Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation

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http://hdl.handle.net/10138/317518

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Valiev , R , Hasan , G , Salo , V-T , Kubecka , J & Kurten , T 2019 , ' Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation ' , Journal of Physical Chemistry A , vol. 123 , no. 30 , pp. 6596-6604 . https://doi.org/10.1021/acs.jpca.9b02559

Title: Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation
Author: Valiev, Rashid; Hasan, Galib; Salo, Vili-Taneli; Kubecka, Jakub; Kurten, Theo
Contributor: University of Helsinki, Department of Chemistry Subunit
University of Helsinki, Department of Chemistry Subunit
University of Helsinki, Department of Chemistry Subunit
University of Helsinki, INAR Physics
University of Helsinki, Department of Chemistry
Date: 2019-08-01
Number of pages: 9
Belongs to series: Journal of Physical Chemistry A
ISSN: 1089-5639
URI: http://hdl.handle.net/10138/317518
Abstract: High molecular weight "ROOR" dimers, likely formed in the gas phase through self- and cross-reactions of complex peroxy radicals (RO2), have been suggested to play a key role in forming ultrafine aerosol particles in the atmosphere. However, the molecular-level reaction mechanism producing these dimers remains unknown. Using multireference quantum chemical methods, we explore one potentially competitive pathway for ROOR' production, involving the initial formation of triplet alkoxy radical (RO) pairs, followed by extremely rapid intersystem crossings (ISC) to the singlet surface, permitting subsequent recombination to ROOR'. Using CH3OO + CH3OO as a model system, we show that the initial steps of this reaction mechanism are likely to be very fast, as the transition states for both the formation and the decomposition of the CH3O4CH3 tetroxide intermediate are far below the reactants in energy. Next, we compute ISC rates for seven different atmospherically relevant (3)(RO center dot center dot center dot R'O) complexes. The ISC rates vary significantly depending on the conformation of the complex and also exhibit strong stereoselectivity. Furthermore, the fastest ISC process is usually not between the lowest-energy triplet and singlet states but between the triplet ground state and an exited singlet state. For each studied (RO center dot center dot center dot R'O) system, at least one low-energy conformer with an ISC rate above 10(8) s(-1) can be found. This demonstrates that gas-phase dimer formation in the atmosphere very likely involves ISCs originating in relativistic quantum mechanics.
Subject: CHEMISTRY
GENERAL FORCE-FIELD
HYDROCARBONS
KINETICS
MECHANISM
SELF-REACTIONS
SETS
SYSTEMS
116 Chemical sciences
114 Physical sciences
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