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

## TL;DR

This study directly observes the equilibrium between chlorine atoms and ethylene using VUV monitoring, revealing new insights into reaction kinetics and thermodynamics.

## Contribution

First direct observation of equilibrium involving Cl atoms and ethylene using VUV laser-induced fluorescence.

## Key findings

- Equilibrium behavior of Cl + C2H4 was observed between 393 and 490 K.
- The enthalpy of reaction was determined as −74.1 ± 0.6 kJ mol–1.
- Ab initio calculations were found to be accurate to within ∼4 kJ mol–1 compared to experimental results.

## Abstract

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, k
1a([M]), 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 (k
1a([M]),k
–1a([M])). 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, Δr
H
R1a

o
. This
analysis yields a Δr
H
0,R1a

o
equal to −74.1 ± 0.6 kJ
mol–1. Ab initio structure calculations
provided input values for MESMER analysis of the equilibrium data.
The range of ab initio calculations carried out returned
consistent values for Δr
H
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 ab initio calculations are good to ∼4 kJ mol–1. Above
500 K, the removal kinetics are dominated by abstraction: Cl + C2H4 → HCl + C2H3 (k
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.