Peroxy radicals are very important reaction intermediates in the oxidation of atmospheric chemistry and combustion. Their reactions can lead to the formation of a great deal of secondary pollutants, such as ozone and secondary organic aerosols, taking a large influence on the atmospheric environment and human health and attracting attentions in the past decades. In particular, with the increase of their mass, the structure of peroxy radicals becomes complex, usually taking several isomers or conformers. The knowledge on the isomers or conformers of peroxy radicals is very limited, and identifying them is still a challenge.

A research group led by Prof. ZHANG Weijun at the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS) has investigated the vacuum ultraviolet (VUV) photochemistry of the ethyl peroxy radical and identified the conformers of it. Relevant study has been published in Physical Chemistry Chemical Physics.

Together with researchers at the National Synchrotron Radiation Laboratory, they investigated C₂H₅O₂, the VUV photoionization of ethyl peroxy radical, which was also the simplest peroxy radical having more than one conformer. By using a home-made photoionization time-of-flight mass spectrometer at Hefei synchrotron, the researchers complemented the experiment with theoretical calculations.

They employed a microwave discharge flow tube as the reactor to produce C₂H₅O₂ radical via reaction of C₂H₅ with oxygen molecules. And a variety of radicals involved in the oxidation reaction were identified and determined in the online photoionization mass spectra.  

In addition, the photoionization spectrum of C₂H₅O₂ was observed and assigned with theoretical Franck–Condon (FC) calculations. The results show that the gauche conformer (G-C₂H₅O₂) of C₂H₅O₂ has favorable FC factor in the ionization transitions, while the trans conformer (T-C₂H₅O₂) contributes little to the photoionization spectrum. 

"This work provides important data and technical methods for online detection of different kinds of peroxy radicals plus their isomers and conformers," said TANG Xiaofeng, one of the researchers.

The work was supported by the National Natural Science Foundation of China, the International Partnership Program of cas and the Key Program of Research and Development of Hefei Science Center of CAS.

Fig. 1. Real-time detection of key species in the oxidation reactions by photoionization mass spectrometry. (Image by LIN Xiaoxiao) 

Fig. 2. The optimized structures of the gauche and the trans conformers of C₂H₅O₂ and their cations. (Image by LIN Xiaoxiao)