Effect of Water on the Structure and Stability of Hydrogen‐Bonded Oxalic Acid Dimer

As the simplest and the most abundant dicarboxylic acids in the atmosphere, oxalic acid (OA) not only plays a key role in aerosol nucleation, but also acts as a prototypical compound for the investigation of intra- and intermolecular hydrogen bonding interactions. In this work, a systematic theoretical study on the hydrated OA dimers were carried out using density functional theory (DFT) at the M06-2X/6-311++G(3df, 2p) level. The properties of the hydrogen bonds in clusters were inspected through topological analysis using atoms in molecules (AIM) theory. The most stable OA dimer involves a cyclic structure with two intermolecular hydrogen bonds. Our calculations show that one H2O has slightly effect on the hydrogen bonds, while two water molecules weaken and three water molecules even break the two intermolecular hydrogen bonds between OAs. Furthermore, there are no hydrogen bond interactions between OAs in almost all stable clusters when the H2O number increases to four and five. Additionally, the ionization and the isomerization of OA through water-assisted proton transfer phenomena were observed in tetrahydrates and pentahydrates. Our work has provided new insights into the conversion process of anhydrous OA to hydrated clusters, which are helpful for further understanding the atmospheric nucleation process and the nature of hydrogen bond.