G-tensor Anisotropy Research Articles

Polythiophene and polyacetylene conductors in the 3-cm and 2-mm ESR bands

This paper compares the magnetic resonance data in the 3-cm and 2-mm ESR bands with the electrical indices of compounds containing polythiophene (PT) and polyacetylene (PA). ESR signals were not found for PT combined with FeC1/sup -//sub 4/, and MnO/sup -//sub 4/ anions. A dependence of the magnetic resonance parameters on the nature of the anion was established for organic polythiophene conductors with various anions. The major values of the anisotropic g-tensor were measured by 2-mm band spectroscopy and the conductivity of polythiophene (BF/sup -//sub 4/) and polyacetylene (I/sup -/) was evaluated at 200 K.

The structure and dynamics of the copper(II)— l-proline 1:2 complex in solution

The 13C relaxation times ( T 1 and T 2) and isotropic contact shifts (Δω) of a one molar aqueous solution of l-proline at pH = 11 (or p D = 11.4) containing ca 10 −4 M copper(II) perchlorate are measured at 62.86 MHz over a temperature range of 26–70°C. The purely dipolar longitudinal relaxation of carbon-13 nuclei contrasting with purely scalar transverse relaxation allowed us to extract carbon-to-metal distances (through T 1 measurements) and hyperfine coupling constants and dynamic parameters (from T 2 and Δω measurements). The structure of the complex in solution is found closely similar to that in the solid state. Curve-fitting procedures allowed us to derive the hyperfine electron—carbon coupling constants A c = −1.95, + 0.42, + 1.90 and −1.70 MHz for carbons α, β, γ, δ, of the pyrrolidinic ring, the reorientation correlation time of the complex, τ R (25°C) = 1.15 × 10 −10 sec, the l-proline exchange rate, k M (25°C) = 4.0 × 10 5 sec −1 (and the corresponding activation parameters Δ H ≠ = 9.0 kcal mol −1 and Δ S ≠ = −0.7 e.u.), and the electronic relaxation time, T 1 e = 1.13 × 10 −8 sec (at 25°C). The latter value was found in agreement with the one computed from ESR data and the above τ R value, showing the predominant contributions of spin—rotation interaction and, to a lesser extent, of the effect of g-tensor anisotropy to the electronic relaxation rate.

Radiation-induced Free Radicals in Solid 5-halouracil Derivatives: Single Crystals of 5-iododeoxyuridine

X-irradiation of single crystals of 5-iododeoxyuridine (IUdR) in the temperature range 8-300 K produces mainly four different radicals which have been studied by electron spin resonance (e.s.r.) and electron nuclear double resonance (ENDOR)-spectroscopy. At low temperatures, a pi-anion is formed which shows predominantly an interaction of the unpaired electron with a proton at carbon C6 of the base (-11.8 G, -23.9 G, -4.6 G). Above 10-20 K, the anion protonates at C6 to yield a RC-I(CH2)-R' radical comprising alpha-iodo and beta-methylene proton hyperfine interactions. The primary oxidation product is an O5'-situated alkoxy radical RCH2O which shows inequivalent beta-proton couplings of about 100 G and 35 G together with a highly anisotropic g-tensor. Upon warming to 265 K, a C2'-located radical on the deoxyribose is formed which is stable at room temperature. A detailed account of its spectral features as obtained by ENDOR exhibits three different alpha-type couplings, two small beta-protons and a dipolar interaction. Other radicals, not reproducibly observed, involve a C5'-hydroxyalkyl radical and a species related to the base cation at low temperatures.

