Organic peroxy radicals (RO₂) are important intermediates in the degradation of atmospheric volatile organic compounds. It not only participates in the cycling of atmospheric radicals and influences oxidizing capacity of the atmosphere, but also controls the formation of secondary pollutants. Under low NOx conditions, peroxy radicals react mainly with HO₂ radicals, as well as with themselves, and their products tend to have low volatility and readily enter the particulate phase. However, the associated double radical reactions are complex, the chemical mechanisms are poorly understood and experimental and theoretical studies are extremely challenging.

A collaborative team led by Prof. Weijun Zhang from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences studied the self-reaction of ethyl peroxy radicals (C₂H₅O₂). They combined advanced vacuum ultraviolet (VUV) photoionization mass spectrometry with theoretical calculations, providing a new insight into the direct measurement of the elusive dimeric product organic peroxides (ROOR).

The results have been published in International Journal of Molecular Sciences.

Together with scientists from the Université de Lille, France, the researchers investigated the self-reactions of C₂H₅O₂. In addition to the main products CH₃CHO, C₂H₅OH, C₂H₅O and C₂H₅OOH, the dimeric product C₂H₅OOC₂H₅ from the self-reaction of C₂H₅O₂ was clearly observed for the first time in the VUV photoionization mass spectrum.

The kinetic experiments of the self-reaction of C₂H₅O₂ and theoretical calculations were performed to verify the reaction mechanism of the ROOR product channel. The adiabatic ionization energy of C₂H₅OOC₂H₅ was also determined by measuring the synchrotron photoionization efficiency spectrum.

Combined with Franck-Condon factor simulations, the neutral and ionic structures of C₂H₅OOC₂H₅ were revealed.

"Our study shows that the ROOR product channel is not negligible in the small RO₂ self-reactions," said LIN Xiaoxiao, member of the team.

VUV lamp photoionization mass spectra measured (a) without O₂ and (b) with O₂ in the microwave discharge flow tube, with five times magnified data in red. (Image by LIN Xiaoxiao)

Photoionization spectrum of C₂H₅OOC₂H₅ and its calculated results in red. (Image by LIN Xiaoxiao)