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

Abstract

As the simplest and most abundant dicarboxylic acid 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. A systematic theoretical study on the hydrated OA dimers performed by using DFT at the M06-2X/6-311++G(3df, 2p) level is discussed herein. The properties of hydrogen bonds in clusters are inspected through topological analysis by using atoms in molecules (AIM) theory. The most stable OA dimer involves a cyclic structure with two intermolecular hydrogen bonds. Calculations show that one H O has a slight effect on the hydrogen bonds, whereas two water molecules weaken and three water molecules break the two intermolecular hydrogen bonds between OAs. Furthermore, there are no hydrogen-bond interactions between OAs in almost all stable clusters as the number of H O molecules increases to four and five. Additionally, ionization and isomerization of OA through water-assisted proton-transfer phenomena are observed in tetra- and pentahydrates. This work provides new insights into the conversion of anhydrous OA into hydrated clusters that are helpful for further understanding the atmospheric nucleation process and nature of hydrogen bond.

Molecular-Scale Mechanism of Sequential Reaction of Oxalic Acid with SO3: Potential Participator in Atmospheric Aerosol Nucleation

“…There are five conformations of OA as shown in Figure 1 along with the difference of Gibbs free energy relative to the most stable conformation. 44 On the basis of the mole fractions for conformers and the total concentration of OA, the concentration of conformations A, B, C, D, and E are estimated to be 1.95 × 10 9 , 4.59 × 10 7 , 1.43 × 10 7 , 9.70 × 10 6 , and 6.06 × 10 5 molecules cm −3 at 298 K, respectively. 24 The most stable of these, conformation A, has two intramolecular hydrogen bonds between two −COOH groups, resulting in no exposed −COOH.…”

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

“…[28][29][30] Quantum chemical studies can elucidate these hydration processes at the molecular level, but primarily the hydration of clusters containing oxalic acid have been investigated. [31][32][33][34][35][36] Presently, there are no studies available on the hydration of atmospherically relevant organic acids of biogenic origin. Water is usually neglected in computational cluster studies, as each additional molecules in the cluster structure makes it more difficult to identify the global minimum.…”

Vibrational Stark Fields in Carboxylic Acid Dimers

“…The importance of high stability of carboxylic acids dimers has been highlighted in various aspects including their ability to dissociate in an appropriate solvent, [19][20][21][22][23] and the dissociation of carboxylic acids in any solvent is a competitive process that involves breaking of the doubly hydrogen-bonded dimer in the presence of the solvent and the interaction with the solvent leading to acid dissociation. 20 Based on such models, the pKa of several carboxylic acids have been estimated using a combination of electronic structure calculations with solvation modes and thermodynamic cycles with reasonable accuracy.…”