Clean Selective Refocusing Sequences With Sensitivity and Resolution Enhancement-A Review

The selective refocusing (SERF) and its modified experiments have permitted the unambiguous assignment of peaks and the straightforward determination of ⁿJ_HH. However, they suffer from the presence of intense axial peaks and the evolution of undesirable couplings in the spectra. In partially addressing these challenges, the Clean-G-SERF sequence, a modified version of the gradient-enhanced SERF-based experiment (G-SERF), has been designed to suppress all the axial peaks and eradicate the unwanted evolution, while retaining only the couplings pertaining to the selectively excited proton. Furthermore, the incorporation of a perfect echo block provided the leverage for increasing slice thickness leading to the increased sensitivity. To additionally enhance the resolution in the direct dimension, the improved sequences have been designed by the incorporation of pure shift, where the homonuclear J couplings are refocused in real time. All these methods permitted the unambiguous assignment of peaks to the coupled partners of the selectively excited proton, thereby enabling the accurate measurement of couplings. The broader utility of the designed sequences, cited in the literature as Clean-G-SERF, Clean-PE-SERF, PS-Clean-G-SERF and PS-Clean-PE-SERF have been demonstrated on several chosen examples including the molecular mixtures for the accurate measurement of J_HH.