As seen from Table , the H-tautomer in the series 3a-c is substantially more stable, which means that the amount of the azo form could be negligible to give a clear signal in the spectrum. The proton NMR spectra, shown in the same publication, can also provide description of the hydrazone form is mentioned as a difficult case. The used pure functionals behave differently, failing strongly in describing azo or hydrazone absorption or both. For instance, the TPSS, used in , overestimates the A-form band by 66 nm and places it at the position of the hydrazone absorption with a separation of only 4 nm (see Figure , Table ).