Reaction of Atmospherically Relevant Sulfur-Centered Radicals with RO₂ and HO₂

Abstract

The atmospheric oxidation of dimethyl sulfide and other emitted sulfur species leads to the formation of the methylthio radical, CH 3 S, which can be further oxidized to the CH 3 SO and CH 3 SO 2 radicals. We investigated computationally the reactions of these three sulfur-centered radicals with the peroxy radicals ROO and HOO. Our results demonstrate that CH 3 S and CH 3 SO react with these peroxy radicals to form short-lived peroxide intermediates, which then decompose via a concerted O− O bond scission and S�O double bond formation that results in an increased valence of the sulfur atom. In contrast, CH 3 SO 2 reacts to form stable CH 3 S(O) 2 OOR and CH 3 S(O) 2 OOH peroxide products, as sulfur is already at its highest valence. Multireference methods were used to describe these reactions in which the valence of the sulfur atom changes.

Atmospheric Gas-Phase Formation of Methanesulfonic Acid

“…Overall, based on the available literature, the gas-phase oxidation of MSIA, MSEA, and CH 3 S all form CH 3 SO or CH 3 SO 2 as intermediates (Figure ). The CH 3 SO radical can react with NO x , RO 2 , and HO 2 , leading to the formation of CH 3 SO 2 . ,,, The CH 3 SO 2 radical is known to rapidly decompose to CH 3 and SO 2 with a measured SO 2 yield of about 98% at 298 K. , Thus, the current known mechanisms leave no obvious route to gas-phase MSA formation to explain the measured gas-phase MSA in the DMS + OH reaction . Here, we propose a temperature-sensitive gas-phase MSA formation mechanism that can explain the measured SA/MSA ratios in ambient field and chamber experiments for a range of atmospherically relevant temperatures. , Global modeling, including the proposed MSA formation mechanism, shows that large amounts of MSA are formed in the gas-phase in polar regions.…”

“…Detailed information about how the complete active space self-consistent field (CASSCF) and the multireference configuration interaction (MRCI) calculations were done is given in S1.1, S1.5, S3.2, and refs , . Briefly, RO-HF wave functions were used as the initial guesses for the CASSCF calculations.…”

“… ,,− This is supported by the high SO 2 yield measured in DMSO + OH experiments where MSIA is the key intermediate. ,, The oxidation of MSEA is mainly initiated by oxidation of O 3 (Figure ). It forms CH 3 S­(O)­OOOH as an intermediate, which mainly decomposes by O–O bond scission forming CH 3 SO 2 + HO 2 and to a lesser extent undergoes a concerted H-shift and O 2 loss reaction forming MSIA. , MSEA can also react with OH by H-abstraction, forming the CH 3 SO radical as a minor pathway. The CH 3 S radical can react with O 3 , NO x , RO 2 , and HO 2 forming the CH 3 SO radical , or react with O 2 forming the CH 3 SOO radical, which further isomerizes to the CH 3 SO 2 radical (Figure ).…”

“…Oxidation mechanism of MSIA, MSEA, and the CH 3 S radical under atmospheric conditions. ,,− The reactions of CH 3 S and CH 3 SO with O 3 and NO 2 can also happen via similar mechanisms with RO 2 and HO 2 (e.g., +RO 2 , −RO).…”

Ab Initio and Statistical Rate Theory Exploration of the CH (X2Π) + OCS Gas-Phase Reaction

“…The choice of a hybrid DFT method and the aug-cc-pV­(Q+d)­Z basis set for geometry and frequency calculation is based on previous work with sulfur-based systems. , Other levels of theory were also explored to calculate stationary point structures (reactants, products, and reaction intermediates), as reported in the Supporting Information, but with little change in relative energies or geometries. Geometries, energies, and vibrational frequencies for all stationary points and transition states reported here can be found in the Supporting Information.…”