W-band Electron Paramagnetic Resonance Research Articles

Probing Methyl Group Tunneling in [(CH3)2NH2][Zn(HCOO)3] Hybrid Perovskite Using Co2+ EPR.

At low temperature, methyl groups act as hindered quantum rotors exhibiting rotational quantum tunneling, which is highly sensitive to a local methyl group environment. Recently, we observed this effect using pulsed electron paramagnetic resonance (EPR) in two dimethylammonium-containing hybrid perovskites doped with paramagnetic Mn2+ ions. Here, we investigate the feasibility of using an alternative fast-relaxing Co2+ paramagnetic center to study the methyl group tunneling, and, as a model compound, we use dimethylammonium zinc formate [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite. Our multifrequency (X-, Q- and W-band) EPR experiments reveal a high-spin state of the incorporated Co2+ center, which exhibits fast spin-lattice relaxation and electron spin decoherence. Our pulsed EPR experiments reveal magnetic field independent electron spin echo envelope modulation (ESEEM) signals, which are assigned to the methyl group tunneling. We use density operator simulations to extract the tunnel frequency of 1.84 MHz from the experimental data, which is then used to calculate the rotational barrier of the methyl groups. We compare our results with the previously reported Mn2+ case showing that our approach can detect very small changes in the local methyl group environment in hybrid perovskites and related materials.

A design of resonant cavity with an improved coupling-adjusting mechanism for the W-band EPR spectrometer

We report a new design of resonant cavity for a W-band electron paramagnetic resonance (EPR) spectrometer. An improved coupling-adjusting mechanism, which is robust, compact, and suits with both solenoid-type and split-pair magnets, is utilized on the cavity, and thus enables both continuous-wave (CW) and pulsed EPR experiments. It is achieved by a tiny metal cylinder in the iris. The coupling coefficient can be varied from 0.2 to 17.9. Furthermore, two pistons at each end of the cavity allow for adjustment of the resonant frequency. A horizontal TE011 geometry also makes the cavity compatible with the two frequently used types of magnets. The coupling-varying ability has been demonstrated by reflection coefficient (S 11) measurement. CW and pulsed EPR experiments have been conducted. The performance data indicates a prospect of wide applications of the cavity in fields of physics, chemistry and biology.

Photoreactivity of Hair Melanin from Different Skin Phototypes-Contribution of Melanin Subunits to the Pigments Photoreactive Properties.

Photoreactivity of melanin has become a major focus of research due to the postulated involvement of the pigment in UVA-induced melanoma. However, most of the hitherto studies were carried out using synthetic melanin models. Thus, photoreactivity of natural melanins is yet to be systematically analyzed. Here, we examined the photoreactive properties of natural melanins isolated from hair samples obtained from donors of different skin phototypes (I, II, III, and V). X-band and W-band electron paramagnetic resonance (EPR) spectroscopy was used to examine the paramagnetic properties of the pigments. Alkaline hydrogen peroxide degradation and hydroiodic acid hydrolysis were used to determine the chemical composition of the melanins. EPR oximetry and spin trapping were used to examine the oxygen photoconsumption and photo-induced formation of superoxide anion, and time-resolved near infrared phosphorescence was employed to determine the singlet oxygen photogeneration by the melanins. The efficiency of superoxide and singlet oxygen photogeneration was related to the chemical composition of the studied melanins. Melanins from blond and chestnut hair (phototypes II and III) exhibited highest photoreactivity of all examined pigments. Moreover, melanins of these phototypes showed highest quantum efficiency of singlet oxygen photogeneration at 332 nm and 365 nm supporting the postulate of the pigment contribution in UVA-induced melanoma.

