Two-dimensional separation of dipolar and scaled isotropic chemical shift interactions in magic angle NMR spectra

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Abstract
Two‐dimensional magic angle NMR methods produce high resolution dipolar‐chemical shift spectra of polycrystalline solids which permit determination of the mutual orientation and principle values of the two coupling tensors. This technique is applied to the amide 15N–1H spin pair in the dipeptide GlyGly ⋅ HCl ⋅ H2O, and we find the nearly axially symmetric 15N shift tensor is tilted 25° away from the NH bond. In many situations, however, only the magnitude of the heteronuclear dipolar coupling is required, and simplification of the spectrum would be desirable. An experiment that projects the dipolar information onto a single rotational sideband pattern for each magnetically inequivalent spin is proposed. The approach involves scaling the isotropic chemical shifts with a multiple‐pulse train, together with rotationally synchronized sampling. The method is demonstrated with spectra of the above mentioned dipeptide.

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