Insulator–metal transition driven by pressure and B‐site disorder in double perovskite La2CoMnO6

The ground state of double perovskite oxide La₂CoMnO₆ (LCMO) and how it is influenced by external pressure and antisite disorder are investigated systematically by first‐principles calculations. We find, on the consideration of both the electron correlation and spin–orbital coupling effect, that the LCMO takes on insulating nature, yet is transformed to half metallicity once the external pressure is introduced. Such tuning is accompanied by a spin‐state transition of Co²⁺ from the high‐spin state (te) to low‐spin state (te) because of the enhancement of crystal‐field splitting under pressure. Using mean‐field approximation theory, Curie temperature of LCMO with Co²⁺ being in low‐spin state is predicted to be higher than that in high‐spin state, which is attributed to the enhanced ferromagnetic double exchange interaction arising from the shrinkage of Co? O and Mn? O bonds as well as to the increase in bond angle of Co? O? Mn under pressure. We also find that antisite disorder in LCMO enables such transition from insulating to half‐metallic state as well, which is associated with the spin‐state transition of antisite Co from high to low state. It is proposed that the substitution of La³⁺ for the rare‐earth (RE) ions with smaller ionic radii could open up an avenue to induce a spin‐state transition of Co, rendering thereby the RE₂CoMnO₆ a promising half‐metallic material. © 2012 Wiley Periodicals, Inc.     

Interactions Magnetiques dans quelques Ferrites Oxyfluores a Structure Spinelle de Formule ZnxMFe2−xO4–xFx (M ? Fe,Co, Ni)

Magnetic interactions in some oxyfluoroferrites of spinel structure with the formula Zn_xMe_2?xO_4?xFx (M = Fe, Co, Ni) Whereas the ferromagnetic spin arrangement of the B-cations is not modified by the Zn²⁺?Fe³⁺ substitution in the ZnFe[Fe₂₊Fe³⁺]O_4?xF_x (0 ≤ x ≤ 0,50) spinel, this same substitution leads to a spin canting in the ZnFe[Co₂₊Fe³⁺]O_4?xF_x and ZnFe[Ni₂₊Fe³⁺]O_4?xF_x (0 ≤ x ≤ 0,80) simples. The difference in the magnetic behaviors with regard to the AB and BB interactions can be explained on the basis of the magnetic exchange theory.     

A Mixed‐Valence {Fe13} Cluster Exhibiting Metal‐to‐Metal Charge‐Transfer‐Switched Spin Crossover

It is promising and challenging to manipulate the electronic structures and functions of materials utilizing both metal‐to‐metal charge transfer (MMCT) and spin‐crossover (SCO) to tune the valence and spin states of metal ions. Herein, a metallocyanate building block is used to link with a Fe^II‐triazole moiety and generates a mixed‐valence complex {[(Tp^4‐Me)Fe^III(CN)₃]₉[Fe^II₄(trz‐ph)₆]}?[Ph₃PMe]₂?[(Tp^4‐Me)Fe^III(CN)₃] ( 1 ; trz‐ph=4‐phenyl‐4H‐1,2,4‐triazole). Moreover, MMCT occurs between Fe^III and one of the Fe^II sites after heat treatment, resulting in the generation of a new phase, {[(Tp^4‐Me)Fe^II(CN)₃][(Tp^4‐Me)Fe^III(CN)₃]₈ [Fe^IIIFe^II₃(trz‐ph)₆]}? [Ph₃PMe]₂?[(Tp^4‐Me)Fe^III(CN)₃] ( 1 a ). Structural and magnetic studies reveal that MMCT can tune the two‐step SCO behavior of 1 into one‐step SCO behavior of 1 a . Our work demonstrates that the integration of MMCT and SCO can provide a new alternative for manipulating functional spin‐transition materials with accessible multi‐electronic states.     

