Raman scattering and DFT calculations used for analyzing the structural features of DMSO in water and methanol

Abstract

We report the concentration dependent Raman spectra of neat dimethyl sulfoxide [(CH₃)₂SO, DMSO] and its binary mixtures with water (W)/methanol (M) in both ν(SO) and ν(C–H) regions. The ν(SO) line profile of neat DMSO was resolved into four component peaks at 1036, 1044, 1054 and 1064 cm⁻¹ and assigned to different dimeric species of neat DMSO. A careful analysis of the Raman spectra of DMSO with water in the ν(SO) region at different concentrations reveals that upon dilution, an additional peak is observed at ~1017 cm⁻¹ (lower side of the peak ~1036 cm⁻¹) which is attributed to the hydrogen bonding of DMSO with water. For the highest dilution case (χ = 0.1 of DMSO), hydrogen bonded species and symmetric stretching of the dimeric ν(SO) mode were obtained which suggest that DMSO exists in dimer form even at low concentration of DMSO. The significant blue shifting of the C–H frequency due to C–H⋯O hydrogen bonding was also obtained in the case of both solvents. Our experimental results imply the existence of the dimer form of DMSO in neat as well as at χ = 0.1 of DMSO. In order to simulate and validate our experimental findings, detailed ab initio and density functional theory (DFT) calculations were also performed to obtain the ground state geometries of neat DMSO, self-associated dimer, trimer and their hydrogen bonded complexes with water and methanol. Our calculated structure of DMSO in dimer and trimer form reports a more accurate and stable geometry, in comparison to earlier calculation on these structures. Overall in this study nice spectra–structure correlations were obtained.