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The effect of solvent has been investigated for the synthesis of bismuth gallate compounds, of which the water-based bismuth subgallate has been used as an active pharmaceutical ingredient (API) for over a century. Using methanol as a solvent, two new bismuth gallates were acquired: first a flexible 3-periodic metal-organic framework (MOF) forms, which transforms upon extended synthesis times into a layered 2-periodic coordination polymer of the same bismuth-to-gallate ratio. The structures were determined by three-dimensional electron diffraction. Synthesis in ethanol resulted in the formation of the MOF phase, but not the layered phase. The layered material of the methanol-based synthesis was used as a Lewis acid catalyst due to its higher stability, showing a comparatively quick and regiospecific conversion of styrene oxide to 2-methoxy-2-phenylethanol, indicating the presence of open metal sites in the material. The acquisition of bismuth gallate structures of varying periodicity highlights the prospect of acquiring novel MOFs and coordination polymers from the same components of APIs. ispartof: DALTON TRANSACTIONS vol:51 issue:37 pages:14221-14227 ispartof: location:England status: published

The monomeric molecular aluminium(I) complex 1 [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl) reacts with a series of terminal and strained alkenes including ethylene, propylene, allylbenzene and norbornene to form alkene bound products. Remarkably all these reactions are reversible under mild conditions (298–353 K) with alkene binding being disfavoured at higher temperatures due to the positive reaction entropy. Van't Hoff analyses have allowed quantification of the binding events with Image ID:c8sc04865g-t1.gif. Calculations and single crystal X-ray diffraction studies are consistent with the alkene bound species being metallocyclopropane complexes. Alkene binding involves a reversible redox process with changes from the +1 to +3 aluminium oxidation state. Under more forcing conditions the metallocyclopropane complexes undergo non-reversible allylic C–H bond activation to generate aluminium(III) allyl hydride complexes. This represents a rare example of redox-based main group reactivity in which reversible substrate binding is followed by a further productive bond breaking event. Analysis of the mechanism reveals a reaction network in which alkene dissociation and reformation of 1 is required for allylic C–H activation, a realisation that has important implications for the long-term goal of developing redox-based catalytic cycles with main group compounds.

International audience; The diversity of synthetic zeolites with silicogermanate composition has grown significantly. Many of these zeolites have large pores, thanks to the double 4-ring (d4r) structural subunits occupied by Ge atoms. The wide pores make them potentially interesting for catalytic transformation of bulky molecules, but the thermal and hydrothermal stability and the acidity are insufficient for practical applications. In the present work, the stability of known silicogermanate zeolite structures and their silicate and aluminosilicate analogues is evaluated using periodic density functional theory calculations. The thermodynamics of isomorphic substitution of Ge atoms for Si and Al via chemical processes are investigated. The study reveals that thermodynamically, (alumino)silicate counterparts of all known silicogermanates are intrinsically stable, and that the energetics of isomorphic substitution reactions of Ge atoms are almost independent of the distribution of d4r units in the different framework topologies. Chlorides are found better isomorphic substitution reagent candidates, at least theoretically. This work opens perspectives for the catalytic use of stable derivatives of silicogermanate zeolites.

Single crystal X-ray diffraction reveals that 4,4'-bis(tetrahydrothiopyranyl) crystallizes in an equatorial-equatorial geometry with a gauche conformation along the central carbon-carbon bond. B3LYP/6-311G** and MP2/6-311G** calculations show that the antiperiplanar conformation is higher in energy than the gauche one because of sulfur induced stretching and widening of the cyclohexane-like rings. Calculations at various levels of theory suggest that in the antiperiplanar region the twisting coordinate of 4,4'-bis(tetrahydrothiopyranyl) exhibits a very shallow double-well potential. The gauche molecular structure of 4,4'-bis(tetrahydrothiopyranyl) thwarts efficient packing of its molecules in the solid state. Crown Copyright 2012 Published by Elsevier B.V. All rights reserved.

