INFRARED AND PROTON MAGNETIC-RESONANCE STUDIES OF 2 BENZENESULFONIC ACIDS IN DIOXANE-WATER MIXTURES .2. ION ASSOCIATION AND SOLVENT STRUCTURE PROBING

Two sulfonic acids, p-toluenesulfonic acid and p-nitrobenzenesulfonic acid, were studied as potential probes of solvent structure. For this purpose their infrared and proton magnetic resonance spectra were examined in 0.5 M solutions in dioxane-water mixtures. By examining the spectra, electrolytic interactions such as ion pairing, anion pairing, and probe effects on water structure were qualitatively elucidated as well as probe-solvent interactions of associational types. Vibrational bands of water and dioxane were also observed. Acid dissociation of the sulfonic acids appeared unaffected by solvent structure. Since water and dioxane proved to interact differentially with the probe aromatic proton pairs, the NMR internal chemical shift Δ, where Δ = |δH-3,5 - δH-2,6|, appeared sensitive to solvent structure. Thus, a broad maximum in Δ observed at 25-35 mol % water supports the existence of a 1:2 water:dioxane complex. The basis for this kind of sensitivity by a para-substituted benzene is examined briefly. © 1979 American Chemical Society.