ESR study of the macromolecular structure of copper-porphyrin-labeled coal

The behavior of the paramagnetic characteristics of low-metamorphized coal and the same coal labeled with copper porphyrin during pyrolysis is interpreted in terms of the microdomain model of the structure of coal In this model vitrainized maceral is regarded as a primarily anisotropic medium and as a natural high-molecular crystal. Anisotropic units or microdomains 6.0 nm in length perform the function of recurrent structural nuclei in different coals. Each microdomain has a complex structure and contains about 15 close-packed crystallites, sequentially passing to the paramagnetic state during coal pyrolysis. Translated fromZhurnal Struktumoi Khimii, Vol. 38, No. 5, pp. 894–901, September-October, 1997.     

Molecular dynamics in paramagnetic materials as studied by magic-angle spinning 2H NMR spectra

A magic-angle spinning (MAS) 2H NMR experiment was applied to study the molecular motion in paramagnetic compounds. The temperature dependences of 2H MAS NMR spectra were measured for paramagnetic [M(H2O)6][SiF6] (M=Ni2+, Mn2+, Co2+) and diamagnetic [Zn(H2O)6][SiF6]. The paramagnetic compounds exhibited an asymmetric line shape in 2H MAS NMR spectra because of the electron-nuclear dipolar coupling. The drastic changes in the shape of spinning sideband patterns and in the line width of spinning sidebands due to the 180 degrees flip of water molecules and the reorientation of [M(H2O)6]2+ about its C3 axis were observed. In the paramagnetic compounds, paramagnetic spin-spin relaxation and anisotropic g-factor result in additional linebroadening of each of the spinning sidebands. The spectral simulation of MAS 2H NMR, including the effects of paramagnetic shift and anisotropic spin-spin relaxation due to electron-nuclear dipolar coupling and anisotropic g-factor, was performed for several molecular motions. Information about molecular motions in the dynamic range of 10(2) s(-1)相似文献   

Determination of the orientation distribution function of anisotropic paramagnetic species from the angular dependence of the ESR spectra

A method for determination of the orientation distribution function of anisotropic paramagnetic species from the angular dependence of the ESR spectra based on simulation of the spectra was developed. The orientation distribution function is represented as a sum of orthonormal functions. The weight coefficients are determined by minimizing the deviations of the theoretical spectra from experimental ones. The method was numerically tested. The method is relatively insensitive to the presence of noise and foreign ESR signals not related to the sample orientation in the magnetic field.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2595–2601, December, 2004.     

Single crystal nuclear magnetic resonance in spinning powders

We present a method for selectively exciting nuclear magnetic resonances (NMRs) from well-defined subsets of crystallites from a powdered sample under magic angle spinning. Magic angle spinning induces a time dependence in the anisotropic interactions, which results in a time variation of the resonance frequencies which is different for different crystallite orientations. The proposed method exploits this by applying selective pulses, which we refer to as XS (for crystallite-selective) pulses, that follow the resonance frequencies of nuclear species within particular crystallites, resulting in the induced flip angle being orientation dependent. By selecting the radiofrequency field to deliver a 180° pulse for the target orientation and employing a train of such pulses combined with cogwheel phase cycling, we obtain a high degree of orientational selectivity with the resulting spectrum containing only contributions from orientations close to the target. Typically, this leads to the selection of between 0.1% and 10% of the crystallites, and in extreme cases to the excitation of a single orientation resulting in single crystal spectra of spinning powders. Two formulations of this method are described and demonstrated with experimental examples on [1-(13)C]-alanine and the paramagnetic compound Sm(2)Sn(2)O(7).     

Identification of static JT in copper(II) doped hexaimidazole M(II) lattices: M=Co and Ni: an EPR study

Single crystal EPR studies on Cu(II) doped paramagnetic host lattices, hexaimidazole M(II) dichloride tetrahydrate (M=Co and Ni), isomorphous with M=Zn, have been carried out from room temperature to 77K to understand the nature of Jahn-Teller (JT) distortion in these paramagnetic host systems. The paramagnetic impurity, doped in the present two paramagnetic host lattices, shows anisotropic EPR spectra with superhyperfine from ligands, even at room temperature. An interesting observation noticed in the EPR spectra at room temperature is that there are more resonances corresponding to the second site in the paramagnetic hosts than in the diamagnetic host at 4.2K. This difference in behavior between the diamagnetic and paramagnetic host lattices indicates a change in the depth of the JT valleys. The spin Hamiltonian parameters are evaluated for Cu(II) ion in both the host lattices and the relaxation times have been calculated for the ion in cobalt host lattice only.     

