Dynamics of Water in NaxCoO2.yH2O

Incoherent inelastic neutron scattering experiments were performed on Na0.7CoO2 and Na0.28CoO2.1.3H2O in order to understand how the dynamics of the hydrogen-bond network of water is modified in the triangular crystalline lattice NaxCoO2.yH2O. Using quasi-elastic neutron scattering (QENS), we were able to differentiate between two types of proton dynamics: a fast process (due to water strongly bound into the sodium cobalt oxyhydrate structure during the hydration process) and a slow process (likely attributable to a collective motion). High-resolution QENS experiments, carried out on Na0.28CoO2.1.3H2O, show that, at temperatures above 310 K, the water dynamics can be well-described by a random jump diffusion model characterized by a diffusion constant equal to 0.9 x 10(-9)m2/s, which is significantly lower than the rate of diffusion for bulk water. Furthermore, our results indicate that, at room temperature, the sodium ions have no influence on the rotational dynamics of the "fast" water molecules.     

Rotational dynamics of methyl groups in m-xylene

Methyl group dynamics of m-xylene was investigated by using incoherent inelastic and quasi-elastic neutron scattering. Inelastic measurements were carried out at the high flux backscattering spectrometer HFBS at the National Institute of Standards, quasi-elastic measurements at the time-of-flight spectrometer NEAT at the Hahn-Meitner-Institute. Rotational potentials are derived which describe the tunnel splittings, first librational, and activation energies of the two inequivalent CH(3) groups. Indications for coupling of the methyl rotation to low-energy phonons have been found. The finite width of one tunneling transition at He temperature is described by direct methyl-methyl coupling. The combined results of the experiments and the calculations allow a unique assignment of rotor excitations to crystallographic sites.     

Tetrahedral order in homologous disaccharide-water mixtures

The present work aims at evidencing the "kosmotrope" nature of trehalose through the analysis of inelastic neutron scattering measurements on trehalose and sucrose water solutions at different temperatures. Neutron spectra were collected by using the spectrometer MARI at the ISIS pulsed neutron source of the Rutherford Appleton Laboratory (Chilton, UK). To study the structural modifications induced on the tetrahedral hydrogen-bond network of water by homologous disaccharides, as a first step, the vibrational properties of pure water at different temperatures have been investigated. In particular, the temperature behavior of the intramolecular OH stretching mode has been analyzed. Successively, the vibrational properties for pure water have been compared with those of the sugar water solutions focusing the attention on the tetrahedral network-forming tendency. Finally, the obtained findings have been compared with previous Raman scattering evidences, and the results interpreted in the frame of recent molecular dynamics simulation works.     

Correlated atomic motions in liquid deuterium fluoride studied by coherent quasielastic neutron scattering

The collective dynamics of liquid deuterium fluoride are studied by means of high-resolution quasielastic and inelastic neutron scattering over a range of four decades in energy transfer. The spectra show a low-energy coherent quasielastic component which arises from correlated stochastic motions as well as a broad inelastic feature originating from overdamped density oscillations. While these results are at variance with previous works which report on the presence of propagating collective modes, they are fully consistent with neutron diffraction, nuclear magnetic resonance, and infrared/Raman experiments on this prototypical hydrogen-bonded fluid.     

Quasielastic and inelastic neutron scattering on hydrated calcium silicate pastes

Using the inverse geometry spectrometer QENS at the Intense Pulsed Neutron Source of the Argonne National Laboratory, we collected quasielastic and inelastic neutron scattering spectra of hydrated tricalcium and dicalcium silicate, the main components of ordinary Portland cement. Data were obtained at different curing time, from a few hours to several months. Both the quasielastic and inelastic spectra have been analyzed at the same time according to the relaxing cage model, which is a model developed to describe the dynamics of water at supercooled temperatures. Short-time and long-time dynamics of hydration water in hydrated cement pastes as a function of the curing time have been simultaneously obtained. The results confirm the findings reported in previous experiments showing that it is possible to fit consistently the quasielastic and inelastic spectra giving insights on the effect of the curing time on the short-time vibrational dynamics of hydration water.     

