Strong Spin-spin Research Articles

The 9 and 33 GHz EPR Spectra of Mn 2+ impurity ions in polycrystalline calcite—Central transition

The 293 K EPR spectra of Mn 2+ impurity ions in polycrystalline natural and synthetic calcite and in the calcite impurity in various coal samples have been studied at 9.2 and 33.3 GHz. The spectra obtained at these two frequencies were found to differ dramatically. At 9.2 GHz, the lineshapes of the central transition hyperfine resonances are in good agreement with previous measurements. The 33.3 GHz EPR spectrum has some features which have not been reported previously at Mn 2+ concentrations below 1000 ppm. Computer simulation programs for both spin-Hamiltonian perturbation and diagonalization techniques were used to describe the mono- and polycrystalline spectra. It was found that the simulated spectrum using the spin-Hamiltonian diagonalization method agrees with previous 9.3 GHz monocrystal data within an error of 0.1 mT for the central transitions and of 0.13 mT for the noncentral transitions, whereas the simulation using the perturbation method had errors of 0.2 and 0.6 mT, respectively. An analysis of the polycrystalline hyperfine resonance lineshapes observed at 9.2 and 33.3 GHz for 1, 50, and 520 ppm Mn 2+ concentrations has been performed using both simulation methods. The hyperfine resonance lineshapes observed at 9.2 GHz are due to the magnetic field angular variation caused by the axial zero field splitting (ZFS) term C 0 2( S). The hyperfine resonance lineshapes observed at 33.3 GHz are shown to be due to the combined effect of A-tensor and g-tensor anisotropy, the axial ZFS, and the spin-spin interaction. The Mn 2+ impurity ion EPR spectrum observed in an Alberta coal and several Argonne coal samples is shown, using lineshape analysis, to originate from their calcite mineral impurity. The lineshape broadening observed in coal samples is characteristic of a strong spin-spin interaction and may be due to interactions with the free radicals and other impurity ions as well as between Mn 2+ ions.

Nuclear magnetic resonance of plane Cu in paramagnetic YBa2Cu3O6

Abstract The first observation of the NMR of plane Cu in the paramagnetic state of YBa 2 Cu 3 O 6 is reported. The NQR frequency and the anisothropies of the magnetic shift tensor and of the spin-lattice and spin-spin relaxation times have been measured. Evidence for a strong indirect spin-spin coupling of plane Cu nuclei mediated by the exchange coupled electrons has been found. The spin-lattice relaxation rate anisotropy is closed to the value expected for dipolar interaction between the nuclear spin and the electronic spin of the Cu(2) 3d x 2 −y 2 hole. An “extra” anisotropy observed in the superconductor YBa 2 Cu 3 O 7 seems to arise from crries and should be therefore dependent on their concentration.

On the identification of cerebral metabolites in localized 1H NMR spectra of human brain In vivo

Localized 1H NMR spectra of human brain in vivo are affected by signal overlap, strong spin-spin coupling, and complex J modulation, and therefore differ considerably from those obtained at higher magnetic fields. This paper deals with the assignment of 1H NMR resonances of cerebral metabolites under the experimental conditions used for human investigations. Conventional 7.0-T FID spectra and 2.0 T localized, short echo time STEAM spectra (TE = 20 ms) of aqueous metabolite solutions are compared to in vivo brain spectra of human volunteers and patients. In addition to singlet resonances from N-acetyl aspartate (NAA), creatines, and cholines, short echo time STEAM spectra exhibit multiplets due to the NAA aspartyl group, glutamate, taurine, and myo-inositol. Enhanced levels of cerebral glutamine are detected in patients with liver cirrhosis. For the first time elevated levels of brain glucose are observed in patients with diabetes mellitus.

