Transition metal atoms on oxide supports density functional calculations

The adsorption properties of Cu, Ag, Ni, and Pd atoms on O^2?, F, and F⁺ sites of MgO, CaO, SrO, and BaO (001) surfaces have been studied by means of density functional calculations. The examined clusters were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The adsorption properties have been analyzed with reference to the basicity and energy gap of the oxide support in addition to orbital interactions. While the free Ni d⁹s¹ triplet ground state is preserved on adsorption on the O^2? sites of MgO, CaO, and SrO surfaces, it is no longer preserved on the O^2? site of BaO. For all adsorbates considered, adsorption is found to be stronger on F⁺ sites compared with regular O^2? sites. While on the O^2? site, Pd and Ni form the most stable complexes, on the F site, Pd and Cu form the most stable counterparts. On the F⁺ site, the single valence electron of Cu and Ag atoms couples with the unpaired electron of the vacancy forming a covalent bond. As a result, the adsorption energies of these atoms on the F⁺ site are stronger than those on the F and O^2? sites. The adsorption properties correlate linearly with the basicity and energy gap of the oxide support in addition to orbital interactions. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Enhanced Surface Interactions Enable Fast Li+ Conduction in Oxide/Polymer Composite Electrolyte

Li⁺‐conducting oxides are considered better ceramic fillers than Li⁺‐insulating oxides for improving Li⁺ conductivity in composite polymer electrolytes owing to their ability to conduct Li⁺ through the ceramic oxide as well as across the oxide/polymer interface. Here we use two Li⁺‐insulating oxides (fluorite Gd_0.1Ce_0.9O_1.95 and perovskite La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.55) with a high concentration of oxygen vacancies to demonstrate two oxide/poly(ethylene oxide) (PEO)‐based polymer composite electrolytes, each with a Li⁺ conductivity above 10^?4 S cm^?1 at 30 °C. Li solid‐state NMR results show an increase in Li⁺ ions (>10 %) occupying the more mobile A2 environment in the composite electrolytes. This increase in A2‐site occupancy originates from the strong interaction between the O^2? of Li‐salt anion and the surface oxygen vacancies of each oxide and contributes to the more facile Li⁺ transport. All‐solid‐state Li‐metal cells with these composite electrolytes demonstrate a small interfacial resistance with good cycling performance at 35 °C.     

An Interplay of Cooperativity between Cation⋅⋅⋅π, Anion⋅⋅⋅π and C?H⋅⋅⋅Anion Interactions

Mixed cation (Li⁺, Na⁺ and K⁺) and anion (F^?, Cl^?, Br^?) complexes of the aromatic π‐surfaces (top and bottom) are studied by using dispersion‐corrected density functional theory. The selectivity of the aromatic surface to interact with a cation or an anion can be tuned and even reversed by the electron‐donating/electron‐accepting nature of the side groups. The presence of a methyl group in the ? OCH₃, ? SCH₃, ? OC₂H₅ in the side groups of the aromatic ring leads to further cooperative stabilization of the otherwise unstable/weakly stable anion???π complexes by bending of the side groups towards the anion to facilitate C? H???anion interactions. The cooperativity among the interactions is found to be as large as 100 kcal mol^?1 quantified by dissection of the three individual forces from the total interaction energy. The crystal structures of the fluoride binding tripodal and hexapodal ligands provide experimental evidence for such cooperative interactions.     

Theoretical study of laser light generation and color image formation: FA1:Cs+ and FA2:Li+ centers at the low coordination (100) and (110) surfaces of AgCl and AgBr

The F_A1:Cs⁺ and F_A2:Li⁺ color centers at the low coordination (100) and (110) surfaces of AgCl and AgBr play important roles in laser light generation and color image formation. Double‐well potentials at these surfaces are investigated by using ab initio calculations. Quantum clusters were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces, and ions that are the nearest neighbors to the F_A ? defect site are allowed to relax to equilibrium. The calculated Stokes shifts suggest that laser light generation is sensitive to the simultaneous effects of the vibrational coupling mode, the impurity cation, the coordination number of the surface ion, the lattice anion, and the choice of the basis set centered on the anion vacancy. An attempt has been made to explain these effects in terms of Madelung potential, electron affinity, and optical–optical conversion efficiency. All relaxed excited states of the defect‐containing surfaces are deep below the lower edges of the conduction bands of the ground‐state defect‐free surfaces, suggesting that the F_A(I):Cs⁺ and F_A(II):Li⁺ centers are suitable laser defects. The dependence of orientational destruction, recording sensitivity, and exciton (energy) transfer on the empty cation; the coordination number of the surface ion; and the lattice anion is clarified. The Glasner–Tompkins empirical rule was generalized to include the impurity cation and the coordination number of the surface ion. As far as color image formation is concerned, the supersensitizer was found to increase the sensitizing capabilities of two primary dyes in the excited states by increasing the relative yield of quantum efficiency. The (110) surfaces of AgBr and AgCl were more sensitive than the corresponding (100) surfaces, and AgBr thin film was found to be more sensitive than that of AgCl. On the basis of quasi‐Fermi levels, the difference in the sensitizing capabilities between the examined dyes in the excited states is determined. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

