A COMBINED NMR IGLO ABINITIO STUDY OF THE C2B10H13-ISOMERS

The C2B10H122- precursor dianion of the two known forms Of C2B10H13-, the ''unreactive' (1, C(s)) and the ''reactive'' isomer (two possibilities: 2, C(s) and 3, C1), have all been characterized computationally. The unsymmetrical, fluctuating structure (3) of the reactive isomer in solution is firmly established by means of IGLO chemical shift calculations. The calculated geometries of 1 and 3 (ab initio: HF/3-21G) are in good agreement with X-ray data. At the MP2/6-31G*//3-21G + ZPE level, the ''unreactive'' isomer (1) is 6.7 kcal/mol more stable than the ''reactive'' isomer (3) which is known to rearrange into the former. IGLO B-11 NMR chemical shifts for 1 and 3 (averaged), but not 2, agree well with experiment. As the energy of 2 is computed to be 11.5 kcal/mol higher than that of 3, it should be possible to ''freeze out' dynamic NMR behavior of the ''reactive'' isomer (3) at lower temperatures.