LR-HSQMBC: A Sensitive NMR Technique To Probe Very Long-Range Heteronuclear Coupling Pathways

HMBC is one of the most often used and vital NMR experiments for the structure elucidation of organic and inorganic molecules. We have developed a new, high sensitivity NMR pulse sequence that overcomes the typical 2,3 J CH limitation of HMBC by extending the visualization of long-range correlation data to 4-, 5-, and even 6-bond long-range n J CH heteronuclear couplings. This technique should prove to be an effective experiment to complement HMBC for probing the structure of proton-deficient molecules. The LR-HSQMBC NMR experiment can, in effect, extend the range of HMBC to provide data similar to that afforded by 1,n-ADEQUATE even in sample-limited situations. This is accomplished by optimizing responses for very small n J CH coupings as opposed to relying on the markedly less sensitive detection of long-range coupled 13C–13C homonuclear pairs at natural abundance. DFT calculations were employed to determine whether the very long-range correlations observed for cervinomycin A2 were reasonable on the basis of the calculated long-range couplings.