Deuterium spin-lattice relaxation times (zH T I ) are reported for the first time as a function of temperature (180-300 K), pressure (p I 225 MPa), and composition (c I 5 m) in undercooled aqueous solutions of tetraalkylammonium ions (R4N+). A comparison with related investigations of undercooled alkali-metal halide solutions allows the competing influence of coulombic, hydrophobic, and H-bond interactions upon the dynamic structure of the random, transient H-bond network to be studied. Solvent dynamics are seen to be closely related to the glass-forming tendency of undercooled tetramethylammonium (Me4N+) and tetrapropylammonium (Pr4N+) bromide solutions and to the clathrate-forming tendency of undercooled tetrabutylammonium (Bu4N+) bromide solutions.

Multinuclear spin-lattice relaxation rates and self-diffusion coefficient measurements are reported over wide ranges of temperature, pressure, and concentration in undercooled aqueous solutions of tetramethylammonium bromide. These dissolved organic cations with apolar surface groups provide model systems to investigate the effect of Coulombic, hydrophobic, and H-bond interactions upon the solvent and solute dynamics within the random, transient H-bond networkof the water. Also reported for the first time areneutron diffraction difference experiments in these undercooled solutions to investigate how changes in dynamic disorder are related to changes in the average static structure of the hydration water as temperature is decreased. A substantial sharpening of the orientational order between water molecules is observed in the undercooled state. IntroductionThe structure and the dynamic properties of a great number of aqueous solutions of atomic ions have been studied thoroughly both with neutron diffraction1 and nuclear magnetic relaxation2 techniques. Aqueous solutions with dissolved organic ions with apolar surface groups like symmetric tetraalkylammonium (R4N+) ions are less well investigated. Despite many thermodynamic data, only very few investigations of the molecular dynamics in these systems exist3v4 and direct structural studies were reported only re~ently.~+6Aqueous solutions of R4N+ ions allow the competing influence of the Coulomb effect of the charge density and the hydrophobic effect of the apolar surface on the dynamic structure of the H-bond network of water to be studied. Though the concept of hydrophobic hydration is widely used in chemistry and biology to describe the reduction in entropy upon solvation of apolar solutes in water, its molecular basis is not well understood yet. Thermodynamic, spectro~copic,3~~ and computer simulation738 results suggest that the solvation of apolar groups (hydrophobic hydration) increases the amount of order in the random, transient H-bond network of the coordinated water molecules and reduces their mobility. The hydration structures involved may resemble clathrate-like cages which would be favored by the network organization of liquid water.g Further insight into the dynamic structure may be obtained in undercooled aqueous R4N+ solutions. Because of reduced thermal excitations hydrophobic hydration structures would become more stable and long-lived and hence would cause a strong

Multinuclear spin−lattice relaxation rate measurements are reported over wide ranges of temperature, pressure, and concentration in undercooled aqueous solutions of tetraethylammonium bromide (Et4NBr). These dissolved organic cations with apolar surface groups provide model systems to investigate their influence upon the dynamics of the water molecules in the hydration shell as well as the influence of the hydration shell upon the dynamics of the dissolved hydrophobic ions. The isobaric relaxation time curves of all nuclei investigated show pronounced minima, indicating a strong slowing down of the dynamics of the solvent molecules in the hydration shell as well as of the dissolved hydrophobic ions. A VTF-like temperature dependence and corresponding scaling behavior allow glass transition temperatures to be deduced which signify the onset of motional arrest in these metastable solutions. Segmental motions of the aliphatic chains of the organic cations are discussed in terms of correlation times, and corresponding order parameters are given characterizing their spatial restrictions. At low temperatures all proton relaxation rates become equal to those of the methyl groups due to strong cross-relaxation effects, indicating that methyl groups act as relaxation sinks in these systems.

Abstract. The dynamic properties of water molecules coordinated to simple ions (alkali and alkali-earth halides) and hydrophobic ions (tetraalkylammonium (TAA) halides) in supercooled solutions have been investigated with KMR. The study of spin-lattice relaxation rates and self-diffusion coefficients as functions of temperature, pressure, Larmor frequency and concentration reveals characteristic features of molecular motions close to the lowtemperature limit of the metastable phase (percolation transition and glass transition) and provides certain details of the local arrangement of water molecules in the coordination sphere of these ions. The intramolecular flexibility of the alkyl chains of the hydrated TAA cations has been investigated also. The dynamics of the methyl group reorientation provide a sensitive probe of structural differences in these solutions.

NMR measurements of the spin-lattice relaxation rates of the 'H, 13C, and 14N nuclei and spin-echo self-diffusion coefficients are reported for tetrapentylammonium bromide in acetonitrile solutions Over a wide range of temperatures and concentrations. The results are discussed in terms of overall and segmental motions of the aliphatic chains of the organic ions. Order parameters characterizing the geometrical constraints upon local segmental motions are given also. Cationic self-diffusion coefficients are found to differ from those obtained with quasi-elastic neutron scattering techniques (QENS). The difference may be explained in terms of fast and slow modes contributing to the diffusion processes and their selective detection with QENS and NMR techniques.