The Vibrational Spectra of Molecules and Complex Ions in Crystals. V. Ammonia and Deutero-Ammonia

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Abstract
Infrared spectra of thin non-scattering films of NH3 and ND3 were obtained at −190°C. All four fundamentals of both compounds were observed with no evidence of inversion doubling of ν2. The crystal spectrum of NH3 suggests a frequency near 3450 cm−1 for ν3 in the gas. The spectra are in agreement with a molecular crystal having the reported x-ray symmetry T4. Two of the five torsional lattice vibrations were probably observed directly and one more strongly indicated in combination bands. A fourth lattice frequency of 53 cm−1 also occurs in combination bands. Since the coupling between the various internal vibrations and between the lattice and internal vibrations proved to be small, the potential function of the crystal was assumed to be separable into an internal and a lattice problem. A normal coordinate treatment of the internal vibrations fits all frequencies to better than one percent.