Structure of the alkali metal salts of hydroxycoumarins and hydroxyfurocoumarins and their MO LCAO calculation

Abstract

Molecules of five isomeric hydroxycoumarins, two hydroxyfurocoumarins, and their anions were calculated by the MO LCAO method in the Huckel approximation. It was shown that the 4-, 5-, and 7-hydroxy derivatives, that is, the compounds free to tautomerism, must possess stronger acid properties than those of the 6- and 8-isomers. The results of a calculation of the keto form of these compounds, which are free to tautomerism, indicated that this form is energetically less favored than that of the lactone. A comparison of the calculated and experimental values of the wavelength of the π1→π1* transition in the UV spectra of the neutral compounds and their salts showed that there is satisfactory agreement between them. The structure of the anion of 4-hydroxycoumarin within the framework of the Huckel method is better described by the molecular diagram of its keto form; for the remaining compounds, the properties of the anions are quite satisfactorily explained by the molecular diagrams of the ions in the lactone form. The results of the calculation for anions of the hydroxycoumarins and the hydroxyfurocoumarins are in agreement with the experimental data and the conclusions obtained in the IR spectral study of their salts.