Vibrational and ultrasonic relaxation spectroscopic study of keto-to-enol tautomerism: The case of acetylacetone

We report on a concentration-dependent Raman and ultrasonic relaxation study of the binary acetylacetone solutions in a wide mole fraction range. We combined vibrational and ultrasonic relaxation spectroscopies with DFT theoretical calculations for the systematic examination of the keto-to-enol tautomerism taking place in acetylacetone solutions with polar and non-polar solvent. Two sets of solutions were prepared with different solvents in terms of polarity. Raman spectra revealed that the keto and enol tautomers of acetylacetone coexist in solution and the keto tautomer is favored as the solvent polarity increases. A single Debye-type relaxation process is detected in the acoustic spectra, which is assigned to the interconversion process between ACAC tautomers. The isentropic volume change associated with the isomerization reaction was estimated from the acoustic data and found to decrease up to x-0.2, while above this mole fraction reaches an almost constant value of-13.6 ml/ mol, close to the theoretically estimated value of Delta Vtheor,CCl4=12.955 ml/mol in CCl4 solvent environment. The keto-to-enol isomerization in the dilute region was found to be strongly affected by the presence of the non-polar solvent molecules, while for mole fractions higher than x-0.3 the process is not considerably altered. Several physical properties of the system including density and acoustic properties, as well as the intermolecular free length and polarizability exhibited an inflection point at a specific mole fraction signifying the cross-over of two different regimes related with different tautomerization rates analogous to that observed in the Raman spectra. The results are analyzed and discussed in the framework of the current phenomenological status of the field.