Ammonia Storage in Hydrogen Bond-Rich Microporous Polymers.

The fascinating structural flexibility of porous polymers is highly attractive because it can result in optimized materials with specific host-guest interactions. Nevertheless, the fundamental mechanisms responsible for controlling the weak interactions of these hydrogen bond-rich networks-essential for developing smart task-specific materials used in recognition, capture, and sequestration processes-remain unexplored. Herein, by systematically comparing performance changes between poly(amic acid) (PAA)- and polycyclic imide (PI)-based porous polymers before and after NH3 adsorption, the role of hydrogen bonds in conformational lability and responsiveness toward guest molecules is highlighted. By combining thermal gravimetric analysis with neutron spectroscopy supported by DFT calculations, we demonstrate that PAA's chemical and physical stability is enhanced by the presence of stronger host-guest interactions. This observation also emphasizes the idea that efficient adsorption relies on having a high number of sites, upon which gas molecules can adsorb with greater affinity via strong hydrogen bonding interactions.

Gapless Quantum Spin Liquid in the Triangular System Sr_{3}CuSb_{2}O_{9}.

We report gapless quantum spin liquid behavior in the layered triangular Sr_{3}CuSb_{2}O_{9} system. X-ray diffraction shows superlattice reflections associated with atomic site ordering into triangular Cu planes well separated by Sb planes. Muon spin relaxation measurements show that the S=1/2 moments at the magnetically active Cu sites remain dynamic down to 65 mK in spite of a large antiferromagnetic exchange scale evidenced by a large Curie-Weiss temperature θ_{CW}≃-143 K as extracted from the bulk susceptibility. Specific heat measurements also show no sign of long-range order down to 0.35 K. The magnetic specific heat (C_{m}) below 5 K reveals a C_{m}=γT+αT^{2} behavior. The significant T^{2} contribution to the magnetic specific heat invites a phenomenology in terms of the so-called Dirac spinon excitations with a linear dispersion. From the low-T specific heat data, we estimate the dominant exchange scale to be ∼36 K using a Dirac spin liquid ansatz which is not far from the values inferred from microscopic density functional theory calculations (∼45 K) as well as high-temperature susceptibility analysis (∼70 K). The linear specific heat coefficient is about 18 mJ/mol K^{2} which is somewhat larger than for typical Fermi liquids.

Materialization of a Geometrically Frustrated Magnet in a Hybrid Coordination Framework: A Study of the Iron(II) Oxalate Fluoride Framework, KFe(C2O4)F.

Here we discuss magnetic hybrid coordination frameworks in relation to the realization of new geometrically frustrated magnets. In particular, we present the nuclear and magnetic structures of one such system-the Fe2+-based oxalate fluoride framework KFe(C2O4)F-through analysis of the powder neutron diffraction and muon spectroscopy data. KFe(C2O4)F retains an orthorhombic Cmc21 structure upon cooling to 2 K composed of quasi-one-dimensional iron fluoride chains connected to a distorted triangular network via oxalate anions. Previous magnetometry measurements of KFe(C2O4)F indicate that it is a strongly interacting system with a Weiss constant θ ≈ -300 K that undergoes a magnetic ordering transition at TN ≈ 20 K, yielding a frustration index, f = |θ|/TN ≈ 15, reflective of high-spin frustration. We determine the nature of this frustrated antiferromagnetic ordering below TN and show that the resulting magnetic structure is best described by a model in the Cmc'21' magnetic space group.

Nanoscale Mobility of Aqueous Polyacrylic Acid in Dental Restorative Cements.

Hydrogen dynamics in a time range from hundreds of femtoseconds to nanoseconds can be directly analyzed using neutron spectroscopy, where information on the inelastic and quasi-elastic scattering, hereafter INS and QENS, can be obtained. In this study, we applied these techniques to understand how the nanoscale mobility of the aqueous solution of polyacrylic acid (PAA) used in conventional glass ionomer cements (GICs) changes under confinement. Combining the spectroscopic analysis with calorimetric results, we were able to separate distinct motions within both the liquid and the GICs. The QENS analysis revealed that the self-diffusion translational motion identified in the liquid is also visible in the GIC. However, as a result of the formation of the cement matrix and its setting, both translational diffusion and residence time differed from the PAA solution. When comparing the local diffusion obtained for the selected GIC, the only noticeable difference was observed for the slow dynamics associated with the polymer chain. Additionally, over short-term aging, progressive water binding to the polymer chain occurred in one of the investigated GICs. Finally, a considerable change in the density of the GIC without progressive water binding indicates an increased polymer cross-linking. Taken together, our results suggest that accurate and deep understanding of polymer-water binding, polymer cross-linking, as well as material density changes occurring during the maturation process of GIC are necessary for the development of advanced dental restorative materials.

