Measurements of two-dimensional NMR powder patterns in rotating solids

Abstract

Sample-spinning NMR techniques for measuring the heteronuclear dipolar/chemical shift two-dimensional (2D) powder patterns are described. For compounds with more than one chemically distinct nucleus, the individual 2D powder patterns can be separately obtained by 3D switching-angle sample-spinning NMR. A practical application of the 3D NMR is demonstrated with the 13C 2D powder patterns of the two crystallographically inequivalent formate ions in calcium formate. The chemical shift tensors and proton positions were obtained for the individual formate ions, which agree with the results already reported by single crystal studies of 13C NMR and neutron diffraction. The 2D powder pattern of polyethylene was also obtained as an example of the CH2 system. The chemical shift principal values, the bond length, and the bond angle were determined. The chemical shift principal axes were found to be only approximately along the symmetry directions of the CH2 group, indicating a strong perturbation of the electric environment by the crystal field.