The vacancy ordering behavior of an A-site deficient perovskite system, Ca1-xLa2x/3TiO3, was studied using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), adopting Pbnm symmetry, there was no indication of long-range ordering. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of displacement maps from A-site cations in the HAADF signal, are shown to be tilt boundaries. At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. These highly ordered domains can be found isolated within a random distribution of vacancies/La3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. The occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems.

The geometry of ethylenedioxy-tetrathiafulvalene, EDO-TTF, plays an important role in the metal-insulator transition in the charge transfer salt (EDO-TTF)(2)PF6. The planar and off-planar geometrical conformations of the EDO-TTF molecules are explained using an extended Debye polarizability model for the bond angle. The geometrical structure of EDO-TTF is dictated by its four sulfur bond angles and these are, in turn, determined by the polarizability of the sulfur atoms. With Hartree-Fock and second-order Moller-Plesset perturbation theory calculations on EDO-TTF, TTF, H2S, and their oxygen and selenium substituted counterparts we confirm this hypothesis. The Debye polarizability model for bond angles relates directly the optimum bond angle with the polarizability of the center atom. Considering the (EDO-TTF)(2)PF6 material in this light proves to be very fruitful.

This study reports a determination of the polarity of WC facets in WC–Co alloys sintered at the liquid state. In these alloys, WC grains are delimited by basal facets and two sets of { 1 0 1 ¯ 0 } prismatic facets, one set of facets being much more developed than the other. A variation of the shape is observed as a function of the composition of the alloy. High resolution transmission electron microscopy is used to investigate the polarity of the prismatic facets owing to the typical triangular patterns appearing on the images. The effect of the composition is investigated using two alloys with different carbon potentials and one containing VC and Cr3C2 as grain growth inhibitors. The interpretation of the images shows that in all cases, the same set of prismatic planes is favoured.

Liquid complex hydrides are a new class of hydrogen storage materials with several advantages over solid hydrides, e.g. they are flexible in shape, they are a flowing fluid and their convective properties facilitate heat transport. The physical and chemical properties of a gaseous hydride change when the molecules are adsorbed on a material with a large specific surface area, due to the interaction of the adsorbate with the surface of the host material and the reduced number of collisions between the hydride molecules. In this paper we report the synthesis and stabilization of gaseous Ti(BH4)(3). The compound was successfully stabilized through adsorption in nanocavities. Ti(BH4)(3), upon synthesis in its pure form, spontaneously and rapidly decomposes into diborane and titanium hydride at room temperature in an inert gas, e.g. argon. Ti(BH4)(3) adsorbed in the cavities of a metal organic framework is stable for several months at ambient temperature and remains stable up to 350 K under vacuum. The adsorbed Ti(BH4)(3) reaches approximately twice the density of the gas phase. The specific surface area (BET, N-2 adsorption) of the MOF decreased from 1200 m(2) g(-1) to 770 m(2) g(-1) upon Ti(BH4)(3) adsorption.

