The mechanism of proton transfer and tautomerism in barbituric acid: A theoretical study

Intermolecular proton transfer in barbituric acid (BA) both in the gas phase and in the presence of water molecule was investigated using the B3LYP method with augccpVDZ basis set. The overall process involves ketoenol and lactamlactim tautomerism that occurs. The most stable form of barbituric acid turns into the intermediates with threestep proton transfer at three different positions of the structure of this compound. Then, these intermediates become to the final products passing through two competing processes. In order to determine the priority of each competitive process, energy changes of all possible tautomeric forms were investigated along with the role of solvent molecules in multiple routes. Also, transposition in the proton transfer process makes changes in the electron transfer energy which was investigated both in the gas phase and in the presence of water molecules. Frequency calculations performed to characterize ground and transition states and calculation of zero point energies. Natural bond orbital analysis and topological property of electron density were investigated using natural bond orbital (NBO) and atoms in molecules (AIM) analysis. From the results, all proton transfer process in the gas phase and in the presence of the solvent molecule (water) is exothermic and nonspontaneous. In the gas phase and in the water assisted condition, the route 1b is kinetically more preferred. Despite the increase of the increase of ring resonance of stronger hydrogen bonds in the product compared to the reactant, the products are more unstable than the reactant. Therefore, the only factor can make the product unstable is the severe decrease of LP (N) →σ* (C-O) electron transfer energy.