Is either direct photolysis or photocatalysed H-shift of peroxyl radicals a competitive pathway in the troposphere?

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http://hdl.handle.net/10138/320230

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Valiev , R R & Kurten , T 2020 , ' Is either direct photolysis or photocatalysed H-shift of peroxyl radicals a competitive pathway in the troposphere? ' , Royal Society Open Science , vol. 7 , no. 9 , 200521 . https://doi.org/10.1098/rsos.200521

Title: Is either direct photolysis or photocatalysed H-shift of peroxyl radicals a competitive pathway in the troposphere?
Author: Valiev, Rashid R.; Kurten, Theo
Contributor: University of Helsinki, Department of Chemistry
University of Helsinki, Department of Chemistry
Date: 2020-09-09
Language: eng
Number of pages: 10
Belongs to series: Royal Society Open Science
ISSN: 2054-5703
URI: http://hdl.handle.net/10138/320230
Abstract: Peroxyl radicals (ROO.) are key intermediates in atmospheric chemistry, with relatively long lifetimes compared to most other radical species. In this study, we use multireference quantum chemical methods to investigate whether photolysis can compete with well-established ROO. sink reactions. We assume that the photolysis channel is always ROO. + h nu => RO + O(P-3). Our results show that the maximal value of the cross-section for this channel is sigma = 1.3 x 10(-18) cm(2) at 240 nm for five atmospherically representative peroxyl radicals: CH3OO., C(O)HCH2OO., CH3CH2OO., HC(O)OO. and CH3C(O)OO.. These values agree with experiments to within a factor of 2. The rate constant of photolysis in the troposphere is around 10(-5) s(-1) for all five ROO.. As the lifetime of peroxyl radicals in the troposphere is typically less than 100 s, photolysis is thus not a competitive process. Furthermore, we investigate whether or not electronic excitation to the first excited state (D-1) by infrared radiation can facilitate various H-shift reactions, leading, for example, in the case of CH3OO. to formation of O.H and CH2O or HOO. and CH2 products. While the activation barriers for H-shifts in the D-1 state may be lower than in the ground state (D-0), we find that H-shifts are unlikely to be competitive with decay back to the D-0 state through internal conversion, as this has a rate of the order of 10(13) s(-1) for all studied systems.
Subject: photolysis
peroxide radicals
complete active space 2nd order perturbation theory (CASPT2)
ab initio
ABSORPTION-SPECTRUM
FLASH-PHOTOLYSIS
RATE CONSTANTS
UV PHOTOLYSIS
KINETICS
RATES
CH3O2
PHOTODISSOCIATION
IMPACTS
SOLAR
114 Physical sciences
116 Chemical sciences
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