Experimental Study of the Reaction between Vinyl and Methyl Radicals in the Gas Phase. Temperature and Pressure Dependence of Overall Rate Constants and Product Yields

The vinyl−methyl cross-radical reaction was studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants and quantitative product yields were obtained in direct real-time experiments in the temperature region 300−900 K and bath gas (He) density (3−12) × 10 16 molecules cm -3 . The observed overall C 2 H 3 + CH 3 rate constant is independent of pressure and demonstrates a slight negative temperature dependence, which can be represented by the expression k 4 = 5.1 × 10 -7 T -1.26 exp(−362/T) molecule -1 cm 3 s -1 (The expression has ±10% uncertainty, see text for details.). Propene, acetylene, and allyl radicals were identified as primary products of the C 2 H 3 + CH 3 reaction. At room temperature propene and acetylene are the major products of the reaction, whereas at 900 K production of allyl radicals becomes the dominating channel. On the basis of these experimental results, a mechanism consisting of two major routes is proposed. One route proceeds via direct abstraction of a hydrogen atom from the vinyl radical by the methyl radical resulting in the formation of acetylene and methane. The other route proceeds via the formation of chemically activated propene that can undergo either collisional stabilization or further decomposition into allyl radical and hydrogen atom.