X-band Electron Paramagnetic Resonance Measurements Research Articles

A Heterometallic Porphyrin Dimer as a Potential Quantum Gate: Magneto-Structural Correlations and Spin Coherence Properties.

In the development of two-qubit quantum gates, precise control over the intramolecular spin-spin interaction between molecular spin units plays a pivotal role. A weak but measurable exchange coupling is especially important for achieving selective spin addressability that allows controlled manipulation of the computational basis states |00⟩ |01⟩ |10⟩ |11⟩ by microwave pulses. Here, we report the synthesis and Electron Paramagnetic Resonance (EPR) study of a heterometallic meso-meso (m-m) singly-linked VIV O-CuII porphyrin dimer. X-band continuous wave EPR measurements in frozen solutions suggest a ferromagnetic exchange coupling of ca. 8 ⋅ 10-3 cm-1 . This estimation is supported by Density Functional Theory calculations, which also allow disentangling the ferro- and antiferromagnetic contributions to the exchange. Pulsed EPR experiments show that the dimer maintains relaxation times similar to the monometallic CuII porphyrins. The addressability of the two individual spins is made possible by the different g-tensors of VIV and CuII -ions, in contrast to homometallic dimers where tilting of the porphyrin planes plays a key role. Therefore, single-spin addressability in the heterometallic dimer can be maintained even with small tilting angles, as expected when deposited on surface, unlocking the full potential of molecular quantum gates for practical applications.

Generation and Characterization of a Tetraradical Embedded in a Curved Cyclic Paraphenylene Unit.

Unique spin-spin (magnetic) interactions, ring-size effects on ground-state spin multiplicity, and in-plane aromaticity has been found in localized 1,3-diradicals embedded in curved benzene structures such as cycloparaphenylene (CPP). In this study, we characterized the magnetic interactions in a tetraradical consisting of two localized 1,3-diradical units connected by p-quaterphenyl within a curved CPP skeleton by electron paramagnetic resonance (EPR) spectroscopy and quantum chemical calculations. Persistent triplet species with zero-field splitting parameters similar to those of a triplet 1,3-diphenylcyclopentane-1,3-diyl diradical were observed by continuous wave (CW) or pulsed X-band EPR measurements. The quintet state derived from the ferromagnetic interaction between the two triplet diradical moieties was not detected at 20 K under glassy matrix conditions. At the B3LYP/6-31G(d) level of theory, the singlet state was lower in energy than the triplet and quintet states. These findings will aid in the development of open-shell species for material science application.

Broadband multimodal THz waveguides for efficient transfer of high-power radiation in space-confined conditions

Terahertz (THz) range, which lies between the microwave and infrared regions of the electromagnetic spectrum, presents a new frontier containing an abundance of technical applications and fundamental research problems. There are several challenges limiting the progress in the THz science and technology. One of them is the limited range ofthe guided-wave propagation of THz radiation, owing to the high loss from the finite conductivity of metals and the high absorption coefficient of dielectric materials in the THz range. In this work, we discuss the design and fabrication of multimodal hollow THz waveguides used at the X-band Electron Paramagnetic Resonance (EPR) endstation located at the Novosibirsk Free Electron Laser facility (NovoFEL). Experiments carried out at the EPR endstation are aimed to investigate the impact of THz and far infrared radiation on the spin system of different inorganic complexes and organic radicals. The EPR probehead and cryostat impose strict constraints on the possible ways of transferring the THz radiation to the sample, requiring the development, manufacturing, and characterization of specific waveguides. The proposed waveguides for X-band EPR measurements under THz radiation resemble hollow metal/dielectric waveguides. They have the shape of ahollow cylinder, tapering towards the end, with a silver-coated inner surface, and allow THz radiation to be transmitted over a distance of about 60 cm in a wide frequency range from THz to middle infrared. The waveguides performance was characterized at different frequencies of NovoFEL radiation and compared with numerical simulations. Following the requirements ofpolarization-sensitive EPR experiments, the waveguides were further modified by a miniature attachment, consisting of a polarizer, a semiconductor mirror, and a quarter-wave plate. The attachment allows preserving the initial polarization properties of THz radiation, turns its propagation vector, and creates the circular-polarized light irradiating the sample – all inside the limited volume of the EPR probehead.

