77Se Nuclear Magnetic Resonance Research Articles

Elucidating Consequences of Selenium Crystallinity on Its Electrochemical Reduction in Aluminum-Selenium Batteries.

Selenium (Se) is an attractive positive electrode material for rechargeable aluminum (Al) batteries due to its high theoretical capacity of 2037 mA h g-1 and its higher electronic conductivity compared to sulfur. Selenium can undergo a series of electrochemical reactions between Se(-II) and Se(IV), resulting in a six-electron capacity per Se atom. However, existing Al-Se battery literature is inconsistent regarding the different electrochemical reactions possible, while the conditions enabling the electrochemical reduction of Se to Al2Se3 are not well understood. Here, we demonstrate that this electrochemical reduction is achievable using amorphous selenium but is suppressed for crystalline selenium. We further show that the electrochemical oxidation of Se to SeCl4, which occurs at higher potentials, reduces the long-range order of crystalline Se and enables its discharge to Al2Se3. Solid-state 77Se nuclear magnetic resonance (NMR) measurements further establish that the local Se helical structures are maintained upon the loss of crystallinity.

Synthesis and Conformational Dynamics of Selenanthrene (Oxides): Establishing an Energetic Flexibility Index for Scaffolds.

The use of new dynamic scaffolds for constructing inorganic and organometallic complexes with enhanced reactivities is an important new research direction. Toward this fundamental aim, an improved synthesis of the dynamic scaffold selenanthrene, along with its monoxide, trans-dioxide and the previously unknown trioxide, is reported. A discussion of the potential reaction mechanism for selenanthrene is provided, and all products were characterized using 1H, 13C, and 77Se nuclear magnetic resonance (NMR) spectroscopy and single-crystal X-ray crystallography. The dynamic ring inversion processes (i.e., "butterfly motion") for selenanthrene and its oxides were investigated using variable-temperature 1H NMR and density functional theory calculations. The findings suggest that selenanthrene possesses a roughly equal barrier to inversion as its sulfur analogue, thianthrene. However, selenanthrene oxides evidently possess larger inversion barriers as compared to their sulfur analogues due to the enhanced electrostatic intramolecular interactions inherent between the highly polar selenium-oxygen bond and adjacent C-H moieties. Finally, we propose a quantitative "flexibility index" in deg/(kcal/mol) for various tricyclic scaffolds to provide researchers with a comparative scale of dynamic motion across many different systems.

Taming the Biological Activity of Pd(II) and Pt(II) Complexes with Triazolato "Protective" Groups: 1H, 77Se Nuclear Magnetic Resonance and X-ray Crystallographic Model Studies with Selenocysteine to Elucidate Differential Thioredoxin Reductase Inhibition.

The biological activity of Pd(II) and Pt(II) complexes toward three different cancer cell lines as well as inhibition of selenoenzyme thioredoxin reductase (TrxR) was modulated in an unexpected way by the introduction of triazolate as a "protective group" to the inner metal coordination sphere using the iClick reaction of [M(N3)(terpy)]PF6 [M = Pd(II) or Pt(II) and terpy = 2,2':6',2″-terpyridine] with an electron-poor alkyne. In a cell proliferation assay using A549, HT-29, and MDA-MB-231 human cancer cell lines, the palladium compound was significantly more potent than the isostructural platinum analogue and exhibited submicromolar activity on the most responsive cell line. This difference was also reflected in the inhibitory efficiency toward TrxR with IC50 values of 0.1 versus 5.4 μM for the Pd(II) and Pt(II) complexes, respectively. UV/Vis kinetic studies revealed that the Pt compound binds to selenocysteine faster than to cysteine [k = (22.9 ± 0.2)·10-3 vs (7.1 ± 0.2)·10-3 s-1]. Selective triazolato ligand exchange of the title compounds with cysteine (Hcys) and selenocysteine (Hsec)─but not histidine (His) and 9-ethylguanine (9EtG)─was confirmed by 1H, 77Se, and 195Pt NMR spectroscopy. Crystal structures of three of the four ligand exchange products were obtained, including [Pt(sec)(terpy)]PF6 as the first metal complex of selenocysteine to be structurally characterized.