Application of pulsed and high-frequency electron paramagnetic resonance techniques to study petroleum disperse systems

The spectral and relaxation characteristics of “free” organic radicals (FR) and vanadyl-porphyrin (VP) complexes in various petroleum disperse systems (PDS) like bitumen, petroleum, their high-molecular components and solutions were studied using stationary (conventional) and pulsed electron paramagnetic resonance (EPR) techniques in two frequency ranges (X- and W-bands, with the microwave radiation frequencies of about 9 GHz and 95 GHz, respectively). The features of the pulsed approaches (electron spin echo, modulation of the electron spin echo signal decay, electronic relaxation times) and high-frequency EPR for PDS investigations were examined. W-band EPR allows to resolve spectrally the lines from the different paramagnetic centers and more accurately determine their spectral characteristics. It is shown that the electron spin echo can be observed at room temperatures even at high magnetic fields of 3.4 T demonstrating the potential of application of pulsed EPR techniques for the low-cost oilfield measurements. Analysis of the VP transverse magnetization decay curve permits to identify electron-nuclear interactions with the 14N and 1H nuclei in situ while in the EPR spectra these hyperfine interactions usually cannot be detected. It is found from the W-band EPR measurements that FR lineshape cannot be fitted with isotropic parameters in contrast to the established X-band results. The observed effect of increasing the rates of electronic transverse relaxation in asphaltenes is described in the framework of a model of spectral diffusion between the fast- and slow-relaxing paramagnetic centers in supramolecular complexes of asphaltenes.

High-Field (3.4 T) ENDOR Investigation of Asphaltenes in Native Oil and Vanadyl Complexes by Asphaltene Adsorption on Alumina Surface

Vanadyl porphyrin complexes in asphaltenes from heavy (Karmalinskoye) oil and in asphaltene films obtained as a result of adsorption on the surface of aluminum oxide were studied by electron paramagnetic resonance (EPR) and double electron-nuclear resonance (ENDOR) in the W-band frequency range (microwave frequency of 95 GHz, magnetic field of 3.4 T). Mims ENDOR spectra from 1H and 27Al nuclei are observed. ENDOR spectra are different for native oil and asphaltenes from one side and the adsorbed samples from the other side while no significant changes in X- (microwave frequency of 9 GHz) or W-band EPR spectra are found. The results allow supposing that vanadyl porphyrin complexes (at least in the studied asphaltene films) participate in the formation of asphaltene aggregates through the functional groups rather than π−π interactions. The data show the feasibility of the commercial pulsed ENDOR approaches for the investigation of crude oils and their constituents under external influence.

W-band EPR of vanadyl complexes aggregates on the surface of Al2O3

Structural characterization of metalloporphyrins, asphaltenes and their aggregates in complex systems such as native hydrocarbons is in the focus of scientific and industrial interests since many years. We present W-band (95 GHz) electron paramagnetic resonance (EPR) study in the magnetic field of about 3.4 T and temperature of 100 K for Karmalinskoe oil, asphaltens and asphaltenes deposited on the surface of Al2O3. Features of the obtained spectra are described. Shift to the higher frequencies allows to separate spectrally the contributions from paramagnetic complexes of different origin and define the EPR parameters more accurately comparing to the conventional X-band (9 GHz). Changes of the EPR parameters are tracked. We suggest that the proposed approach can be used for the investigation of structure of vanadyl complexes aggregates in crude oil and their fractions.

Small Gd(III) Tags for Gd(III)-Gd(III) Distance Measurements in Proteins by EPR Spectroscopy.

