Second-Nearest-Neighbor Effects upon N NMR Shieldings in Models for Solid Si 3 N 4 and C 3 N 4

ISSN： 1090-7807

Source： Journal of Magnetic Resonance, Vol.127, Iss.1, 1997-07, pp. : 49-53

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Abstract

NMR shifts are generally determined mainly by the nearest-neighbor environment of an atom, with fairly small changes in the shift arising from differences in the second-nearest-neighbor environment. Previous calculations on the (SiH 3 ) 3 N molecule used as a model for the local environment of N in crystalline alpha- and beta-Si 3 N 4 gave N NMR shieldings much larger than those measured in the solids and gave the wrong order for the shifts of the inequivalent N sites (e.g., N1 and N2 in beta-Si 3 N 4 ). We have now calculated the N NMR shieldings in larger molecular models for the N2 site of beta-Si 3 N 4 and have found that the N2 shielding is greatly reduced when additional N1 atoms (second-nearest-neighbors to the central N2) are included. The calculated N2 shieldings (using the GIAO method with the 6-31G* basis set and 6-31G* SCF optimized geometries) are 288.1, 244.7, and 206.0 ppm for the molecules (SiH 3 ) 3 N, Si 6 N 5 H 15 , and Si 9 N 9 H 21 (central N2), respectively, while the experimental shielding of N2 in beta-Si 3 N 4 is about 155 ppm. Second-nearest-neighbor effects of only slightly smaller magnitude are calculated for the analog C molecules. At the same time, the effects of molecule size upon Si NMR shieldings and N electric field gradients are small. The local geometries at the N2-like Ns in C 6 N 5 H 15 and C 9 N 9 H 21 are calculated to be planar, consistent with the planar local geometry recently calculated for N in crystalline C 3 N 4 using density functional theory.