Semiempirical study of tautomerism in alloxan

Abstract

103-111A comparative semiempirical study of tautomerism in alloxan is performed with a view to understand the areas of applicability and limitations of the three methods, MNDO, AM1 and PM3. The results are also compared with an ab initio study. It is found that the MNDO method overestimates the stability of lactim forms, while AM1 method seems to underestimate their stabilities. All methods indicate that the tetraketo form is the most stable, followed by the monohydroxy forms. While the PM3 method seems to give the best results regarding relative stabilities of tautomers, it fails in reproducing experimental geometries and charge distributions and in explaining the chemical and biological reactivity of alloxan. The calculated MNDO and AM1 charge densities agree with the results of an ab initio study and with experimental estimates, and are thus able to explain the absence of hydrogen bonding in the crystal structure of alloxan. Nucleophilic attack by water at 5-position leads to a significant increase in the stabilities of the tetraketo and 2-hydroxy forms. Thus, due to the absence of hydrogen bonding interactions in the crystal structure of alloxan, the MNDO geometries are in closest agreement with experiment, and the charge densities are also consistent with experimentally estimated ones