Correction: Spectroscopic characteristics of the OSIRIS near-backscattering crystal analyser spectrometer on the ISIS pulsed neutron source.

Correction for 'Spectroscopic characteristics of the OSIRIS near-backscattering crystal analyser spectrometer on the ISIS pulsed neutron source' by Mark T. F. Telling et al., Phys. Chem. Chem. Phys., 2005, 7, 1255-1261.

How mobile are protons in the structure of dental glass ionomer cements?

The development of dental materials with improved properties and increased longevity can save costs and minimize discomfort for patients. Due to their good biocompatibility, glass ionomer cements are an interesting restorative option. However, these cements have limited mechanical strength to survive in the challenging oral environment. Therefore, a better understanding of the structure and hydration process of these cements can bring the necessary understanding to further developments. Neutrons and X-rays have been used to investigate the highly complex pore structure, as well as to assess the hydrogen mobility within these cements. Our findings suggest that the lower mechanical strength in glass ionomer cements results not only from the presence of pores, but also from the increased hydrogen mobility within the material. The relationship between microstructure, hydrogen mobility and strength brings insights into the material's durability, also demonstrating the need and opening the possibility for further research in these dental cements.

Rapid precipitation: an alternative to solvent casting for organic solar cells.

Rapid precipitation, immersion of a liquid formulation into a nonsolvent, is compared with drop casting for fabricating organic solar cells. Blends comprising poly-3-hexylthiophene (P3HT), phenyl-C61-butyric acid methyl ester (PCBM), and chlorobenzene were processed into bulk samples by using two distinct routes: rapid precipitation and drop casting. The resulting structure, phases, and crystallinity were analyzed by using small-angle neutron scattering, X-ray diffraction, differential scanning calorimetry, and muon spin resonance. Rapid precipitation was found to induce a finely structured phase separation between PCBM and P3HT, with 65 wt % crystallinity in the P3HT phase. In contrast, solvent casting resulted in a mixed PCBM/P3HT phase with only 43 wt % P3HT crystallinity. The structural advantages conferred by rapid precipitation were shown to persist following intense thermal treatments.

Nano-scale hydrogen-bond network improves the durability of greener cements.

More than ever before, the world's increasing need for new infrastructure demands the construction of efficient, sustainable and durable buildings, requiring minimal climate-changing gas-generation in their production. Maintenance-free "greener" building materials made from blended cements have advantages over ordinary Portland cements, as they are cheaper, generate less carbon dioxide and are more durable. The key for the improved performance of blends (which substitute fine amorphous silicates for cement) is related to their resistance to water penetration. The mechanism of this water resistance is of great environmental and economical impact but is not yet understood due to the complexity of the cement's hydration reactions. Using neutron spectroscopy, we studied a blend where cement was replaced by ash from sugar cane residuals originating from agricultural waste. Our findings demonstrate that the development of a distinctive hydrogen bond network at the nano-scale is the key to the performance of these greener materials.

Antiferromagnetic ordering through a hydrogen-bonded network in the molecular solid CuF2(H2O)2(3-chloropyridine).

CuF(2)(H(2)O)(2)(3-chloropyridine) possesses a five-coordinate Cu(2+) center with a slightly distorted trigonal bypyramidal coordination geometry. Strong intermolecular F···H-O hydrogen bonds enable the formation of 2D layers and provide the primary magnetic exchange path that leads to the stabilization of long-range antiferromagnetic (AFM) order below T(N) = 2.1 K.

In situ powder neutron diffraction study of non-stoichiometric phase formation during the hydrogenation of Li3N.

The hydrogenation of Li3N at low chemical potential has been studied in situ by time-of-flight powder neutron diffraction and the formation of a non-stoichiometric Li4-2xNH phase and Li4NH observed. The results are interpreted in terms of a model for the reaction pathway involving the production of Li4NH and Li2NH, which subsequently react together to form Li4-2xNH. Possible mechanisms for the production of Li4NH from the hydrogenation of Li3N are discussed.

Structure and dynamics of a thermoresponsive microgel around its volume phase transition temperature.

