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Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation.
J Phys Chem A 2019; 123(30):6596-6604JP

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···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···R'O) system, at least one low-energy conformer with an ISC rate above 108 s-1 can be found. This demonstrates that gas-phase dimer formation in the atmosphere very likely involves ISCs originating in relativistic quantum mechanics.

Authors+Show Affiliations

Department of Chemistry , University of Helsinki , P.O. Box 55, (A.I. Virtanens Plats 1) , Helsinki FIN-00014 , Finland. Tomsk State University , 36, Lenin Avenue , 634050 Tomsk , Russia.Department of Chemistry , University of Helsinki , P.O. Box 55, (A.I. Virtanens Plats 1) , Helsinki FIN-00014 , Finland. Institute for Atmospheric and Earth System Research , University of Helsinki , Helsinki 00014 , Finland.Department of Chemistry , University of Helsinki , P.O. Box 55, (A.I. Virtanens Plats 1) , Helsinki FIN-00014 , Finland. Institute for Atmospheric and Earth System Research , University of Helsinki , Helsinki 00014 , Finland.Institute for Atmospheric and Earth System Research , University of Helsinki , Helsinki 00014 , Finland.Department of Chemistry , University of Helsinki , P.O. Box 55, (A.I. Virtanens Plats 1) , Helsinki FIN-00014 , Finland. Institute for Atmospheric and Earth System Research , University of Helsinki , Helsinki 00014 , Finland.

Pub Type(s)

Journal Article

Language

eng

PubMed ID

31287685

Citation

Valiev, Rashid R., et al. "Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation." The Journal of Physical Chemistry. A, vol. 123, no. 30, 2019, pp. 6596-6604.
Valiev RR, Hasan G, Salo VT, et al. Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation. J Phys Chem A. 2019;123(30):6596-6604.
Valiev, R. R., Hasan, G., Salo, V. T., Kubečka, J., & Kurten, T. (2019). Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation. The Journal of Physical Chemistry. A, 123(30), pp. 6596-6604. doi:10.1021/acs.jpca.9b02559.
Valiev RR, et al. Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation. J Phys Chem A. 2019 Aug 1;123(30):6596-6604. PubMed PMID: 31287685.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation. AU - Valiev,Rashid R, AU - Hasan,Galib, AU - Salo,Vili-Taneli, AU - Kubečka,Jakub, AU - Kurten,Theo, Y1 - 2019/07/23/ PY - 2019/7/10/pubmed PY - 2019/7/10/medline PY - 2019/7/10/entrez SP - 6596 EP - 6604 JF - The journal of physical chemistry. A JO - J Phys Chem A VL - 123 IS - 30 N2 - 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···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···R'O) system, at least one low-energy conformer with an ISC rate above 108 s-1 can be found. This demonstrates that gas-phase dimer formation in the atmosphere very likely involves ISCs originating in relativistic quantum mechanics. SN - 1520-5215 UR - https://www.unboundmedicine.com/medline/citation/31287685/Intersystem_Crossings_Drive_Atmospheric_Gas-Phase_Dimer_Formation L2 - https://dx.doi.org/10.1021/acs.jpca.9b02559 DB - PRIME DP - Unbound Medicine ER -