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Solid State and Solution Structures of 9-Vertex Arachno Boron Hydride Clusters. An ab Initio/IGLO/NMR Study

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journal contribution
posted on 27.01.1999, 00:00 by Matthias Hofmann, Paul von Ragué Schleyer
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-B9H15 give an energy 110 kcal mol-1 above the fully optimized ab initio geometry. The experimental boron positions were shown to be accurate, but not the hydrogen placements. n-B9H15, which has a 7-vertex open face binding five bridge hydrogens and one endo terminal H, is 2.2 kcal mol-1 lower in energy than the isomer, i-B9H15. i-B9H15 has a C3v symmetric structure in solution with six asymmetric hydrogen bridges on the open B6 face of the 9-vertex arachno cluster. In contrast to the Cs structure reported for the solid state, [B9H14]- is shown to have a fluctional C1 structure in solution with an additional hydrogen bridge. However, the Cs and C1 geometries are very similar and the barriers for endo hydrogen rearrangements are 2.4 kcal mol-1 or less. B9H13·NCCH3 favors C1 symmetry in solution in contrast to the reported Cs solid-state structure which has one fewer hydrogen bridge. In contrast to the experimental solid-state structures, in solution B9H13·NCCH3 and [B9H14]- are isostructural. A solid-state model for [B9H13]2- based on the presence of only three instead of five possible structures is proposed. These three structures may also coexist in a fluctuating mixture in solution.