Water and polymer dynamics in chemically cross-linked hydrogels of poly(vinyl alcohol): a molecular dynamics simulation study

A topologically extended model of a chemically cross-linked hydrogel of poly(vinyl alcohol) (PVA) at high hydration degree has been developed for a molecular dynamics simulation with atomic detail at 323 K. The analysis of the 5 ns trajectory discloses structural and dynamic aspects of polymer solvation and elucidates the water hydrogen bonding and diffusion in the network. The features of local polymer dynamics indicate that PVA mobility is not affected by structural constraints of chemical junctions at the investigated cross-linking density, with a prevailing dumping effect due to water interaction. Simulation results are validated by a favorable comparison with findings of an incoherent quasi-elastic neutron scattering study of the same hydrogel system.     

Inelastic neutron scattering studies of the interaction between water and some amino acids

The interaction between water and some of amino acids (glycine, L-glutamine, L-threonine, L-cysteine and L-serine) was studied by inelastic incoherent neutron scattering (IINS). The vibrational spectra of dry amino acids and amino acids with a water content (e.g., 1 mol water/1 mol amino acid) were recorded. Comparing the difference spectra obtained by subtracting the spectrum of dry sample from those of wet sample with the spectra of ice Ih, we obtained that the difference spectrum for serine changed greatly from normal ice spectrum; but on the other hand, the difference spectra for the other amino acids such as glycine, glutamine, threonine, and cysteine changed slightly. The results demonstrate that serine has stronger hydrophilic character than glycine, glutamine, threonine, and cysteine. This is the first time the hydrophilic or hydrophobic character of amino acids was studied by using inelastic neutron scattering techniques, which provides important information for theoretical modeling and force field refinement for the interaction between water and the amino acids studied here.     

Observation of the neutron quasi-elastic broadening by polymers in soluttion

The quasi-elastic neutron scattering peaks obtained from polytetrahydrofuran and its perdeutero analogue have been observed in solution in CS₂. Thus the incoherent and coherent components can be separated. The results do not support the Rouse model of polymer chain dynamics but the Zimm model, which includes hydrodynamic interactions, may apply.     

Density of phonon states in solid parahydrogen from inelastic neutron scattering

We have measured the inelastic neutron scattering spectrum of solid parahydrogen (at low pressure and T=13.3 K) using the thermal original spectrometer with cylindrical analyzers spectrometer at the ISIS pulsed neutron source (UK). From the experimental spectrum we have obtained the parahydrogen density of phonon states which has been compared with the estimates available in the literature. The present determination improves substantially the previous experimental scenario from the point of view of both statistics and accuracy. The comparison with the most recent estimate obtained from a quantum mechanical simulation of the molecular dynamics calls for an improvement of the computational methods..     

Low-temperature tunneling and rotational dynamics of the ammonium cations in (NH4)2B12H12

Low-temperature neutron scattering spectra of diammonium dodecahydro-closo-dodecaborate [(NH(4))(2)B(12)H(12)] reveal two NH(4)(+) rotational tunneling peaks (e.g., 18.5 μeV and 37 μeV at 4 K), consistent with the tetrahedral symmetry and environment of the cations. The tunneling peaks persist between 4 K and 40 K. An estimate was made for the tunnel splitting of the first NH(4)(+) librational state from a fit of the observed ground-state tunnel splitting as a function of temperature. At temperatures of 50 K-70 K, classical neutron quasi-elastic scattering appears to dominate the spectra and is attributed to NH(4)(+) cation jump reorientation about the four C(3) axes defined by the N-H bonds. A reorientational activation energy of 8.1 ± 0.6 meV (0.79 ± 0.06 kJ/mol) is determined from the behavior of the quasi-elastic linewidths in this temperature regime. This activation energy is in accord with a change in NH(4)(+) dynamical behavior above 70 K. A low-temperature inelastic neutron scattering feature at 7.8 meV is assigned to a NH(4)(+) librational mode. At increased temperatures, this feature drops in intensity, having shifted entirely to higher energies by 200 K, suggesting the onset of quasi-free NH(4)(+) rotation. This is consistent with neutron-diffraction-based model refinements, which derive very large thermal ellipsoids for the ammonium-ion hydrogen atoms at room temperature in the direction of reorientation.     