Improvements in localized proton NMR spectroscopy of human brain. Water suppression, short echo times, and 1 ml resolution

Considerable technical improvements are reported for localized proton NMR spectroscopy using stimulated echoes. When compared to previous results, proton NMR spectra of the human brain are now obtainable (i) with in vivo water suppression factors of ⩾1000, (ii) with only minor T 2 losses and negligible distortions due to J modulation at short echo times of 10–20 ms, and (iii) from volumes of interest as small as 1–8 ml within measuring times of 1–10 min. As a consequence, the detection of cerebral metabolites is greatly facilitated. This particularly applies to the assignment of those resonances (e.g., glutamate, taurine, inositols) that suffer from strong spin-spin coupling at the field strengths commonly in use for NMR in man. Studies of regional metabolite differences, tissue heterogeneity, and focal lesions in patients benefit from the increased spatial resolution and a concomitant reduction of partial volume effects. Localized proton NMR spectroscopy was performed on young healthy volunteers. Experiments were carried out on a 2.0 T whole-body MRI/MRS system using the standard headcoil for both imaging and spectroscopy.

Femtosecond spin-polarization spectroscopy in diluted-magnetic-semiconductor quantum wells.

Charge-carrier spin scattering in diluted-magnetic-semiconductor multiple quantum wells is directly observed through the circular polarization of luminescence in a femtosecond time-resolved measurement. This represents a new short-time regime for direct spin-relaxation studies. Separate observations of electron and hole relaxation are accomplished by varying the applied magnetic field. Surprisingly, the spin-flip scattering rates in these systems appear to be independent of the extremely strong spin-spin--exchange interaction between the charge carriers and the magnetic ions.

Chemical-shift-sensitive NMR imaging using adjusted phase encoding

An analysis of a chemical-shift-sensitive NMR imaging technique using adjusted phase encoding and signal separation by solving systems of equations is given. A procedure for choosing the chemical-shift phase-encoding times for a given spectrum of the object to be imaged is described. This leads to a minimization of the influence of noise on the signal separation, enabling short encoding times which are advantageous in case of short T 2 relaxation times, strong spin-spin couplings, or considerable B 0 inhomogeneities. Examples using simulated as well as experimental data derived from 19F NMR imaging are given.

Langmuir-Blodgett films of pure long-chain nitroxides, their mixtures and mixtures with stearic acid studied by electron spin resonance

In this paper we report information on the spreading isotherms and the electron spin resonance (ESR) spectra of mono- and multilayer films built up with long hydrocarbon chain nitroxides (namely doxyl stearic acid and hexadecanoyl- piperidine-oxyl radicals). These ultrathin molecular assemblies were prepared from (i) single components, (ii) nitroxide binary mixtures, and (iii) a diluted solution of the paramagnetic components in a diamagnetic (stearic acid) host. The spreading isotherms allowed us to establish that the presence of a five- or six-membered ring in the hydrocarbon chain rendered the film more expanded, and this effect depended on the substituent position in the chain. The ESR spectra of undiluted nitroxides were characterized by strong spin-spin (dipole-dipole and Heisenberg spin exchange) magnetic interactions whose extent also depended on the nitroxide type and mixture. The calculation of the parameters of motion governing the ESR lineshape allowed accurate evaluation both of the degree of order and of the chain flexibility, particularly for diamagnetically diluted systems.

NMR measurement of the exchange coupling between Cu(2) atoms in YBa2Cu3O7- delta (Tc=90 K).

The authors report the transverse NMR relaxation of chain [Cu(1)] and plane [Cu(2)] nuclei in a single crystal of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (${T}_{c}=90$ K) in the normal state. They demonstrate the existence of a strong indirect spin-spin coupling between Cu(2) nuclei, and show that it can be explained using a model in which the Cu(2) atoms possess electronic magnetic moments which are exchange coupled to one another. In such a model, the strength of the coupling implies an effective electronic exchange coupling ${J}_{\mathrm{eff}}\ensuremath{\approx}1100$ ${\mathrm{cm}}^{\ensuremath{-}1}$.

Spin-label assay for phospholipase A 2

A rapid and continuous method for measuring phospholipase A 2 activity using electron spin resonance spectroscopy and a spin-labeled phospholipid as a substrate has been developed. The substrate, 1-palmitoyl-2-(4-doxylpentanoyl)glycerophosphocholine, gives rise principally to a broad ESR line in aqueous solution due to strong spin-spin interactions, probably resulting from its micellar formation. Upon addition of bee venom phospholipase A 2, the water-soluble product, 4-doxylpentanoic acid, is released which brings about a sharp three-line spectrum. Thus, the kinetics of phospholipase A 2 activity can be followed by monitoring the increase in the ESR signal amplitude of the three-line spectrum, which is linearly proportional to the amount of 4-doxylpentanoic acid produced; no separation of the product from the substrate is needed during the measurement. The rate of hydrolysis of 1 nmol min −1 can be accurately measured within a 5-min period of time in a sample volume of 100 μl. This new method should be useful for assaying phospholipase A 2 activities in various biological systems.