International audience; The natural speciation of Mn (0.19 g/kg), Ni (46 mg/kg), and Zn (42 mg/kg) in the argillic horizon (120 cm depth, pH = 5.6) of an Ultisol from a paddy soil in northern Taiwan was investigated by advanced X-ray synchrotron techniques. Microchemical associations were imaged by synchrotron-based X-ray microfluorescence, host minerals were identified by standard and micrometer-resolved X-ray diffraction, and the local coordination environment of Mn, Ni, and Zn was probed using extended X-ray absorption fine structure (EXAFS) spectroscopy on a powdered sample and a soil thin section, and polarized EXAFS spectroscopy on a highly textured self-supporting clay film from the < 2 mu m fraction of the soil. Manganese was concentrated in Fe-Mn soft mottles (44.4 g/kg) as turbostratic hexagonal birnessite and lithiophorite having Mn3+/Mn4+ atomic ratios of similar to 20% and 50%, respectively. Quantitative analysis of high-order scattering paths of the EXAFS spectrum for natural and synthetic (Al0.67Li0.32)(Mn0.684+Mn0.323+)O-2(OH)(2) lithiophorite revealed that Mn3+ and Mn4+ are ordered in the [(Mn0.684+Mn0.323+)O-2](0.32-) layer. A structural model is proposed, in which Mn4+ and Mn3+ are ordered similarly to Al and Li in the [(A(0.67)(3+)Li(0.32)(+))(OH2)O-2](0.32+) layer, with Mn3+ cations being surrounded by six Mn4+, and Mn4+ cations by three Mn3+ and three Mn. Similar cation ordering in the manganese and aluminum layers likely provides a more homogeneous local balance of the excess and deficit of charges in each layer and increases the stability of lithiophorite. Ni (r = 0.70 angstrom) substitutes for Mn (r(Mn4+) = 0.54 angstrom, r(Mn3+) = 0.65 angstrom) in the manganese layer in the natural lithiophorite. In contrast, Zn (r = 0.74 angstrom) fills vacant sites in the gibbsitic layer of natural lithiophorite, in a similar manner as lithium (r = 0.74 angstrom) in synthetic lithiophorite. The partitioning of Ni and Zn between the two layers is a result of the general preference of Ni, whose size is intermediate between those of Mn3+ and Li+, for slightly smaller sites. In contrast with nickel, which is detected only where there is lithiophorite, the Zn-lithiophorite association found in Fe-Mn mottles is not representative of the bulk soil. The combined use of X-ray diffraction, and powder and polarized EXAFS spectroscopy revealed that Zn is predominantly bound to hydroxy-Al interlayers sandwiched between 2:1 vermiculite layers in the fine soil matrix. The incorporation of Zn in the gibbsitic layer of both lithiophorite and vermiculite helps increase the stability of these minerals by providing positive charge to balance the negative charge from the 2:1 phyllosilicate layer and the [(Mn0.684+Mn0.323+)O-2](0.32-) layer of lithiophorite. This binding environment for zinc is probably the main mechanism by which zinc is sequestered in acidic to near-neutral aluminum-rich clayey soils.

Platelet-shaped copper sulfide nanocrystals (NCs) with tunable Cu stoichiometry were prepared from Cu-rich covellite (Cu1.1S) nanoplates through their reaction with a Cu(I) complex ([Cu(CH3CN)(4)]PF6) at room temperature. Starting from a common sample, by this approach it is possible to access a range of compositions in these NCs, varying from Cu1.1S up to Cu2S, each characterized by a different optical response: from the metallic covellite, with a high density of free carriers and strong localized surface plasmon resonance (LSPR), up to Cu2S NCs with no LSPR. In all these NCs the valency of Cu in the lattice stays always close to +1, while the average -1 valency of S in covellite gradually evolves to -2 with increasing Cu content; i.e., sulfur is progressively reduced. The addition of copper to the starting covellite NCs is similar to the intercalation of metal species in layered transition metal dichalcogenides (TMDCs); i.e., the chalcogen-chalcogen bonds holding the layers are progressively broken to make room for the intercalated metals, while their overall anion sublattice does not change much. However, differently from the TMDCs, the intercalation in covellite NCs is sustained by a change in the redox state of the anion framework. Furthermore, the amount of Cu incorporated in the NCs upon reaction is associated with the formation of an equimolar amount of Cu(II) species in solution. Therefore, the reaction scheme can be written as: Cu1.1S + 2 gamma Cu(I) -> Cu1.1+gamma S + gamma Cu(II).

The structure of two new phases in the bismuth manganite system are reported. The phases were determined by electron diffraction studies of two oxygen-deficient bulk samples. The first phase, a minority component of bulk BiMnO2.94 forms a n=2 Ruddlesden-Popper phase with space group Cmc21 . The second phase, from bulk BiMnO2.99 , is an orthorhombic structure with spacegroup Pmn21 and a unit cell approximately equal to 4 × √ 2 × 2 √ 2 times the parent perovskite cell. Importantly both phases are non-centrosymmetric and offer further potential for multiferroic studies.

