Synchrotron-radiated X-ray and neutron diffraction study of native cellulose

Precise determination of d-spacings and compositional ratio of cellulose I_α and I_β in various native cellulose samples was successfully carried out by synchrotron-radiated X-ray diffraction and time-of-flight (TOF) neutron diffraction from quasi-powder specimens. X-ray diffraction peaks were separated by the deconvolution method using six types of profile function: Gaussian, Lorentzian, intermediate Lorentzian, modified Lorentzian, pseudo-Voigt, and Pearson VII. In terms of R-factors, the pseudo-Voigt function gave the best fit with the observation, and was used for determination of d-spacings. The numerical results for Valonia cellulose were: dI_α (1 0 0) = 0.613 nm; dI_β (1 1 0) = 0.603 nm; dI_β (1 1 0) = 0.535 nm; dI_α (0 1 0) = 0.529 nm; I_α content = 0.65. The differences determined between dI_α (1 0 0) and dI_β (1 1 0) and between dI_β (1 1 0) and dI_α (0 1 0) were similar to those previously reported. Comparison between unresolved peaks for the two types of cellulose samples revealed a small but definite difference between dI_α (1 1 0) and dI_β (2 0 0). The TOF neutron diffractometry using deuterated samples confirmed this difference. This revised version was published online in August 2006 with corrections to the Cover Date.     

Variable temperature neutron diffraction and X-ray charge density studies of tetraacetylethane

Single crystal neutron diffraction data have been collected on a sample of enolized 3,4-diacetyl-2,5-hexanedione (tetraacetylethane, TAE) at five temperatures between 20 and 298 K to characterize the temperature-dependent behavior of the short, strong, intramolecular hydrogen bond. Upon decreasing the temperature from 298 K to 20 K, the O2-H1 distance decreases from 1.171(11) to 1.081(2) A and the O1...H1 distance increases from 1.327(10) to 1.416(6) A. The convergence of the C-O bond lengths from inequivalent distances at low temperature to identical values (1.285(4) A) at 298 K is consistent with a resonance-assisted hydrogen bond. However, a rigid bond analysis indicates that the structure at 298 K is disordered. The disorder vanishes at lower temperatures. Short intermolecular C-H...O contacts may be responsible for the ordering at low temperature. The intramolecular O...O distance (2.432 +/- 0.006 A) does not change with temperature. X-ray data at 20 K were measured to analyze the charge density and to gain additional insight into the nature of the strong hydrogen bond. Quantum mechanical calculations demonstrate that periodic boundary conditions provide significant enhancement over gas phase models in that superior agreement with the experimental structure is achieved when applying periodicity. One-dimensional potential energy calculations followed by quantum treatment of the proton reproduce the location of the proton nearer to the O2 site reasonably well, although they overestimate the O-H distance at low temperatures. The choice of the single-point energy calculation strategy for the proton potential is justified by the fact that the proton is preferably located nearer to O2 rather than being equally distant to O1 and O2 or evenly distributed (disordered) between them.     

X-ray and neutron diffraction studies on "Li4.4Sn"

A chemical analysis and detailed structural characterization, using X-ray single crystal and neutron powder diffraction, of the binary lithium-tin compound "Li(4.4)Sn" is presented. Phase analyses and subsequent structural refinements result in the reformulation of "Li(4.4)Sn" as Li(17)Sn(4). The lithium-rich binary phase crystallizes with a complex cubic structure in the space group Ffourmacr;3m, with a = 19.6907(11) A, Z = 20. The improved crystal structure determination indicates well-defined lithium atom positions, some of which differ from those previously reported. The nearly Zintl phase Li(17)Sn(4) exhibits poor metallic behavior similar to that of heavily doped semiconductors. Comparisons of the refined crystal structure with previously reported X-ray crystal structures associated with "Li(4.4)Sn" are discussed.     

X-ray and neutron diffraction study of intermediate phases in nonstoichiometric cerium dioxide

Powder samples of reduced ceria, CeO_2?x, of known compositions in the range 0 < x < 0.3 have been examined by X-ray and neutron diffraction techniques in order to determine which intermediate phases belonging to the homologous series Ce_nO_2n?2 (with n = integer) truly exist. Through the appearance of superlattice lines in the neutron diffraction patterns, the existence of four distinct phases, corresponding to n = 7, 9, 10 and 11 was established. Aside from the phase Ce₇O₁₂, the structures of these phases cannot be accounted for with rhombohedral cells based on 〈111〉 vacancy strings, but indicate lower (monoclinic or triclinic) symmetry. The structure of Ce₉O₁₆ and Ce₁₀O₁₈ do not agree with structures proposed for the analogous Pr_nO_2n?2 compounds.     

