Computational and spectroscopic |characterization of thianthrene.

In this work, we have carried out a comprehensive characterization of the vibrational spectroscopy of the non-planar molecule thianthrene. The combination of infrared, Raman and inelastic neutron scattering spectroscopies is highly complementary and allows all of the modes to be observed. Periodic density-functional theory calculations have provided unambiguous assignments of the spectra. The literature states that C-S stretch modes occur in the 600-800 cm-1 range. We find that while there are modes that involve sulfur motion in this region, this is a consequence of motion in the ortho-phenylene rings. The modes that are driven by the C-S stretches are found in the ~400-500 cm-1 range. The C-S-C bending modes occur in the 200-300 cm-1 range; these have not been previously characterized.

Intermolecular Interactions in 3-Aminopropyltrimethoxysilane, N-Methyl-3-aminopropyltrimethoxysilane and 3-Aminopropyltriethoxysilane: Insights from Computational Spectroscopy.

In this work, a computational spectroscopy approach was used to provide a complete assignment of the inelastic neutron scattering spectra of three title alkoxysilane derivatives-3-aminopropyltrimethoxysilane (APTS), N-methyl-3-aminopropyltrimethoxysilane (MAPTS), and 3-aminopropyltriethoxysilane (APTES). The simulated spectra obtained from density functional theory (DFT) calculations exhibit a remarkable match with the experimental spectra. The description of the experimental band profiles improves as the number of molecules considered in the theoretical model increases, from monomers to trimers. This highlights the significance of incorporating non-covalent interactions, encompassing classical NH···N, N-H···O, as well as C-H···N and C-H···O hydrogen bond contacts, to achieve a comprehensive understanding of the system. A distinct scenario emerges when considering optical vibrational techniques, infrared and Raman spectroscopy. In these instances, the monomer model provides a reasonable description of the experimental spectra, and no substantial alterations are observed in the simulated spectra when employing dimer and trimer models. This observation underscores the distinctive ability of neutron spectroscopy in combination with DFT calculations in assessing the structure and dynamics of molecular materials.

In situ real-time neutron imaging of gaseous H2 adsorption and D2 exchange on carbon-supported Pd catalysts.

We use in situ neutron imaging to observe the adsorption/absorption of hydrogen within a packed catalyst bed of a Pd/C catalyst at a spatial and temporal resolution of ∼430 µm and a ∼9 s respectively. Additionally, the H2/D2 exchange process across the catalyst bed is followed in real time.

The adsorption of nitrobenzene over an alumina-supported palladium catalyst: an infrared spectroscopic study.

As part of an on-going programme of development of an aniline synthesis catalyst suitable for operation at elevated temperatures, the geometry of the adsorption complex for nitrobenzene on a 5 wt% Pd/Al2O3 catalyst is investigated by infrared (IR) spectroscopy. Via an appreciation of the reduced site symmetry resulting from adsorption, application of the metal surface selection rule, and observation of in-plane modes only, the adsorption complex (Pd-nitrobenzene) at 28 °C is assigned as occurring vertically or tilted with respect to the metal surface, adopting Csσv(yz) symmetry. Moreover, adsorption occurs via a single Pd-O bond. Single molecule DFT calculations and simulated IR spectra assist vibrational assignments but indicate a parallel adsorption geometry to be energetically favourable. The contradiction between calculated and observed structures is attributed to the DFT calculations corresponding to an isolated molecule adsorption complex, while IR spectra relate to multi molecule adsorption that is encountered during sustained catalytic turnover. Residual hydrogen from the catalyst reduction stage leads to aniline formation on the Pd surface at low nitrobenzene coverages but, on increasing nitrobenzene exposure, the aniline is forced on to the alumina support. A reaction scheme is proposed whereby the nitrobenzene adsorption geometry is inherently linked to the high aniline selectivity observed for Pd/Al2O3 catalysts.

CO2-mineralization and carbonation reactor rig: Design and validation for in situ neutron scattering experiments-Engineering and lessons learned.

