April 3, 2019
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
Coupling constants from a 1D NMR spectrum can be very useful to a chemist when trying to characterize a structure. However, there are cases where the perceived coupling constants can be misleading and thus wrong.
The 1H NMR spectrum below shows several multiplets for a major component tryptophan and a minor component.
For tryptophan, the aromatic multiplets exhibit a dd pattern with a small coupling of ~2 Hz. The CH2 exhibits a ddd pattern with a small coupling of ~2 Hz.
If we look at the multiplets for the minor component, the signals exhibit the same splitting pattern with the same coupling constant of ~2 Hz.
One potential reason for what appears to be extra coupling constants is poor shimming. Shimming can affect line shape and peak resolution.
A special thanks goes to the Ontario NMR group hosted at UTSC.