Antiferromagnetic correlations in almost-localized Fermi liquids.

A Monte Carlo method is used to calculate various properties of one-band Gutzwiller wave functions which are formed by restricting the charge fluctuations in noninteracting wave functions. Gutzwiller's approximate formula for the kinetic energy is tested both for the ground state and excited states. The ground state is found to have strong antiferromagnetic short-range spin-spin correlations for nearly-half-filled bands, thus extending previous work on the half-filled case. These correlations are very sensitive to the choice of occupied Bloch states and when the occupation is distributed uniformly over the band they disappear. From this fact we conclude that correlations are present only at temperatures low compared to the coherence temperature. In the almost-localized limit it is advantageous to describe the system by an effective Hamiltonian which separates into a term due to the kinetic energy of the charge carriers and one due to the Heisenberg spin-spin coupling. We show that the almost-localized Fermi liquid can gain energy from both terms in the effective Hamiltonian. In other words the restrictions on charge fluctuations can cause spin correlations which in turn can stabilize the Fermi-liquid ground state.

Effects of Exchange Interaction in Diluted Magnetic Semiconductor Quantum Wells

AbstractThe presence of transition metal ions (typically Mn2+) in diluted magnetic semiconductors (DMS) results in a strong spin-spin coupling between localized magnetic moments and band electrons. This leads to considerable modifications of the semiconductor band structure in the presence of strong magnetic fields, e.g., to large spin-dependent shifts of the electronic states at the band edge. This feature is of particular interest in the context of quantum wells involving DMS. Starting with the original idea of a “spin-superlattice”, we concentrate on various opportunities which arise due to the tunability of the depth of the quantum wells by the magnetic field and/or temperature associated with the aforementioned spindependent effects. Thus, we discuss boil-off and freeze-out of electrons to and from quantum wells, selective spin tunneling across the barriers, tunable infrared emitters, enhancement of electronic g-factors in shallow non-magnetic wells surrounded by DMS barriers, the possibility of transition from a type-1 to a type-il superlattice induced by the magnetic field, and quantum oscillations anomalies in DMS quantum wells.

Interaction of crotoxin and its isolated subunits with spin-labeled fatty acids.

We have investigated the interaction of crotoxin (component A-component B complex) and of its isolated phospholipase subunit (component B) with hydrophobic compounds by ESR, using spin-labeled fatty acids as probes. The phospholipase subunit alone (component B) binds more than three labeled fatty acid molecules/molecule with different affinities, the highest corresponding to a Kd of 10 microM in the case of 5-doxyl palmitic acid. In contrast, the noncatalytic subunit (component A) and the crotoxin complex do not bind fatty acids. ESR studies of the component B-fatty acid complex reveal a strong immobilization of the whole length of the fatty acid chain, strong spin-spin interactions between bound fatty acids, and nonaccessibility of the bound paramagnetic probe to Ni2+ ions. This suggests that the phospholipase component B possesses a hydrophobic cleft which may contain one or two fatty acids. This hydrophobic cleft is not accessible to spin-labeled fatty acids in the crotoxin complex. An overall rotational correlation time of about 200 ns of the phospholipase component B was determined by saturation transfer ESR. This high value is incompatible with the diffusion of a polypeptide of 14,500 molecular weight. The hydrodynamic analysis of the fatty acid-component B complex led us to estimate an apparent molecular weight of 95,000 which reveals that fatty acids induce the formation of polymers (most probably octamers) of component B. We propose a model in which the phospholipase component B exists in two conformational states which differ by their hydrophobicity.

