Theoretical study of nonrigid rotation of the tetrahedral cation and anion in NH4BH4

Segments of the potential energy surface of NH₄BH₄ corresponding to nonrigid rotations of the NH₄ ⁺ cation and BH₄ ^– anion relative to each other were calculated using the SCF/3–21G approximation with complete geometrical optimization and MP3/6–31G^* approximation for special points. The barriers for these motions are only several kcal/mole and the molecule is very nonrigid relative to several types of intramolecular rotations (rotation about the B-N bond, rotations with change in the dentation of the cation and anion, and rotations of the cation and anion facilitating the formation of some H₃N-N..H-BH₃ bonds and the cleavage of other such bonds). The strong geometric deformation of the NH₄ ⁺ and BH₄ ^– ions, the polarization of the electron density in these ions, and the H-H interaction between the hydrogen atoms belonging to different ions were discussed. The structural nonrigidity of NH₄BH₄ is closely related to its stability relative to decomposition.Institute of New Chemical Products, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 5, pp. 27–34, September–October, 1989.