Search Results (56)

In conventional low-viscosity solvents, magnetic field effects (MFEs) in photoredox catalysis are often negligible because photogenerated radical ion pairs (RIPs) diffuse apart before significant spin evolution occurs. This study reports using ionic liquids (ILs) as a tunable homogeneous “solvent cage” to observe distinct low-field MFEs in the phenothiazine-mediated photoinduced reductive dechlorination of aryl chlorides. Experimental results demonstrate that MFEs increase significantly with bulk viscosity, reaching saturation at approximately 1000 Gs with a maximum enhancement of about 15%, consistent with the hyperfine coupling mechanism (HFCM). Femtosecond transient absorption spectroscopy (fs-TA) reveals that the ionic liquid environment effectively reduces the radical cage escape rate, matching it with the spin evolution rate. This allows the external magnetic field to intervene in the back electron transfer (BET) process. However, unlike strongly confined micellar systems, the contribution of the triplet charge recombination (TCR) pathway here is moderate, intrinsically limiting the magnetic enhancement amplitude. These findings establish that MFE magnitude is determined by both viscosity-controlled cage dynamics and the efficiency of the TCR channel, providing a mechanistic basis for designing spin-modulated homogeneous photoredox systems. Full article

This work presents a comprehensive study of quantum correlations and their degradation under environmental dephasing within the atomic hydrogen system. By analyzing the magnetic coupling between the electron and proton spins in the $1s$ hyperfine state, we elucidate how coherent spin interactions generate entangled states and govern their temporal evolution. The investigation focuses on three key measures of quantum correlations—Bell nonlocality, Einstein–Podolsky–Rosen (EPR) steering, and quantum purity—each reflecting a different level within the hierarchy of nonclassical correlations. Analytical formulations and numerical simulations reveal that, in the absence of decay, all quantities remain steady, indicating the preservation of coherence. When dephasing is introduced, each measure decays exponentially toward a stationary lower bound, with Bell nonlocality identified as the most fragile, followed by steering and purity. A three-dimensional analysis of Werner states under dephasing further establishes the critical purity thresholds required for Bell inequality violations. The results highlight the interdependence between magnetic coupling, decoherence, and initial entanglement, providing a unified framework for understanding correlation dynamics in open quantum systems. These findings have direct implications for the development of noise-resilient quantum information protocols and spin-based quantum technologies, where preserving nonlocal correlations is essential for reliable quantum operations. Full article

Solid-state spin centers are at the forefront of developing advanced quantum technologies, engaging in applications of sensing, communication and computing. A semiconductor host matrix compatible with existing silicon technology provides a robust platform for holding spin defects and an opportunity for external manipulation. In this article, negatively charged nitrogen-vacancy (NV) centers in the hexagonal hh position in a 6H polytype silicon carbide crystal was studied using high-frequency (94 GHz) electron paramagnetic (EPR) and electron nuclear double resonances (ENDOR) spectroscopy. Experimentally determined values of hyperfine and quadrupole interactions of ¹⁴N were compared with the values obtained for the centers in NV_k2k1 positions. The distribution of spin density of the defect within a supercell of the SiC crystal lattice was calculated using the density functional theory approach. The theoretical estimation of electron-nuclear interaction constants turned out to be in close agreement with the experimental values, which allows us to refine the microscopic model of a point defect. The temperature dependence of the spin Hamiltonian values (δA/δT ≅ 180 Hz/K) was studied with the possibility of observing the ¹⁴N NMR signal at room temperature. The fundamental knowledge gained about interactions’ parameters’ behavior lays the foundation for the creation of promising quantum platforms. Full article

In this work, we investigate the quantum coherence and purity in hydrogen atoms under dissipative dynamics, with a focus on the hyperfine structure states arising from the electron–proton spin interaction. Using the Lindblad master equation, we model the time evolution of the density matrix of the system, incorporating both the unitary dynamics driven by the hyperfine Hamiltonian and the dissipative effects due to environmental interactions. Quantum coherence is quantified using the $L_1$ norm and relative entropy measures, while purity is assessed via von Neumann entropy, for initial states, including a maximally entangled Bell state and a separable state. Our results reveal distinct dynamics: for the Bell states, both coherence and purity decay exponentially with a rate proportional to the dissipation parameter, whereas for a kind of separable state, coherence exhibits oscillatory behavior modulated via the hyperfine coupling constant, superimposed on an exponential decay, and accompanied by a steady increase in entropy. Higher dissipation rates accelerate the loss of coherence and the growth of von Neumann entropy, underscoring the environment’s role in suppressing quantum superposition and driving the system towards mixed states. These findings enhance our understanding of coherence and purity preservation in atomic systems and offer insights for quantum information applications where robustness against dissipation is critical. Full article

