Electronic structure of pi systems: XVII. Electronic absorption spectra and tautomerism of 3-acetyl-4-hydroxycoumarin

Electronic absorption spectroscopy and quantum-chemical calculations were u sed to study tautomerism of 3-acetyl-4-hydroxycoumarin. 3-Acetyl-4-hydroxyc oumarin in nonpolar solvents (n-hexane, CCl4) prefers the 3-acetyl-4-hydrox ycoumarin form, while in polar solvents (methanol, ethanol) it exists predo minantly in the 3-(alpha -hydroxy)ethylidene-2,4-chromandion form. As follo ws from quantum-chemical calculations, these two tautomeric forms are more stable that the other four possible tautomers of 3-acetyl-4-hydroxycoumarin . The energies of intramolecular hydrogen bonds in various tautomeric forms were estimated by the AMI semiempirical method. The strongest hydrogen bon ds were found in the 3-acetyl-4-hydroxycoumarin and 3-(alpha -hydroxy)ethyl idene-2,4-chromandione forms.