Microcrystalline Samples Research Articles

A biradical chelate Zn(II) complex with phenoxazin-1-one ligands

A stable biradical bis-chelate complex 3, bis-[2,4,6,8-tetra-(tert-butyl)phenoxazinonato] Zn(II), based on redox-active ligands has been prepared and structurally characterized by X-ray crystallography. Lengths of the coordination Zn–O bonds are equal to 1.938 and 2.008Å and those of the intraligand C–N and C–O bonds (1.356 and 1.308Å, respectively) point to radical-anion form of the phenoxazinone ligand. Effective magnetic moment of a microcrystalline sample of 3 measured at 70K is equal 2.53μB, and steadily decreases to 2.39μB upon elevation of temperature. EPR, UV–Vis measurements and DFT B3LYP∗/6-311++G(d,p) calculations show that 3 has the ground triplet electronic state characterized with weak ferromagnetic coupling of the unpaired electrons localized on the ligands.

Motion-adapted pulse sequences for oriented sample (OS) solid-state NMR of biopolymers

One of the main applications of solid-state NMR is to study the structure and dynamics of biopolymers, such as membrane proteins, under physiological conditions where the polypeptides undergo global motions as they do in biological membranes. The effects of NMR radiofrequency irradiations on nuclear spins are strongly influenced by these motions. For example, we previously showed that the MSHOT-Pi4 pulse sequence yields spectra with resonance line widths about half of those observed using the conventional pulse sequence when applied to membrane proteins undergoing rapid uniaxial rotational diffusion in phospholipid bilayers. In contrast, the line widths were not changed in microcrystalline samples where the molecules did not undergo global motions. Here, we demonstrate experimentally and describe analytically how some Hamiltonian terms are susceptible to sample motions, and it is their removal through the critical π/2 Z-rotational symmetry that confers the "motion adapted" property to the MSHOT-Pi4 pulse sequence. This leads to the design of separated local field pulse sequence "Motion-adapted SAMPI4" and is generalized to an approach for the design of decoupling sequences whose performance is superior in the presence of molecular motions. It works by cancelling the spin interaction by explicitly averaging the reduced Wigner matrix to zero, rather than utilizing the 2π nutation to average spin interactions. This approach is applicable to both stationary and magic angle spinning solid-state NMR experiments.

Out-and-back 13C–13C scalar transfers in protein resonance assignment by proton-detected solid-state NMR under ultra-fast MAS

We present here (1)H-detected triple-resonance H/N/C experiments that incorporate CO-CA and CA-CB out-and-back scalar-transfer blocks optimized for robust resonance assignment in biosolids under ultra-fast magic-angle spinning (MAS). The first experiment, (H)(CO)CA(CO)NH, yields (1)H-detected inter-residue correlations, in which we record the chemical shifts of the CA spins in the first indirect dimension while during the scalar-transfer delays the coherences are present only on the longer-lived CO spins. The second experiment, (H)(CA)CB(CA)NH, correlates the side-chain CB chemical shifts with the NH of the same residue. These high sensitivity experiments are demonstrated on both fully-protonated and 100%-H(N) back-protonated perdeuterated microcrystalline samples of Acinetobacter phage 205 (AP205) capsids at 60 kHz MAS.

Synthesis, magnetic and spectroscopic characterization of a new Fe7 cluster with a six-pointed star topology

A heptanuclear FeIII7 cluster is reported, prepared from the reaction of [Fe3O(O2CPh)6(H2O)3]+ and a combination of aliphatic aminoalcohol ligands, namely apH (3-amino-1-propanol) and hmpipH (2-hydroxymethyl piperidine). The (apH2)[Fe7O3(OH)3(hmpip)6(O2CPh)7] complex possesses a metal core resembling a metal-centered non-planar six-pointed star. Thermogravimetric analysis was used to monitor the thermal stability of 1, and shows stepwise loss of mass for this complex upon heating, while Mössbauer spectroscopy was used to evaluate the chemical environment of the Fe ions. The latter confirmed the existence of only high-spin FeIII ions in the cluster, as also suggested by X-ray crystallography. Dc and ac magnetic susceptibilities were measured on microcrystalline samples of 1, and the ground state for this cluster was found to be S=21/2, with low lying excited states. Attempts to rationalize the observed spin using two different magnetostructural correlation relationships, previously derived for oxide-bridged FeIII dimers, suggested the presence of spin frustrated pathways along the peripheral hexagon, which in turn produce a high-spin ground state. The observed S=21/2 was finally rationalized using the results of this magnetostructural analysis, and assuming a combination of Si=5/2 and intermediate Si=3/2 spin projections. This Fe7 cluster is the third reported example of a heptanuclear Fe cluster with this topology, featuring a tetrahedral Fe ion at the center, and joins a small family of Fe clusters based on the aliphatic ligands apH and hmpipH.

