When carbonyls beat aromaticity: an additive framework for predicting tautomeric preferences.

Aromatic structures are often regarded as privileged stabilising motifs, generally retained even when competing structural effects are present. Here, we show that functional-group energetics can overturn this expectation in heteroaromatic tautomerism. Analysis of the 2-hydroxypyridine/2-pyridone equilibrium reveals a near-cancellation between carbonyl stabilisation and the energetic penalty associated with perturbing the aromatic ring framework, accounting for the small experimental energy gap. Extending this approach to pyridines, diazines, and triazines reveals a simple additive pattern: hydroxy/carbonyl exchange favours carbonylated, non-aromatic tautomers, typically requiring two C[double bond, length as m-dash]O/N-H units to invert the preference, whereas amino/imine and methyl/methylidene exchanges reinforce aromatic tautomers. Spin-coupled generalised valence bond (SCGVB) calculations analysed within the GPF-EP method show that the quasi-classical component favours the aromatic 6c-6e π framework, whereas quantum interference favours localised 2c-2e bonds, especially C[double bond, length as m-dash]O. This provides an energy-decomposition expression of π-distortivity: without interference, the aromatic tautomers would predominate across all series. Complementary NBO analysis suggests that this trend reflects stronger lactam-type donor-acceptor delocalisation in carbonylated N-heteroaromatic tautomers than in the corresponding imine, methylidene, or all-carbon benzenoid keto analogues. The model rationalises benzenoid phenols as boundary cases and predicts tautomeric preferences along the deamination pathway from melamine to cyanuric acid. These results establish a framework for predicting tautomeric preferences and clarify how aromatic-ring stabilisation is balanced against functional-group energetics.