ABSTRACT:The AM1 method is used to study the mechanisms of intra-and intermolecular proton transfer in barbituric acid. In the vapor phase, of the three possibilities, although thermodynamic factors favor the migration of a methylene proton to a neighboring oxygen, kinetic effects favor proton transfers from imine nitrogens. The intervention of a single water molecule has a dramatic effect on the activation barrier, bringing it down significantly, particularly for the methylene proton transfer. Several factors, including the reduction in the strain in the transition state, charge-transfer effects, ionic resonance, and solvent effects were analyzed to explain this. Bulk dielectric effects further reduce the activation energies for the proton transfers. Substituents at the 5-position, however, do not seem to have much effect on the activation barriers.
The electronic structure of barbituric acid has been investigated, keeping in mind the possibility of tautomerism. It has been found that the triketo form is the most stable, followed by the 4-hydroxy tautomer. The difference in their stabilities decreases on substitution at C 5. Substituents that allow a greater degree of deloealization with the ring system (nitro, bromo, thiol) stabilize the 4-hydroxy tautomer to a greater extent since it is planar. The AMI method is found to be the best suited for studying the electronic structure ofbarbituric acid, as it gives the best agreement with the experimental geometries. Of the other two methods investigated, MNDO gives erroneous results for relative energies, while PM3 gives unsatisfactory geometries. The stabilities of the charged species resulting from deprotonation, and the radicals resulting from the removal of a hydrogen atom by heat treatment or irradiation with gamma rays, have also been investigated. Their electronic structures are also discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.