NMR shifts are generally determined mainly by the nearest-For the higher symmetry b form, the N2 site, with threefold neighbor environment of an atom, with fairly small changes in rotational symmetry, showed a chemical shift of 68.9 ppm, the shift arising from differences in the second-nearest-neighbor relative to the ammonium resonance of solid NH 4 NO 3 , while environment. Previous calculations on the (SiH 3 ) 3 N molecule used the N1 site, without threefold symmetry, showed a shift as a model for the local environment of N in crystalline aand around 51 ppm. Earlier calculations on simple (SiH 3 ) 3 N b-Si 3 N 4 gave N NMR shieldings much larger than those measured models for Si 3 N 4 with geometries appropriate to the N1 and in the solids and gave the wrong order for the shifts of the inequiva-N2 sites gave an incorrect order for the shieldings of the lent N sites (e.g., N1 and N2 in b-Si 3 N 4 ). We have now calculated two sites, as well as absolute values of the shielding which the N NMR shieldings in larger molecular models for the N2 site were too high by about 100 ppm (4). Assuming that the of b-Si 3 N 4 and have found that the N2 shielding is greatly reduced when additional N1 atoms (second-nearest-neighbors to the cen-NH / 4 resonance in aqueous NH 4 NO 3 solution and in solid tral N2) are included. The calculated N2 shieldings (using the NH 4 NO 3 is the same, the N absolute shielding scale (5) GIAO method with the 6-31G* basis set and 6-31G* SCF optigives shielding values of 170 and 153 for the N1 and N2 mized geometries) are 288.1, 244.7, and 206.0 ppm for the molesites of b-Si 3 N 4 , while the values calculated for the (SiH 3 ) 3 N cules (SiH 3 ) 3 N, Si 6 N 5 H 15 , and Si 9 N 9 H 21 (central N2), respectively, molecules were around 255-285 ppm (depending upon while the experimental shielding of N2 in b-Si 3 N 4 is about 155 whether one chose an optimized or experimental geometry). ppm. Second-nearest-neighbor effects of only slightly smaller mag-The reported shift for the gas-phase (SiH 3 ) 3 N molecule (6) nitude are calculated for the analog C molecules. At the same translates into a value of 281 for the absolute shielding, in time, the effects of molecule size upon Si NMR shieldings and N reasonable accord with the calculated value at the equilibelectric field gradients are small. The local geometries at the N2like Ns in C 6 N 5 H 15 and C 9 N 9 H 21 are calculated to be planar, consis-rium geometry. Studies on somewhat larger molecules (with tent with the planar local geometry recently calculated for N in both a central N and a second-neighbor N) showed large crystalline C 3 N 4 using density functional theory. α§ 1997 Academic Press conformation-dependent effects upon the shielding of the central nitrogen ( ), but adding up these long range effects for the inequivalent sites in Si 3 N 4 became quite complicated, and it was not clear whether the effects of different second-