Coincidental 13C peaks on an HMBC Spectrum

When interpreting data from a 1H-13C HMBC without a high-resolution 1D 13C NMR, there is the possibility to miss coincidental overlapping carbons. As such, always consider a missing carbon or two as part of the structure elucidation for an unknown.

The 1H-13C HMBC below—note the lack of a high-resolution 1D 13C NMR spectrum—exhibits a 2-3J correlation between a proton at 8.1 ppm (H1) and a carbon at 128 ppm (C2).

Using the fragment information below, a quick structural assumption is that the unknown contains a 5 membered ring. Another possibility is that 2 coincidental carbon peaks are overlapping at 128 ppm. As such, a 6 membered ring is also possible.