Molecular understanding of the interaction of methyl hydrogen sulfate with ammonia/dimethylamine/water

Theoretical calculations at the B3LYP-D3/aug-cc-pVTZ (aug-cc-pV(T+d)Z for sulfur) level were used to investigate the contribution of methyl hydrogen sulfate (MHS) to new particle formation with the common atmospheric aerosol nucleation precursors including water (H₂O), ammonia (NH₃), and dimethylamine (DMA). A typical characteristic feature of the MHS-containing complexes is the formation of six- or eight-membered ring structures via SOH⋯O (MHS donor), OH⋯O/N (H₂O donor) and NH⋯O/N (NH₃/DMA donor). The stability of the complexes was evaluated based on the calculated binding energies. The molecular interactions between three molecules are found to be more thermodynamically favorable than the complexes consisting two molecules. The red shifts of the SOH-stretching (MHS donor) vibrational transitions with respect to the isolated monomers are much larger than the red shifts of the OH (H₂O donor) and NH-stretching (NH₃/DMA donor) vibrational transitions. Topological analysis shows that the electron density and Laplacian at the bond critical points beyond the range of hydrogen bonding criteria for most of the complexes. This is due to the strong acid-base interaction between MHS and DMA or NH₃, thus leads to a proton transfer from MHS to DMA or NH₃. Remarkably, the atmospheric relevance of the MHS-containing complexes is much higher than H₂SO₄, which is evaluated by combining the calculated thermodynamic data and the concentrations of the reactant species. This study reveals the environmental fate of MHS could serve as nucleation centers in new particle formation.