High-field Electron Paramagnetic Resonance Spectroscopy Research Articles

Magnetic Behavior of Trigonal (Bi-)pyramidal 3d8 Mononuclear Nanomagnets: The Case of [Ni(MDABCO)2Cl3]ClO4.

This paper attempts to shed light on the origin of the magnetic behavior specific to trigonal bi- and pyramidal 3d8 mono- and polynuclear nanomagnets. The focus lies on entirely unraveling the system's intrinsic microscopic mechanisms and fundamental quantum mechanical relations governing the underlying electron dynamics. To this end, we develop a self-consistent approach to characterize, in great detail, all electron correlations and the ensuing fine structure of the energy spectra of a broad class of 3d8 systems. The mathematical framework is based on the multiconfigurational self-consistent field method and is devised to account for prospective quantum mechanical constraints that may confine the electron orbital dynamics while preserving the properties of all measurable quantities. We successfully characterize the experimentally observed magnetic anisotropy properties of a slightly distorted trigonal bipyramidal Ni2+ coordination complex, demonstrating that such compounds do not exhibit intrinsic huge zero-field splitting and inherent giant magnetic anisotropy. We reproduce qualitatively and quantitatively the behavior of the low-field magnetic susceptibility, magnetization, low-, and high-field electron paramagnetic resonance spectroscopy measurements and provide an in-depth analysis of the obtained results.

Accumulation and Pulse Electron Paramagnetic Resonance Spectroscopic Investigation of the 4-Oxidobenzyl Radical Generated in the Radical S-Adenosyl-l-methionine Enzyme HydG.

The radical S-adenosyl-l-methionine (SAM) enzyme HydG cleaves tyrosine to generate CO and CN- ligands of the [FeFe] hydrogenase H-cluster, accompanied by the formation of a 4-oxidobenzyl radical (4-OB•), which is the precursor to the HydG p-cresol byproduct. Native HydG only generates a small amount of 4-OB•, limiting detailed electron paramagnetic resonance (EPR) spectral characterization beyond our initial EPR lineshape study employing various tyrosine isotopologues. Here, we show that the concentration of trapped 4-OB• is significantly increased in reactions using HydG variants, in which the "dangler Fe" to which CO and CN- bind is missing or substituted by a redox-inert Zn2+ ion. This allows for the detailed characterization of 4-OB• using high-field EPR and electron nuclear double resonance spectroscopy to extract its g-values and 1H/13C hyperfine couplings. These results are compared to density functional theory-predicted values of several 4-OB• models with different sizes and protonation states, with a best fit to the deprotonated radical anion configuration of 4-OB•. Overall, our results depict a clearer electronic structure of the transient 4-OB• radical and provide new insights into the radical SAM chemistry of HydG.

Compact, tunable polarization transforming reflector for quasi-optical devices used in terahertz science

We describe the design, fabrication, and characterization of a compact polarization transforming reflector (PTR). The device can be easily tuned over a broad frequency range, has very little insertion losses, and can easily be integrated into quasi-optical systems that are based on a half-cube design. By varying the distance between the wire grid and a flat mirror, the polarization state of an arbitrary polarized Gaussian incident beam can be set to an output Gaussian beam in either linear or circular polarization. In addition, by varying the orientation of the wire grid, the PTR can be used as a universal polarizer, a property that has not been discussed or demonstrated in the literature. The ability to control the electric field polarization at Terahertz (THz) frequencies is essential for many applications, such as THz spectroscopy and high-field electron paramagnetic resonance spectroscopy.

Spectroscopic study of volcanic ashes

Volcanic ashes particles are subjected to substantial modification during explosive eruptions. The mineralogical and compositional changes have important consequences on the environment and human health. Nevertheless, the relationship between the speciation of iron (Fe) and the mineralogical composition and particle granulometry of the ashes, along with their interaction with water, are largely unknown. In particular, the Fe oxidation state and the possible formation of new Fe-bearing phases in presence of S, Cl, and F in the plume are key points to assess the impact of the ashes.Fragmental material ejected during volcanic activity (tephra) in 2013, was collected on the Mt. Etna (Italy) and investigated using a multi-technique approach that included conventional Electron Paramagnetic Resonance (EPR), high field EPR (HFEPR), EchoEPR, and Fe K-edge X-ray Absorption Spectroscopy (XAS). These element-selective techniques allowed obtaining a detailed information on the oxidation state and coordination environment of Fe, and of its speciation in the ash samples as a function of the granulometry. A complex mineralogical assemblage, consisting of variable amounts of nanometric crystalline Fe inclusions in a glass matrix, and of Fe-oxides and Fe-sulfur phases was revealed. A risk assessment of the ashes is attempted.

