Through-bond instability in polyphenol tautomers

Unusual instability of keto tautomers in polyphenols is theoretically analyzed. Apart from ordinary resonance energy and bond energy, it is shown that there should be additional keto instability originated from through-bond interactions between the true and pseudo π orbitals. The interactions are characteristic of non-bonding like amplitude pattern in the frontier π orbitals. The magnitude of instability is estimated to be 4 kcal/mol per carbonyl group by modern DFT calculations and experimental results.