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Driving force effects on electron transfer rates in molecular donor–bridge–acceptor systems have been investigated extensively in the past. Significantly less well explored are tunneling energy effects in long-range charge transfer. These effects have to do with the energy differences between the donor/acceptor energy levels and those of the bridging molecules that are in between the two reactants. This contribution summarizes some recent advances in this research field.

Gradual migration of Na from Mg brings about fascinating change in aromatic and magnetic behavior of inorganic MgNa cluster, which is addressed at the B3LYP and QCISD levels. During this process, Na takes away the electron density from Mg causing a net decrease in aromaticity. A tug-of-war between the Pauli repulsion and the aromaticity is shown to be responsible for the observed stability and aromaticity trends in singlet and triplet states. Implications of a spin crossover vis-à-vis a possible superexchange are also explored. [ABSTRACT FROM AUTHOR]

Highlights: [•] The Curie–Weiss law is redefined in the studied temperature range. [•] This will lead to a homogeneous macroscopic electronic state picture. [•] It is in contrast with the coexistence of insulating and metallic regions. [Copyright &y& Elsevier]

Experimental investigations of photoinduced charge transport in synthetic DNA capped hairpins possessing electron acceptor and donor stilbene chromophores at either end have established the mechanism, dynamics, and efficiency of charge transport in DNA. The mechanism for charge transport in repeating A-T base pairs (A-tracts) was found to change from single-step superexchange at short distances to multistep incoherent hole hopping at longer distances. The rate constants for base-to-base hole hopping in longer A- and G-tract sequences are 1.2 x 109 s-1 and 4.3 x 109 s-1, respectively, considerably slower than the rate constants associated with molecular wires. Even slower rate constants are observed for alternating or random base sequences such as those encountered in natural DNA. The efficiency of charge separation in capped hairpins with A-tract sequences is also low as a consequence of the competition of hole hopping with charge recombination. Significantly higher efficiencies for charge separation are possible using diblock purine base sequences consisting of two or three adenines followed by a larger number of guanines. The short A-block serves as a molecular rectifier, slowing down charge recombination. More efficient charge separation can also be achieved using non-natural bases or by using the triplet acceptor anthraquinone for hole injection. [ABSTRACT FROM AUTHOR]

Highlights: [•] We found a parallel relationship between the two correlation lines of ln Δkisc vs. number of bonds. [•] Effects of HOMO of the terminal chromophore appear as the intercept of the correlation lines. [•] Difference of HOMO of the terminal porphyrin does not affect the interaction within the bridge part. [•] An expression involving both HOMO and spacer dependence for EISC rate was derived. [Copyright &y& Elsevier]

Abstract: Intramolecular electron transfer in binuclear mixed valence complexes where the metal atoms are linked by a bridging ligand illustrates a long-known problem: is it a two-step process with a temporary oxidation or reduction of the bridging ligand, or a one-step process best described by the super-exchange mechanism? In the latter case, information on the effective coupling between metal sites, V ab, can be obtained from the intensity of the intervalence transition. It is thus possible to study the decay of the coupling with distance, and also to show the possibility of molecular switching by using a photoactive ligand. However an integrated description of both mechanisms is possible by the explicit introduction of the bridging ligand nuclear degrees of freedom. This is achieved in a three-center model where the usual Hush–Marcus parabolas are replaced by three paraboloids in interaction. It is thus possible to compare different chemical systems and visualize the two limiting mechanisms. An interesting parallel can be made with the case of nanojunctions of the metal–molecule–metal type, which are increasingly available from scanning tunneling microscopy of molecules deposited on a suitable substrate. Such systems present an additional flexibility because the position of one electrode and the potentials can be changed. Thus one can probe again the decay of the electronic effect with distance and the possibility of molecular switching. But in addition, new effects can be observed. A remarkable experiment is the “charge trapping” on a single atom or a single molecule through the application of a voltage pulse. This can be considered as a redox reaction, the charge being stabilized by nuclear relaxation, the same process as the one occurring in polaron formation or in the two-step process invoked above. Finally, by moving the STM tip laterally above a molecule weakly coupled to its substrate, one can image in some cases molecular orbitals. [Copyright &y& Elsevier]

