Chemical Bonding and Aromaticity From Electronic Delocalization Descriptors

Red wine contains a complex mixture of bioactive compounds, including flavonols, monomeric and polymeric flavan-3-ols, highly colored anthocyanins, as well as phenolic acids and the stilbene polyphenol, trans-resveratrol. Some of these compounds, particularly resveratrol, appear to have health benefits. In particular, resveratrol exhibits also therapeutic potential for cancer chemoprevention as well as cardioprotection. In this work, we have carried out a systematic study of the antioxidant activity of transresveratrol towards hydroxyl and hydroperoxyl radicals in aqueous simulated media, using density functional quantum chemistry and computational kinetics methods. All possible mechanisms have been considered: hydrogen atom transfer (HAT), proton coupled electron transfer (PCET), sequential electron proton transfer (SEPT), and radical adduct formation (RAF). Rate constants have been calculated using Conventional Transition State Theory in conjunction with the Collins-Kimball theory. Branching ratios for the different paths contributing to the overall reaction, at 298 K, are reported. For the global reactivity of trans-resveratrol towards •OH radicals in water at physiological pH, several mechanisms contribute to the overall rate constant. Our results indicate that almost all channels are diffusion-controlled, and thus, a mixture of all the possible products will be expected. The calculated branching ratios show that the OH scavenging activity of trans-resveratrol takes place predominantly by a RAF mechanism (~ 85.0 %). However, it is interesting to point out that, because single electron transfer (SET) can occur when the reactants are at much larger distances than either in direct H-abstraction or •OH-addition mechanisms. Thus, in the RV + •OH reaction, in water at physiological pH, the main mechanism is proposed to be the sequential electron proton transfer (SEPT). Regarding the efficiency of trans-resveratrol as an •OOH radicals scavenger, we find that HAT H-abstractions from the phenolic groups are the only thermodynamically feasible reaction channels, in water. The total rate coefficient is predicted to be 1.42 x 10 M s, smaller than the ones for reactions of trans-resveratrol with •OH radicals, but still very fast. Since the •OOH half-life time is several orders larger than the one of the •OH radical, these reactions should contribute significantly to trans-resveratrol oxidation in aqueous biological media. Thus, we can conclude that trans-resveratrol acts as a very efficient •OOH, and presumably •OOR radical scavenger. Physisorption, diffusion and chemisorbed pathways of H2 molecule on Graphene and Single Walled Nanotube by first principle calculations Francesca Costanzo, Francesco Ancilotto, Pier Luigi Silvestrelli Dipartimento di Fisica “G. Galilei”, Università di Padova, Via Marzolo 8, I-35131 Padova Italy. We investigate the interaction of the H2 molecule with a graphene (G) layer and with a single wall carbon nanotube (SCNT) by Density Functional Theory. H2 can interact with a CNT [1,2] both through physisorption and chemisorption, although chemisorption is typically assumed to be irreversible, and thus it is probably technologically less relevant. The physisorption mechanism involves the condensation of the hydrogen molecule inside or between the nanotubes as a result of weak van der Waals forces, while the chemisorption mechanism involves the preliminary dissociation of the hydrogen molecule and the subsequent reaction of hydrogen atoms with the unsatured C-C bonds to form C-H bonds. We take in to account vdW interactions in DFT using the new method (DFT/vdW-WF) [3] recently developed in our group based on the concept of maximally localized Wannier functions. This new technique aims at combining the simplicity of the semiempirical formalism with the accuracy of the first principles approaches. We explore several adsorption sites and orientation of hydrogen molecule relative to the graphene plane: hollow, bridge and top site. The most stable physisorbed state is the hollow site with a binding energy of 54 meV. This value, in agreement with experimental results is also compared to that obtained by other vdW correction methods. The analysis of diffusion paths among physisorbed states show that molecular hydrogen can diffuse at room temperature from one configuration to another with typical energy barriers of ~10 meV. The situation is different when the hydrogen molecules approach the carbon surface and dissociate allowing the chemisorption on the carbon surface (G or SCNT), provided that they are able to overcome the activation energy barriers. In particular, we calculate the potential energy surfaces for the dissociative adsorption of H2 on highly symmetric sites of graphene (orto, meta and para position). The lowest activation barrier of 2.67 eV is that one that describes the process going from the hollow physisorbed state to the para chemisorbed state. Other energy paths are characterized by higher activation barriers (> 3.eV).