Mössbauer study of some Fe-containing M6 and M5 clusters

Isomer shift (δ) and quadrupole splitting (Δ) parameters have been assigned to the iron sites in [FeRh₅(CO)₁₆]⁻, trans- and cis-[Fe₂Rh₄(CO)₁₆]²⁻, [Fe₃-Rh₃(CO)₁₇]³⁻, [FeRh₄(CO)₁₅]²⁻, [Fe₃Pt₃(CO)₁₅]²⁻ and [Fe₄M(CO)₁₆]²⁻ (M = Pd or Pt) from ⁵⁷Fe Mössbauer spectra recorded at 78 K. The data for the closo compounds [FeRh₅(CO)₁₆]⁻ and [Fe₂Rh₄(CO)₁₆]²⁻ are compared with those for [Fe₆(CO)₁₆C]²⁻. In [Fe₃Rh₃(CO)₁₇]³⁻, the three major Fe sites were identified. For both [Fe₄M(CO)₁₆]²⁻ compounds two isomers were shown to be present in the solid state.     

Möbius bis and tris-spiroaromatic systems

We propose that a general class of bis and tris-spiro 7-membered ring systems with a common atom X, of which there are a number of examples characterised crystallographically, can in fact be considered as spiroaromatic molecules in which each ring exhibits some degree of M?bius 4n pi-electron aromaticity. The aromaticity is probed as a function of the spiro-atom using ab initio calculations of the NICS(0) values, which indicate that the M?bius-aromaticity increases as the spiro-atom is changed e.g. from Al to P, and from e.g. P to As.     

Fe- and Cr-substituted mullites: Mössbauer spectroscopy and rietveld structure refinement

Fe-substituted mullite samples Al_4.5-yFe_ySi_1.5O_9.75 (y = 0.25 and 0.50) have been prepared using a sol-gel route involving Si and Al alkoxides and iron nitrate as precursors. Mössbauer spectroscopy reveals the presence of three different Fe(III) sites but does not allow to conclude concerning the ion environments, tetrahedral or octahedral. Rietveld refinements reveal that Fe ions are distributed between octahedral and tetrahedral positions, the proportion of octahedral iron increasing from y = 0.25 to y = 0.50. On the other hand, for Cr-substituted mullite, Cr ions arc mainly located on octahedral positions. This difference between Fe and Cr has been attributed to the fact that Cr shows a clear trend to occupy octahedral sites compared to Fc and that it is incorporated inside the mullite structure only at high temperature.     

M?ssbauer spectroscopy and structural analysis of solids

Mössbauer spectroscopy is reviewed as a method of analysis of hyperfine interactions in the solid state. It is sensitive both to the atomic scale and to phase structures. It utilizes the interactions between the hyperfine fields and nuclei in solids measured by a nuclear technique. The importance of various Mössbauer isotopes is discussed, the ⁵⁷Fe being still the most important. Principles of the qualitative determination of the structure sites and/or phase attachment are explained on the basis of the measurement of hyperfine structure parameters (i.e. the isomer (chemical) shift, the quadrupole and magnetic splittings). The role of the hyperfine field distribution determination is stressed, especially the magnetic hyperfine induction distribution in magnetically ordered solids. Conditions are explained for the feasibility of quantitative estimations of site occupancy and phase abundance. With respect to the predominant role of the magnetic hyperfine structure predestinating Mössbauer spectroscopy to be considered simultaneously as a special magnetic measuring technique, examples are chosen from the field of new magnetic materials. For the substituted hexagonal (M-type) ferrites (aimed, e.g., for the perpendicular magnetic recording), Mössbauer determination of the cation site occupancy is discussed. Structural changes in ion implanted Fe-B-based amorphous alloys detected by the hyperfine field distribution are shown. For the magnetically extremely soft FeCuNbSiB alloys, produced by the controlled crystallization of an amorphous ribbon, the estimation of their rather complicated phase composition by the Mössbauer phase analysis is demonstrated.Common enterprise of the Department of Low Temperature Physics with the Institute of Physics and Institute of Inorganic Chemistry, Czech Academy of Sciences, Prague     

