Are theDmhSymmetric HxqRings with 4n+ 2 σ-Electrons and Hydrogen Clusters Aromatic?

The D m h symmetric H x q rings, H 5 - ( D 5 h, D 4 h ), H 5 + ( C 2 v ), H 6 ( D 6 h, D 5 h ), H 7 + ( D 7 h, D 6 h ), H 8 2+/2- ( D 8 h ), H 9 + ( D 3 h ), H 9 - ( D 9 h, D 8 h ), and H 10 ( D 10 h, D 9 h ) are considered as possible analogs of the Hückel 4 n + 2 electron aromatic annulene systems. While aromatic character (due to ring current effects) is indicated by the magnetic susceptibility exaltations (Λ) and large magnetic susceptibility anisotropies, χ anis (derived from IGLO computations of the magnetic properties), most of these hydrogen ring systems are higher order saddle points. The exceptions are transition structures: H 6 ( D 6 h ), which can be compared with benzene, and H 10 ( D 10 h ) which is even less favorable energetically. On the basis of energetic, structural, and magnetic criteria, aromaticity can result from cyclic delocalization of σ- as well as π-electrons. On the basis of the diamagnetic exaltations, “spherical aromaticity” is illustrated by H 6 2- and H 8, both with O h symmetry and eight σ-electrons as well as H 4 2+ with T d symmetry and only two σ-electrons, even though these species are artificial, higher order saddle points. The Hückel 4 n electron antiaromatic H 3 - ( D 3 h ), H 4 ( D 4 h ), H 5 + ( D 5 h, D 4 h ), and H 8 ( D 8 h ) triplet states have been computed. Both H 4 ( D 4 h ) and H 8 ( D 8 h ) have negative (unfavorable) energies of concert relative to two H atoms and the appropriate number of H 2 molecules.