Measurement of the Intramolecular Hydrogen-Shift Rate Coefficient for the CH₃SCH₂OO Radical between 314 and 433 K

Abstract

The intramolecular hydrogen-shift rate coefficient of the CH3SCH2O2 (methylthiomethylperoxy, MSP) radical, a product formed in the oxidation of dimethyl sulfide (DMS), was measured using a pulsed laser photolysis flow tube reactor coupled to a high-resolution time-of-flight chemical ionization mass spectrometer that measured the formation of the DMS degradation end product HOOCH2SCHO (hydroperoxymethyl thioformate). Measurements performed over the temperature range of 314–433 K yielded a hydrogen-shift rate coefficient of k 1(T) = (2.39 ± 0.7) × 109 exp(−(7278 ± 99)/T) s–1 Arrhenius expression and a value extrapolated to 298 K of 0.06 s–1. The potential energy surface and the rate coefficient have also been theoretically investigated using density functional theory at the M06-2X/aug-cc-pVTZ level combined with approximate CCSD(T)/CBS energies yielding k 1(273–433 K) = 2.4 × 1011 × exp(−8782/T) s–1 and k 1(298 K) = 0.037 s–1 in fair agreement with the experimental results. Present results are compared with the previously reported values of k 1(293–298 K).

Assessing the Isomerization of a Primary Intermediate (CH3SCH2O2• Radical) in Dimethyl Sulfide Degradation in the Marine Boundary Layer

“…14−19 The rate coefficient values (k isom ) estimated from theoretical and laboratory studies for the H-shift reaction of CH The experimentally measured rate coefficient for the first Hshift of the rate-determining step (CH 3 SCH 2 O 2 • ↔ •CH 2 SCH 2 OOH, k 1 ) is in the range of 0.1−0.23 s −1 at 295−298 K, and shows good agreement with each other within the stated uncertainty. 14,17,18 Recent results from theoretical calculations, k 1 = 0.041 s −1 at 293 K in Veres et al 15 and 0.037 s −1 at 298 K in Assaf et al, 19 are somewhat lower but still close to the experimental value. HPMTF has not been previously detected in the atmosphere 20,21 and thus is not widely reflected in atmospheric models describing the eventual impact of sulfur oxidation in the MBL.…”

“…Recently, Wu et al first theoretically proposed the unexpectedly rapid internal isomerization of CH 3 SCH 2 O 2 • under low-NO conditions (typically 2–8 pptv in MBL) to form carbon-centered hydroperoxyalkyl (C•H 2 SCH 2 OOH) radicals. This new study on H-shift has been further validated by laboratory and theoretical studies. − The rate coefficient values ( k isom ) estimated from theoretical and laboratory studies for the H-shift reaction of CH 3 SCH 2 O 2 • are in the range of 0.037–0.23 s –1 in the temperature range of 293–298 K. The radical generated from the H-shift (•CH 2 SCH 2 OOH) undergoes the second O 2 addition in radical chain branching to form oxygen-centered •OOCH 2 SCH 2 OOH radicals, sustaining tropospheric autoxidation to form novel hydroperoxymethylthioformate (HPMTF, HOOCH 2 SCHO) and OH radical recycling. • CH 2 SCH 2 OOH + normalO 2 → • OOCH 2 SCH 2 OOH • OOCH 2 SCH 2 OOH ↔ [ HOOCH 2 SC • HOOH ] → HOOCH 2 SCHO + • OH • OOCH 2 SCH 2 OOH → HOOCH 2 SCHO + • OH …”

“…The atmospheric oxidation of DMS is mainly initiated by OH radicals, , and the reactive species methylthioperoxy radicals (MSP, CH 3 SCH 2 O 2 •) are formed through O 2 addition reaction (). The central role of CH 3 SCH 2 O 2 • peroxy radicals in MBL under low-NO conditions has received considerable attention recently, − and it may ultimately affect the atmospheric global marine sulfur budget. CH 3 SCH 2 • + normalO 2 → CH 3 SCH 2 normalO 2 • CH 3 SCH 2 normalO 2 • ↔ • CH 2 SCH 2 OOH …”

Measurement Report: Carbonyl Sulfide production during Dimethyl Sulfide oxidation in the atmospheric simulation chamber SAPHIR

“…This reaction would be the dominant loss process of the RO2 leading to the formation of hydroperoxymethyl thioformate (HPMTF). Several laboratory studies have been carried out to measure this isomerization rate constant, finding values of 0.23 ± 0.12 s -1 (295 K, Berndt et al, 2019), ~0.1 s -1 (298 K, Jernigan et al, 2022) and 0.06 s -1 (298 K, Assaf et al, 2023). Even with the slower isomerization rate of 0.06 s -1 , this reaction would still be the dominant reaction path in marine environments where nitric oxide (NO) mixing ratios are typically lower than 0.1 ppb.…”

Measurement report: Carbonyl sulfide production during dimethyl sulfide oxidation in the atmospheric simulation chamber SAPHIR

“…This reaction would be the dominant loss process of the RO 2 leading to the formation of hydroperoxymethyl thioformate (HPMTF). Several laboratory studies have been carried out to measure this isomerization rate constant, finding values of 0.23 ± 0.12 s −1 (295 K, Berndt et al, 2019), ∼ 0.1 s −1 (298 K, Jernigan et al, 2022), and 0.06 s −1 (298 K, Assaf et al, 2023). Even with the slower isomerization rate of 0.06 s −1 , this reaction would still be the dominant reaction path in marine environments where nitric oxide (NO) mixing ratios are typically lower than 0.1 ppb.…”