Spin fluctuations from Bogoliubov Fermi surfaces in the superconducting state of S-substituted FeSe

The study of the iron-based superconductor, FeSe, has resulted in various topics, such as the interplay among superconductivity, nematicity, and magnetism, Bardeen-Cooper-Schrieffer Bose-Einstein-condensation (BCS-BEC) crossover, and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity. Recently, topologically protected nodal Fermi surfaces, referred to as Bogoliubov Fermi surfaces (BFSs), have garnered much attention. A theoretical model for the S-substituted FeSe system demonstrated that BFSs can manifest under the conditions of spin-orbit coupling, multi-band systems, and superconductivity with time-reversal symmetry breaking. Here we report the observation of spin fluctuations originating from BFSs in the superconducting (SC) state via 77Se-nuclear magnetic resonance measurements to 100 mK. In a heavily S-substituted FeSe, we found an anomalous enhancement of low-energy spin fluctuations deep in the SC state, which cannot be explained by an impurity effect. Such unusual behavior implies the presence of significant spin fluctuations of Bogoliubov quasiparticles, which are associated with possible nesting properties between BFSs.

Modeling Selenoprotein Se-Nitrosation: Synthesis of a Se-Nitrososelenocysteine with Persistent Stability.

The Se-nitrosation in selenoproteins such as glutathione peroxidase and thioredoxin reductase to produce Se-nitrososelenocysteines (Sec-SeNOs) has been proposed to play crucial roles in signaling processes mediated by reactive nitrogen species and nitrosative-stress responses, although chemical evidence for the formation of Sec-SeNOs has been elusive not only in proteins but also in small-molecule systems. Herein, we report the first synthesis of a Sec-SeNO by employing a selenocysteine model system that bears a protective molecular cradle. The Sec-SeNO was characterized using 1H and 77Se nuclear magnetic resonance as well as ultraviolet/visible spectroscopy and found to have persistent stability at room temperature in solution. The reaction processes involving the Sec-SeNO provide experimental information that serves as a chemical basis for elucidating the reaction mechanisms involving the SeNO species in biological functions, as well as in selenol-catalyzed NO generation from S-nitrosothiols.

Structure of molecule-rich chalcogenide glasses along the join P4Se3-As4Se3: Results from 77Se and 31P NMR and Raman spectroscopy

The atomic structure of unusually molecule-rich glasses along the P4Se3-As4Se3 join with 0 to 70 mol% P4Se3 is studied using Raman, 77Se magic-angle-spinning (MAS) and 2D 31P phase adjusted spinning sideband (PASS) nuclear magnetic resonance (NMR) spectroscopy. When taken together, the spectroscopic results indicate the coexistence of cage-like PxAs4-xSe3 molecular moieties and corner-shared P- and As- containing pyramidal network moieties in the structure of these glasses. Increasing substitution of As with P gives rise to a monotonic increase in the total molecule:network ratio, which is shown to be consistent with the compositional variation in the glass transition temperature.

Interrelationships between nematicity, antiferromagnetic spin fluctuations, and superconductivity: Role of hotspots in FeSe1−xSx revealed by high pressure Se77 NMR study

