NMR Studies on Reorientational Motion of Hydrated D2O Molecules in Tetraalkylammonium Bromide Dilute Aqueous Solutions

The spin-lattice relaxation times of D and 17O nuclei of hydrated D2O molecules in tetraalkylammonium bromide (Me4NBr, Et4NBr, Pr4NBr, and Bu4NBr) dilute aqueous solutions in the concentration range of 0.2 to 1.0 mol kg−1 at 25°C were measured by NMR spectroscopy. The relaxation rates of D and 17O nuclei varied linearly with increasing the concentration below 1.0 mol kg−1. The dynamic hydration number, Bx+, is defined by Bx+=(k·τx+⁄τx0−1)nx+⁄50.0, where τx+ and τx0 (X=D or 17O) are the reorientational correlation times of hydrated D2O molecules (nx+) in R4N+ (R: alkyl group) and pure D2O molecule. The Bx+ increases with increasing the alkyl chain length of R4N+ ions. The abnormally large Bx+ values, compared with those of alkali metal ions, are caused by the hydrophobic hydration around the alkyl group of the R4N+ ion. The reorientational motion of a hydrated D2O molecule in the hydration sphere of R4N+ ions is isotropic, though the motion of a D2O molecule in the ionic hydration of alkali metal ions is anisotropic.