May 20, 2009
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
Peak shape, and subsequently the observed coupling pattern, is highly dependent on a range of factors such as temperature, concentration, pH, FID processing, etc. Relying solely on a 1H NMR spectrum for structure determination can lead into the risky realm of misinterpreting a coupling pattern. One of the simplest solutions to most elucidations is to acquire a 1H-1H COSY in addition to a 1H NMR spectrum and thus validate the coupled systems.
The 1H NMR spectrum below shows 3 multiplets at 3.1 (CH3), 3.5 (CH2) and 4.0 ppm (CH) with identical J-coupling constants. Does the coupling pattern for the CH2 group at 3.5 ppm indicate a coupling to the methine at 4.0 ppm or the methyl group at 3.1 ppm? To rephrase the question: does the 1H NMR spectrum indicate the presence of a CH-CH2 group or a CH2-CH3 group?
The peak shape for the multiplet at 3.5 ppm (CH2) appears to be a doublet and so can be considered to be coupled to the CH triplet at 4.0 ppm. However, upon closer examination of the doublet, it appears to be a poorly-resolved quartet, and thus, be considered coupled to the CH3 triplet at 3.1 ppm. In conclusion, further spectral data is needed to confirm the coupling system.