Unpaired 3d Electrons Research Articles

Structural and Magnetic Property of Ion Irradiated TiO2 Single Crystals

Ferromagnetism is induced in pure TiO2 single crystals by oxygen ion irradiation. The ferromagnetism is observed up to room temperature and is with weak temperature dependence. By combining X-ray diffraction, Rutherford backscattering/channelling, Raman scattering, and electron-spin resonance spectroscopy, supperconducting quantum interference device, displacement per atom, we measured the lattice damage accumulation with increasing fluences. A defect complex, i.e., Ti3+ on the substitutional accompanied by oxygen vacancies, has been identified in the irradiated TiO2. This kind of defect complex results in a local (TiO6−x) stretching Raman mode. We elucidate that Ti3+ with one unpaired 3d electron provide the local magnetic moments.

Reversible Addition of CO to Coordinatively Unsaturated High-Spin Iron(II) Complexes

Several new coordinatively unsaturated iron(II) complexes of the types [Fe(EN-iPr)X2] (E = P, S, Se; X = Cl, Br) and [Fe(ON-iPr)2X]X containing bidentate EN ligands based on N-(2-pyridinyl)aminophosphines as well as oxo, thio, and seleno derivatives thereof were prepared and characterized by NMR spectroscopy and X-ray crystallography. Mössbauer spectroscopy and magnetization studies confirmed their high-spin nature with magnetic moments very close to 4.9 μB, reflecting the expected four unpaired d-electrons in all these compounds. Stable low-spin carbonyl complexes of the types [Fe(PN-iPr)2(CO)X]X (X = Cl, Br) and cis-CO,cis-Br-[Fe(PN-iPr)(CO)2X2] (X = Br) were obtained by reacting cis-Fe(CO)4X2 with the stronger PN donor ligands, but not with the weaker EN donor ligands (E = O, S, Se). Furthermore, the reactivity of [Fe(PN-iPr)X2] toward CO was investigated by IR spectroscopy. Whereas at room temperature no reaction took place, at −50 °C [Fe(PN-iPr)X2] added readily CO to form, depending on the nature of X, the mono- and dicarbonyl complexes [Fe(PN-iPr)(X)2(CO)] (X = Cl) and [Fe(PN-iPr)(CO)2X2] (X = Cl, Br), respectively. In the case of X = Br, two isomeric dicarbonyl complexes, namely, cis-CO,trans-Br-[Fe(PN-iPr)(CO)2Br2] (major species) and cis-CO,cis-Br-[Fe(PN-iPr)(CO)2Br2] (minor species), are formed. The addition of CO to [Fe(PN-iPr)X2] was investigated in detail by means of DFT/B3LYP calculations. This study strongly supports the experimental findings that at low temperature two isomeric low-spin dicarbonyl complexes are formed. For kinetic reasons cis,trans-[Fe(PN-iPr)(CO)2Br2] releases CO at elevated temperature, re-forming [Fe(PN-iPr)Br2], while the corresponding cis,cis isomer is stable under these conditions.

Electronic structure and magnetic properties of Cr monodoped and (Cr, Al) codoped ZnO

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of Cr monodoped and (Cr, Al) codoped in ZnO. The results indicate that Cr monodoped in ZnO favors a spin-polarized state with a total magnetic moment of 7.50μ B per supercell and the magnetic moment mainly comes from the unpaired 3d electrons of Cr atoms. In addition, it was found that the ferromagnetic exchange interaction between Cr atoms is short-ranged in Cr monodoped ZnO. Interestingly, the ferromagnetic stability can be enhanced significantly by codoping Al Zn. We think that the enhancement of ferromagnetic stability should be attributed to the additional electrons introduced by Al Zn codoping.

