Inelastic neutron scattering spectra of NaBH4 and KBH4: reproduction of anion mode shifts via periodic DFT

Inelastic neutron scattering (INS) spectra of NaBH4 and KBH4 are presented to 1500 cm−1. The phonon spectra contain detail unobserved in IR/Raman studies, including combination bands in the phonon and internal modes of KBH4. The BH4− ion vibrational frequencies observed in the solids at 15 K differ significantly from those observed for BH4− in liquid ammonia solution and normal mode frequency calculations for the isolated ion. The INS spectra and ammonia solution frequencies are compared with isolated anion and k=0 periodic DFT calculations to show that the periodic DFT results correctly predict the solid-state molecular vibrational frequencies.