Change In Coordination Number Research Articles

X-ray diffraction study of ground talc Mg3Si4O10(OH)2

A structural analysis of talc ground under dry atmospheric conditions for different grinding times has been made by X-ray diffraction. The SiO4 tetrahedra are found to remain unchanged in the ground talc samples by analysing the radial distribution function. On the other hand, with an increase of the grinding time a change in oxygen coordination number around magnesium has been quantitatively confirmed. This variation is attributed to the change of a hydroxyl induced by the prolonged grinding.

Structure analysis of Tb-Fe amorphous films using extended x-ray absorption fine structure

Extended x-ray absorption fine structure analysis was employed to obtain insight into the influence of structural change on the magnetic and magneto-optical properties of Tb-Fe amorphous films. Films made by bias sputtering show a 20% lower density and a lower apparent coordination number of Fe atoms around an Fe atom than those made by nonbias sputtering. However, their interatomic distances are unaffected by the sputtering methods. After samples made by nonbias sputtering have been annealed, the atomic distances of Fe-Fe and Tb-Fe tend to increase and decrease, respectively, while the apparent coordination number, N*, and the structural disorder parameter, σ, increase and decrease, respectively, in association with a large deterioration in magnetic and magneto-optical properties. On the other hand, in the case of bias-sputtering films, samples annealed at 200 °C show a large decrease in magnetic anisotropy, Ku, but no significant change in individual atomic distances and apparent coordination number. However, the disorder parameter, σ, increases slightly on annealing at 200 °C.

Complex Formation of [Ph2P(O)I2 C=CH2 and [Ph2 P(O) I2 C=PPh3 with Phosphorus and Tantalum Pentafluorides

Abstract Reactions of PF5 and TaF5 with [Ph2P(O)]2 C=CH2 (I) and [Ph2P(O)]2 C=PPh3 (II) in MeCN and CH2Cl2 were studied by means of 19F, 31P, 1H and 13C NMR spectroscopy. It has become evident, that one or two phosphoryl groups in (I) and (II), as well as in cis- and trans-Ph2P(O)CH=CHP(O)Ph2, are involved in complex formation. The formation of tetra-fluoro cations PFL and PF4L along with pentafluorocom-plexes PF5L and TaF5L was found. Ligands are coordinated with central ions of complexes as chelates. The trans-atoms F1 of TaF 5L are nonequivalent because of nonsymmetric position to the Ph3P-group. The F1-atoms in PF4L are supposed to be symmetric to the Ph3P-group. The formation of tri-fluorocomplexes TaOF3L was also observed. Since the position of 19F NMR resonance lines of TaOF3L is near to that of pentafluorocomplexes, it can be supposed that either the change of Ta coordination number takes place, either oxygen atom comes into complex with inner sphere in the reaction with ligand or during hydrolysis.

FT-IR and fluorometric investigation of rare earth and metal ion solvation. 9. Evidence for a coordination number change along the lanthanide series: FT-IR investigation of the solvates [Ln(NO3)3(DMSO)n] in anhydrous acetonitrile

In this paper, we extend our previous work on the interaction between Ln(NO 3 ) 3 (Ln=Nd, Eu, Tb, Er) and DMSO in anhydrous acetonitrile (MeCN) 12,17 to all the remaining members of the Ln series, with the exception of Pm. The reported coordination numbers represent the first consistent series of such data for Ln(III) ions in organic solution. Apparent stability constants for the coordination of DMSO are also evaluated and discussed

Particle Size Effects for Oxygen Reduction on Highly Dispersed Platinum in Acid Electrolytes

The particle size effect for oxygen reduction kinetics on highly dispersed Pt particles in acid electrolytes are discussed. It is suggested that the change in the fraction of surface atoms on the (100) and (111) crystal faces of Pt particles, which are assumed to be cubo‐octahedral structures, can be correlated to the mass activity (A/g Pt) and specific activity (μA/cm2 Pt) of highly dispersed Pt electrocatalysts. The maximum in mass activity that is observed at ∼3.5 nm in several studies is attributed to the maximum in the surface fraction of Pt atoms on the (100) and (111) crystal faces, which results from the change in surface coordination number with a change in the average particle size. The reduction of oxygen on supported Pt particles in acid electrolytes is classified as a demanding or structure‐sensitive reaction; the specific activity increases with an increase in particle size.