CO2 mineralization via aqueous Mg/Ca/Na-carbonate (MgCO3/CaCO3/Na2CO3) formation represents a huge opportunity for the utilization of captured CO2. However, large-scale mineralization is hindered by slow kinetics due to the highly hydrated character of the cations in aqueous solutions (Mg2+ in particular). Reaction conditions can be optimized to accelerate carbonation kinetics, for example, by the inclusion of additives that promote competitive dehydration of Mg2+ and subsequent agglomeration, nucleation, and crystallization. For tracking mineralization and these reaction steps, neutron scattering presents unprecedented advantages over traditional techniques for time-resolved in situ measurements. However, a setup providing continuous solution circulation to ensure reactant system homogeneity for industrially relevant CO2-mineralization is currently not available for use on neutron beamlines. We, therefore, undertook the design, construction, testing and implementation of such a self-contained reactor rig for use on selected neutron beamlines at the ISIS Neutron and Muon Source (Harwell, UK). The design ensured robust attachment via suspension from the covering Tomkinson flange to stabilize the reactor assembly and all fittings (~25 kg), as well as facilitating precise alignment of the entire reactor and sample (test) cell with respect to beam dimension and direction. The assembly successfully accomplished the principal tasks of providing a continuous flow of the reaction mixture (~500 mL) for homogeneity, quantitative control of CO2 flux into the mixture, and temperature and pressure regulation throughout the reaction and measurements. The design is discussed, with emphasis placed on the reactor, including its geometry, components, and all technical specifications. Descriptions of the off-beamline bench tests, safety, and functionality, as well as the installation on beamlines and trial experimental procedure, are provided, together with representative raw neutron scattering results.

Centrohexaindane, a Unique Polyaromatic Hydrocarbon Bearing the Rare Cq (Cq )4 Core: Inelastic Neutron Scattering, Infrared and Raman Spectroscopy.

The structure and vibrational spectroscopy of centrohexaindane, 1, was investigated. This unusual molecule has a quaternary carbon atom that is coordinated to four further such quaternary carbon atoms as its core, each pair of which is bonded to an ortho-phenylene unit. Previous NMR studies have shown that the molecule has tetrahedral (Td ) symmetry in solution. The infrared and Raman spectra of chloroform and deuterochloroform solutions of 1 are completely in agreement with this conclusion, as the only modes that are visible are those allowed for Td symmetry. This is not the case in the solid state: X-ray powder diffraction indicates that the unit cell is triclinic or monoclinic with a volume in excess of 4000 Å3 . The vibrational spectroscopy is consistent with C1 site symmetry and the presence of at least two molecules in the primitive cell. It is likely that the space group is centrosymmetric.

Neutron Scattering Studies of Heterogeneous Catalysis.

Understanding the structural dynamics/evolution of catalysts and the related surface chemistry is essential for establishing structure-catalysis relationships, where spectroscopic and scattering tools play a crucial role. Among many such tools, neutron scattering, though less-known, has a unique power for investigating catalytic phenomena. Since neutrons interact with the nuclei of matter, the neutron-nucleon interaction provides unique information on light elements (mainly hydrogen), neighboring elements, and isotopes, which are complementary to X-ray and photon-based techniques. Neutron vibrational spectroscopy has been the most utilized neutron scattering approach for heterogeneous catalysis research by providing chemical information on surface/bulk species (mostly H-containing) and reaction chemistry. Neutron diffraction and quasielastic neutron scattering can also supply important information on catalyst structures and dynamics of surface species. Other neutron approaches, such as small angle neutron scattering and neutron imaging, have been much less used but still give distinctive catalytic information. This review provides a comprehensive overview of recent advances in neutron scattering investigations of heterogeneous catalysis, focusing on surface adsorbates, reaction mechanisms, and catalyst structural changes revealed by neutron spectroscopy, diffraction, quasielastic neutron scattering, and other neutron techniques. Perspectives are also provided on the challenges and future opportunities in neutron scattering studies of heterogeneous catalysis.

Vibrational spectra of neutral and doped oligothiophenes and polythiophene.

