STABILITY AND 3-DIMENSIONAL AROMATICITY OF CLOSO-MONOCARBABORANE ANIONS, CBN-1HN-, AND CLOSO-DICARBORANES, C2BN-2HN

Comprehensive ab initio calculations RMP2(fc)/6-31G on the closo-mono carbaboranes, CBn-1Hn- (n = 5-12), and the closo-dicarboranes, C2Bn-2H n (n = 5-12), show that the relative energies of all the positional is omers agree with the qualitative connectivity considerations of Willia ms and with the topological charge stabilization rule of Gimarc. The r eaction energies (Delta H) of the most stable positional isomers, 1-CB 4H5-, CB5H6-, 2-CB6H7-, 1-CB7H8-, 5-CB8H9-, 1-CB9H10-, 2-CB10H11-, CB1 1H12-, 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 usi ng the equations, CBH2- + (n - 1)BHincrement --> CBnHn+1- (n = 4-11) a nd C2H2 + nBH(increment) --> C2BnHn+2 (n = 3-10)), show that the stabi lities of closa-CBn-1Hn- and of closo-C2Bn-2Hn generally increase with increasing cluster size from 5 to 12 vertexes. This is a characterist ic of three-dimensional aromaticity. There are variations in stabiliti es of individual closo-CBn-1Hn- and closo-C2Bn-2Hn species, but these show quite similar trends. Moreover, there is rough additivity for eac h carbon replacement. The rather large nucleus independent chemical sh ifts (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 ar omaticity 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 considerat ions are important.