Nonlinear magnetic and optical coherence in coupled two-level systems

The question is raised whether nonlinear coherence can be created in coupled systems whose subsystems interact with distinct radiation fields. The simplest coupled system consisting of two two-level subsystems is treated in interaction with two electromagnetic fields, each close to resonance of one of the subsystems. The problem is treated within the SU(4) symmetries. The generators of SU(4) are defined and their properties under transformations deduced. The Hamiltonian and the density operator of the coupled system are expressed in terms of the generators of SU(4) and the Liouville equation is then used to find the time evolution of the density operator. When the frequencies of the radiation fields are equal to the resonant frequencies of the uncoupled subsystems, one obtains two two-photon nutation processes, provided that the interactions of the subsystems with the radiation fields are weaker than the mutual interaction of the two subsystems. The nutation frequency is proportional to the product of the field strengths and inversely proportional to the coupling parameter which in this case represents the resonance offset of the intermediate levels. Alternatively, the frequencies of the radiation fields can be arranged so that only one two-photon transition is on resonance. The amplitude of the nutation then depends on the strengths of the radiation fields, and acquires maximum value for ${\ensuremath{\omega}}_{1}={\ensuremath{\omega}}_{2}$. The nutation frequency increases when one approaches the resonance with the intermediate level. With a two-photon $\ensuremath{\pi}$ pulse one transfers the population difference from the levels of one subsystem to those of the other. This procedure can be used to polarize rare spins in the case of strong spin-spin coupling with the abundant spins when the method of thermodynamic contact is inapplicable. Both magnetic and optical coupled systems can be described with the same equations.

The rotational diffusion of the acetylcholine receptor in Torpeda marmorata membrane fragments studied with a spin-labelled alpha-toxin: importance of the 43 000 protein(s).

The rotational diffusion of the acetylcholine (ACh) receptor in subsynaptic membrane fragments from Torpedo marmorata electric organ was investigated with a spin-labelled alpha-bungarotoxin. A toxin with two spin labels was first synthesized; the conventional electron spin resonance spectrum (e.s.r.) of this toxin bound to the receptor indicated: (1) a complete immobilization of the probes; and (2) a strong spin-spin interaction that was not, or barely, seen in solution. The modification of the degree of spin-spin interaction is taken as an indication of a toxin conformational change accompanying its binding to the ACh-receptor. To avoid spin-spin interaction a single-labelled toxin was made and used to follow the rotational diffusion of the receptor by saturation transfer e.s.r. (ST-e.s.r.). With native membranes a high immobilization of the ACh-receptor was noticed. Reduction of the membranes by dithiothreitol had little effect on this motion. Only extraction of the 43 000 protein(s) by pH 11 treatment was able to enhance the rotational diffusion of the ACh-receptor protein (rotational correlation time by ST-e.s.r. in the 0.5 - 1 X 10(-4) s range) and to allow its lateral diffusion in the plane of the membrane fragments (observed by electron microscopy after freeze-etching or negative staining).

Strong spin-spin coupling effects arising from fast exchange between weakly coupled spin systems

It is demonstrated that the criterion for the determination of whether a spin-spin interaction ( J ij ) is “strong” or “weak” by comparing J ij with the corresponding chemical shift difference | ω i − ω j | needs modification in the presence of exchange, especially if rapid exchange conditions prevail. In particular, it is shown using computer-generated spectra for ABC ⇄ A′B′ + C′ exchange that two weakly coupled spin systems in fast exchange can yield a spectrum with strong coupling features.

Strong spin-spin coupling in the two-dimensional J-resolved 360-MHz 1H NMR spectra of the common amino acids

The two-dimensional J -resolved H NMR spectra at 360 MHz of the 20 common amino acids have been investigated. The characteristic features of strong coupling are described. Advantages and drawbacks of projections, cross sections, and contour plots for the presentation of spectra including strong coupling are illustrated with selected examples. On the basis of the amino acid data practical aspects of the use of two-dimensional J -resolved spectroscopy to improve the resolution of high-field 1 H NMR spectra of proteins are discussed.