Solid-state NMR spectra of paramagnetic silica-based materials: observation of 29Si and 27Al nuclei in the first coordination spheres of manganese ions

Some silica-based solids, prepared by the sol/gel method in the presence of high Mn2+ concentrations, have been characterized by the 29Si, 27Al MAS NMR spectra and 29Si T1 measurements. The single-pulse 29Si and 27Al MAS NMR spectra have shown broad spinning sideband patterns that are interpreted in terms of anisotropic bulky magnetic susceptibility (BMS) and dipole-field effects. In the absence of paramagnetic isotropic shifts, the 29Si and 27Al nuclei observed in the single-pulse NMR spectra have been assigned to nuclei remote from paramagnetic centers. It has been demonstrated that the 29Si and 27Al nuclei, which are in the vicinity of the manganese ions, can be detected by the Hahn-echo MAS NMR experiments at different carrier frequencies.     

Electronic Circular Dichroism Imaging (ECDi) Casts a New Light on the Origin of Solid-State Chiroptical Properties

Solid-state ECD (ss-ECD) spectra of a model microcrystalline solid, finasteride, dispersed into a KCl pellet were recorded by using the synchrotron radiation source at the Diamond B23 beamline. Scanning a surface of 36 mm² with a step of 0.5 mm, we measured a set of ECD imaging (ECDi) spectra very different from each other and from the ss-ECD recorded with a bench-top instrument (1 cm² area). This is due to the anisotropic part of the ECD (ACD), which averages to zero in solution or on a large number of randomly oriented crystallites, but can otherwise be extremely large. Two-way singular value decomposition (SVD) analysis, through experimental and simulated TDDFT spectra, disclosed that the measured and theoretical principal components are in line with each other. This finding demonstrates that the observed isotropic ss-ECD spectrum is governed by the anisotropy of locally oriented crystals. It also introduces a new quality for ss-ECD measurements and opens a new future for probing and mapping chiral materials in the solid state such as active pharmaceutical ingredients (APIs).     

X-ray measurement of cellulose crystallite orientation in cotton fibers

The orientation of crystallites in a bundle of parallel cotton fibers was studied by x-ray diffraction. The intensity distributions of the 101 and 002 diffraction rings showed the distributions of (101) and (002) planes to be identical within the limits of accuracy. Therefore, the crystallites in the cotton fibers very likely had random orientation about their long axes. The orientation distribution of these axes was calculated by using the intensity distribution of the 002 diffraction ring. The cylindrically symmetrical density distribution J(β) thus obtained was multiplied by sin β to obtain the distribution of relative numbers of crystallites at given angles β to the long axis of the fiber. The average 〈β〉 was found to be in agreement with the value of 〈sin²β〉 measured from the 002 diffraction ring. The intensity distributions on the 101 and 002 diffraction rings showed small fluctuations. These fluctuations appeared much stronger in the J(β) and sin β J(β) distributions, indicating clear discontinuities in the pitch angle distribution.     

Evolution of ordered films of copper phthalocyanine according to EPR data

The procedure for calculating the orientation distribution of molecules using the angular dependence of EPR spectra was employed to study copper(II) phthalocyanine (CuPc) films varying in thickness and obtained by depositing the molecular complex on flat quartz plates. At the first stage of deposition, a layer of the α-CuPc phase with preferable orientation of molecular stacks along the plate surface is formed. At the second stage, a layer with an orthogonal arrangement of molecular stacks is condensed over the first layer. The interaction with NO₂ forms CuPc binuclear associates. Analysis of the EPR spectra made it possible to determine the symmetry of the structure and the distance between the paramagnetic Cu²⁺ ions; the structure of the associates has been proposed. The orientation distribution of CuPc dimers in the film depends both on the initial ordering in the film and on processing conditions. Strong disordering of molecular stacks in ordered films during the α-CuPc to α-CuPc phase transition has been found.     

