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The proton Knight shifts and spin-lattice relaxation times have been measured in crystalline and amorphous Ze2PdH(x). Core polarization from the Zr d-band dominates the proton hyperfine interactions. The density of Fermi level d-electron states is reduced in the amorphous phase relative to the electron density in crystalline Zr2PdH(x).
A series of solid solutions, CuFe 2–x Co x Ge 2 (x = 0, 0.2, 0.4, 0.8, and 1.0), have been synthesized by arc-melting and characterized by powder X-ray and neutron diffraction, magnetic measurements, Mössbauer spectroscopy, and electronic band structure calculations. All compounds crystallize in the CuFe 2 Ge 2 structure type, which can be considered as a three-dimensional framework built of fused MGe6 octahedra and MGe 5 trigonal bipyramids (M = Fe and Co), with channels filled by rows of Cu atoms. As the Co content (x) increases, the unit cell volume decreases in an anisotropic fashion: the b and c lattice parameters decrease while the a parameter increases. The changes in all the parameters are nearly linear, thus following Vegard’s law. CuFe 2 Ge 2 exhibits two successive antiferromagnetic (AFM) orderings, corresponding to the formation of a commensurate AFM structure, followed by an incommensurate AFM structure observed at lower temperatures. Additionally, as the Co content increases, the AFM ordering temperature (TN) gradually decreases, and only one AFM transition is observed for x ≥ 0.2. The magnetic behavior of unsubstituted CuFe 2 Ge 2 was found to be sensitive to the preparation method. The temperature-dependent zero-field 57 Fe Mössbauer spectra reveal two hyperfine split components that evolve in agreement with the two consecutive AFM orderings observed in magnetic measurements. In contrast, the field-dependent spectra obtained for fields ≥ 2 T reveal a parallel arrangement of the moments associated with the two crystallographically unique metal sites. Electronic band structure calculations and chemical bonding analysis reveal a mix of strong M–M antibonding and non-bonding states at the Fermi level, in support of the overall AFM ordering observed in zero field. The substitution of Co for Fe reduces the population of the M–M antibonding states and the overall density of states at the Fermi level, thus suppressing the T N value.
Optically active spin defects in solids are leading candidates for quantum sensing and quantum networking. Recently, single spin defects were discovered in hexagonal boron nitride (hBN), a layered van der Waals (vdW) material. Owing to its two-dimensional structure, hBN allows spin defects to be positioned closer to target samples than in three-dimensional crystals, making it ideal for atomic-scale quantum sensing, including nuclear magnetic resonance (NMR) of single molecules. However, the chemical structures of these defects remain unknown and detecting a single nuclear spin with a hBN spin defect has been elusive. Here we report the creation of single spin defects in hBN using 13 C ion implantation and the identification of three distinct defect types based on hyperfine interactions. We observed both S = 1/2 and S = 1 spin states within a single hBN spin defect. We demonstrated atomic-scale NMR and coherent control of individual nuclear spins in a vdW material, with a π-gate fidelity up to 99.75% at room temperature. By comparing experimental results with density functional theory (DFT) calculations, we propose chemical structures for these spin defects. Our work advances the understanding of single spin defects in hBN and provides a pathway to enhance quantum sensing using hBN spin defects with nuclear spins as quantum memories.
Reaction centers from Rb. sphaeroides with residue M265 mutated from isoleucine to threonine, serine, and asparagine (M265IT, M265IS, and M265IN, respectively) in the Q A –· state are studied by high-resolution ESEEM and ENDOR spectroscopies to investigate the structural characteristics of these mutants influencing the redox properties of the Q A site. All three mutants decrease the redox midpoint potential (E m ) of Q A by ~0.1 V, yet the mechanism for this drop in E m is unclear. In this work, we examine (i) the hydrogen bonding interactions between Q A –· and residues histidine M219 and alanine M260, (ii) the electron spin density distribution of the semiquinone, and (iii) the orientations of the ubiquinone methoxy substituents. 13 C measurements show no significant contribution of methoxy dihedral angles to the observed decrease in E m for the Q A mutants. Instead, 14 N three-pulse ESEEM data suggest electrostatic or hydrogen bond formation between the mutated M265 side chain and His-M219 N δ may be involved in the observed lowering of the Q A midpoint potential. For mutant M265IN, analysis of the proton hyperfine couplings reveals a weakened hydrogen bond network, resulting in an altered Q A –· spin density distribution. Here, the magnetic resonance study presented here is therefore most consistent with an electrostatic or structural perturbation of the His-M219 N δ hydrogen bond in these mutants as a mechanism for the ~0.1 V decrease in Q A E m .