The C7-Gd and C8-Gd tags are compact hydrophilic cyclen-based lanthanide tags for conjugation to cysteine residues in proteins. The tags are enantiomers, which differ in the configuration of the 2-hydroxylpropyl pendant arms coordinating the lanthanide ion. Here, we report the electron paramagnetic resonance (EPR) performance of the C7-Gd ( S configuration) and C8-Gd ( R configuration) tags loaded with Gd(III) on two mutants of the homodimeric ERp29 protein. The W-band EPR spectra were found to differ between the tags in the free state and after conjugation to the protein. In addition, the spectra were sensitive to the labeling position, which may originate from an environment-dependent charge density on the Gd(III)-coordinating oxygens. This is in agreement with previous NMR experiments with different lanthanide ions, which suggested sensitivity to H-bonding. W-band 1H-ENDOR (electron-electron double resonance) experiments detected effects from orientation selection in the central transition, due to a relatively narrow distribution in the ZFS parameters as indicated by simulations. In contrast, the distance distributions derived from DEER (double electron-electron resonance) measurements were insensitive to the R or S configuration of the tags and did not exhibit any orientation selection effects. The DEER measurements faithfully reflected the different widths of the distance distributions at the different protein sites in agreement with previous DEER measurements using other Gd(III) tags. Due to their small size, short tether to the protein, and a broad central EPR transition, the C7-Gd and C8-Gd tags are attractive Gd(III) tags for measurements of relatively short (<4 nm) distances by EPR spectroscopy.

Transition Metal Doping Reveals Link between Electron T1 Reduction and 13C Dynamic Nuclear Polarization Efficiency.

Optimal efficiency of dissolution dynamic nuclear polarization (DNP) is essential to provide the required high sensitivity enhancements for in vitro and in vivo hyperpolarized 13C nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI). At the nexus of the DNP process are the free electrons, which provide the high spin alignment that is transferred to the nuclear spins. Without changing DNP instrumental conditions, one way to improve 13C DNP efficiency is by adding trace amounts of paramagnetic additives such as lanthanide (e.g., Gd3+, Ho3+, Dy3+, Tb3+) complexes to the DNP sample, which has been observed to increase solid-state 13C DNP signals by 100-250%. Herein, we have investigated the effects of paramagnetic transition metal complex R-NOTA (R = Mn2+, Cu2+, Co2+) doping on the efficiency of 13C DNP using trityl OX063 as the polarizing agent. Our DNP results at 3.35 T and 1.2 K show that doping the 13C sample with 3 mM Mn2+-NOTA led to a substantial improvement of the solid-state 13C DNP signal by a factor of nearly 3. However, the other transition metal complexes Cu2+-NOTA and Co2+-NOTA complexes, despite their paramagnetic nature, had essentially no impact on solid-state 13C DNP enhancement. W-band electron paramagnetic resonance (EPR) measurements reveal that the trityl OX063 electron T1 was significantly reduced in Mn2+-doped samples but not in Cu2+- and Co2+-doped DNP samples. This work demonstrates, for the first time, that not all paramagnetic additives are beneficial to DNP. In particular, our work provides a direct evidence that electron T1 reduction of the polarizing agent by a paramagnetic additive is an essential requirement for the improvement seen in solid-state 13C DNP signal.

Tailored Nitroxide Radicals and Biradical Containing 13C Enriched Acetylene Groups: ENDOR and DFT Investigation

A specially synthesized nitroxide biradical R5-C≡13C-(p-C6H4)2-13C≡C-R5 (B4) and two radicals, R5-C≡13CH (RCC) and R5-C≡13C-C6H5 (RCCPh), where R5 is 1-oxyl-2,2,5,5-tetramethyl-pyrroline group, have been studied by X- and W-band electron paramagnetic resonance (EPR) spectroscopy, and by W-band electron-nuclear double resonance (ENDOR). Spin density distribution and hyperfine splitting (hfs) constants on 13C atoms were experimentally determined and also calculated using ORCA 3.0.3 program package. The biradical and radicals geometries were optimized on UKS/B3LYP/cc-pVDZ level. Hfs constants were calculated using density functional theory (DFT) with PBE0 functional and N07D, and were compared with the experimental value of the hfs constant on 13C atoms, measured from ENDOR spectra. It is concluded that at small values of the exchange integral as J ≤ a/2 ≈ 7–8 G, the current quantum chemical approaches do not allow determining precise values of the hfs constants on the 13C atoms in the bridge connecting two paramagnetic nitroxide rings of the biradical.

Enhanced Efficiency of 13C Dynamic Nuclear Polarization by Superparamagnetic Iron Oxide Nanoparticle Doping.

