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Existence of C,3-Me2-closo-1,2-C2B3H3 Refuted by the Ab Initio/IGLO, GIAO-MP2/NMR Method. Attempted Repetition of the Synthesis

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journal contribution
posted on 09.10.1996, 00:00 by Matthias Hofmann, Mark A. Fox, Robert Greatrex, Paul von Ragué Schleyer, Joseph W. Bausch, Robert E. Williams
The 11B chemical shift assignment for the closo-carborane C,3-Me2-1,2-C2B3H3 structure (1a or 1b) is refuted by ab initio/IGLO, GIAO/NMR evaluations. Attempts to reproduce the literature NMR data by examining the computed chemical shifts of many alternative structures failed. Rearrangement of the 1,2-C2B3H5 isomer into the much more stable 1,5-C2B3H5 is computed to involve only a modest energy barrier. Repetition of the thermal (but not the electric discharge) experiments failed to give a compound with the spectroscopic data reported earlier. The original experimental data remain unexplained. Nevertheless, the elimination of the C,3-Me2-1,2-C2B3H3 structural assignment removes the sole exception to four established structural patterns of closo-carboranes:  (a) carbons tend to occupy sites with the lowest coordination, (b) carbons tend to be nonadjacent in the most thermodynamically stable closo-carboranes, (c) the 11B NMR of boron atoms with exo-terminal alkyl groups are always found at lower field than those of otherwise identical boron atoms with exo-terminal hydrogens, and (d) exo-terminal δ(1H)'s normally parallel the δ(11B) of the boron nuclei to which they are bound.