Nonradical Fenton-like catalysis offers opportunities to overcome the low efficiency and secondary pollution limitations of existing advanced oxidation decontamination technologies, but realizing this on transition metal spinel oxide catalysts remains challenging due to insufficient understanding of their catalytic mechanisms. Here, we explore the origins of catalytic selectivity of Fe-Mn spinel oxide and identify electron delocalization of the surface metal active site as the key driver of its nonradical catalysis. Through fine-tuning the crystal geometry to trigger Fe-Mn superexchange interaction at the spinel octahedra, ZnFeMnO 4 with high-degree electron delocalization of the Mn-O unit was created to enable near 100% nonradical activation of peroxymonosulfate (PMS) at unprecedented utilization efficiency. The resulting surface-bound PMS* complex can efficiently oxidize electron-rich pollutants with extraordinary degradation activity, selectivity, and good environmental robustness to favor water decontamination applications. Our work provides a molecule-level understanding of the catalytic selectivity and bimetallic interactions of Fe-Mn spinel oxides, which may guide the design of low-cost spinel oxides for more selective and efficient decontamination applications.

Photoinduced electron transfer (ET) reactions have been investigated in glassy media at 77 K. Distance decay parameters for electron tunneling through water, 2-methyltetrahydrofuran, and toluene have been determined through measurements of donor luminescence quenching by randomly dispersed electron acceptors. Remarkably different long-range ET efficiencies in the three solvents are in accord with the predictions of a super exchange model of distant electronic couplings. We conclude that tunneling energy effects play an important role in long-range ET reactions, and further that the coupling drops off very rapidly across van der Waals gaps betwee nmolecules in glasses.

The magnetic structure of almandine has been investigated by electronic structure calculations in the local spin density approximation in order to arrive at a more detailed understanding of the magnetic structure and the exchange pathways. The calculations are based on experimentally determined geometrical data of the crystal structure at 100 K. The calculated quadrupole splittings, spin-allowed d- d transitions and magnetic moment for iron atoms are in reasonable agreement with the respective experimental values obtained by Mössbauer and absorption spectroscopy, and magnetization measurements demonstrating the reliability of the calculations. The spin structure is derived from the calculated coupling constants for the possible exchange pathways. The competing superexchange pathways exist via oxygen bridges between directly neighboured iron ions and via edges of silicon tetrahedra and aluminium octahedra connecting more distant iron dodecahedra. Careful consideration revealed that almandine structure contains two identical interpenetrative sublattices of Fe dodecahedra connected via Al octahedra and Si tetrahedra. The calculations provide the information about ferromagnetic interaction between the iron spins within each sublattice, whereas the coupling between two magnetic sublattices is weakly antiferromagnetic via Al octahedra and Si tetrahedra. This antiferromagnetic interaction of two identical magnetic sublattices is in good agreement with the experiments and explains the Mössbauer spectra of almandine below the Néel temperature. Since almandine belongs to most abundant crystallized silicates that are main constituents of the earth and main components of cosmic dust, these results have scientific importance of studying the universe. [ABSTRACT FROM AUTHOR]

