Kinetics and products of the self-reaction of propargyl radicals

The kinetics and product branching of the self-reaction of propargyl radicals, C3H3 + C3H3 --> products (1), have been studied as functions of temperature. Rate constants of reaction 1 were obtained in direct real-time experiments by laser photolysis/photoionization mass spectrometry over the temperature interval 500-1000 K and at a bath gas density of (3-6) x 1016 molecules cm(-3). Propargyl radicals were produced by the 248 nm laser photolysis of oxalyl chloride ((CClO)(2)) followed by a fast conversion of the produced chlorine atoms into propargyl radicals and HCl via the reaction with propyne. No active species other than C3H3 were present in the system during the kinetics Of C3H3 decay. The values of the rate constant of reaction 1 were determined from the [C3H3] temporal profiles. The rate constants of reaction 1 decrease from (3.30 +/- 0.35) x 10(-11) cm(3) molecule(-1) s(-1) at 500 K to (2.74 +/- 0.43) x 10(-11) cm(3) molecule(-1) s(-1) at 700 K and to (1.20 +/- 0.14) x 10(-11) cm(3) molecule(-1) s(-1) at 1000 K. The value obtained at 1000 K is likely to be influenced by falloff effects and secondary reactions initiated by the H+ C6H5 products of reaction 1. The rate constants of reaction 1 determined in the current study at elevated temperatures correlate well with the room temperature value of similar to4 x 10(-11) cm(3) molecule(-1) s(-1) obtained in several earlier studies. Combination of the results of the current work with those of earlier room temperature investigations results in the following temperature dependence of the high-pressure-limit rate constant of reaction 1: k(1)infinity = 4.49 x 10(-9)T(-0.75) exp(-128 K/T) cm(3) molecule(-1) s(-1) (295-700 K). Product channels of reaction 1 were studied using final product analysis by gas chromatography/mass spectrometry in the 500-1100 K temperature interval. Several C6H6 isomers were detected as products of the self-reaction of propargyl radicals: 1,5-hexadiyne, fulvene, benzene, and two unknown species identified in the text as unknown 1 and unknown 2. The distribution of products depends on the temperature. At lower temperatures, 1,5-hexadiyne, unknown 1, and benzene were observed. The fraction of benzene increases with temperature; it becomes the major product at 900 K and above. Fulvene and unknown 2 were observed in minor amounts in the 900-1100 K range. The product analysis provides evidence for the appearance of the reaction channel 1b (C6H5 + H) at high temperatures: formation of C8H6 and C9H8 was observed and attributed to the fast reaction of the phenyl radical with the excess of propyne present in the reactor.