How to add a five-membered ring to polycyclic aromatic hydrocarbons (PAHs) – molecular mass growth of the 2-naphthyl radical (C10H7) to benzindenes (C13H10) as a case study

Abstract

The three-ring polycyclic aromatic hydrocarbons (PAHs) 3H-benz[e]indene (C₁₃H₁₀) and 1H-benz[f]indene (C₁₃H₁₀) along with their naphthalene-based isomers 2-(prop-2-yn-1-yl)naphthalene (C₁₃H₁₀), 2-(prop-1-yn-1-yl)naphthalene (C₁₃H₁₀), and 2-(propa-1,2-dien-1-yl)naphthalene (C₁₃H₁₀) were formed through a “directed synthesis” via a high temperature chemical micro reactor under combustion-like conditions (1300 ± 35 K) through the reactions of the 2-naphthyl isomer (C₁₀H₇˙) with allene (C₃H₄) and methylacetylene (C₃H₄). The isomer distributions were probed utilizing tunable vacuum ultraviolet radiation from the Advanced Light Source (ALS) by recording the photoionization efficiency curves at mass-to-charge of m/z = 166 (C₁₃H₁₀) and 167 (¹³CC₁₂H₁₀) of the products in a supersonic molecular beam. Complemented by electronic structure calculations, our study reveals critical mass growth processes via annulation of a five-membered ring from the reaction between aryl radicals and distinct C₃H₄ isomers at elevated temperatures as present in combustion processes and in circumstellar envelopes of carbon stars. The underlying reaction mechanisms proceed through the initial addition of the 2-naphthyl radical with its radical center to the π-electron density of the allene and methylacetylene reactants via entrance barriers between 8 and 14 kJ mol⁻¹, followed by isomerization (hydrogen shifts, ring closure), and termination via atomic hydrogen losses accompanied by aromatization. The reaction mechanisms reflect the formation of indene – the prototype PAH carrying a single five- and a single six-membered ring – synthesized through the reaction of the phenyl radical (C₆H₅˙) with allene and methylacetylene. This leads us to predict that aryl radicals – upon reaction with allene/methylacetylene – may undergo molecular mass growth processes via ring annulation and de facto addition of a five-membered ring to form molecular building blocks essential to transit planar PAHs out of the plane.