Hülya Yekeler

Ab initio study on tautomerism of 2-thiouracil in the gas phase and in solution.

Ab initio geometry optimizations were carried out at the HF/3-21G and HF/6-31+G** levels for the six tautomeric forms of 2-thiouracil (2TU, 2TU1, 2TU2, 2TU3, 2TU4, 2TU5) in the gas phase and in solution. To obtain a more definitive estimate of the relative stabilities for 2-thiouracil tautomers in the gas phase, single-point MP2/6-31+G** calculations were performed on the HF/6-31+G** optimized geometries. The tautomeric equilibria in 1,4-dioxane (ε=2.21), acetonitrile (ε=38), and in water (ε=78.54) were studied using the self-consistent reaction field (SCRF) theory. The calculated relative free energies indicated that 2TU is the energetically preferred tautomer in the gas phase and in solution. The stability order of 2-thiouracil tautomers depends on the level of theory and the dielectric constant of the solvent. The obtained results are compared with the available experimental data.

Hydrogen bonding and dimeric self-association of 2-pyrrolidinone: an ab initio study.

Ab initio calculations on the different associated structures of 2-pyrrolidinone with water and with itself were carried out using 3-21G and 6-31G* basis sets at the Hartree–Fock level, including electron correlation using second-order Møller–Plesset perturbation theory. The calculated free energy changes for the intermolecular hydrogen bonded dimer and hydrated species indicated that the molecular systems with cyclic dimerization and association with two water molecules are dominant. The results are compared to the available experimental data in the literature.

Solvent effects on dimeric self-association of 2-pyrrolidinone: an ab initio study.

Full geometry optimizations were carried out for 2-pyrrolidinone (2-Py) and its dimer structures at the DFT (B3LYP) and HF levels in the gas phase and in solution. Additionally, single-point MP2/6-31G** calculations were performed on the HF/6-31G** optimized geometries. The self-association of 2-Py in 1,4-dioxane (ε=2.21) and in water (ε=78.54) were studied using the self-consistent reaction field (SCRF) theory. The calculated relative free energies (ΔG) for the dimerization processes predict the cyclic dimer (CD) to be more stable than the open dimer (OD) at all levels of theory in the gas phase and in solution. The results show that, at least for the properties of the hydrogen-bonded systems studied in this work, the B3LYP/6-31G** calculations give better results than the HF and MP2 calculations with the same basis set. The data obtained from this study were compared with the corresponding experimental results when available.

An investigation of keto–enol tautomerism in N-vinyl-2-, and 3-pyrrolidinones using ab initio and density functional theory calculations

Abstract We investigated the protomeric tautomerism of N-vinyl-2-pyrrolidinone (NV2P) and N-vinyl-3-pyrrolidinone (NV3P) molecules, and considered the three neutral tautomers for NV2P (1a, 1b, 1c) and the five for NV3P (2a, 2b, 2c, 2d, 2e). Full geometry optimizations were carried out at the HF/6-31G**, HF/6-31+G** and B3LYP/6-31+G** levels in the gas phase and in water. Additionally, single-point MP2/6-31+G** calculations were performed on the HF/6-31+G** optimized geometries. The tautomerization processes in water (e=78.54) were studied using the self-consistent reaction field theory. According to our estimations, the tautomerization of NV2P and NV3P molecules are not strongly influenced by the polar solvents, the improvement of the basis set quality and the electron correlation effects in the gas phase and in water. The calculated relative free energies (ΔG) predict that 1a and 2a are the energetically preferred tautomers in the gas phase and in water.

Prediction of the acidities of organic bases in aqueous solution using AM1 COSMO solvation model

Abstract Potentially tautomeric pyrazole-3-ones were studied, in order to predict their tautomeric equilibrium constants and pKa values, using the semiempirical AM1 COSMO solvation model at the SCF level in aqueous solution. Syn and anti conformers of the hydroxy form were also investigated to find a more stable form. The oxo forms have been found to be more stable than the hydroxy ones. The results obtained from the tautomeric equilibria and acidity calculations are in good agreement with experimental data.