Examples Of Hmbc Spectra

HMBC is just an HMQC experiment with the 1/(2J) delay for evolution into antiphase "tuned" for a much smaller JCH value: typically 8-10 Hz rather than around 150 Hz. This optimizes the experiment for observation of 2D crosspeaks between a 13 C and a "remote" XH nucleus two or three bonds away in the covalent structure: 13C-XH or 13C-X-YH where X and Y can be 12C, O, S, N, and so on. Tuning to a much smaller J coupling means that the 1/(2J) delay will be much longer: 50-62.5 ms rather than around 3.3 ms. Loss of coherence through T2 relaxation becomes significant with these longer delays, so the 1/(2J) refocusing delay at the end is left out. To mimimize T2 losses the 1/(2J) delay is set as short as possible, so for routine work the optimal J value is usually set to the highest expected JCH, or even a bit higher: 8-10 Hz. The FID records a XH signal that is antiphase with respect to the 13 C it correlates with in the 2D spectrum, and 13 C decoupling cannot be used because the antiphase lines would cancel each other, leading to no signal at all. This does simplify things a bit because we do not have to worry about the decoupler duty cycle, and we can use a longer acquisition time (aq/at) if we want. The relative span of J-coupling values is much greater for 2JCH and3 JCH, ranging typically from 0 to 8 Hz, compared to the more predictable 1JCH (125-175 Hz). This means that for smaller JCH values the crosspeak intensity is even weaker because evolution into antiphase occurs to the extent of sin(nJt). All of this contributes to HMBC being one of the less sensitive experiments, perhaps more sensitive than NOESY but much less sensitive than COSY, TOCSY, or HSQC/HMQC. With a few milligrams of a small molecule ("organic") sample on a 600-MHz instrument, a good HSQC spectrum can be obtained in 30 min or 1 h, but a good HMBC will take 3-4 h of acquisition.

The HMBC also incorporates a "low-pass" filter that tries to reject the one-bond correlations seen in HSQC/HMQC. "Low pass" means that only the low values of JCH (0-10 Hz) are allowed to pass through and produce crosspeaks in the 2D spectrum. Because there is no 13 C decoupling, the one-bond correlations appear as wide doublets (J ~ 150 Hz) centered on the XH peak position in F2 (Fig. 11.10—squares). They obscure the weak HMBC crosspeaks and can easily be misinterpreted as long-range correlations, especially if one of the two components of the doublet happens to fall at the position of another peak in the XH spectrum. The low-pass filter is set to reject a particular J value, typically 135 Hz for molecules dominated by saturated hydrocarbon (e.g., 3-heptanone, menthol, cholesterol), 142 Hz for sugars, and 170 Hz for molecules dominated by aromatic carbons. The same

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