AB INITIO IGLO/NMR INVESTIGATION OF NIDO-C4B7H11 CONFIGURATIONS - STRUCTURAL ASSIGNMENT OF THE 3 KNOWN ISOMERS AND RECONFIRMATION OF EMPIRICAL CARBON PLACEMENT PATTERNS/

The ab initio/IGLO/NMR method has been successfully applied to establi sh the structures of the three known isomers of nido-C4B7H11. The meth od confirms the previously proposed structure, nido-7,8,9,10-C4B7H11, 1a, as one of the three known isomers. Of four candidates considered f or the second isomer, one of the previously proposed structures, nido- 1,7,8,10-C4B7H11, 2b, is selected. Of four candidates considered for t he third isomer, structure nido-2,7,9,10-C4B7H11, 3b, which had not be en previously proposed, is established. The relative order of stabilit y is 1a > 2b > 3b. A comparison of the relative energies of the nine c age structures considered in this study shows that, in complete agreem ent with previous empirically determined patterns, the most stable str uctures are those in which the carbons occupy low coordinate sites. Th is preference is more important than avoiding carbon-carbon connection s.