The tautomeric structures of 3(5),3′(5′)-azopyrazole [(E)-1,2-di(1H-pyrazol-3(5)-yl)diazene)]: The combined use of NMR and electronic spectroscopies with DFT calculations

An azo derivative of 1H-pyrazole has been chosen to study the information obtained from different techniques for determining the structure of a non-crystalline compound in the solid-state and in solution. Syn–anti isomerism of the azo group, prototropic tautomerism of the 1H-pyrazole and rotation about the pyrazole–azo group resulted in 20 structures that were analyzed. Energy calculations, 13C and 15N chemical shifts, 1H–1H coupling constants and electronic spectra reduced the 20 possible structures to only one, the 3,3′-Z,Z-anti-azopyrazole.

► An azo derivative of 1H-pyrazole was studied in solution and in the solid-state.
► Syn–anti isomerism, tautomerism and rotation about pyrazole–azo bond were determined.
► Energy of the 20 possible structures and some electronic spectra were calculated.
► 13C and 15N chemical shifts and 1H–1H coupling constants were calculated.