Spectroscopic and semiempirical investigation of the structural features of hetarylazo-5-isoxazolones tautomerism

3-Methyl-4-hetarylazo-5-isoxazolones (I–IV) have been synthesized and investigated on the basis of Fourier transform infrared (FT-IR), UV–visible (UV–Vis), proton nuclear magnetic resonance (1H NMR) spectroscopic techniques and elemental analysis. The measured FT-IR and 1H NMR spectra together with the theoretical calculations (semiempirical) showed that the investigated compounds exist in the hydrazone-keto form in the solid state and chloroform. However, the measured UV–Vis electronic absorption spectra in solutions clarified that the hetarylazo-5-isoxazolone dyes possess equilibrium between hydrazone-keto and common anion forms depending upon the nature of the medium. Furthermore, these dyes display a broad absorption band in the visible region (between 370 and 435 nm) which could be assigned to π–π∗ transition with a considerable intramolecular charge transfer (CT) character