Solid state and solution structures of 9-vertex arachno boron hydride clusters. An ab initio IGLO NMR study

The ab initio/IGLO/NMR method clarifies the structures of several 9-vertex boron hydride clusters by comparing computed chemical shifts using various geometries with NMR data in solution. The experimental coordinates of n-B9H 15 give an energy 110 kcal mol(-1) above the fully optimized ab initio geom etry. The experimental boron positions were shown to be accurate, but not t he hydrogen placements. n-B9H15, which has a 7-vertex open face binding fiv e bridge hydrogens and one endo terminal H, is 2.2 kcal mol(-1) lower in en ergy than the isomer, i-B9H15 i-B9H15 has a C-3v symmetric structure in sol ution with six asymmetric hydrogen bridges on the open B-6 face of the 9-ve rtex arachno cluster. In contrast to the C-s structure reported for the sol id state, [B9H14](-) is shown to have a fluctional C-1 structure in solutio n with an additional hydrogen bridge. However, the C-s and C-1 geometries a re very similar and the barriers for endo hydrogen rearrangements are 2.4 k cal mol(-1) or less. B9H13. NCCH3 favors C-1 symmetry in solution in contra st to the reported C-s solid-state structure which has one fewer hydrogen b ridge. Tn contrast to the experimental solid-state structures, in solution B9H13. NCCH3 and [B9H14](-) are isostructural. A solid-state model for [B9H 13](2-) based on the presence of only three instead of five possible struct ures is proposed. These three structures may also coexist in a fluctuating mixture in solution.