Vibrational spectra and theoretical studies of tautomerism and hydrogen bonding in the violuric acid and 6-amino-5-nitrosouracil system

Violuric acid and related 6-amino-5-nitrosouracil species are typical substituted N-heterocyclic bases displaying a rather complex tautomeric behavior, involving more than 10 possible isomers. In addition, analogously to the nucleic bases, these species can interact by means of hydrogen bonding between the complementary groups, as characterized by vibrational spectroscopy. After the theoretical evaluation at semiempirical level (PM3) and ab initio level (DFT and HF) using the 6-31-G(d) basis set, the most stable tautomer in each case has been identified and its vibrational spectrum calculated and compared with the experimental ones, allowing the identification of the functional groups involved in hydrogen bonding in the associated species.