Spectroscopic characterization of two peroxyl radicals during the O2-oxidation of the methylthio radical

Zhuang Wu; Xin Shao; Bifeng Zhu; Lina Wang; Bo Lu; Tarek Trabelsi; Joseph S Francisco; Xiaoqing Zeng

doi:10.1038/s42004-022-00637-z

Spectroscopic characterization of two peroxyl radicals during the O₂-oxidation of the methylthio radical

Commun Chem. 2022 Feb 17;5(1):19. doi: 10.1038/s42004-022-00637-z.

Authors

Zhuang Wu¹, Xin Shao¹, Bifeng Zhu¹, Lina Wang¹, Bo Lu¹, Tarek Trabelsi², Joseph S Francisco³, Xiaoqing Zeng⁴

Affiliations

¹ Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai, 200433, China.
² Department of Earth and Environment Science, University of Pennsylvania, Philadelphia, PA, 19104-6243, USA.
³ Department of Earth and Environment Science, University of Pennsylvania, Philadelphia, PA, 19104-6243, USA. frjoseph@sas.upenn.edu.
⁴ Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai, 200433, China. xqzeng@fudan.edu.cn.

Abstract

The atmospheric oxidation of dimethyl sulfide (DMS) yields sulfuric acid and methane sulfonic acid (MSA), which are key precursors to new particles formed via homogeneous nucleation and further cluster growth in air masses. Comprehensive experimental and theoretical studies have suggested that the oxidation of DMS involves the formation of the methylthio radical (CH₃S•), followed by its O₂-oxidation reaction via the intermediacy of free radicals CH₃SO_x• (x = 1-4). Therefore, capturing these transient radicals and disclosing their reactivity are of vital importance in understanding the complex mechanism. Here, we report an optimized method for efficient gas-phase generation of CH₃S• through flash pyrolysis of S-nitrosothiol CH₃SNO, enabling us to study the O₂-oxidation of CH₃S• by combining matrix-isolation spectroscopy (IR and UV-vis) with quantum chemical computations at the CCSD(T)/aug-cc-pV(X + d)Z (X = D and T) level of theory. As the key intermediate for the initial oxidation of CH₃S•, the peroxyl radical CH₃SOO• forms by reacting with O₂. Upon irradiation at 830 nm, CH₃SOO• undergoes isomerization to the sulfonyl radical CH₃SO₂• in cryogenic matrixes (Ar, Ne, and N₂), and the latter can further combine with O₂ to yield another peroxyl radical CH₃S(O)₂OO• upon further irradiation at 440 nm. Subsequent UV-light irradiation (266 nm) causes dissociation of CH₃S(O)₂OO• to CH₃SO₂•, CH₂O, SO₂, and SO₃. The IR spectroscopic identification of the two peroxyl radicals CH₃SOO• and CH₃S(O)₂OO• is also supported by ¹⁸O- and ¹³C-isotope labeling experiments.