15N Chemical Shift Tensors of Uracil Determined from 15N Powder Pattern and 15N-13C Dipolar NMR Spectroscopy

Article abstract of DOI:10.1021/j100026a006

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 deg and 9 deg 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.