N-14 EFFECTS ON THE SI-29 SOLID-STATE NMR-SPECTRA OF SILICON-NITRIDE

29Si high-resolution solid-state NMR spectra of the α and β forms of silicon nitride show the effect of residual dipolar coupling to 14N nuclei which is not averaged out by fast magic-angle sample spinning. The influence of 14N is most noticeable in the low-field signal of α-Si3N4 and leads to a complex lineshape with field-dependent partitions. The use of a previously reported first-order perturbative approach allows a successful simulation of 29Si lineshapes at different magnetic fields. As an outcome of the simulation, a value of J(29Si, 14N) = 15 Hz is derived. Comparison with solution scalar coupling constants in Si,N-containing compounds reveals that this value lies within the expected range. Additionally, a 14N quadrupole coupling constant of -2.1 MHz is obtained for Si3N4. This result confirms the fact that NSi σ bond polarization and (d-p)π bonding result in a considerable decrease in χ, as compared with compounds where the nitrogen is bonded to carbon or hydrogen. © 1991.