The sulfur-substituted FeSe, FeSe$_{1-x}$S$_{x} $, is one of the unique systems that provides an independent tunability of nematicity, antiferromagnetism and superconductivity under pressure ($p$). Recently Rana et al. [Phys. Rev. B 101, 180503(R) (2020)] reported, from $^{77}$Se nuclear magnetic resonance (NMR) measurements on FeSe$_{0.91}$S$_{0.09}$ under pressure, that there exists a clear role of nematicity on the relationship between antiferromagnetic (AFM) spin fluctuations and superconducting transition temperature ($T_{\rm c}$) where the AFM spin fluctuations are more effective in enhancing $T_{\rm c}$ in the absence of nematicity than with nematicity. Motivated by the work, we carried out $^{77}$Se NMR measurements on FeSe$_{1-x}$S$_{x}$ with $x$= 0.15 and 0.29 under pressure up to 2.10 GPa to investigate the relationship in a wide range of $x$ in the FeSe$_{1-x}$S$_x$ system. Based on the new results together with the previously reported data for $x$=0 [P. Wiecki et al., Phys. Rev. B 96, 180502(R) (2017)] and 0.09 [K. Rana et al. Phys. Rev. B 101, 180503(R) (2020)], we established a $p$ - $x$ - temperature ($T$) phase diagram exhibiting the evolution of AFM spin fluctuations. From the systematic analysis of the NMR data, we found that the superconducting (SC) state in nematic state arises from a non Fermi liquid state with strong stripe-type AFM spin fluctuations while the SC state without nematicity comes from a Fermi liquid state with mild stripe-type AFM spin fluctuations. Furthermore, we show that the previously reported impact of nematicity on the relationship between AFM fluctuations and superconductivity holds throughout the wide range of $x$ from $x$ = 0 to 0.29 in FeSe$_{1-x}$S$_{x}$ under pressure. We discuss the origin of the role of nematicity in terms of the different numbers of hotspots on Fermi surfaces with and without nematicity.

NMR Evidence for Universal Pseudogap Behavior in Quasi-Two-Dimensional FeSe-Based Superconductors

Recently, by intercalating organic ions into bulk FeSe superconductors, two kinds of layered FeSe-based superconductors [(TBA) x FeSe and (CTA) x FeSe] with superconducting transition temperatures (T c) above 40 K have been discovered. Due to the large interlayer distance (∼ 15 Å), these new layered superconductors have a large resistivity anisotropy analogous to bismuth-based cuprate superconductors. Moreover, remarkable pseudogap behavior well above T c is revealed by nuclear magnetic resonance (NMR) measurements on 77Se nuclei, suggesting a preformed pairing scenario similar to that of cuprates. Here, we report another new kind of organic-ion-intercalated FeSe superconductor, (PY) x FeSe, with a reduced interlayer distance (∼ 10 Å) compared to (TBA) x FeSe and (CTA) x FeSe. By performing 77Se NMR and transport measurements, we observe a similar pseudogap behavior well above T c of ∼ 40 K and a large resistivity anisotropy of ∼10 4 in (PY) x FeSe. All these facts strongly support a universal pseudogap behavior in these layered FeSe-based superconductors with quasi-two-dimensional electronic structures.

Molecular Decomposition Routes of Diaryl Diselenide Precursors in Relation to the Phase Determination of Copper Selenides.

1H nuclear magnetic resonance (NMR), 77Se NMR, and powder X-ray diffraction (XRD) were used to monitor the thermal decomposition of diphenyl and dibenzyl diselenide precursors toward the synthesis of copper selenides. Copper was found to promote the decomposition of both precursors. The inorganic nanocrystals and organic byproducts were sensitive to the specific diaryl diselenides and the presence of oleylamine and copper. Molecular mechanistic routes are proposed. Berzelianite (Cu1.8Se), klockmannite (CuSe), umangite (Cu3Se2), and both petřı́čekite (m-CuSe2) and krutaite (p-CuSe2) were identified as products. Multistep transformations between phases were discovered through reactions with the organoselenium precursors, and organic decomposition products are proposed.

A simple, one-pot method for the synthesis of functionalized selenazole derivatives

A simple, one-pot synthesis of six highly functionalized selenazole derivatives has been achieved in moderate to good yields via a three-component condensation of variously substituted hydrazines, potassium selenocyanate, aroyl chlorides, and dimethyl acetylenedicarboxylate in acetone at room temperature. The structures of the compounds were deduced from elemental analysis and their infrared (IR), 77Se nuclear magnetic resonance (NMR), 1H NMR, and 13C NMR spectra. This new protocol offers advantages such as mild reaction conditions, easy work-up, and high yields.

