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

The formation of low-volatility organic compounds in
the ozonolysis
of α-pinene, the dominant atmospheric monoterpene, provides
an important route to aerosol formation. In this work, we consider
a previously unexplored set of pathways for the formation of highly
oxygenated molecules in α-pinene ozonolysis. Pioneering, direct
experimental observations of Lester and co-workers have demonstrated
a significant production of hydroxycarbonyl products in the dissociation
of Criegee intermediates. Theoretical analyses indicate that this
production arises from OH roaming-induced pathways during the OO fission
of the vinylhydroperoxides (VHPs), which in turn come from internal
H transfers in the Criegee intermediates. Ab initio kinetics computations
are used here to explore the OH roaming-induced channels that arise
from the ozonolysis of α-pinene. For computational reasons,
the calculations consider a surrogate for α-pinene, where two
spectator methyl groups are replaced with H atoms. Multireference
electronic structure calculations are used to illustrate a variety
of energetically accessible OH roaming pathways for the four VHPs
arising from the ozonolysis of this α-pinene surrogate. Ab initio
transition-state theory-based master equation calculations indicate
that for the dissociation of stabilized VHPs, these OH roaming pathways
are kinetically significant with a branching that generally increases
from ∼20% at room temperature up to ∼70% at lower temperatures
representative of the troposphere. For one of the VHPs, this branching
already exceeds 60% at room temperature. For the overall ozonolysis
process, these branching ratios would be greatly reduced by a limited
branching to the stabilized VHP, although there would also be some
modest roaming fraction for the nonthermal VHP dissociation process.
The strong exothermicities of the roaming-induced isomerizations/additions
and abstractions suggest new routes to fission of the cyclobutane
rings. Such ring fissions would facilitate further autoxidation reactions,
thereby providing a new route for producing highly oxygenated nonvolatile
precursors to aerosol formation.