<p><strong>Abstract.</strong> 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 C<sub>5</sub>H<sub>12</sub>O<sub>6</sub>, and the dihydroperoxy hydroxy aldehyde, C<sub>5</sub>H<sub>10</sub>O<sub>6</sub>.

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... 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 10 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 C<sub>5</sub>H<sub>12</sub>O<sub>6</sub>, predicted differences in psat between structural isomers are up to two orders of magnitude, and differences between stereoisomers up to a factor of 20 – though these differences are very likely exaggerated by the overestimation of the effect of intramolecular H-bonds. For C<sub>5</sub>H<sub>10</sub>O<sub>6</sub>, predicted differences between (stereo)isomers are below a factor of 3. 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.</p>