Evidence for the O-H⋅⋅⋅O=C Resonance-Assisted Hydrogen Bond in Tropolones and Quantification of its σ- and π-Components Using Molecular Tailoring Approach

For a series of tropolones, the nature of the intramolecular O-H & sdot;& sdot;& sdot;O=C hydrogen bond closing the five-membered quasi-cycle was studied. Enhancement of conjugation in the hydrogen-bonded rotamer was revealed. Quantification of hydrogen bond energy in tropolones via the molecular tailoring approach yields values in the range from 15 to 20 kcal/mol suggesting that the intramolecular interaction in tropolones has nature of the resonance-assisted hydrogen bond. The total resonance-assisted hydrogen bond energy in the tropolones was divided into sigma- and pi-components. The magnitudes of total energy of resonance-assisted hydrogen bond in the substituted tropolones can be controlled by the electronic properties of the substituents at the tropone ring. In 3-, 4-, and 5-substituted tropolones, the resonance-assisted hydrogen bond energy is raised due to electron-donating substituents and lowered due to electron-withdrawing ones. The opposite trend is observed in 7-substituted tropolones. The size of the pi-shares plays a crucial role in establishing the total energy of resonance-assisted hydrogen bond. The reason for the occurrence of a resonance-assisted hydrogen bond in the tropolones is the molecular backbone aromaticity, since, in accordance with the H & uuml;ckel rule, 10 pi-electrons are delocalized. The intramolecular O-H & sdot;& sdot;& sdot;O=C hydrogen bond in tropolones was quantified. The belonging of this hydrogen bond to the category of resonance-assisted hydrogen bonds was proven. The total energy of this hydrogen was divided into sigma- and pi-components. Control of hydrogen bond energy by substituents at the tropone ring was demonstrated. image