59Co NMR Research Articles

Magnetic Properties of Cen+1Co3n+5B2n (n = 0, 1, 2, and 3) compounds investigated by 59Co NMR

We have performed 59Co spin-echo NMR measurements on Cen+1Co3n+5B2n (n = 0, 1, 2, 3) at T= 1.4K. The NMR spectra corresponding to the inequivalent crystallographic sites were obtained. The resonance frequencies of the 2c-Co have been observed in the range 60-80MHz, and those of the 6i-Co(6i1-Co) around 32MHz. In addition to the zero-field spectra, the field-swept spectra from non-magnetic Co have also been observed for Ce3Co11B4 and Ce2Co7B3. These spectra arise from the 3g-Co in Ce3Co11B4 and the 6i2-Co in Ce2Co7B3. From the Co hyperfine field analysis, the Co magnetic moments at the inequivalent crystallographic sites are estimated. Magnitude of the spin moment for the 2c-Co is estimated to be <1.2μB for Cen+1Co3n+5B2n (n = 0, 1, 2, 3), and to be about 0.4μB for the 6i1-Co in Ce3Co11B4 and Ce2Co7B3, while the orbital moment of the 2c-Co varies from <0.3μB in CeCo5 to <0.1μB in Ce2Co7B3, and those for the 3g-Co and 6i1-Co are found to be 0.08 and 0.01μB, respectively. The Co magnetic moments are found to be strongly influenced by the crystallographic environment around the Co site.

Na order and Co charge disproportionation in NaxCoO2

We have synthesized and characterized different stable phases of sodium cobaltates NaxCoO2 with sodium content 0.65<x<0.80. We demonstrate that 23Na NMR allows us to determine the difference in the susceptibility of the phases and reveals the presence of Na order in each phase. 59Co NMR experiments give clear evidence that Co charge disproportionation is a dominant feature of Na cobaltates. Only a small fraction (≈25%) of cobalt are in a non-magnetic Co3+ charge state whereas electrons delocalize on the other cobalt. The magnetic and charge properties of the different Co sites are highly correlated with each other as their magnetic shift KZZ scales linearly with their quadrupolar frequency νQ. This reflects the fact that the hole content on the Co orbitals varies from site to site. The unusual charge differentiation found in this system calls for better theoretical understanding of the incidence of the Na atomic order on the electronic structures of these compounds.

Efficient hierarchically structured composites containing cobalt catalyst for clean synthetic fuel production from Fischer–Tropsch synthesis

We report a straightforward preparation method to synthesize hierarchical composite consisting of TiO2-coated multi-walled carbon nanotubes (CNTs) decorating a macroscopic host structure of α-Al2O3. The obtained composite possesses moderate specific surface area and very open porous structure, as well as moderate interaction with active sites, which significantly improve the cobalt nanoparticles dispersion and the mass diffusion during the reaction. The Co/TiO2/CNT-α-Al2O3 (CoTiCNTA) catalyst is then used in the Fischer–Tropsch synthesis (FTS) process. This hierarchical catalyst achieves a FTS rate to C5+ of 0.80gC5+gcat−1h−1 along with a long-chain hydrocarbons (C5+) selectivity of 85%, which can be pointed out as the most outstanding noble promoter-free catalyst for the FTS process. The as-synthesized catalyst also exhibits an extremely high stability as a function of time on stream which is also one of the prerequisites for the development of future FTS catalysts, especially for the Biomass-to-Liquids process where trace amount of impurities and/or moisture could have an impact on the catalyst stability. The present work also introduces a new investigation methodology based on the use of zero field 59Co NMR, which allows one to map in a precise manner the cobalt active phase distribution and to correlate it with the FTS performance. It is expected that such technique would be extremely helpful for the understanding of the catalyst structure–performance relationship and for future optimization in the FTS process as well as in other fields of investigation where cobalt particles are involved.

A New Insight into Cobalt Metal Powder Internal Field 59Co NMR Spectra

The authors present a new approach of internal field 59Co NMR spectra assignment leaving apart from “usual” decomposition on “pure” hcp and fcc stackings, and a set of stacking faults (sfs) sf1–sf5 with a certain lines position. The authors propose including into consideration not only cobalt structural features as well as its magnetic nature due to the strong ferromagnetism in Co metal. The last fact supposes an existence of different magnetic species such as magnetic domains, domain walls, and single-domain particles, thereby helping to spectral lines assignment according to both structural and magnetic origin. The examined sample contains fcc and hcp resonance peaks in both domains and domain walls giving the hcp to fcc ratio equal to 1.9, as well a significant amount of Co sfs, or Co in loose coordination, up to 10 %. The research exhibits a good agreement of all implemented techniques.

