Hydrophilic and hydrophobic interactions in concentrated aqueous imidazole solutions: a neutron diffraction and total X-ray scattering study.

The intermolecular interactions in concentrated (5 M) aqueous imidazole solutions have been investigated by combining neutron diffraction with isotopic substitution, total X-ray scattering and empirical potential structure refinement (EPSR) simulations using a box containing 5530 water and 500 imidazole molecules. The structural model with the best fit was used to generate radial distribution functions and spatial density functions. The local volume surrounding imidazole molecules is dominated by water, due to strong hydrogen-bonding between the nitrogen moieties of imidazole and water molecules; within a radius of 6.4 Å from the central imidazole molecule there are, on average, 17 water and only 3 imidazole molecules. Even though imidazole interacts with water it appears to disrupt hydrogen bonding in the surrounding water network only minimally. Hydrogen-bonding between imidazole molecules is negligible. The most probable positions of imidazole nearest-neighbours are above and below the plane of the aromatic ring. At low distances (up to ∼3.5-3.8 Å) these nearest neighbours were found to prefer parallel orientation of the molecular planes, indicating hydrophobic (π-π) stacking. At longer distances (up to ∼5 Å), imidazole neighbours assume both parallel and edge-to-face orientations. Overall, hydrated imidazole molecules are the most probable structural motif in aqueous solutions, with very few direct imidazole-imidazole interactions.

Structure and Dynamics of the Superprotonic Conductor Caesium Hydrogen Sulfate, CsHSO4.

We have investigated caesium hydrogen sulfate, CsHSO4, in all three of its ambient pressure phases by total scattering neutron diffraction, inelastic neutron scattering (INS) and Raman spectroscopies and periodic density functional theory calculations. Above 140 °C, CsHSO4, undergoes a phase transition to a superprotonic conductor that has potential application in intermediate temperature fuel cells. Total scattering neutron diffraction data clearly show that all the existing structures of this phase are unable to describe the local structure, because they have either partial occupancies of the atoms and/or non-physical O-H distances. Knowledge of the local structure is crucial because it is this that determines the conduction mechanism. Starting from one of the previous models, we have generated a new structure that has no partial occupancies and reasonable O-H distances. After geometry optimisation, the calculated radial distribution function is in reasonable agreement with the experimental data, as are the calculated and observed INS and Raman spectra. This work is particularly notable in that we have measured INS spectra in the O-H stretch region above room temperature, which is extremely rare. The INS spectra have the enormous advantage that the electrical anharmonicity that complicates the infrared spectra is absent and the stretch modes are plainly seen.

Dihydrogen vs. hydrogen bonding in the solvation of ammonia borane by tetrahydrofuran and liquid ammonia.

The solvation structures of two systems rich in hydrogen and dihydrogen bonding interactions have been studied in detail experimentally through neutron diffraction with hydrogen/deuterium isotopic substitution. The results were analysed by an atomistic Monte Carlo simulation employing refinement to the experimental scattering data. The systems studied were the hydrogen storage material ammonia borane (NH3BH3, AB) dissolved in tetrahydrofuran (THF), and liquid ammonia (NH3), the latter in which AB shows unusually high solubility (260 g AB per 100 g NH3) and potential regeneration properties. The full orientational and positional manner in which AB-AB, AB-THF and AB-NH3 pairs interact with each other were successfully deciphered from the wide Q-range total neutron scattering data. This provided an unprecedented level of detail into such highly (di)hydrogen bonding solute-solvent interactions. In particular this allowed insight into the way in which H-B acts as a hydrogen bond acceptor. The (di)hydrogen bonding was naturally determined to dictate the intermolecular interactions, at times negating the otherwise expected tendency for polar molecules to align themselves with anti-parallel dipole moments. Several causes for the extreme solubility of AB in ammonia were determined, including the ability of ammonia to (di)hydrogen bond to both ends of the AB molecule and the small size of the ammonia molecule relative to AB and THF. The AB B-H to ammonia H dihydrogen bond was found to dominate the intermolecular interactions, occurring almost three times more often than any other hydrogen or dihydrogen bond in the system. The favourability of this interaction was seen on the bulk scale by a large decrease in AB clustering in ammonia compared to in the dihydrogen bond-less THF.

