Estimating the saturation vapor pressures of isoprene oxidation products C<sub>5</sub>H<sub>12</sub>O<sub>6</sub> and C<sub>5</sub>H<sub>10</sub>O<sub>6</sub> using COSMO-RS

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 C 5 H 12 O 6 , and the dihydroperoxy hydroxy aldehyde, C 5 H 10 O 6 . 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 C 5 H 12 O 6 , predicted differences in p sat 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 C 5 H 10 O 6 , 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.

Thermodynamic properties of isoprene and monoterpene derived organosulfates estimated with COSMO<i>therm</i>

“…Compared to previously calculated saturation vapor pressures for α-pinene autoxidation products using COSMOtherm, the organosulfates studied here are significantly less volatile (Kurtén et al, 2016). It should be noted, however, that in the study of Kurtén et al (2016), the number of intramolecular H-bonds was not limited in the COSMOtherm calculations, which likely led to higher saturation vapor pressure estimates (Kurtén et al, 2018). Furthermore, as we have seen here, the acidic organosulfates readily dissociate in the particle phase, forming ionic species, which will effectively suppress their partitioning to the gas phase.…”

“…Calculated saturation vapor pressures are lower for organosulfates than isoprene derived dihydroxy dihydroperoxides and dihydroperoxy hydroxy aldehydes (Kurtén et al, 2018) and α-pinene derived oxidized compounds (Kurtén et al, 2016). Based on this, organosulfates are more stable in the condensed phase than non-sulfate organic compounds.…”

“…However, none of these methods allow for the systematic conformer sampling of the molecules. The non-systematic conformer generation in COSMOconf has been shown to lead to significantly different results in COSMOtherm depending on the initial geometry with molecules containing hydroxy and hydroperoxy functional groups (Kurtén et al, 2018). Based on the recommendation by Kurtén et al (2018), we therefore used the systematic conformer sampling with the MMFF force fields in the Spartan '14 program (Wavefunction Inc.,…”

“…Based on this, the maximum number of conformers was set to 40 for larger monoterpene derived organosulfates and 50 for the smaller isoprene derived molecules. Kurtén et al (2018) found that COSMOtherm overestimates the effect of intramolecular hydrogen bonds and recommended that only conformers containing no intramolecular hydrogen bonds should be used in saturation vapor pressure calculations.…”

Chamber-based insights into the factors controlling IEPOX SOA yield, composition, and volatility

“…Assuming no particle-phase diffusion limitations but accounting for FIGAERO mass transfer limitations (Schobesberger et al, 2018), we predict a c* of 5-15 µg m -3 for the portion of the C5H12O4 thermogram that evaporates. Utilizing COSMOtherm (2018) with the BP_TZVPD_FINE_18 parameterization as described previously (Kurtén et al, 2018), a Henry's law constant of 4.910 8 -1.110 10 M atm -1 is predicted if all conformers are used (4.910 8 M atm -1 ) or if the number of internal H-bonds is minimized…”