Computational Investigation of the Formation of Peroxide (ROOR) Accretion Products in the OH- and NO3-Initiated Oxidation of α-Pinene.

Galib Hasan,Theo Kurtén,Vili-Taneli Salo,Rashid R. Valiev

doi:10.1021/acs.jpca.1c08969

Abstract

The formation of accretion products (“dimers”) from recombination reactions of peroxyl radicals (RO2) is a key step in the gas-phase generation of low-volatility vapors, leading to atmospheric aerosol particles. We have recently demonstrated that this recombination reaction very likely proceeds via an intermediate complex of two alkoxy radicals (RO···OR′) and that the accretion product pathway involves an intersystem crossing (ISC) of this complex from the triplet to the singlet surface. However, ISC rates have hitherto not been computed for large and chemically complex RO···OR′ systems actually relevant to atmospheric aerosol formation. Here, we carry out systematic conformational sampling and ISC rate calculations on 3(RO···OR′) clusters formed in the recombination reactions of different diastereomers of the first-generation peroxyl radicals originating in both OH- and NO3-initiated reactions of α-pinene, a key biogenic hydrocarbon for atmospheric aerosol formation. While we find large differences between the ISC rates of different diastereomer pairs, all systems have ISC rates of at least 106 s–1, and many have rates exceeding 1010 s–1. Especially the latter value demonstrates that accretion product formation via the suggested pathway is a competitive process also for α-pinene-derived RO2 and likely explains the experimentally observed gas-phase formation of C20 compounds in α-pinene oxidation.

Highlights

Organic peroxyl radicals (RO2) are important molecules in the atmosphere because their reactions play a significant role in the formation of low-volatility products, leading to secondary organic aerosol (SOA) particles
Gas-phase accretion product formation provides an efficient mechanism for atmospheric aerosol formation, as it dramatically lowers the volatility of the participating compounds in a single step
Computational studies by us and others suggest that ROOR′ formation is preceded by intersystem crossing (ISC) in weakly bound 3(RO···OR′) complexes formed as an intermediate step in all peroxyl radical self- and cross-reactions

Summary

■ INTRODUCTION

Organic peroxyl radicals (RO2) are important molecules in the atmosphere because their reactions play a significant role in the formation of low-volatility products, leading to secondary organic aerosol (SOA) particles. Monoterpenes, with elemental composition C10H16, are biogenic hydrocarbons believed to be important especially for the first steps of SOA formation The reason for this is that they are large and complex enough to form oxidation products with very low volatilities, while still having high enough emission rates and atmospheric concentrations. Because of the large size of the α-pinene systems, computational costs prevented us from carrying out calculations at the same level as in our previous studies (coupled cluster singles, doubles and perturbative triples [CCSD(T)] energies on ωB97X-D/aug-cc-pVTZ structures). The matrix element of the spin-orbit coupling interaction between triplet T1-T4 and singlet states S1-S4 was calculated at the CASSCF(6,4)/ 6-311++G** level of theory, but using the XMC-QDPT2/6-311++G** energies, with the general atomic and molecular electronic structure system (GAMESS-US) program.[41]

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES