Interaction of oxalic acid with dimethylamine and its atmospheric implications

Abstract

Oxalic acid and dimethylamine are the most common organic acid and base in the atmosphere, and are recognized as significant precursor species in atmospheric new particle formation. However, the interaction between oxalic acid and dimethylamine in the presence of hydration is not yet understood. In this study, the most stable geometric structures and thermodynamics of (C₂H₂O₄)_m(CH₃NHCH₃)(H₂O)_n (m = 1–2, n = 0–4) clusters are investigated using M06-2X coupled with the 6-311+G(2d,p) basis set. A high level explicitly corrected CCSD(T)-F12/VDZ-F12 method is utilized to benchmark the density functional theory (DFT) methods. Hydration promotes proton transfer from oxalic acid to dimethylamine for (C₂H₂O₄)(CH₃NHCH₃)(H₂O)_n (n = 0–4) clusters, while proton transfer from oxalic acid to dimethylamine occurs without hydration for (C₂H₂O₄)₂(CH₃NHCH₃)(H₂O)_n (n = 0–4) clusters. With regards to the isomer distribution at the potential energy surface, temperature seems not to be an important parameter, since almost all of the global minima for the investigated size range dominate within the investigated temperature range, except for in the (C₂H₂O₄)_m(CH₃NHCH₃)(H₂O)₂ clusters. Under atmospheric conditions, the peak hydration distribution shifts from unhydrated clusters to trihydrates for the (C₂H₂O₄)(CH₃NHCH₃)(H₂O)_n (n = 0–4) clusters, while for the (C₂H₂O₄)₂(CH₃NHCH₃)(H₂O)_n (n = 0–4) clusters, unhydrated clusters clearly dominate the cluster distribution, irrespective of whether the humidity is low or high. Finally, the formation free energies obtained from quantum calculations are used to calculate the evaporation rates. We find that evaporation of dimethylamine is preferred compared to oxalic acid for the (C₂H₂O₄)(CH₃NHCH₃)(H₂O)_n clusters, while the results are reversed for the (C₂H₂O₄)₂(CH₃NHCH₃)(H₂O)_n clusters.