Attainment of high NMR signal enhancements is crucial to the success of in vitro or in vivo hyperpolarized NMR or imaging (MRI) experiments. In this work, we report on the use of a superparamagnetic iron oxide nanoparticle (SPION) MRI contrast agent Feraheme (ferumoxytol) as a beneficial additive in 13C samples for dissolution dynamic nuclear polarization (DNP). Our DNP data at 3.35 T and 1.2 K reveal that addition of 11 mM elemental iron concentration of Feraheme in trityl OX063-doped 3 M [1-13C] acetate samples resulted in a substantial improvement of 13C DNP signal by a factor of almost 3-fold. Concomitant with the large DNP signal increase is the narrowing of the 13C microwave DNP spectra for samples doped with SPION. W-band electron paramagnetic resonance (EPR) spectroscopy data suggest that these two prominent effects of SPION doping on 13C DNP can be ascribed to the shortening of trityl OX063 electron T 1 as explained within the thermal mixing DNP model. Liquid-state 13C NMR signal enhancements as high as 20,000-fold for SPION-doped samples were recorded after dissolution at 9.4 T and 297 K, which is about 3 times the liquid-state NMR signal enhancement of the control sample. While the presence of SPION in hyperpolarized solution drastically reduces 13C T 1, this can be mitigated by polarizing smaller aliquots of DNP samples. Moreover, we have shown that Feraheme nanoparticles (~30 nm in size) can be easily and effectively removed from the hyperpolarized liquid by simple mechanical filtration, thus one can potentially incorporate an in-line filtration for these SPIONS along the dissolution pathway of the hyperpolarizer-a significant advantage over other DNP enhancers such as the lanthanide complexes. The overall results suggest that the commercially-available and FDA-approved Feraheme is a highly efficient DNP enhancer that could be readily translated for use in clinical applications of dissolution DNP.

Single Crystal Electron Paramagnetic Resonance of Dimethylammonium and Ammonium Hybrid Formate Frameworks: Influence of External Electric Field

We present a continuous wave electron paramagnetic resonance (EPR) study of a Mn2+ doped [(CH3)2NH2][Zn(HCOO)3] hybrid dense metal–organic framework (MOF) that exhibits an order–disorder structural phase transition at Tc = 163 K. The W-band EPR measurements of a powder sample are performed to verify the previously reported spin Hamiltonian parameters of the Mn2+ centers in the low-temperature phase. The temperature dependent single crystal X-band EPR experiments reveal that Mn2+ probe ions are susceptible to the phase transition, as the spectrum changes drastically at Tc. The angular dependent EPR spectra of Mn2+ centers are obtained by rotating the single crystal sample about three distinct directions. The simulation of the determined angular dependences reveals six MnO6 octahedra in the ordered phase that originate from a severe crystal twinning of the [(CH3)2NH2][Zn(HCOO)3] MOF. The possible ferroelectric origin of the crystalline twins is investigated by single crystal EPR measurements with an applied...

13C Dynamic Nuclear Polarization Using a Trimeric Gd3+ Complex as an Additive.

