July 22, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
The best approach when dealing with severe ambiguity in assigning a correlation(s) to a 1D resonance(s) in a 1H-13C HMBC spectrum is to take into account all possible assignments. The drawback to this approach is it adds to the complexity of the structure elucidation process rather than simplifying it.
The HMBC spectrum below illustrates a severe case of ambiguity in assigning a correlation to a single 13C resonance. The spectrum exhibits a correlation that can be assigned to a 13C resonance at 129.10 and/or 129.13 ppm.
The elucidator will take the first possible assignment (3.52, 129.10 ppm) and attempt to construct a set of structures (or fragments). Subsequently, the elucidator will take the alternate assignment (3.52, 129.13 ppm) and build a second set of structures. This approach of using multiple assignments will ensure no possibility is overlooked.
TIP: The curved arrows, used to track what nuclei are coupled to each other, are drawn as dotted lines to indicate an ambiguous assignment. Visually, the elucidator can differentiate this ambiguous assignment from curved arrows with solid lines used for definite assignments.