Electron Beam Irradiation of Cellulosic Materials-Opportunities and Limitations.

The irradiation of pulp is of interest from different perspectives. Mainly it is required when a modification of cellulose is needed. Irradiation could bring many advantages, such as chemical savings and, therefore, cost savings and a reduction in environmental pollutants. In this account, pulp and dissociated celluloses were analyzed before and after irradiation by electron beaming. The focus of the analysis was the oxidation of hydroxyl groups to carbonyl and carboxyl groups in pulp and the degradation of cellulose causing a decrease in molar mass. For that purpose, the samples were labeled with a selective fluorescence marker and analyzed by gel permeation chromatography (GPC) coupled with multi-angle laser light scattering (MALLS), refractive index (RI), and fluorescence detectors. Degradation of the analyzed substrates was the predominant result of the irradiation; however, in the microcrystalline samples, oxidized cellulose functionalities were introduced along the cellulose chain, making this substrate suitable for further chemical modification.

Hexanuclear complexes from the use of a series of amino-alcohol ligands in Fe chemistry

Eight novel FeIII6 clusters with the amino-alcohol ligands 3-amino-1-propanol (apH), N-hydroxyethylpiperidine (N-hepipH), 2-hydroxyethylpiperidine (2-hepipH), 2-aminobenzylalcohol (2-abzaH), N-hydroxyethylmorpholine (N-hemorH) and ethanolamine (eaH), [Fe6O2(OH)2L2((CH3)3CCO2)10] (LH=apH, 1; N-hepipH, 2; 2-hepipH, 3; 2-abzaH, 4}, [Fe6O2(OH)2(N-hemor)2((CH3)3CCO2)8(CH3CO2)2]·2MeCN (5·2MeCN), [Fe6O2(OH)2(N-hemor)3((CH3)3CCO2)9]·MeCN (6·MeCN), [Fe6O2(ea)6((CH3)3CCO2)6(N3)2]·2MeCN (7·2MeCN) and [Fe6O3(N-hepip)3(PhCO2)9(PhCO2H)2]·5MeCN (8·5MeCN) are reported. Compound 1 was prepared from the reaction of [Fe3O((CH3)3CCO2)6(H2O)2(OH)]H2O with apH in the presence of NaClO4 in MeCN. Accordingly, compounds 2–4 and 6 were prepared from reactions of trinuclear carboxylate Fe3+ clusters with the aminoalcohol ligands whereas the isolation of 5, involved also the addition of Fe(CH3CO2)2 to the reaction mixture. By using a similar synthetic procedure, but in the presence of N3− ligand, compound 7 was isolated. Finally, the replacement of pivalates with benzoates in the reaction mixture yielded compound 8. To the best of our knowledge, 3, 5 and 6 are the first reported examples containing the ligands 2-hepipH and N-hemorH. Compounds 1–6 are composed of a [Fe6(μ3-O)2(μ-OH)2]12+ structural core, where the two subunits are either arranged trans to each other (1–5), or they adopt the cis conformation (6). Compounds 7 and 8 possess the [Fe6(μ3-O)2(μ-OR)6]8+ and [Fe6(μ3-O)3]12+ structural cores respectively which display topologies rarely found in Fe cluster chemistry. Variable-temperature magnetic susceptibility measurements on powdered microcrystalline samples of 4, 6, 7 and 8 revealed the existence of antiferromagnetic exchange interactions which lead to an ST=5 for 4 and 7, and diamagnetic ground states (ST=0) for 6 and 8. Mössbauer spectroscopy studies on powdered microcrystalline samples of 7 and 8 confirmed that all iron ions of both complexes are in the Fe3+ (S=5/2) state. The variation of the ligand environment in the various iron sites was reflected in their different quadrupole splitting parameters.

