Local atomic and orbital reactivity indices from density functional calculations for hydrogen-bonded 1,2-dihydroxybenzene

Abstract

The Kohn–Sham LCGTO–DFT program deMon-KS3 was used to calculate the geometry, the electronic structure, and the local charge fluctuations in catechol as a model for substituted flavonoids with an internal hydrogen bond. The results bring some new insight into the nature of internal hydrogen bonding and its role in radical stabilization. Particular attention is paid to the effect of the hyperconjugation inherent to these systems. The relative strength of the intramolecular hydrogen bond is about 3.62 kcal/mol in the neutral state, decreases to about 1.42 kcal/mol in the cation radical form, and, remarkably, increases to about 8.9 kcal/mol in the radical formed by proton loss. The hydrogen-bonded oxygen atom shows a larger degree of electron localization in terms of the local charge–charge fluctuations compared to the oxygen without a hydrogen bond. The concept of orbital hardness indices was used to describe in more detail the interrelationship between internal hydrogen bonding and stabilization of neutral or charged radicals. These trends reflect the effect of hyperconjugation in terms of a strong delocalization of the responsible σ orbital and a strong localization of the related π orbital. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 161–173, 2000