Among intense NMR resonances, weak resonances can be easily overlooked. This is commonly seen with 1H NMR spectra whereby 1H resonances with complicated couplings are spread out over a larger area in comparison to uncoupled 1H resonances that appear as sharp signals.
The 1H NMR spectrum below shows 3 intense resonances at 2.39, 2.66 and 3.11 ppm. The weak resonances at 2.33 and 3.26 ppm are barely visible. Upon integration, the weak resonances approximate to whole integral numbers relative to the intense resonances.
The region between 3.0 to 3.4 ppm clearly shows a multiplet at 3.26 ppm. The 1H -13C HSQC spectrum shows a correlation between the 1H at 3.26 ppm and the 13C at 31 ppm. The data from a 2D NMR spectrum serves as an additional flag to the elucidator that there is a 1H resonance in this particular region.