Investigating the Role of CH₂ Radicals in the HACA Mechanism

Abstract

Detailed mechanisms of PAH growth involving methylene (CH₂) were studied using accurate ab initio density functional theory B3LYP/6-311+G­(d,p) calculations, as well as approximate QCISD­(T,full)/6-311++G­(3df,2pd) calculations. The PAH growth can be divided into five essential reaction steps, namely, addition C₂H₂ → intramolecular hydrogen migration → addition CH₂ → cyclization → H-elimination. The aliphatic species of indene and 1H-phenalene are found in the pathways of PAH growth, which is in accord with the experimental results that reveal the formation of aliphatic species in flames. It was found that the simultaneous removal of two H atoms in one reaction step is feasible in PAH evolution, and this can reasonably interpret the absence of a H atom in the post-flame region. The corresponding rate coefficients at 1 atm were evaluated by using TST and RRKM theory by solving the master equations in the temperature range of 500–2500 K. The calculated branching ratios suggest that the pathways involving CH₂ are competitive in PAH growth