Hj. Jiao et al., ARE THE D-MH SYMMETRICAL H-X(Q) RINGS WITH 4N-ELECTRONS AND HYDROGEN CLUSTERS AROMATIC(2 SIGMA), Journal of physical chemistry, 100(30), 1996, pp. 12299-12304
The D-mh symmetric H-x(q) rings, H-5(-) (D-5h, D-4h), H-5(+) (C-2v), H
-6 (D-6h, D-5h), H-7(+) (D-7h, D-6h), H-8(2+/2-) (D-8h), H-9(+) (D-3h)
, H-9(-) (D-9h, D-8h), and H-10 (D-10h, D-9h) are considered as possib
le analogs of the Huckel 4n + 2 electron aromatic annulene systems. Wh
ile aromatic character (due to ring current effects) is indicated by t
he magnetic susceptibility exaltations (Lambda) and large magnetic sus
ceptibility anisotropies, chi(anis) (derived from IGLO computations of
the magnetic properties), most of these hydrogen ring systems are hig
her order saddle points. The exceptions are transition structures: H-6
(D-6h), which can be compared with benzene, and H-10 (D-10h) which is
even less favorable energetically. On the basis of energetic, structu
ral, and magnetic criteria, aromaticity can result from cyclic delocal
ization of sigma- as well as pi-electrons. On the basis of the diamagn
etic exaltations, ''spherical aromaticity'' is illustrated by H-6(2-)
and H-8, both with O-h symmetry and eight sigma-electrons as well as H
-4(2+) With T-d symmetry and only two sigma-electrons, even though the
se species are artificial, higher order saddle points. The Huckel 4n e
lectron antiaromatic H-3(-) (D-3h), H-4 (D-4h), H-5(+) (D-5h, D-4h), a
nd H-8 (D-8h) triplet states have been computed. Both H-4 (D-4h) and H
-8 (D-8h) have negative (unfavorable) energies of concert relative to
two H atoms and the appropriate number of H-2 molecules.