NMR STUDIES ON THE EFFECT OF TEMPERATURE AND PRESSURE ON THE STRUCTURE AND DYNAMICS OF AQUEOUS SYSTEMS

Using a newly constructed NMR machine, we have investigated the effect of temperature and pressure on the structure and dynamics of several interesting aqueous systems, such as pure water, water in organic solvents, ions in water, and organic molecules in water over as wide as possible a range of temperature. New data on molecular orientational correlation times τ_2R in such aqueous systems are discussed in comparison with solvent viscosity μ and translational self-diffusion coefficient D. Arrhenius activation energies E_a are determined to examine how the energy profile depends on the strength of solute-solvent interactions. The E_a value for τ_2R of D₂O is identical to that for D only at room temperature, much higher than that for D or T/μ (μ, solvent bulk viscosity) at lower temperatures. The values of τ_2R as well as E_a for D₂O in organic solvents decrease in the sequence of the solvent polarity. This sequence agrees with that of H chemical shifts but disagrees with that of the solvent viscosity, indicating the invalidity of the Stokes-Einstein-Debye law. It is shown that the directional attractive intermolecular interactions play a key role in controlling the structure and dynamics of water.