CHEMICAL-SHIFTS IN SILATRANE AND ITS DERIVATIVES - A STUDY OF THE TRANSANNULAR INTERACTION

Solid-state C-13, N-15, and Si-29 NMR spectra were obtained on silatrane and a series of derivatives. The isotropic C-13 chemical shifts are largely insensitive to substituent-induced structural changes. In contrast, the isotropic N-15 and Si-29 chemical shifts and chemical shift powder patterns are quite sensitive to substituent-induced structural changes. As the silicon-nitrogen internuclear distance (r(Si-N)) decreases, the isotropic N-15 chemical shift increases and the isotropic Si-29 chemical shift decreases. The magnitude of the N-15 chemical shift anisotropy (CSA) decreases and the Si-29 CSA increases as r(Si-N) decreases. The change in delta(iso) and the CSA for N-15 is primarily due to the change in delta(perpendicular-to), which increases as r(Si-N) decreases. For Si-29, all three principal elements change as r(Si-N) is varied. Variations in the N-15 chemical shift tensor elements are ascribed to changes in the N...Si transannular interaction. For the Si-29 chemical shift tensor, the observed changes are ascribed to a combination of effects due to changes in the transannular interaction and direct substituent effects. A simple molecular orbital treatment is useful in understanding the relationship between the transannular effect and the N-15 chemical shift tensor and in supporting the use of the N-15 chemical shift as an indicator of the transannular interaction.