Anisotropy on the collective dynamics of water confined in swelling clay minerals

Collective excitations of water confined in the interlayer space of swelling clay minerals were studied by means of inelastic neutron scattering. The effect of bidimensional confinement on the dynamics of the interlayer water was investigated by using a synthetic Na-saponite sample with a general formula of Si(7.3)Al(0.7)Mg(6)O(20)(OH)(4)Na(0.7) in a bilayer hydration state. Experimental results reveal two inelastic signals, different from those described for bulk water with a clear anisotropy on the low-energy excitation of the collective dynamics of interlayer water, this difference being stronger in the perpendicular direction. Results obtained for the parallel direction follow the same trend as bulk water, and the effect of the confinement is mainly manifested from the fact that clay interlayer water is more structured than bulk water. Data obtained in the perpendicular direction display a nondispersive behavior below a cutoff wavenumber value, Q(c), indicating a nonpropagative excitation below that value. Molecular dynamics simulations results agree qualitatively with the experimental results.     

Proton dynamics in an extended array of hydrogen bonds: normal coordinates, proton transfer and macroscopic quantum entanglement in the ground state

After a brief introduction to neutron scattering techniques, illustrated with the scattering function for harmonic oscillators, some new aspects of proton dynamics in the KHCO₃ crystal are presented. The full scattering function for the proton modes measured on single crystals provides a graphic view of proton dynamics. Vibrational states are fully characterized with three quantum numbers. The effective oscillator mass of 1 amu confirms the decoupling of protons from the lattice. Combining infrared, Raman and inelastic neutron scattering techniques, the double minimum potential for the transfer of a single proton along hydrogen bonds is totally determined. Elastic neutron scattering techniques probe dynamics in the fully degenerate ground state. Quantum entanglement arising from normal coordinates gives rise to quantum interference. With diffraction techniques, the dynamical structure arising from large-scale quantum coherence is observed as ridges of intensity, well separated from Bragg's peaks. The vibrational wave function in the ground state must be regarded as a superposition of non-factorable macroscopic wave function.     

Anharmonicity in a fragile glass-former probed by inelastic neutron scattering

Within the overall understanding of the glass transition, the relationship between microscopic dynamics and fragility is still to be clarified. Decalin is an organic glass former, for which a cis/trans mixture exhibits the highest known degree of fragility in a molecular system. It is therefore an ideal system for the investigation of microscopic dynamics in fragile systems. In the present study, the microscopic dynamics of pure cis-decalin has been measured by inelastic and quasi-elastic incoherent neutron scattering, giving the single particle self-correlation function. The fast relaxation dynamics and low-frequency vibrational modes are reported here. Both in the glass and in the crystal the vibrations show strong anharmonic behavior. In the glass phase, the short time microscopic dynamics evolve rapidly with temperature, however do not exhibit any significant change around the glass transition temperature T(g). The elastic intensity provides a measure of the mean square displacements which are comparable to those measured in other fragile glass formers, in particular, the archetypical fragile glass former orthoterphenyl. It appears that the microscopic relaxation gets unfrozen, relative to T(g), at much lower temperature than in other fragile systems.     

