Tautomerism in an NH- and CH- acid: A theoretical approach

Intermolecular proton transfer in barbituric acid (BA) both in gas phase and in the presence the water molecule was investigated using B3LYP method with aug-cc-pVDZ basis set. Th e overall process involves keto-enol and lactam-lactim tautomerism that occurs. Th e most stable form of barbituric acid turns into the intermediates with three steps proton transfer at three different positions of the structure of this compound. Th en, these intermediates becomes to the final products passing from two competing processes. In order to determine of the preference of each competitive process, energy changes of all possible tautomerism forms was 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 gas phase and in the presence of water molecule. Frequency calculations performed in order to determination of ground and transition states and also evaluation of zero point energies at B3LYP/aug-cc-pVDZ level of theory. Natural bond orbital analysis and topological property were investigated using natural bond orbital (NBO) and atom in molecule (AIM) analysis.