Spectroscopic characterization of two peroxyl radicals during the O(2)-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 (CH(3)S•), followed by its O(2)-oxidation reaction via the intermediacy of free radicals CH(3)SO(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 CH(3)S• through flash pyrolysis of S-nitrosothiol CH(3)SNO, enabling us to study the O(2)-oxidation of CH(3)S• 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 CH(3)S•, the peroxyl radical CH(3)SOO• forms by reacting with O(2). Upon irradiation at 830 nm, CH(3)SOO• undergoes isomerization to the sulfonyl radical CH(3)SO(2)• in cryogenic matrixes (Ar, Ne, and N(2)), and the latter can further combine with O(2) to yield another peroxyl radical CH(3)S(O)(2)OO• upon further irradiation at 440 nm. Subsequent UV-light irradiation (266 nm) causes dissociation of CH(3)S(O)(2)OO• to CH(3)SO(2)•, CH(2)O, SO(2), and SO(3). The IR spectroscopic identification of the two peroxyl radicals CH(3)SOO• and CH(3)S(O)(2)OO• is also supported by (18)O- and (13)C-isotope labeling experiments.