Rapid formation of HOMs from gas-phase alpha-pinene ozonolysis

Alpha-pinene is the largest globally emitted monoterpene. Its oxidation reaction with ozone leads to peroxy radicals (RO₂) that can subsequently form highly oxygenated organic molecules (HOMs) through the process of autoxidation. HOMs are considered to play a critical role in the growth of early particles as they can have sufficiently low saturation vapor pressures.

Pseudo-unimolecular autoxidation reaction is generally thought to compete with bimolecular reactions of RO₂in the atmosphere. While these bimolecular reactions could potentially lead to radical recycling, [1] they are generally thought to lead to the formation of non-reactive products. In order to compete with these bimolecular reactions, the unimolecular autoxidation reaction must be rapid, especially in high RO₂/NO conditions.

The initial ozonolysis reaction of a-pinene leads to the first-generation RO₂with the 6-member ring broken. Current knowledge dictates the perpetuation of the inner 4-member cylobutyl ring in the first-generation RO₂. This ring has proven to be a hurdle for rapid unimolecular autoxidation reactions as the steric hindrance the ring affords leads to high barriers (and therefore slow reaction rates) for hydrogen-shift (H-shift) reactions central to autoxidation. [2]

In this work, we show that the ozonolysis of a-pinene could directly lead to the formation of a hitherto unexplored completely ring-opened RO₂ product. This pathway is made feasible by considering the large amount of excess energy channeled into the rovibrational modes of the vinoxy product after ozonolysis. This leads to the opening of the cyclobutyl ring of a significant fraction of the “hot” vinoxy radicals under atmospheric conditions, as opposed to all of them adding an O₂molecule as was previously thought. The breaking of the ring potentially leads to the formation of products with up to 8 oxygen atoms after a single hydrogen shift reaction following the formation of the vinoxy.

 

[1] Iyer, S.; Reiman, H.; Møller, K. H.; Rissanen, M. P.; Kjaergaard, H. G.; Kurtén, T. Computational Investigation of RO₂+ HO₂and RO₂+ RO₂Reactions of Monoterpene Derived First-Generation Peroxy Radicals Leading to Radical Recycling. J. Phys. Chem. A2018, 49, 9542-9552.

[2] Kurtén, T.; Rissanen, M. P. Rissanen, Mackeprang, K.; Thornton, J. A.; Jørgensen, S.; Ehn, M.; Kjaergaard, H. G. Computational Study of Hydrogen Shifts and Ring-Opening Mechanisms in a-Pinene Ozonolysis Products. J. Phys. Chem. A2015, 119, 11366-11375.