Complete basis set and Gaussian-2 ab initio computational studies of planar H�ckel and M�bius aromatic hydrogen clusters

Abstract

Energies for highly symmetric cyclic hydrogen clusters (Hn) with Dnh symmetry were evaluated with Gaussian-2 (G2) and quadratic complete basis set (CBSQ) ab initio approaches. The energies were compared with linear hydrogen clusters as well as with their energies of formation from hydrogen and charged hydrogens. To better estimate aromaticity and eliminate ring strain energy, energies of formation for hydrogen clusters of the same ring size but with different electron numbers were compared. In this way, it was confirmed that the planar hydrogen clusters in singlet electronic configurations follow the Huckel aromatic rule, while aromaticity of the planar hydrogen clusters in an excited state (triplet) follows the Mobius 4n aromatic rule. It was also determined that hexagonal H6 is actually the transition-state structure for hydrogen exchange. The activation barrier for this exchange was estimated to be around 75 kcal/mol. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 679–687, 1998