We have measured the infrared, Raman and inelastic neutron scattering (INS) spectra of a series of oligothiophenes (bithiophene, terthiophene, quarterthiophene, sexithiophene and octithiophene) and polythiophene, both pristine and after doping with iodine. The spectra of the pristine (i.e. neutral) systems show a rapid convergence towards the spectrum of polythiophene, such that the spectra of sexithiophene and octithiophene are almost indistinguishable from that of polythiophene. The spectra, in combination with periodic density functional theory calculations, have also provided the first complete assignment of polythiophene. In contrast to the infrared and Raman spectra that show dramatic changes on doping, the INS spectra show only small changes. Isolated molecule DFT calculations show that the molecular structures are not greatly modified on doping and since the INS spectrum largely depends on the structure, this does not change much. In contrast, as shown by others, the electronic structure is greatly modified and this accounts for the major changes in the infrared and Raman spectra.

In-Situ Anaerobic Heating of Human Bones Probed by Neutron Diffraction.

The first neutron diffraction study of in-situ anaerobic burning of human bones is reported, aiming at an interpretation of heat-induced changes in bone, which were previously detected by vibrational spectroscopy, including inelastic neutron scattering techniques. Structural and crystallinity variations were monitored in samples of the human femur and tibia, as well as a reference hydroxyapatite, upon heating under anaerobic conditions. Information on the structural reorganization of the bone matrix as a function of temperature, from room temperature to 1000 °C, was achieved. Noticeable crystallographic and domain size variations, together with O-H bond lengths and background variations, were detected. Above 700 °C, the inorganic bone matrix became highly symmetric, devoid of carbonates and organic constituents, while for the lower temperature range (<700 °C), a considerably lower crystallinity was observed. The present pilot study is expected to contribute to a better understanding of the heat-prompted changes in bone, which can be taken as biomarkers of the burning temperature. This information is paramount for bone analysis in forensic science as well as in archeology and may also have useful applications in other biomaterial studies.

Counting the Acid Sites in a Commercial ZSM-5 Zeolite Catalyst.

This work investigates the acid sites in a commercial ZSM-5 zeolite catalyst by a combination of spectroscopic and physical methods. The Brønsted acid sites in such catalysts are associated with the aluminum substituted into the zeolite lattice, which may not be identical to the total aluminum content of the zeolite. Inelastic neutron scattering spectroscopy (INS) directly quantifies the concentrations of Brønsted acid protons, silanol groups, and hydroxyl groups associated with extra-framework aluminum species. The INS measurements show that ∼50% of the total aluminum content of this particular zeolite is extra framework, a conclusion supported by solid-state NMR and ammonia temperature-programmed desorption (TPD) measurements. Evidence for the presence of extra-framework aluminum oxide species is also seen in neutron powder diffraction data from proton- and deuterium-exchanged samples. The differences between results from the different analytical methods are discussed, and the novelty of direct proton counting by INS in this typical commercial catalyst is emphasized.

Combining quasielastic neutron scattering and molecular dynamics to study methane motions in ZSM-5.

Quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations are applied in combination to investigate the dynamics of methane in H-ZSM-5 zeolite catalysts used for methanol-to-hydrocarbons reactions. Methane is employed as an inert model for the methanol reaction feedstock, and studies are made of the fresh catalyst and used catalysts with varying levels of coke buildup to investigate the effect of coking on reactant mobility. Measurements are made in the temperature range from 5 to 373 K. Methane mobility under these conditions is found to be extremely high in fresh ZSM-5, with the majority of movements occurring too fast to be resolved by the QENS instrument used. A small fraction of molecules undergoing jump diffusion on QENS time scales is identified and found to correspond with short-range jump diffusion within single zeolite pores as identified in MD simulations. Agreement between QENS and MD mobility measurements is found to be within 50%, validating the simulation approach employed. Methane diffusion is found to be minimally affected by moderate levels of coke buildup, while highly coked samples result in the confinement of methane to single pores within the zeolite with minimal long-range diffusion.

Structural Dynamics of Chloromethanes through Computational Spectroscopy: Combining INS and DFT.