Dissolution dynamic nuclear polarization (DNP) is one of the most successful techniques that resolves the insensitivity problem in liquid-state nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) by amplifying the signal by several thousand-fold. One way to further improve the DNP signal is the inclusion of trace amounts of lanthanides in DNP samples doped with trityl OX063 free radical as the polarizing agent. In practice, stable monomeric gadolinium complexes such as Gd-DOTA or Gd-HP-DO3A are used as beneficial additives in DNP samples, further boosting the DNP-enhanced solid-state 13C polarization by a factor of 2 or 3. Herein, we report on the use of a trimeric gadolinium complex as a dopant in 13C DNP samples to improve the 13C DNP signals in the solid-state at 3.35 T and 1.2 K and consequently, in the liquid-state at 9.4 T and 298 K after dissolution. Our results have shown that doping the 13C DNP sample with a complex which holds three Gd3+ ions led to an improvement of DNP-enhanced 13C polarization by a factor of 3.4 in the solid-state, on par with those achieved using monomeric Gd3+ complexes but only requires about one-fifth of the concentration. Upon dissolution, liquid-state 13C NMR signal enhancements close to 20 000-fold, approximately 3-fold the enhancement of the control samples, were recorded in the nearby 9.4 T high resolution NMR magnet at room temperature. Comparable reduction of 13C spin-lattice T1 relaxation time was observed in the liquid-state after dissolution for both the monomeric and trimeric Gd3+ complexes. Moreover, W-band electron paramagnetic resonance (EPR) data have revealed that 3-Gd doping significantly reduces the electron T1 of the trityl OX063 free radical, but produces negligible changes in the EPR spectrum, reminiscent of the results with monomeric Gd3+-complex doping. Our data suggest that the trimeric Gd3+ complex is a highly beneficial additive in 13C DNP samples and that its effect on DNP efficiency can be described in the context of the thermal mixing mechanism.

Influence of Dy3+ and Tb3+ doping on 13C dynamic nuclear polarization.

Dynamic nuclear polarization (DNP) is a technique that uses a microwave-driven transfer of high spin alignment from electrons to nuclear spins. This is most effective at low temperature and high magnetic field, and with the invention of the dissolution method, the amplified nuclear magnetic resonance (NMR) signals in the frozen state in DNP can be harnessed in the liquid-state at physiologically acceptable temperature for in vitro and in vivo metabolic studies. A current optimization practice in dissolution DNP is to dope the sample with trace amounts of lanthanides such as Gd3+ or Ho3+, which further improves the polarization. While Gd3+ and Ho3+ have been optimized for use in dissolution DNP, other lanthanides have not been exhaustively studied for use in C13 DNP applications. In this work, two additional lanthanides with relatively high magnetic moments, Dy3+ and Tb3+, were extensively optimized and tested as doping additives for C13 DNP at 3.35 T and 1.2 K. We have found that both of these lanthanides are also beneficial additives, to a varying degree, for C13 DNP. The optimal concentrations of Dy3+ (1.5 mM) and Tb3+ (0.25 mM) for C13 DNP were found to be less than that of Gd3+ (2 mM). W-band electron paramagnetic resonance shows that these enhancements due to Dy3+ and Tb3+ doping are accompanied by shortening of electron T1 of trityl OX063 free radical. Furthermore, when dissolution was employed, Tb3+-doped samples were found to have similar liquid-state C13 NMR signal enhancements compared to samples doped with Gd3+, and both Tb3+ and Dy3+ had a negligible liquid-state nuclear T1 shortening effect which contrasts with the significant reduction in T1 when using Gd3+. Our results show that Dy3+ doping and Tb3+ doping have a beneficial impact on C13 DNP both in the solid and liquid states, and that Tb3+ in particular could be used as a potential alternative to Gd3+ in C13 dissolution DNP experiments.

Hyperbolic-cosine waveguide tapers and oversize rectangular waveguide for reduced broadband insertion loss in W-band electron paramagnetic resonance spectroscopy. II. Broadband characterization.

