Effect of pressure on deuteron spin-lattice relaxation in several concentrated deuterium oxide diamagnetic electrolyte solutions

The pressure dependence of the deuteron spin‐lattice relaxation times of the deuterium oxide molecules in concentrated 4.5m aqueous solutions of CsBr, RbBr, KBr, LiCl, CaCl₂, and LaCl₃ have been measured at 30°C up to 4.5 kbar. Viscosities of the above solutions, over the same pressure range, at 30°C, have also been determined. From the viscosity data and the deuteron relaxation times at 1 bar, the structure breaking ability of the electrolytes CsBr, RbBr, and KBr is found to increase in the order K⁺ < Cs⁺ ∼ Rb⁺, while the structure making ability of the electrolytes LiCl, CaCl₂, and LaCl₃ increases in the order Li⁺ < Ca⁺² < La⁺³. Values for the rotational correlation time ${\tau }_{\theta }^{+}$ of the D₂O molecules at 1 bar in the hydration spheres of the cations Li⁺, Ca⁺², and La⁺³ are calculated to be 4.5, 10.0, and 23.0 psec, respectively. The differences between ${\tau }_{\theta }^{\mathrm{NMR}}$ and ${\tau }_{\theta }^{\mathrm{Debye}}$ observed as a function of electrolyte and pressure are interpreted in terms of changes in the product of the quadrupole coupling constant and the square root of the effective hydrodynamic volume $\mathrm{[(}\mathrm{QCC}{\mathrm{)(V}}_m^{\mathrm{eff}}{)}^{\text{1/2}}]$ . A decrease in this quantity at 1 bar, due to addition of electrolytes, is explained in terms of a change in the effective hydrodynamic volume of the D₂O molecules, this being related to the degree of coupling between rotational and translational motion. Over the pressure range of 4.5 kbar the quantity $\mathrm{[(}\mathrm{QCC}{\mathrm{)(V}}_m^{\mathrm{eff}}{)}^{\text{1/2}}]$ decreased approximately 10% to 20% for all solutions and approximately 11% for pure D₂O. The data for structure‐breaking electrolytes favor the tentative interpretation that an increase in pressure produces a decrease in the average O–O distance in the O–D⋯O hydrogen bonds resulting in a decrease in the magnitude of the deuteron quadrupole coupling constant. The data for structure making electrolytes can be interpreted as either a change in QCC or $V_m^{\mathrm{eff}}$ or both.