Temperature and Pressure Effects on the Spin-lattice Relaxation Time of Hydrated Water Molecules by NMR

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The temperature effects on the spine-lattice relaxation rates of hydrated D_2O molecules in the dilute solutions of alkali metal bromide (LiBr, NaBr, KBr, and CsBr) and tetraalkylammonium bromide (Me_4NBr, Et_4NBr, n-Pro_4NBr, and n-Bu_4NBr) salts have been studied by NMR spectroscopy, and the high pressure NMR polmer cell has been tested up to 500 bar at room temperature. The concentration dependences of D and ^<17>0 spine-lattice relaxation rates (R_1) of D_2O molecules in D_2O solutions of tetraalkylammonium bromide are repersented by the equation of R_1/R_Rﾟ= 1 + Bm + Cm^2 (R_1ﾟ ; pure D_2O, m ; mol kg^<-1>, B and C ; constants) at lower temperatures below 20 ﾟC. This means the overlap the hydration sphere of tetraalkylammonium ions, when the lower temperature induces the hydrogen bonded network structure of water. Analyzing these results on the model of the solvent-separated hydrophobic hydration, the rotationa motion of the solvent separated hydrophobic hydrated water molecules is faster than those of the hydrated water molecules. The concentration dependences of R_1 in D_2O molecules of alkali metal bromides are represented by the linear equation of R_1/R_1ﾟ = 1 + Bm in the wide range of temperatures of 5 ﾟC to 50 ﾟC. With increasing temperature, the B coefficients of salts solution increase from the negative to positive sign. Therefore, the rotational motion of hydrated water molecules of alkali metal ions is slower than that of the bulk water molecules.