Dimeric Product of Peroxy Radical Self-Reaction Probed with VUV Photoionization Mass Spectrometry and Theoretical Calculations: The Case of C2H5OOC2H5.

Organic peroxy radicals (RO2) as key intermediates in tropospheric chemistry exert a controlling influence on the cycling of atmospheric reactive radicals and the production of secondary pollutants, such as ozone and secondary organic aerosols (SOA). Herein, we present a comprehensive study of the self-reaction of ethyl peroxy radicals (C2H5O2) by using advanced vacuum ultraviolet (VUV) photoionization mass spectrometry in combination with theoretical calculations. A VUV discharge lamp in Hefei and synchrotron radiation at the Swiss Light Source (SLS) are employed as the photoionization light sources, combined with a microwave discharge fast flow reactor in Hefei and a laser photolysis reactor at the SLS. The dimeric product, C2H5OOC2H5, as well as other products, CH3CHO, C2H5OH and C2H5O, formed from the self-reaction of C2H5O2 are clearly observed in the photoionization mass spectra. Two kinds of kinetic experiments have been performed in Hefei by either changing the reaction time or the initial concentration of C2H5O2 radicals to confirm the origins of the products and to validate the reaction mechanisms. Based on the fitting of the kinetic data with the theoretically calculated results and the peak area ratios in the photoionization mass spectra, a branching ratio of 10 ± 5% for the pathway leading to the dimeric product C2H5OOC2H5 is measured. In addition, the adiabatic ionization energy (AIE) of C2H5OOC2H5 is determined at 8.75 ± 0.05 eV in the photoionization spectrum with the aid of Franck-Condon calculations and its structure is revealed here for the first time. The potential energy surface of the C2H5O2 self-reaction has also been theoretically calculated with a high-level of theory to understand the reaction processes in detail. This study provides a new insight into the direct measurement of the elusive dimeric product ROOR and demonstrates its non-negligible branching ratio in the self-reaction of small RO2 radicals.

Dimeric product formation in the self-reaction of small peroxy radicals using synchrotron radiation vacuum ultraviolet photoionization mass spectrometry.

Peroxy radicals (RO2) are key reactive intermediates in atmospheric oxidation processes and yet their chemistry is not fully unraveled. Little is known about their structures and the structures of the dimeric products (ROOR) in the self-reaction of small RO2, which are among the most abundant RO2 in the atmosphere. The product branching ratios of ROOR and their atmospheric roles are still in controversy. Here, the self-reaction of propyl peroxy radicals (C3H7O2), a typical small RO2 radical in the atmosphere, has been studied using synchrotron radiation vacuum ultraviolet photoionization mass spectrometry. Both radical (C3H7O) and closed-shell molecular (C3H6O, C3H7OH, C3H7OOC3H7) products in the self-reaction are observed in photoionization mass spectra and their elusive isomers are definitely identified in mass-selected photoionization spectra. Three isomers of the C3H7OOC3H7 dimeric products, R1OOR1, R1OOR2, and R2OOR2 (R1 and R2 represent 1-C3H7 and 2-C3H7, respectively), as well as their complex structures have been determined for the first time. Kinetic experiments are performed and compared with chemical simulations to reveal the sources of specific products. The branching ratio of the C3H7OOC3H7 dimeric channel is measured at 10 ± 5%. This work demonstrates that the dimeric product formation in the self-reaction of small RO2 radicals is non-negligible and should provide valuable new insight into atmospheric modelling.