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 kcalrmol in the neutral state, decreases to about 1.42 kcalrmol in the cation radical form, and, remarkably, increases to about 8.9 kcalrmol 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 orbital and a strong localization of the related orbital.

Establishment of a Quantitative Structure-Activity Relationship Model for Evaluating and Predicting the Protective Potentials of Phenolic Antioxidants on Lipid Peroxidation

“…This phenomenon can be seen by comparing 4 with 1 , 2 , and 3 ; 11 and 12 with 13 , 14 , and 15 ; and 5 , 6 , 9 , and 10 with 7 and 8 . Another possible explanation of the activity difference is that electronic effects of ortho‐ and para‐heteroatoms, as well as the subtle equilibrium between intra‐ and intermolecular hydrogen bonds, might contribute to the high activity 32–34. The semiquinoneoid free radicals of the molecules produced after H‐abstraction can be stabilized by forming one or more intramolecular hydrogen bonds with OH at the ortho position (see Scheme ).…”

“…It should be noted that only the most stable conformation of antioxidants and their corresponding phenoxyl free radical were taken into consideration in discussing the possible mechanisms of lipid‐derived radical scavenging. For example, conformations possessing hydrogen bonds were selected in the calculation because hydrogen bonds stabilized both the molecules and the corresponding phenoxyl free radicals to a stronger extent 14,15…”

Phenolic antioxidants: electrochemical behavior and the mechanistic elements underlying their anodic oxidation reaction

“…For example, conformations with hydrogen bonds were preferable in the theoretical calculations since they were known to be most stable for both phenolics and their corresponding phenoxyl radicals. 48,49…”

Vibrational state-selective autodetachment photoelectron spectroscopy from dipole-bound states of cold 2-hydroxyphenoxide: o − HO(C6H4)O−

“…[48][49][50] In particular, o-HO(C 6 H 4 )O • has been identified as a persistent environmental free radical formed by decomposition of pyrocatechol in tobacco smoke, which has adverse effects on human health. [51][52][53] There have been a number of theoretical studies about the conformations of o-HO(C 6 H 4 )O • , [54][55][56][57] however, experimental structural information about o-HO(C 6 H 4 )O • has been limited. The o-HO(C 6 H 4 )O • radical was observed in a low-temperature matrix-isolation IR spectroscopy study.…”