Multi-Frequency and Single-Crystal EPR on V4+ in W-Doped \u03b2-Vanadyl(V) Phosphate: Hyperfine Coupling- and g-Tensor Values and Orientation

Different structure types of vanadyl(V) orthophosphate [i.e. (VV ≡O)3+ orthophosphate] have been subjects of research due to their catalytic activity in the oxidation of n-butane to maleic anhydride. Electron paramagnetic resonance (EPR) spectroscopy can be exploited to elucidate the electronic structure of such compounds. When tuning the oxidation state of vanadium in (V1-xWx)OPO4, X-band EPR spectra have confirmed the presence of paramagnetic V4+ ions. However, some of the features in these spectra could not be explained. Here, powder samples of β-(VIV0.01VV.98WVI.01)OPO4 are investigated at S-, X- and Q- band, along with X-band EPR measurements on single crystals. Thereby, the discrepancies between the spectra and their simulations could be resolved. In particular, it could be shown that the g and A tensors are not coaxial. The resulting consistent EPR picture and the refined paramagnetic parameters are reported. The work underlines the indispensability of a multi-frequency approach in EPR for unequivocal conclusions.

A Redox-Innocent Uranium(IV)-Quinoid Metal-Organic Framework.

Quinoid-based ligands constitute the most common class of redox-active ligands used to construct electrically conductive and magnetic metal–organic frameworks (MOFs). Whereas this chemistry is intensively explored for transition-metal and lanthanide ions, any related actinide compound has not received attention. In particular, the MOF chemistry of actinide ions in the lower oxidation states is underexplored. We herein report the synthesis, and structural and physical property characterization of a uranium(IV) quinoid-based MOF, [U(Cl2dhbq)2(H2O)2]·4H2O (1, Cl2dhbq2– = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone). 1 is a rare example of a U(IV)-based coordination solid and the first material to incorporate bona fide reducible bridging ligands. Despite the anticipated thermodynamic driving force, no indications of valence tautomerism are evident from magnetometry, near-IR spectroscopy, and X-band electron paramagnetic resonance measurements. These initial results suggest that reduction potentials alone are insufficient as guidelines for the prediction of the occurrence of electron transfer in uranium–quinoid-based materials.

Hierarchical trimetallic layered double hydroxide nanosheets derived from 2D metal-organic frameworks for enhanced oxygen evolution reaction

Fabrication of monolayer LDHs nanosheets with high electrocatalytic activity and durability is still challenging. In this study, a series of hierarchical trimetallic LDHs composed of monolayer nanosheets are fabricated from oxalate metal-organic frameworks (MOFs) grown on copper foil by in-situ conformal transformation. LDHs maintain the octahedron morphology of MOFs, which is composed of ultrathin nanosheets. TEM and AFM images reveal that the thickness of the LDHs nanosheet approaches to 1 nm. These hierarchical trimetallic LDHs exhibit excellent activity and stability towards oxygen evolution reaction (OER) in alkaline electrolytes. Among them, FeCo0.5Ni0.5-LDH show the smallest overpotential of 248 mV at a current density of 10 mA cm−2 with a small Tafel slope of 38 mV dec-1, which is competitive to most reported catalysts. In-situ Raman and X-band electron paramagnetic resonance measurements reveal that the OER active sites are Ni atoms.

Investigation of TEMPO partitioning in different skin models as measured by EPR spectroscopy - Insight into the stratum corneum.