Size and structural dependence of the magnetic properties of small 3d-transition-metal clusters

The size and structural dependence of magnetic properties of small ${\mathrm{Cr}}_{\mathrm{n}}$, ${\mathrm{Fe}}_{\mathrm{n}}$, and ${\mathrm{Ni}}_{\mathrm{n}}$ clusters are determined by using a tight-binding Hubbard Hamiltonian in the unrestricted Hartree-Fock approximation. Results are given for the average magnetic moment \ensuremath{\mu}${\ifmmode\bar\else\textasciimacron\fi{}}_{n}$, local magnetic moments, and magnetic ordering at T=0. We obtain for ${\mathrm{Fe}}_{\mathrm{n}}$ (n\ensuremath{\le}15) that \ensuremath{\mu}${\ifmmode\bar\else\textasciimacron\fi{}}_{n}$ varies as a function of n due to the interplay between the changes in coordination number and bond length as a function of cluster size, which may be related to recent experiments. The dependence of the structural cluster stability on magnetism is discussed. Results are also given for the cohesive energy, average bond length, and local densities of states.

Elastic constants of the iron oxide doped yttria-stabilized zirconia

Ultrasonic compressional and shear wave velocity measurements have been made on the (ZrO2)1−A (Y2O3)A−X (Fe2O3) X system at 5 MHz. The amounts of Fe2O3 dopant ranging from 0 up to 10 mol% were mixed with the ceramic matrix. In this paper the ultrasonic compressional and shear wave velocity measurements made on this system at room temperature are discussed. The elastic moduli, Poisson's ratio and density are found to be sensitive to the additions of the Fe2O3 dopant and they show an anomalous behaviour. A qualitative explanation of the compositional dependence of the elastic moduli and Poisson's ratio is given in terms of changes in packing density, bond strength, co-ordination number and cross-link density.

Ultrasonic investigations of the complexation between Eu III and NO −3 in aqueous methanol

Ultrasonic absorption measurements were carried out on 0.200 M Eu(NO 3) 3 solutions at 25 °C between 10 and 210 MHz. The absorption data are consistent with those obtained for Er III and Nd III systems and indicate a change in coordination number in aqueous methanol.

ChemInform Abstract: Dynamics of Spin Equilibria in Metal Complexes

Publisher Summary This chapter examines the dynamics of spin-equilibrium processes, principally from studies in solutions. The properties of the complexes that are relevant to the dynamics studies are reviewed. The techniques used to observe these rapid processes are also described in the chapter. The chapter presents some aspects of solid-state dynamics and describes some implications for the description of intersystem crossing processes in excited states and for spin equilibria in heme proteins. Many of the complexes that occur in spin equilibrium possess ligands with complicated structures. Trivial abbreviations are used, with structural formulas presented in the table. Generally the complexes are of low symmetry, but in the description of their electronic structure idealized symmetries are assumed and the appropriate term symbols are used accordingly. Some of the concepts, which are used to describe intramolecular spin equilibria, can be extended to the description of these coordination-spin equilibria. Examples include equilibria among four-, five-, and six-coordinate nickel (II) complexes and equilibria involving coordination number changes in iron porphyrin complexes and in heme proteins. Anomalous magnetic susceptibility is the characteristic feature of spin-equilibrium complexes. The Evans nuclear magnetic resonance (NMR) method gives excellent results provided adequate care is taken. There have been very few reports of the Raman spectra of spin-equilibrium complexes.

Ionic dynamics in computer simulated molten LiNO3. I. Translational and reorientational motion

In order to study the microdynamics of translational and orientational motion of ions in liquid electrolytes, a molecular dynamics simulation of molten LiNO3, which includes all degrees of freedom of vibrating nitrate ions, has been performed. Simple Coulomb pair potentials with Born-type repulsions have been adopted in the simulation. The calculated properties include: translational velocity and force correlation functions, rotational correlation functions, and angular momentum and torque correlation functions. The nature of the ionic motion is analyzed in terms of quasioscillation and quasilibration. Local structures surrounding nitrate ions, which can be characterized by, e.g., the coordination number, are found to persist for a long time (∼1 ps). The coupling between the environmental structure and the translational and orientational behaviors of a given nitrate ion has been examined through selectively sampled correlation functions. We found that the change of coordination number enhances the translational diffusion and the rotational diffusion is slow if the coordination number remains low. This shows that the dynamics are very sensitive to the exact shape of the potential and the dynamics itself. Results of the simulation are compared with the experimental diffusion constants and dynamical behaviors revealed by Raman spectroscopy. While the interionic interaction assumed has been found to describe quite well the static structure of molten LiNO3, simulated translational diffusion and rotational relaxation seem to be somewhat too fast, suggesting too shallow an attractive well in the potential we have assumed.

