RESUMO
The complexation of Al(III) by 2,4(OH)(2)-acetophenone in methanol and ethanol was investigated using spectroscopic methods (UV-vis) and theoretical procedures (DFT) in order to determine its stoichiometry and stability constant and to analyze the effect of temperature, ionic strength and solvent on the reaction rate. The stoichiometric composition of the complex is 1:1 and the stability constant in methanol is greater than in ethanol. The parameters obtained by the Arrhenius equation and the transition-state theory permitted to explain why the reaction proceeds more rapidly in EtOH. The reaction is favoured when the ionic strength and the reaction medium permittivity decrease. Taking into account the kinetic results and the theoretical calculations performed, a reaction mechanism in three steps is proposed that considers the interaction between ionic species of opposite unitary charge to generate the metal complex. In the formation of the complex, the aluminium atom interacts forming a covalent bond with the oxygen atom of the hydroxyl group of the ligand and its carbonyl group by means of a strong Coulombic interaction.
Assuntos
Acetofenonas/química , Alumínio/química , Etanol/química , Cinética , Ligantes , Metanol/química , Modelos Químicos , Concentração Osmolar , Solventes/química , Espectrofotometria Ultravioleta , TemperaturaRESUMO
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.