Access to Heteroleptic Fluorido‐Cyanido Complexes with a Large Magnetic Anisotropy by Fluoride Abstraction

Silicon-mediated fluoride abstraction is demonstrated as a means of generating the first fluorido-cyanido transition metal complexes. This new synthetic approach is exemplified by the synthesis and characterization of the heteroleptic complexes, trans-[MIV F4 (CN)2 ]2- (M=Re, Os), obtained from their homoleptic [MIV F6 ]2- parents. As shown by combined high-field electron paramagnetic resonance spectroscopy and magnetization measurements, the partial substitution of fluoride by cyanide ligands leads to a marked increase in the magnetic anisotropy of trans-[ReF4 (CN)2 ]2- as compared to [ReF6 ]2- , reflecting the severe departure from an ideal octahedral (Oh point group) ligand field. This methodology paves the way toward the realization of new heteroleptic transition metal complexes that may be used as highly anisotropic building-blocks for the design of high-performance molecule-based magnetic materials.

Synthesis, Magnetic and High-Field EPR Investigation of Two Tetranuclear NiII-Based Complexes.

Two tetranuclear compounds with a cubane-like structure were synthesized from a one-pot reaction between NiII and 2,2,6,6-tetramethyl-3,5-heptanedione (Hdpm) for 1 or 4,4,4-trifluoro-1-phenyl-1,3-butanedione (Hbta) for 2 in the presence of sodium methoxide. The crystal structures of both compounds have been determined by single-crystal X-ray diffraction, and their magnetic properties have been studied by SQUID magnetometry as well as by high-field electron paramagnetic resonance (HFEPR) spectroscopy. For 1, the temperature dependence of the magnetic susceptibility can be fitted by taking into account Ni···Ni ferromagnetic interactions, which leads to an S = 4 ground-state spin. For 2, both antiferromagnetic and ferromagnetic interactions are present. However, the latter are dominant, which also leads to an S = 4 ground-state spin, in good agreement with the HFEPR study.

High-Field EPR Spectroscopic Characterization of Mn(II) Bound to the Bacterial Solute-Binding Proteins MntC and PsaA

During infection, the bacterial pathogens Staphylococcus aureus and Streptococcus pneumoniae employ ATP-binding cassette (ABC) transporters to acquire Mn(II), an essential nutrient, from the host environment. Staphylococcal MntABC and streptococcal PsaABC attract the attention of the biophysical and bacterial pathogenesis communities because of their established importance during infection. Previous biophysical examination of Mn(II)-MntC and Mn(II)-PsaA using continuous-wave (≈9 GHz) electron paramagnetic resonance (EPR) spectroscopy revealed broad, difficult-to-interpret spectra (Hadley et al. J. Am. Chem. Soc. 2018, 140, 110-113). Herein, we employ high-frequency (>90 GHz), high-field (>3 T) EPR spectroscopy to investigate the Mn(II)-binding sites of these proteins and determine the spin Hamiltonian parameters. Our analyses demonstrate that the zero-field splitting (ZFS) is large for Mn(II)-MntC and Mn(II)-PsaA at +2.72 and +2.87 GHz, respectively. The measured 55Mn hyperfine coupling values for Mn(II)-MntC and Mn(II)-PsaA of 241 and 236 MHz, respectively, demonstrate a more covalent interaction between Mn(II) and the protein compared to Mn(II) in aqueous solution (≈265 MHz). These studies indicate that MntC and PsaA bind Mn(II) in a similar coordination geometry. Comparison of the ZFS values determined herein with those ascertained for other Mn(II) proteins suggests that the Mn(II)-MntC and Mn(II)-PsaA coordination spheres are not five-coordinate in solution.