Interplay of charge density waves, disorder, and superconductivity in 2H-TaSe2 elucidated by NMR

Single crystals of pristine and 6% Pd-intercalated 2H‐TaSe2 have been studied by means of 77Se nuclear magnetic resonance. The temperature dependence of the 77Se spectrum, with an unexpected line narrowing upon Pd intercalation, unravels the presence of correlated local lattice distortions far above the transition temperature of the charge density wave (CDW) order, thereby supporting a strong-coupling CDW mechanism in 2H‐TaSe2. While, the Knight shift data suggest that the incommensurate CDW transition involves a partial Fermi surface gap opening. As for spin dynamics, the 77Se spin-lattice relaxation rate as a function of temperature shows that a pseudogap behavior dominates the low-energy spin excitations even within the CDW phase, and gets stronger along with superconductivity in the Pd-6% sample. We discuss that CDW fluctuations may be responsible for the pseudogap as well as superconductivity, although the two phenomena are unlikely to be directly linked each other.

Relationship Between Nematicity, Antiferromagnetic Fluctuations, and Superconductivity in FeSe1−xSx Revealed by NMR

The S-substituted FeSe, FeSe1−xSx, under pressure (p), provides a versatile platform for studying the relationship among nematicity, antiferromagnetism, and superconductivity. Here we present a short review of the recent experimental evidence showing that nematicity has a remarkable impact on the relationship between antiferromagnetic fluctuations and superconductivity. This has been revealed by several 77Se nuclear magnetic resonance studies that have tracked the variability of antiferromagnetic fluctuations and superconducting transition temperature (Tc) as a function of x and p. Tc is roughly proportional to antiferromagnetic fluctuations in the presence or absence of nematic order suggesting the importance of antiferromagnetic fluctuations in the Cooper pairing mechanism in FeSe1−xSx. However, the antiferromagnetic fluctuations are more effective in enhancing superconductivity in the absence of nematicity as compared to when it is present. These experimental observations give renewed insights into the interrelationships between nematicity, magnetism, and superconductivity in Fe-based superconductors.

Pressure-induced reconstitution of Fermi surfaces and spin fluctuations in S-substituted FeSe

FeSe is a unique high-T_c iron-based superconductor in which nematicity, superconductivity, and magnetism are entangled with each other in the P-T phase diagram. We performed ^{77}Se-nuclear magnetic resonance measurements under pressures of up to 3.9 GPa on 12% S-substituted FeSe, in which the complex overlap between the nematicity and magnetism are resolved. A pressure-induced Lifshitz transition was observed at 1.0 GPa as an anomaly of the density of states and as double superconducting (SC) domes accompanied by different types of antiferromagnetic (AF) fluctuations. The low-T_{mathrm{c}} SC dome below 1 GPa is accompanied by strong AF fluctuations, whereas the high-T_{mathrm{c}} SC dome develops above 1 GPa, where AF fluctuations are fairly weak. These results suggest the importance of the d_{xy} orbital and its intra-orbital coupling for the high-T_{mathrm{c}} superconductivity.

Selenium‐NMR Spectroscopy in Organic Synthesis: From Structural Characterization Toward New Investigations

AbstractThis review article deals with organic synthesis from the perspective of selenium‐77 nuclear magnetic resonance (NMR) spectroscopy. Different types of organic reactions are briefly discussed, incorporating mechanism aspects through 77Se NMR observable intermediates and by‐products. The 77Se NMR experiments are demonstrated in terms of structural characterization and new insights for organic reactions. The ability to visualize different molecules in a reaction mixture provides a facile and versatile route for the development of synthetic organoselenium compounds. Thus, the review describes the strategies accomplished mainly in the past five years in the syntheses and applications of the organoselenium compounds, focusing on 77Se NMR spectroscopy.

