Chemical shift anisotropies in silicon containing three-membered rings. An ab initio study

✍ Scribed by Gábor Magyarfalvi; Peter Pulay

• Publisher: Elsevier Science
• Year: 1995
• Tongue: English
• Weight: 511 KB
• Volume: 241
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(95)00604-3

✦ Synopsis

Ab initio NMR shielding calculations were carried out for cyclic compounds containing the cyclotrisilane, silaoxirane, disilacyclopropane, silacyclopropane, cyclotetrasilane, cyclopentasilane and cyclohexasilane skeletons. Calculations were performed for all permethylated compounds, as well as for the fully tert-butyl substituted cyclotrisilane. Geometries were optimized at the SCF/3-21G( * 1 level while NMR shieldings were evaluated using large basis sets. The results reproduce the high 29Si chemical shift anisotropies observed in three-membered rings. Calculations on strongly distorted models with local geometries similar to those in small rings suggest that the high anisotropy is due to the distorted geometry around the Si atom, not to special bonding effects in the three-membered rings.

' Part of a Diploma Thesis submitted to the Eijtviis University, Budapest, Hungary.

high anisotropies and Cavalieri et al. consider them unusual, it may be noted that relative to the usual spread of 29Si shieldings (about 600 ppm, see Ref. [SD, the observed anisotropy in the three-membered silicon rings is not larger than the 13C anisotropy in cyclopropane relative to the range of carbon shieldings (= 200 ppm). Recent developments in computational chemistry [6-S] allow the reliable calculation of NMR shieldings of molecules close in size to the actual compounds studied by Zilm and co-workers. The purpose of this study was to determine if the high CSAs of these systems is reproduced by ab initio calculations, and whether they indicate an exceptional electronic structure. The calculation of anisotropies is simpler than that of absolute shieldings since they are relative values, and it is also more instructive, as their experimental determination is more difficult.