Sulfuric acid and aromatic carboxylate clusters H2SO4·ArCOO−: Structures, properties, and their relevance to the initial aerosol nucleation

Abstract

Sulfuric acid and aromatic compounds are abundant in the atmosphere and play an important role in aerosol nucleation and growth. The main objective of this work is to provide molecular level description of pre-nucleation clusters formed by these species. Our approach is based on size- and composition-selective generation of anionic clusters, consisting of sulfuric acid and series of aromatic carboxylates (ArCOO−; Ar = Phenyl, o-, m-, and p-CH3C6H4), and their subsequent characterization using photoelectron spectroscopy and quantum chemical calculations. The photoelectron spectra show that formation of these clusters is accompanied by a significant increase of electron binding energies, implying significant stabilization gained from electron delocalization upon complexation. Comparison between the experimental and calculated results suggests that under low temperature conditions these clusters behave more like a complex of sulfuric acid and aromatic carboxylate, (H2SO4)(ArCOO–), despite the fact that ArCOO– has much higher proton affinity than HSO4–. Similar result has been previously reported for sulfuric acid formate cluster [Hou, Wang, and Valiev, J. Am. Chem. Soc. 139 (2017) 11321–11324], and indicates that interaction energy gained though the formation of these clusters is sufficient to counteract proton affinity imbalance. The calculated intermolecular binding energies (BEs), Gibbs free energies (ΔG), and evaporation rates (γ) show that these (H2SO4)(ArCOO–) clusters are extremely stable, with BEs of ∼54 kcal/mol, ΔG of ∼‒40 kcal/mol, and γ of 10−20 s−1. The experimental and computational results reported in this work suggest that aromatic acids may play a significant role in facilitating the early stages of sulfuric acid nucleation.