January 4, 2011
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
Typical for long range 2D NMR experiments, spectral data may exhibit more than one correlation for two coupled nuclei (e.g. A to B and B to A). The pairwise correlations offer an extra degree of confidence in the interpretation.
For the following fragment, an 1H-13C HMBC correlation exists for the 1H 2.15 ppm to 13C 84.7 ppm (represented by the purple arrow in the diagram below).
Is the pairwise correlation for the 1H 5.31 ppm to 13C 21.1 ppm evident?
A special thanks goes to Gene M. for pointing me to the data.
That kind of correlation, herein between the proton at 2.15 and the carbon at 84.7, i.-e. a 4J correlation, is rare. It can be observed because the signal at 2.15 account for 3H and is singlet. Also, the torsional angle could also explain this observation (W shape). But the signal at 5.31 looks like a doublet accounting for 1H. It’s just normal that an HMBC correlation isn’t observed.