D-HMBC versus LR-HSQMBC: Which experiment provides theoretically and experimentally the best results?

The Long‐Range Heteronuclear Single Quantum Multiple Bond Correlation (LR‐HSQMBC) experiment is the experiment of choice for visualizing heteronuclear long‐range coupling interactions nJCH across 4‐6‐bonds and is experimentally superior to the Decoupled Heteronuclear Multiple‐Bond Correlation (D‐HMBC) experiment. Yet, the exact reasons have not been fully understood and established. On the basis of our recent investigation of the non‐refocused variants LR‐HSQC and HMBC, we have extended a JHH’‐dedicated investigation to the D‐HMBC‐ and LR‐HSQMBC experiments. Unlike the non‐refocused variants, the influence of homonuclear couplings JHH’ on the intensity of long‐range nJCH cross‐peaks is not easily predictable and may be summarized as follows: (i) irrespective of the magnitude and number of JHH’ interactions long‐range nJCH cross‐peaks are more intense in D‐HMBC spectra as long as the evolution delay is not too large, since in contrast to LR‐HSQMBC no JHH’‐caused intensity zeroes will occur. (ii) If JHH‘is small and large, the intensity of cross peaks in D‐HMBC spectra may be weakened or may even vanish at =(0.25+0.5k)/JHH’, while for the LR‐HSQMBC this unwanted effect occurs at = k+0.5/JHH’. Consequently, when is adjusted to visualize weak nJCH long‐range correlations, our findings corroborate that there are potentially more cross‐peaks expected to show up in a LR‐HSQMBC spectrum compared to a D‐HMBC spectrum. This has been indeed noticed experimentally, even though the intensity of a many long‐range nJCH cross‐peaks may still be higher in the spectra of the D‐HMBC experiment correspondingly adjusted for detecting weak nJCH correlations.