Preparation and characterization of hexacyclohexoxido- and trispinacolato-ditungsten (MM). A comparison of staggered and eclipsed ethane-like O3WWO3 units

The reactions between W₂(OBu^t)₆ and each of pinacol (3 equiv) and cyclohexanol (> 6 equiv) yield the title compounds W₂(OCMe₂CMe₂O)₃, I, and W₂(OCy)₆, II (Cy = cyclohexyl) as yellow needles, which are sparingly soluble in hydrocarbon solvents. In the solid-state both compounds form infinite chains. Compound I has three pinacolate ligands which span the WW bond: WW = 2.2738(8) Å, WO = 1.90 Å (av) and WWO = 99° (av). The three bridging ligands lead to a near eclipsed W₂O₆ skeleton having OWWO torsion angles = 10° (av). The infinite chain is caused by weak intermolecular oxygen-to-tungsten bonding, W … O = 2.9 Å. The W₂(OCy)₆ molecule has crystallographically imposed C_2h symmetry and the central W₂O₆ moiety is staggered (virtual D_3d) with WW = 2.340(1) Å, WO = 1.87 Å (av) and WWO = 107° (av). The infinite chain in the solid-state results from the stacking of the cyclohexyl ligands. The intermolecular contacts are essentially identical to the intramolecular ones. In solution W₂(OCMe₂CMe_2O)₃ undergoes rapid enantiomerization such that a time-averaged eclipsed W₂O₆ moiety is seen on the NMR time-scale. The cyclohexyl rings in W₂(OCy)₆ are not inverting (chair ⇌ boat ⇌ chair) on the NMR time-scale.     

Cyclic M（S02） （M=Zn,Cd） and its Anions： Matrix Infrared Spectra and DFT Calculations

Reaction of laser ablated zinc and cadmium atoms with SO2 molecules was studied by low temperature matrix isolation infrared spectroscopy. Cyclic M（SO2） and anion M（SO2）-（M=Zn, Cd） were produced in excess argon and neon, which were identified by 34SO2 and S18O2 isotopic substitutions. The observed infrared spectra and molecular structures were confirmed by density functional theoretical calculations. Natural charge distributions indicated significant electron transfer from s orbitals of zinc or cadmium metal atom to S02 ligand and cyclic M（SO2） complexes favored ＂ion pair＂ M＋（SO2）-formation, which were trapped in low temperature matrices. In addition Zn-O or Cd-O bond in M（SO2） exhibited strong polarized covalent character. Reaction of Hg atom with SO2 was also investigated, but no reaction product was observed, due to the relativistic effect that resulted in the contraction of 6s valence shell and high ionization potential of Hg atom.     

Spectrophotometric Study on the Interaction between Arsenazo M and Proteins

Dye binding protein assays are commonly used in biochemical and clinical laboratories, and the reaction mechanism is still under investigation1-2. In this paper, the interaction of Arsenazo M with serum proteins was studied. Procedure In most experiments, 1.20 mL of Arsenazo M (5.0010-4 mol/L), 2.0 mL of C-L buffer solution, 3.0 mL alcohol and certain amount of protein standard solutions or samples were added to 10 mL volumetric flasks, then diluted to mark with water, and mixed thor…     

Halogen bonds and metal bonds involving superalkalies M2OCN/M2NCO (M = Li,Na) complexes

Structural Chemistry - A new type of halogen bond formed by supermetals or superalkalies with dihalogen molecules was analyzed by means of ab initio at the MP2/aug-cc-pVTZ level. The results reveal...     

Synthesis and properties of the first Möbius annulenes

Heilbronner in 1964 predicted that annulenes with ".. a planar perimeter of N=4r AO's, which would yield an open shell configuration when occupied by 4r electrons, can be twisted into a closed shell M?bius strip perimeter without loss in pi electron energy". We have been able to synthesize the first [4n]annulene with such a M?bius topology and now present further M?bius isomers and the details of their preparation as stable compounds. To address the question whether the twist in the pi system has an effect on the properties we systematically investigate energy, geometry and magnetic parameters of a large number isomers of [16]annulenes. The M?bius twisted annulenes are consistently more aromatic than the non-twisted isomers. This is true for the parent as well as our benzoannelated systems. Our results are in contrast to those published recently by C. Castro, W. L. Karney, P. von R. Schleyer et al.     