Fourier transform infrared spectroscopy study of the effect of pH on anion and cation adsorption onto poly(acrylo‐amidino diethylenediamine)

The role of hydrogen bonding in the chemistry of transition‐metal complexes remains a topic of intense scientific and technological interest. Poly(acrylo‐amidino diethylenediamine) was synthesized to study the effects of hydrogen bonding on complexes at different pHs. The polymer was synthesized through the coupling of diethylene triamine with polyacrylonitrile fiber in the presence of AlCl₃ · 6H₂O addition. The adsorption capacity of this polymer was 11.4 mequiv/g. The ions used for the adsorption test were CrO, PO, Cu²⁺, Ni²⁺, Fe²⁺, and Ag⁺. All experiments were confirmed with Fourier transform infrared. In the study of anion adsorption, at low pHs, only ionic bonds existed, whereas at high pHs, no bonds existed. However, in the middle pH region, both ionic bonds and hydrogen bonds formed between poly(acrylo‐amidino diethylenediamine) and the chromate ion or phosphate ion. When poly(acrylo‐amidino diethylenediamine) and metal ions (Cu²⁺, Ni²⁺, Fe²⁺, and Ag⁺) formed complexes, a hydrogen‐bonding effect was not observed with Fourier transform infrared. The quantity of metal ions adsorbed onto poly(acrylo‐amidino diethylenediamine) followed the order Ag⁺ > Cu²⁺ > Fe²⁺ > Ni²⁺. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2010–2018, 2004     

Determination and comparison of the stability constants of some bivalent ion complexes of 5,7-dichloro-, 5,7-dibromo-, and 5,7-dinitro-8-Hydroxyquinoline and their N-oxides

The stability constants of complexes of Mn⁺⁺, Fe⁺⁺, Co⁺⁺, Ni⁺⁺, Cu⁺⁺, Zn⁺⁺, Cd⁺⁺ and UO with 5,7-dichloro-, 5,7-dibromo- and 5,7-dinitro-8-hydroxyquinoline and their corresponding N-oxides have been determined in 75 + 25 v/v dioxan + water medium at 35°C in presence of 0.20 M sodium perchlorate by pH-titration technique as given by IRVING and Rossotti. A possible explanation for the observed orders of the stability constants of the metal complex with the different ligands, and of the complexes of a particular ligand with different metal ions is also proposed.     

Molecular mechanics,quantum mechanics,potentiometric, and conductometric studies on the complexes of some rare earth metals with 5‐azorhodanine derivatives

The geometry of metal ions (La³⁺, Ce³⁺, UO, and Th⁴⁺) complexes with 5‐azorhodanine derivatives was optimized at the level of molecular mechanics. Two stoichiometric ratios of metal to ligand (i.e., 1:1 and 1:2) were investigated. Tetracoordinate and hexacoordinate of each stoichiometric ratio have been studied. Effect of substitution in the ligand on the geometry of the complexes was discussed in the light of electron donating–accepting properties of these substituents. The influence of the nuclear effective charge of the central metal ions on the metal–ligand (M–L) bonding was discussed and the effect of the number of ligands on the M–L bond length was also discussed and correlated to the experimental results. The total energies of the different metal complexes were computed using the extended Huckel method. The effect of substituents in ligand, metal type, and stoichiometry of the complexes on the complex total energies were discussed. Stability constant of (La³⁺, Ce³⁺, UO, and Th⁴⁺) metal ions with 5‐azorhodanine derivaties have been determined potentiometrically in 0.1 M KCl and 50% (v/v) ethanol–water mixture. The order of the stability constants of the formed complexes was found to be La³⁺ < Ce³⁺ < UO < Th⁴⁺. The influence of substituents on the stability of the complexes was examined on the basis of electron‐repelling property of the substituent. The effect of temperature on the stability of the complexes formed was studied and the corresponding thermodynamic parameters (ΔG, ΔH, and ΔS) were derived and discussed. The stoichiometries of these complexes were determined conductometrically and indicated the formation of 1:1 and 1:2 (metal:ligand) complexes. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Structure and stability of high‐spin Aun(n = 2–8) clusters