Molecular motion in crystalline naphthalene: analysis of multi-temperature X-ray and neutron diffraction data

Single crystals of h8-naphthalene have been examined by both X-ray and neutron diffraction over a range of temperatures from 5 to 295 K. The aim of this case study was to measure the anisotropic displacement parameters (ADPs) of carbons and hydrogens and to interpret them using the model of thermal motion proposed by Bürgi and Capelli (Acta Cryst. 2000, A56, 403). The traditional rigid-body analysis expresses the low-frequency motions in terms of molecular translations and librations only, whereas the Bürgi-Capelli treatment also includes the high-frequency internal modes. We show that a considerable improvement occurs by representing the internal modes by a single second-rank tensor and that a further improvement follows by including a Grüneisen parameter to account for volume thermal expansion. By applying the treatment to multi-temperature diffraction data, there is a considerable reduction in the ratio of number of adjustable parameters/number of independent observations.     

X-ray and neutron imaging with colloids

A series of technical advances are helping to revolutionise the possibilities for X-ray and neutron imaging in colloidal science. These include the development of new imaging modalities with high coherence X-rays such as diffractive imaging, ptychography, and femtosecond holography; and Talbot phase contrast tomography with conventional laboratory based low coherence X-ray sources e.g. standard rotating anodes. A crucial insight is that the available phase contrast with synthetic organic and biological colloids can be two orders of magnitude stronger than the absorption contrast with X-rays, providing large improvements in the signal to noise ratio in the resultant images. Furthermore new developments with the sources of X-rays and neutrons are helping to increase the possibilities for this research as the available coherence, flux and collimation are improved e.g. third generation high brilliance synchrotrons, free electron lasers, high flux pulsed neutron sources and table-top X-ray lasers are being developed. Highlights of the application of these techniques and sources to colloids include: the measurement of the internal strains inside individual crystalline colloidal nanoparticles, the imaging of nanoparticles embedded in opaque solid composite materials, images of defects in the growth of colloidal crystals, and the morphology of nanofoams, intact human chromosomes, protein nanocrystals, viruses, bacteria, and blood cells. The resolution of the reconstructed images can be achieved at the 10–50 nm length scale, without the need for the invasive sample preparation techniques required for transmission electron microscopy e.g. microtoming of specimens is not required. Furthermore fluorescent staining is also not required, as with super-resolution microscopies at visible optical wavelengths (e.g. STED, PALM and STORM), and thick opaque samples can be investigated, although some fragile organic and biological materials require freezing to reduce beam damage with X-rays. Neutron imaging has also benefited from the development of analogous Talbot phase contrast techniques to those possible with low coherence X-rays and a number of useful applications in non-invasive imaging at the 100 μm length scale have been demonstrated e.g. the internal structure of live plants, the inner workings of fuel cells and the three-dimensional domain structure of magnetic materials.     

Low-temperature Na4Ti5O12 from X-ray and neutron powder diffraction data

The crystal structure of the low-temperature Na₄Ti₅O₁₂ (tetra­sodium penta­titanium dodeca­oxide) phase has been solved and refined from X-ray and neutron powder diffraction data at 295 K. The structure is trigonal, space group P3, with Z = 1, although it is pseudo-centrosymmetric. The O and Na atoms form a distorted close-packed structure, where Ti atoms occupy octahedral sites.     

Crystal structures of thermoelectric n- and p-type Ba8Ga16Ge30 studied by single crystal, multitemperature, neutron diffraction, conventional X-ray diffraction and resonant synchrotron X-ray diffraction

Comprehensive single-crystal structural investigations of n- and p-type Ba8Ga16Ge30 have been carried out using multitemperature neutron and conventional X-ray diffraction as well as resonant synchrotron X-ray diffraction. The data show that the guest atom positions and dynamics are very similar in the two structures, although the barium atoms are slightly more displaced from the cage centers in the p-type structure than in the n-type structure (Deltad = 0.025 A). For both structures Fourier difference maps calculated from very high-resolution neutron diffraction data (sin theta/lambda > 2 A-1) show that the Ba nuclear density at lowest temperatures (15 K) is distributed in a torus around the crystallographic 6d site with maxima in the 24j positions. At room temperature the maxima have shifted to the 24k position. Analysis of atomic displacement parameters give Einstein temperatures of approximately 60(1) K for both structures. Thus, the fundamental difference in the low temperature thermal conductivity observed for p- and n-type Ba8Ga16Ge30 appear not to be directly related to the guest atom behavior as is commonly assumed in thermoelectric research. The neutron data and the resonant synchrotron X-ray data facilitate refinement of Ga/Ge framework occupancies. The Ga atoms have a clear preference for the 6c site with the preference being somewhat stronger for the n-type structure.     

