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Intersystem Crossings Drive Atmospheric Gas-Phase Dimer Formation
Rashid R Valiev1,2, Galib Hasan1,3, Vili-Taneli Salo1,3
1Department of Chemistry , University of Helsinki , P.O. Box 55, (A.I. Virtanens Plats 1) , Helsinki FIN-00014 , Finland.
The Journal of Physical Chemistry. A
|July 10, 2019
Summary
High molecular weight dimers form ultrafine aerosol particles. This study reveals a quantum mechanical mechanism involving triplet alkoxy radicals and intersystem crossings, explaining dimer formation in the atmosphere.
Area of Science:
- Atmospheric Chemistry
- Quantum Mechanics
- Chemical Kinetics
Background:
- High molecular weight dimers (ROOR") are implicated in forming ultrafine atmospheric aerosol particles.
- The precise reaction mechanism for their gas-phase formation remains unclear.
Purpose of the Study:
- To elucidate the molecular-level reaction mechanism for ROOR" dimer formation.
- To investigate the role of triplet alkoxy radical pairs and intersystem crossing (ISC) in this process.
Main Methods:
- Employed multireference quantum chemical methods.
- Utilized a CH3OO + CH3OO model system.
- Computed ISC rates for various atmospherically relevant triplet (RO···R'O) complexes.
Main Results:
- Identified a pathway involving triplet alkoxy radical pairs and rapid intersystem crossings to singlet states for ROOR" formation.
- Found transition states for intermediate tetroxide formation and decomposition to be energetically favorable.
- ISC rates are conformation-dependent and stereoselective, with rates exceeding 10^8 s^-1 for some low-energy conformers.
Conclusions:
- Gas-phase dimer formation in the atmosphere likely involves intersystem crossings rooted in relativistic quantum mechanics.
- The proposed mechanism provides a molecular-level understanding of a key process in aerosol formation.
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