4-Carboxyl-2,6-dinitrophenylazohydroxynaphthalenes tautomerism NMR re-explained and other methods verified

• The NMR approach for tautomerism analysis in 1-((2-nitrophenyl)diazenyl)naphthalen-2-ol is verified theoretically.
• There is no concentration effect on the tautomerism in the studied compounds in DMSO.
• In spite of the different approaches used, results from NMR and UV–Vis spectroscopy are in reasonable agreement.

In two consecutive studies the tautomerism in 4-((2-hydroxynaphthalen-1-yl)diazenyl)-3,5-dinitrobenzoic acid and the structurally similar 1-((2-nitrophenyl)diazenyl)naphthalen-2-ol has been considered from viewpoint of theoretical chemistry, UV–Vis spectroscopy and NMR. Although the theoretical data (at M062X level) show that both compounds exist only as a keto tautomer, the experiment proves existence of the enol form. The difference in the results obtained by UV–Vis spectroscopy and NMR requires a deeper consideration and verification of the NMR approach, which is based on using model compounds to provide the NMR signal pattern of individual tautomers.

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