A selective heterogeneous catalyst with the superoxide anion O 2 •– stable at room temperature, generated by treating Ti(OR) 4 (R=iPr, nBu) with H 2 O 2 , was previously reported and used for the oxidation of organic compounds. Furthermore, it was proposed that exceptional stability of the O 2 •– is resulted from its stabilization near Ti +4 cation on the TiO 2 surface with participation of water molecules and/or OH groups. In this article, we performed high-resolution pulsed EPR study on the O 2 •– in this catalyst with the aim to obtain quantitative information about its interactions with the surrounding molecules. Our data has shown that the O 2 •– is involved in weak hyperfine interactions with protons in the nearest environment, which do not exceed a value of 2 MHz. It corresponds to an Os…H distance of ≥ 3 Å that exclude the formation of hydrogen bonds. The analysis of possible structures of O 2 •– adsorption on TiO 2 surface taking into account our results and lack of radical mobility at room temperature previously observed in EPR spectra led us to suggest that the O 2 •– is stabilized in the form of superoxo bridge coordinated with two titanium atoms.
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Abstract The maturation pathway for the nickel-dependent enzyme urease utilizes the protein UreE as a metallochaperone to supply Ni(II) ions. In Helicobacter pylori urease maturation also requires HypA and HypB, accessory proteins that are commonly associated with hydrogenase maturation. Herein we report on the characterization of a protein complex formed between HypA and the UreE2 dimer. Nuclear magnetic resonance (NMR) coupled with molecular modelling show that the protein complex apo, Zn-HypA•UreE2, forms between the rigorously conserved Met-His-Glu (MHE motif) Ni-binding N-terminal sequence of HypA and the two conserved His102A and His102B located at the dimer interface of UreE2. This complex forms in the absence of Ni(II) and is supported by extensive protein contacts that include the use of the C-terminal sequences of UreE2 to form additional strands of β-sheet with the Ni-binding domain of HypA. The Ni-binding properties of apo, Zn-HypA•UreE2 and the component proteins were investigated by isothermal titration calorimetry using a global fitting strategy that included all of the relevant equilibria, and show that the Ni,Zn-HypA•UreE2 complex contains a single Ni(II)-binding site with a sub-nanomolar KD. The structural features of this novel Ni(II) site were elucidated using proteins produced with specifically deuterated amino acids, protein point mutations, and the analyses of X-ray absorption spectroscopy, hyperfine shifted NMR features, as well as molecular modeling coupled with quantum-mechanical calculations. The results show that the complex contains a six-coordinate, high-spin Ni(II) site with ligands provided by both component proteins.
Pyrimidine has two in-plane CH(δ+)/N̈(δ–)/CH(δ+) binding sites that are complementary to the (δ–/2δ+/δ–) quadrupole moment of CO 2 . For this study, we recorded broadband microwave spectra over the 7.5–17.5 GHz range for pyrimidine-(CO 2 ) n with n = 1 and 2 formed in a supersonic expansion. Based on fits of the rotational transitions, including nuclear hyperfine splitting due to the two 14 N nuclei, we have assigned 313 hyperfine components across 105 rotational transitions for the n = 1 complex and 208 hyperfine components across 105 rotational transitions for the n = 2 complex. The pyrimidine-CO 2 complex is planar, with CO 2 occupying one of the quadrupolar binding sites, forming a structure in which the CO 2 is stabilized in the plane by interactions with the C–H hydrogens adjacent to the nitrogen atom. This structure is closely analogous to that of the pyridine-CO 2 complex studied previously by (Doran, J. L. J. Mol. Struct. 2012, 1019, 191–195). The fit to the n = 2 cluster gives rotational constants consistent with a planar cluster of C 2v symmetry in which the second CO 2 molecule binds in the second quadrupolar binding pocket on the opposite side of the ring. The calculated total binding energy in pyrimidine-CO 2 is –13.7 kJ mol –1 , including corrections for basis set superposition error and zero-point energy, at the CCSD(T)/ 6-311++G(3df,2p) level, while that in pyrimidine-(CO 2 ) 2 is almost exactly double that size, indicating little interaction between the two CO 2 molecules in the two binding sites. The enthalpy, entropy, and free energy of binding are also calculated at 300 K within the harmonic oscillator/rigid-rotor model. This model is shown to lack quantitative accuracy when it is applied to the formation of weakly bound complexes.
