Incoherent Quasielastic Neutron Research Articles

ChemInform Abstract: Neutron Investigation of Reorientations in Organic Molecular Compounds

Abstract The application of the Incoherent Quasielastic Neutron Scattering (IQNS) technique to the study of the dynamic disorder in molecular crystals is illustrated by a series of recent experiments. First, some basic principles of incoherent neutron scattering from organic hydrogenous compounds are outlined. The Elastic Incoherent Structure Factor (EISF), a fundamental quantity directly related to the spatial average distribution of the scatterers, is introduced. An illustration of the isotropic rotational model is provided by the case of norbornane. A comparison is made with a related compound, norbornadiene, the latter providing evidence of a strong translation—rotation coupling. The case of deformable molecules is illustrated with different analyses of the dynamics of alkyl chains in various situations. The flexibility of the chains is analysed for tetracarboxylate complexes in liquid-crystal phases, decylammonium and alkylene diammonium molecules in perovskite-type layered crystalline structures, and nonadecane in its rotator phase. Some studies related to alkanes in inclusion compounds are also covered. The final part of the paper is devoted to recent studies of glassy and metastable phases, and also to critical phenomena. We report on the dynamics of the cyanoadamantane molecules in the quenched glassy phase and on the modifications observed by aging of the sample. In the case of p-terphenyl, long-range correlations appear when approaching the phase transition on cooling and contribute to the scattered intensity. We conclude with a description of the rich polymorphism of methoxybenzylidenebutylaniline, where the quenched degrees of freedom are progressively relaxed in a succession of metastable phases.

Hydrated {Mo 72Fe 30} clusters: Low-frequency hydrogen modes and self-aggregation

Using incoherent quasi-elastic and inelastic neutron scattering, we have investigated the hydrogen relaxational dynamics and hydrogen vibrational modes in the polyoxomolybdate specie [Mo 72Fe 30O 252(CH 3COO) 12[Mo 2O 7(H 2O)] 2[H 2Mo 2O 8(H 2O)](H 2O) 91]· ≈ 150 H 2O. The translational dynamics of the water molecules in the compound is profoundly different from that of bulk water at the same temperature showing a non-Debye relaxation behavior. The temperature dependence of the relaxation time can be described in terms of an Arrhenius law, indicating that the dynamics is triggered by the breaking of the bonds connecting the crystal water molecules with the hydrophilic nanocapsule surfaces. Inelastic neutron scattering spectra confirm the attenuation of water translational modes with respect to the bulk water case due to the strong destructuring effect imposed by the nanocage interface and the enhancement of the highest frequency librational mode as already found in hydrated Vycor or Gelsil matrix. Small angle X-ray scattering on freshly prepared aqueous solution evidences the presence of nanocapsule structures proper of the monomer (2.6 nm in diameter) that coexist with a small amount of oligomers. After 1 month the polyoxomolibdate specie self-assembles in a supramolecular structure with a polydisperse distribution of dimensions spanning from the monomer to the “blackberry” vesicular structure already reported in literature.

Thermal Motion in Water + Electrolyte Solutions According to Quasi-Elastic Incoherent Neutron Scattering Data

The main attention of this article is focused on the study of the physical mechanisms of thermal motion in water and water + electrolyte solutions that lead to the broadening of the incoherent neutron scattering peak. It is taken into account that the neutron peak has a diffusion nature and is described by a Lorentzian line shape only for wave vectors k having magnitudes |k| ≡ k ≪ 1/a, where a is the interparticle spacing. A modified version of the theory developed by Singwi and Sjolander (Phys. Rev. 1960, 119, 863) for the description of the Lorentzian half-width is proposed. It is shown that for k > 1/a, the neutron peak is described by a Gaussian line shape whose half-width is proportional to the average thermal velocity of the Lagrange particles. The relevant theoretical parameters can be determined by fitting experimental data for the half-width of the neutron peak. In such a way, the self-diffusion coefficients of water molecules, their collective parts, and the residence times as well as the radii ...