Sustained drug delivery requires the use of multifunctional devices with enhanced properties. These properties include responsiveness to external stimuli (such as temperature, pH, ionic strength), ability to deliver suitably designed ligands to specific receptors, enhanced bioadhesion to cells, and cytocompatibility. Microgels represent one of such multifunctional drug delivery devices. Recently, we described the fabrication of a stable colloidal aqueous suspension of cytocompatible microgel spheres based on a poly(vinyl alcohol)/poly(methacrylate-co-N-isopropylacrylamide) network ( Ghugare, S. Mozetic, P. Paradossi, G. Biomacromolecules 2009 , 10 , 1589 ). These microgel spheres undergo an entropy-driven volume phase transition around the physiological temperature, this phase transition being driven by the incorporation of NiPAAm residues in the network. In that study, the microgel was loaded with the anticancer drug doxorubicin. As the microgel shrank, a marked increase in the amount of doxorubicin released was noted. Indeed, dynamic light scattering measurements showed the diameter reduction to be about 50%. In the present paper, we focus on some fundamental issues regarding modifications of the hydrogel architecture at a nanoscopic level as well as of the diffusive behavior of water associated with the polymer network around the volume phase transition temperature (VPTT). Sieving and size exclusion effects were studied by laser scanning confocal microscopy with the microgel exposed to fluorescent probes with different molecular weights. Confocal microscopy observations at room temperature and at 40 degrees C (i.e., below and above the VPTT) provided an evaluation of the variation of the average pore size (from 5 nm to less than 3 nm). Using quasielastic neutron scattering (QENS) with the IRIS spectrometer at ISIS, UK, the diffusive behavior of water molecules closely associated to the polymer network around the VPTT was investigated. A clear change in the values of diffusion coefficient of bound water was observed at the transition temperature. In addition, the local dynamics of the polymer itself was probed using the QENS spectrometer SPHERES at FRM II, Germany. For this study, the microgel was swollen in D(2)O. An average characteristic distance of about 5 A for the localized chain motions was evaluated from the elastic incoherent structure factor (EISF) and from the Q-dependence of the Lorentzian width.

Investigation of the behavior of ethylene molecular films using high resolution adsorption isotherms and neutron scattering.

The wetting behavior of ethylene adsorbed on MgO(100) was investigated from 83-135 K using high resolution volumetric adsorption isotherms. The results are compared to ethylene adsorption on graphite, a prototype adsorption system, in an effort to gain further insight into the forces that drive the observed film growth. Layering transitions for ethylene on MgO(100) are observed below the bulk triple point of ethylene (T = 104.0 K). The formation of three discrete adlayers is observed on the MgO(100) surface; onset of the second and third layers occurs at 79.2 +/- 1.3 K and 98.3 +/- 0.9 K, respectively. Thermodynamic quantities such as differential enthalpy and entropy, heat of adsorption, and isosteric heat of adsorption are determined and compared to the previously published values for ethylene on graphite. The average area occupied by a ethylene molecule on MgO(100) is 22.6 +/- 1.1 A2 molecule(-1). The locations of two phase transitions are identified (i.e., layer critical temperatures at T(c2)(n=1) at 108.6 +/- 1.7 K and T(c2)(n=2) at 116.5 +/- 1.2 K) and a phase diagram is proposed. Preliminary neutron diffraction measurements reveal evidence of a monolayer solid with a lattice constant of approximately 4.2 A. High resolution INS measurements show that the onset to dynamical motion and monolayer melting take place at approximately 35 K and approximately 65 K, respectively. The data reported here exhibit a striking similarity to ethylene on graphite which suggests that molecule-molecule interactions play an important role in determining the physical properties and growth of molecularly thin ethylene films.

Pressure-dependent deuterium reaction pathways in the Li-N-D system.

Neutron diffraction data from in situ deuteration and dedeuteration of Li(3)N are presented under different pressure regimes, whereby reaction pathways differing from the widely reported stoichiometric pathway of Li(3)N + 2D(2)<--> Li(2)ND + LiD + D(2)<--> LiND(2) + 2LiD are observed. At sufficiently high pressures, where the deuterium chemical potential is comparable with the heat of amide formation, the reaction appears to be driven straight to the amide plus deuteride phase mixture. At lower pressures, a cubic phase exhibiting a concentration-dependent variation in lattice parameter is observed. In dedeuteration, two sets of reflections from cubic structures with distinct lattice parameters are observed, both of which exhibit a continual decrease in cell volume. The reaction pathways are discussed in terms of the compositional variation.

Rotation dynamics of 2-methyl butane and n-pentane in MCM-22 zeolite: a molecular dynamics simulation study.

Molecular dynamics (MD) simulations have been performed to investigate the rotational diffusive motion for 2-methyl butane (2-MeC4) and n-pentane (n-C5) in the cages of MCM-22 zeolite from 200-340 K. The rotation intermediate scattering function I(R) (Q,t) for 2-MeC4 and n-C5 confined in the supercages of MCM-22 was calculated. The dynamic scattering functions S(inc)(Q,omega) obtained via Fourier transformation of this intermediate scattering function are in good agreement with those of quasi-elastic neutron scattering experiments. The rotational intermediate scattering functions of 2-MeC4 decay more rapidly than those of n-C5 before 1.0 ps. In the longer time region, the rotational intermediate scattering functions of both molecules intersect. It is shown that n-C5 molecules move more easily from supercage to supercage compared to 2-MeC4, rotating for a longer time in the confinement of the MCM-22 supercage.

