Nitrogen-15 and oxygen-17 nuclear magnetic resonance chemical shifts and 15N-29Si, 13C-29Si, and 15N–1H coupling constants in cyclosilazoxanes

Abstract

The effects of ring size on ¹⁵N, ¹⁷O, and ²⁹Si n.m.r. chemical shifts and ¹⁵N–²⁹Si, ¹³C–²⁹Si, ¹⁵N–¹H, and ²⁹Si–²⁹Si coupling constants in cyclosilazoxanes have been studied. The opposite shielding sequence in δ(¹⁵N) and δ(²⁹Si) and the decrease in ¹⁵N–²⁹Si coupling with diminishing ring size are consistent with higher Si–N bond ionicity (reduced d_π–p_π interaction) in smaller rings. The ¹⁷O chemical shift is practically independent of ring size in cyclosiloxanes. It has been proposed that the factors governing the ¹⁷O chemical shift act in opposite directions and are balanced. A linear relationship between N–H bond stretching frequency and ¹⁵N n.m.r. parameters (¹⁵N–¹H coupling and ¹⁵N chemical shifts) has been established.