ChemInform Abstract: An Analysis of Complex Alkyl‐Substituent Effects in the Water‐Catalyzed Hydrolysis of 1‐Acyl‐1,2,4‐triazoles

Pseudo-first-order rate constants and thermodynamic activation parameters have been determined for the water-catalyzed hydrolysis of the 1-acyl-1,2,4-triazoles 1–9 in water and in aqueous acetonitrile [x(H2O) = 0.80]. The reaction occurs via water-catalyzed nucleophilic attack of water at the amide carbonyl group. Variation of the alkyl group in the acyl part of the substrate led to the sequence of reactivities (in water): R = t-Bu > i-Pr > Et > Me > n-Pr > s-Bu > i-Pent > neo-Pent. This complex behavior strongly suggests the operation of a composite steric effect most likely involving contributions from conformational preference (SEI), change of coordination number at carbonyl carbon (SE2), steric repulsion between O2 and the alkyl group (SE3), and steric inhibition of solvation (SE4). Modelling of the relative rates in water in terms of the expanded branching equation gave satisfactory results. Two possible transition states are proposed for the neutral hydrolysis, one involving intramolecular hydrogen bonding between the second water molecule and N2 of the 1,2,4-triazole ring. The different sequence of reactivities for neutral hydrolysis in aqueous acetonitrile as well as the rates for hydroxide-ion-catalyzed hydrolysis in water (B AC2 mechanism) support the analysis of substituent effects.

Structural study of binary phosphate glasses with MgO, ZnO, and CaO by x-ray diffraction

The X-ray diffraction profiles of magnesium, and calcium phosphate glasses have been measured and the pair function distributions (PFDs) were calculated. Using the pair function method, distance and coordination numbers for the pairs of PO, OO, and MO (M = Mg, Zn or Ca) were determined. A fundamental unit of the structure in these phosphate glasses is confirmed to be a PO 4 tetrahedron. It is found that in the calcium phosphate glass the PO 4 tetrahedra from long chains and almost all Ca 2+ are surrounded by 6 oxygens, which agrees with the structural model by other workers. In contrast to this normal-type calcium phosphate glass, it is also realized that the structures of the abnormal-type magnesium and zinc phosphate glasses consist of long chains with many branches, and most of Mg 2+ and Zn 2+ are surrounded by 4 oxygens. Thus, the distinct structural difference for abnormal-type phosphate glasses was recognized by the present X-ray diffraction results. However, an abrupt change of an oxygen coordination number around a metallic ion at the metaphosphate concentration was not observed, which is previously considered to be a main reason for the abnormal behavior in the abnormal-type phosphate glasses.

Structure and bonding in gold compounds

Recent developments in chemical applications of197Au Mossbauer spectroscopy are reviewed. For gold(I) and gold(III), systematic variations in isomer shift and quadrupole splitting are seen as the ligands are changed; the effects of change in coordination number of the gold atoms are also systematic. Data for gold(II) systems involving gold-gold bonds lie between those for corresponding gold(I) and gold(III) materials, showing a small increase in isomer shift and a larger increase in quadrupole splitting as the oxidation state decreases; these trends are explained in terms of the structures. Data for mixed-metal cluster compounds are much more sensitive to structural effects than in homonuclear clusters. Both sets of data show systematic changes with increase in the number of metal atoms to which the gold atom is bound. The connectivity also influences the recoil-free fraction.

Raman Spectra and Structure of Glasses in the System Na<sub>2</sub>O-Ga<sub>2</sub>O<sub>3</sub>-B<sub>2</sub>O<sub>3</sub>

The glass-forming region was determined by the water quenching method in the system Na2O-Ga2O3-B2O3. Raman spectra were measured for glasses in the composition range 10-40mol% Na2O, 0-30mol% Ga2O3 and 30-90mol% B2O3. The Raman spectroscopic study revealed that, at a given Ga2O3 concentration, an increase in Na2O content causes changes in boron coordination in the sequence BO3→BO4→BO3, while for a given Na2O concentration an increase in Ga2O3 content reduces the BO4 concentration in the Na2O-Ga2O3-B2O3 glass. Change in coordination number of gallium from 6 to 4 was deduced from the Raman spactra. The compositional dependence of density and refractive index consistent with the deduced structural changes in glass.

