Hydration of Atmospheric Molecular Clusters II: Organic Acid–Water Clusters

Using
computational methods, we study the gas phase hydration of
three different atmospherically relevant organic acids with up to
10 water molecules. We study a dicarboxylic acid (pinic acid) and
a tricarboxylic acid (3-methyl-1,2,3-butanetricarboxylic acid (mbtca))
that are both identified as products from α-pinene oxidation
reactions. We also study a 2-oxohexanediperoxy acid (ohdpa) that has
been identified as a product from cyclohexene autoxidation. To sample
the cluster structures, we employ our recently developed systematic
hydrate sampling technique and identify a total of 551 hydrate clusters.
The cluster structures and thermochemical parameters (at 298.15 K
and 1 atm) are obtained at the ωB97X-D/6-31++G­(d,p) level of
theory, and the single point energy of the clusters have been refined
using a high level DLPNO–CCSD­(T)/aug-cc-pVTZ calculation. We
find that all three tested organic acids interact significantly more
weakly with water compared to the primary nucleation precursor sulfuric
acid. Even at 100% relative humidity (298.15 K and 1 atm), we find
that ohdpa remains unhydrated and only the monohydrate of pinic acid
and mbtca are slightly populated (4% and 2%, respectively). From the
obtained molecular structures, potential implications for the ice
nucleating ability of aerosol particles is discussed.