Vibrational Spectra of Atmospherically Relevant Hydrogen-Bonded MSA···(H₂SO₄) (n = 1-3) Clusters

Methanesulfonic acid (CH₃SO₃H), also known as MSA, has been found to be capable of forming a strong hydrogen-bonded interaction with sulfuric acid (H₂SO₄) under ambient conditions. The energetic stability of the MSA···H₂SO₄ clusters increases with decreasing temperature at higher altitudes in the troposphere, which is relevant in the context of atmospheric aerosol formation. We have performed, in the present work, a detailed and systematic quantum-chemical calculation with high-level density functional theory to characterize the hydrogen bond formation in the binary MSA···H₂SO₄, ternary MSA···(H₂SO₄)₂, and quaternary MSA···(H₂SO₄)₃ clusters. The five different conformations of MSA···(H₂SO₄)₂ and six conformations of MSA···(H₂SO₄)₃, considered in the present work for the spectroscopic analysis, have been taken from our previous work [J. Phys. Chem. A. 2020, 124, 11072-11085]. The hydrogen bonds were analyzed on the basis of infrared vibrational frequencies of different O-H stretching modes and quantum theory of atoms in molecules (QTAIM). A strong positive correlation has been observed between the red shift of the OH groups in MSA and H₂SO₄ and the corresponding O-H elongation as a result of hydrogen bond formation. Topological analysis employing QTAIM shows that most of the charge density and the Laplacian values at bond critical points (BCPs) of the hydrogen bonds of the MSA···(H₂SO₄) (n = 1-3) complexes fall within the standard hydrogen-bond criteria. However, those outside these criteria fall in the category of a very strong hydrogen bond with a hydrogen bond length as low as 1.41 Å and an O-H bond elongation as high as 0.096 Å. In general, the charge densities of the BCPs located on hydrogen bonds increase as the hydrogen-bond lengths decrease. Proportionately, a larger number of hydrogen bonds in ternary MSA···(H₂SO₄)₂ demonstrate a partial covalent character when compared with the quaternary clusters.