Enhancement of the effect of small anisotropies in magic-angle spinning nuclear magnetic resonance

A variety of novel methods in magic-angle spinning NMR is described. The effects have in common the enhancement of the influence of small anisotropies on the NMR spectrum by deliberate intervention, i.e. either by applying pulses, carefully chosen continuous r.f. fields, or by adjustment of the spinning speed. It is shown that rotational sidebands in two-dimensional spin-echo NMR are often much larger than in one-dimensional NMR, owing to the interference of the π-pulse with the rotational echo formation. It is also demonstrated that in systems containing heteronuclear spin pairs the application of a weak continuous r.f. field of carefully chosen intensity can reintroduce small heteronuclear couplings into the spectrum. This is known as rotational resonance recoupling and is due to the interference of coherent spin rotations with the normal averaging effect of the sample rotation. Related effects can occur in homonuclear spin systems when an integer multiple of the spinning speed matches the difference between isotropic chemical shifts. In this case no extra r.f. field is necessary to amplify the effect of the non-secular parts of the dipolar interaction. Greatly enhanced polarization exchange as well as strong spectral effects are demonstrated. The application of these novel methods to the measurement of small interaction tensors and thereby to the extraction of important structural information is discussed.