DFT/B3LYP Study of the O–H Bond Dissociation Enthalpies and Proton Affinities of para- and meta-Substituted Phenols in Water and Benzene
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 30 para- and meta-substituted phenols in two solvents – water and benzene, the reaction enthalpies related to two mechanisms of phenolic antioxidants action, hydrogen atom transfer (HAT) and sequential proton loss electron transfer (SPLET), were calculated using IEF-PCM DFT/B3LYP/6-311++G** method. Phenolic O–H bond dissociation enthalpy (BDE) represents the reaction enthalpy of HAT. Phenoxide anion (ArO–) proton affinity (PA) is related to the first step of SPLET – abstraction of proton from the phenol molecule. Except the comparison of calculated BDEs with available experimental and/or theoretical values, obtained BDEs and PAs were correlated with Hammett constants. We have found that electron-withdrawing groups increase BDE, while electron-donating substituents cause a rise in PA. On the contrary, electron-donating groups lower BDE and induce the increase in PA. Dependences of BDE and PA values on Hammett constants of the substituents are linear. From the thermodynamic point of view, entering SPLET mechanism represents the most probable process in water, where PAs of all studied phenols are considerably lower than BDEs. However, in benzene, BDEs are lower than PAs, i.e. HAT represents the most probable pathway. The only exception is p-nitrophenol – its PA is lower than BDE. In comparison to gas-phase, studied solvents attenuate the substituent effect on PA. On the other hand, substituent induced changes in BDE are larger in the solution-phase.
Keywords: integral equation formalism polarizable continuum model; phenolic antioxidant; salvation; solvent effect; substituent effect
Full paper in Portable Document Format: acs_0045.pdf
Acta Chimica Slovaca, Vol. 2, No. 2, 2009, pp. 37—51