Abstract: Neutron powder diffraction has been performed on the MTbF6 fluorides (M=Cd, Ca, Sr, (α/β)-Ba). Four of these fluorides (Cd, Ca, Sr, β-Ba) are built of a (pseudo-) tetragonal packing of [TbF6]2− chains and only differs by the chains relative orientations. Thus this series represents a valuable opportunity to evaluate the Tb4+-Tb4+ magnetic interactions. All the compounds displayed antiferromagnetic order (T N=2.70K (Cd), 2.15K (Ca), 2.60K (Sr), 2.10K (β-Ba)), except for the α form of BaTbF6. The crystal structure of this latter fluoroterbate has also been investigated by means of high-resolution neutron powder diffraction. From Neutron Powder Diffraction data, CdTbF6 and β-BaTbF6 magnetic structures were determined, together with the metamagnetic behaviour of β-BaTbF6 as a function of an external magnetic field. A tentative phase diagram is then given for β-BaTbF6. Advantage was taken of the polymorphism of the BaTbF6 fluoroterbate to analyse, on the basis of topological parameters such as bond distances and angles, the magnetic behaviour of its α and β forms. It was shown that superexchange interactions are present in β-BaTbF6, and that these interactions may also rule the magnetic behaviour of the other MTbF6 (M=Ca, Sr, Cd) tetravalent terbium fluorides. [Copyright &y& Elsevier]

The magneto-structural correlations of superexchange-coupled doubly hydroxo-bridged Cu(II)-dimers have been investigated. To this end, an analytical approach has been applied to [Cu2(OH) 2F42-1 model complexes. This approach supplies an analytical scheme, based on orbital interactions, for calculating the transfer integral, HAB, which is shown to play the key role in the magnetic coupling constant J for understanding magneto-structural correlations. The single contributions to the transfer integral are calculated and described explicitly. Therefore, this approach supplies a detailed insight into the magnetic behavior and the interaction mechanisms of the hydroxo-bridged Cu-dimers. All analytical results are com- pared with experimental and numerical data [ABSTRACT FROM AUTHOR]

Abstract: Three new C 3-symmetric tritopic ligands with a central phloroglucinol bridging unit have been synthesized and characterized. The ligands are accessible through Schiff-base condensation of 2,4,6-triformylphloroglucinol with 2-aminomethylpyridine (H3tfpg-ampy), N,N-bis(pyridin-2-ylmethyl)-ethylenediamine (H3tfpg-unspenp), and benzhydrazide (H6tfpg-bhy). These ligands differ in nature and number of the donor atoms within the resulting binding pockets. Based on these ligands the synthesis of the first trinuclear phloroglucinol-bridged nickel(II) complexes with three octahedrally coordinated nickel centers is reported. The ligands H3tfpg-ampy and H6tfpg-bhy, which provide tridentate binding pockets, react with nickel(II) perchlorate in the presence of bis(pyridin-2-ylethyl)-amine (bpea) as an additional tridentate capping ligand leading to the formation of the trinuclear complexes [Ni3(tfpg-ampy)(bpea)3](ClO4)3 and [Ni3(tfpg-bhy)(bpea)3](ClO4)3, respectively. Due to the pentadentate binding pocket in ligand H3tfpg-unspenp, no additional coligand is needed and a water molecule occupies the sixth coordination site at the nickel(II) ion resulting in the complex [Ni3(tfpg-unspenp)(H2O)3](ClO4)3. Temperature-dependent magnetic measurements reveal overall weak antiferromagnetic exchange interactions within the trinuclear complex together with a rather strong zero-field splitting (ZFS) for the nickel(II) ions. The observed isotropic coupling constants for the three complexes are in the range of 0.14<− J <0.37cm−1, whereas for the zero-field splitting parameter values between 1.8 and 5.5cm−1 are found. This is indicative for competitive spin-polarization and superexchange mechanisms, with the latter prevailing the interaction between the nickel(II) ions through the meta-phenylene-linkage for the complexes reported. [Copyright &y& Elsevier]

