The "N chemical shift tensors of uracil are reported using I5N powder pattem techniques. The principal values of the I5N uracil tensors are obtained from the spectra of [l-'5N]uracil and [3-'jN]uracil, and the tensor orientations are determined from the spectrum of [ 1,3-'5N2,2-'3C]~ra~i1 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 I5N chemical shielding tensors. The NI nitrogen has principal values of 196, 114, and 30 ppm and the N3 nitrogen 200, 131, and 79 ppm with respect to I5NH4N03. 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 NI-H and N3-H bonds, respectively, rotated toward CZ and Cq. 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.
Complete carbon-13 chemical shift tensors are measured in single crystals of methyl α-d-galactopyranoside monohydrate, methyl α-d-glucopyranoside, methyl α-d-mannopyranoside, methyl β-d-galactopyranoside, methyl β-d-glucopyranoside hemihydrate, and methyl β-d-xylopyranoside. The fits of the experimental data to the second-rank form of shift tensors reflect the accuracy of the measured tensors and yield standard deviations that range between 0.27 and 0.75 ppm. Ab initio gauge-invariant atomic orbital (GIAO) computations using the D-95 double-ζ basis set are used to assign the experimental tensors to the carbons in the unit cell. The root-mean-square (rms) deviation of the diffraction-structure-based GIAO shieldings fitted to all of the experimental shifts is 4.99 ppm. By optimizing the ring and methyl proton positions with the Gaussian-92 program and repeating the GIAO computations, the root-mean-square deviation is reduced to 2.40 ppm. These results illustrate that complete 13C chemical shift tensors measured in single crystals and interpreted with quantum-chemical computations can be used to evaluate differences between crystal structures obtained with X-ray diffraction, neutron diffraction, and structural optimization methods.
The 40 chemical shift tensors of single-crystal perylene in the α crystalline form have been determined with a precision of 0.30 ppm using 13C chemical shift−chemical shift correlation spectroscopy. The in-plane anisotropy of these tensors describes the delocalization of π-electrons at the inner α positions, which is similar to that found in biaryl linkages rather than typical bridgehead carbons. Molecular distortions, originated in intermolecular interactions and associated with chemical shifts of up to 5 ppm in similar carbons of perylene, have been detected indicating that the accuracy in the shift tensors measured in this study is adequate to probe crystalline effects upon the electronic and molecular structure. The systematic tilt observed in the orientation of the smallest principal components, δ33, supports the X-ray observation that the molecules bend about their long axis by 1−2°. Chemical shift calculations, using ab initio methods, are in good agreement with the experimental tensors and provide insight into the observed variation of chemical shifts in terms of molecular geometry.
13C chemical shift tensor measurements on single crystals provide a powerful method to study changes in the electron environment of nuclei with changes in molecular structure. Thus, diffraction structures are critical to an understanding of chemical shift tensors. This work explores the general reliability of using structural data to predict components of the symmetrical chemical shift tensor. Imprecision in the hydrogen positions introduces considerable scatter in the simulated 13C shift tensors, and optimized C-H bond distances in methyl-beta-D-glucopyranoside used with the X-ray positions of the heavier C and O atoms greatly improve the simulated chemical shifts. Acenaphthene, with two crystallographically different molecules per unit cell, offers an excellent example for comparing and contrasting structural differences in the two molecules. A recently improved X-ray structure of naphthalene obtained at low temperature provides chemical shift simulations which are comparable to those from neutron diffraction methods and appear to reflect breaks in the D2h symmetry measured in the NMR chemical shift tensors. These data illustrate the close relationship between NMR and diffraction structures.
A modification is made to the chemical shift−chemical shift, CS-CS, correlation spectroscopy method for measuring shift tensors. This new approach incorporates, in an iterative fashion, the redundancy of information available in the spectrum from congruent nuclei in the unit cell. The redundancy reduces the number of 2D spectra required to determine the full chemical shift tensor. The iterative procedure requires reasonably good starting shift values that may be derived from quantum chemical calculation of nuclear shielding parameters. These theoretical tensor estimates provide approximate spectral patterns used in assigning the experimental peaks. With this technique the 18 unique 13C tensors in a triphenylene unit cell, which describe 72 spectral peaks, were measured with a precision of 0.52 ppm in the tensor components with use of only three 2D spectra instead of the six normally used in the CS-CS method. The chemical shift tensor analysis indicates that, to relieve intramolecular strain, the molecule deforms from planarity, and the molecular symmetry is left with only a single vertical plane. Shift tensors also reflect the major Kekule structures of triphenylene. Further, chemical shift modeling as a function of molecular geometry was used to probe variations between neutron and X-ray diffraction data.
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