We present high-precision multi-configuration Dirac–Hartree–Fock (MCDHF) calculations for the metastable states of ${\mathrm{Pr}}^{9+}$ and ${\mathrm{Nd}}^{10+}$ ions, systematically investigating their energy levels, transition properties, Landé $g_J$ factors, and hyperfine interaction constants. Our results show excellent agreement with available experimental data and theoretical benchmarks, while resolving critical configuration assignment discrepancies through detailed angular momentum coupling analysis. The calculations highlight the significant role of Breit interaction and provide the first theoretical predictions of electric quadrupole hyperfine constants ($B_{\mathrm{hfs}}$). These findings deliver essential atomic data for the development of next-generation optical clocks and establish lanthanide highly charged ions as exceptional candidates for precision tests of fundamental physics. Full article

This study examines experimental data on the influence of the surrounding medium and non-covalent interactions on the isotropic hyperfine coupling constant, A_iso(¹⁴N), of the stable nitroxide radical 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) in solution. The data were used to identify a density functional theory functional/basis set combination that accurately reproduces the experimental A_iso(¹⁴N) values. The variations in A_iso(¹⁴N) due to external factors are two orders of magnitude greater than the accuracy of its experimental measurements, making A_iso(¹⁴N) a highly sensitive experimental probe for quantifying these effects. Additionally, it was found that the proton-accepting ability of the N-O^• moiety in TEMPO resembles that of the P=O moiety, enabling the simultaneous formation of two equally strong hydrogen bonds. Full article

Oxalate decarboxylase is an Mn- and O₂-dependent enzyme in the bicupin superfamily that catalyzes the redox-neutral disproportionation of the oxalate monoanion to form carbon dioxide and formate. Its best-studied isozyme is from Bacillus subtilis where it is stress-induced under low pH conditions. Current mechanistic schemes assume a monodentate binding mode of the substrate to the N-terminal active site Mn ion to make space for a presumed O₂ molecule, despite the fact that oxalate generally prefers to bind bidentate to Mn. We report on X-band ¹³C-electron nuclear double resonance (ENDOR) experiments on ¹³C-labeled oxalate bound to the active-site Mn(II) in wild-type oxalate decarboxylase at high pH, the catalytically impaired W96F mutant enzyme at low pH, and Mn(II) in aqueous solution. The ENDOR spectra of these samples are practically identical, which shows that the substrate binds bidentate (κO, κO’) to the active site Mn(II) ion. Domain-based local pair natural orbital coupled cluster singles and doubles (DLPNO-CCSD) calculations of the expected ¹³C hyperfine coupling constants for bidentate bound oxalate predict ENDOR spectra in good agreement with the experiment, supporting bidentate bound substrate. Geometry optimization of a substrate-bound minimal active site model by density functional theory shows two possible substrate coordination geometries, bidentate and monodentate. The bidentate structure is energetically preferred by ~4.7 kcal/mol. Our results revise a long-standing hypothesis regarding substrate binding in the enzyme and suggest that dioxygen does not bind to the active site Mn ion after substrate binds. The results are in agreement with our recent mechanistic hypothesis of substrate activation via a long-range electron transfer process involving the C-terminal Mn ion. Full article

The effect of nuclear quadrupole coupling of a single nucleus on the rotational spectra of diatomic molecules is given. By careful selection of examples, procedures are given for the analysis of successively more complicated spectra. The microwave spectra of some alkali halides, interhalogen diatomics, and deuterium halides provide excellent examples for analysis and for student exercises. Full article

Avoided level crossing muon spin resonance (ALC-$\mu$SR) is used to characterize muoniated free radicals. These radicals are used as probes of the local environment and reorientational motion of specific components in complex systems. The parameter that provides information about the anisotropic motion is the motionally-averaged muon dipolar-hyperfine coupling constant ($⟨D_{\mu }^{\Vert }⟩$). The ALC-$\mu$SR spectra of muoniated radicals in anisotropic environments frequently have Lorentzian-like ${\mathsf{\Delta }}_1$ resonances, which makes it challenging to extract $⟨D_{\mu }^{\Vert }⟩$. In this paper, we derive a means to estimate$|⟨D_{\mu }^{\Vert }⟩|$ from ALC-$\mu$SR spectra with Lorentzian-like resonances by measuring the amplitude, width, and position of the ${\mathsf{\Delta }}_1$ resonance and the amplitude, width, and position of a ${\mathsf{\Delta }}_0$ resonance. Numerical simulations were used to test this relationship for radicals with a wide range of muon and proton hyperfine parameters. We use this methodology to determine $|⟨D_{\mu }^{\Vert }⟩|$ for the Mu adducts of the cosurfactant 2-phenylethanol in ${\mathrm{C}}_{12}$E₄ bilayers. From this we determined the amplitude of the anisotropic reorientational motion of the cosurfactant. Full article