Abstract: The double perovskite La2VMnO6 with a structure was predicted by Pickett et al. as a half-metal (HM) antiferromagnet (AFM) based on first principle calculations, while Androulakis et al.''s experimental result showed that La2VMnO6 also exhibits the same structure as a ferrimagnetic (FiM) state. For thoroughly understanding what happens in the double perovskite La2VMnO6 with a structure, we first calculated the fixed cubic structure before recalculating the structure through an optimization process. Our results show that La2VMnO6 is a FiM state when found in the fixed and volume-relaxed cubic structure. However, after full relaxation, it becomes a HM-AFM and remains the same state even on-site Coulomb interactions (+U) are taken into consideration. This recalculation illustrates that the magnetic state of the Mn ion is sensitive to the crystal field of MnO6 octahedron, and is the main reason behind the transformation of the magnetic state of La2VMnO6 from a FiM to a AFM state after full relaxation. Because the difference of total energy E FiM−AFM is small (merely about 57.1meV/f.u.) and shows up as nearly the same space group in both FiM and AFM states La2VMnO6, we conclude that in which state it is mainly determined by the crystal field of the MnO6 octahedron, i.e. the distortion magnitude of the MnO6 octahedron, which is related to the process of synthesis. Although Androulakis et al.''s experimental result show that this is a /FiM state, we think that it is still possible for a I4/mmm/AFM state to exists, i.e. a HM-AFM La2VMnO6. [Copyright &y& Elsevier]

Abstract: The infrared and visible cooperative emissions of ytterbium ions are studied in Yb-doped NdVO4 single crystals. The absorption of optical phonons allows the emissions at room temperature when a Nd:YAG laser is used. Low temperature emissions are observed due to the Nd3+→Yb3+ energy transfer following an argon ion laser excitation of the Nd3+ ions. Analysis of the cooperative emission at low doping concentration (1%) indicates that it is generated by distant pair forming Yb3+ ions while at high doping concentration (≥ 5%) close ions magnetically coupled and superexchange mechanisms prevail in the emitting process. [Copyright &y& Elsevier]

An analytical expression for the transfer integral HAB between the localized magnetic orbitals in superexchange-coupled dimers as a function of the type of atoms and geometry of the molecule has been derived by explicitly including orbital interactions. It is shown that HAB plays the key role for understanding magneto-structural correlations. The reliability and capability of this approach is confirmed by comparison with numerical electronic structure calculations in the local spin-density approximation on singly and doubly bridged Cu(ll)-dimers with fluorine ligands. All results can be calculated and understood within the analytical formalism representing, therefore, a powerful tool for understanding the magneto- structural correlations and for constructing magnetic orbitals analytically. [ABSTRACT FROM AUTHOR]

Abstract: A series of diruthenium complexes incorporating bis(imidazo[4,5-f][1,10]phenanthroline) bridging structures were synthesized and their electrochemical properties were investigated. Upon changing the ancillary ligands of one diruthenium complex from 2,2′-bipyridine (bpy) to 1,10-phenanthroline (phen), electrochemical experiments revealed behavior that suggested through-bridge, metal-to-metal communication. By changing the inductive properties of the bridging ligand, the degree of communication could be controlled. The results point toward a hole-type superexchange electron transport mechanism. [ABSTRACT FROM AUTHOR]

Abstract: While it was recently found that La2VTcO6 and La2VCuO6 are promising candidates for half-metallic antiferromagnets (HM-AFM), the search continues for other potential candidates of HM-AFM in the double perovskites structure (B, metal). La2VReO6 is found to be a nearly HM-AFM. Furthermore, considering correlation and spin–orbital coupling (SOC) effects in transition metals, it is still nearly a HM-AFM after generalized gradient approximation with correction of on-site Coulomb interaction and SOC calculations, as reported herein. [ABSTRACT FROM AUTHOR]

The spin dynamics of ultracold bosonic atoms in a double well at finite temperatures is studied in terms of the spin structure factor within the two-site Bose–Hubbard model. The spin structure factor for positive frequencies reveals spin excitations at two frequencies corresponding two transitions among the eigenstates of the Hamiltonian. It has been found that the temperature effect is quite substantial. At zero temperature, only spin excitations at the low frequency survive, while the spectral weights at the two frequencies become approximately equal at high temperatures. As temperature increases, the spectral weight shifts from the low frequency to the high frequency. [ABSTRACT FROM AUTHOR]

