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. We have used COSMO-RS (the conductor-like screening model for real solvents),as implemented in the COSMOtherm program, to compute the saturation vaporpressures at 298 K of two photo-oxidation products of isoprene: the dihydroxydihydroperoxide C5H12O6 , and the dihydroperoxy hydroxyaldehyde, C5H10O6 . The predicted saturation vapor pressureswere significantly higher (by up to a factor of 1000) than recentexperimental results, very likely due to the overestimation of the effects ofintramolecular hydrogen bonds, which tend to increase saturation vaporpressures by stabilizing molecules in the gas phase relative to the liquid.Modifying the hydrogen bond enthalpy parameter used by COSMOtherm can improvethe agreement with experimental results – however the optimal parametervalue is likely to be system-specific. Alternatively, vapor pressurepredictions can be substantially improved (to within a factor of 5 of theexperimental values for the two systems studied here) by selecting onlyconformers with a minimum number of intramolecular hydrogen bonds. Thecomputed saturation vapor pressures were very sensitive to the details of theconformational sampling approach, with the default scheme implemented in theCOSMOconf program proving insufficient for the task, for example bypredicting significant differences between enantiomers, which should haveidentical physical properties. Even after exhaustive conformational sampling,COSMOtherm predicts significant differences in saturation vapor pressuresbetween both structural isomers and diastereomers. For C5H12O6 , predicted differences in psat between structuralisomers are up to 2 orders of magnitude, and differences betweenstereoisomers are up to a factor of 20 – though these differences are very likelyexaggerated by the overestimation of the effect of intramolecular H-bonds.For C5H10O6 , the maximum predicted differences between thethree studied structural isomers and their diastereomer pairs are around afactor of 8 and a factor of 2, respectively, when only conformers lackingintramolecular hydrogen bonds are included in the calculations. In futurestudies of saturation vapor pressures of polyfunctional atmospheric oxidationproducts using COSMOtherm, we recommend first performing thoroughconformational sampling and subsequently selecting conformers with a minimalnumber of intramolecular H-bonds.

atmospheric chemistry and physics

Estimating the saturation vapor pressures of isoprene oxidation products C&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt;H&amp;lt;sub&amp;gt;12&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;6&amp;lt;/sub&amp;gt; and C&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt;H&amp;lt;sub&amp;gt;10&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;6&amp;lt;/sub&amp;gt; using COSMO-RS