Raman spectroscopy of benzenesulfonic and 4-toluenesulfonic acids dissolved in dimethylsulfoxide

Solutions of benzenesulfonic acid (BSA) and 4-toluenesulfonic acid monohydrate (PTSA) in dimethylsulfoxide (DMSO) were studied by FT-Raman spectroscopy in the concentration range 1.0-3.5 mol dm(-3) (BSA) and 1.0-4.8 mol dm(-3) (PTSA). Spectra in the region of the Raman acid complex band (C-S + C-C + SO3) stretches, at 1124 cm(-1) were analysed by band-fitting procedures in order to ascertain the degree of acid dissociation. In BSA solutions, this parameter changes from 0.78 at 1.02 M to 0.47 at 3.5M, despite the strong character of the acid. Interaction of DMSO with undissociated BSA produces a new band in the solvent nu(C-S) Raman spectral region near 671 cm(-1), displaced >15.0 cm(-1), and assigned to DMSO molecules H-bonded to BSA. In PTSA solutions, hydrogen bonds are formed with the oxonium ion (H3O+) dissociated from the acid. In this case, the displacement observed is only >10.0 cm(-1), indicating a weaker interaction. From the concentration of H-bonded DMSO, the solute/solvent coordination number and its inverse, the mean number of H-bonds participating in bonding with each solvent molecule can be calculated. This coordination number changes in BSA solutions in bimodal way, passing through a maximum and reaching a limit of 2 in the most concentrated solution. This number agrees with that found in the solid solvate BSA.2DMSO. In PTSA solutions, the general trend is similar, but low coordination numbers are obtained, in agreement with the low acidity of the oxonium ion. The bimodal behaviour observed in both acids is explained by the self-associated structure of the solvent.