Abstract Here we have performed single-crystal X-ray diffraction (SCXRD) experiments on two high-quality crystal platelets of (Al,Fe)-bearing bridgmanite (Mg0.88Fe0.0653+Fe0.0352+Al0.03)(Al0.11Si0.90)O3 (Fe10-Al14-Bgm) up to 64.6(6) GPa at room temperature in a Boehler-Almax type diamond-anvil cell. Refinements on the collected SCXRD patterns reveal reliable structural information of single-crystal Fe10-Al14-Bgm, including unit-cell parameters, atomic coordinates, and anisotropic displacement parameters. Together with Mössbauer and electron microprobe analyses, our best single-crystal refinement model indicates that the sample contains ~6.5 mol% Fe3+, 3.5 mol% Fe2+, and 3 mol% Al3+ in the large pseudo-dodecahedral site (A site), and ~11 mol% Al3+ in the small octahedral site (B site). This may indicate that Al3+ in bridgmanite preferentially occupies the B site. Our results show that the compression of Fe10-Al14-Bgm with pressure causes monotonical decreases in the volumes of AO12 pseudo-dodecahedron and BO6 octahedron (VA and VB, respectively) as well as the associated A-O and B-O bond lengths. The interatomic angles of B-O1-B and B-O2-B decrease from 145.2–145.8° at 4.2(1) GPa to 143.3–143.5° at 64.6(6) GPa. Quantitative calculations of octahedral tilting angles (Ф) show that Ф increases smoothly with pressure. We found a linear relationship between the polyhedral volume ratio and the Ф in the bridgmanite with different compositions: VA/VB = –0.049Φ + 5.549. Our results indicate an increased distortion of the Fe10-Al14-Bgm structure with pressure, which might be related to the distortion of A-site Fe2+. The local environmental changes of A-site Fe2+ in bridgmanite could explain previous results on the hyperfine parameters, abnormal lattice thermal conductivity, mean force constant of iron bonds and other physical properties, which in turn provide insights into our understanding on the geophysics and geochemistry of the planet.
The synthesis of molecules with strong coupling between electronic and nuclear spins represents an important challenge in molecular quantum information science. Here, we report the synthesis and characterization of the divalent lutetium metallocene complexes Lu(Cp Me 5 )(Cp iPr 5 ) (Cp Me 5 = pentamethylcyclopentadienyl; Cp iPr 5 = pentaisopropylcyclopentadienyl), Lu(Cp iPr 4 Et ) 2 (Cp iPr 4 Et = ethyltetraisopropylcyclopentadienyl), and Lu(Cp iPr 4 ) 2 (Cp iPr 4 = tetraisopropylcyclopentadienyl). The molecular structures of these complexes, as determined through singlecrystal X-ray diffraction, feature a common bent sandwich geometry, with average Cp–Lu–Cp angles ranging from 159.9° to 152.6°. Analysis of continuous-wave electron paramagnetic resonance (EPR) spectra for the complexes reveals nearly isotropic g tensors with only a slight deviation from that of a free electron. Moreover, an extremely large splitting of the eight-line spectra indicates the presence of strong hyperfine coupling, and simulations provide isotropic hyperfine coupling constants of A iso = 4.38, 4.30, and 4.17 GHz across the series, where the value of A iso is found to decrease as the Cp–Lu–Cp angle becomes more acute. Notably, these values are the largest yet observed for any lanthanide complex. Moreover, EPR and computational analysis show that the large values of A iso stem from large s-orbital character up to 41.2% in the corresponding singly occupied molecular orbitals. To our knowledge, this degree of s-character in a molecular orbital is the largest yet reported for an open-shell isolable complex. These results outline a general strategy toward the isolation of paramagnetic molecules with strong hyperfine coupling and highly isotropic doublet electronic ground states.
A deep search for the J = 4-3 and 5-4 fine-structure components of the N(KK) = 4(04)-3(13) rotational transition of methylene toward the hot core of the Orion KL nebula is conducted. An approximate 4-sigma emission feature which is frequency-coincident with a hyperfine blend of the J = 4-3 component is detected, and weak features (about 2-3 sigma) frequency-coincident with the resolved F = 6-5 and 5-4 hyperfine components of the J = 5-4 component are observed. The relative intensities of these spectral features and their observational repeatability suggest that assignment to interstellar CH2 is likely correct, although the result must be confirmed.
