When trying to elucidate an unknown structure using 2D NMR information, an elucidator gains an advantage by analyzing all of the NMR data as a whole rather than as individual pieces. Although complicated at first, this different perspective at viewing the NMR data can facilitate the elucidation process.
The following list are a few advantages for overlaying (also referred to as collect or dual mode) an HSQC with an HMBC:
HMBC correlations can be used to differentiate overlapping HSQC correlations (and vice versa),
by lining-up the correlations for both experiments, fragments can be quickly pieced together,
the number of HMBC correlations per HSQC correlation can be determined easily,
quaternary carbons can be directly differentiated and viewed alongside the protonated carbons.
Shown below are the spectral data of a 1H NMR, an 1H-13C HSQC (green) and an 1H-13C HMBC (red) for an indole group (assigned chemical shifts are shown in blue). The different correlation colours enable the elucidator to simply distinguish the 2D NMR experiments. Carbons 111, 119.4, 119.8, 121 and 125 ppm, shown in green, are the protonated carbons from the HSQC data, thus differentiating the quaternary carbons at 116 and 128.5 ppm. The 13C resonance at 111 ppm shows 2 1H correlations: 7.17 and 7.34 ppm (illustrated with black arrows). The 1H resonance at 7.34 ppm (13C at 111 ppm) shows 3 13C correlations: 119.4, 121 and 128 ppm.