New insights into the competition between antioxidant activities and pro-oxidant risks of rosmarinic acid.

Direct and indirect antioxidant activities of rosmarinic acid (RA) based on HOO˙/CH3OO˙ radical scavenging and Fe(iii)/Fe(ii) ion chelation were theoretically studied using density functional theory at the M05-2X/6-311++G(2df,2p) level of theory. First, four antioxidant mechanisms including hydrogen atom transfer (HAT), radical adduct formation (RAF), proton loss (PL) and single electron transfer (SET) were investigated in water and pentyl ethanoate (PEA) phases. Regarding the free radical scavenging mechanism, HAT plays a decisive role with overall rate coefficients of 1.84 × 103 M-1 s-1 (HOO˙) and 4.49 × 103 M-1 s-1 (CH3OO˙) in water. In contrast to PL, RAF and especially SET processes, the HAT reaction in PEA is slightly more favorable than that in water. Second, the [Fe(iii)(H2O)6]3+ and [Fe(ii)(H2O)6]2+ ion chelating processes in an aqueous phase are both favorable and spontaneous especially at the O5, site-1, and site-2 positions with large negative Δr G 0 values and great formation constant K f. Finally, the pro-oxidant risk of RA- was also considered via the Fe(iii)-to-Fe(ii) complex reduction process, which may initiate Fenton-like reactions forming reactive HO˙ radicals. As a result, RA- does not enhance the reduction process when ascorbate anions are present as reducing agents, whereas the pro-oxidant risk becomes remarkable when superoxide anions are found. The results encourage further attempts to verify the speculation using more powerful research implementations of the antioxidant activities of rosmarinic acid in relationship with its possible pro-oxidant risks.

Antioxidant and UV-radiation absorption activity of aaptamine derivatives - potential application for natural organic sunscreens.

Antioxidant and UV absorption activities of three aaptamine derivatives including piperidine[3,2-b]demethyl(oxy)aaptamine (C1), 9-amino-2-ethoxy-8-methoxy-3H-benzo[de][1,6]naphthyridine-3-one (C2), and 2-(sec-butyl)-7,8-dimethoxybenzo[de]imidazo[4,5,1-ij][1,6]-naphthyridin-10(9H)-one (C3) were theoretically studied by density functional theory (DFT). Direct antioxidant activities of C1-C3 were firstly evaluated via their intrinsic thermochemical properties and the radical scavenging activity of the potential antioxidants with the HOO˙/HO˙ radicals via four mechanisms, including: hydrogen atom transfer (HAT), single electron transfer (SET), proton loss (PL) and radical adduct formation (RAF). Kinetic calculation reveals that HOO˙ scavenging in water occurs via HAT mechanism with C1 (k app, 7.13 × 106 M-1 s-1) while RAF is more dominant with C2 (k app, 1.40 × 105 M-1 s-1) and C3 (k app, 2.90 × 105 M-1 s-1). Antioxidant activity of aaptamine derivatives can be classified as C1 > C3 > C2. Indirect antioxidant properties based on Cu(i) and Cu(ii) ions chelating activity were also investigated in aqueous phase. All three studied compounds show spontaneous and favorable Cu(i) ion chelating activity with ΔG 0 being -15.4, -13.7, and -15.7 kcal mol-1, whereas ΔG 0 for Cu(ii) chelation are -10.4, -10.8, and -2.2 kcal mol-1 for C1, C2 and C3, respectively. In addition, all compounds show UVA and UVB absorption; in which the excitations are determined mostly as π-π* transition. Overall, the results suggest the potential applications of the aaptamines in pharmaceutics and cosmetics, i.e. as a sunscreen and antioxidant ingredient.

Theoretical Investigation of the Reaction of Pyrene Formation from Fluoranthene.

This paper has investigated the reaction process concerning pyrene formation from fluoranthene in their electronic ground states. Both aromatic compounds are considered as direct soot precursors. The geometrical parameters, the vibrational frequencies, and the zero-point energies have been calculated using the BMK (Boese-Martin for kinetics) method and the 6-311++G(d,p) basis set. More accurate single-point energies have been obtained using BMK/6-311++G(3df,2p) to retrieve thermodynamic properties (ΔrH°(T) and ΔrG°(T)) over a wide temperature range (298-2500 K). The isomerization reaction of fluoranthene to pyrene is exothermic and spontaneous in standard conditions. The transition states and the possible intermediate species have been located on the singlet potential energy surface in order to determine the reaction mechanism. Two different reaction channels have been investigated and characterized by entrance reaction barriers of about 419 and 771 kJ mol-1 for the first and the second reaction pathways, respectively. The present work demonstrates that the first reaction channel is the most energetically favored pathway at high temperatures. Therefore, the kinetic parameters of the forward and reverse first step reactions have been determined in sooting flame conditions.

