EPR and ENDOR study of carbon-mineral sorbents

A character of carbon ordering in carbon mineral systems, such as γ-Al₂O₃ and TiO₂ has been investigated using EPR and ENDOR methods. The experimental data have been interpreted at the assumption that a spatial distribution of paramagnetic centers on the surface of aluminum-oxide is 2-dimensional. The results of ENDOR measurements show that no stable covalent bond is formed between the mineral support surface and coke upon carbonization of aluminum oxide by divinyl. The interaction has mainly the ionic character and is easily affected by water.     

High-frequency EPR and ENDOR: Time-domain spectroscopy of ribonucleotide reductase

This paper discusses time-domain electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) experiments aimed at elucidating the enzymatic mechanism of ribonucleotide reductase (RNR), the enzyme responsible for the conversion of ribonucleotides to deoxyribonucleotides. The article begins with a discussion of the current state of the art of instrumentation for high-frequency EPR and ENDOR and some suggestions as to future developments. We then provide an introduction to the chemistry of RNR and a discussion of the high-field EPR and ENDOR spectra of the tyrosyl radical (Y^?) in the R2 subunit of class I RNR. Finally, we describe two examples illustrating the use of high-frequency EPR and ENDOR to elucidate the enzymatic mechanism of RNR. EPR and ENDOR have played an important role for these studies since the mechanism involves several different radical intermediates. These intermediates are all present in low concentrations relative to the Y^? concentration and they possess similarg-values. Spectral overlap, therefore, has been a problem with X-band EPR. At high frequencies the spectra are resolved to the point that individual powder lineshapes are discernible. In addition, we describe our approach, on the basis of differential relaxation, to suppress the spectrum of the dominant Y^?. High-frequency EPR and ENDOR therefore has permitted us to determine the structure of several radical intermediates which in turn have contributed to the understanding of the enzymatic mechanism of RNR.     

Investigation by EPR and ENDOR spectroscopy of the novel 4Fe ferredoxin fromPyrococcus furiosus

The hyperthermophilic archaeonPyrococcus furiosus contains a four-Fe ferredoxin (Pf- Fd) that differs from most other 4Fe-Fd’s in that its [Fe₄S₄] cluster is anchored to protein by only three cysteinyl residues.Pf- Fd also is of interest because in its reduced form, [Fe₄S₄]⁺, the cluster exhibits bothS = 1/2 andS = 3/2 spin states. Addition of excess cyanide ion converts the cluster exclusively to anS = 1/2 state (g₁ = 2.09, g₂ = 1.95, g₃ = 1.92), however dialysis restores the EPR signal of native reduced protein indicating that the cluster is not irreversibly altered by cyanide. Both the native protein and protein in the presence of excess cyanide ion (Pf- Fd 4Fe-CN) were investigated here using the techniques of electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. In particular,Pf- Fd 4Fe-CN was investigated using¹³CN^? and C¹⁵N^? ligands.¹³C and¹⁵N ENDOR indicated that a single cyanide ion bound directly, with the cluster showing an unusually small contact interaction (a_iso(¹³C)～ ?3 MHz, a_iso(¹⁵N) ～ 0). This is in contrast to cyanide bound to monomeric low-spin Fe(III)-containing proteins such as transferrin and myoglobin, for which the¹³C hyperfine coupling has a large isotropic component (a_iso(¹³C) ≈ ?30 MHz). This small contact interaction is not due to low spin density of Fe, as⁵⁷Fe ENDOR of the singly and triply labeledPf- Fd 4FeCN isotopologs, [⁵⁷FeFe₃S₄]⁺ and [Fe⁵⁷Fe₃S₄]⁺, show hyperfine coupling characteristic for [Fe₄S₄]⁺ clusters, particularly for the Fe to which cyanide binds. Thus, the low spin density on¹³C is not due to low spin density on the Fe ion to which it binds. Further theoretical work is needed to explain the contrast between the strong electronic effect of cyanide ion binding with the low spin density on the ligand.     

High-field, multifrequency EPR spectroscopy using whispering gallery dielectric resonators

High-field/high frequency EPR spectroscopy measurements are shown. Experiments were carried out at 240- and 316-GHz frequencies. The employed apparatus uses a novel combination of far infrared molecular lasers and of probehead exploiting dielectric resonators working in the whispering gallery modes. This approach constitutes a relatively simple method of multifrequency EPR spectroscopy and opens appealing perspectives in high-sensitivity EPR spectroscopy up to the THz regime.     

