January 26, 2010
There are many challenges present in deciphering correlations on an HMBC experiment. Barring any data collection issues, most of the challenges will arise from signals that are overlapping, thus, adding an element of ambiguity.
The 1H -13C HMBC spectrum below shows a correlation between the 13C at 71.2 ppm and the overlapping 1H signals at 2.08 (d) and 2.12 (m) ppm. There is nothing usually here, right?
A nearby correlation on the same 1H -13C HMBC spectrum (see below) shows a correlation between the 13C at 75.01 ppm and the 1H at 2.28 ppm. The splitting pattern for this correlation suggests that the previous correlation might also be splitting. If this is the case, then the correlation is centered on 71.26 and 2.10 (m) ppm, and thus, there is no correlation to the doublet at 2.08 ppm. In the end, further analysis of the overlapping multiplets between 2.00-2.15 ppm is needed to resolve the ambiguity.