Fe₇₈B₁₅Sn₇ and (Fe₃Co₁)₇₈B₁₅Sn₇ amorphous metallic alloys were prepared using the method of planar flow casting. The amorphous nature of ribbons containing 7 at. % Sn was verified by X-ray diffraction. The resulting chemical composition was checked by flame atomic absorption spectroscopy and by mass spectrometry with inductively coupled plasma. The microstructure of the as-quenched metallic glasses was investigated by 57-Fe and 119-Sn Mössbauer spectrometry. The experiments were performed with transmission geometry at 300 K, 100 K, and 4.2 K, and in an external magnetic field of 6 T. The replacement of a quarter of the Fe by Co did not cause significant modifications of the hyperfine interactions in the 57-Fe nuclei. The observed minor variations in the local magnetic microstructure were attributed to alterations in the topological short-range order. However, the in-field 57-Fe Mössbauer spectra indicated a misalignment of the partial magnetic moments. On the other hand, the presence of Co considerably affected the local magnetic microstructure of the 119-Sn nuclei. This was probably due to the higher magnetic moment of Co, which induces transfer fields and polarization effects on the diamagnetic Sn atoms. Full article

New iron and cobalt bis(dithiolene) complexes [M(3cbdt)₂] (3cbdt = 3-cyanobenzene-1,2-dithiolate) were prepared as tetraphenylphosphonium (Ph₄P⁺) salts for Fe in the monoanionic state and for Co in both the dianionic and monoanionic states: (Ph₄P)₂[Fe(III)(3cbdt)₂]₂ (1); (Ph₄P)₂[Co(III)(3cbdt)₂]₂ (2); (Ph₄P)₂[Co(II)(3cbdt)₂] (3). These compounds were characterized by single-crystal X-ray diffraction, cyclic voltammetry, EPR, and static magnetic susceptibility. Their properties are discussed in comparison with the corresponding complexes based on the isomer ligand 4-cyanobenzene-1,2-dithiolate (4cbdt) and 4,5-cyanobenzene-1,2-dithiolate (dcbdt), previously described by us. The Fe(III) and the Co(III) compounds (1 and 2) are isostructural, crystallizing in the triclinic $P\overline{1}$ space group, with cis [M(III)(3cbdt)₂] complexes dimerized in a trans fashion, and the transition metal (M = Fe, Co) has a distorted 4+1 square pyramidal coordination geometry. The Co(II) compound (3) crystallizes in the triclinic $P\overline{1}$ space group, with the unit cell containing one cis and three trans inequivalent [Co(II)(3cbdt)₂] complexes with the transition metal (Co) and having a square planar coordination geometry. The Fe(III) complex (1) is EPR-silent, and the static magnetic susceptibility shows a temperature dependence typical of dimers of antiferromagnetically coupled S = 3/2 spins with −J/k_B = 233.6 K and g = 1.8. Static magnetic susceptibility measurements of compound (3) show that this Co(II) complex is paramagnetic, corresponding to an S = ½ state with g = 2, in agreement with EPR spectra showing in solid state a hyperfine structure typical of the I(⁵⁹Co) = 7/2. Static susceptibility measurements of Co(III) complex (2) showed an increase in the paramagnetic susceptibility upon warming above 100 K, which is consistent with strong AFM coupling between dimerized S = 1 units with a constant −J/k_B ~1286 K. Full article

Highly resistant to reduction nitroxides open new opportunities for structural studies of biological macromolecules in their native environment inside living cells and for functional imaging of pH and thiols, enzymatic activity and redox status in living animals. 3,4-Disubstituted nitroxides of 2,2,5,5-tetraethylpyrrolidine and pyrroline series with a functional group for binding to biomolecules and a polar moiety for higher solubility in water and for more rigid attachment via additional coordination to polar sites were designed and synthesized. The EPR spectra, lipophilicities, kinetics of the reduction in ascorbate-containing systems and the decay rates in liver homogenates were measured. The EPR spectra of all 3,4-disubstituted pyrrolidine nitroxides showed additional large splitting on methylene hydrogens of the ethyl groups, while the spectra of similar pyrroline nitroxides were represented with a simple triplet with narrow lines and hyperfine structure of the nitrogen manifolds resolved in oxygen-free conditions. Both pyrrolidine and pyrroline nitroxides demonstrated low rates of reduction with ascorbate, pyrrolidines being a bit more stable than similar pyrrolines. The decay of positively charged nitroxides in the rat liver homogenate was faster than that of neutral and negatively charged radicals, with lipophilicity, rate of reduction with ascorbate and the ring type playing minor role. The EPR spectra of N,N-dimethyl-3,4-bis-(aminomethyl)-2,2,5,5-tetraethylpyrrolidine-1-oxyl showed dependence on pH with pK_a = 3, Δa_N = 0.055 mT and Δa_H = 0.075 mT. Full article

