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

Abstract

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.

Accretion product formation in the self-reaction of ethene-derived hydroxy peroxy radicals

“…Such wall loss likely limited the ability of Barnes et al 14 to observe the ROOR. Additionally, a recent study published on the reactions of ethyl peroxy radicals 12 found a branching ratio to the accretion product of 10±5%, demonstrating that formation of the accretion product is significant even in small unsubstituted peroxy radical systems, lending further support to our results.…”

“…17 The fraction of thermalized alkoxy radicals generated in Reaction (4) may differ from that generated by the reaction between RO 2 and NOc. The analysis of our data, as presented in ESI Appendix C, † indicates that approximately 20% of the alkoxy radicals in our system react with O 2 , whereas 29% of the alkoxy radicals in the system studied by Orlando et al react with O 2 at 298 K. While this As described above, in the limit where F external / 0, Q (eqn (11)) is a measure of the ratio of the radical propagating channels to the radical terminating channels of Reaction (4). In the limit where all HO 2 is produced internally to Reaction (4), >90% of the HO 2 reacts with HOCH 2 CH 2 OOc, and there are no secondary losses,…”

“…1,8,9 In fact, several studies have found that for some RO 2 , Reaction (4d) may proceed at rates approaching the collision rate, suggesting a drastic shi from previous assumptions. 1,8,10 [While this manuscript was under peer-review, Yue et al reported 11 that a peroxide is produced in the RO 2 þ RO 2 chemistry following Habstraction from ethane at approximately 10% yield, using VUV photoionization mass spectrometry at 266 Pa and 298 K].…”

Rate constant and branching ratio of the reaction of ethyl peroxy radicals with methyl peroxy radicals

“…The decomposition of such dimer on reactor walls could lead to formation of aldehydes and thus appear as additional radical formation when measuring stable end products. The dimeric product has very recently been detected in the self-reaction of C 2 H 5 O 2 25 using advanced vacuum ultraviolet (VUV) photoionization mass spectrometry with a yield of 10 AE 5%. The dimer has also been directly detected by CIMS with a yield of 23% in the self-reaction of HOC 2 H 4 O 2 radicals and has been proven to decompose easily on quartz or metal surfaces.…”

“…Methane and ethane are amongst the most abundant hydrocarbons, and their atmospheric oxidation leads to the formation of methyl peroxy (CH 3 O 2 ) and ethyl peroxy (C 2 H 5 O 2 ) radicals. For both radicals, the kinetic and product distribution for the self-reaction has been studied numerous times (for CH 3 O 2 4–12 and for C 2 H 5 O 2 13–26 ), the same is true for their reaction with HO 2 (for CH 3 O 2 5,9,27–32 and for C 2 H 5 O 2 13,14,20,26,30,33–35 ). Their reaction with OH radicals has been the subject of a few studies (for CH 3 O 2 3,36–41 and for C 2 H 5 O 2 42–44 ).…”