Proton NMR Study on Selective Hydration of Anions in Nitrobenzene

The selective hydration of univalent anions (X- = Cl-, Br-, I-, NO3 -, ClO4 -, SCN-) in deuterated nitrobenzene was studied by means of 1H NMR spectroscopy. The dependence of the chemical shift of water protons upon water- and ion-concentrations clearly showed that the selective hydration to the anions can be elucidated in terms of the successive reaction mechanism: X-(H2O) m - 1 + H2O ⇌ X-(H2O) m (m = 1, 2, 3, ...). By curve-fitting analyses, the successive formation constants and the intrinsic chemical shifts could be evaluated for the hydrated anions in different hydration states. Spin−lattice relaxation times were also measured for various water- and ion-concentrations, and likewise analyzed on the basis of the successive reaction mechanism. The intrinsic rotational correlation times evaluated for the hydrated anions in different states were usually larger than ∼3 ps for free waters, suggesting the restriction of the rotational diffusion of water molecules by bonding with the anions.