Selective deuteration as a tool for resolving autoxidation mechanisms in α-pinene ozonolysis

Abstract

Abstract. Highly oxygenated organic molecules (HOMs) from α-pinene ozonolysis have been shown to be significant contributors to secondary organic aerosol (SOA), yet our mechanistic understanding of how the peroxy-radical-driven autoxidation leads to their formation in this system is still limited. The involved isomerisation reactions such as H-atom abstractions followed by O2 additions can take place on sub-second timescales in short-lived intermediates, making the process challenging to study. Similarly, while the end-products and sometimes radical intermediates can be observed using mass spectrometry, their structures remain elusive. Therefore, we propose a method utilising selective deuterations for unveiling the mechanisms of autoxidation, where the HOM products can be used to infer which C atoms have taken part in the isomerisation reactions. This relies on the fact that if a C−D bond is broken due to an abstraction by a peroxy group forming a −OOD hydroperoxide, the D atom will become labile and able to be exchanged with a hydrogen atom in water vapour (H2O), effectively leading to loss of the D atom from the molecule. In this study, we test the applicability of this method using three differently deuterated versions of α-pinene with the newly developed chemical ionisation Orbitrap (CI-Orbitrap) mass spectrometer to inspect the oxidation products. The high mass-resolving power of the Orbitrap is critical, as it allows the unambiguous separation of molecules with a D atom (mD=2.0141) from those with two H atoms (mH2=2.0157). We found that the method worked well, and we could deduce that two of the three tested compounds had lost D atoms during oxidation, suggesting that those deuterated positions were actively involved in the autoxidation process. Surprisingly, the deuterations were not observed to decrease HOM molar yields, as would have been expected due to kinetic isotope effects. This may be an indication that the relevant H (or D) abstractions were fast enough that no competing pathways were of relevance despite slower abstraction rates of the D atom. We show that selective deuteration can be a very useful method for studying autoxidation on a molecular level and likely is not limited to the system of α-pinene ozonolysis tested here.

Lifetimes of pre-reactive complexes of peroxy radicals revisited: thermostat effects, temperature dependence and highly oxygenated molecules

“…Reactions of volatile organic compounds (VOCs) with abundant atmospheric oxidizers (OH, O 3 ) can initiate the formation of peroxy radicals (RO 2 ). 1–4 These radicals are comparatively long-lived, usually undergoing further reactions with NO or HO 2 , or, especially in less polluted regions, undergoing self- or cross-reactions with another peroxy radical. 5 These latter reactions have attracted a great deal of recent research interest, since they can form larger accretion products such as ROOR or R(O)OR species which then can act as seeds for new particle formation leading to the growth of aerosols.…”

OH Roaming during the Ozonolysis of α-Pinene: A New Route to Highly Oxygenated Molecules?

“…The ozonolysis of α-pinene is of considerable interest due its very large molar yields (∼7%) of highly oxygenated molecules (HOMs) and the connection from HOM to atmospheric aerosols . There is a strong desire to understand the molecular mechanism for this HOM formation, and the system has been studied very extensively experimentally (see, e.g., refs – ). Nevertheless, some of the details remain elusive .…”