N-15 CHEMICAL-SHIFT TENSORS OF URACIL DETERMINED FROM N-15 POWDER PATTERN AND N-15-C-13 DIPOLAR NMR-SPECTROSCOPY

The N-15 chemical shift tensors of uracil are reported using N-15 powder pattern techniques. The principal values of the N-15 uracil tensors are obtained from the spectra of [1-N-15]uracil and [3-N-15]uracil, and the tenser orientations are determined from the spectrum of [1,3-N-15(2),2-C-13]uracil by including the effects of the direct dipolar interaction in the spectral fitting routine. Ambiguities in the orientational assignments, which arise from the axial symmetry of the direct dipolar tenser, are resolved using molecular symmetry considerations and results of nb initio calculations of N-15 chemical shielding tensors. The N-1 nitrogen has principal values of 196, 114, and 30 ppm and the N-3 nitrogen 200, 131, and 79 ppm with respect to (NH4NO3)-N-15. Assuming that the smallest (most shielded) chemical shift tenser components are oriented perpendicular to the molecular plane, the largest components are found to lie 18 degrees and 9 degrees off the N-1-H and N-3-H bonds, respectively, rotated toward C-2 and C-4. These orientations are in good agreement with those calculated theoretically. In addition, inclusion of intermolecular hydrogen bond effects in the theoretical calculations significantly improves the correlation between the calculated and experimental principal values.