Microhydration of caesium metaborate: structural and thermochemical properties of CsBO2 + n H2O (n = 1-4) aggregates.

The structures and thermodynamic properties of microhydrates of caesium metaborate (CsBO2) of nuclear safety interest are reported in this work. CsBO2 + n H2O (n = 1-4) molecular complexes were identified on the potential energy surface. The structures were optimized using the ωB97XD DFT method and the aug-cc-pVTZ basis set. Single-point energies were calculated at the CCSD(T)-F12a/awCVTZ and the ωB97XD/aug-cc-pVQZ levels of theory. The standard reaction enthalpies and the standard Gibbs free reaction energies were reported for all molecular complexes. The temperature dependence of ΔrG°(T) was evaluated for all studied structures over the temperature range 300-2000 K. Total hydration reactions were investigated. The results showed that the mono-hydrated form of CsBO2 exists only at temperatures lower than 720 K under standard conditions. The influence on the thermodynamic properties of the number of water molecules in the clusters was described, with successive dehydration from 720 to 480 K. In nuclear severe accident conditions, gaseous CsBO2 will remain unhydrated in the reactor coolant system.

Theoretical Study of the Reactions of H Atoms with CH3I and CH2I2.

High level ab initio methods have been used to provide reliable kinetic data for the H + CH3I and H + CH2I2 gas-phase reactions. The (H, I)-abstraction and I-substitution reaction pathways were identified. The structures were determined on the potential energy surface at the MP2/aug-cc-pVTZ level of theory. The energetics was then refined using the coupled cluster theory. For the iodinated species, the spin-orbit coupling was calculated using the MRCI approach. The core valence and the scalar relativistic corrections were considered. Thermal rate constants were reported using the canonical transition-state theory (TST) and compared to computed values with the canonical variational transition-state theory (CVT) using the zero curvature tunneling (ZCT) and the small curvature tunneling (SCT) corrections over a wide temperature range (250-2500 K) to show the importance of quantum tunneling effects at low temperatures. They are given by the following expressions for the overall reactions using the CVT/SCT method: kH+CH3I( T) = 1.07 × 10-17 × T2.13 exp(2.68 (kJ mol-1)/ RT) and kH+CH2I2( T) = 5.73 × 10-21 × T2.97 exp(3.15 (kJ mol-1)/ RT). The I-abstraction is predicted to be the major pathway for both H + CH3I and H + CH2I2 reactions. The obtained kinetic parameters for the H + CH3I reaction are in excellent agreement with their experimental counterparts over the temperature range 300-750 K. On the basis of our calculated reaction enthalpies, a new evaluation of the standard enthalpy of formation at 298 K of CH2I and CHI2 has been provided. Obtained values are Δf H°298K (CH2I) = 219.5 kJ mol-1 and Δf H°298K(CHI2) = 296.3 kJ mol-1.

Reactivity of Hydrogen Peroxide with Br and I Atoms.

The reaction mechanisms of Br and I atoms with H2O2 have been investigated using DFT and high-level ab initio calculations. The H-abstraction and OH-abstraction channels were highlighted. The geometries of the stationary points were optimized at the B3LYP/aug-cc-pVTZ level of theory, and the energetics were recalculated with the coupled cluster theory. Spin-orbit coupling for each halogenated species was also explicitly computed by employing the MRCI level of theory. Thermochemistry for HOBr and HOI has been revised and updated standard enthalpies of formation at 298 K for HOBr and HOI are the following: ΔfH°298K(HOBr) = (-66.2 ± 4.6) kJ mol-1 and ΔfH°298K(HOI) = (-66.8 ± 4.7) kJ mol-1. The rate constants have been estimated using transition state theory (TST), canonical variational transition state theory (CVT), and CVT with small curvature tunneling (CVT/SCT) over a wide temperature range (250-2500 K). For the direct abstraction mechanism, the overall rate constant at 300 K was predicted to be 2.58 × 10-16 and 7.42 × 10-25 cm3 molecule-1s-1 for the Br + H2O2 and I + H2O2 reactions, respectively. The modified Arrhenius parameters have been estimated for the overall reactions: kBr+H2O2(T) = 4.80 × 10-26 T4.31 exp(-5.51 (kJ mol-1)/RT) and kI+H2O2(T) = 3.41 × 10-23 T3.29 exp(-56.32 (kJ mol-1)/RT).