Time-resolved, broadband UV-absorption spectrometry measurements of Criegee intermediate kinetics using a new photolytic precursor : unimolecular decomposition of CH2OO and its reaction with formic acid

We present a time-resolved broadband cavity-enhanced UV-absorption spectrometer apparatus that we have constructed and utilized for temperature- and pressure-dependent kinetic measurements of formaldehyde oxide (CH2OO) reactions. We also introduce and utilize a new photolytic precursor, bromoiodomethane (CH2IBr), which photolysis at 213 nm in presence of O2 produces CH2OO. Importantly, this precursor appears to be free from secondary reactions that may regenerate CH2OO in kinetic experiments. The unimolecular decomposition rate coefficient of CH2OO has been measured over wide pressure (5–400 Torr) and temperature (296–600 K) ranges and master equation simulations of the decomposition kinetics have been performed using MESMER program. The MESMER simulations of the experimental data with the calculated zero-point energy corrected transition state energy 85.9 kJ mol−1 for decomposition required no adjustment and returned 〈ΔE〉down = 123.2 × (T/298 K)0.74 cm−1 for temperature-dependent exponential-down model of the collisional energy transfer in He. A very good agreement between results of simulations and experiments is obtained. The results are compared with the previously reported unimolecular decomposition study by Stone et al. (Phys. Chem. Chem. Phys., 2018, 20, 24940–24954). Current master equation simulations suggest about 61% decomposition yield for the predominant H2 + CO2 channel, whereas the yields of two other channels, H2O + CO, and HCO + OH, are sensitive on the parameters involved in the simulations. The kinetics of CH2OO reaction with formic acid has also been investigated as function of pressure (5–150 Torr) and temperature (296–458 K). The bimolecular rate coefficient for CH2OO + HCOOH reaction shows a negative temperature dependency, decreasing from (1.0 ± 0.03) × 10−10 cm3 molecule−1 s−1 at 296 K to (0.47 ± 0.05) × 10−10 cm3 molecule−1 s−1 at 458 K with an Arrhenius activation energy of −4.9 ± 1.6 kJ mol−1, where statistical uncertainties shown are 2σ. Estimated overall uncertainty in the measured rate coefficients is about ±20%. Current bimolecular rate coefficient at room temperature agrees with the previously reported rate coefficients from the direct kinetic experiments. The reaction is found to be pressure independent over the range between 5 and 150 Torr at 296 K in He.We present a time-resolved broadband cavity-enhanced UV-absorption spectrometer apparatus that we have constructed and utilized for temperature- and pressure-dependent kinetic measurements of formaldehyde oxide (CH2OO) reactions. We also introduce and utilize a new photolytic precursor, bromoiodomethane (CH2IBr), which photolysis at 213 nm in presence of O-2 produces CH2OO. Importantly, this precursor appears to be free from secondary reactions that may regenerate CH2OO in kinetic experiments. The unimolecular decomposition rate coefficient of CH2OO has been measured over wide pressure (5-400 Torr) and temperature (296-600 K) ranges and master equation simulations of the decomposition kinetics have been performed using MESMER program. The MESMER simulations of the experimental data with the calculated zero-point energy corrected transition state energy 85.9 kJ mol(-1) for decomposition required no adjustment and returned (down) = 123.2 x (T/298 K)(0.74) cm(-1) for temperature-dependent exponential-down model of the collisional energy transfer in He. A very good agreement between results of simulations and experiments is obtained. The results are compared with the previously reported unimolecular decomposition study by Stone et al. (Phys. Chem. Chem. Phys., 2018, 20, 24940-24954). Current master equation simulations suggest about 61% decomposition yield for the predominant H-2 + CO2 channel, whereas the yields of two other channels, H2O + CO, and HCO + OH, are sensitive on the parameters involved in the simulations. The kinetics of CH2OO reaction with formic acid has also been investigated as function of pressure (5-150 Torr) and temperature (296-458 K). The bimolecular rate coefficient for CH2OO + HCOOH reaction shows a negative temperature dependency, decreasing from (1.0 +/- 0.03) x 10(-10) cm(3) molecule(-1) s(-1) at 296 K to (0.47 +/- 0.05) x 10(-10) cm(3) molecule(-1) s(-1) at 458 K with an Arrhenius activation energy of -4.9 +/- 1.6 kJ mol(-1), where statistical uncertainties shown are 2 sigma. Estimated overall uncertainty in the measured rate coefficients is about +/- 20%. Current bimolecular rate coefficient at room temperature agrees with the previously reported rate coefficients from the direct kinetic experiments. The reaction is found to be pressure independent over the range between 5 and 150 Torr at 296 K in He.Peer reviewe

Huippututkimus ei ole välttämättä kapea-alaista

Terveyden ja hyvinvoinnin laitoksen (THL) tulevaisuutta sosiaali- ja terveysministeriön toimeksiannosta pohtinut selvitysmies asetti raportissaan mielenkiintoisella tavalla vastinpareiksi tutkimuksen tieteelliset ansiot ja tutkimuksen kautta saavutetun asiantuntemuksen hyödynnettävyyden. Raportin mukaan valtion tutkimuslaitoksena THL:n tulee tavoitella pikemminkin laaja-alaista asiantuntemusta kuin pyrkiä kurottautumaan tutkimuksen kansainväliseen kärkeen, sillä se johtaa päätöksenteon tukemisen kannalta liian kapea-alaiseen asiantuntemukseen

Solving the discrepancy between the direct and relative-rate determinations of unimolecular reaction kinetics of dimethyl- substituted Criegee intermediate (CH3)2COO using a new photolytic precursor

We have performed direct kinetic measurements of thermal unimolecular reaction of (CH3)2COO in the temperature 243– 340 K and pressure 5–350 Torr ranges using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, 2-bromo-2-iodopropane ((CH3)2CIBr), which photolysis at 213 nm in presence of O2 produces acetone oxide, (CH3)2COO. The results show that the thermal unimolecular reaction is more important main loss process of (CH3)2COO in the atmosphere than direct kinetic studies hitherto suggest. The current experiments show that the unimolecular reaction rate of (CH3)2COO at 296 K and atmospheric pressure is 899 ± 42 s-1. Probably more importantly, current measurements bring the direct and relative rate measurements of thermal unimolecular reaction kinetics of (CH3)2COO in quantitative agreement.Peer reviewe