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Identifying Peak Overlap on an HMBC Spectrum

Carbon peaks that overlap on an 1H -13C HMBC experiment can be tricky to deal with especially when additional experiments do not help to clarify the situation. A good approach is to keep note of any high correlation counts for a carbon resonance, and subsequently, treat the carbon resonance as possibly multiple carbons with coincidental chemical shifts.

The 1H -13C HMBC spectrum and attached 1D 1H NMR spectrum below illustrate a carbon resonance at 76 ppm with 7 proton correlations. Treating this as a single carbon will result in a structural dead end to the elucidation process. The next step is to treat the carbon as 2 carbons resonances.

HMBCOverlap_Oct282008     

The corresponding structure is shown below to illustrate the point.

HMBCOverlap_Str_Oct282008