A comparison of choline:urea and choline:oxalic acid deep eutectic solvents at 338 K.

1:2 choline chloride:urea and 1:1 choline chloride:oxalic acid deep eutectic solvents are compared at 338 K using liquid-phase neutron diffraction with H/D isotopic substitution to obtain differential neutron scattering cross sections and fitting of models to the experimental data using Empirical Potential Structure Refinement. In comparison to the previously reported study of choline chloride:urea at 303 K, we observed significant weakening and lengthening of choline-OH⋯Cl- and choline-OH⋯hydrogen-bond acceptor correlations.

Confinement of Iodine Molecules into Triple-Helical Chains within Robust Metal-Organic Frameworks.

During nuclear waste disposal process, radioactive iodine as a fission product can be released. The widespread implementation of sustainable nuclear energy thus requires the development of efficient iodine stores that have simultaneously high capacity, stability and more importantly, storage density (and hence minimized system volume). Here, we report high I2 adsorption in a series of robust porous metal-organic materials, MFM-300(M) (M = Al, Sc, Fe, In). MFM-300(Sc) exhibits fully reversible I2 uptake of 1.54 g g-1, and its structure remains completely unperturbed upon inclusion/removal of I2. Direct observation and quantification of the adsorption, binding domains and dynamics of guest I2 molecules within these hosts have been achieved using XPS, TGA-MS, high resolution synchrotron X-ray diffraction, pair distribution function analysis, Raman, terahertz and neutron spectroscopy, coupled with density functional theory modeling. These complementary techniques reveal a comprehensive understanding of the host-I2 and I2-I2 binding interactions at a molecular level. The initial binding site of I2 in MFM-300(Sc), I2I, is located near the bridging hydroxyl group of the [ScO4(OH)2] moiety [I2I···H-O = 2.263(9) Å] with an occupancy of 0.268. I2II is located interstitially between two phenyl rings of neighboring ligand molecules [I2II···phenyl ring = 3.378(9) and 4.228(5) Å]. I2II is 4.565(2) Å from the hydroxyl group with an occupancy of 0.208. Significantly, at high I2 loading an unprecedented self-aggregation of I2 molecules into triple-helical chains within the confined nanovoids has been observed at crystallographic resolution, leading to a highly efficient packing of I2 molecules with an exceptional I2 storage density of 3.08 g cm-3 in MFM-300(Sc).

Dissolved chloride markedly changes the nanostructure of the protic ionic liquids propylammonium and ethanolammonium nitrate.

The bulk nanostructure of 15 mol% propylammonium chloride (PACl) dissolved in propylammonium nitrate (PAN) and 15 mol% ethanolammonium chloride (EtACl) in ethanolammonium nitrate (EtAN) has been determined using neutron diffraction with empirical potential structure refinement fits. For both the PAN:PACl and EtAN:EtACl mixtures, data for three different scattering contrasts were simultaneously fit, and the structures determined and compared to that of the pure ionic liquids. Strong electrostatic interactions between chloride and cation charged groups, as well as the alcohol moiety of EtAN, lead to marked changes in local ion packing that alter the liquid structure. In PAN, the addition of chloride modifies but does not significantly disrupt the bicontinuous amphiphilic nanostructure of the IL. Tight packing of ammonium groups around chloride favours a gauche conformer for the cation which shrinks the apolar domains and brings the terminal methyls nearer the polar domains. The weakly-clustered nanostructure of EtAN, a consequence of the terminal hydroxyl, is overwhelmed by strong chloride-cation interactions. Ethanolammonium binds tightly to chloride in a monodentate fashion via either its alcohol or ammonium charge centre, or through both in a bidentate arrangement by adopting a gauche or eclipsed conformer.

On the atomic structure of cocaine in solution.