Incoherent Quasielastic Neutron Scattering study of hydrogen diffusion in thorium–zirconium hydrides

Monophase thorium–zirconium hydrides (ThZr 2H x ) have been fabricated starting from a metallic alloy and the hydrogen stoichiometry determined by X-ray diffraction. Incoherent Quasielastic Neutron Scattering (IQNS) on the hydrides was conducted over the temperature range 650–750 K at the Backscattering Silicon Spectrometer (BASIS) at the Spallation Neutron Source (SNS) at ORNL. The isotropic Chudley–Elliott model was utilized to analyze the quasielastic linewidth broadening data as function of momentum transfer. The diffusion coefficient and average jump distance of hydrogen atoms in ThZr 2H 5.6 and ThZr 2H 6.2 were extracted from the measurements.

Investigation of the Relationship between Hydrogen Bonds and Macroscopic Properties in Hybrid Core−Shell γ-Fe2O3−P(NIPAM-AAS) Microgels

We investigate in a hybrid material the interactions existing between magnetic nanoparticles of gamma-Fe(2)O(3) and the polymer matrix constituted by core-shell poly(N-isopropylacrylamide-sodium acrylate) microgels. These interactions provoke the shifting of the microgel volume phase transition to higher temperatures when the amount of gamma-Fe(2)O(3) increases. The study was performed using different techniques such as incoherent quasi-elastic neutron scattering (IQNS), infrared spectroscopy (FTIR-ATR), and dynamic light scattering (DLS). Below the low critical solution temperature (LCST) of the polymer, the IQNS data confirm that the presence of inorganic nanoparticles affects the PNIPAM chain motions. Thus, in the swollen state both the mean-square displacement of the polymer segments and the diffusive motion of the polymer chains decrease as the iron oxide content increases. The FTIR-ATR study indicates that the reduction of vibrational and diffusional motions of the polymer chains is due to the formation of hydrogen bonds between the amide groups of the polymer matrix and the OH groups of the magnetic nanoparticles. The creation of this hybrid complex would explain the reduction of the swelling capacity with increasing the iron content in the microgels. Furthermore, this interaction could also explain the shift of the polymer LCST to higher temperatures as due to the extra energy required by the system to break the hydrogen bonds prior to the PNIPAM collapse.

Anomalous Proton Dynamics in Ice at Low Temperatures

We present incoherent quasielastic neutron scattering measurements on ice Ih (ordinary ice) and Ic (cubic ice) which show the existence of nonharmonic motion of hydrogen at low temperatures, down to 5 K. We show that this dynamics is localized, nonvibrational, and related to the hydrogen disorder since it is absent in ordered ice VIII. A main jump distance of 0.75 A is identified, hence close to the distance between the two possible proton sites along the oxygen-oxygen bond. The dynamics is non-Arrhenius, has a large time rate of 2.7x10(11) s-1, and affects only a few percent of the total number of hydrogen atoms in the crystal. These results give evidence for the existence of concerted proton tunneling in these ice phases.

Protein dynamics investigated by neutron scattering

This contribution describes incoherent quasielastic neutron scattering (QENS) as a suitable tool for investigations of protein dynamics with special emphasis on applications in photosynthesis research. QENS characterizes protein dynamics via the measurement of energy and momentum exchange between sample system and incident low-energy neutrons (1 meV<E<20 meV). This method is especially sensitive for picosecond motions of hydrogen atoms because it makes use of the exceptionally large incoherent neutron scattering cross section of protons and their almost homogeneous distribution in proteins. After a short introduction into the basic principles of neutron scattering, a more detailed description of QENS will be presented including a short overview on instrumentation and theory. Recent QENS results will be discussed for the antenna complex LHC II and PS II membrane fragments. It is shown that diffusive protein dynamics is indispensable for enabling Q(A)(-·) reoxidation by Q(B) at temperatures above 240 K, which explains the strong dependence of this electron transfer step on temperature and hydration level of the sample. Finally, a new laser-QENS pump-probe technique will be introduced which permits in situ monitoring of protein dynamics correlated with a change of the functional state of the sample, i.e. a direct observation of structure-dynamics-function relationships in real time.

