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Figure 11.21

sin(nJr) dependence applies to the rejection process, so any 1JCH values that do not match the filter setting will "leak" through and show up in the HMBC spectrum as wide doublets centered on the position of the 13C-decoupled HSQC (HMQC) crosspeak. Because a one-bond 2D spectrum (HSQC or HMQC) is almost always acquired along with the HMBC, it is a good idea to superimpose them and find and label the one-bond artifacts right away, so you will not mistake them for true HMBC crosspeaks.

HMBC spectra have traditionally been processed and displayed in magnitude mode, but more recently HMBC sequences have been developed that permit a phase-sensitive display mode. To avoid "phase twists" the sequence is designed to refocus all but the desired chemical-shift evolution: 13C in F1. In this case, the HMBC crosspeaks appear as "candy-canes": alternating positive and negative intensities along F2 due to the relatively small JCH coupling in antiphase. This makes them easy to distinguish from noise and from one-bond artifacts, which are also antiphase but with respect to the very wide 1JCH coupling. An HMBC crosspeak is shown in Figure 11.21 for the traditional HMBC experiment (magnitude mode) and an experiment designed to allow phase-sensitive display. Note the much sharper peaks in F2 and the greater contrast with noise in the phase-sensitive (right side) spectrum. The crosspeak is a rare four-bond (4JCH) correlation observed for the natural product andrographolide (Fig. 11.22). Horizontal (F2) slices from phase-sensitive HMBC spectra can also be analyzed by curve fitting to extract the exact coupling constant 3JCH from the antiphase splitting, for conformational or stereochemical studies using the Karplus relation.

Figure 11.23

11.5.1 HMBC Spectrum of 3-Heptanone

Figure 11.23 shows the HMBC spectrum of 3-heptanone, presented in phase-sensitive mode with positive intensities shown in black and negative intensities in gray. Note that the F1 spectral window is much wider (0-220 ppm) than in the HSQC (Fig. 11.16: 5-45 ppm) because long-range correlations can be seen to quaternary carbons, including the carbonyl carbon at 212 ppm. Strong one-bond artifacts (indicated by squares) centered on the HSQC peak positions are observed for the "strong" XH peaks: H-1 and H-7 CH3 triplets and H-2 and H-4 CH2 triplets. The H-5 and H-6 proton signals have more complex splitting patterns that reduce their 1H peak height. This same splitting effect will reduce the peak heights of the HMBC correlations from H-5 and H-6. The one-bond artifacts could have been minimized if the low-pass filter were "tuned" to reject the narrow range of 1/CH values (125-128), but instead it was set to a more "generic" natural product value of 135 Hz. The ketone carbonyl carbon C-3 has a weak crosspeak to H-2 (bottom left, 2/CH) and a strong crosspeak to H-1 (bottom right, 3JCH). When counting the number of bonds between a *H and a 13C, do not forget to count the C-H bond! There appears to be a weak crosspeak between C-3 and H-7 (bottom right), but closer examination reveals this to be part of a vertical streak at the F2 frequency of H-7. The strong methyl triplets in the *H spectrum give these subtraction artifact streaks. In phase-sensitive spectra look for crosspeaks with alternating positive and negative intensity—even one tiny positive spot at the side of a negative spot, at the intersection of a known XH shift and a known 13C shift, can be unambiguously assigned. No correlation is observed from C-3 to H-4 or H-5, even though these are two-bond and three-bond relationships, respectively. Many crosspeaks will be missing in HMBC spectra due to the lack of sensitivity and sometimes very small J values. Lack of a crosspeak should never be used to rule out a possible structure, but the presence of a crosspeak that is more than three bonds distant in the proposed structure is a serious problem.

Moving up to the C-2 frequency in F1, we see a strong HMBC crosspeak to H-1 (2 JCH). At the F1 = C-1 horizontal line, in addition to the one-bond artifacts, there is a weak correlation to H-2 at the far left. This is complementary to the C-2/H-1 crosspeak. Moving to the other side of the carbonyl group, start with the F1 = C-4 (42 ppm) horizontal line: There is a strong crosspeak to H-5 (2JCH) and a very weak crosspeak (arrow) to H-6 (3JCH). Close examination of the H-6 crosspeak shows a negative spot just to the right side of the positive spot (inset). On the F1 = C-5 horizontal line, there is a weak crosspeak to H-4 (left side) and a strong crosspeak to H-7 (right side). The F1 = C-6 line also has a strong crosspeak to H-7, just above the C-5/H-7 crosspeak. Often you will see these crosspeaks "stretched" in the vertical (F1) dimension because they are spanning two nearby 13C chemical-shift positions. Be careful not to interpret them as a single crosspeak at an intermediate F1 shift value. Finally, on the F1 = C-7 line there are no true HMBC crosspeaks, only the one-bond artifacts.

A table of predicted and observed HMBC correlations confirms that many crosspeaks are missed, even in a fairly concentrated sample:

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