Experimental results have been reported on an oversize rectangular waveguide assembly operating nominally at 94 GHz. It was formed using commercially available WR28 waveguide as well as a pair of specially designed tapers with a hyperbolic-cosine shape from WR28 to WR10 waveguide [R. R. Mett et al., Rev. Sci. Instrum. 82, 074704 (2011)]. The oversize section reduces broadband insertion loss for an Electron Paramagnetic Resonance (EPR) probe placed in a 3.36 T magnet. Hyperbolic-cosine tapers minimize reflection of the main mode and the excitation of unwanted propagating waveguide modes. Oversize waveguide is distinguished from corrugated waveguide, overmoded waveguide, or quasi-optic techniques by minimal coupling to higher-order modes. Only the TE10 mode of the parent WR10 waveguide is propagated. In the present work, a new oversize assembly with a gradual 90° twist was implemented. Microwave power measurements show that the twisted oversize waveguide assembly reduces the power loss in the observe and pump arms of a W-band bridge by an average of 2.35 dB and 2.41 dB, respectively, over a measured 1.25 GHz bandwidth relative to a straight length of WR10 waveguide. Network analyzer measurements confirm a decrease in insertion loss of 2.37 dB over a 4 GHz bandwidth and show minimal amplitude distortion of approximately 0.15 dB. Continuous wave EPR experiments confirm these results. The measured phase variations of the twisted oversize waveguide assembly, relative to an ideal distortionless transmission line, are reduced by a factor of two compared to a straight length of WR10 waveguide. Oversize waveguide with proper transitions is demonstrated as an effective way to increase incident power and the return signal for broadband EPR experiments. Detailed performance characteristics, including continuous wave experiment using 1 μM 2,2,6,6-tetramethylpiperidine-1-oxyl in aqueous solution, provided here serve as a benchmark for other broadband low-loss probes in millimeter-wave EPR bridges.

Toward increased concentration sensitivity for continuous wave EPR investigations of spin-labeled biological macromolecules at high fields

High-field, high-frequency electron paramagnetic resonance (EPR) spectroscopy at W-(∼94GHz) and D-band (∼140GHz) is important for investigating the conformational dynamics of flexible biological macromolecules because this frequency range has increased spectral sensitivity to nitroxide motion over the 100ps to 2ns regime. However, low concentration sensitivity remains a roadblock for studying aqueous samples at high magnetic fields. Here, we examine the sensitivity of a non-resonant thin-layer cylindrical sample holder, coupled to a quasi-optical induction-mode W-band EPR spectrometer (HiPER), for continuous wave (CW) EPR analyses of: (i) the aqueous nitroxide standard, TEMPO; (ii) the unstructured to α-helical transition of a model IDP protein; and (iii) the base-stacking transition in a kink-turn motif of a large 232nt RNA. For sample volumes of ∼50μL, concentration sensitivities of 2–20μM were achieved, representing a ∼10-fold enhancement compared to a cylindrical TE011 resonator on a commercial Bruker W-band spectrometer. These results therefore highlight the sensitivity of the thin-layer sample holders employed in HiPER for spin-labeling studies of biological macromolecules at high fields, where applications can extend to other systems that are facilitated by the modest sample volumes and ease of sample loading and geometry.

EPR Study of TiO2 (Rutile) Doped with Vanadium

Samples of titanium dioxide, TiO2, doped with vanadium ions (0.1 ≤ [V n+]0 ≤ 5.0 at. %) at high temperature, were prepared in a polycrystalline state and investigated using X-, Q-, and W-band electron paramagnetic resonance, EPR, spectroscopy. Substitutional and interstitial V4+ centers in TiO2 lattice have been both observed in EPR spectra, and their spin-Hamiltonian parameters were calculated. Portions of paramagnetic and diamagnetic species of V n+ ions were estimated. The effect of additional high temperature annealing on the valence state of vanadium centers is discussed.

High-Frequency EPR Studies on Polymer Chain Morphology in Dip-Coated Films of Poly(3-hexylthiophene)/Fullerene Bulk-Heterojunctions

Abstract Polymer chain morphology and its response to annealing were investigated in terms of g-tensor anisotropy in poly(3-hexylthiophene) (P3HT)/fullerene blend films fabricated by dip coating. The g-anisotropy highly resolved by W-band (94 GHz) electron paramagnetic resonance (EPR) spectroscopy enabled us to precisely determine the molecular orientation of P3HT. The substrate dependence of the morphology was also investigated by W-band EPR spectroscopy. The determined morphology and EPR line width provided insight into the growth of P3HT crystalline domains, which was correlated with solar cell performance.

W-band EPR studies of high-spin nitrenes with large spin-orbit contribution to zero-field splitting.

