Okazaki Takao, Galembeck Sérgio E, Laali Kenneth K
Department of Chemistry, Kent State University, Ohio 44242, USA.
J Org Chem. 2002 Dec 13;67(25):8721-5. doi: 10.1021/jo026133f.
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.
对迄今难以捉摸的1,3 - 脱氢硅金刚烷双阳离子2(2+) - 5(2+)进行了密度泛函理论（DFT）研究。计算得到的核独立化学位移（NICS）值强烈表明，在所得的笼状双阳离子中存在三维杂芳香性（通过2电子、4中心同共轭作用）。在优化结构中，硅呈金字塔形。尽管电荷计算（NPA和MKS）表明硅原子上有显著的正电荷积累，但(29)Si GIAO NMR化学位移却异常屏蔽。后一发现与最近对7 - 硅降冰片二烯 - 7 - 鎓单阳离子10(+)的DFT计算结果一致，表明硅屏蔽是双阳离子中异常键合和同共轭作用的结果。从2(2+)到5(2+)，NICS值和硅屏蔽都降低。利用自然键轨道（NBO）分析来揭示这些双阳离子中三维杂芳香性的起源。