Cocaine is an amphiphilic drug which has the ability to cross the blood-brain barrier (BBB). Here, a combination of neutron diffraction and computation has been used to investigate the atomic scale structure of cocaine in aqueous solutions. Both the observed conformation and hydration of cocaine appear to contribute to its ability to cross hydrophobic layers afforded by the BBB, as the average conformation yields a structure which might allow cocaine to shield its hydrophilic regions from a lipophilic environment. Specifically, the carbonyl oxygens and amine group on cocaine, on average, form ∼5 bonds with the water molecules in the surrounding solvent, and the top 30% of water molecules within 4 Šof cocaine are localized in the cavity formed by an internal hydrogen bond within the cocaine molecule. This water mediated internal hydrogen bonding suggests a mechanism of interaction between cocaine and the BBB that negates the need for deprotonation prior to interaction with the lipophilic portions of this barrier. This finding also has important implications for understanding how neurologically active molecules are able to interact with both the blood stream and BBB and emphasizes the use of structural measurements in solution in order to understand important biological function.

On the structure of an aqueous propylene glycol solution.

Using a combination of neutron diffraction and empirical potential structure refinement computational modelling, the interactions in a 30 mol. % aqueous solution of propylene glycol (PG), which govern both the hydration and association of this molecule in solution, have been assessed. From this work it appears that PG is readily hydrated, where the most prevalent hydration interactions were found to be through both the PG hydroxyl groups but also alkyl groups typically considered hydrophobic. Hydration interactions of PG dominate the solution over PG self-self interactions and there is no evidence of more extensive association. This hydration behavior for PG in solutions suggests that the preference of PG to be hydrated rather than to be self-associated may translate into a preference for PG to bind to lipids rather than itself, providing a potential explanation for how PG is able to enhance the apparent solubility of drug molecules in vivo.

How the surface structure determines the properties of CuH.

CuH is a material that appears in a wide diversity of circumstances ranging from catalysis to electrochemistry to organic synthesis. There are both aqueous and nonaqueous synthetic routes to CuH, each of which apparently leads to a different product. We developed synthetic methodologies that enable multigram quantities of CuH to be produced by both routes and characterized each product by a combination of spectroscopic, diffraction and computational methods. The results show that, while all methods for the synthesis of CuH result in the same bulk product, the synthetic path taken engenders differing surface properties. The different behaviors of CuH obtained by aqueous and nonaqueous routes can be ascribed to a combination of very different particle size and dissimilar surface termination, namely, bonded hydroxyls for the aqueous routes and a coordinated donor for the nonaqueous routes. This work provides a particularly clear example of how the nature of an adsorbed layer on a nanoparticle surface determines the properties.

Association and liquid structure of pyridine-acetic acid mixtures determined from neutron scattering using a 'free proton' EPSR simulation model.

The liquid structure of pyridine-acetic acid mixtures have been investigated using neutron scattering at various mole fractions of acetic acid, χHOAc = 0.33, 0.50, and 0.67 and compared to the structures of neat pyridine and acetic acid. Data has been modelled using empirical potential structure refinement (EPSR) with a 'free proton' reference model, which has no prejudicial weighting towards either the existence of molecular or ionised species. Analysis of the neutron scattering results shows the existence of hydrogen-bonded acetic acid chains with pyridine inclusions, rather than the formation of an ionic liquid by proton transfer.

Conformation and interactions of dopamine hydrochloride in solution.

The aqueous solution of dopamine hydrochloride has been investigated using neutron and X-ray total scattering data together with Monte-Carlo based modelling using Empirical Potential Structure Refinement. The conformation of the protonated dopamine molecule is presented and the results compared to the conformations found in crystal structures, dopamine-complexed protein crystal structures and predicted from theoretical calculations and pharmacophoric models. It is found that protonated dopamine adopts a range of conformations in solution, highlighting the low rotational energy barrier between different conformations, with the preferred conformation being trans-perpendicular. The interactions between each of the species present (protonated dopamine molecules, water molecules, and chloride anions) have been determined and are discussed with reference to interactions observed in similar systems both in the liquid and crystalline state, and predicted from theoretical calculations. The expected strong hydrogen bonds between the strong hydrogen bond donors and acceptors are observed, together with evidence of weaker CH hydrogen bonds and π interactions also playing a significant role in determining the arrangement of adjacent molecules.

