Mechanisms and Thermochemistry of Reactions of SiO and Si2O2 with OH and H2O

This paper reports on computational studies of gas-phase reactions of SiO and Si₂O₂. The oxidation of SiO can initiate efficient formation of silica or silicate dust particles in a wide range of environments. Both OH radicals and H₂O molecules are often present in these environments, and their reactions with SiO and the smallest SiO cluster, Si₂O₂, affect the efficiency of eventual dust formation. Density functional theory calculations on these reactions, benchmarked against accurate coupled cluster calculations, indicate that the Si₂O₂ + OH reaction should be faster than SiO + OH. The reaction SiO + H₂O → SiO₂ + H₂ is both endothermic and has high activation energies to reaction. Instead, the formation of molecular complexes is efficient. The reaction of Si₂O₂ with H₂O, which has been suggested as efficient for producing Si₂O₃, might not be as efficient as previously thought. If the H₂O molecules dissociate to form OH radicals, oxidation of SiO and Si₂O₂ could be accelerated instead.

Oxidation and nitration of tyrosine by ozone and nitrogen dioxide: reaction mechanisms and biological and atmospheric implications

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.