Determination of nuclear shielding tensor orientations in polycrystalline materials by dipolar modulation. The dilute I₂S system

Solid-state NMR methods of dipolar modulation and separated local field spectroscopy are extended to magnetically dilute, heteronuclear I₂S moieties in polycrystalline systems, and then used to measure the principal directions of the ¹⁵N chemical shielding interactions in an NH ₂ group. Characteristic features of the dipolar-modulated chemical shift anisotropy powder spectrum are analyzed, and the dependence of the modulation pattern on the relative orientation of the two tensors is discussed. The effect of homonuclear interactions is considered briefly.