DFT Studies on Structures, Stabilities, and Electron Affinities of closo-Supercarboranes C2Bn–2Hn (n = 13–20)

Structures, stabilities, and electron affinities of closo-supercarboranes C2Bn–2Hn (n = 13–20) were studied with the aid of DFT calculations. The results regarding the relative stability of positional isomers for each closo-supercarborane can be well understood with the qualitative rules established on the basis of early studies on 5- to 12-vertex carboranes C2Bn–2Hn (n = 5–12). The calculated cumulative BH addition energies for the most stable CAd (CAd = carbon-atoms-adjacent) and CAp (CAp = carbon-atoms-apart) positional isomers (using the equation 1,2-C2B10H12 + (n – 12) BHinc → C2Bn–2Hn (n = 13–20), where BHinc is set as the energy difference between B6H10 and B5H9) suggest that the thermodynamic stability should not be the reason for nonobservation of 15-vertex CAp closo-carborane and other larger closo-supercarboranes C2Bn–2Hn (n = 16–20), and effort toward their synthesis is worth spending. The calculated HOMO–LUMO energy gaps show the relatively low chemical stability of 15-vertex closo-carboranes, explaining the difficulty of their experimental synthesis. Among these closo-supercarboranes studied, the 17-vertex carborane is predicted to be the most stable one, thus the most plausible target for experimental synthesis.