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

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 (CHS•), followed by its O-oxidation reaction via the intermediacy of free radicals CHSO• (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 CHS• through flash pyrolysis of S-nitrosothiol CHSNO, enabling us to study the O-oxidation of CHS• 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 CHS•, the peroxyl radical CHSOO• forms by reacting with O. Upon irradiation at 830 nm, CHSOO• undergoes isomerization to the sulfonyl radical CHSO• in cryogenic matrixes (Ar, Ne, and N), and the latter can further combine with O to yield another peroxyl radical CHS(O)OO• upon further irradiation at 440 nm. Subsequent UV-light irradiation (266 nm) causes dissociation of CHS(O)OO• to CHSO•, CHO, SO, and SO. The IR spectroscopic identification of the two peroxyl radicals CHSOO• and CHS(O)OO• is also supported by O- and C-isotope labeling experiments.