Electronic absorption spectra of benzoquinone and its hydroxy substituents and effect of solvents on their spectra.

The electronic absorption spectra of 1,4-benzoquinone (BQ) and its 2,5-dihydroxy and tetrahydroxy derivatives have been studied in detail. The interpretation of the electronic bands is made on the basis of PPP and CNDO calculations. It is found that the pi --> pi* transitions are well predicted by the PPP method. The predictions of the CNDO method are however superior both in their accuracy as well as ability to predict the n --> pi* transitions. The effect of solvents on the electronic absorption bands have also been investigated in detail. Linear correlations are found between the solvent's dielectric constant and wavelength of the absorption bands. The solvent shifts are explained on the basis of the polarities of the solute and solvent molecules as well as due to hydrogen bonding.