Electron paramagnetic resonance (EPR) spectroscopy represents an established tool to study properties of microenvironments, e.g. to investigate the structure and dynamics of biological and artificial membranes. In this study, the partitioning of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in ex vivo human abdominal and breast skin, ex vivo porcine abdominal and ear skin as well as normal and inflammatory in vitro skin equivalents was investigated by EPR spectroscopy. Furthermore, the stratum corneum (SC) lipid composition (as determined by high-performance thin-layer chromatography), SC lipid chain order (probed by infrared spectroscopy) and the SC thickness (investigated by histology) were determined in the skin models. X-band EPR measurements have shown that TEMPO partitions in the lipophilic and hydrophilic microenvironment in varying ratios in different ex vivo and in vitro skin models. Ex vivo human abdominal skin exhibited the highest amount of TEMPO in the lipophilic microenvironment. In contrast, the lowest amount of TEMPO in the lipophilic microenvironment was determined in ex vivo human breast skin and the inflammatory in vitro skin equivalents. Individual EPR spectra of epidermis including SC and dermis indicated that the lipophilic microenvironment of TEMPO mainly corresponds to the most lipophilic part of the epidermis, the SC. The amount of TEMPO in the lipophilic microenvironment was independent of the SC lipid composition and the SC lipid chain order but correlated with the SC thickness. In conclusion, EPR spectroscopy could be a novel technique to determine differences in the SC thickness, thus suitably complementing existing methods.

Elucidating the Formation and Transformation Mechanisms of the Switchable Metal-Organic Framework ELM-11 by Powder and Single-Crystal EPR Study.

The effect of the synthesis conditions on the structure and guest-responsive properties of a "gate pressure" metal-organic framework (MOF) with composition [Cu(4,4'-bipy)2(BF4)2] n (4,4'-bipy = 4,4'-bipyridine), also known as ELM-11 (ELM = elastic layer material) was investigated. Two different batches of ELM-11, synthesized from water-methanol and water-acetonitrile solutions, have been entirely characterized by PXRD, nitrogen (77 K) and carbon dioxide (195 K) physisorption, elemental analysis, DRIFT, TG, and SEM. Both ELM-11 samples were studied by electron paramagnetic resonance (EPR) spectroscopy in order to follow the change in the local structure of the copper ion during the activation and resolvation. Continuous wave X-band EPR measurements on powder samples provided an elongated octahedral coordination symmetry of the cupric ions and revealed different axial ligands in the as-synthesized and activated forms in both bulk samples of ELM-11. One of the procedures was amended in order to slow down the crystallization that allows isolation of single crystals of two polymorphic modifications of Cu-4,4'-bipyridine coordination polymers, namely [Cu(4,4'-bipy)2(CH3CN)2](BF4)2 and [Cu2O(4,4'-bipy)3(CH3CN)4](BF4)2, one of which shows a crystal structure similar to that of ELM-11. Further single-crystal EPR experiments on the as-synthesized material [Cu(4,4'-bipy)2(CH3CN)2](BF4)2 revealed the orientation of the g tensor of the cupric ions and proved that layers of acetonitrile-synthesized ELM-11 are arranged perpendicularly to the crystallographic c axis.

Substituted versus Naked Thiourea Ligand Containing Pseudotetrahedral Cobalt(II) Complexes: A Comparative Study on Its Magnetization Relaxation Dynamics Phenomenon.

A series of mononuclear tetrahedral cobalt(II) complexes with the general molecular formula [Co(L1)2X2] [where L1 = tetramethylthiourea ([(CH3)2N]2C═S) and X = Cl (1), Br (2), and I (3)] were isolated, and their structures were characterized by single-crystal X-ray diffraction. The experimental direct-current magnetic data are excellently reproduced by fitting both χM T( T) and M( H) simultaneously using the spin Hamiltonian (SH) parameters D1 = -18.1 cm-1 and g1,iso = 2.26, D2 = -16.4 cm-1 and g2,iso = 2.33, and D3 = -22 cm-1 and g3,iso = 2.4 for 1-3, respectively, and the sign of D was unambiguously confirmed from X-band electron paramagnetic resonance measurements. The effective energy barrier extracted for the magnetically diluted complexes 1-3 (10%) is larger than the barrier observed for the pure samples and implies a nonzero contribution of dipolar interaction to the magnetization relaxation dynamics. The SH parameters extracted for the three complexes drastically differ from their respective parent complexes that possess the general molecular formula [Co(L)2X2] [where L = thiourea [(NH2)2C═S] and X = Cl (1a), Br (2a), and I (3a)], which is rationalized by detailed ab initio calculations. An exhaustive theoretical study reveals that both the ground and excited states are not pure but rather multideterminental in nature (1-3). Noticeably, the substitution of L by L1 induces structural distortion in 1-3 on the level of the secondary coordination sphere compared to 1a-3a. This distortion leads to an overall reduction in | E/ D| of 1-3 compared to 1a-3a. This may be one of the reasons for the origin of the slower relaxation times of 1-3 compared to 1a-3a.

