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How to deal with ambiguity in an HMBC spectrum? … Part 3

July 24, 2008

Nine times out of ten, the consequence of an incorrect assignment of a 2D correlation to a 1D resonance is a structural dead end. That is, the fragments cannot be pieced together to present a clear-cut candidate(s). Dealing with ambiguity in an HMBC spectrum is a common area where the structure elucidation process can go off-track and hit that structural dead end.

The 1H-13C DEPT-HSQC spectrum below illustrates 2 terminal alkene CH2 groups with coinciding 13C resonances at 112.0 ppm.


The 1H-13C HMBC spectrum (optimized to 8 Hz) illustrates a correlation between the 13C resonance at 112.0 ppm and a 1H resonance at 5.38 ppm for a CH group.


Recognizing the overlapping 13C resonances, there are two possible fragments to consider. The fragment below illustrates such a scenario where a misassignment can inhibit an elucidation.


Note: three fragments are possible if you consider both carbons correlated to the same 1H resonance.


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