Quasi-Elastic Neutron Scattering Study on Water and Polymer Dynamics in Thermo/Pressure Sensitive Polymer Solutions

Dynamics of water and poly(N-isopropylacrylamide) (PNIPA) in concentrated aqueous solutions, where the majority of water molecules are attached to polymer chains, has been investigated with use of incoherent quasi-elastic neutron scattering (QENS) and dynamic light scattering (DLS) measurements as functions of temperature, T, and hydrostatic pressure, P. It was observed by QENS that the self-diffusion coefficient, D(water), of water in PNIPA/H(2)O solutions increased by P at temperatures below the lower critical solution temperature (LCST) of PNIPA aqueous solutions. However, above the LCST, D(water) decreased by P, as is often reported in non-hydrogen bonding solutions. In isobaric heating runs, therefore, the jump in D(water) at LCST decreased with increasing pressure. On the other hand, the mean-square displacement, <u(2)>, of the local vibrational motion of PNIPA in PNIPA/D(2)O solutions, where the incoherent scattering signal of PNIPA was predominantly observed, was reduced due to the aggregation behavior of PNIPA by pressurizing, which was also confirmed by using DLS. The jump in <u(2)> at the LCST became gradual by pressurizing, which was consistent with the changes of the dynamics of water obtained in PNIPA/H(2)O solutions.

Molecular dynamics of glycerol and glycerol-trehalose bioprotectant solutions nanoconfined in porous silicon

Glycerol and trehalose-glycerol binary solutions are glass-forming liquids with remarkable bioprotectant properties. Incoherent quasielastic neutron scattering is used to reveal the different effects of nanoconfinement and addition of trehalose on the molecular dynamics in the normal liquid and supercooled liquid phases, on a nanosecond time scale. Confinement has been realized in straight channels of diameter D=8 nm formed by porous silicon. It leads to a faster and more inhomogeneous relaxation dynamics deep in the liquid phase. This confinement effect remains at lower temperature where it affects the glassy dynamics. The glass transitions of the confined systems are shifted to low temperature with respect to the bulk ones. Adding trehalose tends to slow down the overall glassy dynamics and increases the nonexponential character of the structural relaxation. Unprecedented results are obtained for the binary bioprotectant solution, which exhibits an extremely non-Debye relaxation dynamics as a result of the combination of the effects of confinement and mixing of two constituents.

To What Extent Is Water Responsible for the Maintenance of the Life for Warm-Blooded Organisms?

In this work, attention is mainly focused on those properties of water which are essentially changed in the physiological temperature range of warm-blooded organisms. Studying in detail the half-width of the diffusion peak in the quasi-elastic incoherent neutron scattering, the behavior of the entropy and the kinematic shear viscosity, it is shown that the character of the translational and rotational thermal motions in water radically change near TH ~ 315 K, which can be interpreted as the temperature of the smeared dynamic phase transition. These results for bulk pure water are completed by the analysis of the isothermic compressibility and the NMR-spectra for water-glycerol solutions. It was noted that the non-monotone temperature dependence of the isothermic compressibility (βT) takes also place for the water-glycerol solutions until the concentration of glycerol does not exceed 30 mol%. At that, the minimum of βT shifts at left when the concentration increases. All these facts give us some reasons to assume that the properties of the intracellular and extracellular fluids are close to ones for pure water. Namely therefore, we suppose that the upper temperature limit for the life of warm-blooded organisms [TD = (315 ± 3) K] is tightly connected with the temperature of the dynamic phase transition in water. This supposition is equivalent to the assertion that the denaturation of proteins at T ≥ TH is mainly provoked by the rebuilding of the H-bond network in the intracellular and extracellular fluids, which takes place at T ≥ TH. A question why the heavy water cannot be a matrix for the intracellular and extracellular fluids is considered. The lower physiological pH limit for the life of warm-blooded organisms is discussed.

Observation of high-temperature dynamic crossover in protein hydration water and its relation to reversible denaturation of lysozyme

The diffusive dynamics of hydration water in lysozyme is studied by high-resolution incoherent quasielastic neutron scattering spectroscopy and molecular dynamics (MD) simulations in a temperature range of 290 K<T<380 K. The hydration level of the protein powder sample is kept at h=0.35 gram of water per gram of dry protein to provide monolayer of water coverage on the protein surfaces. Two lysozyme samples, the H(2)O hydrated and the D(2)O hydrated, are measured in the experiments. The difference spectra of the two are used to extract the diffusive dynamics of the hydration water. The self-diffusion constant D of the hydration water is obtained from the analyses of the low-Q spectra. The Arrhenius plot of the inverse diffusion constant [i.e., log(1/D) versus 1/T] shows a dynamic crossover from a super-Arrhenius behavior at low temperatures to an Arrhenius behavior at high temperatures bordered at T(D)=345+/-5 K. We also observe a pronounced increase in the migration distance d of the hydration water molecules above T(D). We present evidence from the neutron scattering experiment that this dynamic crossover temperature in the hydration water coincides with that of the reversible denaturation of lysozyme determined by specific heat measurements. We further performed MD simulations of hydrated lysozyme powder to offer a plausible reason for this coincidence of the crossover phenomenon with the reversible denaturation of the protein.

