Computerized electronic spectral properties of some azobarbiturate compounds in presence of different solvents

The solvent effects on the spectra of 5-(p-substituted phenylazo) barbituric acid have been analyzed by the multiple linear regression technique using a Fortran TV program on a PC computer. The correlations based on one parameter equation gave that the solute permanent dipole-solvent induced dipole interaction (parameter M) plays the important role in determining the spectral shifts. The data based on two parameter equation gave that the solvent-solute hydrogen bonding combined with the solvent permanent dipole-solute permanent dipole interactions (E,N) are the important factors to explain the spectral shifts. For the three parameter correlations, the compounds with nitro, sulphonic, acetyl and iodo substituents gave that the combination (E,M,K) gives a good fit to the observed spectral shifts. The HMO theory and the PPP method explain the phenomena of tautomerism of barbituric acid. The data predict that the diketo tautomer is the most stable one. Using the calculated PPP binding energy and singlets transition for the dienol tautomer which are nearly the same as those of the diketo tautomer to assign that a mixture of the diketo and dienol tautomers coexist. The agreement between the calculated and the experimental values of transition energies is reasonably good. The phenomena of tautomerism of barbituric acid is studied based on the Complete Neglect of Differential Overlap (CNDO) method. Calculations have shown that the dioxo form is the most stable tautomer. The singlets electronic transitions for 5-(p-substituted phenylazo) barbituric acid have been calculated by the CNDO method. The oscillator strength values of 5-(p-substituted phenylazo) barbituric acid are calculated and found to be affected by other factors e.g., the dielectric constant of the solvent, the electronic property of the substituent and the solvation energy.