One of the trickiest parts of interpreting a 1H-13C HMBC is deciding whether a 13C resonance is coinciding with another 13C resonance, i.e. overlapping 13C peaks. A past blog, Part 1, describes a specific case where the possibility of two coinciding 13C resonances can be deciphered based on a high carbon correlation count. Herein, we present a specific case for a 1H-13C HMBC with paired 1J responses to distinguish peak overlap.
The 1H -13C HMBC spectrum below illustrates 2 protons multiplets at 1.89 and 2.37 ppm and 2 carbon resonances at 26.8 and 43.0 ppm. The paired 1J responses at 43.0 ppm indicate a CH2 group with the proton multiplet at 1.89 ppm. Similarly, the carbon at 26.8 ppm shows a paired 1J response with the proton multiplet at 2.37 ppm (CH group). In addition, a single correlation at exactly 2.37, 26.8 ppm indicates a quaternary carbon coinciding with the carbon resonance for the CH group.
The quaternary carbon and CH group are in the vicinity of each other.