DFT/B3LYP Study of the Enthalpies of Homolytic and Heterolytic O–H Bond Dissociation in Sterically Hindered Phenols
Institute of Physical Chemistry and Chemical Physics, Slovak University of Technology, Radlinského 9, SK-812 37 Bratislava, Slovak Republic
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Abstract: For 15 para-substituted sterically hindered phenols, i.e. phenols with large tert-butyl groups in the two ortho positions, the reaction enthalpies related to three mechanisms of phenolic antioxidants action: (i) hydrogen atom transfer (HAT), (ii) single-electron transfer – proton transfer (SET-PT), and (iii) sequential proton loss electron transfer (SPLET) in gas-phase, were calculated using DFT/B3LYP/6-311++G** method. Computed enthalpies were compared with available experimental values and with data obtained for para-substituted phenols. Obtained reaction enthalpies were also correlated with Hammett constants, sp. Electron-donating groups lower BDE, IP and ETE and induce an increase in PA and PDE. Electron-withdrawing groups cause a decrease in PA and PDE and a rise in BDE, IP and ETE. Dependences of studied reaction enthalpies on Hammett constants can be considered linear. In the case of HAT and SPLET mechanisms, we have found linear dependences between corresponding enthalpies (BDE, PA, ETE) and length of phenolic C–O bond. Linear dependence between this bond length and Hammett constant, sp, has been obtained, too.
Keywords: sterically hindered phenol, antioxidant, substituent effect, reaction mechanism
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Acta Chimica Slovaca, Vol. 4, No. 2, 2011, pp. 55—71