title: Novel examples of three-dimensional aromaticity: 1,3-dehydro-silaadamantane dications. A theoretical (DFT, GIAO NMR, NICS) study.
abstract: A DFT study of the hitherto elusive 1,3-dehydro-silaadamantane dications 2(2+)-5(2+) has been carried out. Computed nucleus independent chemical shift (NICS) values are strongly indicative of three-dimensional heteroaromaticity in the resulting caged dications (via 2-electron, 4-center homoconjugation). In the optimized structures, silicon is pyramidalized. Although charge calculations (NPA and MKS) indicate significant positive charge build-up at silicon(s), the (29)Si GIAO NMR chemical shifts are unusually shielded. The latter finding agrees with the recent DFT calculations on 7-silanobornadien-7-ylium monocation 10(+), suggesting that silicon shielding is a consequence of unusual bonding and homoconjugation in the dications. Both NICS values and silicon shielding decrease in going from 2(2+) to 5(2+). Natural bond orbital (NBO) analysis was utilized to shed light on the origin of the three-dimensional heteroaromaticity in these dications.
authors.affiliation: Department of Chemistry, Kent State University, Ohio 44242, USA.
authors.forename: Takao
authors.initials: T
authors.lastname: Okazaki
authors.forename: Sérgio E
authors.initials: SE
authors.lastname: Galembeck
authors.forename: Kenneth K
authors.initials: KK
authors.lastname: Laali