Supplementary Information for α-pinene Autoxidation. Products May not Have Extremely Low Saturation Vapor

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1 Supplementary Information for α-pinene Autoxidation Products May not Have Extremely Low Saturation Vapor Pressures Despite High O:C Ratios by Theo Kurtén, Kirsi Tiusanen, Pontus Roldin, Matti Rissanen, Jan-Niclas Luy, Michael Boy, Mikael Ehn and Neil Donahue The chemical structures of the test compounds are shown in Figure S1, and their computed group contribution vapor pressures in Table S1. Figure S1. Structures of test compounds. a)fluoranthene, b)4-t.butylcatechol, c)pimelic acid, d)adipic acid, e)levoglucosan, f)peroxyformic acid, g)peroxyacetic acid. S1

2 Table S1. Group contribution vapor pressures for the test compounds, in bar, at K. (See main text for experimental and method references). molecule p sat, Nannoolal/ Nannoolal p sat, EVAPORATION p sat, SIMPOL Experimental Fluoranthene t.butylcatechol Pimelic acid Adipic acid Levoglucosan Peroxyformic acid Peroxyacetic acid Possible chemical mechanism leading to the studied 16 HOM compounds (see Figure 2 and Figure 3 in the main text) For an overview of the initial and well-established steps of α-pinene ozonolysis (implemented e.g. in the MCM model 1 ), see Figure 2 in Kurtén et al. 2 or Scheme 7 in Rissanen et al. 3 The starting point for the HOM-candidate structures (with a few exceptions detailed below) are the four first-generation peroxy radicals, with the elemental formula C 10 H 15 O 4. The first set of products in our study (denoted C 10 H 16 O 4 - iso1, C 10 H 16 O 4 -iso2 and C 10 H 16 O 4 -iso3) correspond to bimolecular termination products of these radicals from e.g. RO 2 + HO 2 ROOH + O 2 reactions (henceforth, ROOH termination ). We have omitted the fourth possible C 10 H 16 O 4 structure, which is identical to C 10 H 16 O 4 -iso3 except for the relative positions of the OOH and H substituents on the cyclobutyl ring, as this isomery is unlikely to have a significant effect on the vapor pressure. Most of the subsequent, more highly oxidized structures are obtained via alkoxy ring-opening from either of the two C 10 H 15 O 4 peroxy radicals with the OO S2

3 group attached to a carbon atom on the cyclobutyl ring (carbon number 1 in the numbering scheme of Kurtén et al. 2 ). Alkoxy ring opening here refers to the reaction where the peroxy radical first reacts with e.g. NO or another RO 2 to form an alkoxy radical, which promptly undergoes a ring-opening reaction to form an alkyl radical with a keto group (see Figure 11 in Kurtén et al. 2 ). This alkyl radical then rapidly undergoes O 2 addition to form a second-generation C 10 H 15 O 5 peroxy radical. The C 10 H 16 O 5 structure is simply the ROOH termination product of this peroxy radical. The C 10 H 16 O 6 -iso1 structure could be formed by another alkoxy formation (where C 10 H 15 O 5 reacts with e.g. NO or RO 2 to form a C 10 H 15 O 4 alkoxy radical), followed by an alkoxy H-shift from the aldehydic carbon (number 3 in the numbering scheme of Kurtén et al. 2 ), forming an OH group, O 2 addition to the ensuing QOOH alkyl radical, and ROOH termination. The C 10 H 16 O 6 -iso2 structure is formed by a similar reaction sequence but in a different order, with a H-shift (by the C 10 H 15 O 5 peroxyradical from the C3 aldehydic carbon atom) and O 2 addition occurring first, followed by alkoxy formation, alkoxy H-shift from the COOH group to form a carboxylic acid, and finally ROOH termination regenerating the COOH moiety. The C 10 H 16 O 7 -iso1 structure is formed similarly to C 10 H 16 O 6 -iso2 but without the second alkoxy step, resulting in a peroxy acid moiety instead of a carboxylic acid. The formation of C 10 H 16 O 7 -iso2 requires two H-shifts, O 2 additions and alkoxy-forming reactions (in addition to the ring-opening reaction and associated O 2 addition). Due to rapid scrambling of the alkoxy/peroxy radical groups by CO H OOC and COO...H OOC H-shifts, the order of these reactions can vary. One example pathway is alkoxy formation from the C 10 H 15 O 5 peroxyradical, followed by alkoxy H- shift from the C3 aldehydic carbon, followed by O 2 addition, another alkoxy formation, alkoxy H-shift from the tertiary carbon atom (C5 in the notation of Kurtén S3

