The Infra-Red Absorption Spectra of Some Metal Borohydrides

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Abstract
The absorption spectra of aluminum, lithium, and sodium borohydride have been obtained in the region from 1–25μ. The aluminum borohydride was investigated as gas and an analysis of the infra-red and Raman bands fits a bridge-type structure of symmetry D3 (probably D3h). The spectra of lithium and sodium borohydrides were obtained from finely ground slurries of these materials. Their spectra indicate how the bridge structure of the H2BH2 metal groups passes over to the tetrahedral structure in a completely ionic lattic. Using certain bond-distance force-constant relationships in conjunction with data from the diatomic hydrides and accepting electron and x-ray diffraction data for bond distances between the heavier atoms, values for the geometrical parameters are calculated for beryllium, aluminum, and sodium borohydride. These seem more plausible than any so far suggested. Observations on the electronic spectrum of aluminum borohydride have also been made down to 1000A. The diffuseness of the absorption obtained at short wave-lengths prevented any detailed interpretation. The spectrum of a compound formed between beryllium borohydride and sodium chloride has also been obtained and a consideration of its absorption bands has given information concerning the nature of this material. A spectrum of a slurry of the methyl derivative of lithium borohydride (LiBH3CH3) indicates considerable change in the structure relative to that of lithium borohydride. A table of frequencies of related bands in the metal and other borohydrides is given which illustrates the changes referred to above. It includes a number of boron-nitrogen hydrides and shows that the BH frequency, when it is associated with analogous electronic groups, behaves in much the same way as that of CH when it is attached to saturated, olefinic or aromatic groups.

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