Stability and three-dimensional aromaticity of closo-monocarbaborane anions, CBn-1Hn-, and closo-dicarboranes, C2Bn-2Hn

Comprehensive ab initio calculations RMP2(fc)/6-31G* on the closo-monocarbaboranes, CBn-1Hn- (n = 5-12), and the closo-dicarboranes, C2Bn-2Hn (n = 5-12), show that the relative energies of all the positional isomers agree with the qualitative connectivity considerations of Williams and with the topological charge stabilization rule of Gimarc. The reaction energies (Delta H) of the most stable positional isomers, 1-CB4H5-, CB5H6-, 2-CB6H7-, 1-CB7H8-, 5-CB8H9-, 1-CB9H10-, 2-CB10H11-, CB11H12-, as well as 1,5-C2B3H5, 1,6-C2B4H6, 2,3-C2B5H7, 1,7-C-2B-6H(8), 4,5-C2B7H9, 1,10-C2B8H10, 2,3-C2B9H10, and 1,12-C2B10H12 (computed using the equations, CBH2- + (n - 1)BHincrement --> CBnHn+1- (n = 4-11) and C2H2 + nBH(increment) --> C2BnHn+2 (n = 3-10)), show that the stabilities of closa-CBn-1Hn- and of closo-C2Bn-2Hn generally increase with increasing cluster size from 5 to 12 vertexes. This is a characteristic of three-dimensional aromaticity. There are variations in stabilities of individual closo-CBn-1Hn- and closo-C2Bn-2Hn species, but these show quite similar trends. Moreover, there is rough additivity for each carbon replacement. The rather large nucleus independent chemical shifts (NICS) and the magnetic susceptibilities (chi), which correspond well with one another, also show all closo-CBn-1Hn- and closo-C2Bn-2Hn species to exhibit "three-dimensional aromaticity". However, the aromaticity ordering based on these magnetic properties does not always agree with the relative stabilities of positional isomers of the same cluster, when other effects such as connectivity and charge considerations are important.