The only proton expected to correlate with three nonequivalent protons is proton 9! Chemical Shift Look at the table for any clear differences in correlation and begin there! In this example, all unassigned protons show one or two COSY correlations-except the proton at 4.233 ppm, which correlates to three other protons by COSY. My favorite way to analyze a COSY spectrum with many unassigned protons is to make a table of correlations, like the one seen here. The remaining protons are doublets, triplets, and multiplets that can be assigned by 2-dimensional COSY. The peak also integrates to 1 proton, supporting the assignment. The chemical shift of 11.256 ppm supports this assignment, as imide protons often show up far downfield. The only proton that should show up as a singlet is proton 6, as it has no neighboring protons that would split the peak (the nearest proton is 5 bonds away!). There is only one singlet in the ¹H-NMR spectrum. Therefore, the peak at 7.690 ppm must represent proton 4! The integration and chemical shift support the assignment, as proton 4 is the only aromatic proton in the structure. The long-range coupling constant observed for proton 3 (J=1.2 Hz, split into a doublet by proton 4) is reflected in the coupling constant for proton 4 (J=1.2 Hz, split into a quartet by proton 3). Protons that are coupled to each other should exhibit the same coupling constant. The peak is split into a doublet with a coupling constant of 1.2 Hz, reflecting the long-range coupling between protons 3 and 4, which also supports this assignment. The high field chemical shift supports this assignment. The only peak with an integration of 3 is the doublet at 1.770 ppm. Proton 3 is the only methyl group in the structure, and therefore must integrate to 3 protons.
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