Synthesis and computationally assisted spectroscopic study of tautomerism in 3-(phenyl(2-arylhydrazineylidene)methyl)quinoxalin-2(1H)-ones

The recently developed efficient protocol combining implicit and explicit, accurate quantum mechanical modeling of the condensed state [Katsyuba et al., J. Chem. Phys., 2021, 155, 024507] is used to describe the tautomeric/isomeric/conformational composition and the IR and UV-Vis spectra of a series of 3-(phenyl(2-arylhydrazineylidene)methyl)quinoxalin-2(1H)-ones and their solutions in dimethylformamide (DMF) and dichloromethane (DCM). Only Z-isomers of oxo and hydroxy tautomers of the compounds are shown to exist in the solutions in perceptible quantities in conformations stabilized by OHMIDLINE HORIZONTAL ELLIPSISO or NHMIDLINE HORIZONTAL ELLIPSISN and/or NHMIDLINE HORIZONTAL ELLIPSISO intramolecular hydrogen bonds. Oxo tautomers dominate in DMF and in the solid state, while DCM stabilizes hydroxy forms. Convenient condensed-state spectroscopic markers of the tautomers are revealed.