Prompt carbon structure rearrangements enable rapid highly oxygenated organic molecule formation from a-pinene Matti Rissanen 1,2,3 , Siddharth Iyer 1 , Rashid Valiev 3,4 , Shawon Barua 1 , Jordan E. Krechmer 5 , Joel Thornton 6 , Mikael Ehn 7 & Theo Kurtén 3 1 Aerosol Physics Laboratory, Tampere University, Finland, 2 Karsa Inc., Finland, 3 Department of Chemistry, University of Helsinki, Finland, 4 Tomsk State University, Russia, 5 Aerodyne Research, Inc., USA, 6 Department of Atmospheric Science, University of Washington Seattle, USA, 7 Institute for Atmospheric and Earth System Research (INAR/Physics), University of Helsinki, Finland The formation of highly oxygenated organic molecules (HOM) by rapid autoxidation of hydrocarbons and their relation to the formation and growth of atmospheric secondary organic aerosol (SOA) has been known now for a decade. From the pioneering studies it was already apparent how the general process can yield highly oxygenated material in very short time-scales, yet the instrumentation and the methodologies used were not able to record the details of these fast events - and neither could the theory be used to track the path by computations. Now, with the recently developed Multi-scheme chemical ionization inlet (MION) [1], and the ability to account for excess energy retained from exothermic reaction steps [2], we are able to study the fast production with evermore shorter reaction times, allowing to inspect the details of the rapid HOM formation processes [3]. HOM formation from ozonolysis initiated autoxidation of the most common monoterpene a-pinene was investigated under atmospheric conditions at variable short reaction times from 75 to 500 milliseconds. Several quartz flow tube reactors equipped with movable injectors and coupled to a chemical ionization atmospheric pressure interface chemical ionization mass spectrometer (CI-APi-ToF-MS), and the MION inlet, were applied for the work. The experimental work was heavily augmented by quantum chemical computations of the step-by-step reaction mechanism starting from the early ozonolysis intermediates, and subsequent master equation modelling to obtain the all-important reaction rates. Key for tracking the reaction path by computations was to account for the energy non-accommodation, which lead to the rearrangement of the carbon structure, and to the opening of the oxidation hindering cyclobutyl ring. The coupled application of the MION inlet and the master equation computations allowed to inspect the dynamics of the evolving radical product distribution at variable short reaction times. With increasing reaction time we were able to record an increasing amount of reaction products, together with the shift from radical dominated product distribution to closed-shell product domination, under these relatively high reactant concentration experiments. Most importantly, we were able to illustrate how the 8 oxygen atom containing peroxy radical ( i.e. , C 10 H 15 O 8 ) forms in less than 100 milliseconds timeframe, amply illustrating the high efficiency of the atmospheric autoxidation 1. M. P. Rissanen, J. Mikkilä, S. Iyer, and J. Hakala, Multi-scheme chemical ionization inlet (MION) for fast switching of reagent on chemistry in atmospheric pressure chemical ionization mass spectrometry (CIMS) applications, Atmos. Meas. Tech. (2019) 12, 6635-6646. 2. J. Shannon, S. H. Robertson, M. A. Blitz, and P. W.Seakins, Bimolecular reactions of activated species: An analysis of problematic HC(O)C(O) chemistry, Chem. Phys. Lett. (2016) 661, 58-64. 3. Iyer, M. P. Rissanen, R. Valiev, S. Barua, J. E. Krechmer, J. Thornton, M. Ehn, and T. Kurtén, Molecular mechanism for rapid autoxidation in α-pinene ozonolysis, Nat. Commun. (2021) 12, 1-6. process [3]. References
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