O- radical ions on MgO: a tool for a structural description of the surface

The present contribution aims to show that detailed information on structure and location of surface defective sites (surface vacancies, low co-ordination sites) can be obtained by combining data on the chemical reactivity of polycrystalline surfaces and EPR data. The use of nitrous oxide for bleaching trapped electron centres at the surface of MgO produces a remarkable variety of O^- species, which differ in their values of the energy splitting (δE) term between the p_z and the other two p orbitals of the radical ion. This indicates the existence of a corresponding variety of surface traps having different structural features.     

O<Superscript>−</Superscript> radical anions on oxide catalysts: Formation,properties, and reactions

A systematic in situ EPR study of processes yielding O⁻ radical anions on the surface of oxide dielectrics (MgO, CaO), semiconductors (ZnO, TiO₂), supported systems (V/SiO₂), and zeolite FeZSM-5 is reported. Methodological approaches to the study of O-radical anions are considered for the cases in which these species are directly undetectable by EPR. Particular attention is focused on the development of methods of investigation of so-called α-oxygen on the FeZSM-5 surface, which is an O⁻ radical anion stabilized on the paramagnetic ion Fe³⁺. The reactions involving α-oxygen and the analogous reactions known for O-radical anions stabilized on the oxide surface are demonstrated to occur in similar ways. The photostimulated formation of spatially separated electron and hole centers on the surface of oxide systems is most likely due not to charge separation, but to the spatial separation of the radicals resulting from the homolytic photodissociation of chemisorbed water. A scheme is suggested for this process on the partially hydroxylated MgO surface.     

Ab initio study of the positronation of the CaO and SrO molecules including calculation of annihilation rates

Ab initio multireference single‐ and double‐excitation configuration interaction calculations have been performed to compute potential curves for ground and excited states of the CaO and SrO molecules and their positronic complexes, e⁺CaO, and e⁺SrO. The adiabatic dissociation limit for the ²Σ⁺ lowest states of the latter systems consists of the positive metal ion ground state (M⁺) and the OPs complex (e⁺O^?), although the lowest energy limit is thought to be e⁺M + O. Good agreement is found between the calculated and experimental spectroscopic constants for the neutral diatomics wherever available. The positron affinity of the closed‐shell X ¹Σ⁺ ground states of both systems is found to lie in the 0.16–0.19 eV range, less than half the corresponding values for the lighter members of the alkaline earth monoxide series, BeO and MgO. Annihilation rates (ARs) have been calculated for all four positronated systems for the first time. The variation with bond distance is generally similar to what has been found earlier for the alkali monoxide series of positronic complexes, falling off gradually from the OPs AR value at their respective dissociation limits. The e⁺SrO system shows some exceptional behavior, however, with its AR value reaching a minimum at a relatively large bond distance and then rising to more than twice the OPs value close to its equilibrium distance. © 2012 Wiley Periodicals, Inc.     

A comparison between the absorption properties of the regular and F<Subscript><Emphasis Type="Italic">s</Emphasis></Subscript>-defected MgO (100) surface

The electron density, the electrostatic potential and the electric field of the MgO (100) surface, both regular and containing an oxygen vacancy (F _s center), are compared in order to understand the modifications induced in the surface-absorbate interaction by the presence of the defect, with particular attention to the metal-oxide case. The spin-density for a gold atom absorbing on the most characteristic sites of the regular and F _s -defected surface is also shown. It is found that in the defected surface the electron pair in the vacancy protrudes appreciably out of the surface, thus shifting the electrostatic potential to negative values (but producing a similar electric field) and being able to chemically interact with neighboring absorbed species. These results rationalize the rotational invariance and double frustration effects previously described for the metal/F _s -defected MgO (100) surface.     

