Formation of C₅H₆ isomers: a combination of experimental and computational investigation

A propagation mechanism originating from 1,3-cyclopentadienyl, a prearomatic resonantly stabilized radical, makes a significant contribution to the growth of polycyclic aromatic hydrocarbons and soot. 1,3-Cyclopentadiene, as the simplest 5-membered carbon closed-shell molecule that could generate 1,3-cyclopentadienyl via photolysis and H-elimination, is attracting attention from astrochemistry and combustion chemistry communities. The reaction of propargyl (˙C₃H₃) with ethylene (C₂H₄) was investigated in a micro SiC reactor under low-pressure (<100 Torr) conditions coupled with tunable synchrotron radiation photoionization and molecular beam mass spectrometry techniques. Their potential energy surfaces were explored by ab initio electronic structure calculations. Subsequently, microscopic kinetics were demonstrated by RRKM/master equation theory in consideration of temperature- and pressure-dependent effects. The analysis of the photoionization efficiency (PIE) analysis at m/z = 66 has confirmed the formation of C₅H₆ molecules with a cyclic structure, i.e. 1,3-cyclopentadiene, as well as its linear isomer 3-penten-1-yne. Supported by ionization energies and Franck-Condon factors from theoretical predictions, this work proposes the possible formation of C₅H₆ molecules with two linear isomers 1,2,4-pentatriene and 4-penten-1-yne. Kinetics reveal the discrepancy of product selectivity under diverse temperatures and pressures. Notably, the generation of 1,2,4-pentatriene prevails at high temperatures corresponding to combustion environments, followed closely by 4-penten-1-yne and 3-penten-1-yne formations. Conversely, 1,3-cyclopentadiene shows a strong yield predominance in a vacuum environment within 300-600 K. This finding provides a potential pathway to aromatic hydrocarbon formation, especially in the planetary nebulae and circumstellar envelopes of carbon-rich stars.