Kinetic modeling of the reactions of C3H3+ with acetylene,deuteroacetylene, and diacetylene

The reactions of ?-C₃H₃⁺ (propargylium cation) with acetylene and diacetylene have been modeled kinetically. Data were obtained from Fourier Transform Ion Cyclotron Resonance (FTICR) experiments on these systems, which are themselves models for soot particle initiation. Acetylene forms an encounter complex with ?-C₃H₃⁺, but, in the absence of a third body collision, the complex decomposes to acetylene and c-C₃H₃⁺ (cyclopropenylium cation) at about 1/3 the rate it decomposes to acetylene and ?-C₃H₃⁺, in spite of the fact that c-C₃H₃⁺ is ca. 115 kJ/mol more stable than ?-C₃H₃⁺. The encounter complex is long enough lived, and energetic enough, to scramble deuterium in reactions between ?-C₃H₃⁺ and C₂D₂. These reactions have been successfully modeled, yielding a nearly statistical distribution of deuterium, and a rather large kinetic isotope effect. The more complex reactions of ?-C₃H₃⁺ with diacetylene have also been modeled.