Magnetochemistry, Volume 4, Issue 2 (June 2018) – 10 articles

The aim of the present work was to study the magnetic properties of ZrO₂(Mn) nanocrystals prepared by microwave-assisted hydrothermal synthesis using three different precursors: KMnO₄, MnCl₂, and Mn(NO₃)₂. The structural characterization was performed by means of X-ray diffraction. The morphology of the samples was studied by using STEM microscopy. The magnetic properties were studied by means of alternating current (AC) susceptibility (at a small AC magnetic field with amplitude not exceeding 5 Oe) and direct current (DC) magnetization (up to 9 T). All of the samples demonstrated Curie–Weiss behavior at higher temperatures with negative values of the Curie–Weiss temperature θ. It was shown that the conditions of the synthesis, e.g., pH and the type of precursor, can be adjusted to decrease the value of the Curie–Weiss temperature and reduce antiferromagnetic interactions. Full article

We studied the electron spin resonance (ESR) line width for localized moments within the framework of the Kondo lattice model. Only for a sufficiently small Kondo temperature can an ESR signal be observed for a Kondo impurity. On the other hand, for a Kondo lattice representing a heavy fermion compound, short-range ferromagnetic correlations (FM) between the localized moments are crucial to observe a signal. The spin relaxation rate (line width) and the static magnetic susceptibility are inversely proportional to each other. The FM enhance the susceptibility and hence reduce the line width. For most of the heavy fermion systems displaying an ESR signal, the FM order arises in the $ab$-plane from the strong lattice anisotropy. CeB${}_6$ is a heavy fermion compound with cubic symmetry having a ${\mathsf{\Gamma }}_8$ ground-quartet. Four transitions are expected for individual Ce ions with a ${\mathsf{\Gamma }}_8$ ground-multiplet, but only one has been observed. Antiferro-quadrupolar order (AFQ) arises below 4 K due to the orbital content of the ${\mathsf{\Gamma }}_8$-quartet. We addressed the effects of the interplay of AFQ and FM on the ESR line width and the phase diagram. It is usually difficult to distinguish among ESR resonances due to localized moments and conducting heavy electron spins, especially for anisotropic Ce and Yb compounds. However, for CeB${}_6$, an itinerant picture within the AFQ phase is necessary to explain the electron spin resonances. The longitudinal magnetic susceptibility has a quasi-elastic central peak of line width 1/T${}_1$ and inelastic peaks for the absorption/emission of excitations. The latter are measured via inelastic neutron scattering (INS) and provide insights into the magnetic order. We briefly summarize some of the INS results for CeB${}_6$ in the context of the picture that emerged from the ESR experiments. Full article

A one-dimensional chain-type dicopper(II) coordination polymer with 1,4-di(4-pyridyl)-benzene (dpybz), [Cu₂(O₂C-tBu)₄(dpybz)] (1), is synthesized and characterized by single crystal X-ray diffraction, infrared spectroscopy, and CHN elemental analysis. Single crystal X-ray diffraction confirms that the one-dimensional chains of 1 are assembled with CH···π interactions at the dpybz moieties to form a brick-like porous network structure. Magnetic susceptibility measurement and broken-symmetry density functional theory (BS-DFT) calculations indicate that (i) antiferromagnetic interactions are present between two copper ions through the bridging carboxylate ligands; the observed exchange integral value (J) of 1 is −175.3 cm⁻¹, which is consistent with the DFT-calculated value for 1 (−174.5 cm⁻¹), and (ii) the magnetic interaction between two Cu₂ units through the dpybz ligand is negligible. N₂ adsorption measurements indicate that the porous structure of 1 is retained even after evacuation of the guest solvents from the pores of 1, and 1 adsorbs N₂ molecules into its pores (the Langmuir surface area of 1 is estimated as 538.0 m²/g). Full article

