Intermolecular Hydrogen-Bonded Interactions of Oxalic\nAcid Conformers with Sulfuric Acid and Ammonia

Oxalic acid is one\nof the simplest naturally occurring dicarboxylic\nacids that is abundantly found in the atmosphere, and it has several\nstable structural conformers. Hydrogen-bonded interactions of oxalic\nacid with other atmospheric molecules are important, as they might\ninfluence the chemical composition of the atmosphere, thereby impacting\natmospheric chemistry and environmental processes. In this work, we\nused density functional calculations with the M06–2<i>X</i>/6-311++G(3df,3pd) model to examine the interaction of\nfive oxalic acid conformers with sulfuric acid and ammoniatwo\nwidely recognized atmospheric nucleation precursor moleculeswith\nthe aim of observing the hydrogen-bonding characteristics of the conformers\nindividually. An extensive and systematic quantum-chemical calculation\nhas been conducted to analyze the structural, thermodynamical, electrical,\nand spectroscopic characteristics of several binary and ternary clusters\nmediated by five oxalic acid conformers. Our analysis of the electronic-binding\nenergies and free energy changes associated with the formation of\nthe clusters at ambient temperature reveals that multiple conformations\nof oxalic acid have the potential to engage in stable cluster formation\nin the atmosphere. In fact, the highest energy oxalic acid conformer\nexhibits the lowest bonding free energy in most cases. According to\nour calculations, clusters of oxalic acid with sulfuric acid demonstrate\ngreater thermodynamic stability, a higher probability of formation,\nand more intense light scattering compared to clusters with ammonia.\nFurthermore, the analysis of successive cluster formation reveals\nthat clusters formed between sulfuric and oxalic acids are more likely\nto grow spontaneously than those formed between ammonia and oxalic\nacid.