THEORETICAL REFINEMENTS OF THE ENDO STRUCTURES AND THE EXO STRUCTURES OF TETRABORANE(8) CARBONYL

The molecular structures of the endo (1a) and exo (1b) isomers of B4H8CO have been optimized at the ab initio MP2(Full)/6-31G* level of theory. The agreement of the computed geometrical parameters with the recently published electron-diffraction (GED) data is very good, even though a number of geometrical constraints were applied in the experimental determination. The IGLO (individual gauge for localized orbitals) B-11 NMR chemical shifts, calculated at the II'//MP2/6-31G* level, are also in accord with experiment. The formation of 1a and 1b by association of B4H8 and CO is computed to be exothermic by 22.8 and 22.2 kcal/mol, respectively, at the MP2(Full)/6-31G*//MP2(Full)/6-31G* + ZPE(6-31G*) level of theory. The Lewis acid strength of B4H8 toward CO is comparable to that of BH3.