Dimethyl Sulfide Oxidation at 400 – 545 K: Mass Spectrometric Characterization of Hydroperoxymethyl Thioformate (HPMTF) and Measurement of the CH ₃ SCH ₂ O ₂ → CH ₂ SCH ₂ OOH Rate Coefficient

We studied the Cl-initiated oxidation of dimethyl sulfide (DMS) at <i>p</i> = 800 Torr and <i>T</i> = 400 - 545 K, using a flow reactor coupled to a microsecond time-resolved tunable vacuum-ultraviolet photoionization mass spectrometer. Following the initiating photolysis laser pulse, we observed complex distributions of transient chemical intermediates and products, which included the peroxy radical RO₂ (CH₃SCH₂O₂), the atmospherically important compound hydroperoxymethyl thioformate (HPMTF, HOOCH₂SCHO), and 15 other products of competing secondary reactions. We characterized unknown transient chemical species with help from theoretical calculations of ionization energies and cation dissociation channels, demonstrating that HPMTF does not have a stable parent cation and fully fragments into the <i>m</i>/<i>z</i> = 46, 47, 75, and 80 ion channels even at ionization threshold. Similarly, RO₂ is detectable solely by the <i>m</i>/<i>z</i> = 61 fragment ion. Using a global time-resolved C and S atom balance tracking approach, we constrained the concentrations of HPMTF and RO₂ and determined their absolute photoionization cross sections up to 11.2 eV. The yield of HPMTF increased with higher O₂ concentrations and peaked at 70% at <i>T</i> = 475 K and at constant [O₂] = 1.5 × 10¹⁸ cm^-3. Furthermore, we determined the key rate-limiting reaction rate coefficient for HPMTF formation, <i>k</i>₃, at 430 - 510 K. Combining our values of <i>k</i>₃ with those of Assaf et al. (Assaf, E.; Finewax, Z.; Marshall, P.; Veres, P. R.; Neuman, J. A.; Burkholder, J. B., <i>J. Phys. Chem. A,</i> <b>2023,</b> <i>127,</i> 2336-2350) at 314-433 K results in an Arrhenius expression, <i>k</i>₃ = (2.44 ± 0.5) × exp (-(7248 ± 78)/T), which is in nearly perfect agreement with the results of Assaf et al. Our characterization of RO₂ and HPMTF enables their future quantitative probing in other experimental reactors by photoionization mass spectrometry.