Isotropic high field NMR spectra of Li-ion battery materials with anisotropy >1 MHz

The use of a magic-angle turning and phase-adjusted spinning sideband NMR experiment to resolve and quantify the individual local environments in the high field (7)Li and (31)P NMR spectra of paramagnetic lithium-ion battery materials is demonstrated. The use of short radio frequency pulses provides an excitation bandwidth that is sufficient to cover shift anisotropy of >1 MHz in breadth, allowing isotropic and anisotropic components to be resolved.     

Changes in the NMR characteristics of 3,6‐di‐tert‐butylquinone on formation of paramagnetic complexes

High‐resolution NMR spectra of 3,6‐di‐tert‐butylquinone were recorded and analyzed for the first time in a wide range of temperatures. The spectra were transformed by paramagnetic additives of cobalt, nickel, and copper complexes synthesized on the basis of metal semiquinolates. Chloroform, dimethylsulfoxide, toluene, and acetone were used as solvents. It was shown that the spectra changed by paramagnetic additives can contain valuable information on the nature of a superfine interaction in paramagnetic complexes and on peculiarities of intramolecular dynamics inherent in these compounds. Copyright © 2013 John Wiley & Sons, Ltd.     

Magnetic and EPR Studies of α-, β-, and γ-Fe2WO6 Phases at Low Temperatures

The polymorphic modifications α-, β-, and γ-Fe₂WO₆ of the iron tungstate system were studied by means of magnetic susceptibility and EPR measurements at low temperatures. Both methods revealed a significant paramagnetic contribution, probably resulting from local distortions of the antiferromagnetic bulk structure induced by a disturbed cation ordering or the presence of Fe²⁺ ions. The magnetic susceptibility revealed a peak at 260 K for all samples which can be related with an AF phase transition. The EPR spectra comprised the contribution of various isolated paramagnetic iron centers, one arising from high-spin Fe³⁺ ions in rhombic crystal field symmetry with E/D ≈ 1/3 and D ≈ 0.22 cm^-1, an anisotropic EPR signal consistent with an S= 3/2 ground state with large zero-field splitting, and a dominant component in the g ≈ 2 region presumably arising from an S = 1/2; spin state. The latter spectra were tentatively attributed to the formation of multi-iron clusters, one of them invoking the presence of Fe²⁺ ions as well. For the βFe₂WO₆ phase an additional EPR spectrum was observed, which probably results from high-spin Fe³⁺ ions in a weak crystal field.     

Solid‐State 17O NMR Spectroscopy of Paramagnetic Coordination Compounds

High‐quality solid‐state ¹⁷O (I=5/2) NMR spectra can be successfully obtained for paramagnetic coordination compounds in which oxygen atoms are directly bonded to the paramagnetic metal centers. For complexes containing V^III (S=1), Cu^II (S=1/2), and Mn^III (S=2) metal centers, the ¹⁷O isotropic paramagnetic shifts were found to span a range of more than 10 000 ppm. In several cases, high‐resolution ¹⁷O NMR spectra were recorded under very fast magic‐angle spinning (MAS) conditions at 21.1 T. Quantum‐chemical computations using density functional theory (DFT) qualitatively reproduced the experimental ¹⁷O hyperfine shift tensors.     

Aggregation of nitrosubstituted spiropyranes in polymeric films

Absorption, luminescence, and luminescence polarization spectra of associates of photochronic molecules of nitrosubstituted indolinospiropyranes in the closed form in polymeric films at high temperatures were studied. The efficiency of the formation of the associates depends on the structure of the photochrome molecules and the nature of the polymeric matrix. A scheme of phototransformations of the associates taking into account the exciting light wavelength was suggested. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 946–949, May. 1997.     

Electron paramagnetic resonance spectroscopy of γ-irradiated cellulose and related compounds

EPR studies were carried out on different irradiated cellulosic materials. The shape of the EPR spectrum was found to depend upon the fine structure of cellulose. The peak-to-peak distances of the large peaks on the differential curves were found to be 23.3 and 24.8 gauss, respectively, for the amorphous and recrystallized cellulose. The number of free radicals present per unit weight in the recrystallized cellulose and in the amorphous cellulose were found to be in the ratio of 2.1:1. The EPR spectrum of a sample of irradiated cotton crystallites which was soaked in acetone before being dried was markedly symmetrical, in contrast to the EPR spectra of samples of cotton linters and of cotton crystallites dried from water, and was considered to be a reflection of the lower intermolecular bonding in the former. For corresponding doses, the free radicals on cotton crystallites have the highest g value, followed by cotton linters, followed by regenerated cellulose I. The g value decreased as the irradiation dose was increased.     

