UV solvatochromic shifts of 4-hydroxy-benzophenone and 2,4-dihydroxy-benzophenone in ethanol-acetonitrile mixtures.

The anomalous UV spectroscopic behavior of 4-hydroxy-benzophenone and 2,4-dihydroxy-benzophenone in ethanol-acetonitrile mixtures has been investigated using theoretical and experimental methods. Band I of these compounds, associated to strong pi-->pi* transitions, suffers a blue shift when the polarity of the ethanol-acetonitrile solutions increases. The magnitude of the solvatochromic shifts suffered by the analyzed benzophenones (BPs) changes inversely with the planarity of their molecules. The experimental solvatochromic shifts of the compounds were correlated with the permittivity and the solvation parameters alpha and pi* that characterize the ethanol-acetonitrile mixtures used. In order to explain theoretically the observed solvatochromic shifts, it was proposed that in solution, the two compounds form an association complex of stoichiometry 1:1 with a molecule of ethanol. These complexes are formed by means of an intermolecular hydrogen bond between the hydrogen atom of 4-OH group of the solutes and the oxygen atom of ethanol. The calculations performed at the HRF/6-31G(d) level of theory using Onsager's and Tomasi's models showed that these solute-ethanol association complexes have an elevated thermodynamic stability. Good linear relations were obtained between the experimental absorption frequencies of BPs and the theoretical absorption frequencies of the association complexes. These frequencies were also very satisfactorily correlated with the properties of the above mentioned ethanol-acetonitrile mixtures. It was concluded that the magnitude of the analyzed solvatochromic shifts is determined by the degree of occurrence of solute-solvent interactions, which essentially depend on the polarity, polarizability and hydrogen bond donating ability of the ethanol-acetonitrile mixtures.