In the course of four funding cycles of this project, its research focused on the exploration and understanding of spin-dependent electronic processes in organic semiconductors. It aimed to investigate the potential of harnessing the spin degree of freedom in organic materials for various applications such as spin electronics, quantum information, sensors, and as well as quantum coherent spin phenomena of charge carrier states in organic semiconductors, with the aim to develop room-temperature-based quantum applications. Specifically, the project aimed to: Apply new diagnostic tools, i.e. spin spectroscopy techniques, for paramagnetic charge carrier states in organic semiconductors, based on coherent control with pulsed electron spin resonance to understand spin relaxation and spin mixing mechanisms in organic semiconductor materials and devices such as OLEDs. Explore the nature of charge carrier spin-coupling and pathways for its manipulation to control the physical behaviors of these materials. This involves investigating spin interactions through materials structure, composition, morphological dependencies, isotopic effects, and externally applied electric and magnetic fields, with the goal to manipulate electrical and optical material properties by adjusting spin-orbit, spin-dipolar, spin-exchange, as well as spin-hyperfine interactions to open up this materials class for new applications. Study spin-dependent charge carrier transport and recombination and how these observables are governed by spin-propagation, especially coherent spin propagation as well as collective spin-effects such as the spin-Dicke effect and resonant multi-photon magnetic dipole transitions. The project utilized pulsed electrically detected magnetic resonance (pEDMR) and pulsed optically detected magnetic resonance (pODMR) techniques over a wide frequency range. These techniques enabled the observation and quantification of various spin-spin coupling types, including spin-exchange, spin-dipolar, hyperfine, and spin-orbit interactions. The project will leverage low- to mid-frequency pEDMR/pODMR facilities and collaboration with the National High Magnetic Field Laboratory.
Mo-dependent nitrogenase is a major contributor to global biological N 2 reduction, which sustains life on Earth. Its multi-metallic active-site FeMo-cofactor (Fe 7 MoS 9 C-homocitrate) contains a carbide (C 4– ) centered within a trigonal prismatic CFe 6 core resembling the structural motif of the iron carbide, cementite. The role of the carbide in FeMo-cofactor binding and activation of substrates and inhibitors is unknown. Here, to explore this role, the carbide has been in effect selectively enriched with 13 C, which enables its detailed examination by ENDOR/ESEEM spectroscopies. 13 C-carbide ENDOR of the S = 3/2 resting state (E 0 ) is remarkable, with an extremely small isotropic hyperfine coupling constant, C a = +0.86 MHz. Turnover under high CO partial pressure generates the S = 1/2 hi-CO state, with two CO molecules bound to FeMo-cofactor. This conversion surprisingly leaves the small magnitude of the 13 C carbide isotropic hyperfine-coupling constant essentially unchanged, C a = –1.30 MHz. This indicates that both the E 0 and hi-CO states exhibit an exchange-coupling scheme with nearly cancelling contributions to C a from three spin-up and three spin-down carbide-bound Fe ions. In contrast, the anisotropichyperfine coupling constant undergoes a symmetry change upon conversion of E 0 to hi-CO that may be associated with bonding and coordination changes at Fe ions. In combination with the negligible difference between CFe 6 core structures of E 0 and hi-CO, these results suggest that in CO-inhibited hi-CO the dominant role of the FeMo-cofactor carbide is to maintain the core structure, rather than to facilitate inhibitor binding through changes in Fe-carbide covalency or tretching/breaking of carbide–Fe bonds.
The primary electron donor P 700 of the photosystem I ( PS I) is a heterodimer consisting of two chlorophyll molecules. A series of electron-transfer events immediately following the initial light excitation leads to a stabilization of the positive charge by its cation radical form, P 700 +• . Here, the electronic structure of P 700 +• and, in particular, its asymmetry with respect to the two chlorophyll monomers is of fundamental interest and is not fully understood up to this date. Here, we apply multifrequency X- (9 GHz) and Q-band (35 GHz) hyperfine sublevel correlation (HYSCORE) spectroscopy to investigate the electron spin density distribution in the cation radical P 700 +• of PS I from a thermophilic cyanobacterium Thermosynechococcus elongatus . Six 14 N and two 1 H distinct nuclei have been resolved in the HYSCORE spectra and parameters of the corresponding nuclear hyperfine and quadrupolar hyperfine interactions were obtained by combining the analysis of HYSCORE spectral features with direct numerical simulations. Based on a close similarity of the nuclear quadrupole tensor parameters, all of the resolved 14 N nuclei were assigned to six out of total eight available pyrrole ring nitrogen atoms (i.e., four in each of the chlorophylls), providing direct evidence of spin density delocalization over the both monomers in the heterodimer. Using the obtained experimental values of the 14 N electron-nuclear hyperfine interaction parameters, the upper limit of the electron spin density asymmetry parameter is estimated as R A/B upper = 7.7 ± 0.5, while a tentative assignment of 14 N observed in the HYSCORE spectra yields R B/A = 3.1 ± 0.5.
