Lattice frequencies and rotational barriers for inorganic carbonates and nitrates from low temperature infrared spectroscopy

Abstract
Infrared absorption spectra of inorganic nitrates and carbonates have been obtained on single crystals at room temperature, under liquid nitrogen refrigeration and under liquid helium refrigeration. Diffuse absorption observed in the room temperature spectra between about 1400 cm−1 and 700 cm−1 is resolved under liquid helium refrigeration into a large number of bands not attributable to fundamentals or combinations of fundamental frequencies of the anions. Almost all of these bands are readily interpreted as summation bands of fundamental frequencies with successive levels of a librating oscillator. The libration is considered to represent a planar torsional oscillation of the anion about the trigonal axis. The librational frequency depends in part on the crystal structure and on the ions. In a given material the librational frequency as determined from successive bands is reasonably constant. In the substances studied the librational frequency varied from about 15 cm−1 to 30 cm−1.

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