The cytochrome P450 family consists of ubiquitous monooxygenases with the potential to perform a wide variety of catalytic applications. Among the members of this family, CYP116B5hd shows a very prominent resistance to peracid damage, a property that makes it a promising tool for fine chemical synthesis using the peroxide shunt. In this meticulous study, we use hyperfine spectroscopy with a multifrequency approach (X- and Q-band) to characterize in detail the electronic structure of the heme iron of CYP116B5hd in the resting state, which provides structural details about its active site. The hyperfine dipole–dipole interaction between the electron and proton nuclear spins allows for the locating of two different protons from the coordinated water and a beta proton from the cysteine axial ligand of heme iron with respect to the magnetic axes centered on the iron. Additionally, since new anti-cancer therapies target the inhibition of P450s, here we use the CYP116B5hd system—imidazole as a model for studying cytochrome P450 inhibition by an azo compound. The effects of the inhibition of protein by imidazole in the active-site geometry and electron spin distribution are presented. The binding of imidazole to CYP116B5hd results in an imidazole–nitrogen axial coordination and a low-spin heme Fe^III. HYSCORE experiments were used to detect the hyperfine interactions. The combined interpretation of the gyromagnetic tensor and the hyperfine and quadrupole tensors of magnetic nuclei coupled to the iron electron spin allowed us to obtain a precise picture of the active-site geometry, including the orientation of the semi-occupied orbitals and magnetic axes, which coincide with the porphyrin N-Fe-N axes. The electronic structure of the iron does not seem to be affected by imidazole binding. Two different possible coordination geometries of the axial imidazole were observed. The angles between g_x (coinciding with one of the N-Fe-N axes) and the projection of the imidazole plane on the heme were determined to be −60° and −25° for each of the two possibilities via measurement of the hyperfine structure of the axially coordinated ¹⁴N. Full article

5-Deazaflavins are analogs of naturally occurring flavin cofactors. They serve as substitutes for natural flavin cofactors to investigate and modify the reaction pathways of flavoproteins. Demethylated 5-deazaflavins are potential candidates for artificial cofactors, allowing us to fine-tune the reaction kinetics and absorption characteristics of flavoproteins. In this contribution, demethylated 5-deazariboflavin radicals are investigated (1) to assess the influence of the methyl groups on the electronic structure of the 5-deazaflavin radical and (2) to explore their photophysical properties with regard to their potential as artificial cofactors. We determined the proton hyperfine structure of demethylated 5-deazariboflavins using photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy, as well as density functional theory (DFT). To provide context, we compare our findings to a study of flavin mononucleotide (FMN) derivatives. We found a significant influence of the methylation pattern on the absorption properties, as well as on the proton hyperfine coupling ratios of the xylene moiety, which appears to be solvent-dependent. This effect is enhanced by the replacement of N5 by C5-H in 5-deazaflavin derivatives compared to their respective flavin counterparts. Full article

Nuclear Magnetic Resonance (NMR) spectroscopy is the ideal tool to address the structure, reactivity and dynamics of both inorganic and biological substances. The knowledge of nuclear spin interaction and spin dynamics is increasingly consolidated, and this allows for tailoring pulse sequences. When dealing with paramagnetic systems, several decades of research have led to the development of rule-of-the-thumb criteria for optimizing the experiments, allowing for the detection of nuclei that are in very close proximity to the metal center. In turn, the observation of these systems, coupled with the development of robust and accessible quantum chemical methods, is promising to provide a link between the spectra and the structural features through the interpretation of the electronic structure. In this review, we list the challenges encountered and propose solutions for dealing with paramagnetic systems with the greatest satisfaction. In our intentions, this is a practical toolkit for optimizing acquisition and processing parameters for routine experiments aimed at detecting signals influenced by the hyperfine interaction. The implications of paramagnetic shift and line broadening are examined. With this endeavor, we wish to encourage non-expert users to consider the application of paramagnetic NMR to their systems. Full article