Diffusion of water in clays on the microscopic scale: modeling and experiment

Diffusion of water in montmorillonite clays at low hydration has been studied on the microscopic scale by two quasi-elastic neutron scattering techniques, neutron spin-echo (NSE) and time-of-flight (TOF), and by classical microscopic simulation. Experiment and simulation are compared both directly on the level of intermediate scattering functions, I(Q, t), and indirectly on the level of relaxation times after a model of atomic motion is applied. Regarding the dynamics of water in Na- and Cs-monohydrated montmorillonite samples, the simulation and NSE results show a very good agreement, both indicating diffusion coefficients of the order of (1-3) x 10(-10) m(2) s(-1). The TOF technique significantly underestimates water relaxation times (therefore overestimates water dynamics), by a factor of up to 3 and 7 in the two systems, respectively, primarily due to insufficiently long correlation times being probed. In the case of the Na-bihydrated system, the TOF results are in closer agreement with the other two techniques (the techniques differ by a factor of 2-3 at most), giving diffusion coefficients of (5-10) x 10(-10) m(2) s(-1). Attention has been also paid to the elastic incoherent structure factor, EISF(Q). Simulation has played a key role in understanding the various contributions to EISF(Q) in clay systems and in clearly distinguishing the signatures of "apparent" and true confinement. Indirectly, simulation highlights the difficulty in interpreting the EISF(Q) signal from powder clay samples used in experiments.     

The vibrations of hydrogen bonds

I present recent inelastic incoherent neutron scattering (INS) work involving the study of the H-bond vibrations. These vibrations have spectral consequences which are strongly dependent upon spectroscopic technique, and these will be indicated for IR and Raman, and detailed for INS. The vibrational dynamics of the simple FHF⁻ will be used by way of an example. These INS results demonstrate that the latest generation of neutron spectrometers provides new insights into H-bonding dynamics.     

Molecular dynamics in triglycine sulphate by cold neutron spectroscopy

Molecular dynamics of the ferroelectric compound triglycine sulphate (TGS) was studied by quasi-elastic neutron scattering. From among the three glycine molecules forming the unit cell the non-planar GI molecule is responsible for the polarisation which occurs in crystal and for the order–disorder ferroelectric phase transition at T_c = 49 °C. This molecule also shows a complex dynamics much faster than that performed by the other glycine molecules. The key group of this molecule is the amino group that, on one hand, seems to be responsible for the phase transition and, on the other hand, performs motional processes spread on a wide time scale. These motions have been detailed studied by means of different energy resolution windows within the quasi-elastic scattering range. We found that the dynamics of this group develops in a restricted spatial volume and consists in a faster rotational diffusion around the molecular axis of the group and a slower flipping dynamics between two symmetrical positions on both sides of a mirror plane containing the molecular axis of GI molecule.     

Diffusion coefficients of small molecules at the gas-solid interface as measured by the nuclear magnetic resonance pulsed field gradient method

The nuclear magnetic resonance pulsed field gradient method has been used to measure the diffusion coefficients for some low molecular weight compounds adsorbed at the gas-solid interface. The systems studied are ammonia adsorbed upon graphite, methane adsorbed upon graphite, neopentane adsorbed upon graphite and neopentane adsorbed upon titanium dioxide. Results are compared with values obtained from quasi-elastic neutron scattering where available.     

Quasielastic neutron scattering experiments including activation energies and mathematical modeling of methyl halide dynamics

Quasielastic neutron scattering experiments were carried out using the multichopper time-of-flight spectrometer V3 at the Hahn-Meitner Institut, Germany and the backscattering spectrometer at Forschungszentrum Julich, Germany. Activation energies for CH(3)X, X=F, Cl, Br, and I, were obtained. In combination with results from previous inelastic neutron scattering experiments the data were taken to describe the dynamics of the halides in terms of two different models, the single particle model and the coupling model. Coupled motions of methyl groups seem to explain the dynamics of the methyl fluoride and chloride; however, the coupling vanishes with the increase of the mass of the halide atom in CH(3)Br and CH(3)I.     

Aqueous peptides as experimental models for hydration water dynamics near protein surfaces

We report quasi-elastic neutron scattering experiments to contrast the water dynamics as a function of temperature for hydrophilic and amphiphilic peptides under the same level of confinement, as models for understanding hydration dynamics near chemically heterogeneous protein surfaces. We find that the hydrophilic peptide shows only a single non-Arrhenius translational process with no evidence of spatial heterogeneity unlike the amphiphilic peptide solution that exhibits two translational relaxations with an Arrhenius and non-Arrhenius dependence on temperature. Together these results provide experimental proof that heterogeneous dynamical signatures near protein surfaces arise in part from chemical heterogeneity (energy disorder) as opposed to mere topological roughness of the protein surface.