The electronic and magnetic structure of the chain silicate orthoferrosilite FeSi2O6 has been investigated by electronic structure calculations in the local spin density approximation. All calculations are based on experimentally determined geometrical data at room temperature. The calculated spin-allowed d– d excitation energies and hyperfine parameters are in quantitative agreement with the respective experimental data from optical absorption and Mössbauer spectroscopy. Inside one ribbon that is parallel to the crystallographic c axis and contains two non-equivalent M1 and M2 sites, all iron spins are ferromagnetically coupled with coupling constants of about +16 cm−1. Between these ribbons within the ( b, c)-plane a weak ferromagnetic coupling of about +2 cm−1 is obtained. Neighboured ( b, c)-planes are coupled antiferromagnetically via chains of Si B-tetrahedra but ferromagnetically via chains of Si A-tetrahedra. Such a theoretically determined "double-plane antiferromagnetic" spin structure is at variance with an experimentally derived magnetic structure. This discrepancy is attributed to differences between the geometry at room temperature and at temperatures below the Néel temperature currently not available. [ABSTRACT FROM AUTHOR]

The magnon excitation spectrum of ferromagnetic quasicubic manganites is studied by using the linearized spin-wave approximation for the double-exchange and the superexchange interactions. Two situations are distinguished: 1) a rigid coupling of the t2g and eg spins via a large Hund's rule coupling and 2) a softened coupling between these spins (e.g., due to coupling of the eg electrons to Jahn-Teller modes). Case 2) leads to a softening of the magnon dispersion (compared to the Heisenberg model) close to the zone boundary. [ABSTRACT FROM AUTHOR]

Abstract: The new double-Schiff-base ligand H6ipa-bhea has been synthesized by condensation of a 4,6-diformylresorcinol derivative (ipa) with two equivalents of N,N-bis-(2-hydroxyethyl)ethylenediamine (bhea). Reaction with copper(II) perchlorate leads to the formation of two different products depending on the reaction conditions. The directed synthesis of either a mononuclear or dinuclear copper(II) complex is reported. The reaction in methanol results in the formation of a dinuclear complex [Cu2(H4ipa-bhea)](ClO4)2 (1). Whereas in the presence of water as solvent for the reaction, one imine side chain of the ligand is hydrolyzed regenerating the formyl moiety with the mononuclear complex [Cu(H3hyforsa-bhea)]ClO4 ·2H2O (2) as final product. Subsequent reaction of complex 2 with N,N-bis-(pyridin-2-ylmethyl)ethylenediamine (unspenp) as additional amine component results in the formation of the mononuclear complex [Cu(Hhyforsa-unspenp)]ClO4 (3). All complexes are characterized by IR spectroscopy, elemental analysis and X-ray crystallography. Temperature-dependent magnetic measurements on the dinuclear complex indicate weak antiferromagnetic exchange interactions between the copper(II) ions with a coupling constant of J =−16.4cm−1. Density functional calculations have been used to evaluate the magnetic properties. The exchange coupling constant can be nicely reproduced with the use of the broken symmetry approach. The exchange pathway through the meta-phenylene-linkage is discussed in terms of a competitive spin-polarization and superexchange mechanism as well as geometrical changes at the copper(II) ions. [Copyright &y& Elsevier]

Two approximate formulae are derived for the magnetic exchange parameter, J = ½{E(singlet) - E(triplet)}, for an O-Cu-Cu-O component of CuII carboxylate dimers. The derivations use standard valence bond and molecular orbital techniques, but the effects of superexchange are incorporated in different ways. A superexchange parameter k measures the extent of delocalization of oxygen lone-pair electrons into singly occupied 3dx²- y² atomic orbitals of the CuII ions. The two formulae for J are equivalent only when k = 0 and k = ∞. The formulae are compared, and suggestions are made for elaborations of them. The roles of two types of "cis O-O overlap" for stabilizing (via covalent-ionic resonance) the antiferromagnetic spin state are re-stated for 10-electron 6-centre (with two carboxylate ligands) as well as the 6-electron 4-centre bonding units [ABSTRACT FROM AUTHOR]

