Thermodynamics of the hydroxyl radical addition to isoprene

Optical Identification of the Resonance-Stabilized para-Ethynylbenzyl Radical

We report the spectroscopic observation of the jet-cooled para-ethynylbenzyl (PEB) radical, a resonance-stabilized isomer of C₉H₇. The radical was produced in a discharge of p-ethynyltoluene diluted in argon and probed by resonant two-color two-photon ionization (R2C2PI) spectroscopy. The origin of the D₀(²B₁)-D₁(²B₁) transition of PEB appears at 19,506 cm^-1. A resonant two-color ion-yield scan reveals an adiabatic ionization energy (AIE) of 7.177(1) eV, which is almost symmetrically bracketed by CBS-QB3 and B3LYP/6-311G++(d,p) calculations. The electronic spectrum exhibits pervasive Fermi resonances, in that most a₁ fundamentals are accompanied by similarly intense overtones or combination bands of non-totally symmetric modes that would carry little intensity in the harmonic approximation. Under the same experimental conditions, the m/z = 115 R2C2PI spectrum of the p-ethynyltoluene discharge also exhibits contributions from the m-ethynylbenzyl and 1-phenylpropargyl radicals. The former, like PEB, is observed herein for the first time, and its identity is confirmed by measurement and calculation of its AIE and D₀-D₁ origin transition energy; the latter is identified by comparison with its known electronic spectrum (J. Am. Chem. Soc., 2008, 130, 3137-3142). Both species are found to co-exist with PEB at levels vastly greater than might be explained by any precursor sample impurity, implying that interconversion of ethynylbenzyl motifs is feasible in energetic environments such as plasmas and flames, wherein resonance-stabilized radicals are persistent.

Emerging Nonvalence Anion States of [Isoprene-H·]·H2O Accessed via Detachment of OH-·Isoprene

The anion photoelectron imaging spectra of an ion with m/z 85, generated under ion source conditions that optimize ^•OH production in a coexpansion with isoprene, are presented and analyzed with supporting calculations. A spectroscopic feature observed at a vertical electron detachment energy of 2.45 eV, which dominates the photoelectron spectrum measured at 3.495 eV photon energy, is consistent with the OH^-·isoprene ion-molecule complex, while additional signal observed at lower electron binding energy can be attributed to other constitutional isomers. However, spectra measured over a 2.2-2.6 eV photon energy range, i.e., from near threshold of the predominant OH^-·isoprene detachment feature through the vertical detachment energy, exhibit sharp features with common electron kinetic energies, suggesting autodetachment from a temporary anion prepared by photoexcitation. The photon energy independence of the electron kinetic energy of these features along with the low dipole moment predicted for the neutral ^•OH·isoprene van der Waals complex, suggest a complex photon-driven process. We present calculations supporting a hypothesis that near-threshold production of the ^•OH···isoprene reactive complex results in hydrogen abstraction of the isoprene molecule. The newly formed activated complex anion supports a dipole bound state that temporarily traps the near zero-kinetic energy electron and then autodetaches, encoding the low-frequency modes of the dehydrogenated neutral isoprene radical in the electron kinetic energies.

Kinetic and mechanistic insight into the OH-initiated atmospheric oxidation of 2,3,7,8-tetrachlorodibenzo-p-dioxin via OH-addition and hydrogen abstraction pathways: A theoretical investigation

The 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic polychlorinated dibenzo-p-dioxin. The OH-initiated oxidation of TCDD has been studied using the density functional, canonical transition state, and canonical Rice-Ramsperger-Kassel-Marcus theories. The kinetic data were corrected for quantum tunneling by the Wigner and Eckart models. All OH addition and hydrogen atom abstraction channels were thermodynamically exergonic. The kinetic and thermodynamic data analysis at the reliable level MPWB1K/MG3S//M06-2X/MG3S indicate that the addition of OH to the carbon atom adjacent to the oxygen atom in dioxin ring leads to the formation of predominant adduct. The calculated bimolecular rate constant for the formation of predominant adduct was ~5.97-6.75 × 10^-13 cm³ molecule^-1 s^-1, its branching ratio was ~0.955, and the overall rate constant for the OH-initiated oxidation of TCDD was ~6.25-7.08 × 10^-13 cm³ molecule^-1 s^-1. The atmospheric lifetime of TCDD determined by OH was ~8.17-9.26 days indicating the TCDD can be categorized as medium lifetime organic pollutant.

Modelling the chemical repair of protein carbon-centered radicals formed via oxidative damage with dihydrolipoic acid

Dihydrolipoic acid repairs carbon-centred radicals at diffusion-controlled rates via HAT mechanism.

Role of allyl group in the hydroxyl and peroxyl radical scavenging activity of S-allylcysteine

S-Allylcysteine (SAC) is the most abundant compound in aged garlic extracts, and its antioxidant properties have been demonstrated. It is known that SAC is able to scavenge different reactive species including hydroxyl radical (•OH), although its potential ability to scavenge peroxyl radical (ROO•) has not been explored. In this work the ability of SAC to scavenge ROO• was evaluated, as well as the role of the allyl group (-S-CH(2)-CH═CH(2)) in its free radical scavenging activity. Two derived compounds of SAC were prepared: S-benzylcysteine (SBC) and S-propylcysteine (SPC). Their abilities to scavenge •OH and ROO• were measured. A computational analysis was performed to elucidate the mechanism by which these compounds scavenge •OH and ROO•. SAC was able to scavenge •OH and ROO•, in a concentration-dependent way. Such activity was significantly ameliorated when the allyl group was replaced by benzyl or propyl groups. It was shown for the first time that SAC is able to scavenge ROO•.

Canolol: a promising chemical agent against oxidative stress

The OOH radical scavenging activity of canolol (CNL) has been studied in aqueous and lipid solutions, using the density functional theory. CNL is predicted to react about 3.6 times faster in aqueous solution than in lipid media. The overall rate coefficients are predicted to be 2.5 × 10(6) and 6.8 × 10(5) M(-1) s(-1), respectively. The OOH radical scavenger activity of canolol is predicted to be similar to that of carotenes, higher than that of allicin, and much higher than that of melatonin. Branching ratios for the different channels of reaction are reported for the first time. It was found that the reactivity of canolol toward OOH radicals takes place almost exclusively by H atom transfer from the phenolic moiety in canolol, regardless of the polarity of the environment. Taking into account that the reactivity of peroxyl radicals is significantly lower than that of other reactive oxygen species, canolol is proposed to be a very good antioxidant.

Quantum chemistry and TST study of the mechanisms and branching ratios for the reactions of OH with unsaturated aldehydes

A theoretical study is presented on the mechanism of OH reactions with three unsaturated aldehydes, relevant to atmospheric chemistry. Using acrolein as test molecule, several methods were tested in conjunction with the 6-311 ++ G(d,p) basis set. Based on the results from this study, the MPWB1K and M05-2X functionals were selected for the further study of acrolein, methacrolein and crotonaldehyde. All possible reaction channels have been modeled. Calculated overall rate coefficients at M05-2X/6-311 ++ G(d,p) are in excellent agreement with experimental data, supporting the proposed mechanisms. The previously proposed global mechanisms were confirmed, and specific mechanisms were identified. The causes of the mechanism for crotonaldehyde being different from the one of acrolein and methacrolein were clarified. The agreement between experiment and calculations validates the use of the chosen DFT methods for kinetic calculations, especially for large systems and cases in which spin contamination is an important issue.