First Direct Observation of Equilibrium Involving Cl Atoms: Cl + C ₂ H ₄ ⇔ ClCH ₂ CH ₂ by VUV Monitoring

The kinetics between Cl and ethylene, R1, have been determined between 298 and 822 K in time-resolved experiments, where the Cl atoms were monitored for the first time via laser-induced fluorescence at 118.877 nm. A key advantage of this method of Cl detection is that there is limited absorption of 118 nm radiation by oxygen, and we report the first Cl reactivity measurements. The kinetics of Cl + C₂H₄ at room temperature are simple association, <i>k</i>_{1a([<i>M</i>])}, and have been used in a master equation analysis (via the MESMER application) to show consistency with most of the literature. Between 393 and 490 K, the kinetics exhibited equilibrium behavior: Cl + C₂H₄ ⇔ Cl-C₂H₄ (<i>k</i>_{1a([<i>M</i>])},<i>k</i>_{-1a([<i>M</i>])}). These forward and reverse rate coefficients have been used in van't Hoff and reaction rate theory (MESMER) analysis to determine the enthalpy of reaction, Δ_r<i>H</i>_R1a^o. This analysis yields a Δ_r<i>H</i>_0,R1a^o equal to -74.1 ± 0.6 kJ mol^-1. <i>Ab initio</i> structure calculations provided input values for MESMER analysis of the equilibrium data. The range of <i>ab initio</i> calculations carried out returned consistent values for Δ_r<i>H</i>_0,R1a^o, but the values are consistently more exothermic than the experimental value. Via comparison between theory and experiment, it is estimated that these <i>ab initio</i> calculations are good to ∼4 kJ mol^-1. Above 500 K, the removal kinetics are dominated by abstraction: Cl + C₂H₄ → HCl + C₂H₃ (<i>k</i>_1b). This reaction occurs on an endothermic potential surface, where the energy of the transition state is below that of the products. Analysis of this kinetic data and the literature highlights that the location of the transition state along the reaction coordinate varies with temperature, becoming more reagent-like with increased temperature.