EPR and ODMR investigations of defect centres in ZnTe:Cl

We report EPR and ODMR (optically detected EPR) studies of Cl doped ZnTe that show a fraction of the chlorine participating in complex, deep-level defects. Photoexcitation at T < 50 K creates an EPR spectrum corresponding to an S = 12 centre with nearly isotropic g factor (g = 1.96) and hyperfine interaction with a Cl nucleus. Weak, satellite EPR lines correspond to interaction with three equivalent Te nuclei and a low abundance, I = 12 nucleus X, which could be either an Sn, Pb or Cd nucleus. Our model for this trigonal symmetry defect is a Cl on a Te site with X on a neighbouring Zn site. ODMR studies at 2 K of an emission band peaking near 770 nm gives the Cl centre EPR spectrum and in addition the spectrum of a trigonal acceptor centre with highly anisotropic g-tensor (g″ = 2.65, g⊥ ≅ 0). The ODMR work shows that the 770 nm emission results from recombination of the electron of the deep Cl centre with the hole of the trigonal acceptor centre.

Zeeman and optical experiments on the lowest triplet state of palladium porphin

Zeeman and variable temperature experiments on the lowest (optically excited) triplet state of palladium porphin (PdP) in single crystals of n-octane (n-C 8) and n-decane (n-C10) performed at low temperatures are reported. The orbital degeneracy of the 3 Eu state of the free molecule appears to be lifted in the crystal and the splitting between the two orbital components |x> and |y> amounts to 57 cm-1 for n-C8 and 41 cm-1 for n-C10. The triplet state parameters for PdP in n-C8 are D = - 23 ± 5 GHz, g ⊥ = 2·00 ± 0·02, g ‖ = 1·68 ± 0·04, and for PdP in n-C10 they are D = -42 ± 5 GHz, g ⊥ = 2·00 ± 0·01, g ‖ = 1·51 ± 0·03. The zero field splitting and g-tensor anisotropy are dominated by contributions from the spin-orbit coupling which connects the τ x , τ y levels of the 3|x> with those of the 3|y> state by a matrix element which has a magnitude of Z′ = 14·6 ± 0·3 cm-1. The orbital angular momentum in units of ħ in the 3 Eu state amounts to Λ′ = 1·4 ± 0·1. In accordance with theory Z′ and Λ′ appear to be ind...

Electron paramagnetic resonance linewidth studies of (π-C 5H 5) 2TaCl 2, (π-C 5H 5) 2NbCl 2 and (π-C 5H 5) 2VCl 2 complexes

The electron paramagnetic relaxation phenomena of (π-C 5H 5) 2TaCl 2, (π-C 5H 5) 2NbCl 2 and (π-C 5H 5) 2 VCl 2 in dilute chloroform solutions have been investigated. This has enabled the separate contributions to the observed EPR linewidths from spin-rotational interaction, g-tensor anisotropy, hyperfine coupling anisotropy, and unresolved chlorine hyperfine coupling to be evaluated. These results have been used to estimate the size of the chlorine isotropic hyperfine coupling constants and thus the extent of delocalisation of the unpaired electron on to the chlorine ligands.

Line broadenings within the alkali metal hyperfine multiplets of ion pairs and triple ions. A theoretical approach

The effect of rotational diffusion on the metal hyperfine multiplets in the e.s.r. spectra of ion pairs and cationic ion triplets of alkali metal cations and aromatic radical anions is investigated theoretically. The triple ion of lithium and pyrazine is chosen as a model system; the broadening parameters are calculated for various structures of the triple ion. The results indicate that the anisotropic lithium hyperfine dipolar couplings, being modulated simultaneously with the anisotropic g-tensor, produce high-field broadening of the lithium septet, irrespective of the positions of the cations. Furthermore, it is concluded that there is no triple ion structure for which all the lithium broadening parameters vanish. Finally it is found that the calculated parameters are very small. In general, the anisotropy of the alkali hyperfine coupling in ion pairs and triple ions will be insufficient to account for the linewidth variations observed in these systems.