Mössbauer Spectroscopy of Iron Oxides and Related Compounds

The superparamagnetic iron oxide particles with a diameter of about 10 and 16 nm were obtained by the reaction of Fe²⁺ and Fe³⁺ ions in a water pool of reversed microemulsions. The obtained particles were ascribed to -Fe₂O₃ due to the oxidation of Fe₃O₄. Very fine particles of -Fe and Fe₃O₄ were also obtained by the thermal decomposition of FeC₂O₄2H₂O. The decomposition products and their particle size depended on the heat treatments.     

Möbius function and characteristic monomials for combinatorial enumeration

After the definitions of amplified representations and number-theoretical vectors, the markaracter table of a cyclic subgroup is converted into the corresponding Q-conjugacy character table. The conversion is shown to necessitate an interconversion matrix that contains M?bius functions as elements. Since the interconversion matrix gives characteristic monomials for cyclic groups, all the powers appearing in each of the characteristic monomials are shown to be integers. Characteristic monomials for finite groups are then built up by starting from those of cyclic groups. This procedure clarifies the fact that all the powers appearing in each characteristic monomial for finite groups are integers. The relationship between characteristic monomial tables and unit-subduced-cycle-index tables is discussed with respect to their application to isomer enumeration. Received: 15 July 1998 / Accepted: 16 December 1998 / Published online: 16 March 1999     

A 3D Supramolecular Pd（Ⅱ） Complex via Hydrogen-bonding and Weak M…M Interactions

The reaction of 2,4,6-tris（2-pyridyl）-1,3,5-triazine （tptz） and K2PdCl4 in THF/H2O （10：1） results in the hydrolysis of tptz to bis（2-pyridylcarbonyl）amide anion （bpca）, and affords complex Pd（bpca）Cl 1, which is the first example of Pd（Ⅱ）-promoted hydrolysis of ligands. Crystal data for 1： orthorhombic, space group Pbcn, a = 12.0136（17）, b = 14.180（2）, c = 6.9747（11）A, V= 1188.2（3） A3, Z = 4, Mr = 368.06, Dc = 2.058 g/cm3, F（000） = 720,μ = 1.786 mm^-1, λ（MoKα） = 0.71073 A, T= 293（2） K, 2θmax = 54.9°, GOOF = 1.085, the final R= 0.0647 and wR = 0.1051 for 1234 observed reflections with I 〉 2σ（I） （refinement on F2）. Complex 1 is connected through hydrogen bonding to give a 2D network. And weak Pd...Pd interactions are also found between adjacent molecules with the distance of 3.6074（5） A, so the complex is further extended into a 3D supramolecular structure. Thermal gravity analysis （TGA） shows that 1 exhibits high thermal stability below 310℃. X-ray powder diffraction （XRD） and UV/Vis spectrum of 1 are also discussed.     

Möbius and twisted graphene nanoribbons: stability, geometry, and electronic properties

Results of classical force field geometry optimizations for twisted graphene nanoribbons with a number of twists N(t) varying from 0 to 7 (the case N(t)=1 corresponds to a half-twist M?bius nanoribbon) are presented in this work. Their structural stability was investigated using the Brenner reactive force field. The best classical molecular geometries were used as input for semiempirical calculations, from which the electronic properties (energy levels, HOMO, LUMO orbitals) were computed for each structure. CI wavefunctions were also calculated in the complete active space framework taking into account eigenstates from HOMO-4 to LUMO+4, as well as the oscillator strengths corresponding to the first optical transitions in the UV-VIS range. The lowest energy molecules were found less symmetric than initial configurations, and the HOMO-LUMO energy gaps are larger than the value found for the nanographene used to build them due to electronic localization effects created by the twisting. A high number of twists leads to a sharp increase of the HOMO-->LUMO transition energy. We suggest that some twisted nanoribbons could form crystals stabilized by dipolar interactions.     