In this work, the structural dynamics of the chloromethanes CCl4, CHCl3 and CH2Cl2 were evaluated through a computational spectroscopy approach by comparing experimental inelastic neutron scattering (INS) spectra with the corresponding simulated spectra obtained from periodic DFT calculations. The overall excellent agreement between experimental and calculated spectra allows a confident assignment of the vibrational features, including not only the molecular fundamental modes but also lattice and combination modes. In particular, an impressive overtone sequence for CHCl3 is fully described by the simulated INS spectrum. In the CCl4 spectrum, the splitting of the ν3 mode at ca. 765-790 cm-1 is discussed on the basis of the Fermi resonance vs. crystal splitting controversy.

Structure and Spectroscopy of Iron Pentacarbonyl, Fe(CO)5.

We have re-investigated the structure and vibrational spectroscopy of the iconic molecule iron pentacarbonyl, Fe(CO)5, in the solid state by neutron scattering methods. In addition to the known C2/c structure, we find that Fe(CO)5 undergoes a displacive ferroelastic phase transition at 105 K to a P1̅ structure. We propose that this is a result of certain intermolecular contacts becoming shorter than the sum of the van der Waals radii, resulting in an increased contribution of electrostatic repulsion to these interactions; this is manifested as a strain that breaks the symmetry of the crystal. Evaluation of the strain in a triclinic crystal required a description of the spontaneous strain in terms of a second-rank tensor, something that is feasible with high-precision powder diffraction data but practically very difficult using strain gauges on a single crystal of such low symmetry. The use of neutron vibrational spectroscopy (which is not subject to selection rules) has allowed the observation of all the fundamentals below 700 cm-1 for the first time. This has resulted in the re-assignment of several of the modes. Surprisingly, density functional theory calculations that were carried out to support the spectral assignments provided a poor description of the spectra.

The spontaneous self-assembly of a molecular water pipe in 3D space.

The self-assembly and self-organization of water molecules are relevant in many fields of research. When water spontaneously reacts with 2,2,6,6-tetra-methyl-piperidine (TMP) to form colourless and crystalline discrete needles, only in the exact ratio of 2:1, it is important to understand the phenomenon. Single-crystal X-ray and neutron diffraction data have unveiled that TMP self-assembles around columns of water molecules, and as such, the resulting adduct may be described as a series of molecular water pipes.

Vibrational Spectroscopy of Hexahalo Complexes.

Halogenated inorganic complexes Ax[MHaly] (A = alkali metal or alkaline earth, M = transition or main group metal, x = 1-3, and y = 2-9) are an archetypal class of compounds that provide entry points to large areas of inorganic and physical chemistry. All of the hexahalo complexes adopt an octahedral, Oh, symmetry (or nearly so). Consequently, one of the bending modes is forbidden in both the infrared and Raman spectra. In the solid state, many of the complexes crystallize in the cubic space group Fm3̅m, which preserves the octahedral symmetry. Even for those that are not cubic, the octahedral symmetry of the [MHal6]n- ion is largely retained and, to a good approximation, so are the selection rules. In the present work, we show that by using the additional information provided by neutron vibrational spectroscopy, in combination with conventional optical spectroscopies, we can generate complete and unambiguous assignments for all the modes. Comparison of the experimental and calculated transition energies for the systems where periodic-density functional theory was possible (i.e., those for which the crystal structure is known) shows that the agreement is almost quantitative. We also provide a linear relationship that enables the prediction of the forbidden mode.

Spectroscopic Signatures of Hydrogen-Bonding Motifs in Protonic Ionic Liquid Systems: Insights from Diethylammonium Nitrate in the Solid State.

Diethylammonium nitrate, [N0 0 2 2][NO3], and its perdeuterated analogue, [N D D 2 2] [NO3], were structurally characterized and studied by infrared, Raman, and inelastic neutron scattering (INS) spectroscopy. Using these experimental data along with state-of-the-art computational materials modeling, we report unambiguous spectroscopic signatures of hydrogen-bonding interactions between the two counterions. An exhaustive assignment of the spectral features observed with each technique has been provided, and a number of distinct modes related to NH···O dynamics have been identified. We put a particular emphasis on a detailed interpretation of the high-resolution, broadband INS experiments. In particular, the INS data highlight the importance of conformational degrees of freedom within the alkyl chains, a ubiquitous feature of ionic liquid (IL) systems. These findings also enable an in-depth physicochemical understanding of protonic IL systems, a first and necessary step to the tailoring of hydrogen-bonding networks in this important class of materials.

