Ab Initio/IGLO/NMR Investigation of nido-C₄B₇H₁₁ Configurations: Structural Assignment of the Three Known Isomers and Reconfirmation of Empirical Carbon Placement Patterns

The ab initio/IGLO/NMR method has been successfully applied to establish the structures of the three known isomers of nido-C₄B₇H₁₁. The method confirms the previously proposed structure, nido-7,8,9,10-C₄B₇H₁₁, 1a, as one of the three known isomers. Of four candidates considered for the second isomer, one of the previously proposed structures, nido-1,7,8,10-C₄B₇H₁₁, 2b, is selected. Of four candidates considered for the third isomer, structure nido-2,7,9,10-C₄B₇H₁₁, 3b, which had not been previously proposed, is established. The relative order of stability is 1a > 2b > 3b. A comparison of the relative energies of the nine cage structures considered in this study shows that, in complete agreement with previous empirically determined patterns, the most stable structures are those in which the carbons occupy low coordinate sites. This preference is more important than avoiding carbon-carbon connections.