Radical Thermometers, Thermochemistry, and Photoelectron Spectra: A Photoelectron Photoion Coincidence Spectroscopy Study of the Methyl Peroxy Radical
Poster Number
8a
Introduction/Abstract
Reactive intermediates, such as alkylperoxy (ROO) radicals, play a crucial role in combustion and atmospheric processes. The study of such intermediates is also of fundamental value, providing rich opportunities to understand biradical vs. zwitterion reactivity, conformer-resolved chemistry, and autooxidation mechanisms.
Purpose
We investigated the simplest alkylperoxy radical, CH3OO, using the new CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence) apparatus at the Swiss Light Source to derive highly accurate thermochemical data to improve the preciseness of predictive models in combustion and atmospheric processes.
Method
The new double-imaging CRF-PEPICO apparatus has been used to record the first threshold photoelectron spectrum (TPES) of the methyl peroxy radical. This prototypical free radical was formed by hydrogen atom abstraction from methane with photolytically generated chlorine atoms and the subsequent reaction with oxygen molecules. We used Franck–Condon simulations including a hindered rotor treatment to extract the adiabatic ionization energy of the methyl peroxy radical from the TPES. Oxygen molecule loss by dissociative ionization from the internal energy selected methyl peroxy cations was also investigated using statistical energy distribution modeling of the experimental breakdown curve.
Results
Modeling the experimental photoion mass-selected threshold photoelectron spectrum using Franck–Condon simulations including transitions to triplet and singlet cationic states yielded the adiabatic ionization energy of 10.265 ± 0.025 eV of the methyl peroxy radical (CH3OO). Its dissociative photoionization generates the CH3+ fragment ion at the appearance energy of 11.164 ± 0.010 eV. Combining these two values with the heat of formation of the CH3 radical yields the heat of formation of the CH3OO radical, 22.06 ± 0.97 kJ/mol, reducing the uncertainty of the previously determined value by a factor of 5. In addition, the statistical simulation of the CH3OO breakdown diagram provides a molecular thermometer of the free radical’s internal temperature, which we measured to be 330 ± 30 K.
Significance
The oxidation of organic molecules, particularly of hydrocarbons, is a crucial process in combustion and atmospheric chemistry. Although decades of research has provided a solid understanding of many reaction mechanisms, there is an ever-increasing need for reliable experimental data to improve the accuracy of predictive models and to extend these models to more complex molecules and reaction conditions. Our derived heat of formation of the methylperoxy radical reduced the uncertainty of the previously determined value by a factor of 5. In addition, we showed that modeling a high signal-to-noise breakdown curve could also serve as molecular thermometer of free radicals, which can be used to monitor whether these species have reached thermal equilibrium, a contentious issue in kinetics studies.
Location
DeRosa University Center
Format
Poster Presentation
Poster Session
Afternoon 1pm-3pm
Radical Thermometers, Thermochemistry, and Photoelectron Spectra: A Photoelectron Photoion Coincidence Spectroscopy Study of the Methyl Peroxy Radical
DeRosa University Center
Reactive intermediates, such as alkylperoxy (ROO) radicals, play a crucial role in combustion and atmospheric processes. The study of such intermediates is also of fundamental value, providing rich opportunities to understand biradical vs. zwitterion reactivity, conformer-resolved chemistry, and autooxidation mechanisms.