Investigating the thermally- and light-induced interconversion of bisdithiazolyl radicals and dimers with high-field EPR

Materials with magnetic properties that can be modulated via optical excitation offer enticing opportunities for the development of next generation technologies. An exciting new class of photomagnetic materials based on bisdithiazolyl radicals has recently been developed. The dimorphic system in this study crystallizes in two phases, one composed of diamagnetic dimers and the other of paramagnetic radicals. Here we report on the use of high-field electron paramagnetic resonance spectroscopy to characterize both the thermally- and light-induced transitions in the dimer phase. During the course of this study we show that signals originating from residual radical defects in the dimer phase can be differentiated from those arising from the radical phase.

Synthesis, Crystal Structures, and EPR Studies of First MnIIILnIII Hetero-binuclear Complexes.

A new family of binuclear complexes [MnIIILnIII(dpm)4(MeO)2(MeOH)2] is reported (where Ln = LaIII (1), PrIII (2), and EuIII(3)). These compounds were obtained from a one-pot reaction between 2,2,6,6-tetramethyl-3,5-heptanodione (Hdpm), MnII, and the respective LnIII salt in the presence of sodium methoxide. The derivative containing the diamagnetic ion LaIII has been synthesized in order to characterize the local anisotropy of the MnIII ion. High-field electron paramagnetic resonance (HFEPR) spectroscopy shows that the MnIII ion, with an elongated octahedral geometry in all compounds, has a significant axial zero-field splitting and a small rhombic anisotropy. Additionally, the HFEPR measurements indicate that there is almost no exchange between the spin carriers in these compounds, all of which exhibit field-induced slow relaxation of the magnetization.

In-Depth Magnetic Characterization of a [2 × 2] Mn(III) Square Grid Using SQUID Magnetometry, Inelastic Neutron Scattering, and High-Field Electron Paramagnetic Resonance Spectroscopy.

A tetranuclear [2 × 2] grid-like manganese(III) Schiff base complex, Mn4, has been synthesized and characterized by single-crystal X-ray crystallography. Direct-current magnetization measurements were performed on the system and proved to be insufficient for an accurate magnetic model to be deduced. Combined inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR) experiments provided the necessary information in order to successfully model the magnetic properties of Mn4. The resulting model takes into account both the magnitude and the relative orientations of the single-ion anisotropy tensors.

A two-dimensional cobalt(ii) network with a remarkable positive axial anisotropy parameter exhibiting field-induced single-ion magnet behavior

With pyridyl-1,2,4-triazole (3,3′-Hbpt), {[Co(3,3′-Hbpt)2(SCN)2]·2H2O}n (1) and {[Ni(3,3′-Hbpt)2(SCN)2]·2H2O}n(2) are prepared and magnetically characterized.

High field electron paramagnetic resonance characterization of electronic and structural environments for paramagnetic metal ions and organic free radicals in Deepwater Horizon oil spill tar balls.

In the first use of high-field electron paramagnetic resonance (EPR) spectroscopy to characterize paramagnetic metal-organic and free radical species from tar balls and weathered crude oil samples from the Gulf of Mexico (collected after the Deepwater Horizon oil spill) and an asphalt volcano sample collected off the coast of Santa Barbara, CA, we are able to identify for the first time the various paramagnetic species present in the native state of these samples and understand their molecular structures and bonding. The two tar ball and one asphalt volcano samples contain three distinct paramagnetic species: (i) an organic free radical, (ii) a [VO](2+) containing porphyrin, and (iii) a Mn(2+) containing complex. The organic free radical was found to have a disc-shaped or flat structure, based on its axially symmetric spectrum. The characteristic spectral features of the vanadyl species closely resemble those of pure vanadyl porphyrin; hence, its nuclear framework around the vanadyl ion must be similar to that of vanadyl octaethyl porphyrin (VOOEP). The Mn(2+) ion, essentially undetected by low-field EPR, yields a high-field EPR spectrum with well-resolved hyperfine features devoid of zero-field splitting, characteristic of tetrahedral or octahedral Mn-O bonding. Although the lower-field EPR signals from the organic free radicals in fossil fuel samples have been investigated over the last 5 decades, the observed signal was featureless. In contrast, high-field EPR (up to 240 GHz) reveals that the species is a disc-shaped hydrocarbon molecule in which the unpaired electron is extensively delocalized. We envisage that the measured g-value components will serve as a sensitive basis for electronic structure calculations. High-field electron nuclear double resonance experiments should provide an accurate picture of the spin density distribution for both the vanadyl-porphyrin and Mn(2+) complexes, as well as the organic free radical, and will be the focus of follow-up studies.

