Combined crossed-beam studies of C(3PJ) + C2H4 -> C3H3 + H reaction dynamics between 0.49 and 30.8 kJ mol-1

The reaction C(3PJ) + C2H4(X1A1)  C3H3 + H(2S1/2) has been studied using complementary crossed molecular beam techniques. Integral cross sections have been obtained in the range of relative translational energies ET = 0.49-24.9 kJ mol–1 in experiments conducted with pulsed supersonic beams coupled with laser-induced fluorescence detection of H(2S1/2) atoms. The major reaction pathway leading to HCCCH2 (propargyl) + H has been found without any barrier, with relative integral cross sections that are proportional to (ET)–0.60 ± 0.03 below 8 kJ mol-1. Threshold for a minor pathway, leading also to H formation, occurs around 6 kJ mol-1; the relative importance of this second pathway increases with relative translational energy. Differential cross sections have been obtained at three relative translational energies: ET = 9.1, 17.2, and 30.8 kJ mol–1 in experiments conducted with continuous supersonic molecular beams coupled with universal mass spectrometric detection and time-of-flight analysis. At the lowest ET of 9.1 kJ mol–1 formation of HCCCH2 (propargyl) + H is observed to be the dominant channel with a nearly forward-backward symmetric angular distribution in the center-of-mass (CM) frame; about 35% of the total available energy is channelled into translation indicating that the propargyl radical is highly internally excited; formation of less stable C3H3 isomer(s) is minor (2%). As ET increases, formation of appreciable, increasingly larger fractions of less stable propyn-1-yl and/or cyclopropenyl isomers is also observed. These findings are consistent with the integral cross section measurements. While formation of propargyl is thought to proceed via an osculating complex mechanism following addition of C(3PJ) to the double bond of ethylene, the dynamics of formation of the less stable isomers is going through a long-lived complex, as witnessed by an isotropic CM angular distribution. The H2 elimination channel leading to C3H2 formation has not been found to occur, which suggests that inter-system-crossing to the ground singlet C3H4 potential energy surface manifold has low probability and/or the H2-elimination process on the triplet surface is characterized by a very large exit potential barrier.