title: On the way to understand antioxidants: chromanol and dimethoxyphenols gas-phase acidities.
abstract: 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 OCH(3)) which induce further stabilization of the geometries.
authors.affiliation: Centro de Química e Bioquímica, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, Lisbon, Portugal.
authors.forename: Paulo J Amorim
authors.initials: PJ
authors.lastname: Madeira
authors.forename: Mohammed
authors.initials: M
authors.lastname: Faddoul
authors.forename: Marta B
authors.initials: MB
authors.lastname: Afonso
authors.forename: Pedro D
authors.initials: PD
authors.lastname: Vaz
authors.forename: M Tereza
authors.initials: MT
authors.lastname: Fernandez
authors.forename: João Paulo
authors.initials: JP
authors.lastname: Leal
mesh_terms: D000975:Antioxidants / Q000737:chemistry*; D002839:Chromans / Q000737:chemistry*; D005740:Gases / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D016014:Linear Models; D013058:Mass Spectrometry / Q000379:methods*; D008956:Models, Chemical; D010636:Phenols / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D000975:Antioxidants; D002839:Chromans; D005740:Gases; D010636:Phenols; C029141:2,2,5,7,8-pentamethyl-1-hydroxychroman