A multifrequency EPR approach to travertine characterisation

The understanding of processes that give rise to travertine deposits is important. This is so because of its widespread use as decorative material, but more so in environmental studies due to the significance, by proxy, of travertine in climatology. In this paper, a multifrequency EPR spectroscopy study of the behaviour of an ubiquitary vicariant of Ca in calcite, Mn(II), is presented. EPR spectra were obtained from a natural sample at 9.5 (X-band), 95, 190, and 285GHz, and interpreted through numerical simulation. An analysis of the distribution of the zero-field splitting interaction revealed the source of some unexpected spectral features in the width of the lines in the X-band. By contrast, the homogeneous broadening plays only a minor role. Moreover, field-dependent anisotropies of the Zeeman and hyperfine tensors were observed at higher frequency. On the basis of results garnered in this study, the ZFS interaction of Mn(II) has been ascribed to the microstructural anomalies of the Mn(II) distribution in calcite. This may be considered as the fingerprint of the physical-chemical conditions at the time of travertine deposition. As a consequence, X-band EPR spectroscopy represents a specific tool to investigate the genesis, and to check the homogeneity of Mn(II) distribution in travertines as well as in other calcite-based materials.     

Study of As-Sb-Se chalcogenide glasses by NQR and EPR spectroscopy methods

In this paper experimental results obtained by both ⁷⁵As NQR and EPR spectroscopy are presented for the three-component system As-Sb-Se. The ⁷⁵As NQR spectra of glasses of structures (As₂Se₃)_0.78 (Sb₂Se₃)_0.22, (As₂Se₃)_0.75 (Sb₂Se₃)_0.25, (As₂Se₃)_0.5 (Sb₂Se₃)_0.5 have broad lines with two Sb-NQR lines (corresponding to the Sb₂Se₃ units) and two ⁷⁵As-NQR lines (corresponding to the As₂Se₃ units). Differences in the EPR spectra of the different glasses arise because of the different amounts of arsenic and antimony in their structure.     

Accurate determination of the spin Hamiltonian parameters for Mn2+ ions in cubic ZnS nanocrystals by multifrequency EPR spectra analysis

Accurate determination of the spin Hamiltonian (SH) parameters, describing the electron paramagnetic resonance (EPR) spectra of paramagnetic impurity ions in wide band gap semiconductor nanocrystals, is essential for determining their localization and quantum properties. Here we present a procedure, based on publicly available software, for determining with higher accuracy the SH parameters of isolated Mn(2+) impurity ions in small cubic ZnS nanocrystals. The procedure, which can be applied to other cubic II-VI semiconductor nanocrystals as well, is based on the analysis of both low and high frequency EPR spectra with line shape simulation and fitting computing programs, which include the hyperfine forbidden transitions and line broadening effects. The difficulties, limitations and errors which can affect the accuracy in determining some of the SH parameters are also discussed.     

Anatase TiO2 nanocrystals prepared by mechanochemical synthesis and their photochemical activity studied by EPR spectroscopy

Anatase TiO₂ has been prepared by mechanochemical synthesis using TiOSO₄·xH₂O and Na₂CO₃ as starting reactants. The reaction was performed in high-energy ball mill using steel and corundum jars, respectively. The final products were obtained by annealing the milled powder in the temperature range of 300–700 °C and subsequently by washing out the water-soluble byproduct Na₂SO₄·xH₂O. When steel jars were used, the annealing in the range of 300–600 °C led to anatase. For products milled in corundum, the stability of anatase increased up to 700 °C. Transition electron microscopy (TEM) showed that crystallites with a size in the range of 20–50 nm with equiaxed morphology were obtained after milling in corundum and annealing at 600 and 700 °C. The process of photoinduced reactive hydroxyl radical generation in aerated aqueous titania suspensions was studied by EPR spectroscopy using spin trapping technique. The presence of iron impurities in the samples milled in steel substantially decreases the radical formation. The rate of radical formation is substantially affected by particle size development of TiO₂ nanopowders. The product milled in corundum and annealed at 700 °C outperforms more than twice the photochemical activity of TiO₂ Degussa P25 standard.     

A Q-band pulse EPR/ENDOR spectrometer and the implementation of advanced one- and two-dimensional pulse EPR methodology

A versatile high-power pulse Q-band EPR spectrometer operating at 34.5--35.5 GHz and in a temperature range of 4--300 K is described. The spectrometer allows one to perform one- and two-dimensional multifrequency pulse EPR and pulse ENDOR experiments, as well as continuous wave experiments. It is equipped with two microwave sources and four microwave channels to generate pulse sequences with different amplitudes, phases, and carrier frequencies. A microwave pulse power of up to 100 W is available. Two channels form radiofrequency pulses with adjustable phases for ENDOR experiments. The spectrometer performance is demonstrated by single crystal pulse ENDOR experiments on a copper complex. A HYSCORE experiment demonstrates that the advantages of high-field EPR and correlation spectroscopy can be combined and exploited at Q-band. Furthermore, we illustrate how this combination can be used in cases where the HYSCORE experiment is no longer effective at 35 GHz because of the shallow modulation depth. Even in cases where the echo modulation is virtually absent in the HYSCORE experiment at Q-band, matched microwave pulses allow one to get HYSCORE spectra with a signal-to-noise ratio as good as at X-band. Finally, it is shown that the high microwave power, the short pulses, and the broad resonator bandwidth make the spectrometer well suited to Fourier transform EPR experiments.     

