MOLECULAR-STRUCTURE OF 1-AZA-CLOSO-DODECABORANE(12) - EXPERIMENTAL AND THEORETICAL REFINEMENT

The gas-phase electron diffraction data (GED) for 1-aza-closo-dodecaborane, 1-NB11H12, can be fit by four models with C5v symmetry almost equally well. The MP2/6-31G* single point calculations, as well as IGLO (individual gauge for localized orbitals) computed chemical shifts were able to decide among these possibilities. The final experimental geometry was selected on the basis of the agreement between the IGLO B-11 chemical shifts calculated for the various GED models and the experimental values. This optimal choice was supported by the energetic criteria. The shorter N-B separation [r(g) = 1.716(9) angstrom] and the expansion of the pentagonal belt adjacent to nitrogen [r(g) = 1.825(6) angstrom] are the main distortions, compared with the icosahedral ''parent'' B12H122- [r = 1.77 angstrom]. The mean length of the B-B bonds in the hemisphere opposite to nitrogen is 1.791(5) angstrom. The IGLO delta B-11's computed for the best model agree with the experimental values. In particular, the chemical shift of the antipodal B-12 is reproduced reasonably well (IGLO delta = 5.5 ppm; experimental delta = 2.8 ppm).