Unusual hydrogen bonding in L‐cysteine hydrogen fluoride

L‐Cysteine hydrogen fluoride, or bis(L‐cysteinium) difluoride–L‐cysteine–hydrogen fluoride (1/1/1), 2C₃H₈NO₂S⁺·2F⁻·C₃H₇NO₂S·HF or L‐Cys⁺(L‐Cys...L‐Cys⁺)F⁻(F⁻...H—F), provides the first example of a structure with cations of the `triglycine sulfate' type, i.e.A⁺(A...A⁺) (where A and A⁺ are the zwitterionic and cationic states of an amino acid, respectively), without a doubly charged counter‐ion. The salt crystallizes in the monoclinic system with the space group P2₁. The dimeric (L‐Cys...L‐Cys⁺) cation and the dimeric (F⁻...H—F) anion are formed via strong O—H...O or F—H...F hydrogen bonds, respectively, with very short O...O [2.4438 (19) Å] and F...F distances [2.2676 (17) Å]. The F...F distance is significantly shorter than in solid hydrogen fluoride. Additionally, there is another very short hydrogen bond, of O—H...F type, formed by a L‐cysteinium cation and a fluoride ion. The corresponding O...F distance of 2.3412 (19) Å seems to be the shortest among O—H...F and F—H...O hydrogen bonds known to date. The single‐crystal X‐ray diffraction study was complemented by IR spectroscopy. Of special interest was the spectral region of vibrations related to the above‐mentioned hydrogen bonds.     

Luminescence and defect centres in Tb3+ doped LaMgAl11O19 phosphors

Terbium (Tb) doped LaMgAl₁₁O₁₉ phosphors have been prepared by the combustion of corresponding metal nitrates (oxidizer) and urea (fuel) at furnace temperature as low as 500 °C. Combustion synthesized powder phosphor was characterized by X-ray diffraction and field emission scanning electron microscopy techniques. LaMgAl₁₁O₁₉ doped with trivalent terbium ions emit weakly in blue and orange light region and strongly in green light region when excited by the ultraviolet light of 261 nm. Electron Spin Resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the defect centres responsible for the thermally stimulated luminescence (TSL) process. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of at least two defect centres. One of the centres (centre I) with principal g-values g_∥ = 2.0417 and g_⊥ = 2.0041 is identified as O₂^? ion while centre II with an axially symmetric g-tensor with principal values g_∥= 1.9698 and g_⊥ = 1.9653 is assigned to an F⁺ centre (singly ionized oxygen vacancy). An additional defect centre is observed during thermal annealing experiments and this centre (assigned to F⁺ centre) seems to originate from an F centre (oxygen vacancy with two electrons). The F centre and also the F⁺ centre appear to correlate with the observed high temperature TSL peak in LaMgAl₁₁O₁₉:Tb phosphor.     

Stabilization of Substituted Triazenide Oxides: Synthesis and X‐ray Structural Features of Polymeric Potassium 3‐(4‐carboxylatophenyl)‐1‐methyltriazene N‐oxide‐hydrate, {K[O2C‐C6H4‐N(H)NN(CH3)O]·4H2O}n

3‐(4‐carboxyphenyl)‐1‐methyltriazene N‐oxide reacts with KOH in methanol/pyridine to give {K[O₂C‐C₆H₄‐N(H)NN(CH₃)O]·4H₂O}_n, Potassium‐3‐(4‐carboxylatophenyl)‐1‐methyltriazene N‐oxide). The terminal carboxylato group of the anion does not interact with the cation. In the crystal lattice of {K(C₈H₈N₃O₃)·4H₂O}_n each three of the four water molecules interact with two potassium cations, every K⁺ ion being the centre of six bridging K···O interactions. Potassium cations interact further with the terminal N‐oxigen atom of single [C₈H₈N₃O₃]^? anions achieving two parallel {C₈H₈N₃O₃^?K⁺}_n chains, which are linked through water molecules. The resulting polymeric, one‐dimensional chain, is operated by a screw axis 2₁ parallel to the crystallographic direction [010], along and equidistant to the K⁺ centres. The coordination of the K⁺ centres involves a distortion of the boat conformation of elementary sulfur (S₈) with the ideal C_2v symmetry.     