Stabilization effects of kosmotrope systems on ornithine carbamoyltransferase.

In the present article the influence of salts and additives, such as trehalose, NaCl, ornithine, sodium phosphate and ammonium sulphate, on ornithine carbamoyltransferase (OCTase) is investigated in order to study the OCTase stabilization process as a function of solutes and to point out the fundamental role played by an enhancement of hydrophobic interactions. The synergic use of different techniques, such as neutron spectroscopy, UV-vis spectroscopy, activity and thermal measurements, allows to highlight the cosolute capability to avoid thermal inactivation, to induce important changes in secondary and tertiary enzyme structure and to stabilize biological macromolecules.

Anharmonic behavior in the multisubunit protein apoferritin as revealed by quasi-elastic neutron scattering.

Quasi-elastic neutron scattering (QENS) has been used to study the deviation from Debye-law harmonic behavior in lyophilized and hydrated apoferritin, a naturally occurring, multisubunit protein. Whereas analysis of the measured mean squared displacement (msd) parameter reveals a hydration-dependent inflection above 240 K, characteristic of diffusive motion, a hydration-independent inflection is observed at 100 K. The mechanism responsible for this low-temperature anharmonic response is further investigated, via analysis of the elastic incoherent neutron scattering intensity, by applying models developed to describe side-group motion in glassy polymers. Our results suggest that the deviation from harmonic behavior is due to the onset of methyl group rotations which exhibit a broad distribution of activated processes ( E a,ave = 12.2 kJ.mol (-1), sigma = 5.0 kJ x mol (-1)). Our results are likened to those reported for other proteins.

Thermodynamic investigation of n-hexane thin films adsorbed on magnesium oxide.

The thermodynamic properties of n-hexane adsorption on MgO(100) were determined using high-resolution volumetric adsorption isotherms in the temperature range 195-255 K. Two distinct layering transitions are observed in the isotherms. The isotherms are used to calculate the two-dimensional compressibility, the differential enthalpy and entropy, the heat of adsorption, and the isosteric heat of adsorption. Neutron Diffraction is used to identify where melting of the n-hexane monolayer takes place.

Stochastic molecular motions in the nematic, smectic-A, and solid phases of p,p'-di-n-heptyl-azoxybenzene as seen by quasielastic neutron scattering and 13C cross-polarization magic-angle-spinning NMR.

Molecular rotational dynamics in p,p'-di-n-heptyl-azoxybenzene was studied by means of quasielastic neutron scattering (QENS) and 13C cross-polarization magic-angle-spinning (CPMAS) NMR. Fast reorientation of the hydrogen nuclei was observed by QENS in the two liquid crystalline (LC) phases nematic and smectic A, as well as in the crystalline phase. The latter could not be restricted to the -CH3 rotations alone, and a clear indication was found of some other reorientation motions persisting in the crystal. Two Lorentz-type components convoluted with the resolution function gave an excellent fit to the QENS spectra in both LC phases. The narrow (slow) component was attributed to the reorientation of the whole molecule around the long axis. The corresponding characteristic time of approximately 130 ps agreed well with the values obtained in recent dielectric relaxation and 2H NMR studies. The full width at half maximum of the broader (fast) component shows a quadratic Q dependence (Q is the momentum transfer). Hence the corresponding motions could be described by a stretched exponential correlation function and were interpreted as various "crankshaft-type" motions within the alkyl tails. The 13C CPMAS experiments fully corroborated the QENS results, sometimes considered ambiguous in complex systems.

Structure determination and phase transition behaviour of dimethyl sulfate.

The crystal structures of phase I and phase II of dimethyl sulfate, (CH3O)2SO2, have been determined using complementary high-resolution neutron powder and single-crystal X-ray diffraction techniques. Below its melting point of 241 K dimethyl sulfate crystallizes in an orthorhombic structure (I) in the space group Fdd2. On cooling below approximately 175 K the crystal transforms to a monoclinic structure (II) in the space group I2/a. The molecule is located on a twofold axis (Z' = 1/2) in both structures. The phase transition is of first order with strong hysteresis. The phase transition results in changes to both the intra- and the intermolecular coordination environment.

Alpha,alpha-trehalose-water solutions. VIII. Study of the diffusive dynamics of water by high-resolution quasi elastic neutron scattering.

The present paper shows high-resolution quasi-elastic neutron scattering (QENS) findings on homologues disaccharides (i.e. trehalose, maltose, and sucrose)-water mixtures as a function of temperature. The QENS measurements were performed on both partially deuterated disaccharides in D2O and on hydrogenated disaccharides in H2O to separate the solute dynamics from that of the solvent. The results highlight a noticeable disaccharide kosmotrope character, with results more marked for trehalose. Such evidence accounts for its higher bioprotective effectiveness.