On the formation of cyclopentadiene in the C3H5˙ + C2H2 reaction

Abstract

The reaction between the allyl radical (C₃H₅˙) and acetylene (C₂H₂) in a heated microtubular reactor has been studied at the VUV beamline of the Swiss Light Source. The reaction products are sampled from the reactor and identified by their photoion mass-selected threshold photoelectron spectra (ms-TPES) by means of imaging photoelectron photoion coincidence spectroscopy. Cyclopentadiene is identified as the sole reaction product by comparison of the measured photoelectron spectrum with that of cyclopentadiene. With the help of quantum-chemical computations of the C₅H₇ potential energy surface, the C₂H₂ + C₃H₅˙ association reaction is confirmed to be the rate determining step, after which H-elimination to form C₅H₆ is prompt in the absence of re-thermalization at low pressures. The formation of cyclopentadiene as the sole product from the allyl + acetylene reaction offers a direct path to the formation of cyclic hydrocarbons under combustion relevant conditions. Subsequent reactions of cyclopentadiene may lead to the formation of the smallest polycyclic aromatic molecule, naphthalene.