Electron Paramagnetic Resonance in the Experimental Oncology: Implementation Examples of the Conventional Approaches

This paper presents a mini-review of application of electron paramagnetic resonance (EPR) in the experimental oncology studies based on the authors’ experience acquired at the R.E. Kavetsky Institute of Experimental Pathology, Oncology, and Radiobiology of the National Academy of Science of Ukraine in cooperation with the EPR group at Kazan Federal University. We show some applications of the conventional X-band EPR measurements combined with the spin trapping techniques to study the mechanisms of the chemical, radiation-induced, and hormonal carcinogenesis; to investigate the influence of the regulators of the cell proliferation; to diagnose and differentiate the tumors; and to monitor the effectiveness of the therapeutic treatment of the malignant tumors.

Magnetic properties of La3Ga5.5Ta0.5O14 single crystals doped with Sm3+

Electron Paramagnetic Resonance (EPR) and magnetic susceptibility measurements have been performed for La3Ga5.5Ta0.5O14 single crystal doped intentionally with 3 wt% of Sm3+ ions. X-band EPR measurements have been carried out with covering the field range 0–1.4 T and the temperature range 3–300 K. It has been found that two magnetically inequivalent sites having at least C2 symmetry are responsible for the EPR spectra. The static magnetization measurements as a function of temperature and magnetic field have been done using a vibrating sample magnetometer (VSM). The field and temperature dependences of magnetization are measured in zero-field cooled (ZFC) and field cooled (FC) regimes in an applied field of 100 Oe. It is seen from magnetization measurements that the strong coupling regime is due to the presence of a strong coupling of the Sm3+ ions to the lattice of LGT crystals.

Effect of Ligand Substituent Coordination on the Geometry and the Electronic Structure of Cu(II)-Diradical Complexes

Two organic moieties, known as ligands, having -OMe and -SePh as the ortho substituent attached to the aniline moiety of the parent 2-anilino-4,6-di-tert-butylphenol ligand, were synthesized. The ligands reacted with CuCl2·2H2O in a 2:1 ratio in CH3CN in the presence of Et3N and provided the corresponding mononuclear Cu(II)-diradical complexes 1 (-OMe) and 2 (-SePh). Complex 1 was square planar, while complex 2 was in distorted square planar geometry due to the secondary coordination between the Se atom and the central Cu(II) center. Both complexes were comprised of multi-paramagnetic centers and exhibited an St = 1/2 ground state as established by variable-temperature magnetic susceptibility measurements. X-band electron paramagnetic resonance measurements indicated the presence of an unpaired electron at the Cu(II) center in complex 1 and at the ligand center (π-radical) in complex 2. The extent of the secondary interaction was found to be dependent on the "softness" of the donor atom belonging to the ortho substituent.

Electron paramagnetic resonance studies of manganese and cobalt ions in YAlO3 crystals

The results of X-band electron paramagnetic resonance measurements of Mn2+ and Mn4+ ions in YAlO3 (YAP) crystals in the temperature range 100–300 K are presented. Additionally, we observe electron paramagnetic resonance spectra of unintentional impurities Fe3+ and Cr3+ in YAP crystals. The angular dependence for Mn ions in YAP crystals can be presented. We aim at obtaining a better insight into the spectroscopic properties of paramagnetic Mn centers and structural information about their nearest environment in YAP host and at determining the valence of manganese ion. In this paper we compare electron paramagnetic resonance spectra for Co and Mn ions in YAP crystals.