Oxygen Vacancies in Amorphous InOx Nanoribbons Enhance CO2 Adsorption and Activation for CO2 Electroreduction

Tuning surface electron transfer process by oxygen (O)‐vacancy engineering is an efficient strategy to develop enhanced catalysts for CO₂ electroreduction (CO₂ER). Herein, a series of distinct InO_x NRs with different numbers of O‐vacancies, namely, pristine (P‐InO_x), low vacancy (O‐InO_x) and high‐vacancy (H‐InO_x) NRs, have been prepared by simple thermal treatments. The H‐InO_x NRs show enhanced performance with a best formic acid (HCOOH) selectivity of up to 91.7 % as well as high HCOOH partial current density over a wide range of potentials, largely outperforming those of the P‐InO_x and O‐InO_x NRs. The H‐InO_x NRs are more durable and have a limited activity decay after continuous operating for more than 20 h. The improved performance is attributable to the abundant O‐vacancies in the amorphous H‐InO_x NRs, which optimizes CO₂ adsorption/activation and facilitates electron transfer for efficient CO₂ER.     

An optically resolved crystal of thiomalate: (S)‐1‐phenylethanaminium (R)‐thiomalate

The asymmetric unit of the optically resolved title salt, C₈H₁₂N⁺·C₄H₅O₄S⁻, contains a 1‐phenylethanaminium monocation and a thiomalate (3‐carboxy‐2‐sulfanylpropanoate) monoanion. The absolute configurations of the cation and the anion are determined to be S and R, respectively. In the crystal, cation–anion N—H...O hydrogen bonds, together with anion–anion O—H...O and S—H...O hydrogen bonds, construct a two‐dimensional supramolecular sheet parallel to the ab plane. The two‐dimensional sheet is linked with the upper and lower sheets through C—H...π interactions to stack along the c axis.     

Oxythiamine hexafluorophosphate monohydrate,a thiamine antagonist with the same conformation as ­thiamine

In the title compound, 3‐[(3,4‐di­hydro‐2‐methyl‐4‐oxopyrimidin‐5‐yl)­methyl]‐5‐(2‐hydroxy­ethyl)‐4‐methyl­thia­zolium hexa­fluoro­phosphate monohydrate, C₁₂H₁₆N₃O₂S⁺·PF₆^?·H₂O, oxy­thi­amine is a monovalent cation with a neutral oxo­pyrimidine ring. The mol­ecule assumes the F conformation, which is a common form for thi­amine but which is substantially different from the unusual V conformation found in the chloride and hydro­chloride salts of oxy­thi­amine. The anion‐bridging interaction, C—H?anion?pyrimidine, is emphasized as being important for stabilization of the F conformation.     

Influence of basic residues on dissociation kinetics and dynamics of singly protonated peptides: time‐resolved photodissociation study

Product ion yields in postsource decay and time‐resolved photodissociation at 193 and 266 nm were measured for some peptide ions with lysine ([KF₆ + H]⁺, [F₆K + H]⁺, and [F₃KF₃ + H]⁺) formed by matrix‐assisted laser desorption ionization. The critical energy (E₀) and entropy (ΔS^?) were determined by RRKM fitting of the data. The results were similar to those found previously for peptide ions with histidine. To summarize, the presence of a basic residue, histidine or lysine, inside a peptide ion retarded its dissociation by lowering ΔS^?. On the basis of highly negative ΔS^?, presence of intramolecular interaction involving a basic group in the transition structure was proposed. Copyright © 2009 John Wiley & Sons, Ltd.     

The hydrochloride and hydrobromide salt forms of (S)‐amphetamine

Despite the high profile of amphetamine, there have been relatively few structural studies of its salt forms. The lack of any halide salt forms is surprising as the typical synthetic route for amphetamine initially produces the chloride salt. (S)‐Amphetamine hydrochloride [systematic name: (2S)‐1‐phenylpropan‐2‐aminium chloride], C₉H₁₄N⁺·Cl⁻, has a Z′ = 6 structure with six independent cation–anion pairs. That these are indeed crystallographically independent is supported by different packing orientations of the cations and by the observation of a wide range of cation conformations generated by rotation about the phenyl–CH₂ bond. The supramolecular contacts about the anions also differ, such that both a wide variation in the geometry of the three N—H...Cl hydrogen bonds formed by each chloride anion and differences in C—H...Cl contacts are apparent. (S)‐Amphetamine hydrobromide [systematic name: (2S)‐1‐phenylpropan‐2‐aminium bromide], C₉H₁₄N⁺·Br⁻, is broadly similar to the hydrochloride in terms of cation conformation, the existence of three N—H...X hydrogen‐bond contacts per anion and the overall two‐dimensional hydrogen‐bonded sheet motif. However, only the chloride structure features organic bilayers and Z′ > 1.     

