A DFT approach for simple and solvent assisted-proton movement: Biurea as a case of study

Abstract

306 conformers of Biurea as a biologically important molecule were optimized and frequency calculation was performed for each conformer. Then, these conformers were classified in seven tautomeric categories. Five most stable tautomers were selected and their stabilities were investigated carefully. Results showed that none of the tautomers are planar and tautomer 2 having two carbonyl function trans to each other is the most stable one. A full scheme transformation of these tautomers to each other was also conveyed through proton exchange routs. Solvent effects were studied for all tautomers and the results were compared with the gas phase. The obtained data showed that all the solvents stabilize all the available tautomers and order of solvent effect is as follow as: water>DMSO>acetone>toluene. But solvents did not have significant effect on the relative Gibbs free energy values. Then, prototropic tautomerism was performed using one-water molecule both in the gas and water phases. Explicit water indicated very important role in proton jumping. Explicit water molecule can lower energy barrier of tautomerism around 20–26kcal/mol, whereas implicit water lowers such an energy by an amount of 1–2kcal/mol.