Molecular-level insight into uptake of dimethylamine on hydrated nitric acid clusters

Abstract

Mixed nitric acid/water clusters with dimethylamine (DMA) represent a suitable model system for understanding acid–base chemistry in atmospherically relevant clusters. We investigate these clusters in a detailed molecular-beam experiment accompanied by ab initio calculations. The (HNO₃)_M(H₂O)_N clusters are produced by supersonic expansion into vacuum and doped by DMA molecules in a pickup process. Two complementary mass spectrometry approaches are employed to analyze the resulting (DMA)_K(HNO₃)_M(H₂O)_N clusters: (i) electron impact ionization at 70 eV to form positive cluster ions and (ii) low-energy electron attachment at 0–10 eV to form negative clusters. The positive ion spectra contain mainly protonated (DMA)_k(HNO₃)_mH⁺ clusters with k = m + 1, whereas the negative ones are dominated by (DMA)_k(HNO₃)_mNO⁻₃ with m ≥ k, followed by (DMA)_k(HNO₃)_mNO₂⁻ (m > k) ions with low abundances. These observations are rationalized by our calculations, which exhibit the tendency of the mixed clusters to maximize the number DMA·H⁺ and NO₃⁻ ions in the clusters. In the neutral clusters, this is fulfilled for 1 : 1 ratio of DMA and HNO₃, while the positively charged (DMA)_k(HNO₃)_mH⁺ clusters satisfy this condition for k = m + 1. The protonated clusters always contain the DMA·H⁺ moiety. For the negatively charged cluster ions, thermochemistry supports the prevailing formation of NO⁻₃ and m ≥ k ion composition. Furthermore, the NO⁻₃-containing cluster ions can form when an electron attaches to the protonated moiety of the DMA·H⁺⋯NO⁻₃ ion pair in the cluster, which leads to H atom evaporation. From the gas phase HNO₃ molecule, where NO⁻₂ is formed exclusively upon an electron attachment, the tendency to form NO⁻₃ increases to hydrated HNO₃ clusters, where both NO⁻₂ and NO⁻₃ ions are generated in approximately equal abundances, to the DMA doped clusters, where NO⁻₃ strongly prevails NO⁻₂.