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 ImplicationsArticle link copied!

• NCJ Number: 310644
• Journal: The Journal of Physical Chemistry A Volume: 129 Issue: 30 Dated: July 2025
• Date Published: July 2018
• Grant Number(s): 15PNIJ-22-GG-04423-SLFO
• Agencies: NIJ-Sponsored
• Publication Type: Research (Applied/Empirical)
• Award: Research and Development Using High-level Quantum Chemistry Calculations for the Creation of a Rapid Field Test for the Identification and Differentiation of Hemp and Marijuana

Abstract

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. (Publisher abstract provided.)