Double Hydrogen-Atom Exchange Reactions of HX (X = F, Cl, Br, I) with HO2

Kinetic Study of Gas-Phase Reactions of Pyruvic Acid with HO2

Gas-phase reactions between pyruvic acid (PA) and HO₂ radicals were examined using ab initio quantum chemistry and transition state theory. The rate coefficients were determined over a temperature range of 200-400 K including tunneling contributions. Six potential reaction pathways were identified. The two hydrogen abstraction reactions yielding the H₂O₂ product were found to have high barriers. The HO₂ radical was also found to have a catalytic effect on the intramolecular hydrogen transfer reactions occurring by three distinct routes. These hydrogen-shift reactions are very interesting mechanistically although they are highly endothermic. The only reaction that contributes significantly to the consumption of PA is a multistep pathway involving a peroxy-radical intermediate, PA + HO₂ → CH₃COOH + OH + CO₂. This exothermic process has potential atmospheric relevance because it produces an OH radical as a product. Atmospheric models currently have difficulty predicting accurate OH concentrations for certain atmospheric conditions, such as environments free of NOx and the nocturnal boundary layer. Reactions of this sort, although not necessary with PA, may account for a portion of this deficit. The present study helps settle the issue of the relative roles of reaction and photolysis in consumption of PA in the troposphere.

Reactions of Criegee Intermediates are Enhanced by Hydrogen-Atom Relay Through Molecular Design

We report a type of highly efficient double hydrogen atom transfer (DHAT) reaction. The reactivities of 3-aminopropanol and 2-aminoethanol towards Criegee intermediates (syn- and anti-CH₃ CHOO) were found to be much higher than those of n-propanol and propylamine. Quantum chemistry calculation has confirmed that the main mechanism of these very rapid reactions is DHAT, in which the nucleophilic attack of the NH₂ group is catalyzed by the OH group which acts as a bridge of HAT. Typical gas-phase DHAT reactions are termolecular reactions involving two hydrogen bonding molecules; these reactions are typically slow due to the substantial entropy reduction of bringing three molecules together. Putting the reactive and catalytic groups in one molecule circumvents the problem of entropy reduction and allows us to observe the DHAT reactions even at low reactant concentrations. This idea can be applied to improve theoretical predictions for atmospherically relevant DHAT reactions.

Theoretical studies of reaction mechanisms for potential reactions of HNCO with HO2 radicals

The reaction mechanism of HNCO with HO2 radicals is investigated by means of the B3PW91/6-311+G(d,p) method to determine a more reasonable pathway. Four possible entrance patterns are designed; however, only three situations are finally confirmed. When HNCO and HO2 are close to each other, they first form an intermediate a2. Then, the terminal O atom in HO2 connects with the C atom of HNCO along with the H atom transferring from HO2 to the O atom of HNCO. After that the C–O bond ruptures again to form i. Finally, product P6(NCO + HOOH) is generated from i via H atom transfer. P6 is the most accessible product with the simplest steps and lowest barrier height. In addition, the energy of several of the routes is refined at the CCSD(T)/6-311+G(d,p) level based on the optimized geometries. Although there are some differences, the most favorable product is still P6. It is expected that this study would be helpful in further studies on HNCO and in understanding its reactions.