Tetra­phenyl­phos­phonium 4‐hydroxy‐2,2,6,6‐tetra­methyl­piperidinyl­oxy‐4‐sulfonate

The title compound, C₂₄H₂₀P⁺·C₉H₁₇NO₅S⁻, consists of an organic monovalent cation and an organic monovalent anion, the latter being derived from the TEMPO radical (TEMPO is 2,2,6,6‐tetra­methyl­piperidin‐1‐oxyl). Two inversion‐related anions interact via two –O—H⃛O—S– hydrogen bonds, forming a dimer in which there are no short contacts between the spin centres (–N—O) of the TEMPO(OH)SO₃⁻ anions. Furthermore, no significant magnetic interaction is observed between the dimers because the dimer is surrounded by cations. These results are consistent with the paramagnetic behaviour of the title salt.     

10‐Methyl‐ and 9,10‐dimethyl­acridinium methyl sulfate

The title compounds, C₁₄H₁₂N⁺·CH₃O₄S^?, (I), and C₁₅H₁₄N⁺·CH₃O₄S^?, (II), respectively, crystallize with the planar 10‐methylacridinium or 9,10‐di­methyl­acridinium cations arranged in layers, parallel to the twofold axis in (I) and perpendicular to the 2₁ axis in (II). Adjacent cations in both compounds are packed in a `head‐to‐tail' manner. The methyl sulfate anion only exhibits planar symmetry in (II). The cations and anions are linked through C—H?O interactions involving three O atoms of the anion, six acridine H atoms and the CH₃ group on the N atom in (I), and the four O atoms of the anion, three acridine H atoms and the carbon‐bound CH₃ group in (II). The methyl sulfate anions are oriented differently in the two compounds relative to the cations, being nearly perpendicular in (I) but parallel in (II). Electrostatic interaction between the ions and the network of C—H?O interactions leads to relatively compact crystal lattices in both structures.     

Influence of SrO content on microstructure and crystallization of glazes in the SiO2–Al2O3–CaO–MgO–K2O system

The effect of the SrO addition on the microstructure and structure of the glazes from the SiO₂–Al₂O₃–CaO–MgO–K₂O system was investigated in this study. The results were obtained by testing the ability of the frits crystallization, the stability of the crystallizing phases during the single-step fast-firing cycle depending on their chemical composition and the effect of addition of strontium oxide. Differential scanning calorimetry (DSC) curves showed that all glazes crystallized, and diopside and anorthite were mainly identified as dominant phases in the obtained glazes, while the size and amount of each depended on the amount of SrO introduced. The thermal characteristic of the frits was carried out using DSC, and crystalline phases were determined by X-ray diffractometry. The glaze microstructure was investigated by scanning electron microscopy and transmission electron microscopy. Additional information on the microstructure of frits was derived from spectroscopic studies in the mid-infrared range.

     

Molecular structures of 3‐[(2,2,3,3‐tetrafluoropropoxy)methyl]‐ and 3‐[(2,2,3,3,3‐pentafluoropropoxy)methyl]pyridinium saccharinates

The salts 3‐[(2,2,3,3‐tetrafluoropropoxy)methyl]pyridinium saccharinate, C₉H₁₀F₄NO⁺·C₇H₄NO₃S⁻, (1), and 3‐[(2,2,3,3,3‐pentafluoropropoxy)methyl]pyridinium saccharinate, C₉H₉F₅NO⁺·C₇H₄NO₃S⁻, (2), i.e. saccharinate (or 1,1‐dioxo‐1λ⁶,2‐benzothiazol‐3‐olate) salts of pyridinium with –CH₂OCH₂CF₂CF₂H and –CH₂OCH₂CF₂CF₃meta substituents, respectively, were investigated crystallographically in order to compare their fluorine‐related weak interactions in the solid state. Both salts demonstrate a stable synthon formed by the pyridinium cation and the saccharinate anion, in which a seven‐membered ring reveals a double hydrogen‐bonding pattern. The twist between the pyridinium plane and the saccharinate plane in (2) is 21.26 (8)° and that in (1) is 8.03 (6)°. Both salts also show stacks of alternating cation–anion π‐interactions. The layer distances, calculated from the centroid of the saccharinate plane to the neighbouring pyridinium planes, above and below, are 3.406 (2) and 3.517 (2) Å in (1), and 3.409 (3) and 3.458 (3) Å in (2).