The structures and relative stability of the maximum‐spin ⁿ⁺¹Au_n and ⁿAu (n = 2–8) clusters have been determined by density‐functional theory. The structure optimizations and vibrational frequency analysis are performed with the gradient‐corrections of Perdew along with his 1981 local correlation functional, combined with SBKJC effective core potential, augmented in the valence basis set by a set of f functions. We predicted the existence of a number of previously unknown isomers. The energetic and electronic properties of the small high‐spin gold clusters are strongly dependent on sizes. The high‐spin clusters tend to holding three‐dimensional geometry rather than planar form preferred in low‐spin situations. In whole high‐spin Au_n (n = 2–8) neutral and cationic species, ⁵Au₄, ²Au, and ⁴Au are predicted to be of high stability, which can be explained by valence bond theory. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Thermal,morphology, and NMR characterizations on phase behavior and miscibility in blends of isotactic polystyrene with poly(cyclohexyl methacrylate)

The miscibility and phase behavior in a binary blend of isotactic polystyrene (iPS) and poly(cyclohexyl methacrylate) (PCHMA) were investigated by differential scanning calorimetry, optical microscopy (OM), and solid‐state ¹³C cross‐polarity/magic‐angle spinning NMR. The iPS/PCHMA blend was miscible when all compositions showed a single composition‐dependent glass‐transition temperature (T_g) and when the blend went through a thermodynamic phase transition upon heating to above the lower critical solution temperature as determined by OM measurements. The ¹H NMR spin‐relaxation times in the laboratory frame (T) and in the rotating frame (T) for iPS/PCHMA blends with various compositions and neat components were directly measured through solid‐state¹³C NMR. The results of T indicated that the blends are homogeneous, at least on a scale of 75–85 nm, confirming the miscibility of the system. The single decay and composition‐dependent T values for each blend further demonstrated the blends are homogeneous on a scale of 2.5–3.5 nm. The results suggested that iPS and PCHMA are intimately mixed at the molecular level within the blends at all compositions. The tacticity of polystyrene does not seem to adversely influence the miscibility in blends of iPS/PCHMA. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 772–784, 2003     

Theoretical spectroscopic studies of InI and InI+

Relativistic configuration interaction calculations are carried out to study the electronic structure and spectroscopic properties of InI and InI⁺. Potential energy curves of the ground and a number of low‐lying states are constructed. Spectroscopic parameters of the bound states of both species are computed and compared with the experimental and other theoretical data. Effects of spin‐orbit coupling on the spectroscopic properties are studied. Because of the presence of the heavy atoms the effect is large. The spin‐orbit splitting of the ground state (X²Π) of InI⁺ is more than 8350 cm^?1. As a result of the strong spin‐orbit interaction between X²Π and A²Σ⁺ of InI⁺, the potential energy curve of A²Σ becomes repulsive. Radiative lifetimes for the spin‐forbidden transitions such as A³Π?X¹Σ and B³Π₁ ?X¹Σ of InI and spin‐allowed transitions such as B²Σ⁺?A²Σ⁺, C²Π?A²Σ⁺, and B²Σ⁺?X²Π are calculated. Vertical and adiabatic ionization energies of InI and the electric dipole moments of both the neutral and ionic species are estimated. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Redox‐Inert Fe3+ Ions in Octahedral Sites of Co‐Fe Spinel Oxides with Enhanced Oxygen Catalytic Activity for Rechargeable Zinc–Air Batteries