Elucidating Reversible Electrochemical Anionic Redox in Intercalation Electrodes for Rechargeable Multivalent-Ion Batteries

Battery electrodes that enable electrochemical intercalation of multivalent ions result in multiple electron transfers per ion, respectively, yielding significant enhancements in electric charge storage capacity. When multivalent cathodes are paired with their corresponding metal anodes, potentially transformative gains in energy density are possible. However, few positive electrode materials exist that intercalate multivalent ions, in part due to the slow solid-state diffusion of highly charged multivalent ions. The chevrel phases Mo6X8, where X is a chalcogen atom (e.g., S or Se) are among the few materials known to reversibly and electrochemically intercalate both divalent (e.g. Mg2+, Zn 2+, etc.) and trivalent (Al3+) ions. Thus, understanding the ion intercalation and electron charge transfer mechanisms within these unique compounds may provide insights into the molecular-level design of new intercalation electrodes for rechargeable multivalent ion batteries.Here, we couple electrochemical and solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) measurements to reveal the concurrent aluminum-ion intercalation and electron charge transfer mechanisms in chevrel electrodes Mo6S8 and Mo6Se8 up from the molecular level. Solid-state 27Al single-pulse MAS NMR measurements were performed on the thio-chevrel Mo6S8 electrodes at different states-of-charge to observe changes in the environments and to quantify the local populations of aluminum ions within the host crystal structure, revealing both intercalated ions and additional aluminum species associated with surface layers and/or decomposition products. Al-Mo6Se8 batteries were made for the first time and their electrochemical properties were compared to the thio-chevrel Mo6S8. Solid-state 77Se and 95Mo MAS NMR measurements on the seleno-chevrel Mo6Se8 at charged and discharged states revealed that electrons are transferred preferentially to the anionic chalcogen framework upon aluminum ion intercalation, as opposed to the transition metals (Mo). Anionic redox was further confirmed by density functional theory (DFT) calculations and X-ray absorption near-edge structure (XANES) measurements. Lastly, we generalize the results to other ion valencies, demonstrating reversible electrochemical anionic redox in the Chevrel phase upon divalent Zn2+ cations in aqueous Zn batteries.Overall, for the first time, we demonstrate reversible electrochemical anionic redox as a charge storage mechanism for rechargeable batteries, which preserves the crystalline framework structure and does not involve the breaking and forming of chemical bonds. This charge storage mechanism is fundamentally different than that observed for lithium-ion intercalation into transition metal oxides, where the electrons are stored within the d-orbitals of the transition metal. The results suggest materials design principles aimed at designing multivalent-ion intercalation electrodes with improved electrochemical properties. Figure 1

Chemical order in binary Se-Te glasses: Results from high-resolution 2D 77Se and 125Te MATPASS NMR spectroscopy

The structure of binary TexSe100-x (0 ≤ x ≤ 35) glasses is studied using high-resolution two-dimensional magic-angle-turning phase-adjusted spinning sidebands (2D MATPASS) 77Se and 125Te nuclear magnetic resonance (NMR) spectroscopy. The 77Se (125Te) isotropic NMR spectra are consistent with the presence of three different kinds of Se (Te) sites, which correspond to Se–Se (Te)–Se, Te–Se (Te)–Se, and Te–Se (Te)–Te environments. The compositional variation of the relative fractions of the three Se sites indicates that the Se and Te atoms are randomly distributed to form [Se, Te]n copolymer chains. The 77Se and 125Te NMR isotropic chemical shifts for Se and Te atoms in isostructural environments are found to display a linear correlation. The systematic compositional variation of the CSA of Se–Se–Se sites can be related to the appearance of Te atoms as next-nearest neighbors of Se atoms in the [Se, Te]n chains.

A Novel Synthesis of Poly(Ester-Alt-Selenide)s by Ring-Opening Copolymerization of γ-Selenobutyrolactone and Epoxy Monomer.

Ring-opening copolymerization (ROCOP) is an effective means to prepare functionalized polyester. In this work, a type of selenide-containing polyesters with controllable structure, molecular weight, and molecular weight distribution was successfully prepared by ROCOP of γ-selenobutyrolactone and epoxy compounds. The influence of the catalyst, solvent, and reaction temperature on the reaction efficiency was examined. Then, kinetic study was investigated under an optimized condition. The structure of the copolymers was carefully characterized by nuclear magnetic resonance (NMR), 1H NMR, 13C NMR, and 77Se NMR, Matrix-assisted laser-desorption-ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and size exclusion chromatography (SEC). The resulting polymers showed a linear structure with a sequence regulated backbone repeating unit of ester-selenide. On this basis, some typical epoxides were investigated to verify the scope of the polymerization system. Due to the “living”/controlled characteristics of this ROCOP, multiblock, amphiphilic, and stereotactic copolymers could be prepared with a pre-designed structure. As expected, the selenide-containing amphiphilic copolymer could self-assemble to micelles and showed an oxidative response.

