Some aspects of transition selective NMR involving rare spins

Satellite transition selective excitation/inversion of abundant spins across chemical shifts offers a robust opportunity for sensitivity enhancement of rare spins that are coupled to them. It is also well established that reconversion of rare spin double quantum coherence (DQC) to single quantum-single transitions (SQ-ST's) over a wide range of chemical shifts offers sensitivity enhancement in INADEQUATE-style experiments. The present contribution gives an overview of the latter category of experiments, including a brief summary of the literature, and the contributions from our Lab. In particular, the transition selective “composite refocusing” approach of Sørensen and his group is discussed for reconversion of DQC to SQ-ST's. A shorter transition selective DQC reconversion module introduced from our Lab is also described. A 2D rare spin correlation experiment introduced by us is then reviewed, in which we replace rare spin DQ evolution with immediate reconversion of DQC to SQ-ST's, followed by evolution of SQ-ST's, that is then terminated by a mixing period to deliver a diagonal-free COSY-like correlation map. Finally, our ‘indirect’, ¹H detected version of this experiment is reviewed. Transition selective reconversion in ADEQUATE-style experiments was also introduced by us and is briefly mentioned. Interestingly, partial ¹H transition selectivity is shown to result as a consequence of reconversion of rare spin DQC to SQ-ST's, followed by coherence order selective heteronuclear reverse transfer. The performance of these experiments when applied to small molecules is illustrated.