Bimetallic cobalt‐based spinel is sparking much interest, most notably for its excellent bifunctional performance. However, the effect of Fe³⁺ doping in Co₃O₄ spinel remains poorly understood, mainly because the surface state of a catalyst is difficult to characterize. Herein, a bifunctional oxygen electrode composed of spinel Co₂FeO₄/(Co_0.72Fe_0.28)_Td(Co_1.28Fe_0.72)_OctO₄ nanoparticles grown on N‐doped carbon nanotubes (NCNTs) is designed, which exhibits superior performance to state‐of‐the‐art noble metal catalysts. Theoretical calculations and magnetic measurements reveal that the introduction of Fe³⁺ ions into the Co₃O₄ network causes delocalization of the Co 3d electrons and spin‐state transition. Fe³⁺ ions can effectively activate adjacent Co³⁺ ions under the action of both spin and charge effect, resulting in the enhanced intrinsic oxygen catalytic activity of the hybrid spinel Co₂FeO₄. This work provides not only a promising bifunctional electrode for zinc–air batteries, but also offers a new insight to understand the Co‐Fe spinel oxides for oxygen electrocatalysis.     

Fluorescence enhancement of cationic diacetylene‐contained polyelectrolyte by anions and cations and application for sensitive and selective detection of Hg2+

An unusual phenomenon, the fluorescence enhancement of cationic conjugated fluorene‐co‐carbazole‐co‐diacetylene polymers (CPFC) by both anionic and cationic ions, was reported. The fluorescence enhancement of CPFC strongly depended on the nature of the ions and the counterions. In the solution of DMF/H₂O, PO, CO and Hg²⁺ showed the most pronounced fluorescence enhancement response of CPFC (more than 10‐fold). The fluorescence of CPFC was quenched by K₄[Fe(CN)₆, and then could be recovered with the addition of Hg²⁺. Based on these observations, a simple and sensitive fluorescent “turn‐on” sensor in aqueous solution for Hg²⁺ was developed by use of a system of CPFC/ K₄[Fe(CN)₆]. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

High- und Low-Spin-Verhalten des Ni3+ -Ions in oktaedrischer Koordination. (A) NiF-Polyeder

High and Low Spin Behaviour of Ni³⁺?Ions in Octahedral Coordination The compounds Cs₂NaNiF₆, Cs₂KNiF₆, Rb₂KNiF₆, K₃NiF₆, and Na₃NiF₆ were investigated by ligand field and EPR spectroscopy between 298 and 4,2 K. These fluorides ? with the exception of the first one – crystallise in the cubic elpasolite lattice or in distorted modifications of this structure type and contain the Ni³⁺ ions in the low spin configuration te. This configuration is stabilised versus the high spin alternative te by an appreciable Jahn-Teller splitting of the ²E_g-state of about 7000 cm^?1. The NiF₆-octahedra are tetragonally elongated, the distortion being dynamical at 298 K. In case of the cubic compound Rb₂KNiF₆ a transition to a tetragonal structure with c/a > 1 as a consequence of a ferrodistortive Jahn-Teller ordering is observed at lower temperatures. It is calculated from the anisotropic g-parameters, that the first excited quartet level ⁴A_2g(⁴T_1g ? te) has an energy which is about 1000 cm^?1 higher than that of the ²A_1g(²E_g ? te) groundstate. Spin-orbit interactions between the energetically neighboured ²A_1g(²E_g) and ⁴A_2g, ⁴E_g(⁴T_1g) states lead to third order contributions to the g-factors, which are very sensitive with respect to the doublet-quartet separation. In the hexagonal compound Cs₂NaNiF₆ finally, in which half of the Ni³⁺ ions occupy octahedral sites connected by common corners as in the other fluorides, while the other half is located in octahedral sites with common faces, high and low spin Ni³⁺ ions are found side by side. Obviously the latter half of these Ni³⁺ ions is geometrically restricted with respect to a Jahn-Teller distortion and hence the high spin configuration energetically favoured.     

