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In the table, the predicted crosspeaks are indicated with 2 for 2JCH and 3 for 3JCH, and the intensity is indicated in parentheses: s for strong, m for medium, and w for weak. The strong 1H methyl signals (H-1 and H-7) virtually never fail to give crosspeaks, and usually these are strong. The worst performance is seen for the most "split" proton resonance: H-6, which is a sextet. The more splittings there are, the smaller each individual line appears, so the multiplet tends to "fall" into the noise and disappear. In rigid molecules, the 3Jch depends on the stereochemical relationship of 13C and 1H (Fig. 11.24) with an anti relationship giving a near maximum J value (~ 8 Hz) and a gauche relationship giving a small J value (~ 2-3 Hz). Because of the low sensitivity of HMBC, the gauche relationships give weak crosspeaks or no crosspeak at all. In 3-heptanone the molecule is flexible and the 3 JCH values are probably conformationally averaged to a "medium" value (~ 4 Hz).

In this example, we had already assigned all of the carbon peaks in the 13C spectrum, so there was no mystery. In the most difficult case of a complete unknown, the peaks in the13 C spectrum are numbered arbitrarily from upfield to downfield, and the protons are numbered correspondingly according to the carbon they correlate to in the HSQC/HMQC spectrum. Then the HMBC correlations are tabulated (e.g., C-5-H-16a) and the puzzle solving begins.

Figure 11.24

11.5.2 HMBC Spectrum of Cholesterol

The HMBC spectrum of cholesterol, displayed in phase-sensitive mode, is shown in Figure 11.25 with positive contours black and negative contours gray and with a portion of the HSQC spectrum at the left side for reference. The full HMBC spectrum has a very large number of crosspeaks, so for simplicity we show here only the upper right portion at a very high contour threshold. The most intense crosspeaks occur at the 1H chemical shifts of the methyl groups in F2, for the same reason that these are the most intense peaks in the HSQC spectrum: We are directly observing proton and the methyl groups have three equivalent protons. The protons of a methyl group can be correlated to no more than four carbons in an HMBC, and these are always observed due to their high intensity (Fig. 11.26). If the methyl group is attached to a carbonyl group, it can only correlate to two carbons (one if it is an ester or amide). If it is a methoxy group, it can only correlate to one carbon. If it is attached to a saturated carbon, that carbon can be a methylene (leading to a maximum of

(Number of HMBC correlations)

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