Looking for Minor Phenolic Compounds in Extra Virgin Olive Oils Using Neutron and Raman Spectroscopies.

Extra virgin olive oil (EVOO) is defined as a functional food as it contains numerous phenolic components with well-recognized health-beneficial properties, such as high antioxidant and anti-inflammatory capacity. These characteristics depend on their structural/conformational behavior, which is largely determined by intra- and intermolecular H-bond interactions. While the vibrational dynamics of isolated compounds have been studied in a number of recent investigations, their signal in a real-life sample of EVOO is overwhelmed by the major constituent acids. Here, we provide a full characterization of the vibrational spectroscopic signal from commercially available EVOO samples using Inelastic Neutron Scattering (INS) and Raman spectroscopies. The spectra are dominated by CH2 vibrations, especially at about 750 cm-1 and 1300 cm-1. By comparison with the spectra from hydroxytyrosol and other minor phenolic compounds, we show that the best regions in which to look for the structure-activity information related to the minor polar compounds is at 675 and 1200 cm-1 for hydroxytyrosol, and around 450 cm-1 for all minor polar compounds used as reference, especially if a selectively deuterated sample is available. The regional origin of the EVOO samples investigated appears to be related to the different amount of phenolic esters versus acids as reflected by the relative intensities of the peaks at 1655 and 1747 cm-1.

Control of zeolite microenvironment for propene synthesis from methanol.

Optimising the balance between propene selectivity, propene/ethene ratio and catalytic stability and unravelling the explicit mechanism on formation of the first carbon-carbon bond are challenging goals of great importance in state-of-the-art methanol-to-olefin (MTO) research. We report a strategy to finely control the nature of active sites within the pores of commercial MFI-zeolites by incorporating tantalum(V) and aluminium(III) centres into the framework. The resultant TaAlS-1 zeolite exhibits simultaneously remarkable propene selectivity (51%), propene/ethene ratio (8.3) and catalytic stability (>50 h) at full methanol conversion. In situ synchrotron X-ray powder diffraction, X-ray absorption spectroscopy and inelastic neutron scattering coupled with DFT calculations reveal that the first carbon-carbon bond is formed between an activated methanol molecule and a trimethyloxonium intermediate. The unprecedented cooperativity between tantalum(V) and Brønsted acid sites creates an optimal microenvironment for efficient conversion of methanol and thus greatly promotes the application of zeolites in the sustainable manufacturing of light olefins.

Structure and spectroscopy of methionyl-methionine for aquaculture.

The amino acid L-methionine is an essential amino acid and is commonly used as a feed supplement in terrestrial animals. It is less suitable for marine organisms because it is readily excreted. It is also highly water soluble and this results in loss of the feed and eutrophication of the water. To address these problems, the dipeptide DL-methionyl-DL-methionine (trade name: AQUAVI Met-Met) has been developed as a dedicated methionine source for aquaculture. The commercial product is a mixture of a racemic crystal form of D-methionyl-D-methionine/L-methionyl-L-methionine and a racemic crystal form of D-methionyl-L-methionine/L-methionyl-D-methionine. In this work, we have computationally, structurally, spectroscopically and by electron microscopy characterised these materials. The microscopy and spectroscopy demonstrate that there is no interaction between the DD-LL and DL-LD racemates on any length scale from the macroscopic to the nanoscale.

Characterisation of hydration water in Nafion membrane.

Nafion, a polytetrafluoroethylene polymer with perfluorinated-vinyl-polyether side chains ending in sulfonic acid groups, is widely used as the proton-exchange membrane in polymer electrolyte fuel cells, particularly low temperature hydrogen-oxygen fuel cells. The state of hydration of the sulfonic acid groups is crucial to its operation. By using a combination of inelastic neutron scattering (INS) and infrared spectroscopies, and by comparison to a series of trifluoromethanesulfonic acid hydrates of well-defined stoichiometry, we characterise how the hydration changes as a function of water content.