4 et al. 2 ), O 2 addition and finally ROOH termination. C 10 H 16 O 8 -iso1 and C 10 H 16 O 8 -iso2 are formed analogously to C 10 H 16 O 7 -iso2, but with one less alkoxy-forming reaction, resulting in one more oxygen atom, as either of the OH groups in C 10 H 16 O 7 -iso2 is replaced by OOH. C 10 H 16 O 8 -iso3 is a more speculative species, which could be formed from the singlet diradical product of the high-energy Criegee intermediate ring opening reaction (see Figure 9 in Kurtén et al 2 ). To form C 10 H 16 O 8 -iso3, this diradical would need to undergo (not necessarily in this order) an O 2 addition to the alkyl radical on C6, a ring-closure reaction where the newly formed RO 2 group on C6 attacks the double bond (between C2 and C1) formed in the ring-opening reaction, a H-shift by the RO 2 group on C2 from the aldehydic C3, O 2 addition to C3, OH elimination from the C2 OOH group (producing two radicals from the diradical) another H-shift by the RO 2 group now on C3 from C1, another O 2 addition (to C1), and finally ROOH termination. While this scheme may seem somewhat implausible, we note that COOC ring closure reactions are currently the least improbable explanation for the surprisingly low number of acidic (OH or OOH) hydrogen atoms in the HOM molecules measured by H/D exchange experiments. C 10 H 16 O 8 -iso3 is thus intended to be a representative autoxidation product containing a COOC group, not necessarily a realistic exact structure. C 10 H 16 O 9, in contrast, is a straightforward (3 peroxy radical H-shifts, 3 O 2 additions, followed by ROOH termination) autoxidation product of the C 10 H 15 O 5 peroxyradical, as shown in Figure 12 of Kurtén et al. 2 C 10 H 16 O 10 is another ring-closure product, formed analogously to C 10 H 16 O 8 but with one more autoxidation (peroxyradical H-shift + O 2 addition) step. The dimer structures C 20 H 30 O 10 -iso1, C 20 H 30 O 10 -iso2 and C 20 H 30 O 12 are formed by combining C 10 peroxyradicals through RO 2 + RO 2 ROOR + O 2 reactions, as suggested by experiments (though so far unverified by direct kinetic measurements or S4

5 calculations). C 20 H 30 O 10 -iso1 could be formed from two C 10 H 15 O 6 peroxyradicals, both of which are formed from C 10 H 15 O 5 via an alkoxy-forming reaction, an alkoxy H-shift (from the aldehydic C3) and an O 2 addition. C 20 H 30 O 10 -iso2 could be formed from one C 10 H 15 O 5 and one C 10 H 15 O 7 peroxyradical (the latter forming from the former through a H-shift and O 2 addition). C 20 H 30 O 12 could be formed from the reaction of two C 10 H 15 O 7 peroxyradicals. In addition to this pdf, the Supporting Information contains a zip archive containing.cosmo and.energy files at both BP/TZVP and BP/TZVPD-FINE levels. These contain all the data on HOM candidate structures computed as part of the project, and can be used as input for COSMOTherm runs to obtain e.g. vapor pressures at different temperatures, or other desired properties such as solubilities or activity coefficients. 1 Saunders, S. M.; Jenkin, M. E.; Derwent, R. G.; Pilling, M. J. Protocol for the Development of the Master Chemical Mechanism, MCM v3 (Part A): Tropospheric Degradation of Non- Aromatic Volatile Organic Compounds. Atmos. Chem. Phys. 2003, 3, Kurtén, T.; Rissanen, M. P.; Mackeprang, K.; Thornton, J. A.; Hyttinen, N.; Jørgensen, S.; Ehn, M.; Kjaergaard, H. A Computational Study of Hydrogen Shifts and Ring-Opening Mechanisms in α-pinene Ozonolysis Products. J. Phys. Chem. A 2015, 119, Rissanen, M.; Kurtén, T.; Sipilä, M.; Thornton, J. A.; Kausiala, O.; Garmash, O.;. Kjaergaard, H. G.; Petäjä, T.; Worsnop, D. R.; Ehn, M.; Kulmala, M. Effects of Chemical Complexity on the Autoxidation Mechanisms of Endocyclic Alkene Ozonolysis Products: From Methylcyclohexenes toward Understanding α-pinene. J. Phys. Chem. A 2015, 119, S5