Characterization of (Boc-Cys/Sec-NHMe)2 and (Boc-Cys/Sec-OMe)2 : Evidence of local conformational difference between disulfide and diselenide.

Conformations of disulfide and diselenide were compared in (Boc-Cys/Sec-NHMe)2 and (Boc-Cys/Sec-OMe)2 using X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, density functional theory (DFT), and circular dichroism (CD) spectroscopy. Conformations of disulfide/diselenide in polypeptides are defined based on the sign of side chain torsion angle χ3 (-CH2 -S/Se-S/Se-CH2 -); negative indicates left-handed and positive indicates right-handed orientation. In the crystals of (Boc-Cys-OMe)2 and (Boc-Sec-OMe)2 , the disulfide exhibits a left-handed and the diselenide a right-handed orientation. Characterization of cystine and selenocystine derivatives in solution using 1 H-NMR, natural abundant 77 Se NMR, 2D-ROESY, and chemical shift analysis coupled to DMSO titration has indicated the symmetrical nature and antiparallel orientation of Cys/Sec residues about the disulfide/diselenide bridges. Structural calculations of cystine and selenocystine derivatives using DFT further support the antiparallel orientation of Cys/Sec residues about disulfide/diselenide. The far-ultraviolet (UV) region CD spectra of cystine and selenocystine derivatives have exhibited the negative Cotton effect (CE) for disulfide and positive for diselenide confirming the difference in the conformational preference of disulfide and diselenide. In the previously reported polymorphic structure of (Boc-Sec-OMe)2 , the diselenide has right-handed orientation. In the X-ray structures of disulfide and diselenide analogues of Escherichia coli protein encoded by curli specific gene C (CgsC) retrieved from Protein Databank (PDB), disulfide has left-handed and the diselenide right-handed orientation. The current report provides the evidence for the local conformational difference between a disulfide and a diselenide group under unconstrained conditions, which may be useful for the rational replacement of disulfide by diselenide in polypeptide chains.

Spin-Orbital-Intertwined Nematic State in FeSe

The importance of the spin-orbit coupling (SOC) effect in Fe-based superconductors (FeSCs) has recently been under hot debate. Considering the Hund's coupling-induced electronic correlation, the understanding of the role of SOC in FeSCs is not trivial and is still elusive. Here, through a comprehensive study of 77Se and 57Fe nuclear magnetic resonance, a nontrivial SOC effect is revealed in the nematic state of FeSe. First, the orbital-dependent spin susceptibility, determined by the anisotropy of the 57Fe Knight shift, indicates a predominant role from the 3dxy orbital, which suggests the coexistence of local and itinerant spin degrees of freedom (d.o.f.) in the FeSe. Then, we reconfirm that the orbital reconstruction below the nematic transition temperature (Tnem ~ 90 K) happens not only on the 3dxz and 3dyz orbitals but also on the 3dxy orbital, which is beyond a trivial ferro-orbital order picture. Moreover, our results also indicate the development of a coherent coupling between the local and itinerant spin d.o.f. below Tnem, which is ascribed to a Hund's coupling-induced electronic crossover on the 3dxy orbital. Finally, due to a nontrivial SOC effect, sizable in-plane anisotropy of the spin susceptibility emerges in the nematic state, suggesting a spin-orbital-intertwined nematicity rather than simply spin- or orbital-driven nematicity}. The present work not only reveals a nontrivial SOC effect in the nematic state but also sheds light on the mechanism of nematic transition in FeSe.

Separate tuning of nematicity and spin fluctuations to unravel the origin of superconductivity in FeSe

The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high-temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc.