skeletal vibrations of the carbamate ion are discussed in terms of a plane symmetric XY3model distorted toC2vsymmetry. Approximate force constants are calculated for the two ends of the ion assuming the NH2group behaves as a triatomic molecule and the NCO2skeleton vibrates as a ZXY2molecule withC2vsymmetry. To aid in the assignments of the carbamate ion the Tellermdash;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 NH2CO2mdash;ion, rather than a weak bond in either species.
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