Generally, a 1H-13C HMBC experiment offers a wealth of connectivity information about an unknown structure(s). However, an elucidator may be faced with the issue of ambiguity in assigning a correlation(s) to a 1D resonance(s).
A correlation with an ambiguous assignment may be assessed in one of 5 ways:
-reprocess the HMBC data using a different weighing function to try and narrow down an assignment,
-look for additional information among the other experiments to confirm one assignment over an other,
-ignore the ambiguous correlation and see if there is adequate HMBC data to continue the elucidation,
-make an assumption and assign the correlation to a 1D resonance at the risk of being incorrect, or
-consider multiple assignments at the risk of complicating the elucidation process.
The case below illustrates a mild form of ambiguity in an HMBC spectrum. (A subsequent blog will demonstrate how to deal with a severe case of ambiguity.) The HMBC spectrum exhibits 2 adjacent CH2 resonances, at 41 and 42 ppm, whereby one or both carbons are correlated to the 1H resonance at 2.37 ppm. From afar, the correlation appears to be linked to both carbons. However, upon closer examination of the correlation, the carbon at 42 ppm seems to be the better choice.
TIP: Zooming-in on a correlation can sometimes help resolve the uncertainty associated with the assignment.