Magnetic behaviors of cobalt(II) complex chains connected with a 1,3-phenylenebis(4-pyridylcarbene) magnetic coupler

One-to-one mixtures of Co(hfpip and hfnip)2; hfpip=1,1,1,5,5,5-hexafluoro-4-(phenylimino)-2-pentanonato and hfnip=1,1,1,5,5,5-hexafluoro-4-(1-naphthylimino)-2-pentanonato, and 1,3-phenylenebis(4-pyridyldiazomethane), D2py2, gave diazo-cobalt complexes, 1 and 2, respectively, formulated by [Co(hfpip and hfnip)2(D2py2)]n. Complex 2 was obtained as a single crystal and was found to have a zig-zag chain structure in the cis coordination of pyridines in D2py2 to the cobalt ions by X-ray crystallography. After irradiation of microcrystalline samples of 1 and 2, the generated carbene-cobalt complexes, 1c and 2c, showed magnetic properties due to ferromagnetic chains. Those complexes exhibited slow magnetic relaxations with Ueff/kB=109 and 129K and temperature-dependent hysteresis loops with Hc=11 and 9kOe at 1.9K for 1c and 2c, respectively.

Activation energy for formation of nickel-aluminide thin film on nanocrystalline and microcrystalline nickel

In this paper, activation energy and time needed for formation of Al3Ni2 thin film with thickness of ∼1 μm on nanocrystalline (NC) nickel was investigated and compared with microcrystalline (MC) nickel. NC and MC samples were prepared by electrodeposition and annealing, respectively. The samples were aluminized by pack cementation method in the temperature range of 475–600 °C. The surface morphology and cross section of coated specimens were characterized by means of optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was observed that the time needed for formation of Al3Ni2 thin film on NC samples was shorter than that for MC specimens at all investigated temperatures. In addition, for both, time needed for formation of the thin film was decreased by increasing temperature according to exp (Q/RT). It was found that activation energy for formation of the thin film on NC samples was 35% lower than that for MC specimens. Activation energy for formation of thin film of Al3Ni2 were ∼50% and ∼ 120% lower than the activation energy for growth of thick layer of aluminide coating on MC and NC nickel, respectively.

Na3SbS3: Single Crystal X‐ray Diffraction, Raman Spectroscopy, and Impedance Measurements

AbstractNa3SbS3 was prepared by the reaction of anhydrous Na2S, antimony, and sulfur in a ratio of 3:2:3 at 870 K. The pale yellow compound is air and moisture sensitive. A microcrystalline sample was obtained after annealing Na3SbS3 for two weeks at 720 K. The crystal structure of Na3SbS3 was determined by single‐crystal X‐ray diffraction at 123 K. Na3SbS3 crystallizes in the cubic space group P213 (No. 198) with a = 8.6420(1) Å, V = 645.42(1) Å3 and Z = 4. The structure refinement converged to R = 0.0099 (wR = 0.0181) for 592 independent reflections and 23 parameters. Na3SbS3 is isotypic with Na3AsS3. Sodium atoms are located on three different sites, which show a strongly distorted octahedral coordination sphere of sulfur. The coordination polyhedra of equivalent sodium sites share common vertices, whereas those of different sodium sites share common faces. Antimony and sulfur form trigonal SbS3 pyramids, which coordinate sodium as a mono‐, bi‐, or tridentate ligand. Raman spectroscopic investigations result in stretching modes ν(Sb–S) at 334, 321, and 312 cm–1, respectively. Thermoanalytical studies do not show any additional thermal effects up to the melting point of 875 K. Impedance spectroscopy on Na3SbS3 in a range from 325 to 570 K shows a temperature dependent Na+ conductivity, which is 1.9 × 10–6 Ω–1·cm–1 at 570 K.

An X-ray absorption spectroscopy study of the inversion degree in zinc ferrite nanocrystals dispersed on a highly porous silica aerogel matrix

The structural properties of zinc ferrite nanoparticles with spinel structure dispersed in a highly porous SiO(2) aerogel matrix were compared with a bulk zinc ferrite sample. In particular, the details of the cation distribution between the octahedral (B) and tetrahedral (A) sites of the spinel structure were determined using X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the zinc ferrite spinel structures varies with particle size. In particular, in the bulk microcrystalline sample, Zn(2+) ions are at the tetrahedral sites and trivalent Fe(3+) ions occupy octahedral sites (normal spinel). When particle size decreases, Zn(2+) ions are transferred to octahedral sites and the degree of inversion is found to increase as the nanoparticle size decreases. This is the first time that a variation of the degree of inversion with particle size is observed in ferrite nanoparticles grown within an aerogel matrix.

Bis[(E)-2,6-bis(1H-pyrazol-1-yl)-4-styrylpyridine]iron(II) Complex: Relationship between Thermal Spin Crossover and Crystal Solvent

The crystal structures and thermal spin-crossover (SCO) behavior of [Fe(E-dpsp)(2)](BF(4))(2)·X [E-dpsp = (E)-2,6-bis(1H-pyrazol-1-yl)-4-styrylpyridine; X = crystal solvent] are investigated. The titled iron(II) complex features polymorphology induced by crystal solvents, which is identified by means of single-crystal X-ray diffraction analysis: For [Fe(E-dpsp)(2)](BF(4))(2)·acetone and [Fe(E-dpsp)(2)](BF(4))(2)·4MeNO(2), detailed analyses at various temperatures are conducted. The magnetic properties of bulk microcrystalline samples of [Fe(E-dpsp)(2)](BF(4))(2)·X are assessed using a SQUID magnetometer. Among the series, only [Fe(E-dpsp)(2)](BF(4))(2)·acetone undergoes peculiar thermal SCO, such as a precipitous and hysteretic spin-state change (T(1/2↑) = 179 K, T(1/2↓) = 164 K, and ΔT(1/2) = 15 K) and frozen-in effect. All single crystals of [Fe(E-dpsp)(2)](BF(4))(2)·X are free from intermolecular interaction except for [Fe(E-dpsp)(2)](BF(4))(2)·acetone: One of the phenyl rings in [Fe(E-dpsp)(2)](BF(4))(2)·acetone is twisted appreciably and features an intermolecular H-H short contact with one of the neighboring complexes to form a one-dimensional network. The twisted phenyl group also participates in π-π stacking with one of the pyrazolyl rings of another neighboring molecule, constructing a dimeric couple. These intermolecular interactions would induce cooperative effects, which leads to the good thermal SCO phenomenon of [Fe(E-dpsp)(2)](BF(4))(2)·acetone.

Peculiar size effect in nanocrystalline BaTiO3

Dense BaTiO3 ceramics with grain sizes of 35nm to 5.6μm were prepared, and the electrical properties investigated in the temperature range 500–700°C by means of impedance spectroscopy. Charge carriers (oxygen vacancies and holes) are depleted in the space charge regions at the BaTiO3 grain boundaries. When the grain size is ⩾250nm, the width of the space charge region was determined to be ∼40nm. Therefore, the depletion regions were expected to overlap when the grain size decreases to 35nm; in such a situation, charge carriers would be depleted over the entire grain, resulting in depressed conductivity. However, the conductivity of the 35nm grain size sample was measured to be one to two orders of magnitude higher than those of the microcrystalline samples, and the activation energy markedly lower. Moreover, we determined a width of ∼7nm for the space charge regions in the 35nm grain size sample; therefore, the space charge regions do not overlap. The enhanced conductivity is ascribed to a reduced oxidation enthalpy in nanocrystalline BaTiO3, and the distorted grain boundaries in nanocrystalline BaTiO3 are believed to be the atomic level origin of the reduced oxidation enthalpy.

Solid state NMR of proteins at high MAS frequencies: symmetry-based mixing and simultaneous acquisition of chemical shift correlation spectra

We have carried out chemical shift correlation experiments with symmetry-based mixing sequences at high MAS frequencies and examined different strategies to simultaneously acquire 3D correlation spectra that are commonly required in the structural studies of proteins. The potential of numerically optimised symmetry-based mixing sequences and the simultaneous recording of chemical shift correlation spectra such as: 3D NCAC and 3D NHH with dual receivers, 3D NC'C and 3D C'NCA with sequential (13)C acquisitions, 3D NHH and 3D NC'H with sequential (1)H acquisitions and 3D CANH and 3D C'NH with broadband (13)C-(15)N mixing are demonstrated using microcrystalline samples of the β1 immunoglobulin binding domain of protein G (GB1) and the chicken α-spectrin SH3 domain.

