Chelation of the bare and hydrated iron(II) cation by quercetin has been investigated at the DF/B3LYP level in the gas phase. Several complexed species arising from neutral and anionic forms of the ligand have been taken into account. Both 1:1 and 1:2 metal/flavonoid stoichiometries have been considered. Results indicate that among the potential sites of chelation present on quercetin, the oxygen atoms belonging to the 3-hydroxy and 4-oxo, and to the 5-hydroxy and 4-oxo groups, are the preferred ones. Time-dependent density functional theory (TDDFT) calculations, used to reproduce the electronic UV-vis spectra of isolated quercetin and its complexes with Fe2+, were also performed in methanol and dimethylsulfoxide.

The potential energy surfaces corresponding to the dehydrogenation reaction of H2O, NH3, and CH4 molecules by Fe+(6D, 4F) cation have been investigated in the framework of the density functional theory in its B3LYP formulation and employing a new optimized basis set for iron. In all cases, the low-spin ion−dipole complex, which is the most stable species on the respective potential energy hypersurfaces, is initially formed. In the second step, a hydrogen shift process leads to the formation of the insertion products, which are more stable in a low-spin state. From these intermediates, three dissociation channels have been considered. All of the results have been compared with existing experimental and theoretical data. Results show that the three insertion pathways are significantly different, although spin crossings between high- and low-spin surfaces are observed in all cases. The topological analysis of the electron localization function has been used to characterize the nature of the bonds for all of the minima and transition states along the paths.

In this paper we report recontracted LANL2DZ basis sets for first-row transition metals. The valence-electron shell basis functions were recontracted using the PWP86 generalized gradient approximation functional and the hybrid B3LYP one. Starting from the original LANL2DZ basis sets a cyclic method was used in order to optimize variationally the contraction coefficients, while the contraction scheme was held fixed at the original one of the LANL2DZ basis functions. The performance of the recontracted basis sets was analyzed by direct comparison between calculated and experimental excitation and ionization energies. Results reported here compared with those obtained using the original basis sets show clearly an improvement in the reproduction of the corresponding experimental gaps.

The effectiveness of naturally occurring antioxidant caffeic acid in the inactivation of the very damaging hydroxyl radical has been theoretically investigated by means of hybrid density functional theory. Three possible pathways by which caffeic acid may inactivate free radicals were analyzed: hydrogen abstraction from all available hydrogen atoms, hydroxyl radical addition to all carbon atoms in the molecule, and single electron transfer. The reaction paths were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory including the contribution of tunneling. The more reactive sites in caffeic acid are the C4OH phenolic group and the C4 carbon atom, for the hydrogen abstraction and radical addition, respectively. The single electron transfer process seems to be thermodynamically unfavored, in both polar and nonpolar media. Both hydrogen abstraction and radical addition are very feasible, with a slight preference for the latter, with a rate constant of 7.29 × 10(10) M(-1) s(-1) at 300 K. Tunnel effects are found to be quite unimportant in both cases. Results indicate caffeic acid as a potent natural antioxidant in trapping and scavenging hydroxyl radicals.

The effectiveness of naturally occurring antioxidant quercetin in the inactivation of the damaging lipid peroxide radical was investigated by means of hybrid density functional based approach, using the direct dynamics method, where the thermal rate constants were calculated using variational transition-state theory with multidimensional tunneling. H-atom abstraction in quercetin by CH(3)OO peroxide occurs preferentially at the 4'OH phenolic site, from both kinetic and thermodynamic points of view. In principle, the narrowness of the obtained adiabatic potential-energy profile makes the occurrence of a significant tunnelling contribution possible. In fact, this contribution enhances the value of the computed rate constant at 300 K from 1.94 x 10(1) to 9.63 x 10(3) M(-1) s(-1) indicating that quercetin is a potent natural antioxidant in trapping and scavenging free radicals.