Inelastic neutron scattering spectrum of Cs ₂[B ₁₂H ₁₂]: Reproduction of its solid-state vibrational spectrum by periodic DFT

The inelastic neutron scattering (INS) spectrum of polycrystalline Cs ₂[B ₁₂H ₁₂] is assigned through 1200 cm ^-1 on the basis of aqueous and solid-state Raman/IR measurements and normal mode analyses from solid-state density functional theory. The Cs ⁺ cations are responsible for frequency shifts of the internal cage vibrational modes and I _h cage mode splittings due to the crystal T _h site symmetry. These changes to the [B ₁₂H ₁₂] ^2- molecular modes make isolated-molecule calculations inadequate for use in complete assignments. Solid-state calculations reveal that 30/40 cm ^-1 shifts of T _g/H _g molecular modes are responsible for structure in the INS spectrum unobserved by optical methods or in aqueous solutions.