Sub-magnetic ordering of CoFe 2 O 4 spinel was elaborated through the electronic structure via ab-initio calculations. PBE-GGA was preferred to obtain band structure and density of states applying Hubbard correction. Considered magnetic states, including distinctive sub-orders, exhibited several band gaps and magnetic features. Applying the magnetic force theorem to the ground state solution, exchange coupling energies were obtained. We double-checked O(p)-Fe/Co(d) hybridizations and exchange energies of considered magnetic states contributing to FM/FiM phases. Co–O hybridizations were stronger than Fe–O. Rest couples, obviously overshadowed the contribution of oxygen-connected ions, herewith, assuring different strengths of exchange energy. Initially, FM coupled Co–Co and Fe–Fe pairs had the strongest values. Magnetocrystalline anisotropy had close-set values consistent with previous studies (0.09210385eV for Co–Co, AFM, and Fe–Fe, FM). Curie temperatures (T c) were obtained from temperature-dependent normalized magnetization and magnetic susceptibility curves using the Monte Carlo method on a classical Heisenberg model. Exchange energies, magnetocrystalline anisotropy, and magnetic moments obtained from DFT calculations were implemented as inputs to the simulation process. The highest T c was found to be 725 K for the FM sub-order state. Other states undergo a phase transition under 500 K and 445 K temperatures, respectively. Considered magnetic orders can occur concerning experimental conditions and production method/procedure of CoFe 2 O 4 spinel ferrites. • FM sub-order of Co-Fe pairs lead to the highest band-gap. • Co-O hybridizations are stronger than Fe-O for all magnetic orders. • A non-linear distance-dependent relation in exchange energies is clear. • The mixed state of considered configurations or a single one can be responsible for the magnetic behavior. • Curie temperature is highest when FM sub-order formed. [ABSTRACT FROM AUTHOR]

Here we compare two kinds of doping in transition metal oxides by considering defects due to the substitution of the transition metal or rare earth element. The focus is on insulating t 2 g orbital systems and we present various mechanisms that lead to reorganization of spin–orbital order. Substitutional doping of d 4 -transition metal by d 3 ions generally results in removal of the local orbital degrees of freedom and the exchange on hybrid d 4 − d 3 bonds modifying locally (or globally) spin–orbital order. Different routes of rearrangement of spin–orbital patterns are also found when considering doping by d 2 ions. Here we demonstrate that in the regime of strong impurity-host coupling ferro-orbital type ordering is favored. This analysis can find application to ruthenate oxides with Mn or Cr replacing the Ru atoms. In contrast, charged (Sr,Ca) defect replacing R ion in the R 1 − x (Ca,Sr) x VO 3 (R =La,Y) perovskites generates a spin–orbital polaron in the defect cube. We show that t 2 g orbital rotations are then induced near the charged defect and a doped hole disturbs orbital order. As a result, the collapse of G -type orbital order occurs but C -AF spin order stays robust under increasing doping. • Orbital dilution for d 3 doping in d 4 host. • Charge dilution for d 2 doping in d 4 host. • Spin–orbital polarons in doped vanadium perovskites. • t 2 g orbital rotations near the charged defect. • Orbital order melting by charged defects. [ABSTRACT FROM AUTHOR]