The orientation of membrane bound radicals. An EPR investigation of magnetically ordered spinach chloroplasts

The orientation of membrane-bound radicals in spinach chloroplasts is examined by electron paramagnetic resonance (EPR) spectroscopy of chloroplasts oriented by magnetic fields. Several of the membrane-bound radicals which possess g-tensor anisotropy display EPR signals with a marked dependence on the orientation of the membranes relative to the applied EPR field. The fraction of oxidized and reduced plastocyanin, P-700, iron-sulfur proteins A and B, and the X center, an early acceptor of Photosystem I, can be controlled by the light intensity during steady-state illumination and can be trapped by cooling. The X center can be photoreduced and trapped in the absence of strong reductants and high pH, conditions previously found necessary for its detection. These results confirm its role as an early electron acceptor in P-700 photo-oxidation. X is oriented with its smallest principal g-tensor axis ( g x) predominantly parallel to the normal to the thylakoid membrane, the same orientation as was found for an early electron acceptor based on time-resolved electron spin polarization studies. We propose that the X center is the first example of a high potential iron-sulfur protein which functions in electron transfer in its ‘;superreduced’; state. We present evidence which suggests that iron-sulfur proteins A and B are 4Fe-4S clusters in an 8Fe-8S protein. Center B is oriented with g y predominantly normal to the membrane plane. The spectra of center A and plastocyanin do not show significant changes with sample orientation. In the case of plastocyanin, this may indicate a lack of molecular orientation. The absence of an orientation effect for reduced center A is reconcilable with a 4Fe-4S geometry, provided that the electron obtained upon reduction can be shared between any pair of Fe atoms in the center. Orientation of the ‘;Rieske’; iron-sulfur protein is also observed. It has axial symmetry with g ∥ close to the plane of the membrane. A model is proposed for the organization of these proteins in the thylakoid membrane. A new EPR signal was observed in oriented chloroplasts. This broad unresolved resonance displays a g value of 3.2 when the membrane normal is parallel to the field. It shifts to g = 1.9 when the membrane normal is perpendicular to the field. The signal is sensitive to illumination and to washing of the thylakoid membranes of broken chloroplasts. We suggest that there is a relation between this signal and the water-oxidizing enzyme system.

Extra absorption peaks in EPR spectra of systems with anisotropic g-tensor and hyperfine structure in powders and glasses

Analytical solutions for locating extra absorption peaks in “polycrystalline” EPR spectra of systems with anisotropic g-factor and hyperfine structure are obtained, and general conditions for the existence of these peaks are formulated. The effect of second-order terms of perturbation theory on these conditions is revealed. As an example, the frequency intervals of the occurrence of extra absorption peaks and their positions in the EPR spectrum of a frozen solution of Cu acetylacetonate are calculated.

Electron spin polarization in photosynthesis and the mechanism of electron transfer in photosystem I. Experimental observations

Transient electron paramagnetic resonance (EPR) methods are used to examine the spin populations of the light-induced radicals produced in spinach chloroplasts, photosystem I particles, and Chlorella pyrenoidosa. We observe both emission and enhanced absorption within the hyperfine structure of the EPR spectrum of P700+, the photooxidized reaction-center chlorophyll radical (Signal I). By using flow gradients or magnetic fields to orient the chloroplasts in the Zeeman field, we are able to influence both the magnitude and sign of the spin polarization. Identification of the polarized radical and P700+ is consistent with the effects of inhibitors, excitation light intensity and wavelength, redox potential, and fractionation of the membranes. The EPR signal of the polarized P700+ radical displays a 30% narrower line width than P700+ after spin relaxation. This suggests a magnetic interaction between P700+ and its reduced (paramagnetic) acceptor, which leads to a collapse of the P700+ hyperfine structure. Narrowing of the spectrum is evident only in the spectrum of polarized P700+, because prompt electron transfer rapidly separates the radical pair. Evidence of cross-relaxation between the adjacent radicals suggests the existence of an exchange interaction. The results indicate that polarization is produced by a radical pair mechanism between P700+ and the reduced primary acceptor of photosystem I. The orientation dependence of the spin polarization of P700+ is due to the g-tensor anisotropy of the acceptor radical to which it is exchange-coupled. The EPR spectrum of P700+ is virtually isotropic once the adjacent acceptor radical has passed the photoionized electron to a later, more remote acceptor molecule. This interpretation implies that the acceptor radical has g-tensor anisotropy significantly greater than the width of the hyperfine field on P700+ and that the acceptor is oriented with its smallest g-tensor axis along the normal to the thylakoid membranes. Both the ferredoxin-like iron-sulfur centers and the X- species observed directly by EPR at low temperatures have g-tensor anisotropy large enough to produce the observed spin polarization; however, studies on oriented chloroplasts show that the bound ferredoxin centers do not have this orientation of their g tensors. In contrast, X- is aligned with its smallest g-tensor axis predominantly normal to the plane of the thylakoid membranes. This is the same orientation predicted for the acceptor radical based on analysis of the spin polarization of P700+, and indicates that the species responsible for the anisotropy of the polarized P700+ spectrum is probably X-. The dark EPR Signal II is shown to possess anisotropic hyperfine structure (and possibly g-tensor anisotropy), which serves as a good indicator of the extent of membrane alignment.

The temperature dependencies of lanthanide-induced NMR shifts: Evaluation of theoretical approaches and experimental evidence

Bleaney's theory represents an excellent approximation in the calculation of the variation of magnetic susceptibility anisotropy as a function of f-electron configuration in complexes of trivalent lanthanide ions. Owing to certain approximations employed in his approach, Bleaney's prediction of a linear dependence of the anisotropy upon T −2 is not strictly correct. The more complete calculational procedure of Horrocks et al. predicts significant deviations from the simple T −2 temperature dependence in some cases. Experimental single-crystal magnetic anisotropy data on two axially symmetric Yb(III) complexes and a nonaxial Yb(III) shift reagent adduct reveal significant deviations from a linear dependence on T −2. Bleaney's assumption of an isotropic Landé splitting factor, g j is shown to be consistent with large g-tensor anisotropies of individual lanthanide ion levels and with large first-order Zeeman contributions to the observed shifts.

Influence of the fine-structure interaction and of the g-tensor anisotropy on the ESR lineshape of triplet excitons in molecular crystals with two molecules in the unit cell

Recently a set of equations has been derived describing the influence of the quasi-incoherent exciton motion on the ESR lineshape of triplet excitons in molecular crystals with two molecules per unit cell. Starting from these equations in the case of rapid exciton motion analytic expressions for the positions and widths of the ESR lines are calculated by second order perturbation theory taking into account the different orientations of the fine-structure interaction tensors and of the g-tensors at the two inequivalent molecules in the unit cell. Explicit expressions of the angular dependence of these quantities for rotation of the magnetic high-field around the a −, b − , c − -axes of crystals of the anthracene type are given. Finally contact to experimental results is made.

Electron spin resonance study of the superoxide ion in melt-recrystallized strontium chloride

An e.s.r. spectrum in melt-recrystallized SrCl2 single crystals has a fully anisotropic g-tensor with principal values 2.0028, 2.0093 and 2.053 in directions oriented in the crystal as [110], and [1text-decoration:overline10] and [001]. The spectrum is ascribed to O–2 in a site of D2h symmetry between two anion vacancies, and interacting with two Sr2+ ions so that the unpaired electron resides in a B2g orbital localized mainly on the O–2. The crystal field splitting Δ between B2g and B3g(O2πg(x) and πg(y) mixed with Sr 4d) is about 40 times the spin-orbit coupling constant, i.e. about 0.9 eV. When Δ is so large, the orbital angular momentum reduction parameter cannot be estimated accurately from the g-tensor.

ESR and NMR in Aqueous and Methanol Solutions of Copper(II) Solvates. Temperature and Magnetic Field Dependence of Electron and Nuclear Spin Relaxation

ESR line-shape measurements of Cu(ClO4)2 solutions in methanol and water at X- and Q-band frequencies are reported. In both frequencies, above room temperature, the over-all linewidth increases with increasing temperature, while at low temperatures the slopes level off and (at Q band) even change signs. The results are quantitatively interpreted in terms of two main electronic relaxation mechanisms, viz., modulation of the spin rotation and anisotropic g-tensor interactions. From the magnetic parameters of the complexes determined in frozen solution and the field dependence of the linewidth, the correlation times are determined. It is found that in both solvents the anisotropic g tensor is modulated by the fast hopping of the complex distortion axis rather than by the molecular tumbling of the complex. Assuming a random jump of the distortion axis between the three octahedral principal directions, the kinetic parameters of this hopping process are calculated to be τi(25°C)=1.6× 10−11 sec−1, ΔEi=1.2 kcal/mole for the aqueous complex and τi(25°C)=1.2× 10−11 sec−1, ΔEi=1.1 kcal/mole for the methanol complex. NMR relaxation measurements of Cu(ClO4)2 solutions in methanol between −90°C and 100°C are also reported and are interpreted in terms of kinetic and magnetic parameters of the Cu2+ solvation complex. Since there is fast intramolecular inversion of the complex, only weighted averaged parameters over the axial and equatorial ligand molecules are obtained. The pseudo-first-order rate constant for the exchange of solvation shell molecules is 1/ τM(25°C)=7.4× 107 sec−1 with an activation energy ΔE=6.6 kcal/mole; the molecular tumbling time is τR(25°C)=4.6× 10−11 sec with ΔER=3.0 kcal/mole; the isotropic ligand hyperfine interaction constants are AOH/h=8.8× 105 Hz, ACH3/h=10.6× 105 Hz for the protons and AO/h=2.9× 107 Hz for 17O. Previous NMR results in aqueous solutions are discussed and reinterpreted.

ELECTRON PARAMAGNETIC RESONANCE IN ION IMPLANTED SILICON

Electron paramagnetic resonance spectra of radiation damage centers in silicon have been observed following implantation with nitrogen and phosphorous ions. Two of these spectra have narrow lines and can be fitted to anisotropic g-tensors and zero field splitting tensors. One is a new spectrum with approximately [111] axial symmetry and will be called the Si-B2 spectrum. The other can be identified as the Si-P3 spectrum which has been known previously to result from neutron irradiation. Both spectra are attributed to defects in the crystal containing broken silicon bonds.

The Mechanism of ESR Relaxation in Peroxylamine Disulfonate Ion ·ON(SO3)22−

Abstract The mechanism of ESR relaxation in ·ON(SO3)22− is discussed. As the line width in an aqueous solution is plotted against the concentration, it is almost concentration independent in the range lower than 7×10−3 mol/l, but the dependence increases in the higher range. Powles and Moseley showed that the anisotropy of the g-value contributes to the line width in the lower concentration range, but the line width in the higher-concentration range, which is investigated in the present investigation, can not be accounted for by the g-tensor anisotropy. Hence, it is supposed that electron spin exchange or nuclear quadrupole interaction contributes in the higher-concentration range. The latter can, however, be eliminated by the line width measurement in a deuterium measurement in a deuterium oxide solution, and so it may be concluded that electron spin exchange is the main contribution to the line width in the higher-concentration range. This is in accordance with the proposal of Jones. Furthermore, we could show that, by calculation of the line shape for Bloch’s equation, including exchange interactions, the exchange frequency of 5.4×107 sec−1 can give a line fitting the observed line shape for the 7.8×10−2 mol/l solution. This result can be taken as another evidence showing that the electron spin exchange is the main source of the line width in the higher-concentration range.

