Abstract
Abstract. We have used COSMO-RS (the conductor-like screening model for real solvents), as implemented in the COSMOtherm program, to compute the saturation vapor pressures at 298 K of two photo-oxidation products of isoprene: the dihydroxy dihydroperoxide C5H12O6, and the dihydroperoxy hydroxy aldehyde, C5H10O6. The predicted saturation vapor pressures were significantly higher (by up to a factor of 1000) than recent experimental results, very likely due to the overestimation of the effects of intramolecular hydrogen bonds, which tend to increase saturation vapor pressures by stabilizing molecules in the gas phase relative to the liquid. Modifying the hydrogen bond enthalpy parameter used by COSMOtherm can improve the agreement with experimental results – however the optimal parameter value is likely to be system-specific. Alternatively, vapor pressure predictions can be substantially improved (to within a factor of 5 of the experimental values for the two systems studied here) by selecting only conformers with a minimum number of intramolecular hydrogen bonds. The computed saturation vapor pressures were very sensitive to the details of the conformational sampling approach, with the default scheme implemented in the COSMOconf program proving insufficient for the task, for example by predicting significant differences between enantiomers, which should have identical physical properties. Even after exhaustive conformational sampling, COSMOtherm predicts significant differences in saturation vapor pressures between both structural isomers and diastereomers. For C5H12O6, predicted differences in psat between structural isomers are up to 2 orders of magnitude, and differences between stereoisomers are up to a factor of 20 – though these differences are very likely exaggerated by the overestimation of the effect of intramolecular H-bonds. For C5H10O6, the maximum predicted differences between the three studied structural isomers and their diastereomer pairs are around a factor of 8 and a factor of 2, respectively, when only conformers lacking intramolecular hydrogen bonds are included in the calculations. In future studies of saturation vapor pressures of polyfunctional atmospheric oxidation products using COSMOtherm, we recommend first performing thorough conformational sampling and subsequently selecting conformers with a minimal number of intramolecular H-bonds.
Highlights
Atmospheric aerosol particles play a key role in regulating the Earth’s climate, and are responsible for most air pollution-related mortality (Pachauri and Meyer, 2014; Brauer et al, 2016)
The computed saturation vapor pressures were very sensitive to the details of the conformational sampling approach, with the default scheme implemented in the COSMOconf program proving insufficient for the task, for example by predicting significant differences between enantiomers, which should have identical physical properties
While COSMOtherm saturation vapor pressure predictions can be modified to agree better with experiments by scaling the hydrogen bond enthalpy parameter c1, the optimal value for this parameter is likely to depend on the system, and this approach can not generally be recommended
Summary
Atmospheric aerosol particles play a key role in regulating the Earth’s climate, and are responsible for most air pollution-related mortality (Pachauri and Meyer, 2014; Brauer et al, 2016). A large fraction of these particles consists of secondary organic aerosol (SOA) material formed by oxidation reactions (Jimenez et al, 2009). Until recently, both the amount and the condensability of SOA were severely underestimated by atmospheric chemistry models, leading to large gaps in the understanding of atmospheric aerosols (Pierce et al, 2011; Zhang et al, 2014). Unlike the sequential oxidation reactions which dominate the chemistry of simple “textbook” species with few functional groups, autoxidation only needs a single initial hydrocarbon–oxidant reaction; the subsequent cascade of RO2 hydrogen shift (H-shift) and O2 addition reactions can add up to 10 oxygen atoms to a hydrocarbon backbone without needing any additional low-concentration oxidants such as OH, O3, or NO3 (Ehn et al, 2014)
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