EXISTENCE OF C,3-ME(2)-CLOSO-1,2-C2B3H3 REFUTED BY THE AB-INITIO IGLO, GIAO-MP2 NMR METHOD - ATTEMPTED REPETITION OF THE SYNTHESIS/

The B-11 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 ev aluations. Attempts to reproduce the literature NMR data by examining the computed chemical shifts of many alternative structures failed. Re arrangement of the 1,2-C2B3H5 isomer into the much more stable 1,5-C2B 3H5 is computed to involve only a modest energy barrier. Repetition of the thermal (but not the electric discharge) experiments failed to gi ve a compound with the spectroscopic data reported earlier. The origin al experimental data remain unexplained. Nevertheless, the elimination of the C,3-Me(2)-1,2-C2B3H3 structural assignment removes the sole ex ception to four established structural patterns of closo-carboranes: ( a) carbons tend to occupy sites with the lowest coordination, (b) carb ons tend to be nonadjacent in the most thermodynamically stable closo- carboranes, (c) the B-11 NMR of boron atoms with exo-terminal alkyl gr oups are always found at lower field than those of otherwise identical boron atoms with exo-terminal hydrogens, and (d) exo-terminal delta(H -1)'s normally parallel the delta(B-11) of the boron nuclei to which t hey are bound.