The tautomerization of salicylate anion in the presence of A1(III) in ethanol was studied by UV ? visible absorption spectroscopy and fluorescence spectroscopy, anisotropy, and lifetime measurements from 100 to 298 K. Complexation with A1(III) causes an equilibrium shift from the normal form of the salicylate anion toward the tautomer form, demonstrating that the presence of a highly charged cation, A1(III), stabilizes the tautomer form of salicylate. Spectra and fluorescence lifetimes of salicylate and other salicyl derivatives in the presence of A1(III) reveal three types of A1(III)-salicylate complexes. In type I complexes, salicylate binds to A1(III) through the carboxylate group, preserving the intramolecular hydrogen bond between the carbonyl oxygen and the phenol group, as indicated by the largely Stokes-shifted fluorescence emission following the excited state pr oton transfer process. In type II complexes, salicylate binds to A1(III) through the carboxylate group, but the phenol proton is oriented away from the carbonyl oxygen so that the complex shows short wavelength fluorescence emission characteristic of substituted phenolic compounds. In type III compleses, A1(III) stabilizes and binds to the tautomer form of salicylate through the phenolate oxyten, in which salicylate exists in its proton transferred tautomer form. Absorption spectra recorded at temperatures between 100 K and 298 K indicate that the type III tautomer complex is energetically favored at low temperature, although type I is the dominant species at room temperature. All three types of complexes are interconvertible above the ethanol glass transition temperature. However, below the glass transition temperature interconversion ceases, indicating large amplitude atomic motion is involved in the conversion.
Revised: September 18, 2002 |
Published: January 16, 2001
Wang Z., D. Friedrich, C.C. Ainsworth, S.L. Hemmer, A.G. Joly, and M.R. Beversluis. 2001.Ground-State Proton Transfer Tautomer of Al(III)-Salicylate Complexes in Ethanol Solution.Journal of Physical Chemistry A 105, no. 5:942-950.PNNL-SA-33421.