We report the DFT study of the vibrational spectroscopy properties of Mg(B₃H₈)₂, a potential intermediate in the decomposition of Mg(BH₄)₂, as well as those of CB₁₁H₁₂^− and CB₉H₁₀^−, whose salts can exhibit high ionic conductivities. Because the inclusion of anharmonicity is key to the accurate description of the vibrational properties of BH species [D. Sethio, L. M. Lawson Daku, H. Hagemann. Int. J. Hydrogen Energy, 41 (2016) 6814], the calculations were performed both in the harmonic and in the anharmonic approximation. The IR and Raman spectra of Cs(CB₁₁H₁₂) and Na₂(B₁₀H₁₀) have also been measured. The calculated and experimental spectra are in good agreement. A comparative analysis of the vibrational spectroscopy properties is made for B₃H₈^− and Mg(B₃H₈)₂, B₁₂H₁₂^2− and CB₁₁H₁₂^−, and for B₁₀H₁₀^2− and CB₉H₁₀^−.

The characterization of boron-hydrogen compounds is an active research area which encompasses subjects as diverse as the chemistry and structures of closoboranes or the thermal decomposition mechanism of the borohydrides. Due to their high gravimetric hydrogen content, borohydrides are considered as potential hydrogen storage materials. Their thermal decompositions are multistep processes, for which the intermediate products are not easily identified. To help address this issue, we have extensively investigated the vibrational and NMR properties of 21 relevant B_mHnz− boron-hydrogen species (m = 1–12; n = 1–14; z = 0–2) within density functional theory. We could thus show that the B3LYP-D2 dispersion-corrected hybrid can be used in combination with the large cc-pVTZ basis set for the reliable prediction of the ¹¹B and ¹H NMR spectra of the boron-hydrogen species, and also for the reliable prediction of their IR and Raman spectra while taking into account the anharmonicity of their molecular vibrations.

B₁₂H₁₂^2− can be formed during the thermal decomposition of metal borohydrides (M(BH₄)_x). Halogen ions such as fluoride or chloride can contribute to destabilize the BH₄^− ions. Hydride and fluoride mixed species like B₁₂H_nF_(12−n)^2− will be probable products after hydrogen release from mixed boro-hydride-fluoride (BH_xF_(4−x)^−) or borohydride-borofluoride systems (BH₄^−, BF₄^−). Various number of isomers are possible for B₁₂H_nF_(12−n)^2− (n = 2–11). DFT calculations were performed on isolated ions of all the possible isomers for (n = 0–3, 9–12), using B3LYP functionals and 6-31G(d,p) basis set. Relative stability, vibrational and NMR spectroscopy of these isomers are discussed and compared with available experimental data.

Borohydrides are actively considered as potential hydrogen storage materials. In this context fundamental understanding of breaking and forming B-H bond is essential. Isotope exchange reactions allow isolating some parts of this reaction without introducing major structural or chemical changes. Experiments were performed on Ca(BH₄)₂and Ca(BD₄)₂ as a function of temperature and pressure. A complete exchange can be realized in about 9h at 200 °C using a deuterium pressure of 20 bar. The activation energy, estimated using first order kinetics, for the forward reaction (Ca(BH₄)₂ → Ca(BD₄)₂) was found to be 82.1 ± 2.7 kJ/mol (P = 35 bar) and the one for the backward reaction (Ca(BD₄)₂ → Ca(BH₄)₂) was found to be 98.5 ± 8.3 kJ/mol (P = 35 bar). Pressure dependent study shows that the reaction rate increases with increasing pressure up to 35 bar. This behavior is consistent with first adsorption step prior to diffusion into the solid and isotope exchange according to the scheme described below.