Preferential Adsorption from Binary Mixtures on Graphite: The n-Decane−n-Heptan-1-ol System

The competitive adsorption of n-decane and n-heptan-1-ol adsorbed from the binary liquid mixture onto graphite has been studied using differential scanning calorimetry, incoherent quasielastic neutron scattering, and 1H and 2H nuclear magnetic resonance. A solid monolayer is identified at all bulk solution compositions with a melting temperature that varies with bulk composition in a manner resembling the bulk behavior. Incoherent elastic neutron scattering, IQNS, and nuclear magnetic resonance, NMR, data indicate that decane is preferentially adsorbed onto the surface over most of the composition range, heptanol being the principal surface component only at very high heptanol concentrations. NMR is proved, for the first time, to be an efficient tool to provide independent information on each component of the system.

Dynamics of hydration water in proteins

Thermodynamic and transport properties of liquid water are determined essentially by inter-molecular hydrogen bonds and their dynamics. Because the molecular dynamics depends mostly on geometric constrains and dynamics of hydrogen bonds, it is shown that a simple statistics on the number of bonds, including at the vicinity of hydrophilic substrates describes well the temperature dependence of water diffusion on the surface of a small peptide. Experiments were performed by incoherent quasi-elastic neutron scattering giving information about the hydrogen bond lifetime and the residence time of the water molecules. In order to study the interactions between water molecules and a small hydrophobic peptide (an assembly of five monomers of alanine), the hydration level is changed step by step. The only motion of the first single water molecule of the structure is due to hydrogen libration. The first two added water molecules bind with the hydrophilic site of the peptide and have only the dynamics of the bond breaking. The other hydration molecules show fast diffusive motions on the surface of the peptide.

Cytoplasmic Water and Hydration Layer Dynamics in Human Red Blood Cells

The dynamics of water in human red blood cells was measured with quasielastic incoherent neutron scattering in the temperature range between 290 and 320 K. Neutron spectrometers with time resolutions of 40, 13, and 7 ps were combined to cover time scales of bulk water dynamics to reduced mobility interfacial water motions. A major fraction of approximately 90% of cell water is characterized by a translational diffusion coefficient similar to bulk water. A minor fraction of approximately 10% of cellular water exhibits reduced dynamics. This slow water fraction was attributed to dynamically bound water on the surface of hemoglobin which accounts for approximately half of the hydration layer.

Dynamic Properties of Solid Ammonium Cyanate

Ammonium cyanate is well-known to undergo a solid-state reaction to form urea. Knowledge of fundamental physicochemical properties of solid ammonium cyanate is a prerequisite for understanding this solid-state chemical transformation, and this paper presents a comprehensive study of the dynamic properties of the ammonium cation in this material. The techniques used—incoherent quasielastic neutron scattering (QENS) and solid-state 2H NMR spectroscopy—provide insights into dynamic properties across a complementary range of time scales. The QENS investigations (carried out on a sample with natural isotopic abundances) employed two different spectrometers, allowing different experimental resolutions to be probed. The 2H NMR experiments (carried out on the deuterated material ND4+OCN−) involved both 2H NMR line shape analysis and 2H NMR spin−lattice relaxation time measurements. The results of both the QENS and 2H NMR studies demonstrate that the ammonium cation exhibits reorientational dynamics across a wide ...

