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Computational investigation at the BHandHLYP/6-311+G(d,p) level of theory of the gas-phase tautomerism of 2- and 4-pyridones confirmed the slight prevalence of lactim in the case of the former, but its dominance in the case of the latter, as shown previously. Examination of aromaticity by using HOMA, EDDB, NBOdel, NICS and AICD led to the conclusion that tautomerization of 4-pyridone results in greater aromaticity gain. It is also driven by the Pauli repulsion relief, which was revealed by the tautomerization energy decomposition analysis. By contrast, in the case of 2-pyridone, lactim is favoured by orbital and electrostatic interactions and disfavoured by the Pauli repulsion. Aromaticity gain in this case is smaller. The position of the tautomeric equilibrium can be modulated by substituent inductive effects (Cl and F), inductive and resonance effects (NH2 and NO2), hydrogen bonding (NO2), and medium polarity, the increase of which increases lactam population.

This work was initiated by the increasing interest in BN/CC isosterism and by the long-lasting interest in the concepts of aromaticity and substituent effects. We have theoretically examined the aromaticity and stability of monosubstituted BN isosters of benzene, the three isomeric azaborines. The results provide insight into the effect of substitution on two basic molecular properties, which are influenced, here, by the substituent effects and by the B/N relationship in the ring. The results, along with other examples in the literature, also warn chemists that the general belief that aromaticity accounts for enhanced thermodynamic stability is not always true. The stability of cyclic, conjugated compounds depends on several effects, and only one of them is aromaticity. In addition, our calculations predict a switching of electronic properties of the NH2 group from the usual p-electron donor to a p-electron acceptor when it is moved from the B/C atoms to the nitrogen atom in all isomers, or C6 in 1,3-azaborine. This is the result of the conformational change that places the NLP in the plane of the ring and the NH bonds in a favourable spatial position to act as acceptors of p-electron density