Proton Transfer in Mixed Clusters of Methanesulfonic Acid, Methylamine, and Oxalic Acid: Implications for Atmospheric Particle Formation

Understanding the properties of atmospheric particles made of several components is a very challenging problem. In this paper, we perform quantum chemical calculations for small multicomponent clusters of atmospheric relevance that incorporate methanesulfonic acid (MSA), methylamine (MA), oxalic acid (OxA), and water (H₂O). Potential correlations between theoretical predictions of proton transfer in the small clusters and findings of recent experiments on formation of particles of detectable sizes (>2 nm) from the same components are studied. It is proposed that proton transfer from the acid to the amine in the 1:1 clusters correlates with experiments on particle formation in systems, such as MSA-MA and MSA-MA-OxA. In the case of OxA + MA, which has been observed to give few particles, proton transfer does not occur for the 1:1 cluster but does for the 2:2 cluster. Adding H₂O to OxA-MA promotes the occurrence of proton transfer, and corresponding particles are slightly enhanced. The partial charge on the MA component increases by adding OxA or H₂O to MSA-MA, which is correlated with enhanced particle formation compared to MSA-MA alone. Ab initio molecular dynamics simulations show that proton transfer at room temperature (T = 298 K) and high temperature (T = 500 K) is little affected compared with the equilibrium structure (T = 0 K). These results suggest that small cluster calculations may be useful in predicting the formation of multicomponent particles in the atmosphere.