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Tautomerism in barbituric and thiobarbituric acids
Author(s) -
Millefiori Salvatore,
Millefiori Arcangelo
Publication year - 1989
Publication title -
journal of heterocyclic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.321
H-Index - 59
eISSN - 1943-5193
pISSN - 0022-152X
DOI - 10.1002/jhet.5570260324
Subject(s) - tautomer , chemistry , barbituric acid , computational chemistry , thiobarbituric acid , phase (matter) , ionization , medicinal chemistry , organic chemistry , ion , antioxidant , lipid peroxidation
The molecular geometry of barbituric and thiobarbituric acid tautomers have been fully optimized using the AMI method in order to estimate the relative energies of the tautomers. The results are in agreement with available experimental data and indicate that in the vapour the barbiturate ring is essentially planar. In both unsubstituted compounds the trioxo structure is found to be the most stable one, in agreement with experimental findings in the solid. Tautomeric equilibria are sensitive to phase change and to substitution at the C 5 position. On passing from the vapour to the water the population of the most polar structure increases, although the order of stability remains unchanged and only for the Br and I derivatives it can be suggested the coexistence of two forms in solution. The substitution at C 5 does not alter the order of stability except for the 5‐nitro derivatives where the dioxo form predominates, in agreement with experimental results in the solid. Electron affinities and ionization potentials of the tautomers have been evaluated and briefly discussed. It is suggested that gas‐phase uv photoelectron spectroscopy should be able in analysing the tautomeric equilibria of barbituric and thiobarbituric acids in the vapour.