Hydrogen production from ammonia using sodium amide.

This paper presents a new type of process for the cracking of ammonia (NH3) that is an alternative to the use of rare or transition metal catalysts. Effecting the decomposition of NH3 using the concurrent stoichiometric decomposition and regeneration of sodium amide (NaNH2) via sodium metal (Na), this represents a significant departure in reaction mechanism compared with traditional surface catalysts. In variable-temperature NH3 decomposition experiments, using a simple flow reactor, the Na/NaNH2 system shows superior performance to supported nickel and ruthenium catalysts, reaching 99.2% decomposition efficiency with 0.5 g of NaNH2 in a 60 sccm NH3 flow at 530 °C. As an abundant and inexpensive material, the development of NaNH2-based NH3 cracking systems may promote the utilization of NH3 for sustainable energy storage purposes.

In situ X-ray powder diffraction studies of hydrogen storage and release in the Li-N-H system.

We report the experimental investigation of hydrogen storage and release in the lithium amide-lithium hydride composite (Li-N-H) system. Investigation of hydrogenation and dehydrogenation reactions of the system through in situ synchrotron X-ray powder diffraction experiments allowed for the observation of the formation and evolution of non-stoichiometric intermediate species of the form Li1+xNH2-x. This result is consistent with the proposed Frenkel-defect mechanism for these reactions. We observed capacity loss with decreasing temperature through decreased levels of lithium-rich (0.7 ≤ x ≤ 1.0) non-stoichiometric phases in the dehydrogenated material, but only minor changes due to multiple cycles at the same temperature. Annealing of dehydrogenated samples reveals the reduced stability of intermediate stoichiometry values (0.4 ≤ x ≤ 0.7) compared with the end member species: lithium amide (LiNH2) and lithium imide (Li2NH). Our results highlight the central role of ionic mobility in understanding temperature limitations, capacity loss and facile reversibility of the Li-N-H system.

Understanding composition-property relationships in Ti-Cr-V-Mo alloys for optimisation of hydrogen storage in pressurised tanks.

The location of hydrogen within Ti-Cr-V-Mo alloys has been investigated during hydrogen absorption and desorption using in situ neutron powder diffraction and inelastic neutron scattering. Neutron powder diffraction identifies a low hydrogen equilibration pressure body-centred tetragonal phase that undergoes a martensitic phase transition to a face-centred cubic phase at high hydrogen equilibration pressures. The average location of the hydrogen in each phase has been identified from the neutron powder diffraction data although inelastic neutron scattering combined with density functional theory calculations show that the local structure is more complex than it appears from the average structure. Furthermore the origin of the change in dissociation pressure and hydrogen trapping on cycling in Ti-Cr-V-Mo alloys is discussed.

Synthesis and structure of pillared molybdates and tungstates with framework layers.

Six new layered lanthanide molybdate and tungstate phases pillared by either naphthalenedisulfonate (NDS) or fumarate anions have been synthesized hydrothermally and structurally characterized. Five of these materials, [Nd(H(2)O)MoO(4)](2)[2,6-NDS] (1), [Nd(H(2)O)MoO(4)](2)[1,5-NDS] (2), [La(H(2)O)WO(4)](2)[1,5-NDS] (3), [La(H(2)O)WO(4)](2)[2,6-NDS] (4), and [Ce(H(2)O)MoO(4)](2)[fumarate] (6), have a closely related cationic inorganic layer structure which comprises a bilayer of polyhedra leading to the formation of a framework layer containing small, inaccessible pores. These layers are pillared by the organic anions which also bridge between the lanthanide cations within the layers. In the La/WO(4)/2,6-NDS system, a second polymorph, [La(2)(H(2)O)(2)W(2)O(8)][2,6-NDS] (5), is observed. In this compound, the tungstate anions have dimerized, forming W(2)O(8)(4-). This dimer is unique and comprises two square-based pyramidal tungsten centers which are opposed to each other.

