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The recently reported LR-HSQMBC experiment has been optimized for 1H–15N long-range heteronuclear couplings. Several previously unreported four-bond correlations, consistent with the predicted by DFT calculations (0.2–0.3 Hz 4JNH couplings), have been observed for strychnine using 2 Hz optimization of the LR-HSQMBC experiment. This experiment offers an advantage over accordion-optimized experiments such as IMPEACH and CIGAR for the observation of long-range 1H–15N correlations in that the experiment is refocused and employs a CLIP pulse sequence element to bring the long-range correlations into phase, allowing broadband X-decoupling to be employed during acquisition.  相似文献   
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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 nonrefocused variants LR-HSQC and HMBC, we have extended a JHH′-dedicated investigation to the D-HMBC and LR-HSQMBC experiments. Unlike the nonrefocused 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: (a) 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 Δ is not too large, because in contrast to LR-HSQMBC no JHH′-caused intensity zeroes will occur. (b) If JHH′ is small and Δ large, the intensity of cross peaks in D-HMBC spectra may be weakened or may even vanish at Δ = (0.25+0.5k)/JHH′, whereas for the LR-HSQMBC this unwanted effect occurs at Δ = k + 0.5/JHH′. Consequently, when Δ 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 with 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.  相似文献   
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