Transformation of an Embedded Five-Membered Ring in Polycyclic Aromatic Hydrocarbons via the Hydrogen-Abstraction-Acetylene-Addition Mechanism: A Theoretical Study.

Five-membered rings are constituents of many polycyclic aromatic hydrocarbons (PAHs), and their presence on the edges of large PAHs has been repeatedly observed experimentally. However, modern kinetic combustion models often do not consider the growth of PAHs through the transformation of the five-membered rings. In connection with the above, we carried out a theoretical study of the mechanism of hydrogen-abstraction-acetylene-addition (HACA) transformation of an embedded five-membered ring on the armchair PAH edge to a six-membered ring, considering cyclopenta[,,]phenanthrene (4,5-methylenephenanthrene) as a prototype system for this process.

On the mechanism of soot nucleation. II. E-bridge formation at the PAH bay.

A recently proposed mechanism of soot nucleation (M. Frenklach and A. M.

Mechanism and rate constants of the CH + CH CO reactions in triplet and singlet states: A theoretical study.

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH with ketene CH CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice-Ramsperger-Kassel-Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH and C H + CO products, where the former channel is preferable at higher temperatures from 1000 K and above.

Rate constants for H abstraction from benzo(a)pyrene and chrysene: a theoretical study.

Density functional B3LYP/6-31G(d) and ab initio G3(MP2,CC) calculations have been carried out to determine thermal rate constants of direct H abstraction reactions from four- and five-ring polycyclic aromatic hydrocarbons (PAH) chrysene and benzo[a]pyrene by various radicals abundant in combustion flames, such as H, CH, CH, and OH, using transition state theory. The results show that the H abstraction reactions with OH have the lowest barriers of ∼4 kcal mol, followed by those with H and CH with barriers of 16-17 kcal mol, and then with propargyl radicals with barriers of 24-26 kcal mol. Thus, the OH radical is predicted to be the fastest H abstractor from PAH.

Primary biliary cirrhosis and coronary atherosclerosis: protective antioxidant effect of bilirubin.

Increased concentration of lipid peroxidation products in patients with primary biliary cirrhosis is related to elevation of serum lipid content, but not to activation of lipid peroxidation. Hyperbilirubinemia in patients with primary biliary cirrhosis is accompanied by a decrease in the concentration of lipid peroxidation products and increase in antioxidant activity of blood serum. Antioxidants play a major role in the prevention of atherosclerosis.