April 7, 2009
A common misinterpretation of 2D NMR data can occur when dealing with weak correlations. Weak correlations are commonly introduced in how the sample is prepared or how the data is collected or processed. Examining the spectrum down to level of the density matrix can ensure all correlations are picked up.
The 1H-13C HMQC below shows a protonated carbon at 3.2 and 32 ppm. Since the proton multiplet at 3.6 ppm has no carbon correlation, the proton is most likely from an exchangeable group such as NH or OH group.
When the spectrum threshold is lowered to 1 % relative to the most intense correlation, a weak correlation at 2.6 and 41.9 ppm is seen. Therefore, the proton at 3.6 ppm is actually a CH group and not an exchangeable one.