Field-induced slow relaxation in a monometallic manganese(III) single-molecule magnet.

High-field electron paramagnetic resonance spectroscopy shows that the structurally distorted Mn(III) ion in Na5[Mn(L-tart)2]·12H2O (1; L-tart = L-tartrate) has a significant negative axial zero-field splitting and a small rhombic anisotropy (∼1% of D). Alternating-current magnetic susceptibility measurements demonstrate that 1, which contains isolated Mn(III) centers, displays slow relaxation of its magnetization under an applied direct-current magnetic field.

A High-Field EPR Study of the Accelerated Dynamics of the Amorphous Fraction of Semicrystalline Poly(dimethylsiloxane) at the Melting Point

The reorientation of a small paramagnetic tracer in poly(dimethylsiloxane) (PDMS) has been investigated by high-field electron paramagnetic resonance spectroscopy at a Larmor frequency of 285 GHz. The tracer is confined in the disordered phase of the semicrystalline PDMS. A sudden change of the rotational dynamics is observed close to the melting point (213 K) of the crystallites. This points to strong coupling between the crystalline and the disordered fractions of PDMS. Below the glass transition ( $$T_\mathrm{g} \sim 150 \mathrm{K}$$ ), the tracer reorientation occurs via small angle jumps, with no apparent distribution of the correlation times. Above $$T_\mathrm{g}$$ , a power-law distribution of correlation times is evidenced.

High-frequency and high-field electron paramagnetic resonance (HFEPR): a new spectroscopic tool for bioinorganic chemistry

This minireview describes high-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy in the context of its application to bioinorganic chemistry, specifically to metalloproteins and model compounds. HFEPR is defined as frequencies above ~100GHz (i.e., above W-band) and a resonant field reaching 25T and above. The ability of HFEPR to provide high-resolution determination of g values of S=1/2 is shown; however, the main aim of the minireview is to demonstrate how HFEPR can extract spin Hamiltonian parameters [zero-field splitting (zfs) and g values] for species with S>1/2 with an accuracy and precision unrivalled by other physical methods. Background theory on the nature of zfs in S=1, 3/2, 2, and 5/2 systems is presented, along with selected examples of HFEPR spectroscopy of each that are relevant to bioinorganic chemistry. The minireview also provides some suggestions of specific systems in bioinorganic chemistry where HFEPR could be rewardingly applied, in the hope of inspiring workers in this area.

Tuning Transverse Anisotropy in CoIII–CoII–CoIII Mixed-Valence Complex toward Slow Magnetic Relaxation

Two cobalt mixed-valence complexes with different substituents have been prepared and structurally characterized by single-crystal X-ray diffraction to alter slow magnetic relaxation by tailoring the transverse anisotropy. The trinuclear complexes [(L(1))4Co3(H2O)2](NO3)4·CH3OH·5H2O (1-NO3) and [(L(2))4Co3(H2O)2](NO3)4·6H2O (2-NO3) feature a distorted octahedral Co(II) strongly hindered in a trinuclear Co(III)-Co(II)-Co(III) mixed-valence array. Detailed magnetic studies of 1-NO3 and 2-NO3 have been conducted using direct- and alternating-current magnetic susceptibility data. In accordance with variable-field magnetic susceptibility data at low temperatures, high-field electron paramagnetic resonance (HF-EPR) spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in complexes 1-NO3 and 2-NO3. These findings indicate that the onset of the variation of distortion within complex 2-NO3 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Consequently, the anisotropy energy scale associated with the relaxation barrier, 5.46 cm(-1) (τo = 1.03 × 10(-5) s), is determined by the transverse E term. The results demonstrate that slow magnetic relaxation can be switched through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy.

