First Direct Observation of Equilibrium Involving Cl Atoms: Cl + C₂H₄ ⇔ ClCH₂CH₂ by VUV Monitoring

Area of Science:

• Chemical Kinetics and Reaction Dynamics
• Atmospheric Chemistry
• Physical Chemistry

Background:

• Understanding the reaction between chlorine atoms (Cl) and ethylene (C2H4) is crucial for atmospheric chemistry.
• Previous kinetic studies have provided some data, but a comprehensive understanding across a wide temperature range is lacking.
• Accurate kinetic data is essential for atmospheric modeling and predicting chemical reaction pathways.

Purpose of the Study:

• To determine the kinetics of the Cl + C2H4 reaction over a broad temperature range (298–822 K).
• To measure the enthalpy of reaction for the association pathway using experimental and theoretical methods.
• To investigate the temperature dependence of the abstraction reaction pathway and its transition state.

Main Methods:

• Time-resolved experiments utilizing laser-induced fluorescence (LIF) at 118.877 nm for Cl atom detection.
• Master equation analysis (MESMER) to model reaction kinetics and compare with literature data.
• Van't Hoff and reaction rate theory analysis combined with ab initio calculations to determine thermodynamic properties.

Main Results:

• Room temperature kinetics show simple association (k1a[M]).
• Equilibrium behavior observed between 393–490 K, yielding an enthalpy of reaction (ΔrH°R1a) of -74.1 ± 0.6 kJ mol⁻¹.
• Above 500 K, abstraction (k1b) dominates, with the transition state location varying with temperature.

Conclusions:

• The study provides the first reactivity measurements for Cl + C2H4 using LIF detection.
• Experimental and theoretical results for the enthalpy of reaction are in good agreement, with ab initio calculations accurate to ~4 kJ mol⁻¹.
• The temperature-dependent transition state location in the abstraction channel highlights complex reaction dynamics.