Solvent and lipid dynamics of hydrated lipid bilayers by incoherent quasielastic neutron scattering

The microscopic dynamics of the planar, multilamellar lipid bilayer system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) has been investigated using quasielastic neutron scattering. The DMPC was hydrated to a level corresponding to approximately nine water molecules per lipid molecule. Selective deuteration has been used to separately extract the dynamics of the water, the acyl chains, and the polar head groups from the strong incoherent scattering of the remaining hydrogen atoms. Furthermore, the motions parallel and perpendicular to the bilayers were probed by using two different sample orientations relative to the incident neutron beam. For both sample orientations, the results showed an onset of water motions at 260 K on the experimental time scale of about 100 ps. From lack of wave-vector dependence of the onset temperature for water motions, it is evident that the observed water dynamics is of mainly rotational character at such low temperatures. At 290 K, i.e., slightly below the gel-to-liquid transition around 295 K, the nature of the water dynamics had changed to a more translational character, well described by a jump-diffusion model. On the limited experimental time and length (about 10 A) scales, this jump-diffusion process was isotropic, despite the very anisotropic system. The acyl chains exhibited a weak onset of anharmonic motions already at 120 K, probably due to conformational changes (trans-gauche and/or syn-anti) in the plane of the lipid bilayers. Other anharmonic motions were not observed on the experimental time scale until temperature had been reached above the gel-to-liquid transition around 295 K, where the acyl chains start to show more substantial motions.

Polymer Chain Dynamics of Core−Shell Thermosensitive Microgels

Microgels of poly(N-isopropylacrylamide) (PNIPAM) have the ability to change size in response to temperature. PNIPAM microgels with 0.25 wt % cross-linker content were used to investigate the molecular dynamics of the polymer chains in the swollen and collapsed states. The study was performed using incoherent elastic (IES) and quasielastic neutron scattering (IQNS), and pulsed field gradient NMR spectroscopy (PFG−NMR). From IES the volume transition is characterized by a sharp increase of the elastic intensity at the transition temperature. Using IQNS, a diffusive motion of the polymer chains was identified with self-diffusion coefficient D = 1.1 ± 0.2 × 10−11 m2/s at 290 K which decreases down to D = 8.6 ± 0.1 × 10−13 m2/s when the microgel deswells at 327 K. With PFG−NMR spectroscopy we measured two diffusion coefficients in the swollen state that are associated with regions of high and low cross-linking content in the microgel. IQNS and PFG−NMR spectroscopy measurements demonstrate that in the swollen state the polymer is behaving as if it is in solution whereas in the collapsed state it resembles as a solid material.

Novel PVA-Based Hydrogel Microparticles for Doxorubicin Delivery

Micro- and nanoparticles are considered suitable drug delivery systems for their unique features, such as a large surface to volume ratio, and for the possibility to tune their size and hydrophobicity. A polymer/polymer/water emulsion method was used for producing a chemically cross-linked hydrogel made of poly(vinyl alcohol) and of poly(methacrylate) moieties. Mesoscopic investigation of the microparticles was accomplished by laser scanning confocal microscopy. Dynamics of confined water within the gel meshes was studied by quasi-elastic incoherent neutron scattering. Succinoylation of these particles allowed an efficient loading with a maximum doxorubicin payload of about 50% (w/w) of dry microparticles. To evaluate the potentials of such a microdevice for drug delivery, LoVo colon cancer cells have been exposed to doxorubicin loaded microparticles to study the in vitro efficiency of the payload release and the consequent cytotoxic effect.

Dynamics of functionalized single wall carbon nanotubes in solution studied by incoherentneutron scattering experiments

We have studied, by incoherent neutron scattering experiments, the dynamics of acolloidal suspension of functionalized single wall carbon nanotubes (SWNTs).The nanotubes have been functionalized with pentyl ester groups attached atthe ends and suspended in deuterated toluene with a concentration of 2.6 mgSWNT/1 ml of deuterated toluene. The experimental techniques were incoherent elastic neutron scattering(IENS) and incoherent quasielastic neutron scattering (IQNS). In the temperature rangebetween 4 K and 300 K, three phases were observed by IENS measurements: a solid phase forT<Tg, an undercooledliquid phase for Tg<T<Tm anda liquid phase for T>Tm. Furthermore, in the high temperature range of the undercooled liquidphase, hysteresis loops in the heating and cooling scans were observed. Thelower limit of the hysteresis loop defines the critical crossover temperatureTc. IQNS measurements in the liquid phase and a cooling scan of the undercooledliquid phase were performed. Three different quasielastic peaks were identified,two in the liquid phase and another one in the undercooled liquid phase. Thewidths of the quasielastic peaks are discussed as a generalized diffusion functionwhich can be factorized as a temperature dependent diffusion function and aQ dependent structure function. From the comparison of the diffusion function with theviscosity of toluene, we conclude that two components are in the long-time rangeBrownian motion and the other one in the short-time range Brownian motion.

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.