Absolute Configuration of Highly Flexible Natural Products by the Solid‐State ECD/TDDFT Method: Ximaolides and Sinulaparvalides

AbstractThe quantum‐mechanical calculation of chiroptical properties such as electronic circular dichroism (ECD) is an increasingly popular tool for establishing the absolute configuration of organic compounds, including natural products. However, this approach is often limited by conformational flexibility, which can be overcome by considering the solid crystalline state instead of solution. By knowing the solid‐state geometry, one may employ it as the input structure for ECD calculations and then comparing this with the ECD spectrum measured on a microcrystalline sample. We report here the application of this solid‐state ECD approach to establish the absolute configuration of three natural products, namely ximaolide A, sinulaparvalide A and B; all display pronounced flexibility and belong to two families of compounds with remarkable biological activity. The present configurational assignment may also be extended to several analogues whose absolute configurations have not yet been reported.

Heterometallic FeIII–CeIV complexes from the use of aliphatic aminoalcohol ligands

Six novel heteronuclear FeIII/CeIV clusters with the ligands 3-amino-1-propanol (Hap) and 2-(hydroxymethyl)piperidine (Hhmpip) [FeIII4CeIV6O8(L)4((CH3)3CCO2)12(RCO2)4] (R=CH3, L=ap−, 1; R=ClCH2, L=ap−, 2; R=BrCH2, L=ap−, 3; R=C2H5, L=ap−, 4; R=CH3, L=hmpip−, 5) and ([Fe2Ce2Na2O2((CH3)3CCO2)8(N3)2(ap)2])∞ (6)∞ are reported. Compounds 1–5 were prepared from reactions of preformed trinuclear Fe3+ clusters with the ligands, in the presence of (NH4)2Ce(NO3)6 and RCO2M (R=CH3, M+=H+, 1; R=ClCH2, M+=Na+, 2; R=BrCH2, M+=Na+, 3; R=CH3CH2, M+=Na+, 4; R=CH3, M+=H+, 5) in 1:5:1:1 molar ratio in CH3CN. Complex (6·2CH3CN)∞ was prepared by a similar to the above described synthetic procedure that involved the use of NaN3 instead of RCO2M. To the best of our knowledge, compounds 1–6 are the initial examples of FeIII/CeIV species and rare examples of metal complexes with the ligands Hap and Hhmpip. The structures of 1–5 are composed of a distorted supertetrahedral [FeIII4CeIV6(μ4-O)4(μ3-O)4]20+ structural core, while (6·2CH3CN)∞ displays a [FeIII2CeIV2Na2(μ4-O)2]12+ core consisting of two tetrahedral units sharing one common edge. These two structural types are rather uncommon for mixed metal 3d/4f compounds although they are well-known for 3d metal cluster chemistry. Variable temperature dc magnetic susceptibility studies on powdered microcrystalline samples of 1 and (6·2CH3CN)∞ revealed the presence of weak antiferromagnetic interactions as expected on the basis of the large separations (>6Å) between the paramagnetic Fe3+ ions in both complexes.

Magnetic and cytotoxic properties of hot-filament chemical vapour deposited diamond

Microcrystalline (MCD) and nanocrystalline (NCD) magnetic diamond samples were produced by hot-filament chemical vapour deposition (HFCVD) on AISI 316 substrates. Energy Dispersive X-ray Spectroscopy (EDS) measurements indicated the presence of Fe, Cr and Ni in the MCD and NCD samples, and all samples showed similar magnetisation properties. Cell viability tests were realised using Vero cells, a type of fibroblastic cell line. Polystyrene was used as a negative control for toxicity (NCT). The cells were cultured under standard cell culture conditions. The proliferation indicated that these magnetic diamond samples were not cytotoxic.

