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Theoretical prediction ability of M06-2X functional was tested for thermodynamics of phenol, 15 para and 15 meta phenol derivatives. Calculations were done for gas phase as well as for polar and nonpolar solvents. Although predicted values might be shifted from the experimental ones in the framework of the employed DFT functional and basis set, the calculated and experimental data sets correlate well together. Very good linearity was found especially for the correlation of experimental and theoretical proton affinities. Hammett type correlations between the environments considered were compared. The phenolic C—O bond length was also tested as an alternative substituent effect descriptor while the type and position of the functional group on the aromatic ring have a direct effect on the phenolic bond.

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Coumarins represent a broad class of compounds with pronounced pharmacological properties and therapeutic potential. The pursuit of the commercialization of these compounds requires the establishment of controlled and highly efficient degradation processes, such as advanced oxidation processes (AOPs). Application of this methodology necessitates a comprehensive understanding of the degradation mechanisms of these compounds. For this reason, possible reaction routes between HO• and recently synthesized aminophenol 4,7-dihydroxycoumarin derivatives, as model systems, were examined using electron paramagnetic resonance (EPR) spectroscopy and a quantum mechanical approach (a QM-ORSA methodology) based on density functional theory (DFT). The EPR results indicated that all compounds had significantly reduced amounts of HO• radicals present in the reaction system under physiological conditions. The kinetic DFT study showed that all investigated compounds reacted with HO• via HAT/PCET and SPLET mechanisms. The estimated overall rate constants (koverall) correlated with the EPR results satisfactorily. Unlike HO• radicals, the newly formed radicals did not show (or showed negligible) activity towards biomolecule models representing biological targets. Inactivation of the formed radical species through the synergistic action of O2/NOx or the subsequent reaction with HO• was thermodynamically favored. The ecotoxicity assessment of the starting compounds and oxidation products, formed in multistage reactions with O2/NOx and HO•, indicated that the formed products showed lower acute and chronic toxicity effects on aquatic organisms than the starting compounds, which is a prerequisite for the application of AOPs procedures in the degradation of compounds.

Systematic quantum chemical investigation of quercetin and selected eight mono- and bihydroxyflavonols is presented. Structural analysis based on the Density Functional Theory showed that the energetically preferred conformation of flavonols substituted at the C5 and C3 atoms by a hydroxyl group is stabilised via intramolecular hydrogen bonds occurring between the (C4)O···HO(3 or 5) atomic pairs. Depending on the hydroxyl group positions, energetically preferred torsional orientation of the phenyl ring with respect to the planar benzo-γ-pyrone moiety changed from 0 to 180 degrees. Gas-phase electron transitions were investigated using the time-dependent DFT treatment. The dependence of maximal wavelengths on the torsional deformation of the phenyl ring is of a similar shape, i.e. minima observed for the perpendicular orientation and maxima for the planar one. Shape and energies of the Highest Occupied (HOMO) and Lowest Unoccupied (LUMO) Molecular Orbitals were compared. The obtained theoretical results were compared with available experimental data.

Antioxidant activity represents one of the important features of phenolic acids, such as hydroxyderivatives of cinnamic acid. However, in the case of cis-cinnamic acid derivatives, corresponding thermochemistry data can be still considered scarce. This work is focused on the two most relevant mechanisms of primary antioxidant action in gas-phase, non-polar benzene and in aqueous solution. Reaction enthalpies describing thermodynamics of Hydrogen Atom Transfer (HAT) and Sequential Proton-Loss – Electron Transfer (SPLET) mechanisms were theoretically investigated using (SMD) M06-2X/6-311++G(d,p) method for cis-ortho-coumaric, cis-meta-coumaric, cis-para-coumaric, cis-ferulic, cis-sinapic and cis-caffeic acid and their carboxylate anions. The effect of carboxyl COOH group deprotonation on the thermodynamics of studied mechanisms was assessed for the three environments.