Nanometer-Scale Distance Measurements in Proteins Using Gd3+ Spin Labeling

Methods for measuring nanometer-scale distances between specific sites in proteins are essential for analysis of their structure and function. In this work we introduce Gd(3+) spin labeling for nanometer-range distance measurements in proteins by high-field pulse electron paramagnetic resonance (EPR). To evaluate the performance of such measurements, we carried out four-pulse double-electron electron resonance (DEER) measurements on two proteins, p75ICD and tau(C)14, labeled at strategically selected sites with either two nitroxides or two Gd(3+) spin labels. In analogy to conventional site-directed spin labeling using nitroxides, Gd(3+) tags that are derivatives of dipicolinic acid were covalently attached to cysteine thiol groups. Measurements were carried out on X-band (approximately 9.5 GHz, 0.35 T) and W-band (95 GHz, 3.5 T) spectrometers for the nitroxide-labeled proteins and at W-band for the Gd(3+)-labeled proteins. In the protein p75ICD, the orientations of the two nitroxides were found to be practically uncorrelated, and therefore the distance distribution could as readily be obtained at W-band as at X-band. The measured Gd(3+)-Gd(3+) distance distribution had a maximum at 2.9 nm, as compared to 2.5 nm for the nitroxides. In the protein tau(C)14, however, the orientations of the nitroxides were correlated, and the W-band measurements exhibited strong orientation selection that prevented a straightforward extraction of the distance distribution. The X-band measurements gave a nitroxide-nitroxide distance distribution with a maximum at 2.5 nm, and the W-band measurements gave a Gd(3+)-Gd(3+) distance distribution with a maximum at 3.4 nm. The Gd(3+)-Gd(3+) distance distributions obtained are in good agreement with expectations from structural models that take into account the flexibility of the tags and their tethers to the cysteine residues. These results show that Gd(3+) labeling is a viable technique for distance measurements at high fields that features an order of magnitude sensitivity improvement, in terms of protein quantity, over X-band pulse EPR measurements using nitroxide spin labels. Its advantage over W-band distance measurements using nitroxides stems from an intrinsic absence of orientation selection.

Tridentate Facial Ligation of Tris(pyridine-2-aldoximato)nickel(II) and Tris(imidazole-2-aldoximato)nickel(II) To Generate NiIIFeIIINiII, MnIIINiII, NiIINiII, and ZnIINiIIand the Electrooxidized MnIVNiII, NiIINiIII, and ZnIINiIIISpecies: A Magnetostructural, Electrochemical, and EPR Spectroscopic Study

Eight hetero- and homometal complexes 1-6, containing the metal centers Ni(II)Fe(III)Ni(II) (1), Mn(III)Ni(II) (2), Ni(II)Ni(II) (3a-c and 4), Zn(II)Ni(II) (5), and Zn(II)Zn(II) (6), are described. The tridentate ligation property of the metal complexes tris(pyridine-2-aldoximato)nickel(II) and tris(1-methylimidazole-2-aldoximato)nickel(II) with three facially disposed pendent oxime O atoms has been utilized to generate the said complexes. Complex 1 contains metal centers in a linear arrangement, as is revealed by X-ray diffraction. Complexes were characterized by various physical methods including cyclic voltammetry (CV), variable-temperature (2-290 K) magnetic susceptibility, electron paramagnetic resonance (EPR) measurements, and X-ray diffraction methods. Binuclear complexes 2-6 are isostructural in the sense that they all contain a metal ion in a distorted octahedral environment MN(3)O(3) and a second six-coordinated Ni(II) ion in a trigonally distorted octahedral NiN(6) geometry. Complexes 1-4 display antiferromagnetic exchange coupling of the neighboring metal centers. The order of the strength of exchange coupling in the isostructural Ni(II)2 complexes, 3a-c, and 4, demonstrates the effects of the remote substituents on the spin coupling. The electrochemical measurements CV and square wave voltammograms (SQW) reveal two reversible metal-centered oxidations, which have been assigned to the Ni center ligated to the oxime N atoms, unless a Mn ion is present. Complex 2, Mn(III)Ni(II), exhibits a reduction of Mn(III) to Mn(II) and two subsequent oxidations of Mn(III) and Ni(II) to the corresponding higher states. These assignments of the redox processes have been complemented by the X-band EPR measurements. That the electrooxidized species [3a]+, [3b]+, [3c]+, and [4]+ contain the localized mixed-valent NiIINiIII system resulting from the spin coupling, a spin quartet ground state, S(t) = 3/2, has been confirmed by the X-band EPR measurements.

