Temperature dependence of the infrared spectrum of ammonia borane: Librations, rotations, and molecular vibrations

The absorbance of solid ammonia borane (AB) was measured in the energy range between 30 and 5000 cm-1 and in the temperature range between 10 and 300 K. The intramolecular vibrations and their evolution through the structural phase transition around Tt ? 220 K fairly agree with previous measurements performed by means of Raman spectroscopy. In addition, we observed new vibrations centered in the far-infrared range, which can be tentatively ascribed to rotations and librations. The number of such modes does not agree with the calculations based on the group theory for both the tetragonal and the orthorhombic crystal structure of AB. We suggest that such a discrepancy is due to local reduction of the crystal symmetry compared to the one reported by X-ray diffraction and used to compute the number of IR active modes.