In a previous work [Lakshmanan, S.;et al. J. Phys. Chem. A 2018, 122, 4808−4818], direct dynamics simulations at the M06/6-311++G(d,p) level of theory were reported for 3 CH 2 (X 3 B 1 ) + 3 O 2 (X 3 ∑ g − ) reaction on its ground-state singlet potential energy surface (PES) at 300 K. However, further analyses revealed the simulations are unstable for the 3 CH 2 (X 3 B 1 ) + 3 O 2 (X 3 ∑ g − ) reactants on the groundstate singlet surface and the trajectories reverted to an excited-state singlet surface for the 1 CH 2 (ã1A 1 ) + 1 O 2 (b 1 ∑ g + ) reactants. Thus, the dynamics reported previously are for this excited-state singlet PES. The PESs for the 3 CH 2 (X 3 B 1 ) + 3 O 2 (X 3 ∑ g − ) and 1 CH 2 (ã1A 1 ) + 1 O 2 (b 1 ∑ g + ) reactants are quite similar, and this provided a means to perform simulations for the 3 CH 2 (X 3 B 1 ) + 3 O 2 (X 3 ∑ g − ) reactants on the groundstate singlet PES at 300 K, which are reported here. The reaction dynamics are quite complex with seven different reaction pathways and nine different products. A consistent set of product yields have not been determined experimentally, but the simulation yields for the H atom, CO, and CO 2 are somewhat lower, higher, and lower respectively, than the recommended values. The yields for the remaining six products agree with experimental values. Product decomposition was included in determining the product yields. The simulation 3 CH 2 + 3 O 2 rate constant at 300 K is only 3.4 times smaller than the recommended value, which may be accommodated if the 3 CH 2 + 3 O 2 → 1 CH 2 O 2 potential energy curve is only 0.75 kcal/mol more attractive at the variational transition state for 3 CH 2 + 3 O 2 → 1 CH 2 O 2 association. The simulation kinetics and dynamics for the 3 CH 2 + 3 O 2 and 1 CH 2 + 1 O 2 reactions are quite similar. Their rate constants are statistically the same, an expected result, since their transition states leading to products have energies lower than that of the reactants and the attractive potential energy curves for 3 CH

The reaction of CH with O is of fundamental importance in combustion, and the reaction is complex as a result of multiple extremely exothermic product channels. In the present study, direct dynamics simulations were performed to study the reaction on both the singlet and triplet potential energy surfaces (PESs). The simulations were performed at the UM06/6-311++G(d,p) level of theory. Trajectories were calculated at a temperature of 300 K, and all reactive trajectories proceeded through the carbonyl oxide Criegee intermediate, CHOO, on both the singlet and triplet PESs. The triplet surface leads to only one product channel, HCO + O(P), while the singlet surface leads to eight product channels with their relative importance as CO + HO > CO + OH + H ∼ HCO + O(D) > HCO + OH ∼ CO + H ∼ CO + H + O(D) > CO + H + H > HCO + O(D) + H. The reaction on the singlet PES is barrierless, consistent with experiment, and the total rate constant on the singlet surface is (0.93 ± 0.22) × 10 cm molecule s in comparison to the recommended experimental rate constant of 3.3 × 10 cm molecule s. The simulation product yields for the singlet PES are compared with experiment, and the most significant differences are for H, CO, and HO. The reaction on the triplet surface is also barrierless, inconsistent with experiment. A discussion is given of the need for future calculations to address (1) the barrier on the triplet PES for CH + O → CHOO, (2) the temperature dependence of the CH + O reaction rate constant and product branching ratios, and (3) the possible non-RRKM dynamics of the CHOO Criegee intermediate.

For the (aut)oxidation of toluene to benzyl hydroperoxide, benzyl alcohol, benzaldehyde, and benzoic acid, the thermochemical profiles for various radical-generating reactions have been compared. A key intermediate in all of these reactions is benzyl hydroperoxide, the heat of formation of which has been estimated by using results from CBS-QB3, G4, and G3B3 calculations. Homolytic O-O bond cleavage in this hydroperoxide is strongly endothermic and thus unlikely to contribute significantly to initiation processes. In terms of reaction enthalpies the most favorable initiation process involves bimolecular reaction of benzyl hydroperoxide to yield hydroxy and benzyloxy radicals along with water and benzaldehyde. The reaction enthalpy and free energy of this process is significantly more favorable than those for the unimolecular dissociation of known radical initiators, such as dibenzoylperoxide or dibenzylhyponitrite.

The nitration of tyrosine by atmospheric oxidants, O3 and NO2, is an important cause for the spread of allergenic diseases. In the present study, the mechanism and pathways for the reaction of tyrosine with the atmospheric oxidants O3 and NO2 are studied using DFT-M06-2X, B3LYP, and B3LYP-D methods with the 6-311+G(d,p) basis set. The energy barrier for the initial oxidation reactions is also calculated at the CCSD(T)/6-31+G(d,p) level of theory. The reaction is studied in gas, aqueous, and lipid media. The initial oxidation of tyrosine by O3 proceeds by H atom abstraction and addition reactions and leads to the formation of six different intermediates. The subsequent nitration reaction is studied for all the intermediates, and the results show that the nitration affects both the side chain and the aromatic ring of tyrosine. The rate constant of the favorable oxidation and nitration reaction is calculated using variational transition state theory over the temperature range of 278-350 K. The spectral properties of the oxidation and nitration products are calculated at the TD-M06-2X/6-311+G(d,p) level of theory. The fate of the tyrosine radical intermediate is studied by its reaction with glutathione antioxidant. This study provides an enhanced understanding of the oxidation and nitration of tyrosine by O3 and NO2 in the context of improving the air quality and reducing the allergic diseases.

Dimethylphenols are highly reactive in the atmosphere, and their oxidation plays a vital role in the autoignition and combustion processes. The dominant oxidation process for dimethylphenols is by gas-phase reaction with OH radical. In the present study, the reaction of OH radical with dimethylphenol isomers is studied using density functional theory methods, B3LYP, M06-2X, and MPW1K, and also at the MP2 level of theory using 6-31G(d,p) and 6-31+G(d,p) basis sets. The activation energy values have also been calculated using the CCSD(T) method with 6-31G(d,p) and 6-311+G(d,p) basis sets using the geometries optimized at the M06-2X/6-31G(d,p) level of theory. The reactions subsequent to the principal oxidation steps are studied, and the different reaction pathways are modeled. The positions of the OH and CH3 substituents in the aromatic ring have a great influence on the reactivity of dimethylphenol toward OH radical. Accordingly, the reaction is initiated in four different ways: H-atom abstraction from the phenol group, H-atom abstraction from a methyl group, H-atom abstraction from the aromatic ring by OH radical, or electrophilic addition of OH radical to the aromatic ring. Aromatic peroxy radicals arising from initial H-atom abstraction and subsequent O2 addition lead to the formation of hydroperoxide adducts and alkoxy radicals. The O2 additions to dimethylphenol-OH adduct results in the formation of epoxide and bicyclic radicals. The rate constants for the most favorable reaction pathways are calculated using canonical variational transition state theory with small curvature tunneling corrections. This study provides thermochemical and kinetic data for the oxidation of dimethylphenol in the atmosphere and demonstrates the mechanism for the conversion of peroxy radical into aldehydes, hydroperoxides, epoxides, and bicyclic radicals, and their lifetimes in the atmosphere.