Crystal structure and hydrogen-bonding system in cellulose Ibeta from synchrotron X-ray and neutron fiber diffraction

The crystal and molecular structure together with the hydrogen-bonding system in cellulose Ibeta has been determined using synchrotron and neutron diffraction data recorded from oriented fibrous samples prepared by aligning cellulose microcrystals from tunicin. These samples diffracted both synchrotron X-rays and neutrons to better than 1A resolution (>300 unique reflections; P2(1)). The X-ray data were used to determine the C and O atom positions. The resulting structure consisted of two parallel chains having slightly different conformations and organized in sheets packed in a "parallel-up" fashion, with all hydroxymethyl groups adopting the tg conformation. The positions of hydrogen atoms involved in hydrogen-bonding were determined from a Fourier-difference analysis using neutron diffraction data collected from hydrogenated and deuterated samples. The hydrogen atoms involved in the intramolecular O3...O5 hydrogen bonds have well-defined positions, whereas those corresponding to O2 and O6 covered a wider volume, indicative of multiple geometry with partial occupation. The observation of this disorder substantiates a recent infrared analysis and indicates that, despite their high crystallinity, crystals of cellulose Ibeta have an inherent disorganization of the intermolecular H-bond network that maintains the cellulose chains in sheets.     

The structure of calcium-ammonia solutions by neutron diffraction

The microscopic structures of calcium-ammonia solutions have been established by using neutron diffraction. Total structure factors measured at 230 K reveal immediately the evolution of an uncommonly intense diffraction prepeak in the metallic solutions. As concentration is increased from 4 mole percent metal to 10 mole percent metal (i.e., saturation), this feature intensifies and shifts from 0.6 to 0.9 A(-1). It is therefore evidence of well developed intermediate-range ordering among the solvated cations, and is a microstructural signature of the observed strong phase separation of metallic (concentrated) and nonmetallic (dilute) solutions. The technique of isotopic labelling of *N by 15N was then used in conjunction with difference analysis to focus on the solvent structure in metallic solutions at 4 and 10 mole percent metal. These nitrogen-centered functions are analyzed in conjunction with classical Monte Carlo computer simulation techniques, to provide us with detailed insight into the calcium solvation and the extent of hydrogen bonding. We find that calcium is solvated by approximately 6-7 ammonia molecules, with a Ca-N distance of around 2.45 A. There is evidence of hydrogen bonding among the solvent molecules, even in the saturated 10 mole percent metal solution.     

A structural development of an organogel explored by synchrotron time-resolved small-angle X-ray scattering

We investigated the gelation process of 12-hydroxystearic acid (12-HSA)/dodecane gel (turbid gel), 12-HSA/xylene and 12-HSA/toluene gels (transparent gels) by using time-resolved synchrotron small-angle X-ray scattering technique. The fibers were developed via nucleation and growth mechanism, and the induction period was longer at higher temperatures. After the induction period, the fiber growth can be divided into two regimes. In the first stage, the scattering intensity increased with time without appearance of any crystalline peak, and in the second stage (001) peak appeared due to crystalline nucleation. In the former regime, the fibers grew with increasing the cross-sectional size, while in the latter regime, they grew with keeping it almost constant, i.e., with keeping the fiber thickness constant. The fiber thickness for the turbid gel (radius of the gyration R _c of ～100 Å) was thicker than that for the transparent gel (R _c of ～82 Å). The fractal dimensions of the fibrillar aggregates for the turbid gels (2.0–2.3) at various temperatures were larger than those for the transparent gels (1.4–1.6), suggesting that the structure of the former gels is more branching or more compact in comparison with the latter.     

Histidine and deuterium-labelled histidine by asymmetric catalytic reduction and assignment of the absolute stereochemistry by neutron diffraction

We describe an efficient and convenient route for the preparation of enantiomerically pure d-labelled histidine by asymmetric hydrogenation with a Rh–diphosphine complex in the presence of strong non-coordinating acids. A single-crystal neutron diffraction study of the selectively dideuterated N-benzoyl-histidine methyl ester, tetrafluoroborate salt determined the absolute stereochemistry. This allowed us to clarify the role of H⁺ in the whole reduction process.     

Kinetics and mechanism of decarboxylation of aspartic acid with ninhydrin

The kinetic study of the decarboxylation of aspartic acid has been carried out at various [ninhydrin], [H⁺] and at different temperature ranging from 60–95°C. The reaction follows an irreversible first-order reaction path under pseudo first-order kinetic conditions. The variation of pseudo first-order rate constant (k_obs) with ninhydrin concentration was found to be in agreement with equation 1/k_obs = B₁ + B₂/[Ninhydrin]. One mol of carbondioxide evolved from decarboxylation of α-COOH and second mol of carbondioxide comes from the decarboxylation of β-keto acid which is an intermediate and formed during the course of ninhydrin and aspartic acid reaction. On the basis of the observed data, a possible mechanism has been proposed.     

The detailed mechanism of the Dushman reaction explored by computer

Since 1926 investigations of the Dushman reaction have relied mainly upon “constant-rate” measurements, usually by instrumental methods. Since about that time, the confusion surrounding this venerable reaction has been growing. In part, the confusion arises because the reaction involves reactive intermediate species that have not been studied directly—and may never be. Alternative detailed mechanisms have been assumed with little restraint. One of these, which is built around H₂I₂O₃, has been explored by computer with promising results. The mechanism seems capable of reducing the confusion now attending the Dushman reaction and others related to it.