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A Critical Evaluation of the Factors Determining the Effect of Intramolecular Hydrogen Bonding on the OH Bond Dissociation Enthalpy of Catechol and of Flavonoid Antioxidants
Author(s) -
Lucarini Marco,
Pedulli Gian Franco,
Guerra Maurizio
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305311
Subject(s) - chemistry , intramolecular force , enthalpy , bond dissociation energy , radical , hydrogen bond , catechol , dissociation (chemistry) , photochemistry , electron paramagnetic resonance , semiquinone , medicinal chemistry , computational chemistry , molecule , organic chemistry , redox , thermodynamics , nuclear magnetic resonance , physics
New experimental results on the determination of the bond dissociation enthalpy (BDE) value of 3,5‐di‐ tert ‐butylcatechol, a model compound for flavonoid antioxidants, by the EPR radical equilibration technique are reported. By measurement of the equilibrium constant for the reaction between 3,5‐di‐ tert ‐butylcatechol and the 2,6‐di‐ tert ‐butyl‐4‐methylphenoxyl radical, in UV irradiated isooctane solutions at different temperatures, it has been shown that the thermodynamic parameters for this reaction are Δ H ° = −2.8±0.1 kcal mol −1 and Δ S ° = +1.3±0.2 cal mol −1 K −1 . This demonstrates that the entropic variations in the hydrogen exchange reaction between phenols and the corresponding phenoxyl radicals are also negligible when one of the reacting species is a polyphenol and that the EPR radical equilibration technique also allows the determination of the OH BDEs in intramolecularly hydrogen‐bonded polyphenols. The BDE of 3,5‐di‐ tert ‐butylcatechol (78.2 kcal mol −1 ) was determined to be identical to that of α‐tocopherol. Through use of the group additivity rule, this piece of data was also used to calculate the strength of the intramolecular hydrogen bond between the hydroxyl proton and the oxygen radical centre in the corresponding semiquinone radical (5.6 kcal mol −1 ), which is responsible both for the excellent antioxidant properties of catechols and for the BDE of catechol (81.8 kcal mol −1 ). These values are in poor agreement with those predicted by DFT calculations reported in the literature (9.5 kcal mol −1 and 77.6 kcal mol −1 , respectively). Extensive theoretical calculations indicate that the BDE of catechol is reproduced well (81.6 kcal mol −1 ) by use of diffuse functions on oxygen and the CCSD method.