In earlier work, we previously established a formalism that allows to express the exchange energy J vs. fundamental molecular integrals without crystal field, for a fragment A–X–B, where A and B are 3d1 ions and X is a closed-shell diamagnetic ligand. In this article, we recall this formalism and give a physical interpretation: we may rigorously predict the ferromagnetic (J < 0) or antiferromagnetic (J > 0) character of the isotropic (Heisenberg) spin-spin exchange coupling. We generalize our results to ndm ions (3 ≤ n ≤ 5, 1 ≤ m ≤ 10). By introducing a crystal field we show that, starting from an isotropic (Heisenberg) exchange coupling when there is no crystal field, the appearance of a crystal field induces an anisotropy of exchange coupling, thus leading to a z-z (Ising-like) coupling or a x-y one. Finally, we discuss the effects of a weak crystal field magnitude (3d ions) compared to a stronger (4d ions) and even stronger one (5d ions). In the last step, we are then able to write the corresponding Hamiltonian exchange as a spin-spin one. [ABSTRACT FROM AUTHOR]

Abstract: Ytterbium ions infrared and visible cooperative luminescences, resulting from YAG laser and selective site excitations, in (6%) Yb-doped Y2SiO5 thin film are analyzed. Magnetically coupled Yb–Yb ion pairs seem to play a major role in energy transfer and cooperative emission, confirming the prevalence of superexchange mechanisms. [Copyright &y& Elsevier]

The results of electron paramagnetic resonance and optical spectroscopy studies of Yb3+ ions in LiNbO3:1% Yb host crystal are reported. Also some results of Raman spectra measurements are presented. The observed features are assigned to single Yb3+ ions and pairs of dissimilar ions (with different g value) of the type of evenYb3+–evenYb3+. The values of the components of the g-tensor for both ions and interaction exchange tensors are estimated. The value of J = -0.0283 cm-1 shows that the exchange interactions are of antiferromagnetic nature. The most probably the evenYb3+–evenYb3+ pairs substitute for neighborhood Li+–Nb5+ positions what induces gain of distance between ions of about 1 Å. There are no observed dissimilar ion pairs of the type oddYb3+–oddYb3+. [ABSTRACT FROM AUTHOR]

Multicopper oxidases (MCOs) catalyze the 4e- reduction of O2 to H2O. The reaction of the fully reduced enzyme with O2 generates the native intermediate (NI). which undergoes a slow decay to the resting enzyme in the absence of substrate. NI is a fully oxidized form, but its spectral features are very different from those of the resting form (also fully oxidized), because the type 2 and the coupled-binuclear type 3 Cu centers in the O2-reducing trinuclear Cu cluster site are isolated in the resting enzyme, whereas these are all bridged by a μ3-oxo ligand in NI. Notably, the one azide-bound NI (NIAz) exhibits spectral features very similar to those of NI, in which the μ3-oxo ligand in NI has been replaced by a μ3-bridged azide. Comparison of the spectral features of NI and NIAz, combined with density functional theory (DFT) calculations, allows refinement of the NI structure. The decay of NI to the resting enzyme proceeds via successive proton-assisted steps, whereas the rate- limiting step involves structural rearrangement of the μ3-oxo-bridge from inside to outside the cluster. This phenomenon is consistent with the slow rate of NI decay that uncouples the resting enzyme from the catalytic cycle, leaving NI as the catalytically relevant fully oxidized form of the MCO active site. The all-bridged structure of NI would facilitate electron transfer to all three Cu centers of the trinuclear cluster for rapid proton-coupled reduction of NI to the fully reduced form for catalytic turnover. [ABSTRACT FROM AUTHOR]

Abstract: We report the low temperature magnetic behavior of a complex with nominal formula “Mn(OAc)(OBz)(μ-O)0.1”. It is presumed that the phase responsible for the interesting magnetic phenomena is related to {Mn5(OAc)4(OBz)6}∞ – a recently reported complex comprised of two dimensional sheets of Mn12 loops arranged in a honeycomb-like fashion via carboxylate bridging in the plane. Temperature and field dependent magnetic studies show exchange transitions between 1.7 and 12K which are similar to those found in other Mn carboxylate complexes. The point of interest we wish to explore in this paper, that differs from similar manganese carboxylate-based complexes, is the rather strong two-dimensional antiferromagnetic exchange and the possible role of interplanar exchange on ordering. [Copyright &y& Elsevier]