The Magnetocaloric Effect in Two-Phase Y-Fe Nanocrystalline Alloys

Synthesis of nanocomposite based on binary compound and determination of dependence between the values of magnetocaloric effect (MCE) and the grain size of such specimens are the purposes of this work. The results of the investigation in microcrystalline and nanocrystalline samples Y25Fe75 and Y35Fe65 alloys show, that reduction of the grain size down to 80-100 nanometers on nanocrystalline samples of Y25Fe75 and Y35Fe65 alloys leads to increasing of value of MCE by 20-25 % from comparison with values on microcrystalline samples. It can be explained by intergrain exchange interaction in nanocrystalline alloys.

Grain Boundary Blocking Effect in Yttria Stabilized Zirconia Thin Films

The cross-plane oxygen ion conductivity of yttria stabilized zirconia thin films prepared by aerosol assisted chemical vapor deposition is strongly influenced by the thin film microstructure. Thin films with highly textured columnar grains oriented parallel to the current direction exhibit significantly higher conductivity than thin films with randomly oriented nanocrystalline microstructure, where ionic transport proceeds through the numerous grain boundaries. The total conductivity of columnar AA-CVD thin films is consistent with literature values on oxygen ion conduction through chemically pure YSZ grains as determined for microcrystalline samples, while that of the nanocrystalline specimens is similar to corresponding specific grain boundary conductivities.

Combination of DQ and ZQ Coherences for Sensitive Through‐Bond NMR Correlation Experiments in Biosolids under Ultra‐Fast MAS

A double-zero quantum (DZQ)-refocused INADEQUATE experiment is introduced for J-based NMR correlations under ultra-fast (60 kHz) magic angle spinning (MAS). The experiment records two spectra in the same dataset, a double quantum-single quantum (DQ-SQ) and zero quantum-single quantum (ZQ-SQ) spectrum, whereby the corresponding signals appear at different chemical shifts in ω(1). Furthermore, the spin-state selective excitation (S(3)E) J-decoupling block is incorporated in place of the second refocusing echo of the INADEQUATE scheme, providing an additional gain in sensitivity and resolution. The two sub-spectra acquired in this way can be treated separately by a shearing transformation, producing two diagonal-free single quantum (SQ-SQ)-type spectra, which are subsequently recombined to give an additional sensitivity enhancement, thus offering an improvement greater than a factor of two as compared to the original refocused INADEQUATE experiment. The combined DZQ scheme retains transverse magnetization on the initially polarized (I) spin, which typically exhibits a longer transverse dephasing time (T(2)') than its through-bond neighbour (S). By doing so, less magnetization is lost during the refocusing periods in the sequence to give even further gains in sensitivity for the J correlations. The experiment is demonstrated for the correlation between the carbonyl (CO) and alpha (CA) carbons in a microcrystalline sample of fully protonated, [(15)N,(13)C]-labelled Cu(II),Zn(II) superoxide dismutase, and its efficiency is discussed with respect to other J-based schemes.

Photoluminescence and Cathodoluminescence Properties of Nanocrystalline BaFCl:Sm3+ X-ray Storage Phosphor

Nanocrystalline BaFCl:Sm3+ is an efficient photoluminescent X-ray storage phosphor. To gain a better understanding of the storage mechanism, the photoluminescence and cathodoluminescence properties of nanocrystalline BaFCl:Sm3+ before and after X-irradiation were investigated. The results were compared with those obtained for microcrystalline BaFCl:Sm3+ prepared by high-temperature sintering. Significant differences in the local coordination environment of Sm3+ ions between the two samples were revealed by comparing the photoluminescence spectra at 2 and 293 K. From the cathodoluminescence microanalysis, it follows that the Sm3+ ions in the as-prepared nanocrystalline BaFCl:Sm3+ are mainly located on or close to the surface of the nanoparticles, whereas those in the microcrystalline sample are distributed homogeneously throughout the microcrystallites. For nanocrystalline BaFCl:Sm3+, the X-irradiation-induced reduction of Sm3+ to Sm2+ ions and the photoionization of Sm2+ to Sm3+ ions during photobleaching were investigated by monitoring the photoluminescence intensities of both Sm2+ and Sm3+ ions. The two processes can be modeled well assuming dispersive first-order kinetics, where the rate constant of the electron transfer is given by an exponential function of the distance between the shallow electron traps and the Sm3+ centers.