A new polymorph of 2-methyl-6-nitroaniline.

A new crystal form of 2-methyl-6-nitroaniline, C7H8N2O2, crystallizing with Z' = 2 in the space group P2(1)/c, has been identified during screening for salts and cocrystals. The different N-H...O hydrogen-bonding synthons result in linear V-shaped chains in the new polymorph, rather than the helical chain arrangement seen in the known form where Z' = 1. The presence of a second component during crystallization appears to have determined the resultant crystal form of 2-methyl-6-nitroaniline.

The effect of particle size, morphology and support on the formation of palladium hydride in commercial catalysts.

The relative amounts of hydrogen retained by a range of supported palladium catalysts have been investigated by a combination of electron microscopy and spectroscopic techniques, including incoherent inelastic neutron scattering. Contrary to expectation, the hydrogen capacity is not determined solely by the metal particle size, but it is a complex interaction between the particle size and its state of aggregation. The nature of the support is not only integral to the amount of hydrogen held by the catalyst, it also causes a marked difference in the rate of release of stored hydrogen from palladium. It is more difficult to fully dehydrogenate palladium on/in the porous activated carbon than on the non-porous carbon black based catalyst. The type of support also results in differences in the form of the residual hydrogen: whether it is α- or ß-hydride phase, subsurface or in the threefold surface site. Our data on the supported catalysts reinforces what has only been seen previously with palladium black and our computational study provides confirmation of the empirical assignments. We also report the first vibrational spectroscopic study of hydrogen adsorbed at the surface of ß-PdH and have observed for the first time hydrogen in the on-top site. This has enabled the relative proportion of bulk- to surface-H occupation in calculated model and in industrial nanoparticles to be estimated.

The reaction of formic acid with RaneyTM copper.

The interaction of formic acid with RaneyTM Cu proves to be complex. Rather than the expected generation of a monolayer of bidentate formate, we find the formation of a Cu(II) compound. This process occurs by direct reaction of copper and formic acid; in contrast, previous methods are by solution reaction. This is a rare example of formic acid acting as an oxidant rather than, as more commonly found, a reductant. The combination of diffraction, spectroscopic and computational methods has allowed this unexpected process to be characterized.

Structure and spectroscopy of CuH prepared via borohydride reduction.

Copper(I) hydride (cuprous hydride, CuH) was the first binary metal hydride to be discovered (in 1844) and is singular in that it is synthesized in solution, at ambient temperature. There are several synthetic paths to CuH, one of which involves reduction of an aqueous solution of CuSO4·5H2O by borohydride ions. The product from this procedure has not been extensively characterized. Using a combination of diffraction methods (X-ray and neutron) and inelastic neutron scattering spectroscopy, we show that the CuH from the borohydride route has the same bulk structure as CuH produced by other routes. Our work shows that the product consists of a core of CuH with a shell of water and that this may be largely replaced by ethanol. This offers the possibility of modifying the properties of CuH produced by aqueous routes.

Supramolecular binding and separation of hydrocarbons within a functionalized porous metal-organic framework.

Supramolecular interactions are fundamental to host-guest binding in many chemical and biological processes. Direct visualization of such supramolecular interactions within host-guest systems is extremely challenging, but crucial to understanding their function. We report a comprehensive study that combines neutron scattering, synchrotron X-ray and neutron diffraction, and computational modelling to define the detailed binding at a molecular level of acetylene, ethylene and ethane within the porous host NOTT-300. This study reveals simultaneous and cooperative hydrogen-bonding, π···π stacking interactions and intermolecular dipole interactions in the binding of acetylene and ethylene to give up to 12 individual weak supramolecular interactions aligned within the host to form an optimal geometry for the selective binding of hydrocarbons. We also report the cooperative binding of a mixture of acetylene and ethylene within the porous host, together with the corresponding breakthrough experiments and analysis of adsorption isotherms of gas mixtures.