Arathala Parandaman, Kumar Avinash, Musah Rabi A
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States.
Aerosol Physics Laboratory, Physics Unit, Faculty of Engineering and Natural Sciences, Tampere University, 33720 Tampere, Finland.
J Phys Chem A. 2025 Jul 31;129(30):6866-6882. doi: 10.1021/acs.jpca.5c00743. Epub 2025 Jul 18.
The reaction of 3-methyl-2-butene-1-thiol (MBT; (CH)C═CHCHSH) with the OH radical is reported to proceed via the addition to either of the sp hybridized C atoms, forming the two distinct C-centered radicals: (CH)C(OH)CHCHSH (R1) and (CH)CCH(OH)CHSH (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 O to form R1O ((CH)C(OH)CH(OO)CHSH). In contrast, R2 can either undergo isomerization to form the sulfur-centered MBT-OH radical or add O to form R2O ((CH)C(OO)CH(OH)CHSH). These radicals undergo HO 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 for R1O and R2O, respectively, calculated relative to those of the R1 + O and R2 + O reactants. Rate coefficients for key pathways, including unimolecular dissociation and O addition followed by subsequent reactions, were calculated and analyzed. The kinetics results suggest that the intramolecular H atom transfer paths of R1O and R2O are significantly faster by ∼3 orders of magnitude compared to their bimolecular reactions with NO/HO, respectively. The findings suggest that under low NO concentrations R1O and R2O 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)CHSH, (CH)C(OH)C(═O)H, CHC(O)CH, and various S- and C-centered alkyl radicals in the atmosphere.
据报道，3-甲基-2-丁烯-1-硫醇（MBT；(CH₃)₂C═CHCH₂SH）与羟基自由基的反应是通过加成到sp²杂化的两个碳原子中的任意一个上进行的，形成两种不同的以碳为中心的自由基：(CH₃)₂C(OH)CH₂CH₂SH（R1）和(CH₃)₂CCH(OH)CH₂SH（R2）。了解这些自由基的归宿对于阐明MBT的大气转化机制和反应产物很重要。通过量子化学计算和动力学建模，我们表明，由于高能垒，R1的单分子解离以及异构化反应在动力学上是不利的，并且R1最有可能与大气中的O₂反应形成R1O₂（(CH₃)₂C(OH)CH(OO)CH₂SH）。相比之下，R2可以进行异构化形成以硫为中心的MBT-OH自由基，或者与O₂加成形成R2O₂（(CH₃)₂C(OO)CH(OH)CH₂SH）。这些自由基会经历羟基消除和分子内氢原子转移（HAT）途径。具体来说，分子内HAT从-SH基团转移到R-OO的末端氧原子上形成以硫为中心的QOOH自由基，相对于R1 + O₂和R2 + O₂反应物，R1O₂和R2O₂的能垒高度分别为-18.6和-18.3 kcal mol⁻¹。计算并分析了包括单分子解离和O₂加成随后的后续反应等关键途径的速率系数。动力学结果表明，与它们分别与NO/HO₂的双分子反应相比，R1O₂和R2O₂的分子内氢原子转移路径明显快约3个数量级。研究结果表明，在低NO浓度下，R1O₂和R2O₂能够经历氢转移驱动的自氧化机制。讨论了其大气影响。结果表明，源自MBT的过氧自由基通过在大气中产生高氧化态的过氧自由基、HC(O)CH₂SH、(CH₃)₂C(OH)C(═O)H、CH₃C(O)CH₃以及各种以硫和碳为中心的烷基自由基等活性物种，对对流层化学有贡献。
