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The Effect of Water and Bases on the Clustering of a Cyclohexene Autoxidation Product C6H8O7 with Sulfuric Acid.

We investigate the molecular interaction between sulfuric acid and a C6H8O7 ketodiperoxy acid compound (a proxy for highly oxidized products of, e.g., monoterpene autoxidation) in the presence of water, ammonia, or dimethylamine. The molecular geometries are obtained using density functional theory (M06-2X, PW91, and ωB97X-D) with the 6-31++G(d,p) basis set, and the binding energy is corrected utilizing a high-level DLPNO-CCSD(T)/def2-QZVPP calculation. The formation free energies were calculated (ΔG at 298 K and 1 atm), and the stability of the molecular clusters was evaluated. The presence of bases is found to enhance the interaction between ketodiperoxy acid compounds and sulfuric acid. The addition of C6H8O7 compounds to (H2SO4)(NH3) or (H2SO4)((CH3)2NH) clusters is, however, not able to compete with the corresponding uptake of another sulfuric acid molecule, even at a high loading of organic compounds. We furthermore investigate the origin of the weak binding of peroxyacid compounds using atoms in molecules and natural bonding orbital analysis. The weak binding is caused by an internal hydrogen bond and the lack of a strong hydrogen bond acceptor in the peroxyacid group. These findings indicate that autoxidation products containing solely or mainly hydroperoxide and carbonyl functional groups do not participate in the initial step of new particle formation and thereby only contribute to the growth of atmospheric particles.