UV-induced photoisomerization of maleic hydrazide

Monomers of maleic hydrazide (3-hydroxypyridazin-6-one) were studied using the experimental matrix-isolation technique as well as DFT and QCISD methods of quantum chemistry. The oxo-hydroxy tautomer was theoretically predicted to be the most stable form of the compound. The energies of the dihydroxy and dioxo forms were calculated (at the QCISD level) to be higher by more than 20 kJ mol−1. Only the oxo-hydroxy form was trapped from the gas phase into low-temperature Ar matrices. UV irradiation of matrix-isolated maleic hydrazide induced two isomerization processes: (i) hydrogen-atom transfer converting the oxo-hydroxy form into the dihydroxy tautomer (λ > 234 nm and λ > 200 nm); (ii) transformation to N-aminomaleimide (λ > 200 nm). Both photoproducts were identified by comparison of their experimental FTIR spectra with the spectra theoretically predicted at the DFT level.