Infrared Spectra and Vapor Pressure Isotope Effect of Crystallized Ammonia and Its Deuterium Derivatives

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Abstract
We have reinvestigated the infrared spectra of crystalline NH3, NH2D, ND2H, and NH3, which were throughly studied by Reding and Hornig in 1951 and 1955. The antisymmetric stretching bands of NH3 and ND3 are found to be split. Their shifts relative to the values for the gas phase found to be approximately equal by the previous measurements have now been shown to be distinctly different. The antisymmetric stretching vibrations of NH2D and ND2H are observed separately from those of NH3 and ND3. The ND stretching vibration of NH2D consists of a triplet composed of the unperturbed vibrational center band and of two satellite bands due to correlation field splitting. It has been derived from this splitting that intermolecular coupling between the ammonia molecules is weak. The equality of the NH and ND stretching force constants in NH2D and ND2H as well as in NH3 and ND3 follows from two facts: (1) The NH stretching vibration of ND2H lies in the middle of the NH2 stretching vibrations of NH2D, also the ND stretching vibration of NH2D lies in the middle of the ND2 stretching vibrations of ND2H; (2) only single NH and ND stretching bands are observed. Therefore, the unequal shifts of the symmetric and of the antisymmetric stretching vibrations of NH3 and ND3 relative to the values observed in the gas phase cannot be explained by an asymmetric change of stretching force constants during crystallization. They must be explained by Fermi resonance of the symmetric stretching vibration with the parallel component of the overtone of the deformational vibration. The calculation of the vapor pressure isotope effect of crystalline NH3 and ND3 by using the corrected vibrational frequencies shows further evidence in support of these explanations.