Specifically labeled NaBD₃H has been synthesized and characterized using X-ray diffraction, NMR, and vibrational spectroscopy. The isotopic purity of the compound, as estimated from NMR spectra, was found to be about 85% with the compound NaBD₂H₂ as the second product. IR spectra confirm the relatively strong intensity of the single B–H stretching mode predicted from DFT calculations. Anharmonic DFT calculations show that for the BD₃H^- ion Fermi resonances with the single B–H stretching mode are very limited, making this mode a promising structural probe for complex borohydrides which can be prepared by metathetical reactions.

Vibrational spectra of BH₄^- and its isotopic analogues in a crystalline environment of alkali metals cations (K⁺, Rb⁺, Cs⁺) have been investigated beyond the harmonic approximation using a variational approach supported by computations of B3LYP type anharmonic force fields. From the comparison of the observed and simulated IR spectra, the influence of the anharmonic couplings on the band position and on the relative intensity of the allowed vibrational transitions is discussed. Here, the effect of the crystalline environment induces a blue shift of about 50 and 100 cm^-1 respectively for the bending and stretching modes of BH₄^-. Furthermore, anharmonic effects, which are exclusively well reproduced by a variational approach, are needed to yield reliable positions and relative amplitudes of IR allowed combination and overtone transitions. This leads to theoretical results fitting their experimental counterpart between 6 and 30 cm^-1 in the investigated series.