Temperature Dependent Kinetics (195−798 K) and H Atom Yields (298−498 K) from Reactions of ¹CH₂ with Acetylene, Ethene, and Propene

The rate coefficients for the removal of the excited state of methylene, ¹CH₂ (a¹A₁), by acetylene, ethene, and propene have been studied over the temperature range 195−798 K by laser flash photolysis, with ¹CH₂ being monitored by laser-induced fluorescence. The rate coefficients of all three reactions exhibit a negative temperature dependence that can be parametrized as k_¹CH2+C2H2 = (3.06 ± 0.11) × 10⁻¹⁰ T ^{(−0.39±0.07)} cm³ molecule⁻¹ s⁻¹, k_¹CH2+C2H4 = (2.10 ± 0.18) × 10⁻¹⁰ T ^{(−0.84±0.18)} cm³ molecule⁻¹ s⁻¹, k_¹CH2+C3H6 = (3.21 ± 0.02) × 10⁻¹⁰ T ^{(−0.13±0.01)} cm³ molecule⁻¹ s⁻¹, where the errors are statistical at the 2σ level. Removal of ¹CH₂ occurs by chemical reaction and electronic relaxation to ground state triplet methylene. The H atom yields from the reactions of ¹CH₂ with acetylene, ethene, and propene have been determined by laser-induced fluorescence over the temperature range 298−498 K. For the reaction with propene, H atom yields are close to the detection limit, but for acetylene and ethene, the fraction of H atom production is approximately 0.88 and 0.71, respectively, at 298 K, rising to unity by 398 K, with the balance of the reaction with acetylene presumed to be electronic relaxation. Experimental constraints limit studies to a maximum of 1 Torr of bath gas; master equation calculations using an approach that allows treatment of intermediates with deep energy wells have been carried out to explore the role of collisional stabilization for the reaction of ¹CH₂ with acetylene. Stabilization is calculated to be insignificant under the experimental conditions, but does become significant at higher pressures. Between pressures of 100 and 1000 Torr, propyne and allene are formed in similar amounts with a slight preference for propyne. At higher pressures propyne formation becomes about a factor two greater than that of allene, and above 10⁵ Torr (300 < T (K) < 600) cyclopropene formation starts to become significant. The implications of temperature-dependent ¹CH₂ relaxation on the roles of ¹CH₂ in chemical mechanisms for soot formation are discussed.