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.

Jari Peltola,Arkke Eskola,Prasenjit Seal,Niko Vuorio,Petri Heinonen

doi:10.1039/d1cp02270a

Jari Peltola, Arkke Eskola + Show 3 more

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https://doi.org/10.1039/d1cp02270a

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Abstract

We have performed direct kinetic measurements of the thermal unimolecular reaction of (CH3)2COO in the temperature range 243-340 K and pressure range 5-350 Torr 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 the presence of O2 produces acetone oxide, (CH3)2COO. The results show that the thermal unimolecular reaction is even more important main loss process of (CH3)2COO in the atmosphere than direct kinetic studies have suggested hitherto. 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 into quantitative agreement.

Highlights

Gas phase ozonolysis is one of the major degradation pathways of biogenic and anthropogenic alkenes in the atmosphere
O3 reacts with a double bond of an alkene forming a highly excited primary ozonide, which subsequently decomposes to an aldehyde and a Criegee intermediate
The geometry optimization and vibrational frequency calculations of the stationary points on the potential energy surface (PES) were performed using Truhlar’s Minnesota functional, MN1523 and def2-TZVP basis set as implemented in Gaussian 16.24 The energies of the stationary points were refined with coupled cluster method, CCSD(T) as employed in ORCA code[25] and extrapolated to the complete basis set limit (CBS) using Dunning’s correlation consistent basis sets, i.e., cc-pVXZ (X=T and Q) following the scheme proposed by Neese and Valeev.[26]

Summary

Introduction

Gas phase ozonolysis is one of the major degradation pathways of biogenic and anthropogenic alkenes in the atmosphere. O3 reacts with a double bond of an alkene forming a highly excited primary ozonide, which subsequently decomposes to an aldehyde and a Criegee intermediate. The reactions of sCIs are sources of hydroxyl radicals (OH), organic acids, hydroperoxides, and aerosols in the troposphere.[2,3,4,5,6] In particular, the competition between the unimolecular and bimolecular reactions (mainly with water and SO2) of sCIs play an important role since the oxidation of SO2 by sCIs can be a significant source of sulfuric acid in alkene rich environments.[7]

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