June 20, 2008
A 1H-1H COSY dataset provides information on coupled spin systems. A coupled spin system does not necessarily imply that the protons are bonded to only carbon atoms . An example of an exchangeable proton (OH) coupled to a proton from a CH group is shown below.
Three spectral datasets, 1H NMR, HMQC and a COSY-45 dataset, are shown below. The 1H NMR spectrum shows 2 multiplets: 4.12 and 5.29 ppm with both integrating to 1 proton each. For the time being, we will not consider the coupling constants of the multiplets. The HMQC spectrum shows that only the proton at 4.12 ppm has a correlation to a carbon peak and therefore the proton peak at 5.29 ppm is most likely an exchangeable proton. The COSY spectrum shows the off-diagonal correlations between protons 4.12 and 5.29 ppm.