An NMR signal must be substantially bigger than the noise level to investigate. A minimum requisite for an S/N level might be 3:1. In standard 1 H NMR experiments of small organic molecules.

So if you run an HMBC and either have expected correlations missing or you want to know if an observed signal is over 2 bonds or more, you should give H2BC a try. STD The saturation transfer ...

Image Credit: Nanalysis Corp. In the amide, N,N–dimethylacetamide (DMA), two different signals are observed on the 1 H NMR spectrum as two methyl groups bond to the nitrogen (Figure 1).