ARE THE D-MH SYMMETRICAL H-X(Q) RINGS WITH 4N-ELECTRONS AND HYDROGEN CLUSTERS AROMATIC(2 SIGMA)

Citation
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
Citations number
62
Categorie Soggetti
Chemistry Physical
ISSN journal
00223654
Volume
100
Issue
30
Year of publication
1996
Pages
12299 - 12304
Database
ISI
SICI code
0022-3654(1996)100:30<12299:ATDSHR>2.0.ZU;2-O
Abstract
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.