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

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 CH3SOx• (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.

Atmospheric Gas-Phase Formation of Methanesulfonic Acid

“…Under atmospheric conditions, most hydrocarbon-derived alkyl radicals (R) add O 2 forming peroxy radicals (RO 2 ), and similar reactions can, in principle, also happen for the sulfur-centered radicals. However, the O 2 additions to CH 3 SO and CH 3 SO 2 remain speculative. ,,, Therefore, we calculated the O 2 addition to CH 3 S, CH 3 SO, and CH 3 SO 2 forming the corresponding peroxy radicals CH 3 S­(O) n OO ( n = 0–2) using multireference (MR) methods (S1.1). The O 2 addition to the CH 3 S radical has been studied experimentally, and we use it to benchmark our theoretical approach .…”

“…It follows that the recently proposed MSA formation from CH 3 S(O)OO isomerization and subsequent bimolecular reactions is unlikely. 61 Thus, the only remaining candidate to explain atmospheric gas-phase MSA formation is through the CH 3 SO 2 intermediate.…”

“…However, the O 2 additions to CH 3 SO and CH 3 SO 2 remain speculative. 3,11,13,61 Therefore, we calculated the O 2 addition to CH 3 S, CH 3 SO, and CH 3 SO 2 forming the corresponding peroxy radicals CH 3 S(O) n OO (n = 0−2) using multireference (MR) methods (S1.1). The O 2 addition to the CH 3 S radical has been studied experimentally, and we use it to benchmark our theoretical approach.…”

“…We find that, at a given temperature, the equilibrium constant, K eq , for O 2 addition to CH 3 S, CH 3 SO, and CH 3 SO 2 differs by more than 10 orders of magnitude, in contrast to the assumptions in some literature that O 2 additions to different sulfur radicals are rather similar. 13,61 At 300 K, our calculated K eq for CH 3 SOO is about a factor of 34 lower than the available experimental results. 41 Therefore, we scaled our calculated K eq for the three systems up by a factor of 34 (S1.4) based on the consistency of relative stability of CH 3 S(O) n OO (n = 0−2) calculated with different methods (S1.3).…”

Arctic observations of Hydroperoxymethyl Thioformate (HPMTF) – seasonal behavior and relationship to other oxidation products from Dimethyl Sulfide at the Zeppelin Observatory, Svalbard

“…This compound is produced via the H-abstraction pathway, leading to the methylthiomethyl-peroxy radical (CH3SCH2OO • ), quickly followed by an H-shift to form intermediate radicals and then HPMTF (Berndt et al, 2019;R. Wu et al, 2015;Z. Wu et al, 2022).…”