Communication: Energetics of reaction pathways for reactions of ethenol with the hydroxyl radical: The importance of internal hydrogen bonding at the transition state

Atmospheric Chemistry of Enols: The Formation Mechanisms of Formic and Peroxyformic Acids in Ozonolysis of Vinyl Alcohol

Vinyl alcohol (VA), for a long time, is thought to be a missing source of formic acid (FA) in the atmospheric models. However, a recent study has shown that FA is just a byproduct in the OH-initiated oxidation of VA, which stimulates investigation on the other sinks of VA in the atmosphere. In this study, the detailed ozonolysis mechanism of VA was investigated theoretically for the first time. The results show that two primary ozonides (syn- and anti-POZ) can be formed in the ozonolysis of VA and that FA coupled with the simplest Criegee intermediate (CH₂OO) can be produced as the main nascent products. Thus, the ozonolysis of VA is predicted to be a more efficient process to produce FA in the atmosphere compared with its OH-initiated oxidation. Moreover, it is found that the syn-POZ can directly decompose to peroxyformic acid plus formaldehyde, breaking the known "Criegee mechanism" to form carbonyl oxide with carbonyl compound. This special mechanism by providing a new source of peroxy acids in the atmosphere enriches the atmospheric chemistry of enols. The atmospheric lifetime of VA by ozonolysis is predicted to be 30 h, comparable with its prevalent reaction with the OH radical. Therefore, the obtained theoretical results can be usefully incorporated into a future modeling study of enols.

Infrared Spectra of Gas-Phase 1- and 2-Propenol Isomers

Fourier transform infrared spectra of isolated 1-propenol and 2-propenol in the gas-phase have been collected in the range of 900-3800 cm^-1, and the absolute infrared absorption cross sections reported for the first time. Both cis and trans isomers of 1-propenol were observed with the trans isomer in greater abundance. Syn and anti conformers of both 1- and 2-propenol were also observed, with abundance consistent with thermal population. The FTIR spectrum of the smaller ethenol (vinyl alcohol) was used as a benchmark for our computational results. As a consequence, its spectrum has been partially reassigned resulting in the first report of the anti-ethenol conformer. Electronic structure calculations were used to support our experimental results and assign vibrational modes for the most abundant isomers, syn-trans-1-propenol and syn-2-propenol.

Lipoic acid and dihydrolipoic acid. A comprehensive theoretical study of their antioxidant activity supported by available experimental kinetic data

The free radical scavenging activity of lipoic acid (LA) and dihydrolipoic acid (DHLA) has been studied in nonpolar and aqueous solutions, using the density functional theory and several oxygen centered radicals. It was found that lipoic acid is capable of scavenging only very reactive radicals, while the dehydrogenated form is an excellent scavenger via a hydrogen transfer mechanism. The environment plays an important role in the free radical scavenging activity of DHLA because in water it is deprotonated, and this enhances its activity. In particular, the reaction rate constant of DHLA in water with an HOO(•) radical is close to the diffusion limit. This has been explained on the basis of the strong H-bonding interactions found in the transition state, which involve the carboxylate moiety, and it might have implications for other biological systems in which this group is present.

Investigation of plausible mechanistic pathways in hydrogenation of η⁵-(C₅H₅)₂Ta(H)=CH₂: an analysis using DFT and AIM techniques

In this manuscript, we investigate two plausible pathways for addition of H₂ across the bond Ta=C in η⁵-(C₅H₅)₂Ta(H)=CH₂. One of the investigated reaction pathways involves a single concerted step with a four-membered transition state keeping the oxidation state of tantalum unaltered, where as the other pathway deals with a two step reaction with α-insertion of H₂ to produce a 16e⁻ Ta(III)-methyl species and a subsequent oxidative addition. We must emphasize that an experimental study by Bregel et al. [J Am Chem Soc 2002, 124:13827-13832] on a derivative of the investigated chemical system in the present study showed that the two step strategy of α-insertion followed by subsequent oxidative addition is the preferred one. Our numerical investigations using DFT and AIM calculations lead to a similar conclusion. To establish our conclusion, we employ various basis sets to obtain the free energy of activation of the reaction. The AIM technique especially helps us to characterize the bond critical points at the optimized geometries of the reactants, products, transition states, and intermediates for the two step mechanism.

New pathways for formation of acids and carbonyl products in low-temperature oxidation: the Korcek decomposition of γ-ketohydroperoxides

We present new reaction pathways relevant to low-temperature oxidation in gaseous and condensed phases. The new pathways originate from γ-ketohydroperoxides (KHP), which are well-known products in low-temperature oxidation and are assumed to react only via homolytic O-O dissociation in existing kinetic models. Our ab initio calculations identify new exothermic reactions of KHP forming a cyclic peroxide isomer, which decomposes via novel concerted reactions into carbonyl and carboxylic acid products. Geometries and frequencies of all stationary points are obtained using the M06-2X/MG3S DFT model chemistry, and energies are refined using RCCSD(T)-F12a/cc-pVTZ-F12 single-point calculations. Thermal rate coefficients are computed using variational transition-state theory (VTST) calculations with multidimensional tunneling contributions based on small-curvature tunneling (SCT). These are combined with multistructural partition functions (Q(MS-T)) to obtain direct dynamics multipath (MP-VTST/SCT) gas-phase rate coefficients. For comparison with liquid-phase measurements, solvent effects are included using continuum dielectric solvation models. The predicted rate coefficients are found to be in excellent agreement with experiment when due consideration is made for acid-catalyzed isomerization. This work provides theoretical confirmation of the 30-year-old hypothesis of Korcek and co-workers that KHPs are precursors to carboxylic acid formation, resolving an open problem in the kinetics of liquid-phase autoxidation. The significance of the new pathways in atmospheric chemistry, low-temperature combustion, and oxidation of biological lipids are discussed.

Kinetics of the Hydrogen Abstraction from Carbon-3 of 1-Butanol by Hydroperoxyl Radical: Multi-Structural Variational Transition-State Calculations of a Reaction with 262 Conformations of the Transition State

We estimated rate constants for the hydrogen abstraction from carbon-3 of 1-butanol by hydroperoxyl radical, a critically important reaction in the combustion of biofuel. We employed the recently developed multi-structural variational transition-state theory (MS-VTST), which utilizes a multifaceted dividing surface that allows us to include the contributions of multiple structures for reacting species and transition states. First, multiconfigurational Shepard interpolation-based on molecular-mechanics-guided interpolation of electronic-structure Hessian data obtained by the M08 HX/jun-cc-pVTZ electronic model chemistry-was used to obtain the portion of the potential energy surface needed for single-structure variational transition-state theory rate constants including multidimensional tunneling; then, the M08-HX/MG3S electronic model chemistry was used to calculate multi-structural torsional anharmonicity factors to complete the MS-VTST rate constant calculations. The lowest-energy structures of the transition state have strongly bent hydrogen bonds. Our results indicate that neglect of multi-structural anharmonicity would lead to errors of factors of 0.3, 46, and 171 at 200, 1000, and 2400 K for this reaction.

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.