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Intermolecular frequencies below 900 cm−1 of H2O molecules in water and aqueous solutions of LaCl3, CaCl2, MgCl2, CsI, CsCl, KSCN, KI, KBr, KCl, KF, NaCl, LiNO3, and LiCl have been measured by slow neutron inelastic scattering. The diffusive kinetics of H2O molecules in aqueous solutions of LaCl3, MgCl2, CsCl, KSCN, KCl, KF, NaCl, and LiCl have also been investigated. Changes were observed simultaneously in both the intermolecular frequencies and the diffusive motions of H2O in ionic solutions relative to water, and they were specific to the size and charge of the ions, the concentrations, and the temperature. Ionic solutions containing a small and/or highly charged ion (e.g., LaCl3, MgCl2, LiCl, and KF) showed vibrational maxima at frequencies similar to rocking, wagging, and twisting librational modes and to ion–water stretching modes of H2O molecules in the corresponding solid salt hydrates. These maxima intensify with increasing concentration. In general, these frequencies lose intensity and broaden with increasing temperature, but many persist to 75°C. Solutions containing large singly charged ions (e.g., CsCl, KCl) also show new frequencies at lower temperatures, but these are generally broader and weaker than those characteristic of smaller or highly charged ions. Many of these maxima appear better defined at higher temperatures. At lower temperatures (1° and 25°C) the diffusive motions of water molecules in most salt solutions are in accord with a delayed diffusion model. Small and/or highly charged ions decrease the self-diffusion coefficient D and increase the residence time τ0 relative to water. D increases and τ0 decreases with increasing temperature, but they remain smaller and larger, respectively, than for pure water. At higher temperatures the diffusion kinetics depart from delayed diffusion behavior. The neutron spectra indicate that ions of high charge-to-radius ratio disrupt the water structure and form complexes having local ordering and bonding similar to that of H2O molecules in solid salt hydrates. These strong ion–water interactions give rise to higher activation energies for the movement of H2O molecules relative to pure water; thus, these salts act as “positive hydrators.” In contrast, salts of ions of low charge-to-radius ratio increase D and decrease τ0 relative to water and act as “negative hydrators.” The values of the intermolecular frequencies, diffusion coefficients, and residence times obtained during this study are in agreement with those obtained by other techniques. A tentative explanation for the diffusion kinetics is given for cases where the delayed diffusion model is not valid.

The dependences of intermolecular frequencies and of diffusive kinetics on concentration, on cation, and on anion have been investigated using neutron inelastic scattering for H20 molecules in aqueous solutions. In CsCl, NaCl, and MgCl2 solutions, water frequencies are observed up to concentrations of about 0.5 m. In contrast, characteristic frequencies of water persist to concentrations above 4.6 m for KC1, but for LiCl any correspondence with water frequencies is lost above 0.02 m. With further increases in concentration, intermolecular frequencies characteristic of primary ion-water hydration complexes appear and intensify. Below 25°, the diffusive kinetics were in accord with a delayed-diffusion mechanism. For dilute solutions (especially <0.5 m), the values of the self-diffusion coefficients (D) and residence times (to) were nearly identical with those for water. At Io, with increasing concentration, the D's decrease and the t0's increase relative to water for KF, NaCl, LiCl, MgCla, MgSCh, and CrClg ("positive hj^dration"), while the reverse is true for CsCl, CsBr, KSCN, KI, KBr, and KC1 ("negative hydration"). However, for concentrated solutions of certain strongly hydrating cations (e.g.: Cr3+ and Li+), jump reorientations of individual waters in the primary layers are sufficiently restricted, so that diffusion of entire cation-water complexes is observed. For CsCl and CsBr, the D's increase and the t0's decrease initially with increasing concentration, but become nearly constant, due to increased ion pairing. For KSCN solutions, with increasing concentration D increases and t0 decreases initially relative to water. They then go through a maximum and minimum, respectively, and approach the values for water corresponding to a decrease in "negative hydration." Thus, while SCN~ions increase the diffusive mobility in the solvent, reorientations of waters in primary hydration coordinations are at least as restricted as those in water. In concentrated solutions of small and/or highly charged cations (í.e., Li+, Mg2+, La3+), the frequencies and diffusive characteristic parameters are primarily determined by the cation, and only secondarily by 1-anions. The replacement of Cl" anions by NOsanions primarily causes small broadenings and loss of resolution of the intermolecular frequencies together with a slight increase in D and a decrease in t0. Relative to MgCL, in MgSO, solution, the SO42-ion decreases slightly the anharmonicity of the vibrations of waters coordinated to the cations, but mainly restricts the diffusive mobility of water molecules beyond the primary layers. In contrast, for solutions of larger, singly charged cations, the intermolecular frequencies and the diffusive kinetics show a strong dependence on anion. Thus, in potassium halide solutions, the "structurebreaking" influence of the anions increases as Cl-< Br~< I", and F" acts as a "structure maker." Experimental evidence and a tentative explanation are given for abrupt variations with temperature in intermolecular frequencies and in the diffusi...

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The intrachain and interchain vibrations below 900 cm−1 of polydimethylsiloxane (PDMS) have been studied by slow neutron inelastic scattering. A composite motion observed at +25°C for the methyl groups corresponds to nearly free rotation about the threefold axis of symmetry together with a large‐amplitude rotation of the entire methyl group. At −123°C, rotation about the threefold axis evolves to a torsional oscillation. The large‐amplitude rotation evolves to the skeletal vibrations of a helical conformation. Vestiges of the cooperative skeletal vibrations of the conformation at −123°C persist into the 25°C spectrum. The results indicate the presence of interrupted helical conformations at 25°C, which result from thermal disordering of the low temperature helices. The effects of crosslinking, low molecular‐weight oils, and silica filler on the freedom of the methyl group motions and on skeletal vibrations have been determined. The effects of different crosslinking agents and different relative amounts of filler and oil on both the macroscopic physical properties and the observed molecular motions of PDMS can also be interpreted in terms of an interrupted helix.