Ab initio study on the electronic structure and vibration modes of alkali and alkaline-earth amides and alanates

We study the electronic structure and vibrational modes of several amides M(NH2)(n) and alanates M(AlH4)(n) (M = K, Na, Li, Ca and Mg), focusing on the role of cation states. Calculated breathing stretching vibration modes for these compounds are compared with measured infrared and Raman spectra. In the amides, we find a significant tendency such that the breathing mode frequencies and the structural parameters of NH2 vary in accordance with the ionization energy of cation. The tendency may be explained by the strength in hybridization between cation orbitals and molecular orbitals of (NH2)(-). The microscopic mechanism of correlations between the vibration frequencies and structural parameters is elucidated in relation to the electronic structure. A possible similar tendency in the alanates is also discussed.