Κβαντοχημική μελέτη με μοντέλα SCRF της επίδρασης του διαλύτη σε παραμέτρους που σχετίζονται με την αντιοξειδωτική δραστικότητα φαινολικών ενώσεων

Abstract

Modeling of the solution environment is a growing area of interest within the computational chemistry. The continuum reaction field models (SCRF), representing a simple and popular approach to simulate the solution environment, have been selected for the solvent effect study of some 2-substituted phenols, 2-X-ArOH and/or their cation radicals 2-X-ArOH·+. In particular, four electron-donating (edg), and three electron-withdraw-ing (ewg) substituents were chosen, involved in a variety of biochemical transformations and phenolic antioxidants. Seven solvents, differing in their H-bonding ability and polarity, were selected to model different environmental situations. The solvent and substituent effects on the conformers, intramolecular hydrogen bond (HB) enthalpies, (DHintras), O-H Bond Dissociation Enthalpies, (BDEs) and Ionization Energies (Εis), were studied for the 2-X-ArOH and/or the 2-X-ArOH·+, in the liquid-phase, with the PCM continuum model, at the DFT/B3LYP level. Finally, the investigation of the antioxidant activity of the above phenols (or bigger antioxidant molecules) is discussed as far as the two main mechanisms, HAT and SET, are concerned. There is an absence of theoretical data for anyone of the two mechanisms, in the liquid phase. The PCM continuum model and the DFT/B3LYP with the 6-31+G(,3pd) basis set and, is found to be the optimum level for the accurate calculation of the absolute and relative values of both BDE and Εi, in solution. The investigation of the optimum level-basis set could be implemented in the gas- than in the liquid phase, in order to save computational work. The use of the UAHF model for the description of the cavity, in the PCM model, seems to be valid, in contrast to the use of the isodensity surface cavity, in the SCIPCM. Only the PCM model describes well both the “bulk” solvent effects and the solute-solvent interactions. For the PCM calculation of the hydrogen radical, a Bondi’s radius was assigned to the hydrogen, to build its cavity, hence, the differences of its solvation enthalpy was calculated theoretically in each solvent, for the first time. The seven solvents could be divided into three groups: (A) non-polar, aprotic, (B) dipolar, aprotic, and (C) polar, protic. The solvent effect study could be derived on one solvent of each group, because the values of both BDE and Εi are almost the same, in solvents belonging in the same group. In all of the HB-phenols and their cation radicals, along with the intramolecular HB, the formation of an intermolecular one should not be excluded, resulting to a weakening of the former and in some phenols, eliminating it, in the protic solvents. The main conformations of all phenols are the HB ones, in the groups A and B of solvents, while in the group C, both HB and non-HB ones are present. The strength of the intramolecular HB depends on both the solvent and the 2-substituent, appearing bigger in the case of the ewg ones, due to the presence of the six membered ring, formed.