A study of the molecular structure of bacterial lysodektose by quantum-chemical calculations and EPR spectroscopy

The standard redox potentials of the sequential oxidation of lysodektose to the corresponding nitrone were estimated by quantum chemistry methods. It follows from these estimates that the experimentally observed accumulation of the intermediate nitroxyl radical in substantial amounts during the oxidation of lysodektose can be explained by high medium reorganization energy in the oxidation of the nitrosyl radical with simultaneous proton abstraction. The EPR spectra of the radical lysodektose form were modeled. Arguments in favor of the suggestion that one nonequivalent proton appeared in the formation of an intramolecular H-bond were presented. Quantum-chemical calculations of the hyperfine structure constants were in satisfactory agreement with experiment.     

A novel loop-gap resonator probehead for EPR and ENDOR at X-band

An EPR and ENDOR probehead with a loop-gap resonator for X-band is described. The novel feature of the construction is that an iris-type coupling of the resonator is used instead of the conventional antenna coupling. The ENDOR coil combines the role of creating the radio frequency field and that of a shield for the microwave loop-gap structure. Hence, in order to accommodate the iris and waveguide, a pair of RF coils is used in conjunction with a reduced waveguide with dielectric filling. This arrangement simplifies matching the resonator to the microwave bridge, and standard EPR cryostats can be used making sample manipulation more convenient.     

EPR,ENDOR and optical spectroscopy of Yb3+ ion in KZnF3 single crystals

The paramagnetic center of tetragonal symmetry formed by the Yb³⁺ ion in the KZnF₃ crystal has been studied using methods of EPR, ENDOR and optical spectroscopy. The location of the impurity ion and the structural model of the complex differing from the model of the Yb³⁺ center in KMgF₃ have been established. The empirical scheme of the energy levels of the Yb³⁺ ion has been found. The parameters of its interaction with the crystal electrostatic field and the hyperfine interaction with ligands of the nearest environment have been determined. The parameters of the crystal field were used for the analysis of the distortions of the crystal lattice in the vicinity of Yb³⁺. The parameters of the transferred hyperfine interaction have been calculated for the distances between Yb³⁺ and F⁻ ions of the nearest environment obtained taking into account the found distortions. They are in good agreement with the experimental values.     

Multifrequency Probe for Pulsed EPR and ENDOR Spectroscopy

The design, construction, and performance of a multifrequency pulsed EPR and ENDOR probe for use at cryogenic temperatures are described. Interchangeable resonators based on a folded strip line design allow variation of the resonance frequency over a range of 5-11 GHz. Variable coupling to the resonator is achieved capacitively via a simple mechanical adjustment which is thermally and mechanically stable. The entire assembly is robust and easily fabricated. Common methods of analyzing the resonator parameters such as the Q-factor and coupling coefficient are discussed quantitatively. Probe performance data and multifrequency pulsed ENDOR spectra are presented.     

EPR and ENDOR investigations of Sn impurities in CdS

Wurtzite type CdS single crystals with tin impurities have been reinvestigated by means of magnetic resonance. The by far strongest neighbour interaction with the rare isotope³³S is detectable by electron paramagnetic resonance (EPR), while nine weaker cadmium interactions can be resolved with the electron nuclear double resonance (ENDOR) technique. Three distinct shell symmetries are detected and can be explained by the wurtzite lattice symmetry. The parameters evaluated are interpreted in terms of the LCAO approach. An additional Sn-related spectrum, being not resolvable by ordinary EPR was identified by means of ENDOR-induced-EPR (EI-EPR) and turned out to be associated with Lithium.     

EPR and ENDOR Studies of Shallow Donors in SiC

Recent progress in the investigation of the electronic structure of the shallow nitrogen (N) and phosphorus (P) donors in 3C–, 4H– and 6H–SiC is reviewed with focus on the applications of magnetic resonance including electron paramagnetic resonance (EPR) and other pulsed methods such as electron spin echo, pulsed electron nuclear double resonance (ENDOR), electron spin-echo envelope modulation and two-dimensional EPR. EPR and ENDOR studies of the ²⁹Si and ¹³C hyperfine interactions of the shallow N donors and their spin localization in the lattice are discussed. The use of high-frequency EPR in combination with other pulsed magnetic resonance techniques for identification of low-temperature P-related centers in P-doped 3C–, 4H– and 6H–SiC and for determination of the valley–orbit splitting of the shallow N and P donors are presented and discussed.     