Solid‐state nuclear magnetic resonance relaxation times in crosslinked macroporous polymer particles of divinylbenzene homopolymers

Monodisperse porous particles of poly(divinylbenzene) prepared by the activated swelling method have been investigated by solid‐state ¹³C crosspolarization magic‐angle spinning (CPMAS) nuclear magnetic resonance (NMR) relaxation measurements. Homopolymeric combinations of two porogens (toluene and 2‐ethylhexanoic acid) and two monomers (meta‐ and para‐divinylbenzene) were studied. Residual vinyl groups were systematically reacted with increasing amounts of bromine, producing 20 different polymers samples for which we measured crosspolarization times, T_CH, proton rotating frame spin‐lattice relaxation, T, ¹³C spin‐lattice relaxation, T, and proton spin‐lattice relaxation, T. These parameters were chosen to reflect expected changes in a wide range of frequencies of motion as a function of structure. Relative differences in the molecular mobility of the major functional groups (aromatic, vinyl and aliphatic) is related to initial reactants used, vinyl concentration, relative reactivity of vinyl groups, distribution of vinyl groups, pore structure, and degree of crosslinking. Variable temperature ¹H combined rotation and multiple pulse NMR (CRAMPS) was used to derive activation energies for selected samples via measurement of the proton spin‐lattice relaxation time, T. Irreversible thermal effects were observed in ambient temperature relaxation after heating to temperatures in the range of 393–418 K. Simple univariate statistical analyses failed to reveal consistent correlations among the known variables. However, the application of more sophisticated multivariate and neural network analyses allowed excellent structure–property predictions to be made from the relaxation time data. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1307–1328, 1999     

Dimethylformamide-Exchange Mechanism on Some First-Row Divalent Transition-Metal Ions

The effect of temperature on the dimethylformamide exchange on Mn(DMF) and Fe(DMF) has been studied by ¹³C- and ¹⁷O-NMR, respectively, yielding the following kinetic parameters: k²⁹⁸ equals; (2.2±0.2). 10⁶ S^?1, ΔH^≠ = 34.6 ± 1.3 kJ mol^?1, ΔS^≠ = ?7.4 ± 4.8 J K^?1mol^?1 for Mn²⁺ and K²⁹⁸ = (9.7 ± 0.2).10⁵ S^?1, Delta;H^≠ = 43.0 ± 0.9 kJ mol^?1, ΔS^≠ = + 13.8 ± 2.8 J K^?1mol^?1 for Fe²⁺. The volumes of activation, ΔV^≠ in cm³mol^?1, derived from high-pressure NMR on these metal ions, together with the previously published activation volumes for Co²⁺ and Ni²⁺ (+2.4 ± 0.2 (Mn²⁺), +8.5 ± 0.4 (Fe²⁺) +9.2 ± 0.3 (Co²⁺), + 9.1 ± 0.3 (Ni²⁺)) give evidence for a dissociative activation mode for DMF exchange on these high-spin first-row transition-metal divalent ions. The small positive ΔV^≠ value observed for DMF exchange on Mn²⁺ seems to indicate that a mechanistic changeover also occurs along the series, (probably from I_d to D), as for the other solvents previously studied (I_a to I_d, for H₂O, MeOH, MeCN). This changeover is shifted to the earlier elements of the series, due to more pronounced steric crowding for dimethylformamide hexasolvates.     

Density functional theory of the iron–nitrosyl (S = 3/2) complex

On the basis of density functional theory (DFT), the iron–nitrosyl complex Fe[Me₃TACN](NO)(N₃)₂ (S = 3/2) is studied via the B3LYP hybrid method. Its Raman vibrational frequencies, atomic net charges, and spin densities are analyzed. The related complexes Fe(NH₃) (n = 1, 2, and 3) are employed as reference compounds to determine the characteristics of the central iron. Our results indicate that the S = 3/2 spin ground state of Fe[Me₃TACN](NO)(N₃)₂ is best described by the presence of Fe^II (S = 2) anti‐ferromagnetically coupled to NO⁰ (S = 1/2) yielding Fe^II[Me₃TACN](NO⁰)(N)₂. This is clearly different from the previous Fe^III‐NO^? theoretical assignment. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Local density functional theory applied to spin coupling in Fe6S supercluster