The incommensurately modulated structure of copper telluride Cu3-xTe2(Cu2.91Te2) with modulation wavevector q = 0.397 (1)a* + 1/2c* has been determined by X-ray diffraction at room temperature (974 unique reflections). The lattice parameters of the primitive orthorhombic cell of the average structure are: a = 3.9727 (4), b = 4.0020 (5), c = 6.1066 (3) angstrom, V = 97.1 angstrom3, Z = 2, mu = 304 cm-1 (lambda = 0.7107 angstrom), M(r) = 440.1. The symmetry of the structure is given by the superspace group C111BAR(Pmmn). The final R(F) factor is 0.045. The modulated structure is related to the structure of Ni3+/-xTe2 (Schutte & de Boer (1993). Acta Cryst. B49, 392-398] and, as in the latter compound, the occupation of the octahedral sites by metal atoms varies throughout the range 0 to 1. Different from the nickel telluride structure is the antiphase ordering along c together with a different displacement pattern of the atoms, which can be ascribed to a different interaction strength and a different charge transfer between the metal atom and Te.

The size of a series of typical substituents has been probed by dynamic NMR measurements of the barriers to aryl-aryl rotation of the corresponding biphenyls. The resulting B values are meaningful because only mono-ortho substituted compounds were investigated and thus the results are not compromised by the non-additivity of multiple steric effects. On the basis of the chosen model system ethynyl and cyano groups were found to be slightly smaller than a phenyl ring. In contrast, vinyl and, in particular, formyl groups proved to be larger than phenyl. The latter difference is due to the loss of conjugation forces at the planar transition state. Alpha-Hydroxyhexafluoroisopropyl is slightly more bulky than tert-butyl. Pentafluorophenyl and trifluoromethoxy exhibit nearly the same effective size as phenyl and methoxy, respectively. Trifluoromethyl is somewhat smaller than isopropyl.

Pt deposited on a Ge(001) surface spontaneously forms nanowire arrays. These nanowires are thermodynamically stable and can be hundreds of atoms long. The nanowires only occur on a reconstructed Pt-Ge-surface where they fill the troughs between the dimer rows on the surface. This unique connection between the nanowires and the underlying substrate make a thorough understanding of the latter necessary for understanding the growth of the nanowires. In this paper we study possible surface reconstructions containing 0.25 and 0.5 of a monolayer of Pt. Comparison of calculated scanning tunneling microscope (STM) images to experimental STM images of the surface reconstruction reveal that the Pt atoms are located in the top layer, creating a structure with rows of alternating Pt-Ge and Ge-Ge dimers in a $c(4\ifmmode\times\else\texttimes\fi{}2)$ arrangement. Our results also show that Pt atoms in the second or third layer cannot be responsible for the experimentally observed STM images.

The molecular structure and conformational properties of S -ethyl trifluorothioacetate, CF3 COSCH2 CH3, were determined in the gas phase by electron diffraction and vibrational spectroscopy (IR and Raman). The experimental investigations were supplemented by ab initio (Møller Plesset of second order) and density functional theory quantum chemical calculations at different levels of theory. Both experimental and theoretical methods reveal two structures with Cs (anti, anti) and C1 (anti, gauche) symmetries, although there are disagreements about which is more stable. The electron diffraction intensities are best interpreted with a mixture of 51(3)% anti, anti and 49(3)% anti, gauche conformers. This conformational preference was studied using the total energy scheme and the natural bond orbital scheme. In addition, the infrared spectra of CF3 COSCH2 CH3 are reported for the gas, liquid and solid phases as well as the Raman spectrum of the liquid. Using calculated frequencies as a guide, evidence for both Cs and C1 structures is obtained in the IR spectra. Harmonic vibrational frequencies and scaled force fields have been calculated for both conformers. Fil: Defonsi Lestard, Maria Eliana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; Argentina. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química Física; Argentina Fil: Tuttolomondo, María Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; Argentina. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química Física; Argentina Fil: Wann, Derek A.. University of Edinburgh; Reino Unido Fil: Robertson, Heather E.. University of Edinburgh; Reino Unido Fil: Rankin, David W. H.. University of Edinburgh; Reino Unido Fil: Ben Altabef, Aída. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; Argentina. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química Física; Argentina

