Thermodynamic Study and Total Energy Calculation for three systems of Enol↔Keto Tautomerism

Using Hartree–Fock (HF) and Density Functional Theory (DFT) calculations the thermodynamic properties such as thermal energy , , thermal enthalpy , , thermal entropy , , thermal Gibbs free energy , , heat capacity ,Cv, and molecular structures of several species involving in keto↔enol tautomerism related to acetaldehyde (A), 5,5-dimethyl-1,3-cyclohexanedione (dimedone) and  acetylacetone (AA) have been investigated. In addition, the geometric structure of all stages intervening in enol ↔ keto conversion related to three above mentioned compounds have been investigated on ab initio calculation and DFT level by using  STO-3G, 6-31G, 6-31++G and 6-31++G** basis sets. The results showed that the enol structure related to acetylacetone is more stable because of intramolecular hydrogen bond formation in the enol form while the keto structure is more stable in acetaldehyde and dimedone because of strong C=O bond existence in the keto tautomers. The computed HF/DFT results are in a good agreement with the experimental studies. Moreover,the values of , , and thermodynamic equilibrium constant, Kth, of tautomerization phenomenon related to acetaldehyde, dimedone and acetylacetone are calculated on the basis of DFT-IR method.