Using density functional theory (DFT), the multiplicity of the ground state was determined for Fe₆S₆Cl ions as well as the order of the excited spin states. A method to determine the exchange integrals J of the Fe₆S cluster is presented based on these results and a spin‐coupling algebra. The following order of the spin states was established with respect to the total spin 5/2, 1/2, 7/2, 3/2, 9/2, 11/2, 15/2, 13/2, and 17/2. We also calculated the Heisenberg coupling parameters J₁, J₂, J₃, and J₄ as 22, 146, −130, and 81 cm⁻¹, respectively. The possibility of the ground state of high multiplicity as well as the necessary conditions for such state are discussed. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 39–51, 1999     

The N(4S) + N2O(X̃1∑) reaction

The title reaction, which is spin‐forbidden for N₂(X¹∑) + NO(X²Π) production, has been studied from 960 to 1130 K in a high‐temperature photochemistry reactor. No reaction could be observed, indicating k < 1 × 10^?15 cm³ molecule^?1 s^?1. It is concluded that there is no significant contribution from the spin‐allowed exothermic path leading to N₂(X¹∑) + NO(a⁴Π). © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 387–389, 2001     

Synthesis,characterization and applications of polysiloxane networks with immobilized pyrogallol ligands

A porous, solid insoluble polysiloxane‐immobilized ligand system bearing pyrogallol active sites of the general formula P? (CH₂)₃? NH(CH₂)₃OC₆H₃(OH)₂ (where P represents [Si? O]_n siloxane network) has been prepared by the reaction of 3‐aminopropylpolysiloxane with 1,3‐dibromopropane followed by the reaction with pyrogallol. ¹³C CP‐MAS NMR and X‐ray photoelectron spectroscopy confirmed that the pyrogallol is chemically bonded to the siloxane backbone. Thermal analysis showed that the ligand system is stable under nitrogen at relatively high temperature. The polysiloxane–pyrogallol ligand system exhibits high potential for the uptake of the metal ions (Fe³⁺, Co²⁺, Ni²⁺ and Cu²⁺). Complexation of the pyrogallol ligand system for the metal ions at the optimum conditions was found to be in the order Fe³⁺ > Cu²⁺ > Ni²⁺ > Co²⁺. Copyright © 2005 John Wiley & Sons, Ltd.     

Photon-Triggered Complex Formation: Radical Complexes of o-Benzosemiquinone,Dopa, Dopamine and Adrenaline Formed by Electron-Transfer Reaction from Excited Tris (2,2′-bipyridyl)ruthenium (II)

The pyrocatechol derivatives, dopamine ( 2 ), adrenaline ( 3 ) and L -dopa (4) form in their semiquinoid oxidation states complexes with several closed-shell metal ions like Zn²⁺, Cd²⁺ and Y³⁺. This complex formation can be triggered by visible light via a one-electron transfer reaction from the pyrocatechol derivative to (³CT)Ru(bpy), which is thereby reduced (Schemes 1 and 2), The quenching of the triplet charge-transfer state, (³CT)Ru(bpy) cannot be measured by conventional methods (Stern-Volmer plot), because the bimolecular electron-transfer rate is too slow (k_q < 10^?7 M^?1 s^?1). ESR. spectroscopy shows, however, clearly the paramagnetic reaction products. The concentration of these paramagnetic species is strongly enhanced by complex formation. The hindered rotation of the side chain in L -dopa and in its Y³⁺-complex is described within a two-jump model, which gives best agreement between calculated and measured spectra assuming an energy of 37 kJmol^?1 for the rotational barrier.     

Zur Kenntnis von Ba6La2Co4O15

About Ba₆La₂Co₄O₁₅ Ba₆La₂Co₄O₁₅ were prepared and investigated by X-ray single crystal work. It crystallizes with hexagonal symmetry, space group C–P6₃mc; a = 11.8082; c = 7.0019 Å; Z = 2. Ba²⁺ show face connected BaO₆-octahedra and larger polyhedra of C.N. = 10 and 12. Co³⁺ is surrounded by four and six (tetrahedra, octahedra) oxygen. The Ba²⁺ and La³⁺ ions occupy one point position statistically.