"This paper is published as part of a Dalton Transactions themed issue on: The Synergy between Theory and Experiment" Ni-61 chemical shifts of Ni(all-trans-cdt) L (cdt = cyclododecatriene, L = none, CO, PMe3), Ni(CO)(4), Ni(C2H4)(2)(PMe3), Ni(cod)(2) (cod = cyclooctadiene) and Ni(PX3)(4) (X = Me, F, Cl) are computed at the GIAO (gauge-including atomic orbitals), BPW91, B3LYP and BHandHLYP levels, using BP86-optimised geometries and an indirect referencing scheme. For this set of compounds, substituent effects on delta(Ni-61) are better described with hybrid functionals than with the pure BPW91 functional. On going from Ni(all-trans-cdt) to Ni(all-cis-cdt) the computations predict substantial shielding of the Ni-61 nucleus by nearly 700 ppm, as well as a sharp increase of the electric field gradient at this position. The latter result is predicted to afford an undetectably broad Ni-61 NMR line for the all-cis-cdt complex, rationalizing the apparent failure to record the NMR spectrum experimentally. Postprint

The structures of 1-trimethylsilyl-1,2,3-benzotriazole and 2-trimethylsilyl-1,3-thiazole have been determined by gas electron diffraction and computational methods. While 1-trimethylsilyl-1,2,3-benzotriazole shows a significant asymmetry in the way the SiMe3 groups bonds to the ring system, the same is not true for 2-trimethylsilyl-1,3-thiazole. However, it has been shown that when the positions of formal single and double bonds in the rings systems are considered, the silyl groups in both compounds are displaced towards the neighbouring ring nitrogen atom. Calculated structures of a series of analogous compounds with different substituents on silicon show only minor variations in the extent of the distortion, although with hydrogen instead of a silyl group the displacement is significantly smaller.

International audience; The dynamics of the enolic form of acetylacetone (E-AcAc) was investigated using a femtosecond pump-probe experiment. The pump at 266 nm excited E-AcAc in the first bright state, S-2(pi pi*). The resulting dynamics was probed by multiphoton ionization at 800 rim. It was investigated for 80 ps on the S-2(pi pi*) and S-1(n pi*) potential energy surfaces. An important step is the transfer from S-2 to S-1 that occurs with a time constant of 1.4 +/- 0.2 ps. Before, the system had left the excitation region in 70 +/- 10 fs. An intermediate step was identified when E-AcAc traveled on the S-2 surface. Likely, it corresponds to an accidental resonance in the detection scheme that is met along this path. More importantly, some clues are given that an intramolecular vibrational energy relaxation is observed, which transfers excess vibrational energy from the enolic group O-H to the other modes of the molecule. The present multistep evolution of excited E-AcAc probably also describes, at least qualitatively, the dynamics of other electronically excited beta-diketones.

International audience; Sodium hydroxo titanate nanowires with NaTi2O4(OH) formulation are fabricated by chimie-douce from TiO2 powders in a hot concentrated base solution. In the present work, we investigate, in detail, the structure and morphology of the titanate, acid-exchanged materials, and phases formed during dehydration of these compounds; we then discuss the relationship between the initial titanate structure and TiO2 formation. XRD, ED, HRTEM, EDS, density measurements, and TG/MS results suggest that the titanate and acid-exchanged structures were closely related to the pseudolepidocrocite arrangement ("Cs2Ti6O12" type). The average structure of NaTi2O4(OH) nanowires consists of noncorrugated Ti2O3(OH) and NaO layers stacked along the [0 10] direction and an energy minimization was further conducted by theoretical methods. From TEM study, we demonstrated that NaTi2O4(OH) exhibits a modulated structure (q wave vector equals to 1/7 [001]*), which originates from the misfit between the NaO and Ti2O3(OH) sublattices. NaTi2O4(OH) structure has a monoclinic distortion (I2/m space group) with refined unit cell a = 0.37000(6) nm, b = 1.7816(4) nm, c = 2.2077(3) nm, beta = 92.70(1)degrees. Proton exchange followed by heat treatment convert the sodium hydroxo titanate into highly photoactive TiO2 nanowires. The activity is attributed to the presence of preferentially exposed (001) surface plane which is explained by the topotactic phase transformation of TiO2(B) into anatase.

