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 (epsilon = 2.21), acetonitrile (epsilon = 38), and in water (epsilon = 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.
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.
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