First W-band 94 GHz EPR spectra of randomly oriented triplet, quintet, and septet nitrenes formed during the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene in cryogenic matrices are reported. In comparison with conventional X-band 9 GHz electron paramagnetic resonance (EPR) spectroscopy, W-band EPR spectroscopy allows the detection and complete spectroscopic characterization of all paramagnetic species formed at different stages of the photolysis of aromatic polyazides. This type of spectroscopy is of paramount importance for experimental determination of the sign of the zero-field splitting (ZFS) parameters of high-spin molecules with large spin-orbit contribution to the ZFS, caused by the effect of heavy atoms. The study shows that triplet 1,3-diazido-2,4,6-tribromo-5-nitrenobenzene (T1) has DT = 1.369 cm(-1), ET = 0.093 cm(-1), and g = 2.0033, quintet 1-azido-2,4,6-tribromo-3,5-dinitrenobenzene (Q1) shows DQ = - 0.306 cm(-1), EQ = 0.0137 cm(-1), and g = 2.0070, and septet 2,4,6-tribromo-1,3,5-trinitrenobenzene (S1) has DS = - 0.203 cm(-1), ES = 0, and g = 2.0073. The experimental ZFS parameters agree well with the results of density functional theory calculations at the PBE/Ahlrichs-DZ level of theory, showing that such calculations adequately describe the magnetic properties of bromine-containing high-spin nitrenes. Both experimental and theoretical data indicate that, in contrast to all known to date quintet dinitrenes, dinitrene Q1 has the negative sign of magnetic anisotropy due to the "heavy atom effect." This dinitrene along with septet trinitrene S1 possess the largest negative value of D among all known quintet and septet organic polyradicals.

Multi-frequency (S, X, Q and W-band) EPR and ENDOR Study of Vanadium(IV) Incorporation in the Aluminium Metal-Organic Framework MIL-53.

Doping the well-known metal-organic framework MIL-53(Al) with vanadium(IV) ions leads to significant changes in the breathing behaviour and might have repercussions on the catalytic behaviour as well. To understand the properties of such a doped framework, it is necessary to determine where dopant ions are actually incorporated. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are applied to reveal the nearest environment of the paramagnetic vanadium(IV) dopant ions. EPR spectra of as-synthesised vanadium-doped MIL-53 are recorded at S-, X-, Q- and W-band microwave frequencies. The EPR spectra suggest that at low dopant concentrations (1.0-2.6 mol %) the vanadium(IV) ions are well dispersed in the matrix. Varying the vanadium dopant concentration within this range or the dopant salt leads to the same dominant EPR component. In the ENDOR spectra, hyperfine (HF) interactions with (1) H, (27) Al and (51) V nuclei are observed. The HF parameters extracted from simulations strongly suggest that the vanadium(IV) ions substitute Al in the framework.

Revealing the Cu2+ ions localization at low symmetry Bi sites in photorefractive Bi12GeO20 crystals doped with Cu and V by high frequency EPR

The sites of incorporation of Cu2+ impurity ions in Bi12GeO20 single crystals co-doped with copper and vanadium have been investigated by electron paramagnetic resonance (EPR). While the X-band EPR spectra consist of a simple broad (ΔB ∼50mT) line with anisotropic lineshape, the W-band EPR spectra exhibit well resolved, strongly anisotropic lines, due to transitions within the 3d9–2D ground manifold of the Cu2+ ions. The most intense group of lines, attributed to the dominant Cu2+(I) center, displays a characteristic four components hyperfine structure for magnetic field orientations close to a 〈110〉 direction. The g and A tensor main axes are very close to one of the 12 possible sets of orthogonal 〈1−10〉, 〈00−1〉 and 〈110〉 crystal directions. Several less intense lines, with unresolved hyperfine structure and similar symmetry properties, mostly overlapped by the Cu2+(I) spectrum, were attributed to Cu2+(II) centers. The two paramagnetic centers are identified as substitutional Cu2+ ions at Bi3+ sites with low C1 symmetry, very likely resulting from different configurations of neighboring charge compensating defects.