Quantitative Spectra–Structure Relations for Borohydrides

Abstract

Among the different potential hydrogen storage materials, borohydrides have been largely investigated because of their high gravimetric and volumetric hydrogen content. In the analysis of borohydrides, vibrational spectroscopy plays an important role since it gives information on the local structure of the BH4– ion inside the solid. Here the GF method, developed by Wilson, is used in order to determine the local symmetry of BH4– in solid borohydrides starting from their vibrational spectra. Two different cases of deformations of BH4– are considered. In the first case, the effects of small angular variations on the vibrational spectra of borohydrides will be taken into account; starting from the splitting of the bands corresponding to the deformation modes, the angular deformations will be estimated. In the second one, the BH4– under chemical pressure (in different cubic alkali halides) is considered; in this case, the symmetry of the BH4– remains Td, while the bond lengths change according to the pressure experienced. Different practical examples will be illustrated

Correlating Boron–Hydrogen Stretching Frequencies with Boron–Hydrogen Bond Lengths in Closoboranes: An Approach Using DFT Calculations

“…It is interesting to note that the magnitude of this value is significantly larger than the one obtained for in cubic crystals (14.7 cm −1 /pm) …”

“…In contrast, the empirical correlation between frequency and bond length of ions in cubic crystals yielded the expression …”

“…In a previous study, we have determined quantitative vibrational spectra‐structure relations for borohybrides and, in particular, we have established a semi‐empirical correlation between B–H stretching IR frequencies and B–H bond lengths for tetrahedral ions (diluted) in cubic crystals . In this correlation, the strong Fermi resonances which appear for B–H stretching vibrations are implicitly included …”

“…[16] In a previous study, we have determined quantitative vibrational spectra-structure relations for borohybrides and, in particular, we have established a semi-empirical correlation between B-H stretching IR frequencies and B-H bond lengths for tetrahedral BH À 4 ions (diluted) in cubic crystals. [17] In this correlation, the strong Fermi resonances which appear for B-H stretching vibrations are implicitly included. [18] In this work, we investigate the correlation between the stretching frequency and the length of the B-H bond from the results of DFT calculations on the closoboboranes B x H 2À…”

Halide Free M(BH4)2 (M = Sr, Ba, and Eu) Synthesis, Structure, and Decomposition

“…As all 4 B−H bond lengths are very similar, we have applied the GF method to calculate the splitting of the deformation modes using the bond angles from the optimized DFT structure. 59 The ν 2 deformation modes are 35,36 and CaCl 2 type (rutile-like) Ba(BH 4 ) 2 transforms at 395°C also into this polymorph. Rutile to α-PbO 2 type is a very well-known transition which may for example be induced in rutile by applying a pressure.…”