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|Language of final thesis:||Slovak|
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|Title of the thesis:||Theoretical Study of Energetics of Phenols Antioxidant Effect|
|Summary:||The reaction enthalpies related to the individual steps of three phenolic antioxidants action mechanisms, hydrogen atom transfer (HAT), single electron transfer - proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET), for 15 meta- and 15 para-substituted phenols (ArOH) in water, ethanol and benzene were calculated using DFT/B3LYP method in 6-311++G** basis set and polarized continuum model (PCM). Except the comparison of calculated reaction enthalpies with available experimental and/or theoretical values, obtained enthalpies were correlated with Hammett constants. We have found that electron-donating substituents induce the rise in the enthalpy of proton dissociation (PDE) from ArOH+ radical cation (second step in SET-PT) and in the proton affinities of phenoxide ions ArO- (reaction enthalpy of the first step in SPLET). Electron-withdrawing groups cause the increase in the reaction enthalpies of phenolic O-H bond homolytic dissociation and of the processes where electron is abstracted, i.e. in the ionization potentials of ArOH (first step in SET-PT) and in the enthalpy of electron transfer from ArO- (second step in SPLET). Found results indicate that dependences of reaction enthalpies on Hammett constants of the substituents can be considered linear or roughly linear. The calculations of liquid-phase reaction enthalpies indicate that substituent effects on the reaction enthalpies related to SET-PT and SPLET mechanisms are in water and ethanol weaker in comparison to published gas-phase data. On the other hand, water and ethanol strengthen the substituent effect on phenolic O-H bond dissociation enthalpy. Benzene attenuates substituent effects in lesser extent. Significant attenuation was found only in the case of proton affinities of meta- and para-substituted phenols and electron transfer enthalpies of para-substituted phenols. Benzene also strengthens the substituent effect on phenolic O-H bond dissociation enthalpy in some extent. From the thermodynamic point of view, entering SPLET mechanism represents the most probable process in water and ethanol, while in benzene HAT represents the most probable reaction pathway. Abstraction of the electron from the studied substituted phenols, characterized by ionization potential (first step of SET-PT mechanism), is the process with the highest energy requirement in all solvents. We found that proton affinities, electron transfer ethalpies and O-H bond dissociation enthalpies can be successfully correlated with the neighboring C-O bond length.|
|Key words:||BDE, IP, PDE, PA, ETE, phenols, antioxidant, DFT/B3LYP, PCM|
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