Dimensional stability of regenerated cellulose film in relation to orientations of crystalline and noncrystalline phases

The dimensional stability of regenerated cellulose film on swelling with water is discussed in relation to the biaxial orientation of the two kinds of structural units, cellulose II crystallites and noncrystalline chain segments, and their anisotropic swelling (anisotropic absorption of water). Considerable dimensional stability in the plane of the film but enormous instability of thickness on swelling in water of some commercial cellophanes is qualitatively interpreted in terms of the planar orientation of crystal (101) planes along the film surface and the orientation of the noncrystalline chain segments parallel to the film surface. The dimensional changes on swelling from the completely dry state to 10% moisture regain were further interpreted quantitatively in terms of the degrees of biaxial orientation of the two kinds of structural units and their degrees of anisotropic swelling by modifying the Hermans monophase model for crystalline and noncrystalline biphase structures. The following degrees of anisotropic swelling of the structural units were thus obtained: q_{c, [101]} = 0.40%, q_{c, [101 ]} = ?0.33%, and q_a = 2.42%.     

Complexes of the 14th group elements with tridentate redox-active ligand

New methods of synthesis of the 14th group metal (Si, Sn, Pb) complexes containing a redox-active ligand, 3,5-di-tert-butyl-1,2-quinone-1-(2-hydroxy-3,5-di-tert-butylphenyl)imine, in various redox states were developed. New compounds were studied by the ESR spectroscopy in solutions. The redox properties of the paramagnetic tin compounds are characterized by the cyclic voltammetry. The solvent and temperature effect on the ESR spectra parameters of paramagnetic complexes of Sn(IV)and Pb(IV) was revealed.     

The effect of ionizing radiation of 1,4-dihydropyridine derivatives in the solid state

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate the gamma-radiation damage in the crystalline powder form of nine calcium channel blockers from the 1,4-dihydropyridine derivatives, which are in clinical use for treatment of arteria hypertension and ischemic heart disease. EPR studies have been carried out, showing the influence of irradiation and storage parameters on the nature and concentration of the free radicals trapped. EPR spectra of isardipine and felodipine showed single EPR line. EPR spectra of nifedipine, nisoldipine, nitrendipine, nimodipine, nicardipine and nilvadipine reveal a broad anisotropic signal of hyperfine interaction. No EPR signal was observed from amlodipine.     

Local structure analysis of Cu(II)-diazepam complexes by ESR spectroscopy

CuL₂X₂ (L = 7-chloro-1,3-dihydro-1-methyl-5-phenyl-3H-1,4-benzodiazepin-2-one, also known as diazepam, X = Cl, Br) complexes have been prepared and investigated by ESR spectroscopy. Powder ESR spectra of these complexes suggest a planar-rhombic distorted local symmetry. The CH₃Cl solutions spectra show the presence of pseudo-tetrahedral species with a 3d_xy+4p_z mixture ground state for the paramagnetic electron. The anisotropic spectra obtained for the Cu(II)-diazepam solution adsorbed on NaY zeolite confirm the existence of a CuN₂X₂ chromophore.     

Size effects in the conduction electron spin resonance of anthracite and higher anthraxolite

Electron paramagnetic resonance spectroscopy of conduction electrons, i.e. Conduction Electron Spin Resonance (CESR), is a powerful tool for studies of carbon samples. Conductive samples cause additional effects in CESR spectra that influence the shape and intensity of the signals. In cases where conduction electrons play a dominant role, whilst the influence of localized paramagnetic centres is small or negligible, the effects because of the spins on conduction electrons will dominate the spectra. It has been shown that for some ratios of the bulk sample sizes (d) to the skin depth (δ), which depend on the electrical conductivity, additional size effects become visible in the line asymmetry parameter A/|B|, which is the ratio of the maximum to the absolute, minimum value of the resonance signal. To study these effects the electrical direct current–conductivity and CESR measurements are carried out for two amorphous bulk coal samples of anthracite and a higher anthraxolite. The observed effects are described and discussed in terms of the Dyson theory. Copyright © 2015 John Wiley & Sons, Ltd.