A Computational Study on the Atmospheric Fate of Carbon-Centered Radicals from the 3-Methyl-2-butene-1-thiol + ^•OH Reaction: Mechanistic Insights and Atmospheric Implications

The reaction of 3-methyl-2-butene-1-thiol (MBT; (CH3)2C═CHCH2SH) with the OH radical is reported to proceed via the addition to either of the sp2 hybridized C atoms, forming the two distinct C-centered radicals: (CH3)2C(OH)C•HCH2SH (R1) and (CH3)2C•CH(OH)CH2SH (R2). Understanding the fate of these radicals is important for elucidating MBT's atmospheric transformation mechanisms and the reaction products. Using quantum chemical calculations and kinetic modeling, we show that the unimolecular dissociation as well as isomerization reactions of R1 are kinetically unfavorable due to high energy barriers, and that R1 most likely reacts with atmospheric O2 to form R1O2 ((CH3)2C(OH)CH(OO•)CH2SH). In contrast, R2 can either undergo isomerization to form the sulfur-centered MBT-OH radical or add O2 to form R2O2 ((CH3)2C(OO•)CH(OH)CH2SH). These radicals undergo HO2 elimination and intramolecular hydrogen atom transfer (HAT) pathways. Specifically, intramolecular HAT from the -SH group to the terminal oxygen atom of R-OO forms S-centered QOOH radicals, with barrier heights of -18.6 and -18.3 kcal mol-1 for R1O2 and R2O2, respectively, calculated relative to those of the R1 + O2 and R2 + O2 reactants. Rate coefficients for key pathways, including unimolecular dissociation and O2 addition followed by subsequent reactions, were calculated and analyzed. The kinetics results suggest that the intramolecular H atom transfer paths of R1O2 and R2O2 are significantly faster by ∼3 orders of magnitude compared to their bimolecular reactions with NO/HO2, respectively. The findings suggest that under low NO concentrations R1O2 and R2O2 are capable of undergoing H-shift-driven autoxidation mechanisms. The atmospheric implications are discussed. Results indicate that MBT-derived peroxy radicals contribute to tropospheric chemistry by generating reactive species such as highly oxygenated peroxy radicals, HC(O)CH2SH, (CH3)2C(OH)C(═O)H, CH3C(O)CH3, and various S- and C-centered alkyl radicals in the atmosphere.