Hydrogen bonding in cyclic complexes of carboxylic acid–sulfuric acid and their atmospheric implications†
The interactions of three common carboxylic acids (glyoxylic acid, oxalic acid and pyruvic acid) with an atmospheric nucleation precursor (sulfuric acid) have been investigated with density functional theory, atoms in molecules and localized molecular orbital energy decomposition analysis methods. A typical feature of the complexes is the formation of cyclic ring systems via two types of hydrogen bonds: SO–H⋯O and CO–H⋯O. Based on the geometric parameters, the SO–H⋯O hydrogen bonds are classified as strong to medium-strength hydrogen bonds, and all the CO–H⋯O hydrogen bonds belong to medium-strength hydrogen bonds. The carboxylic acid–sulfuric acid complexes possess larger binding energies in the nine- and eight-membered rings than in the seven- and six-membered rings. The red shifts of the OH-stretching transitions of both the SO–H⋯O and CO–H⋯O hydrogen bonds are much larger in the nine- and eight-membered rings than those in the seven- and six-membered rings with respect to the isolated monomers. The localized molecular orbital energy decomposition analysis shows that the electrostatic interaction is the major contribution to the total interaction energy. Topological analysis shows that the charge density at the bond critical points of the carboxylic acid–sulfuric acid complexes falls in the range of hydrogen bonding criteria. The Gibbs free energy of formation calculated within the atmospheric temperature and pressure range (atmospheric height 0–12 km) could help to further validate the potential importance in atmospheric particle nucleation and growth.