LR-HSQMBC versus LR-selHSQMBC: Enhancing the Observation ofTiny Long-Range Heteronuclear NMR Correlations
Kumar Motiram-Corral, Pau Nolis, Josep Saurí, and Teodor Parella
ABSTRACT:The detection of ultra-long-range (4JCHand higher) heteronuclear connectivities can complement the conventionaluse of HMBC/HSQMBC data in structure elucidation NMR studies of proton-deficient natural products, where two-bond andthree-bond correlations are usually observed. The performance of the selHSQMBC experiment with respect to its broadbandHSQMBC counterpart is evaluated. Despite its frequency-selectivity nature, selHSQMBC efficiently prevents any unwanted signalphase and intensity modulations due to passive proton−proton coupling constants typically involved in HSQMBC. As a result,selHSQMBC offers a significant sensitivity enhancement and provides pure in-phase multiplets, improving the detection levels forshort- and long-range cross-peaks corresponding to small heteronuclear coupling values. This is particularly relevant for experimentsoptimized to smallnJCHvalues (2−3 Hz), referred to as LR-selHSQMBC, where key cross-peaks that are not visible in the equivalentbroadband LR-HSQMBC spectrum can become observable in optimum conditions.