Hyperfine and g-tensor anisotropy of the tropenyl radical

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHyperfine and g-tensor anisotropy of the tropenyl radicalWalter Victor Volland and Gershon VincowCite this: J. Phys. Chem. 1969, 73, 4, 1147–1149Publication Date (Print):April 1, 1969Publication History Published online1 May 2002Published inissue 1 April 1969https://doi.org/10.1021/j100724a067RIGHTS & PERMISSIONSArticle Views34Altmetric-Citations6LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (341 KB) Get e-Alerts

Antiferromagnetic Resonance in Copper Formate Tetrahydrate

It has been suggested 1–3 that monoclinic Cu (HCOO)2·4H2O may closely approximate a two-dimensional antiferromagnetic. Magnetic susceptibility and magnetization measurements have indicated that antiferromagnetic ordering occurs at 17°K and have also shown evidence for weak ferromagnetism.2,3 An additional broad peak in the susceptibility has been observed near 65°K.3,4 Recent ESR measurements5 have shown a unique linear temperature dependence of the paramagnetic linewidth. In order to obtain more information on the magnetic parameters characterizing this crystal, we have made AFMR measurements between 1.4° and 15°K. Resonance transitions have been observed at 9 GHz, from 32 to 36 GHz, from 50 to 80 GHz, and from 107 to 124 GHz with magnetic fields up to 26 kOe. The minimum resonance field for the low-frequency mode in the ac plane is observed at a position (called the z axis) 3° from the c axis. The AFMR modes (frequency vs magnetic field) for the field along the z and b axis suggest that z and b are respectively the easy and intermediate axes for the spins. However, the following differences from the behavior of a normal antiferromagnet are observed: (1) Along the z direction, the high and lowfield branches intersect at a nonzero frequency. (2) Along the b axis a discontinuity is observed at 5.3 kOe at which a weak signal is observed at various frequencies between 9 and 72 GHz; (3) Above 5.3 kOe, an additional resonance is observed in the bc′ plane. We have attempted to fit the data with a two-sublattice model with z as the easy axis and weak moment along the b axis. The model contains isotropic exchange, symmetric and antisymmetric exchange, and g-tensor anisotropy. Good agreement between the calculation and the data is obtained for the low-frequency mode along the z and b axes above 5.3 kOe. From this fit and the static measurements2,3 consistent values of the exchange parameters are obtained. However, below 5.3 kOe the calculated resonance frequencies are considerably lower than the experimental ones. Furthermore, this two-sublattice model is inadequate to explain the discontinuity along the b axis and the additional resonance in the bc′ plane. A detailed paper will be published elsewhere.

Electron Spin Relaxation in the N,N′-Dimethylpyrazine Cation Radical

Line shapes in the ESR spectrum of the N,N′-dimethylpyrazine cation radical have been studied from 20° to −80°C, and an analysis has been carried out using the relaxation matrix theory. It turns out that anisotropic dipole and g-tensor interactions provide the major relaxation mechanisms. Classical dipole tensor components have been calculated for all of the magnetic nuclei, and it is shown that the hyperfine splitting constants for the nitrogen nuclei and the methyl protons are positive. Partially resolved second-order structure is observed in some of the lines and a linewidth correction has been introduced to take this into account. Spin relaxation resulting from modulation of the isotropic dipole interactions through contact with the counter ion is discussed on the basis of an electrostatic model and a steric model. The predictions of the steric model are the most acceptable, but neither has been confirmed experimentally.

Electron Spin Resonance Absorption Spectrum of the AsO22− Molecule–Ion in γ-Irradiated Single-Crystal Calcite

An electron spin resonance absorption spectrum corresponding to S = 12, I = 32, and exhibiting orthorhombic symmetry has been observed in γ-irradiated single-crystal calcite. At 4.2°K, the spin-Hamiltonian parameters are gzz = 2.0150, gxx = 1.9991, gyy = 1.9910, Azz = −152 MHz, Axx = 614 MHz, Ayy = −173 MHz, (gnβn/h) = −7.24 × 10−4MHz/Oe, e2qQ = 16.2 MHz, and η ≃ 0. The paramagnetic defect associated with this spectrum is identified as the AsO22− molecule–ion which is located at carbonate ion sites with its plane coincident with that of the carbonate ions. From the hyperfine structure splittings and the g-tensor anisotropy, it is deduced that the unpaired electron has approximately 1% 4s, 96% 4p, and 3% 4d character and that it is almost completely localized on the arsenic atom.