15N chemical shift tensors of uracil determined from 15N powder pattern and 15N-13C dipolar NMR spectroscopy

The ¹⁵N chemical shift tensors of uracil are reported using ¹⁵N powder pattern techniques. The principal values of the ¹⁵N uracil tensors are obtained from the spectra of [1-¹⁵N]uracil and [3-¹⁵N]uracil, and the tensor orientations are determined from the spectrum of [1,3-¹⁵N2,2-¹³C]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 tensor, are resolved using molecular symmetry considerations and results of ab initio calculations of ¹⁵N chemical shielding tensors. The N1 nitrogen has principal values of 196, 114, and 30 ppm and the N3 nitrogen 200, 131, and 79 ppm with respect to ¹⁵NH4NO3. Assuming that the smallest (most shielded) chemical shift tensor components are oriented perpendicular to the molecular plane, the largest components are found to lie 18° and 9° off the N1-H and N3-H bonds, respectively, rotated toward C2 and C4. 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. © 1995 American Chemical Society.

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Author Julio Cesar Facelli Biostatistics & Bioinformatics, Division of Translational Bi ...