FLUXIONAL BEHAVIOR IN B8H82- - THEORETICAL-STUDY

Molecular orbital calculations using the PRDDO approximation are reported for various geometries of BgH82- in order to study its fluxional behavior. A low-energy path with a barrier less than 4 kcal/mol has been found for rearrangement. The low-energy path connects D2d and C2v geometries; the D2d is predicted to be more stable. No computational evidence in support of a stable D4d structure was found. Wave functions and energies are also reported for four additional structures of high symmetry, none of which are energetically competitive with the D2d, C2v, and D4d triumvirate. Environmental factors such as solvation and ion pairing were also considered by using a supermolecule approach. Solvation by hydrogen fluoride was found to have little effect on the relative stabilities of the various structures. Some support for the concept that cations stabilize square faces is found from computations on Li+B8H82- However, addition of a second Li+ ion firmly establishes the stability order: D2d>C2v>D4d. Correlation diagrams for the D2d⇆C2v and C2v⇆D4d transformations are constructed. Localized molecular orbitals for the highly symmetric structures are presented and correlations among the LMO's are discussed. Finally the temperature-dependent NMR spectra of the octaborane dianion are given an alternative explanation in the light of our computations. © 1979, American Chemical Society. All rights reserved.