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

Abstract

Using computational methods we study the gas phase hydration of three different atmospherically relevant organic acids with up to 10 water molecules. We study the 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 an 2-oxohexanediperoxy acid (ohdpa) that have 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)/augcc-pVTZ calculation. We find that all three tested organic acids interact significantly weaker 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 ice nucleating ability of aerosol particles is discussed. 2

Hygroscopic behavior of aerosols generated from solutions of 3-methyl-1,2,3-butanetricarboxylic acid, its sodium salts, and its mixtures with NaCl

“…This is the first study reporting the hygroscopic properties of MBTCA. A previous study showed that MBTCA was not hydrated significantly in the ambient atmosphere (Kildgaard et al, 2018), implying that the MBTCA solids stay in the air once they effloresced, based on our results.…”

Systematic microsolvation approach with a cluster‐continuum scheme and conformational sampling

“…Regarding the first requirement, we apply efficient generalized-gradient-approximation density functional theory with density fitting for the optimization of solvent-solute clusters, but note that semi-empirical quantum chemical methods [60] can very much reduce the increasing computational cost caused by considering solvent molecules explicitly. [39,59] As these methods struggle to properly describe important interactions in solution such as dispersion and hydrogen-bonding, corrections have been developed to alleviate such shortcomings. [61][62][63] Hence, their application for the conformational sampling of large clusters will be beneficial.…”

Technical note: Estimating aqueous solubilities and activity coefficients of mono- and <i>α</i>,<i>ω</i>-dicarboxylic acids using COSMO<i>therm</i>

“…In carboxylic acid dimers, the hydrogen bond donors and acceptors are saturated, which means that carboxylic acids are unlikely to form larger clusters than dimers (Vawdrey et al, 2004;Elm et al, 2014Elm et al, , 2019. Computational studies (Aloisio et al, 2002;Weber et al, 2012;Kildgaard et al, 2018) have shown that, in the gas phase, the energetically most favorable dihydrate is formed by two water molecules attaching to the same carboxylic acid group. Therefore, adding a second water molecule to the cluster does not significantly change the probability distribution of the screening charge density (σ profile) of the acid in the cluster compared to the acid in a monohydrate or dimer.…”

“…Group contribution methods, such as UNIFAC (Fredenslund et al, 1975) and AIOMFAC (Zuend et al, 2008), are often used to estimate activity coefficients of atmospherically relevant compounds. More recently, a quantumchemistry-based COnductor-like Screening MOdel for Real Solvents (COSMO-RS; Klamt, 1995;Klamt et al, 1998;Eckert and Klamt, 2002) has been used to predict thermodynamic properties of multifunctional compounds. Solubilities and activity coefficients of carboxylic acids have also been estimated using the COSMO-RS theory implemented in the COSMOtherm program (COSMOtherm, 2019).…”