Modeling formation of molecules in the interstellar medium by radical reactions with polycyclic aromatic hydrocarbons

Adsorptions of CH°₂, CH°₃, NH°₂, and OH° radicals and molecule formation on a partially hydrogenated surface of a polycyclic aromatic hydrocarbon (PAH) (C₂₄H₂₇⁺) were modeled. It was found that radical adsorptions are feasible with important modifications of surface bond strengths and bond distances. Adsorbed hydrogen may diffuse due to adsorbate‐surface interactions. Formations of CH₄, NH₃, H₂O, CH₃NH₂, and CH₃OH were studied by Eley‐Rideal (ER) and Langmuir‐Hishelwood (LH) mechanisms. Potential energetic surfaces were performed for both mechanisms and the ER presents lower reaction energy barriers than the LH one, in all cases. Parametric quantum program (CATIVIC) was employed and comparisons with DFT results were performed. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2560–2572, 2010     

The keto–enol equilibrium and thermal conversion kinetics of 2- and 4-hydroxyacetophenone in the gas phase: a DFT study

Keto–enol tautomeric equilibrium and the mechanism of thermal conversion of 2- and 4-hydroxyacetophenone in gas phase have been studied by means of electronic structure calculations using density functional theory (DFT). A topological analysis of electron density evidence that the structure of keto and enol forms of 2-hydroxyacetophenone are stabilised by a relatively strong intramolecular hydrogen bond. 2- and 4-hydroxyacetophenone undergo deacetylation reactions yielding phenol and ketene. Two possible mechanisms are considered for these eliminations: the process takes place from the keto form (mechanism A), or occurs from the enolic form of the substrate (mechanism B). Quantum chemical calculations support the mechanism B, being found a good agreement with the experimental activation parameters. These results suggest that the rate-limiting step is the reaction of the enol through a concerted, non-synchronous, semi-polar, four-membered cyclic transition state (TS). The most advanced reaction coordinate in the TS is the rupture of O₁···H₁ bond, with an evolution in the order of 79.7%–80.9%. Theoretical results also suggest a three-step mechanism for the phenyl acetate formation from 2-hydroxyacetophenone.