On the way to understand antioxidants: chromanol and dimethoxyphenols gas-phase acidities

• Published in: Journal of mass spectrometry. 2011-07, Vol.46 (7), p.640-648
• Main Authors: Amorim Madeira, Paulo J., Faddoul, Mohammed, Afonso, Marta B., Vaz, Pedro D., Fernandez, M. Tereza, Leal, João Paulo
• Format: Article
• Language: English
• Subjects: Antioxidants; Antioxidants - chemistry; Atomic and molecular physics; Bonding; Chemistry; Chromans - chemistry; Cooks kinetic method; DFT calculations; Exact sciences and technology; Free energy; gas-phase acidity; Gases - chemistry; Heat of formation; Hydrogen bonds; Hydrogen-Ion Concentration; Ionization potentials, electron affinities, molecular core binding energy; Kinetics; Linear Models; Mass spectrometry; Mass Spectrometry - methods; Mathematical analysis; Models, Chemical; Molecular properties and interactions with photons; Organic chemistry; Phenol; Phenols - chemistry; Physics; Properties of molecules and molecular ions; Quantum chemistry; Reactivity and mechanisms; Thermodynamics
• ISSN: 1076-5174; 1096-9888; 1096-9888
• DOI: 10.1002/jms.1933

Some antioxidant mechanisms displayed by several phenolic compounds relate with OH bond dissociation energy. One way for its determination, in the gas‐phase, relies on acidity measurements. Gas‐phase acidities were determined experimentally, applying the kinetic method, for chromanol and four dimethoxyphenols, and theoretically through quantum chemical DFT calculations for chromanol, six dimethoxyphenols and 3,4,5‐trimethoxyphenol. The experimental acidity order, 2,3‐dimethoxyphenol > 3,5‐dimethoxyphenol > 2,6‐dimethoxyphenol > 3,4‐dimethoxyphenol ≈ phenol > chromanol shows good agreement with the theoretical acidity order, 2,5‐dimethoxyphenol > 2,3‐dimethoxyphenol > 3,4,5‐trimethoxyphenol > 3,5‐dimethoxyphenol ≈ 2,4‐dimethoxyphenol > 2,6‐dimethoxyphenol > 3,4‐dimethoxyphenol > phenol > chromanol. These acidity trends were rationalized in terms of the substituent effects on the thermodynamic stabilities both of the parent phenols and the corresponding phenoxide ions. DFT calculations also evidenced the occurrence of intramolecular CH···O hydrogen bonds whenever there are vicinal substituents (either OH, O− or OCH3) which induce further stabilization of the geometries. Copyright © 2011 John Wiley & Sons, Ltd.