Formation of the propargyl radical in the reaction of 1CH2 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 radicals are produced during combustion by the reaction of (CH2)-C-1 with acetylene, which proceeds via a three well mechanism. A master equation model of this system is constructed with the aim of determining the branching ratio 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 necessary 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 fluorescence of (CH2)-C-1. Experimental results are presented and used in the master equation model, which shows that the yield, gamma, of dissociation products H + C3H3 decreases with increasing pressure and that the onset of the decrease shifts to higher pressures as the temperature increases. At higher pressures and temperatures, there is an overlap in the timescales of dissociation of thermalised C3H4 and of the nascent C3H4* formed from (CH2)-C-1 + C2H2, so that a simple description through time independent rate coefficients is no longer possible.