Formation of bicyclic polycyclic aromatic hydrocarbons (PAHs) from the reaction of a phenyl radical with cis-3-penten-1-yne

Abstract

The formation of polycyclic aromatic hydrocarbons (PAHs) on the C₁₁H₁₁ potential energy surface involved in the reactions of a phenyl radical (C₆H₅) with cis-3-penten-1-yne (cis-C¹HC²–C³HC⁴H–C⁵H₃, referred to as C₅H₆) and its three radicals (CHC–ĊCH–CH₃, CHC–CHĊ–CH₃, and cis-CHC–CHCH–ĊH₂, referred to as the C³-, C⁴-, and C⁵-radicals with the same chemical components, C₅H₅) assisted by H atoms is investigated by performing combined density functional theory (DFT) and ab initio calculations. Five potential pathways for the formation of PAHs have been explored in detail: Pathways I–II correspond to the reaction of C₆H₅ with C₅H₆ at the C¹ and C² position, and Pathways III–V involve the reaction of C₆H₅ with the C³-, C⁴-, and C⁵-radicals with the assistance of H atoms. The initial association of C₆H₅ with C₅H₆ or C₅H₅ is found to be highly exothermic with only minor barriers (1.4–7.1 kcal mol⁻¹), which provides a large driving force for the formation of PAHs. The hydrogen atom is beneficial for the ring enlargement and ring formation processes. The present calculations predict 9 potential PAHs, six (CS6, CS10, CS13, CS26, CS28 and CS29) of which are indicated to be energetically more favorable along Pathways I, III, IV and V at low temperature. The calculated barriers for the formation of these PAHs are around 19.2–38.0 kcal mol⁻¹. All PAHs products could be formed at flame temperature, for the medium barriers are easily overcome in various flame conditions. The theoretical results supplement the PAH formation pathway and provide help to understand PAH growth mechanism.