Computational Study of the Clustering of a Cyclohexene Autoxidation Product C₆H₈O₇ with Itself and Sulfuric Acid

Abstract

We investigate the molecular interactions between sulfuric acid and a recently reported C6H8O7 ketodiperoxy acid formed through autoxidation from cyclohexene and ozone. Structurally similar but larger ELVOC (extremely low volatility organic compound) products formed from autoxidation of monoterpenes are believed to play a major role in the formation and early growth of atmospheric aerosol particles. Utilizing density functional theory geometries, with a DLPNO-CCSD(T)/def2-QZVPP single point energy correction, the stepwise Gibbs free energies of formation have been calculated, and the stabilities of the molecular clusters have been evaluated. C6H8O7 interacts weakly with both itself and sulfuric acid, with standard free energies of formation (ΔG at 298 K and 1 atm) around or above 0 kcal/mol. This is due to the presence of strong intramolecular hydrogen bonds in the peroxyacid groups of C6H8O7. These stabilize the isolated molecule with respect to its clusters, and lead to unfavorable interaction energies. The addition of sulfuric acid to clusters containing C6H8O7 is somewhat more favorable, but the formed clusters are still far more likely to evaporate than to grow further in atmospheric conditions. These findings indicate that the O/C ratio cannot exclusively be used as a proxy for volatility in atmospheric new particle formation involving organic compounds. The specific molecular structure, and especially the number of strong hydrogen binding moieties, are equally important. The interaction between the C6H8O7 compound and aqueous phase sulfate ions indicates that ELVOC-type compounds can contribute to aerosol mass by effectively partitioning into the condensed phase.

What Is Required for Highly Oxidized Molecules To Form Clusters with Sulfuric Acid?

“…from the -OOH group to the O=C). This internal hydrogen bond needs to be broken to form a new hydrogen bond with sulfuric acid 19 and causes the interaction of the peroxy acid to be significantly weaker (by 2.1 kcal/mol) than the individual contributions from an peroxide (-1.8 kcal/mol) and a carbonyl group (-1.7…”

“…Using the CCSD(T)-F12a/VDZ-F12 binding energy calculations of the organic acids ( Figure 1) as a benchmark set we assess the performance DLPNO-CCSD(T) using the cc-pVXZ (X= D,T,Q), aug-cc-pVXZ (X= D,T) and Def2-QZVPP basis sets as shown in Table S5 in the Supporting Information. The Def2-QZVPP basis set, which we have previously used in connection with the DLPNO method for studying the interaction between sulfuric acid and highly oxidized organic compounds, 19,20,71 We calculate the reaction free energy of sulfuric acid clustering with the common monoterpene markers: pinic acid, 72-76 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), 77,78 terebic acid, 79-81 3-hydroxy glutaric acid 79,82 and terpenylic acid. [79][80][81]83 The pinic acid and MBTCA cluster structures have been taken from previous studies.…”

“…18 For instance recent computational studies have shown that a C 6 H 8 O 7 autoxidation product from cyclohexene ozonolysis binds very weakly to sulfuric acid. 19,20 The identification of the participating species is complicated by the fact that directly measuring the initial steps in sulfuric acid cluster formation is very difficult. A promising technique is the combination of Fourier transform infrared (FT-IR) spectroscopy experiments and quantum chemical calculations.…”

Effect of Bisulfate, Ammonia, and Ammonium on the Clustering of Organic Acids and Sulfuric Acid

“…[4][5][6][7][8][9] It has recently been suggested that low-volatile organic compounds participate in the first steps of new-particle formation, 10,11 but molecular-level explanation and details concerning the involvement of oxidized organic compounds are still missing. [12][13][14] Specifically, recent laboratory studies have indicated that ions can play a major role in organics-driven particle formation by enhancing the initial molecular cluster formation. 15 Volatile organic compounds (VOCs) are emitted into atmosphere from both anthropogenic and natural sources.…”

“…) yields binding energies with lower stability than canonical coupled cluster methods, and therefore it can be used as a lower bound for the "true" cluster binding energies 12. As…”