We present a strategy for stereospecific NMR assignment of H^β2 and H^β3 protons in mid-size proteins (~150 residues). For such proteins, resonance overlap in standard experiments is severe, thereby preventing unambiguous assignment of a large fraction of β-methylenes. To alleviate this limitation, assignment experiments may be run in high static fields, where higher decoupling power is required. Three-bond H^α–H^β J-couplings (³J_Hα–Hβ) are critical for stereospecific assignments of β-methylene protons, and for determining rotameric χ₁ states. Therefore, we modified a pulse sequence designed to measure accurate ³J_Hα–Hβ couplings such that probe heating was reduced, while the decoupling performance was improved. To further increase the resolution, we applied non-uniform sampling (NUS) schemes in the indirect ¹H and ¹³C dimensions. The approach was applied to two medium-sized proteins, odorant binding protein 22 (OBP22; 14.4 kDa) and Pin1 (18.2 kDa), at 900 MHz polarizing fields. The coupling values obtained from NUS and linear sampling were extremely well correlated. However, NUS decreased the overlap of H^β2/3 protons, thus supplying a higher yield of extracted ³J_Hα-Hβ coupling values when compared with linear sampling. A similar effect could be achieved with linear prediction applied to the linearly sampled data prior to the Fourier transformation. Finally, we used ³J_Hα–Hβ couplings from Pin1 in combination with either conventional or exact nuclear Overhauser enhancement (eNOE) restraints to determine the stereospecific assignments of β-methylene protons. The use of eNOEs further increased the fraction of unambiguously assigned resonances when compared with procedures using conventional NOEs. Full article

Three new dinuclear copper(II) complexes [Cu₂(μ–HLⁿ)₂(μ-Cl)₂Cl₂]Cl₂ (1–3) have been synthesized and structurally characterized by single-crystal X-ray diffraction, where HL^x, (HL¹ = N⁶-propyladeninium, HL² = N⁶-butyladeninium and HL³ = N⁶-isobutyladeninium) are N⁶-alkyl bidentate NN donor adenine bases. Complexes 1–3 exhibit a coplanar arrangement of both N⁶-alkyladeninium moieties with UD conformation, with the terms U(up) or D(down) referring to the coordination of each pyrimidinic N³ atoms to the upper or lower metal center. In the three complexes, both copper atoms are five-coordinated (N₂Cl₃ donor set), resembling a compressed trigonal bipyramid. Each adenine moiety is protonated in N¹ and the positive charge balanced by chloride counterions. Magnetic measurements of complexes 1 and 3 in the 2–300 K temperature range indicate antiferromagnetic coupling with J = −156.1(7) and J = −151(2) cm⁻¹, respectively. Density functional theory calculations have also been performed in order to estimate the exchange coupling constants in these complexes. The theoretically calculated J values are in good agreement with the experimental values. Full article

Chromium (VI) is carcinogenic through intermediates formed in the cellular milieu by reduction with small reductants like glutathione (GSH), ascorbate, cysteine, and NADPH. Although the reduction of chromate by thiols has been investigated, the participation of Cr(IV) intermediates has been inferred only indirectly due to the Cr(IV) refractive behavior towards EPR spectroscopy. Biological data from numerous reports indicate that Cr(IV) is the species most likely responsible for the carcinogenicity of Cr(VI). Our kinetic studies suggested that in acidic solutions, glycine buffer at pH 2.8, the reduction of chromate with GSH involves mostly a chromium(IV) intermediate. As a step towards the full characterization of the paramagnetic species involved in the reduction of chromate by thiols at neutral pH, we embarked on an investigation of the reduction of chromate with GSH in glycine buffer at pH 2.8 using a Superconducting QUantum Interference Device (SQUID) magnetometer. Our results indicate a strong influence of temperature and confirm the presence of Cr(IV). At 2 K, the saturation magnetization method was applied to the frozen reaction when it reached the peak of formation of intermediates and the contributions were calculated to be 30% of Cr(IV) and 69% of Cr(V). When the Curie–Weiss method was applied to determine the effective magnetic moment, the use of the linear portion of the curve, 100–200 K, yielded 58% Cr(IV) and 42% Cr(V); when data from the region below the temperature of liquid N₂ (77 K) is employed, the intermediate is exclusively Cr(IV). Full article