Relationship between Ferromagnetic Criticality and the Enhancement of Superconductivity Induced by Transverse Magnetic Fields in UCoGe

We have performed 59Co NMR experiments on the ferromagnetic superconductor UCoGe under magnetic fields (H) along the a- and b- axes to investigate the relationship between ferromagnetic properties and superconductivity. The ferromagnetic ordering temperature TCurie is suppressed and the nuclear spin-lattice relaxation rate 1/T1 at 2 K is enhanced in H || b, although TCurie and 1/T1 are unchanged in H || a, indicating that the ferromagnetic criticality is induced only when H is applied along the b axis. We show the close relationship between the magnetic anisotropies and the superconducting ones reported by Aoki et al.: the superconductivity is gradually suppressed in H || a, but enhanced in H || b above 5 T. We strongly suggest that the enhancement of the superconductivity observed in H || b originates from the field induced ferromagnetic criticality, as pointed out by Aoki et al and Mineev.

EFfect of alumina modification on the structure of cobalt-containing Fischer-Tropsch synthesis catalysts according to internal-field 59Co NMR data

An internal-field 59Co NMR study of cobalt-containing Fischer-Tropsch synthesis catalysts supported on different alumina modifications was reported. The Co/δ-Al2O3 sample was shown to contain single-domain fcc packing and stacking faults, whereas Co/γ-Al2O3 gave signals from the fcc domain walls, hcp and stacking faults, thus indicating differences in the particle size of the studied samples. T 2 relaxation times were measured; their distribution in a spectrum is non-uniform, which allows signals to be distinguished by their relaxation times. Quantitative measurements of the relative atoms content in different packings revealed that the catalysts have mostly a defect structure. A brief historical background was presented to characterize the internal-field 59Co NMR technique, the related problems, and different approaches to acquired data interpretation.

Pressure evolution of the metamagnetic transition in UCoAl As measured using 59Co NMR

We have performed NMR measurements under pressure in UCoAl with a quantum critical endpoint of the metamagnetic transition from the paramagnetic phase to the ferromagnetic (FM) phase. 59Co-NMR sensitively detects the evolution of the internal field caused by applying the external field. The metamagnetic field H m increases with increasing pressure consistently with other experimental methods, accompanied by the suppression of the magnetization in the field-induced FM phase and the magnetization jump at H m . The loss of the NMR signal on approaching the QCEP indicates the development of the spin fluctuations.

Titania-Decorated Silicon Carbide-Containing Cobalt Catalyst for Fischer–Tropsch Synthesis

The metal–support interactions of titanium dioxide decorated silicon carbide (β-SiC)-supported cobalt catalyst for Fischer–Tropsch synthesis (FTS) were explored by a combination of energy-filtered transmission electron microscopy (EFTEM), 59Co zero-field nuclear magnetic resonance (59Co NMR), and other conventional characterization techniques. From the 2D elemental maps deduced by 2D EFTEM and 59Co NMR analyses, it can be concluded that the nanoscale introduction of the TiO2 into the β-SiC matrix significantly enhances the formation of small and medium-sized cobalt particles. The results revealed that the proper metal–support interaction between cobalt nanoparticles and TiO2 led to the formation of smaller cobalt particles (<15 nm), which possess a large fraction of surface atoms and, thus, significantly contribute to the great enhancement of conversion and the reaction rate. The cobalt time yield of the catalyst after modification increased to 7.5 × 10–5 molCO gCo–1 s–1 at 230 °C, whereas the C5+ selecti...

Synthesis of C-8 alkyl xanthines by pentaamminecobalt(III) complex

Alkyl xanthines underwent selective homolytic aromatic substitution at C-8 position with alkyl groups of pentaamminecobalt(III) complex. In this process of synthesis, we used monoalkyl hydrazines as the radical source in aqueous ammonia solution. Evidence supporting coordination of the alkyl hydrazine to pentaamminecobalt(III) complex by radical trapping was in good agreement with literature. The products were characterized using GC-MS and 1H, 14N and 59Co NMR spectroscopy.

Spin-State Transition in RCoO3 (R = La, Pr, and Nd): Single-Crystal 59Co NMR Measurements

A thermally-induced spin-state transition in RC0O3 (R = La, Pr, and Nd) is investigated by single-crystal 59Co NMR measurements. In contrast to LaCoO3, PdCoC3 and NdCoC3 exhibit no clear transition but a crossover from a nonmagnetic state to a paramagnetic phase. Subtracting a rare-earth contribution to the 59Co Knight shift in PrCoC3 and NdCoC3, we obtain the hyperfine coupling constant between a 59Co nuclear spin and 3d spins in the paramagnetic state with the finite 3d spin susceptibility.