Diffraction studies of the liquid structure of AlCl3, AlBr3, GaBr3 and GaI3 close to their respective freezing points have revealed fourfold coordination of the trivalent metal ions, consistent with dimeric M2X6 molecules being the dominant species. We evaluate the species-resolved pair distribution functions and liquid structure factors in all these melts by carrying out classical molecular-dynamics simulations, based on polarizable-halogen force laws that were determined on isolated molecular monomers and dimers in the gaseous phase. We also report results for mean-square displacements and diffusion coefficients of the two species in each melt. The model reproduces the main features of the total neutron-diffraction structure factors, showing peaks due to intermediate-range order and to charge and density short-range order, and accounts for the experimental data at a good quantitative level. Direct simulation of the pair distribution functions yields agreement with the diffraction data on metal-halogen and halogen-halogen bond lengths in the melt and on the stability of the first-neighbour shell of the metal ions. We examine the temperature dependence of the liquid structure in our models for GaBr3 and AlCl3 and emphasize the structural role of van der Waals interactions between the halogens.

Lanthanum hexaboride is one of the cathode materials most used in high-power electronics technology, but the many experimental results do not provide a consistent picture of the surface properties. Therefore, we report the first ab initio calculations of the work functions and surface energies of the (001), (011), (111), (112), and (012) surfaces by considering the different surface terminations and structural relaxation. Either the (111)B- or the (001) La-terminated surface is the most stable, depending on La chemical potential. The work function of the latter is the lowest (2.07 eV) of the surfaces considered. Both the work function and surface energy decrease further when surface La is replaced by Ba and become, respectively, 1.43 and 7.7 eV/nm(2) at the chemical potentials of elemental lanthanum and barium bulk. These results compare favorably with previous work on the intermetallics BaAl4, CaAl4, and BaAuIn3. Their most stable surfaces possess the lowest work function. Now, we study a compound with a decidedly different crystal type and with its constituting elements from column 3 of the periodic table, of which one is nonmetallic.

International audience; Multiwalled carbon nanotubes are grown by microwave plasma chemical vapor deposition with CH4 and H2 as precursor gases. Ni and Ni/Pt electroplated layers are used as catalysts for the synthesis of the tubes. We observe that a very efficient filling of the tubes takes place with Ni. In some cases Ni/Pt filling is also observed inside the tubes. High-resolution transmission electron microscopy (HRTEM) studies, coupled with energy-dispersive X-ray analyses of the tubes, indicate Ni nanorods with a highly symmetrical cylindrical structure. The diameter of the cylindrical nanorods is on the order of 40 nm, and their length is 660 nm. The nano area diffraction pattern of the nanorods reveals the cubic structure of nickel, and electron diffraction spots corresponding to (111), (200), (220) planes are evident. The lattice constant of Ni measured from the diffraction spots was found to be 0.347 ± 0.0013 nm. This should be compared with 0.352 nm, the value of "a" in bulk Ni. The decrease in the lattice constant may be due to the strain experienced inside the tubes. Raman spectroscopy shows the typical signature of the tangential breathing mode present in the tubes at 1580 cm-1 that shifts to a new position when the C12 is replaced by 13C. The shift, however, is too small and is difficult to explain on the basis of mass difference. HRTEM experiments indicate the presence of Ni3C in the samples dominantly in the interfacial region.