Chain compounds of paddle-wheel Cu₂-clusters of 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (Hcaproxy) and 4-carboxy-2,2,6,6-tetramethylpiperidinyloxy (Hcatempo) and N,N’-bidentate ligands (L = 4,4′-bipyridine (4,4′-bpy), 1,2-bis(4-pyridyl)ethane (bpe), trans-1,2-bis(4-pyridyl)ethylene (bpel), 4,4′-dipyridyl disulfide (pds), 1,4-diazabicyclo[2.2.2]octane (dabco), and pyrazine (pyz)), [Cu₂(caproxy)₄(L)]_n, and [Cu₂(catempo)₄(L)]_n, were synthesized and characterized by elemental analysis, infrared and UV-vis spectra and temperature dependence of magnetic susceptibilities (4.5–300 K). The crystal structures of [Cu₂(caproxy)₄(pds)]_n, [Cu₂(catempo)₄(4,4′-bpy)]_n, and [Cu₂(catempo)₄(bpe)]_n revealed zigzag or linear chains consisting of alternate arrangement of the dinuclear cluster bearing four nitroxide radicals and N,N’-bidentate ligand. Temperature dependence of magnetic susceptibilities showed a considerable antiferromagnetic interaction between the two copper(II) ions within the dinuclear cluster, and weak antiferromagnetic interaction between the dinuclear clusters and/or the radical and dinuclear cluster. Full article

A lantern-type diruthenium(II,III) complex [Ru₂(HNOCPh)₄(BF₄)(H₂O)] was prepared from [Ru₂(HNOCPh)₄Cl]_n by removal of the axial chlorido-bridge using AgBF₄ in THF. The room temperature magnetic moment (per Ru₂⁵⁺ unit) of [Ru₂(HNOCPh)₄(BF₄)(H₂O)] is 3.84 μ_B, which is similar to that (4.15 μ_B) of [Ru₂(HNOCPh)₄Cl]_n, for which magnetic measurement was newly performed in this study. These results indicate that both of the complexes have a spin state of S = 3/2, although temperature-variable (VT) magnetic moments (2–300 K) showed that considerable antiferromagnetic interaction (zJ = −2.8 cm⁻¹) exists through the axial chlorido-bridge for [Ru₂(HNOCPh)₄Cl]_n, but such a large interaction (zJ = −0.08 cm⁻¹) does not exist for [Ru₂(HNOCPh)₄(BF₄)(H₂O)], where the large zero-field splitting D = 61 cm⁻¹ is operative for both complexes, like other lantern-type diruthenium(II,III) complexes. The X-ray single-crystal structure analysis of [Ru₂(HNOCPh)₄(BF₄)(H₂O)]·2(acetone) showed that the axial positions of the complex were occupied by a fluorine atom of the BF₄⁻ ion and an oxygen atom of the water molecule, with distances of Ru-F_ax = 2.3265(19) Å and Ru-O_ax = 2.280(2) Å, respectively. The Ru-Ru bond distance was 2.2793(4) Å, which is shorter than those (2.295(2) and 2.290(2) Å) reported for [Ru₂(HNOCPh)₄Cl]_n. The quartet ground states (S = 3/2) were reasonably interpreted for [Ru₂(HNOCPh)₄(BF₄)(H₂O)] and [Ru₂(HNOCPh)₄Cl]_n, as well as the theoretically modeled complex cation [Ru₂(HNOCPh)₄]⁺, by DFT calculation results. A Ru₂⁶⁺/Ru₂⁵⁺ redox couple was observed at 1.12 V (vs. SCE) for [Ru₂(HNOCPh)₄(BF₄)(H₂O)] in dichloromethane containing Bu₄NPF₆ as electrolyte. Full article

The effect of Al substitution on microstructure, martensitic transformation and magnetocaloric properties in Ni₄₈Mn_39.5Sn_12.5−xAl_x (x = 0, 1, 2, 3) alloys is reported. At room temperature, depending on Al concentration, the alloys have typical Heusler L2₁ austenite structure and/or orthorhombic martensite structure with Pmma space group. A secondary Ni-Mn-Al phase also appears already for low Al concentrations (x ≥ 1). On cooling, irrespective of Al substitution, all the samples show ferromagnetic type ordering below 303 K in the austenite phase. The martensitic transition temperature varies with Al content. All the alloys undergo magnetic field-induced reverse martensitic transformation giving rise to an inverse magnetocaloric effect. The largest magnetic entropy change (8.5 J·kg⁻¹·K⁻¹) is observed near 280 K for the Ni₄₈Mn_39.5Sn_12.5 alloy. Full article