First direct kinetic measurement of i-C4H5 (CH2CHCCH2) + O2 reaction: Toward quantitative understanding of aromatic ring formation chemistry

Abstract

The kinetics of the i-C4H5 (buta-1,3-dien-2-yl) radical reaction with molecular oxygen has been measured over a wide temperature range (275–852 K) at low pressures (0.8–3 Torr) in direct, time-resolved experiments. The measurements were performed using a laminar flow reactor coupled to photoionization mass spectrometer (PIMS), and laser photolysis of either chloroprene (2-chlorobuta-1,3-diene) or isoprene was used to produce the resonantly stabilized i-C4H5 radical. Under the experimental conditions, the measured bimolecular rate coefficient of i-C4H5 + O2 reaction is independent of bath gas density and exhibits weak, negative temperature dependency, and can be described by the expression k3 = (1.45 ± 0.05) × 10−12 × (T/298 K)−(0.13±0.05) cm3 s−1. The measured bimolecular rate coefficient is surprisingly fast for a resonantly stabilized radical. Under combustion conditions, the reactions of i-C4H5 radical with ethylene and acetylene are believed to play an important role in forming the first aromatic ring. However, the current measurements show that i-C4H5 + O2 reaction is significantly faster under combustion conditions than previous estimations suggest and, consequently, inhibits the soot forming propensity of i-C4H5 radicals. The bimolecular rate coefficient estimates used for the i-C4H5 + O2 reaction in recent combustion simulations show significant variation and are up to two orders of magnitude slower than the current, measured value. All estimates, in contrast to our measurements, predict a positive temperature dependency. The observed products for the i-C4H5 + O2 reaction were formaldehyde and ketene. This is in agreement with the one theoretical study available for i-C4H5 + O2 reaction, which predicts the main bimolecular product channels to be H2CO + C2H3 + CO and H2CCO + CH2CHO.