Formation of the propargyl radical in the reaction of (CH2)-C-1 and C2H2: experiment and modelling

The propargyl radical, C3H3, is thought to be an important precursor to the formation of aromatic compounds and of soot in combustion systems. These r adicals are produced during combustion by the reaction of (CH2)-C-1 with ac etylene, which proceeds via a three well mechanism. A master equation model of this system is constructed with the aim of determining the branching ra tio for formation of the propargyl radical as a function of temperature and pressure. The rate limiting step is the initial formation of cyclopropene from the reactants and a knowledge of the rate of this reaction is necessar y for accurate modelling. The rate coefficient for the overall reaction was measured, as a function of temperature, using laser flash photolysis of a ketene-acetylene mixture. The reaction was monitored by laser induced fluor escence of (CH2)-C-1. Experimental results are presented and used in the ma ster equation model, which shows that the yield, gamma, of dissociation pro ducts H + C3H3 decreases with increasing pressure and that the onset of the decrease shifts to higher pressures as the temperature increases. At highe r pressures and temperatures, there is an overlap in the timescales of diss ociation of thermalised C3H4 and of the nascent C3H4* formed from (CH2)-C-1 + C2H2, so that a simple description through time independent rate coeffic ients is no longer possible.