Tryptophan-Based Radical in the Catalytic Mechanism of Versatile Peroxidase from Bjerkandera adusta

Versatile peroxidase (VP) from Bjerkandera adusta is a structural hybrid between lignin (LiP) and manganese (MnP) peroxidase. This hybrid combines the catalytic properties of the two above peroxidases, being able to oxidize typical LiP and MnP substrates. The catalytic mechanism is that of classical peroxidases, where the substrate oxidation is carried out by a two-electron multistep reaction at the expense of hydrogen peroxide. Elucidation of the structures of intermediates in this process is crucial for understanding the mechanism of substrate oxidation. In this work, the reaction of H(2)O(2) with the enzyme in the absence of substrate has been investigated with electron paramagnetic resonance (EPR) spectroscopy. The results reveal an EPR signal with partially resolved hyperfine structure typical of an organic radical. The yield of this radical is approximately 30%. Progressive microwave power saturation measurements indicate that the radical is weakly coupled to a paramagnetic metal ion, suggesting an amino acid radical in moderate distance from the ferryl heme. A tryptophan radical was identified as a protein-based radical formed during the catalytic mechanism of VP from Bjerkandera adusta through X-band and high-field EPR measurements at 94 GHz, aided by computer simulations for both frequency bands. A close analysis of the theoretical model of the VP from Bjerkandera sp. shows the presence of a tryptophan residue near to the heme prosthetic group, which is solvent-exposed as in the case of LiP and other VPs. The catalytic role of this residue in a long-range electron-transfer pathway is discussed.

Synthesis of Novel Aromatic Nitroxides as Potential DNA Intercalators. An EPR Spectroscopical and DFT Computational Study.

The synthesis and electron paramagnetic resonance (EPR) spectroscopic properties of three novel aromatic nitroxides and potential DNA intercalators, the carbazole-based 3,6-dimethylcarbazole-9-oxyl, as well as the acridane-based 9-acridanylidenemalonitrile-10-oxyl and 9-ethylacridanylidenecyanoacetate-10-oxyl, are described. The two acridane-based nitroxides can be isolated and are stable in solution as well as in the solid state for several days. Continuous wave X-band EPR measurements and density functional theory (DFT) calculations demonstrated that the spin density is delocalized over the whole molecule in all three cases. Furthermore, the DFT calculations provided insight into the molecular and electronic structures of these nitroxides and yielded hyperfine coupling constants which are in very good agreement with the experimental data allowing therefore an unambiguous assignment of the hyperfine couplings.

Synthesis of novel aromatic nitroxides as potential DNA intercalators. An EPR spectroscopical and DFT computational study.

The synthesis and electron paramagnetic resonance (EPR) spectroscopic properties of three novel aromatic nitroxides and potential DNA intercalators, the carbazole-based 3,6-dimethylcarbazole-9-oxyl, as well as the acridane-based 9-acridanylidenemalonitrile-10-oxyl and 9-ethylacridanylidenecyanoacetate-10-oxyl, are described. The two acridane-based nitroxides can be isolated and are stable in solution as well as in the solid state for several days. Continuous wave X-band EPR measurements and density functional theory (DFT) calculations demonstrated that the spin density is delocalized over the whole molecule in all three cases. Furthermore, the DFT calculations provided insight into the molecular and electronic structures of these nitroxides and yielded hyperfine coupling constants which are in very good agreement with the experimental data allowing therefore an unambiguous assignment of the hyperfine couplings.

Electron paramagnetic resonance of Mn 2+ in Cs 2M(SO 4) 2s6H 2O (M = Mg, Zn, Co, Ni) single crystals

X-band electron paramagnetic resonance measurements are reported on single crystals of Cs 2M(SO 4) 2s6H 2O (M = Mg, Zn, Co, Ni) doped with Mn 2+ ions at 290 K and 77K. Mn 2+ substitutes for divalent cation sites. The spin-Hamiltonian parameters are evaluated. The observation regarding the temperature dependence of linewidth in cobalt salt is discussed in terms of the host spin-lattice relaxation narrowing. The anisotropic and field dependent linewidths of Mn 2+ in nickel salt are discussed in terms of the spin quenching ideas.