EPR and ENDOR study of porphyrins and their covalently linked dimers in the photoexcited triplet state

The triplet states of several substituted porphyrins (Tetraphenylporphyrin (H₂TPP), Zinc-Tetramethylporphyrin (ZnTMP), Octaethylporphyrin (H₂OEP) and the Dication of H₂TPP (H₄TPP²⁺)) and two covalently linked dimers with H₂TPP-subunits in disordered solid solution were studied by EPR and ENDOR at liquid helium temperature. The measurement yields theA _zz component of the hyperfine tensors of all α-protons in the reference frame of the zero field splitting tensor. Dipolar and isotropic contributions toA _zz are discussed and spin densities derived. The spin densities are compared with results of all-valence-electrons self-consistent field molecular orbital calculations (RHF-INDO/S). One of the dimers shows indications of triplet energy transfer between the porphyrin subunits. The order of magnitude of the transfer rate is estimated to be 5 · 10⁵ s^?1.     

Probeheads and instrumentation for pulse EPR and ENDOR spectroscopy with chirped radio frequency pulses and magnetic field steps

Probeheads and instrumentation for modern X-band pulse EPR and ENDOR experiments with chirped radio-frequency pulses and rapidB ₀-field pulses are described. The resonant frequency, the quality factor and, for the first time, the response of a pulse ENDOR resonator structure to a microwave pulse in the subnanosecond time scale have been calculated. The performance of the probeheads for time-domain chirp ENDOR and electron Zeeman-resolved EPR is demonstrated.     

High-Frequency EPR and ENDOR Spectroscopy on Semiconductor Quantum Dots

It is shown that high-frequency electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy are excellent tools for the investigation of the electronic properties of semiconductor quantum dots (QDs). The great attractions of these techniques are that, in contrast to optical methods, they allow the identification of the dopants and provide information about the spatial distribution of the electronic wave function. This latter aspect is particularly attractive because it allows for a quantitative measurement of the effect of confinement on the shape and properties of the wave function. In this contribution EPR and ENDOR results are presented on doped ZnO QDs. Shallow donors (SDs), related to interstitial Li and Na and substitutional Al atoms, have been identified in this material by pulsed high-frequency EPR and ENDOR spectroscopy. The shallow character of the wave function of the donors is evidenced by the multitude of ENDOR transitions of the ⁶⁷Zn nuclear spins and by the hyperfine interaction of the ⁷Li, ²³Na and ²⁷Al nuclear spins that are much smaller than for atomic lithium, sodium and aluminium. The EPR signal of an exchange-coupled pair consisting of a shallow donor and a deep Na-related acceptor has been identified in ZnO nanocrystals with radii smaller than 1.5 nm. From ENDOR experiments it is concluded that the deep Na-related acceptor is located at the interface of the ZnO core and the Zn(OH)₂ capping layer, while the shallow donor is in the ZnO core. The spatial distribution of the electronic wave function of a shallow donor in ZnO semiconductor QDs has been determined in the regime of quantum confinement by using the nuclear spins as probes. Hyperfine interactions as monitored by ENDOR spectroscopy quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. In addition, the effect of confinement on the g-factor of SDs in ZnO as well as in CdS QDs is observed. Finally, it is shown that an almost complete dynamic nuclear polarization (DNP) of the ⁶⁷Zn nuclear spins in the core of ZnO QDs and of the ¹H nuclear spins in the Zn(OH)₂ capping layer can be obtained. This DNP is achieved by saturating the EPR transition of SDs present in the QDs with resonant high-frequency microwaves at low temperatures. This nuclear polarization manifests itself as a hole and an antihole in the EPR absorption line of the SD in the QDs and a shift of the hole (antihole). The enhancement of the nuclear polarization opens the possibility to study semiconductor nanostructures with nuclear magnetic resonance techniques.     

Application of the Raman heterodyne technique for the detection of EPR and ENDOR

Raman heterodyne detection is a coherent optical-RF double resonance technique where the optical and RF fields induce coherence within a three level system and a resultant Raman field is measured using heterodyne detection. This approach has been used previously to detect NMR and more recently EPR. In this paper the parameters that affect the amplitude and signal to noise ratio of the Raman heterodyne signals are considered. The power levels in relation to the oscillator strength and dephasing times, the amplitude and spectrum of the laser frequency jitter in relation to the optical homogeneous linewidths and holeburning rates, and the sample properties such as absorption strength and optical quality, are all factors that affect the Raman signal. The presentation is focused on the Raman heterodyne detected EPR of the nitrogen-vacancy pair centre in diamond making comparisons with Raman heterodyne detected NMR signals obtained for rare earth ion systems. RF-RF double resonance studies, RF holeburning and ENDOR, which give information about the hyperfine levels are also reported for the nitrogen-vacancy centre. The resonance frequencies are in agreement with those predicted from the spin Hamiltonian. The factors affecting the lineshapes and relative intensities of the double resonance signals are discussed.