Temperature dependence of a resonance frequency of 59Co NMR in a ferromagnetic Heusler alloy Co2FeSi

The temperature dependence of the resonance frequency of 59Co NMR in a ferromagnetic Heusler alloy, Co2FeSi, in zero applied field is reported. The temperature dependence in the range 4.2–300K has been found to be well described by ν=ν(0)(1-BT3/2) with ν(0)=139.1MHz, and B=5.32×10-6K-3/2, which corresponds to the spin-wave stiffness constant D of 2.60meVnm2. The behavior is roughly in agreement with the temperature dependence of the macroscopic magnetization and is consistent with the dominance of the spin-wave excitations in Co2FeSi at low temperatures.

A solid-state NMR investigation of the colossal expansion material, Ag3Co(CN)6

Presented here is a solid-state NMR investigation of the so-called “colossal expansion” material, Ag3Co(CN)6, a compound that exhibits some of the largest positive and negative thermal expansion properties reported. This study explores the 13C, 15N, and 59Co NMR properties of this material at room temperature and at variable temperatures with the goal of probing the effects of this colossal expansion behaviour on these properties. We found that the flexible nature of the crystal framework leads to a distribution of electric field gradients, and that, oddly enough, no strong correlation is observed between the NMR parameters of Ag3Co(CN)6 and its colossal expansion nature. The 59Co isotropic chemical shift increased and the 59Co nuclear quadrupolar coupling constant decreased with increasing temperature, but neither of these relationships were extraordinary when compared to other octahedral Co(III) complexes. The link between the colossal expansion and the NMR properties of Ag3Co(CN)6 may be the distribution of lattice parameters and hence unusually broad features in the 59Co NMR spectra. The high order of symmetry at the cobalt site resulted in a small quadrupolar coupling constant less than 1 MHz in magnitude. We also observed a |1J(107/109Ag,15N)| value of 96 Hz, the largest 107/109Ag–15N coupling constant reported to date.

Determination of 1J( 59Co– 59Co) scalar coupling constants in the tetrahedral mixed-metal cluster HFeCo 3(CO) 10(PCyH 2)(PPh 2[CH 2C(O)Ph]) using COSY-type NMR experiments

Two-dimensional 59Co correlation spectroscopy (COSY) and double-quantum-filtered (DQF) COSY NMR experiments are reported for the tetrahedral mixed-metal cluster HFeCo 3(CO) 10(PCyH 2)(PPh 2[CH 2C(O)Ph]), which consists from the point of view of 59Co NMR spectroscopy, of an AMX system of three-spin S = 7/2. Both 2D NMR spectra prove the existence of a J scalar coupling constant between non-equivalent 59Co nuclei. By contrast to what happens with the conventional 2D 59Co COSY NMR spectrum, it was possible to simulate the 2D 59Co DQF-COSY NMR spectrum by density matrix calculations in order to extract the values of the 1J( 59Co– 59Co) coupling constants. The comparison between experimental and theoretical 2D NMR spectra gives spin-couplings constants of several hundreds Hertz for this cluster.

Role of weak interactions in binding of aryldicarboxylate ion: Synthesis, spectroscopic characterization, single-crystal X-ray structure determination and packing analysis of [Co(phen) 2CO 3] 2(C 8H 4O 4)·15H 2O

To explore the role of weak interactions in aryldicarboxylate binding, the hydrated complex salt, [Co(phen) 2CO 3] 2(C 8H 4O 4)·15H 2O, 1, ( C 8 H 4 O 4 2 - = terephthalate ) was synthesized and characterized by elemental analyses, spectroscopic studies (IR, UV/Visible, 1H, 13C and 59Co NMR), physical measurements and single-crystal diffraction. Compound 1 crystallizes in space group P 1 ¯ and the unit cell contains two complex cations, one terephthalate anion and fifteen water molecules. Two N,N-bidentate 1,10-phenanthroline ligands and one O,O-bidentate carbonate dianion lead to a distorted cis-CoO 2N 4 octahedral geometry for the metal ion. In the crystal, a supramolecular network built up from C H⋯O water/carboxylate (through second sphere interactions) and O H⋯O water/carboxylate, π–π stacking and C H⋯π interactions besides electrostatic forces of attraction help to consolidate the packing. Interestingly a chain of water hexamers and tetramers running along the a-axis also have been observed which is trapped in between the complex cations and terephthalate anions framework.

Calculation of 59Co shielding tensor [formula omitted] using LF–DFT

59Co NMR shielding tensor σ calculation using perturbation theory within the framework of Ligand Field–Density Functional Theory (LF–DFT) [1] is implemented for transition-metal complexes as an extension of the LF–DFT methodology. We give some first results for absolute and relative shielding tensors, which are in a good agreement with experimental results as well as with other theoretical calculations.

