Co-pyrolysis of 1-naphthylmethyl radical with allene and propyne: Radical-molecule (C3H4) vs. radical-radical (C3H3) reaction pathways

Abstract

In this work, we conducted a combined experimental and theoretical investigation into the reactivity of naphthylmethyl radical (1-A2CH₂, C₁₁H₉) with allene (aC₃H₄) and propyne (pC₃H₄). Co-pyrolysis experiments of 1-(chloromethyl)naphthalene (1-A2CH₂Cl) with aC₃H₄ or pC₃H₄ were studied using a flash pyrolysis microreactor coupled with synchrotron vacuum ultraviolet photoionization mass spectrometry. Significant signals at m/z = 39 (C₃H₃), attributed to the rapid H-abstraction and/or pyrolysis of C₃H₄ molecules, were detected in both experiments, indicating concurrent radical-radical (1-A2CH₂ + C₃H₃) and radical-molecule (1-A2CH₂ + aC₃H₄/pC₃H₄) reactions. Theoretical calculations reveal that the radical-radical reaction is kinetically favored under experimental conditions compared to the radical-molecule reaction, dominantly forming two C₁₄H₁₂ adducts, 1-(1,2-butadiene-4-yl)naphthalene (1-W1) and 1-(but‑yn-3-yl)naphthalene (1-W2), along with minor amounts of other C₁₄H₁₂ isomers and C₁₄H₁₁ + H products. The computed rate coefficients and branching ratios of various reaction channels expand the analysis to broader temperature and pressure ranges. The radical-radical products (1-W1 and 1-W2) dominate at atmospheric-to-high pressures (>1 atm) or relatively lower temperatures (<1500 K). At higher temperatures and low pressures, competition arises between radical-radical and radical-molecule pathways. Furthermore, the structures of bimolecular products differ significantly between the 1-A2CH₂ + aC₃H₄ and 1-A2CH₂ + pC₃H₄ reaction systems, with implications for the formation of larger polycyclic aromatic hydrocarbons.