Theoretical and spectroscopic study of ethyl 1,4-dihydro-4-oxoquinoline-3-carboxylate and its 6-fluoro and 8-nitro derivatives in neutral and radical anion forms

A systematic comparative theoretical and spectroscopic study has been performed on a series of four recently prepared ethyl 4-oxoquinoline-3-carboxylate derivatives possessing a variety of biological activities. The most probable oxo- and hydroxy-tautomeric neutral molecular forms were identified using density functional theory (DFT). Vertical optical transitions were calculated for global minima using time-dependent version of DFT. Calculated spectra were compared with the experimental electronic spectra of quinolones measured in various aprotic solvents (toluene, acetonitrile, dimethylsulfoxide). Finally, the structures and spin density distributions of radical anions obtained upon photoinduced reduction of two nitro-substituted derivatives in titania suspension were deduced from the comparison of calculated isotropic hyperfine coupling constants with experimental data determined from EPR spectra.