59Co NMR at Pulsed High Magnetic Fields

We report the 59Co-NMR observation by spin-echo method at pulsed high magnetic fields up to 48 T. The 22 mm bore-resistive coil magnet with an inductance of 7.59 mH has a total time duration of 50 ms. After discharging the capacitor bank, NMR signals were observed near the maximum field. By repeatedly pulsing the RF sequence around the maximum field, spin echoes at different fields were recorded to give the NMR spectrum.

59Co NMR study on local magnetic properties of Ca1−xNaxCo2O4

59Co NMR measurements have been performed to study the local magnetic properties of Ca1−xNaxCo2O4 with x = 0-0.6, which is a new system synthesized for the first time under a high pressure. The Knight shift of the 59Co NMR spectra for these materials is small and almost T-independent even when the Ca ions are substituted by Na up to x = 0.6. Furthermore, a magnetic order is not observed at least down to 4.2 K for the samples with x = 0-0.6 prepared in this work. In order to explain these peculiar magnetic behaviors, site-selective distribution of holes has been discussed.

Second sphere coordination in anion binding: Synthesis, spectroscopic and X-ray structural study of [Co(phen) 3] 2[Hg(SCN) 4] 3·3H 2O

The complex salt [Co(phen) 3] 2[Hg(SCN) 4] 3·3H 2O was synthesized by mixing the separately dissolved tris(1,10-phenanthroline)cobalt(III) chloride, mercury(II) chloride and ammonium thiocyanate in aqueous medium in 2:3:12 molar ratio. The newly synthesized complex salt was characterized by elemental analyses, spectroscopic studies (IR, 1H, 13C and 59Co NMR). Single-crystal X-ray structure determination of [Co(phen) 3] 2[Hg(SCN) 4] 3·3H 2O revealed that the complex salt crystallizes in the monoclinic crystal system with space group P2 1/n where a = 12.0150(4), b = 20.1916(5), c = 36.1151(9) Å, β = 94.408(2)°, V = 8735.7(4) Å 3, Z = 4. The asymmetric unit consists of two complex cations, three anions and three lattice water molecules. Crystal lattice is stabilized by hydrogen bonding interactions of type C–H…O, C–H…N through second sphere coordination and O–H…O interactions besides the electrostatic forces of attraction. The structural studies suggest that [Co(phen) 3] 3+ is a potential anion receptor for the [Hg(SCN) 4] 2− ion.

C59oandA75sNMR investigation of lightly dopedBa(Fe1−xCox)2As2(x=0.02,0.04)

We investigate the electronic properties of Ba[Fe(1-x)Co(x)]2As2 (x =0.02, 0.04) in the lightly electron-doped regime by 59Co and 75As NMR. We demonstrate that Co doping significantly suppresses the magnetic ordering temperature to the SDW state, T_{SDW}. Furthermore, ordered moments below T_{SDW} exhibit large distribution. Strong spin fluctuations remain even below T_{SDW}, persisting all the way down to 4.2 K. We find no signature of additional freezing of spin degrees of freedom unlike the case of the lightly hole-doped stripe phase of the cuprates.

Co(III) complexes of the type [(L)Co(O 2CO)] + (L = tripodal tetraamine ligand): Synthesis, structure, DFT calculations and 59Co NMR

The syntheses and characterisation of the Co(III) complexes [(L)Co(O 2CO)]ClO 4 (L = a tripodal tetraamine ligand = baep, abap, uns-penp, dppa, trpn) are reported. Geometric isomers are possible for all but the trpn complex, owing to the non-equivalence of the three arms on the tripodal ligand, and both NMR and X-ray crystallography are used to identify the single isomer formed. X-ray crystal structures of the complexes [(L)Co(O 2CO)]ClO 4 · xH 2O (L = baep, x = 0.5; L = abap, x = 0; L = uns-penp, x = 1; L = dppa, x = 0; L = trpn, x = 1) are reported; little variation is observed in the geometry of the carbonate chelate ring while significant lengthening of bonds and expansion of angles involving the cobalt ion occurs as the number of six-membered chelate rings in the complex cations increases. 59Co NMR chemical shift data for the complexes show the expected linear relationship between λ max, the wavelength of the lowest energy d– d transition, and γ, the magnetogyric ratio of the 59Co nucleus. An excellent correlation between Δ, the d orbital splitting parameter, and δ( 59Co) also exists for these complexes. Rate data for the acid hydrolysis of [(L)Co(O 2CO)] + (L = uns-penp, dppa) in 1.0 M HClO 4 differ by two orders of magnitude, and this is attributed to the differing steric accessibility of the endo O atoms in each complex. DFT calculations on the complexes reproduce the isomeric preferences, UV–Vis and 59Co NMR spectroscopic data well, provided that solvent effects are included.