EPR concentration dependence in magnetically diluted crystals with strong spin-spin interaction

Experimental investigations of electron paramagnetic resonance have been made in ionic crystals using diamagnetic samples with paramagnetic impurities and also in solid solutions in a large range of concentration. There exist cases in which neither the moment nor the statistical method could until now give even a qualitative explanation of the EPR linewidth and amplitude concentration dependence experimentally observed. In particular, this is true for the MnOMgO solid solutions observed in the concentration range 10 - ̊ 3 to 0.8. It is believed that it is necessary to take into account spin-spin interactions (although not very small interactions) and magnetic clusters. A numerical statistical method, making no use of perturbation theory with respect to the interaction, is presented for considering multiparticle spin-spin interactions. A universal computer program using the ALGOL-60 language has been elaborated and applied, and the results for MnOMgO solid-solution polycrystalline powders are given. Despite ambiguity concerning the literature data on the exchange constants and the small number of neighbors used ( n = 5), the nonmonotonic character of the experimental curves giving the linewidth and amplitude concentration dependence may be qualitatively explained by the influence of the dipolar and exchange interactions.

Analysis of ESR spectra of radicals having large hyperfine interactions

Methods of analysis of isotropic ESR spectra are presented along with examples for S = 1 2 systems where one of the hyperfine interactions is very large. Simple algebraic expressions derived from the Breit-Rabi equations are of great value in assigning the observed transitions or in predicting the appearance of a spectrum, and in many instances they may be used to obtain quite accurate magnetic parameters. Due to the effects of strong spin-spin coupling, the superhyperfine structure of transitions detected at low magnetic fields may take on an unexpected appearance. Such effects can often be used to advantage in determining the relative signs of the hyperfine interactions.

Deposition of radiation energy in solids as visualized by the distribution, structure and properties of alkyl radicals in γ-irradiated n-alkane single crystals

This paper summarizes results obtained earlier from E.S.R. studies of γ-irradiated n-alkane single crystals. It also contains some new experimental results that serve to give a more complete picture of the deposition of radiation energy in solid alkanes. The experiments performed with solid n-alkanes have thus far provided structural data that permit the nature and even the conformation of alkyl radicals to be clearly understood. Two types of radical exist, namely, one where the unpaired electron is located next to the end methyl group and one with the unpaired electron in the interior of the chain. The first type has a conformation which differs from that of the undamaged molecule. Microwave saturation data show that there is a difference in relaxation properties of these radicals which can be understood in terms of a difference in mobility. Relative yield measurements give the distribution of isomeric alkyl, the result differing from that obtained using product analysis in liquids. For protiated n-alkanes n-alkyl is lacking and the 2-alkyl concentration is higher than expected. For deuterated n-alkanes the E.S.R. spectrum is mainly that of radicals with the unpaired electron located in the interior of the carbon chain. This isotope effect is again contrary to observations in liquid n-alkanes. The broad lines observed in protiated alkanes irradiated at 77 K and deuterated alkanes irradiated at 4.2 K are not believed to arise from strong spin-spin interactions. They are thought instead to arise from distorted crystal and radical structures relating to the damage regions of the crystals. Radical pairs exist with different stability, yield and structure. Our estimate, in deuterated alkanes, that as much as 40% of the radicals are formed in pairs or clusters at 4.2 K shows that radical pair formation is an important radiolytic process in solid n-alkanes. Energy transfer from deuterated to protiated molecules has also been experimentally verified. This energy transfer is temperature dependent and occurs efficiently at 77 K, but less efficiently at 4.2 and 273 K. A relationship also exists for deuterated alkanes between the amount of radical pairs formed at 4 K and the long-range energy transfer at 77 K. This can be readily accounted for by an exciton transfer mechanism analogous to that in aromatic crystals. It might also be qualitatively described by a hydrogen abstraction process. In this case, however, the properties of the deuterium atoms are not understood.

EPR study of the nature and removal of iron(III) impurities in ammonium-exchanged NaY-zeolite

Abstract EPR studies of the Fe 3+ impurity in NH 4 -faujasite show that iron can be simultaneously present in the zeolite as three forms: Fe 3+ species in the aluminosilicate framework, Fe 3+ ions (possibly in S 1 , sites) acting as counterions, and Fe 3 O 4 or another Fe 3+ compound with strong exchange spin-spin interactions precipitated on the zeolite. All of these are incorporated in the zeolite in the course of its synthesis. Chemical extraction and EPR data show that both the precipitated iron and the Fe 3+ counterions can be easily removed from the zeolite. Substitutional Fe 3+ cations in the aluminosilicate framework are probably also extracted, although this point is rather less conclusive. The chemical extraction procedure that is proposed provides a useful means of purifying commercial synthetic zeolites without altering their crystallographic structure.