Heat capacities and thermodynamic properties of M(HBTC)(4,4′-bipy)·3DMF (M?=?Ni and Co)

Two metal?Corganic frameworks (MOFs) of M(HBTC)(4,4??-bipy)·3DMF [M?=?Ni (for 1) and Co (for 2); H₃BTC?=?1,3,5-benzenetricarboxylic acid (1,3,5-BTC); 4,4??-bipy?=?4,4??-bipyridine; DMF?=?N,N??-dimethylformamide] were synthesized by a one-pot solution reaction and a solvothermal method, respectively, and characterized by powder X-ray diffraction and FT-IR spectra. The low-temperature molar heat capacities of M(HBTC)(4,4??-bipy)·3DMF were measured by temperature-modulated differential scanning calorimetry (TMDSC) for the first time. The thermodynamic parameters such as entropy and enthalpy relative to reference temperature 298.15?K were derived based on the above molar heat capacity data. Moreover, the thermal stability and the decomposition mechanism of M(HBTC)(4,4??-bipy)·3DMF were investigated by thermogravimetry analysis (TGA). The experimental results through TGA measurement demonstrate that both of the two compounds have a three-stage mass loss in air flow.     

Transition-metal-centered nine-membered boron rings: MⓒB9 and MⓒB9(-) (M = Rh, Ir)

We report the observation of two transition-metal-centered nine-atom boron rings, Rh?B(9)(-) and Ir?B(9)(-). These two doped-boron clusters are produced in a laser-vaporization supersonic molecular beam and characterized by photoelectron spectroscopy and ab initio calculations. Large HOMO-LUMO gaps are observed in the anion photoelectron spectra, suggesting that neutral Rh?B(9) and Ir?B(9) are highly stable, closed shell species. Theoretical calculations show that Rh?B(9) and Ir?B(9) are of D(9h) symmetry. Chemical bonding analyses reveal that these complexes are doubly aromatic, each with six completely delocalized π and σ electrons, which describe the bonding between the central metal atom and the boron ring. This work establishes firmly the metal-doped B rings as a new class of novel aromatic molecular wheels.     

M≡E and M=E Complexes of Iron and Cobalt that Emphasize Three-fold Symmetry (E = O, N, NR)

Mid-to-late transition metal complexes that feature terminal, multiply bonded ligands such as oxos, imides, and nitrides have been invoked as intermediates in several catalytic transformations of synthetic and biological significance. Until about ten years ago, isolable examples of such species were virtually unknown. Over the past decade or so, numerous chemically well-defined examples of such species have been discovered. In this context, the presentreview summarizes the development of 4- and 5-coordinate Fe(E) and Co(E) species under local three-fold symmetry.     

X-Ray Diffraction and Mössbauer Characterization of Raney Fe-Ni Catalysts

Rancy type catalysts were prepared by means of a two step procedure: (1) Mechanical alloying of the metals and, (2) alkaline aluminum leaching. Mechanical alloying is a novel atlernative related to the synthesis of skeletal Ni catalysts. X-ray diffraction, and Mössbauer spectroscopies were performed for catalysts characterization. Binary Al-Ni and ternary Al-Ni-Fe alloys were produced by mechanical alloying from pure metallic powders; in particular, the intermetallic -(AlNi) phase was formed with a fine microstructure as a non-equilibrium phase; then, aluminum was selectively removed. After aluminum leaching the -(AlNi) phase was transformed into the more stable nickel fee structure.     

Magnetic and Mössbauer Studies of Magnetite-Loaded Polyvinyl Alcohol Hydrogels

Materials producing strain in a magnetic field are known as magnetoelastic or magnetostrictive materials. A new type of material that is able to produce giant strain in a nonhomogeneous magnetic field has been developed. In these magnetic-field-sensitive gels (ferrogels) fine colloidal particles having superparamagnetic behavior are incorporated into a highly swollen elastic polymer network. Magnetic properties of ferrogels have been investigated using electron microscopy, static magnetization measurements, and M?ssbauer spectroscopy. Analysis of the data yielded information on the superparamagnetic behavior of ferrogels and made it possible to estimate the size distribution of the magnetic cores of magnetite particles made by chemical precipitation and built into a chemically cross-linked polyvinyl alcohol matrix. The results are interpreted on the basis of a core-shell model. Copyright 2000 Academic Press.