SPACE-CHARGE LAYER, INTRINSIC "BULK" AND SURFACE COMPLEX DEFECTS IN ZnO NANOPARTICLES — A HIGH-FIELD ELECTRON PARAMAGNETIC RESONANCE ANALYSIS

The defect structure of ZnO nanoparticles is characterized by means of high-field electron paramagnetic resonance (EPR) spectroscopy. Different point and complex defects could be identified, located at the "bulk" or the surface region of the nanoparticles. In particular, by exploiting the enhanced g-value resolution at a Larmor frequency of 406.4 GHz, it could be shown that the resonance commonly observed at g = 1.96 is comprised of several overlapping resonances from different defects. Based on the high-field EPR analysis, the development of a space-charge layer could be monitored that consists of (shallow) donor-type [Formula: see text] defects at the "bulk" and acceptor-type [Formula: see text] and complex [Formula: see text] defects at the surface. Application of a core-shell model allows to determine the thickness of the depletion layer to 1.0 nm for the here studied compounds [J.J. Schneider et al., Chem. Mater.22, 2203 (2010)].

Hydrogen bonding to the nitroxide of protein bound spin labels

Deuterium electron spin-echo envelope modulation spectroscopy (2H-ESEEM) (9.3 GHz, 0.33T) and continuous wave high-field (95 GHz, 3.4T) electron paramagnetic resonance (EPR) spectroscopy were applied to disentangle the sources of protons which are hydrogen bonded to nitroxides of protein bound spin labels. For a set of 12 colicin A mutants with nitroxide spin labels site-specifically introduced at residue positions 26, 33, 42, 62, 91, 105, 115, 154, 166, 169, 181 and 192 of the channel forming domain we analysed the heterogeneity in the gxx region of EPR spectra detected at 95 GHz and quantified the fractions of nitroxide populations engaged in H-bonding to water and protein protons. The amplitudes of the narrow and broad components of 2H-ESEEM spectra arising from non-bonded and bonded water (D2O) molecules or deuterated protein components reflect the local H-bonding equilibria. The combination of both methods allowed estimating the fraction of nitroxide populations hydrogen bonded to non-exchangeable protein protons. Hydrogen bonds are known to play a governing role in protein folding and function, and the presented approach extends the information obtainable from site-directed spin labelling in the investigation of complex biomolecular systems and their conformational changes.

Field-induced slow relaxation of magnetization in a tetrahedral Co(ii) complex with easy plane anisotropy

The mononuclear Co(II) complex CoBr (dmph = 2,9-dimethyl-1,10-phenanthroline) was obtained and X-ray structurally characterized as a distorted tetrahedron environment that is responsible for the moderately strong positive anisotropy of high spin Co(II). In combination with variable-field magnetic susceptibility data at low temperature, high-field electron paramagnetic resonance (HF-EPR) spectroscopy reveals the presence of easy-plane anisotropy (D > 0) in complex CoBr. Slow magnetic relaxation effects were observed for CoBr in the presence of a dc magnetic field. At very low temperatures, ac magnetic susceptibility data show the magnetic relaxation time, τ, to be temperature-independent, while above 2.4 K thermally activated Arrhenius behavior is dominated with Ueff = 22.8(8) cm(-1) and τ0 = 3.7(5) × 10(-10) s. Upon dilution of the complex within a matrix of the isomorphous compound ZnBr, ac susceptibility data reveal the individual molecular nature of the slow magnetic relaxation and indicate that the quantum tunneling pathway observed at low temperatures is likely mediated by intermolecular dipolar interactions.

High-Field EPR Spectroscopic Characterization of Spin Probes in Aqueous Ionic Liquid Mixtures

Abstract Binary mixtures of the hydrophilic room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim+][BF4 -]) and water have been probed using high-field/high-frequency electron paramagnetic resonance (EPR) spectroscopy. By adding nitroxide radicals as paramagnetic tracers, the solution structure could be assessed from typical solutes' points of view. Three probe molecules, differing in charge and hydrogen bonding affinity, have been employed: the dianionic Fremy's salt, amphiphilic TEMPO, and hydrophilic TEMPOL. High-field pulse EPR spectroscopy has been used to determine hyperfine interaction tensors and g-matrices under cryogenic conditions. Polarity-proticity plots reveal the interaction of the nitroxide moiety with apolar (TEMPO) and ionic (FS) nano-domains, depending on the probe structure. Only small variations are observed on changing the water content suggesting that the probes are well shielded from aqueous subdomains. The results indicate that addition of water in fact stabilizes IL-rich mesostructures by embedding them in pools of water.