Understanding the Formation of 5-(Diethylammoniothio)-1,3-dimethylbarbituric Acid: Crystal Structure and DFT Studies

5-(Diethylammoniothio)-1,3-dimethylbarbituric acid ( 8 ) was obtained in good yield from the reaction of 5-bromo-1,3-dimethylbarbituric acid ( 5 ) and 1,3-diethylthiourea. The obtained product has been characterized using different techniques including single-crystal X-ray diffraction, FTIR, MS and NMR spectroscopy. For this reaction, a detailed computational study of the reaction steps has been performed using density functional theory (DFT). Both thermodynamic and kinetic parameters were calculated. Step B is the most favorable reaction with an activation energy of 33 kJ mol −1 using the solvation model that based on the solvent model density (SMD) calculated at B3LYP/6-31G(d). Graphic abstract The rate-determining step of the most plausible mechanism involves the formation of C-S bond via proton transfer to oxygen (intramolecular attraction) with an activation barrier of 33 kJ mol-1 using the solvation model that based on density (SMD) solvent model calculated at B3LYP/6-31G(d).