The low-temperature Si(111)7 x 7-Pb interface has been investigated using surface X-ray diffraction. The Schottky barrier height of the Pb/Si diode is known to depend on the atomic reconstruction of the interface. We have studied the structure of the evolving interface using X-ray growth oscillations and by interrupting the growth and measuring the specular reflectivity. Both techniques reveal that the film is initially highly disordered with irregular oscillations in the reflected intensity during growth. At a critical coverage of approximately five Pb layers, the whole overlayer crystallizes to a well-ordered Pb(111) structure with subsequent growth being layer-by-layer. Detailed models of the mature interface are consistent with the preservation, after burial, of the Si(111)7 x 7-Pb reconstruction originally formed during growth of the first monolayer. (C) 1999 Elsevier Science B.V. All rights reserved.

A composite multilayer consisting of semiconducting CdS distributed in Cd-arachidate/arachidic acid layers was prepared by exposing the precursor Cd-arachidate multilayers to H2S. The structures of the precursor multilayers and the modified multilayers were studied by grazing incidence reflection/diffraction using X-rays from a conventional generator and a synchrotron source. The formation of CdS in arachidic acid/Cd-arachidate composite multilayers resulted in the introduction of a new layered structure with a period of 4.37 nm compared to the precursor multilayer period of 5.55 nm. The reduction in the period of the multilayers is consistent with the observation of a tilted structure found in alkanethiol and H2S exposed Cd-stearate multilayers. The lateral morphology of CdS which is replicated between the successive layers in the vertical direction is different when compared to the head group morphology in the precursor multilayer. The molecular arrangement in the plane of the Cd-arachidate precursor multilayer is found to be orthorhombic while the CdS molecules in the modified multilayer are arranged in a hexagonal array. The lateral size of the CdS domains/particles is found to be similar to 13 nm, at least an order of magnitude greater than the layer thickness. This clearly indicates that the CdS particles are quasi-2D in nature and are disk shaped.

The equilibrium molecular structure of the hexasilsesquioxane, Si6O9(OSiMe3)6, has been determined in the gas phase by electron diffraction. With OSi-containing substituents on the cage silicon atoms, this molecule closely resembles the moiety that if reproduced in a periodic manner would yield a zeolite-type structure. Semi-empirical molecular-dynamics (SE-MD) calculations were used to give amplitudes of vibration, vibrational distance corrections (differences between interatomic distances in the equilibrium structure and the vibrationally averaged distances that are given directly by the diffraction data) and anharmonic constants. A number of different SE-MD methods were tested, and their results are compared. The inclusion of d-type orbitals in the SE-MD method is crucial for obtaining accurate vibrational quantities for Si6O9(OSiMe3)6, with the PM6 and MNDO/D methods both giving acceptable values

The structure of 1,4-B4S2(NMe2)4 has been determined by gas-phase electron diffraction and quantum chemical calculations and is compared with the known solid-state structure. While these structures are similar, with a twisted ring geometry [the dihedral angle S-B-B-S from electron diffraction is 75.4(16)°], they are strikingly different to the solid-state structure of 1,4-B4O2(OH)4, which is planar. Using quantum chemical calculations, the combinations of O or S in the ring and OH or NMe2 as the substituent have been studied and it has been shown that there are two separate causes of the twisted ring. Since the calculated (and observed) structure of 1,4-B4O2(OH)4 is planar but that of 1,4-B4S2(OH)4 is twisted, it is concluded that the inclusion of sulfur in the ring twists the structure by approximately 40°. By comparing the structures of 1,4-B4S2(OH)4 and 1,4-B4S2(NMe2)4 it has been determined that the twist caused by the NMe2 groups is around 30°.

The ability to visualize molecular structure in the course of a chemical reaction or a biological function has been a dream of scientists for decades. X-ray absorption spectroscopy (XAS) is ideal in this respect because it is chemically selective and can be implemented in any type of medium. Furthermore, using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) in laser pump/X-ray probe experiments allows the retrieval of not only the local geometric structure of the system under study, but also the underlying electronic structure changes that drive the structural dynamics. We review recent developments in picosecond and femtosecond XAS applied to molecular systems in solution. Examples on ultrafast photoinduced processes such as intramolecular electron transfer, low-to-high spin change, and bond formation are presented.