An analysis of complex alkyl‐substituent effects in the water‐catalyzed hydrolysis of 1‐acyl‐1,2,4‐triazoles

AbstractPseudo‐first‐order rate constants and thermodynamic activation parameters have been determined for the water‐catalyzed hydrolysis of the 1‐acyl‐1,2,4‐triazoles 1–9 in water and in aqueous acetonitrile [x(H2O) = 0.80]. The reaction occurs via water‐catalyzed nucleophilic attack of water at the amide carbonyl group. Variation of the alkyl group in the acyl part of the substrate led to the sequence of reactivities (in water): R = t‐Bu > i‐Pr > Et > Me > n‐Pr > s‐Bu > i‐Pent > neo‐Pent. This complex behavior strongly suggests the operation of a composite steric effect most likely involving contributions from conformational preference (SEI), change of coordination number at carbonyl carbon (SE2), steric repulsion between O2 and the alkyl group (SE3), and steric inhibition of solvation (SE4). Modelling of the relative rates in water in terms of the expanded branching equation gave satisfactory results. Two possible transition states are proposed for the neutral hydrolysis, one involving intramolecular hydrogen bonding between the second water molecule and N2 of the 1,2,4‐triazole ring. The different sequence of reactivities for neutral hydrolysis in aqueous acetonitrile as well as the rates for hydroxide‐ion‐catalyzed hydrolysis in water (B AC2 mechanism) support the analysis of substituent effects.

A glass formation study of aqueous rare earth electrolyte solutions

Glass formations of aqueous rare earth chloride, nitrate, perchlorate, and trifluoroacetate solutions were investigated. The series behavior of glass transition temperatures (T/sub g/'s) was discussed in conjunction with the hydration (coordination) number change in the middle of the series. It is concluded that the change of the inner-sphere coordination (hydration) number is the major cause for most of the irregularities observed in aqueous rare earth electrolyte solutions across the rare earth series. They confirm again that the population of the Eu/sup 3 +/ ions holding nine water molecules in their innermost hydration spheres increases with increasing salt concentration in a glassy EuCl/sub 3/ solution.

NEAR-EDGE STRUCTURE OF OXYGEN IN INORGANIC OXIDES : EFFECT OF LOCAL GEOMETRY AND CATION TYPE

NEXAFS measurements at the oxygen K-edge have been carried out on a variety of crystalline oxide samples to examine the effects of differences in local coordination environment of oxygen on near-edge structure. All spectra display an intense white line at about 543 eV and a strong shape resonance about 20 eV above the edge. The white line shifts to higher energies with increasing oxygen coordination number, although differences in nearest-neighbor type can affect this correlation. In addition, the Ca and transition-metal contain- ing oxides display features 6-10 eV below and 6-9 eV above the white line. The intensities of these features vary inversely with the number of 3d-electrons but their energies are independent of cation oxidation state. Large changes in coordination number and geometry affect edge structure less than variations in types of nearest- neighbor cations. The inverse distance-energy relationship is poorly obeyed in these compounds when oxygen- cation bond lengths are compared with the position of the strongest shape resonance for a given structure type. The near-edge structure is qualitatively interpreted using the results of recent Xa multiple scattered-wave calcu- lations.

Structural and energetic aspects of spin pairing in lanthanides

Numerical Hartree-Fock calculations on the lanthanide ions confirm the earlier results of Marcantonatos and coworkers, by showing that the excitation from the ground state of a given f n - configuration to a state with lower spin multiplicity is accompanied by a change in size and shape of the orbitals. This orbital change is also shown to be at the basis of a new interpretation of the spin pairing phenomenon in lanthanides. This interpretation is significantly different from the picture offered by conventional multiplet theory; indeed, the spin pairing energy, calculated on the basis of Hartree- Fock theory is accompanied by a decrease in interelectronic repulsion. Although the relevant energetic (repulsion) effects are obviously quite large, the underlying size effects are very much smaller. Therefore, it seems highly unlikely that they are responsible for the change in coordination number of Gd(H 2O) 8 3+ upon excitation, as suggested by Marcantonatos.

Changes in Zr coordination number during the pyrolysis of a SiO 2ZrO 2 gel

Zirconia-silica gels were prepared and their structures studied by EXAFS. From the ZrO distances, the coordination numbers were calculated. Heat treatment appears to cause the coordination number of the zirconium to increase.

Physical properties of glasses containing several glass-forming oxides

Physical properties, such as dielectric loss, acoustic velocity and electrical conductivity of glasses containing several glass-forming oxides were measured at temperatures in the range from −200°C to 200°C. The relationships between the structure and properties of the glasses, and the change in coordination number of individual glass-forming cations in alkali-containing glasses were analysed. It is shown that the glasses containing several glass-forming oxides are characterized by an inhomogeneous structure.