Experimental studies of the ethyl and propargyl radicals in the gas phase

The self-reactions of ethyl and propargyl radicals play important roles in the combustion of organic fuels. These reactions are involved in the termination of chain reactions and the growth of hydrocarbon molecules leading to soot formation.; The ethyl radical self-reaction (reaction 1) was studied over the temperature interval 301–800 K and bath gas (He) density (3–12) × 10 16 molecule cm−3 and the propargyl radical self reaction (reaction 2) was studied over the temperature interval 500–1000 K and bath gas density (3–6) × 1016 molecule cm −3 using Laser Photolysis/Photoionization Mass Spectrometry. The products of reaction 2 were studied by GC/MS analysis over the temperature interval 500–1100K.; The radicals were produced by the 193/248-nm laser photolysis of oxalyl chloride followed by an instantaneous conversion of the produced chlorine atoms into ethyl or propargyl radicals by the reaction with corresponding substrate. Initial concentrations of the ethyl radical were determined using the measured photolytic depletion of oxalyl chloride. Initial concentrations of the propargyl radical were determined using the measured production of HCl. The kinetics of ethyl/propargyl decay was monitored in real time and the values of the rate constant of the reaction were determined from the detected temporal ion signal profiles.; The high-pressure-limit rate constants of reaction 1 were determined to be k₁∞ = 2.29 × 10−6 T−1.66exp(−552 K/T) cm3 molecule−1 s−1. The disproportionation to recombination branching ratio was determined at 297 and 400 K in agreement with the well-established value of 0.14.; The rate constant of reaction 2 decreased from (3.3 ± 0.4) × 10−11 cm3 molecule−1 s−1 at 500K to (2.7 ± 0.4) × 10−11 cm3 molecule−1 s−1 at 700 K and to (1.2 ± 0.1) × 10−11 cm3 molecule−1 s−1 at 1000 K. The low value at 1000 K is likely to be due to falloff effects. Products of reaction 2 were studied in real-time experiments as well as by GC/MS analysis. In the real-time experiments, formation of products at the mass of C₆H₆ was observed. GC/MS analysis demonstrated the dependence of the distribution of the products of reaction 2 on temperature with benzene becoming the major product at temperatures above 900 K. Evidence for phenyl radical formation was observed.