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Chasing 1H-1H Coupling Constants

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.

ChasingCouplingConstants_image1_apr2019

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.

ChasingCouplingConstants_image2_apr2019

One potential reason for what appears to be extra coupling constants is poor shimming. Shimming can affect line shape and peak resolution.

For more information, I recommend the following posts by Glenn. Post#1, post#2 and post#3.

A special thanks goes to the Ontario NMR group hosted at UTSC.