Tautomerisation of thymine acts against the Hückel 4N + 2 rule. The effect of metal ions and H-bond complexations on the electronic structure of thymine.

The stability and aromaticity of thirteen known thymine tautomers were studied in the gas phase at the B3LYP/6-311++G(2d,2p) computational level. It was found that they do not follow the Hückel 4N + 2 rule when the energetic criterion is considered, but they follow it when aromaticity indices, such as NICS, HOMA and the sum of the Wiberg bond indices, are applied. It was shown that the stability of a given tautomer is strongly dependent on the number of C[double bond, length as m-dash]O groups attached to the ring. The most stable tautomer i.e. with two carbonyl groups exhibits low π-electron delocalization (HOMA = 0.490, NICS(0) = -1.5). Its stability results from specific interactions between N(δ-)H(δ+) and C(δ+)O(δ-) bond dipoles. A qualitative rule, which implies an increase in stability and a loss of aromaticity with increasing number of C[double bond, length as m-dash]O groups, holds in the case of thymine tautomers. Effects of intermolecular interactions (H-bonding and metal ion complexation) on the geometry and π-electron structure were analyzed for the five most stable tautomers with the following partners: HF/F(-) and Li(+)/Na(+)/K(+). The magnitude of these effects strongly depends on the site and type of intermolecular interaction. The electronic structure of the most aromatic tautomers is more weakly influenced by external perturbations such as H-bonding and is almost entirely resistant to metal complexation.