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Logic Puzzle #21: COSY 3J or 4J or 5J? … Solution

Tying together the information from a 1H NMR and COSY spectrum, an elucidator can piece together certain fragments. Below is the solution for Logic Puzzle #21.

The 1H NMR spectrum shows four aromatic methine multiplets at 6.61, 7.15, 7.22 and 7.58 ppm with coupling constants less than 2.5 Hz. The small coupling constants points to an aromatic system exhibiting meta coupling or geminal coupling for an unsaturated system. Note the 2JHH coupling response was not part of the question.

The 1H-1H COSY spectrum shows that proton signals at 6.61 and 7.15 ppm are coupled to each other and likewise for the signals at 7.22 and 7.58 ppm. Since the intensity of the off-diagonal correlations are significantly weaker in relation to the diagonal, meta coupling is most likely the case here.

As such, the off-diagonal correlations on the COSY experiment relate to 4JHH coupling responses.

Thank you Djalma for your comment.