Abstract
The hindered rotational energy levels of the borohydride ion in the high temperature phases of NaBH4 and KBH4 have been computed. The Hamiltonian which describes the motion of the borohydride ion in NaBH4 is invariant under the direct product group Ō × O, while the Hamiltonian for the borohydride ion in KBH4 is invariant under the direct product group T̄ × T. The rotational heat capacity of the borohydride ion computed from the energy levels is compared with the rotational heat capacity derived from the observed heat capacity for both NaBH4 and KBH4. The torsional frequency of the borohydride ion in NaBH4 is found to be 240 cm−1 and in KBH4 244 cm−1.