Modeling the Clustering of Fumaric/Maleic Acid with Water and Na⁺, Cl^- Ions

Organic dicarboxylic acids are common atmospheric compounds and are important constituents of secondary organic aerosols (SOA) with implications for air quality and climate. Both fumaric acid (FA) and maleic acid (MA) are found in substantial amounts in aerosol particles and have widespread applications throughout industry. While there have been extensive studies of the isomerization and spectroscopic properties of FA and MA in aqueous solution, there has, to the best of our knowledge, not been any studies of the thermodynamic and spectroscopic properties of microhydrated gas-phase and solvated clusters of FA and MA together with Na⁺ and Cl^- ions. Here, we present a study of the gas-phase and aqueous-phase clusters of FA and MA together with ions by probing cluster geometries, binding and solvation free energies, and infrared (IR) absorption spectra using quantum chemical methods. We performed a detailed configurational sampling protocol to obtain gas-phase structures calculated at the DLPNO-CCSD(T₀)/aug-cc-pVTZ//ωB97X-D/6-31++G(d,p) level followed by a reoptimization in implicit solvent to obtain aqueous-phase structures. We find that only negligible shifts in geometry occur upon solvation of the gas-phase clusters containing FA and MA, while slightly larger rearrangements of the structures occur when solvating the ion-containing clusters. The binding free energies reveal that it is thermodynamically unfavorable to hydrate FA and MA in the gas phase but favorable to hydrate their conjugate bases (FA^-, MA^-, FA^2-, and MA^2-), while it is favorable to solvate all of the clusters. Furthermore, we find that it is both favorable to hydrate the ion-containing clusters in the gas phase and solvate them. Finally, in order to identify weakly bound clusters and guide future experimental work, our IR absorption analysis reveals that the harmonic frequencies of the FA and MA carboxylic O-H stretching modes of the microhydrated FA and MA clusters are red-shifted in the spectrum upon solvation. On the other hand, we find no clear trend in the spectra of the (FA)₁(Na⁺)₁(Cl^-)₁(w)_0-5 and (MA)₁(Na⁺)₁(Cl^-)₁(w)_0-5 systems, but interestingly we find consistent red-shifts in the spectra for (FA^-)₁(Na⁺)₁(w)_0-5, (MA^-)₁(Na⁺)₁(w)_0-5, (FA^-)₁(Na⁺)₁(Cl^-)₁(w)_0-5, and (MA^-)₁(Na⁺)₁(Cl^-)₁(w)_0-5.