Relative orientation of chemical shielding and dipolar coupling tensors: Mixed single- and double-quantum homonuclear rotary resonance nuclear magnetic resonance of rotating solids

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Abstract
A novel two-dimensional magic-angle spinning nuclear magnetic resonance (NMR) method for determination of relative orientation of dipolar and chemical shielding tensors for dipolar-coupled homonuclear spin-1/2-pairs of powder samples is described. Simultaneous recoupling of anisotropic chemical shielding and dipolar coupling interactions is accomplished using a homonuclear rotary resonance pulse sequence with the amplitude ωrf of a rf irradiation field matched to the spinning frequency ωr according to ωrf=ωr. Employing this technique in the first dimension of a two-dimensional experiment leads to powder spectra exhibiting strong dependence on the magnitudes and the relative orientation of the two shielding tensors and the dipolar coupling tensor correlated to a high-resolution spectrum in the sampling dimension. Various aspects of the recoupling experiment are described theoretically and the applicability of the method for determination of relative orientation of these three anisotropic tensors through numerical simulation is demonstrated on basis of experiments for a doubly C13-labeled powder of L-alanine. With reference to this sample (and aminoacids in general), minor effects from simultaneous recoupling of the heteronuclear dipolar coupling between Cα and the amide N14 nucleus are evaluated. In the present case, the C13-N14 dipolar interaction is used in numerical simulations to refine our structural analysis and to obtain information about the absolute orientation of the C13 chemical shielding tensors relative to the molecular coordinate system.

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