Special Features of Singlet Oxygen Generation Photosensitized by Magnesium Complexes of Tetrapyrrolic Macrocycles

Triplet excited states of photosensitive molecules play a special role in the photocatalytic and photosensitized processes.[1] Phosphorescence (most often, it is the radiative transition T1-S0) is unresolved and the lifetime of the triplet states is 10-3 s and above usually, whereas the characteristic lifetime of singlet states is estimated as 10-9-10-8 s. In the case of tetrapyrrole photosensitizers (TPS), the generation of long-lived triplet excited states with high quantum yield is typical for metal-free porphyrins and related compounds, as well as their complexes with d0and d10-metals, i.e. elements containing no unpaired d-electrons. The most photobiologically important example of TPS with d0-metals is chlorophyll containing magnesium ion. Under photoexcitation of chlorophyll a (Mg-Chl) generation of the first triplet state T1 with a high quantum yield (60%) is observed.[2] Energy of excited state T1 is 11325 cm -1

Electronic structure and magnetism of Cr-doped ZnO nanowires

According to the spin-polarized density functional theory, we study the electronic structures, the magnetic and the optical properties of Cr-doped ZnO nanowires. The calculated results show ferromagnetic coupling for Cr atoms substitution for Zn atoms in ZnO nanowires along the [0001] direction, and the antiferromagnetic coupling with Cr-doped in ZnO nanowires along the [1010] and [0110] directions. The results reveal that the magnetic coupling state near the Fermi level gives rise to such a spin splitting phenomenon near the Fermi level, which indicates that Cr 3d and O 2p orbitals have intense hybrid effects. In addition, the spin electronic density results indicate that system magnetic moments are generated mainly by the unpaired 3d electrons of Cr atoms and are also related to the electron configuration. Moreover, the results of optical properties show that the obvious absorption peaks are observed in the far ultraviolet and the near ultraviolet regions and there is a red shift phenomenon in the ultraviolet region. These results indicate that the Cr-doped ZnO nanowires could be a promising dilute magnetic semiconductor material.

Ferromagnetically Coupled Cobalt−Benzene−Cobalt: The Smallest Molecular Spin Filter with Unprecedented Spin Injection Coefficient

Here, we predict that the ferromagnetically coupled cobalt-benzene-cobalt system will act as the smallest molecular spin filter with unprecedented spin injection coefficient. To validate our in-silico observation, we have performed state-of-the-art nonequilibrium Green's function calculations and analyzed the density of states of cobalt at the relativistic and nonrelativistic level of theory. Remarkably, we found that unpaired 3d electrons of cobalt are not participating in the spin transport process like other transition metal containing multidecker complexes. Instead, an admixture of the outer-sphere 4s and 4p orbitals of cobalt along with the 2p orbital of carbon of the benzene moiety is contributing to the singly occupied highest molecular orbital in the majority spin channel that creates a path for coherent spin transport leading to the extremely high spin injection coefficient of the system. The absence of the 3d electrons of cobalt in the spin transport process has been carefully examined, and it was found that the nodal structure of the 3d orbital of cobalt is not at all suitable for bonding in the cobalt-benzene-cobalt system. The whole study indicates that the underlying mechanism of the spin filter action in cobalt-benzene-cobalt is completely distinctive from the other known materials.

Electronic structure and magnetism of Fe-doped SiC nanotubes

The electronic structure and magnetic properties of Fe-doped SiC nanotubes are investigated by using the first-principles method based on density functional theory (DFT) in the local spin density approximation (LSDA). The calculation results indicate that the SiC nanotube of Fe substitution for C exhibits antiferromagnetism while ferromagnetism features prominently when Fe substitutes Si. This is a kind of half-metal magnetic material. The formation energy calculation results show that the formation energy of ferromagnetic structure is 3.2 eV lower than that of antiferromagnetic structure. Fe atoms are more likely to replace Si atoms. Spin-orbit coupling induces electron spin polarization in the ground state. Also, the doping Fe atoms make relaxation towards the outside of the tube to some extent and larger geometric distortion occurs when Fe substitutes C, but the whole geometric structure of SiC nanotubes is not damaged due to the doping. It is revealed in the calculation of energy band structure and density of states that more dispersed distribution of energy levels is produced near the Fermi level. For Fe substitution for Si, obviously there are spin-split and intense p-d hybrid effects by Si 3p electron spins and Fe 3d electron spins localized at the exchanging interactions between magnetic transitional metal (TM) impurities. Spin electronic density results indicate that system magnetic moments are mainly generated by the unpaired 3d electrons of Fe atoms. All these results show that the transition metal doping SiC nanotube could be a potential route to fabricating the promising magnetic materials.

Mn2+Complexes with Pyridine-Containing 15-Membered Macrocycles: Thermodynamic, Kinetic, Crystallographic, and1H/17O Relaxation Studies

Given its five unpaired d-electrons, long electronic relaxation time, and fast water exchange, Mn(2+) is a potential candidate for contrast agent application in medical magnetic resonance imaging. Nevertheless, the design of chelators that ensure stable Mn(2+) complexation and optimal relaxation properties remains a coordination chemistry challenge. Here, we report the synthesis of two pyridine-containing ligands L1 and L2, with 15-membered triaza-dioxa-crown and pentaaza-crown ether macrocycles, respectively, and the characterization of their Mn(2+) complexes. Protonation constants of the ligands and stability constants of various metal complexes were determined by potentiometry. The presence of the pyridine in the macrocyclic ring induces rigidity of the complexes which results in a greater thermodynamic stability with respect to the nonpyridine analogues. Solid-state structures of MnL1 and MnL2 confirmed seven-coordination of Mn(2+) with Cl(-) and H(2)O in axial positions. The dissociation kinetics of MnL2 in the presence of Zn(2+) were followed by relaxometric measurements. They proved the prime importance of the proton-assisted dissociation while the zinc(II)-assisted pathway is not important at physiological pH. For MnL1, the dissociation was too fast to be studied by conventional relaxivity measurements under pH 6. A combined (17)O NMR and (1)H NMRD study on MnL1 and MnL2 yielded the parameters that govern the relaxivity of these complexes. The water exchange rate for MnL1, k(ex)(298) = 0.38 x 10(7) s(-1), is the lowest value ever reported for a Mn(2+) complex, while a considerably higher value was obtained for MnL2 (k(ex)(298) = 6.9 x 10(7) s(-1)). Anion binding was studied by relaxometric titrations. They revealed weak interactions between MnL2 and phosphate or citrate, leading to the formation of monohydrated species. Overall, the incorporation of a pyridine into a polyaza macrocycle scaffold has several beneficial effects on the Mn(2+) chelates with respect to potential MRI contrast agent applications: (i) The thermodynamic and the kinetic stability of the complexes is increased. (ii) The rigidified ligand backbone results in higher coordination numbers of the metal ion, allowing for two inner-sphere water molecules in aqueous solution.

Striking Differences between the Solution and Solid-State Reactivity of Iron PNP Pincer Complexes with Carbon Monoxide

Several new iron(II) complexes of the types [Fe(PNP)X2] (X = Cl, Br) containing tridentate PNP pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine have been prepared. They all exhibit intermolecular Fe−X···H−N hydrogen bonds, forming supramolecular networks in the solid state. In the case of X = Cl these compounds react readily with gaseous CO both in the solid state and in solution to give selectively the octahedral complexes cis- and trans-[Fe(PNP)(CO)(Cl)2], respectively, whereas with X = Br mixtures of cis and trans isomers are obtained. These transformations are accompanied by color and spin-state changes. CO binding is fully reversible in all cases, and heating solid samples of either cis- or trans-[Fe(PNP)(CO)(X)2] leads to complete regeneration of analytically pure [Fe(PNP)(X)2]. Mössbauer spectroscopy confirmed the high-spin nature of the parent five-coordinate Fe(II) complex (δ = 0.80(1) mm s−1) and the shift to two different low-spin octahedral species after reaction with CO in the solid (δ = 0.13(1) mm s−1) or in solution (δ = 0.15(1) mm s−1). Magnetization studies led to a magnetic moment close to 4.9 μB, reflecting the expected four unpaired d-electrons in [Fe(PNP)Cl2], which were confirmed by DFT calculations. The DFT study of the reaction pathway for CO capture led to low energy barriers (≤3.4 kcal mol−1). The cis−trans isomerization reaction was found to take place with a low energy barrier (10.8 kcal mol−1), after initial loss of chloride, and involves also spin-state changes

Magneto-optical properties of transition metal compounds XPt 3 (X = V, Cr, Mn, Fe, Co) and X 3Pt (X = Fe, Co)

We have studied theoretically the electronic structure and magneto-optical Kerr effect (MOKE) of XPt 3 (X = V, Cr, Mn, Fe, Co) and X 3Pt (X = Fe, Co) compounds from first principles using the full potential linearized augmented plane wave (FPLAPW) method within the local spin density approximation. The calculated magneto-optical spectra show the features of the experimental spectra which can be attributed to electronic transitions. For the Pt based compounds of 3d transition metals, we find the largest Kerr angle of −1.17° for MnPt 3 in the polar geometry which is in good agreement with the experimental data. Our calculations show an increase in Kerr rotation for XPt 3 series which can be correlated to an increase in the unpaired valence d-electrons of 3d transition metal (X) up to MnPt 3 and then a decrease for FePt 3 and CoPt 3. The Kerr spectrum is analyzed to ascertain the origin of the various peaks in terms of optical transitions. In addition, the effect of interchanging X and Pt atoms in XPt 3 is also studied in order to have greater insight for the dependence of Kerr spectra on configuration and local environment of these two atoms.

Exchange coupling in alkoxy-polyoxovanadates [VIV n VV 6−n O7(OR)12]4−n (n = 4, 3, 2)

The molecular structure and magnetic properties of alkoxy-polyoxovanadates [VIVnVV6−nO7(OR)12]4−n (n = 4, 3, 2) were studied within the framework of the DFT approach. The equilibrium geometric configurations of all complexes studied in this work are characterized by a distorted octahedral hexavanadate core; the unpaired d-electrons are localized on the metal centers (VIV). The localized spin density distribution is also retained in the low-temperature crystal structures of the compounds whose magnetic properties are described by the Heisenberg-Dirac-van Vleck exchange spin Hamiltonian. The exchange parameters calculated using the broken symmetry formalism suggest predominance of ferromagnetic coupling between vanadium(IV) ions in the μ-OR bridged dimeric units {VIVO(OR)VIV} and in the diagonal pairs {VIVOVIV} (n = 4). The results obtained indicate that the magnitude and sign of the exchange parameters in the isostructural dimeric units within the hexavanadate core depend on the total number of unpaired electrons in the system.

Structure and Codeposition Behavior of Ni–W Alloys Electrodeposited from Ammoniacal Citrate Solutions

The microstructure of Ni–W alloys electrodeposited at from ammoniacal citrate solutions was discussed in terms of their codeposition behavior. When the W content in the alloys was increased, the partial current efficiency of W first increased toward a maximum value and then decreased. This showed the existence of two characteristic alloy compositions, i.e., a W content at a maximum partial current efficiency of W and an upper limiting content of W. The deposition behavior of Ni–W alloys was explained by the multistep mechanism of the reduction of the electrochemically reduced W(IV) oxide to W by atomic hydrogen adsorbed using the unpaired 3d electrons of the freshly deposited Ni. X-ray diffraction (XRD) analysis revealed that alloys with a low W content consisted of a Ni solid solution, and the diffraction peaks became broader with an increase in W content. Ni–W alloys with high W contents showed a smooth surface composed of fine grains. Transmission electron microscopy studies revealed that the major phase in Ni–W alloys was the Ni solid solution, and the XRD pattern of the Ni–53.9 mass % W alloy revealed a halo pattern characteristic of an amorphous structure. This alloy consisted of crystal grains smaller than in diameter.

157 T internal magnetic field in Fe[C(SiMe3)3]2 compound at 20 K

Fe[C(SiMe3)3]2 compound, in which iron is coordinated by two carbons, was prepared using the reaction of FeCl2 with (Me3Si)3CLi, and investigated by XRD, 57Fe Mossbauer spectroscopy and DFT calculations. 157.5 T hyperfine magnetic field was found at the site of the iron nucleus of this compound at 20 K. DFT calculations predict the quintet states to be clearly favored energetically over the lower spin states. The population analysis reveals considerable 4s as well as large unpaired 3d electron contributions, which can be responsible for the extremely high hyperfine field.

X-ray photoelectron spectra and structure of polynuclear nickel complexes

Mono- and binuclear nickel complexes of different stoichiometry have been studied by X-ray photoelectron spectroscopy (XPS). The Ni2p, Ni3p, and N1s X-ray photoelectron spectra have been examined, and the role of a ligand in their formation has been determined. As distinct from a low-spin Ni(II) complex, the Ni2p spectra of high-spin Ni(II) compounds show strong satellite lines. For high-spin Ni(II) complexes, which have unpaired 3d electrons, the Ni2p 1/2-Ni2p 3/2 spin-orbit splitting is larger than that for a low-spin Ni(II) compound. The presence or absence of the satellite structure has made it possible to classify these complexes with regard to their magnetic properties. The difference between the Ni2p 3/2 and N1s binding energies has made it possible to estimate the covalence of the metal-ligand bond. The XPS results are consistent with X-ray crystallography data.

Dinuclear Complexes Formed with the Triazacyclononane Derivative ENOTA4-: High-Pressure 17O NMR Evidence of an Associative Water Exchange on [MnII2(ENOTA)(H2O)2

Mn2+ has five unpaired d-electrons, a long electronic relaxation time, and labile water exchange, all of which make it an attractive candidate for contrast agent application in medical magnetic resonance imaging. In the quest for stable and nonlabile Mn2+ complexes, we explored a novel dimeric triazacyclononane-based ligand bearing carboxylate functional groups, H4ENOTA. The protonation constants of the ligand and the stability constants of the complexes formed with some endogenously important metals (Ca2+, Cu2+, Zn2+), as well as with Mn2+ and Ce3+, have been assessed by NMR methods, potentiometry, and UV-vis spectrophotometry. Overall, the thermodynamic stability of the complexes is lower as compared to that of the corresponding NOTA analogues (H3NOTA, 1,4,7-triaazacyclononane-1,4,7-triacetic acid). The crystal structure of Mn2(ENOTA)(H2O) x 5H2O contains two six-coordinated Mn2+, in addition to the three amine nitrogens and the two oxygens from the pendent monodentate carboxylate groups, and one water (Mn2) or one bridging carboxylate oxygen (Mn1) completes the coordination sphere of the metal ion. In an aqueous solution, this bridging carboxylate is replaced by a water molecule, as evidenced by the 17O chemical shifts and proton relaxivity data that point to monohydration for both metal ions in the dinuclear complex. A variable-temperature and -pressure 17O NMR study has been performed on [Mn2(ENOTA)(H2O)2] to assess the rate and, for the first time on a Mn2+ chelate, also the mechanism of the water exchange. The inner sphere water is slightly more labile in [Mn2(ENOTA)(H2O)2] (k298ex = 5.5 x 107 s-1) than in the aqua ion (2.1 x 107 s-1, Merbach, A. E.; et al. Inorg. Chem. 1980, 19, 3696). The water exchange proceeds via an almost limiting associative mechanism, as evidenced by the large negative activation volume (deltaV = -10.7 cm3 mol-1). The proton relaxivities measured on [Mn2(ENOTA)(H2O)2] show a low-field dispersion at approximately 0.1 MHz arising from a contact interaction between the MnII electron spin and the water proton nuclear spins.

X-ray photoelectron spectra and structure of cobalt compounds with N-heterocyclic ligands

The electronic structure of cobalt complexes with bi-, tri-, and tetradentate ligands and the mutual influence of ligands in them have been studied by X-ray photoelectron spectroscopy. The Co2p, N1s, and O1s photoelectron spectra have been studied. Unlike low-spin Co(III) complexes, the high-spin Co(II) compound exhibits a strong satellite line in the Co2p spectrum. For the high-spin Co(II) compound having unpaired 3d electrons, the Co2p1/2-Co2p3/2 spin-orbit splitting is larger than that in the low-spin Co(III) complexes. All cobalt complexes under consideration contain strongly bound dioxygen, which can be considered an inherent structural unit.

7Li NMR and Two‐Dimensional Exchange Study of Lithium Dynamics in Monoclinic Li3V2(PO4)3.

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7Li NMR and Two-Dimensional Exchange Study of Lithium Dynamics in Monoclinic Li3V2(PO4)3

High-resolution solid-state (7)Li NMR was used to characterize the structure and dynamics of lithium ion transport in monoclinic Li(3)V(2)(PO(4))(3). Under fast magic-angle spinning (MAS) conditions (25 kHz), three resonances are clearly resolved and assigned to the three unique crystallographic sites. This assignment is based on the Fermi-contact delocalization interaction between the unpaired d-electrons at the vanadium centers and the lithium ions. One-dimensional variable-temperature NMR and two-dimensional exchange spectroscopy (EXSY) are used to probe Li mobility between the three sites. Very fast exchange, on the microsecond time scale, was observed for the Li hopping processes. Activation energies are determined and correlated to structural properties including interatomic Li distances and Li-O bottleneck sizes.

The β-carotene dilemma: ESR study with coordinated peroxyl radicals

AbstractThe apparently unpredictable behaviour of β-carotene in the supplementation of the diet of smokers is discussed in the light of the reactions of peroxyl radicals with β-carotene in the absence of oxygen. The decay of tert-butylperoxyl radicals in the presence of β-carotene was studied at ambient temperature in non-polar solvents by ESR spectroscopy. The primary reaction in the absence of oxygen is interpreted as a spin-trapping effect of a peroxyl radical by β-carotene producing an intermediate labile free radical, which disappears after recombination with a second tert-butylperoxyl radical. The result is the transformation of β-carotene to a diamagnetic compound with two peroxy bonds. In the presence of chelating transition metals with unpaired d-electrons as electron donors the peroxy group of the oxidized β-carotene can be split to alkoxyl free radicals. The primary attack of tert-butylperoxyl radicals is completely inhibited in the presence of vitamin E followed by production of free aryloxy radicals and the presence of oxygen has no significant effect on this reaction. Spin-trapping of peroxyl radicals by the double bond of vitamin A leads to its oxidation in the absence of vitamin E.Transition metal ions such as Co, Cr, Fe, and Mn, known to be present in the aerosol of cigarette smoke, homolyse the peroxyl bonds of peroxidised β-carotene, which results in cell damage.

Periodic table of 3d-metal dimers and their ions.

The ground states of the mixed 3d-metal dimers TiV, TiCr, TiMn, TiFe, TiCo, TiNi, TiCu, TiZn, VCr, VMn, VFe, VCo, VNi, VCu, VZn, CrMn, CrFe, CrCo, CrNi, CrCu, CrZn, MnFe, MnCo, MnNi, MnCu, MnZn, FeCo, FeNi, FeCu, FeZn, CoNi, CoCu, CoZn, NiCu, NiZn, and CuZn along with their singly negatively and positively charged ions are assigned based on the results of computations using density functional theory with generalized gradient approximation for the exchange-correlation functional. Except for TiCo and CrMn, our assignment agrees with experiment. Computed spectroscopic constants (r(e),omega(e),D(o)) are in fair agreement with experiment. The ground-state spin multiplicities of all the ions are found to differ from the spin multiplicities of the corresponding neutral parents by +/-1. Except for TiV, MnFe, and MnCu, the number of unpaired electrons, N, in a neutral ground-state dimer is either N(1)+N(2) or mid R:N(1)-N(2)mid R:, where N(1) and N(2) are the numbers of unpaired 3d electrons in the 3d(n)4s(1) occupation of the constituent atoms. Combining the present and previous results obtained at the same level of theory for homonuclear 3d-metal and ScX (X=Ti-Zn) dimers allows one to construct "periodic" tables of all 3d-metal dimers along with their singly charged ions.