Direct Trace Fitting of Experimental Data Using the Master Equation: Testing Theory and Experiments on the OH + C₂H₄...

Area of Science:

• Chemical Kinetics and Dynamics
• Combustion Chemistry
• Atmospheric Chemistry

Background:

• Understanding the reaction mechanism between hydroxyl radicals (OH) and ethylene (C2H4) is crucial for combustion and atmospheric chemistry.
• Accurate determination of the binding energy for the hydroxyethyl radical (HOC2H4) adduct is essential for modeling these processes.
• Previous studies have provided estimates, but precise experimental validation is needed.

Purpose of the Study:

• To experimentally determine the binding energy (ΔRH00) of the HOC2H4 adduct formed from the OH + C2H4 reaction.
• To investigate the influence of OH abstraction pathways at elevated temperatures.
• To validate and refine theoretical calculations for this important reaction system.

Main Methods:

• Utilized laser flash photolysis coupled with laser-induced fluorescence detection of OH radicals.
• Studied the OH + C2H4 reaction over a temperature range of 563–723 K and N2 pressures from 58–250 Torr.
• Employed a master equation approach, fitting time-resolved OH decay traces to extract thermodynamic data and kinetic parameters.

Main Results:

• Precisely determined the binding energy of the HOC2H4 adduct to be 111.8 ± 2 kJ mol⁻¹.
• The experimental binding energy is in excellent agreement with high-level ab initio calculations (111.4 kJ mol⁻¹).
• Identified and quantified the role of the OH abstraction channel at higher temperatures.

Conclusions:

• The study provides a highly accurate experimental value for the HOC2H4 adduct binding energy.
• The MESMER master equation code was successfully extended to include secondary chemistry, improving analysis.
• Results contribute to a more robust understanding of OH radical reactions with alkenes in combustion and atmospheric environments.