Ni and nitrogen-doped carbons are selective catalysts for CO 2 reduction to CO (CO 2 R), but the hypothesized NiNx active sites are challenging to probe with traditional characterization methods. Here, we synthesize 61 Ni-enriched model catalysts, termed 61 NiPACN, in order to apply 61 Ni Mössbauer spectroscopy using synchrotron radiation ( 61 Ni-SR-MS) to characterize the structure of these atomically dispersed NiNx sites. First, we demonstrate that the CO 2 R results and standard characterization techniques (SEM, PXRD, XPS, XANES, EXAFS) point to the existence of dispersed Ni active sites. Then, 61 Ni-SR-MS reveal significant internal magnetic fields of ~5.4 T, which is characteristic of paramagnetic, high-spin Ni 2+ , in the 61NiPACN samples. Finally, theoretical calculations for a variety of Ni-N x moieties confirm that high-spin Ni 2+ is stable in non-planar, tetrahedrally distorted geometries, which results in calculated isotropic hyperfine coupling that is consistent with 61 Ni-SR-MS measurements.
Amorphous-oxide layers in thin-film capacitors are linked to reduced transmon-qubit T 1 coherence times. Ta -based capacitors outperform Nb -based ones, suggesting that amorphous Ta 2 O 5 − x is less lossy than Nb 2 O 5 − x . We investigate the microscopic features of these amorphous oxides using ab initio molecular dynamics and density functional theory, revealing the origins of the superior performance of Ta 2 O 5 − x . We establish that oxygen deficiency is less likely to occur in amorphous Ta 2 O 5 − x than in Nb 2 O 5 − x for 0 ≤ x ≤ 0.25 and that for a given oxygen deficiency x , metal Ta — Ta bond formation is enhanced. Such bonds, which are accommodated by structural flaws in the amorphous network, capture electrons better than in amorphous Nb 2 O 5 − x . These thermochemical differences quench or highly suppress magnetic moments in amorphous Ta 2 O 5 − x and eliminate a potential source of quasiparticles and magnetic flux noise. We also show that hyperfine couplings between Nb nuclei and local magnetic moments in Nb 2 O 5 − x can form “two-level systems” (TLSs) or “two-level fluctuators” with energy splittings of 100–1000 MHz or higher. This reveals a TLS mechanism in amorphous Nb 2 O 5 − x oxide layers that is likely inactive in Ta 2 O 5 − x . Our work provides a fundamental understanding of the materials chemistry and limitations imposed by native oxides of superconducting qubits that can be used to guide materials selection and processing.
Pr is the only rare earth which, when substituted for Y in YBa2Cu3O7, significantly alters the superconducting transition temperature T(sub c) without changing the crystal structure. For YxPr1-xBa2Cu3O7-delta with delta approx. equal to 0, T(sub c) is reduced rapidly as x is increased, reaching zero for x about 0.5. For x above 0.5 the compound is antiferromagnetic with a Neel temperature that increases with increasing x, rising to above room temperature for x near 1. A similar behavior is observed when the oxygen deficit delta is increased from zero to 1 with x=0. For the case of Pr substitution, the drop in T(sub c) is believed due to magnetic interactions. For the case of varying delta with x=0, the drop can be attributed to a combination of magnetic interactions, band filling, and changes in crystal structure. To study these effects, the Mossbauer effect of 57 Fe atoms substituted for the Cu atoms has been observed as a function of delta, x, and temperature. The observed spectra are all well described by a two quadrupole-split pairs, a central singlet, and a six-line magnetic hyperfine field pattern. For several Pr compositions both delta and temperature were varied, and the results support the hypothesis that a magnetic interaction exists between the Fe in the Cu lattice and the substitutional Pr atoms.
Velocity-resolved spectra of the C-12 II 157.8-micron 2P3/2-2P1/2 fine-structure line in the M42 region of Orion have been obtained. Observations at 0.8 km/s resolution with a laser heterodyne spectrometer show multiple velocity components in some locations, with typical linewidths of 3-5 km/s. Spectra of theta C and BN-KL also show weak emission from the F = 2-1 hyperfine component of the equivalent C-13 II line. From the observed C-12 II/C-13 II line intensity ratios, it is deduced that the C-12 II emission is optically thick with tau of about 5 at both positions.