Se NMR Measurements Research Articles

Selenate—An internal chemical shift standard for aqueous 77Se NMR spectroscopy

The 77 Se NMR spectra of selenate were studied under various circumstances, such as concentration, pH, temperature, ionic strength, and D2 O:H2 O ratio, in order to examine its potential as a water-soluble internal chemical shift standard. The performance of selenate as a chemical shift reference and that of other attempted ones from the literature (dimethyl selenide, tetramethylsilane/TMS, and 3-(trimethylsilyl)propane-1-sulfonate/DSS) was also explored. The uncertainty in the resulting chemical shift relative to the effective spectral width is comparable to that of DSS. Compared to the currently prevalent water-soluble external chemical shift reference, selenic acid solution, the properties of internal selenate are much more favorable in terms of ease of use. We have also demonstrated that selenate can be used in reducing media, which is inevitable for the analysis of selenol compounds. Thus, it can be stated that sodium selenate is a robust internal chemical shift reference in aqueous media for 77 Se NMR measurements; the chemical shift of this reference in a solution containing 5V/V% D2 O at 25°C and 0.15 mol·dm-3 ionic strength is 1048.65 ppm relative to 60 V/V% dimethyl selenide in CDCl3 and 1046.40 ppm relative to the 1 H signal of 0.03 V/V% TMS in CDCl3 . In summary, a water-soluble, selenium-containing internal chemical shift reference compound was introduced for 77 Se NMR measurements for the first time in the literature, and with the aforementioned results all previous 77 Se measurements can be converted to a unified scale defined by the International Union of Pure and Applied Chemistry.

Pressure-induced Lifshitz transition in FeSe$_{0.88}$S$_{0.12}$ probed via $^{77}$Se-NMR

Recently, FeSe$_{1-x}$S$_x$ systems have received much attention because of the unique pressure-temperature phase diagram. We performed $^{77}$Se-NMR measurements on a single crystal of FeSe$_{0.88}$S$_{0.12}$ to investigate its microscopic properties. The shift of $^{77}$Se spectra exhibits anomalous enhancement at $1.0~\mathrm{GPa}$, suggesting a topological change in the Fermi surfaces, so-called Lifshitz transition, occurs at $1.0~\mathrm{GPa}$. The magnetic fluctuation simultaneously changes its properties, which implies a change in the dominant nesting vector.
 Edited by: A. Goñi, A. Cantarero, J. S. Reparaz

77Se-NMR Study under Pressure on 12%-S Doped FeSe

The 12%-S doped FeSe system has a high Tc of 30 K at a pressure of 3.0 GPa. We have successfully investigated its microscopic properties for the first time via $^{77}$Se-NMR measurements under pressure. The antiferromagnetic (AFM) fluctuations at the optimal pressure (~3 GPa) exhibited unexpected suppression compared with the AFM fluctuations at ambient pressure, even though the optimal pressure is close to the phase boundary of the AFM phase induced at the high-pressure region. In addition, we revealed that the SC phase at an applied field of 6.02 T exhibited a remarkable double-dome structure in the pressure-temperature phase diagram, unlike the SC phase at zero field.

Persistent correlation between superconductivity and antiferromagnetic fluctuations near a nematic quantum critical point in FeSe1−xSx

We present $^{77}$Se-NMR measurements on FeSe$_{1-x}$S$_x$ samples with sulfur content $x=0,9,15$ and $29\%$. Twinned nematic domains are observed in the NMR spectrum for all samples except $x=29\%$. The NMR spin-lattice relaxation rate shows that magnetic fluctuations are initially enhanced between $x=0\%$ and $x=9\%$, but are strongly suppressed for higher $x$ values. The observed behavior of the magnetic fluctuations parallels the superconducting transition temperature $T_c$ in these materials, providing strong evidence for the primary importance of magnetic fluctuations for superconductivity, despite the presence of nematic quantum criticality in this system.

NMR evidence for static local nematicity and its cooperative interplay with low-energy magnetic fluctuations in FeSe under pressure

We present $^{77}$Se-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, $T_{\rm s}$. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe.

(77)Se chemical shift tensor of L-selenocystine: experimental NMR measurements and quantum chemical investigations of structural effects.

The genetically encoded amino acid selenocysteine and its dimeric form, selenocystine, are both utilized by nature. They are found in active sites of selenoproteins, enzymes that facilitate a diverse range of reactions, including the detoxification of reactive oxygen species and regulation of redox pathways. Due to selenocysteine and selenocystine's specialized biological roles, it is of interest to examine their (77)Se NMR properties and how those can in turn be employed to study biological systems. We report the solid-state (77)Se NMR measurements of the L-selenocystine chemical shift tensor, which provides the first experimental chemical shift tensor information on selenocysteine-containing systems. Quantum chemical calculations of L-selenocystine models were performed to help understand various structural effects on (77)Se L-selenocystine's chemical shift tensor. The effects of protonation state, protein environment, and substituent of selenium-bonded carbon on the isotropic chemical shift were found to be in a range of ca. 10-20 ppm. However, the conformational effect was found to be much larger, spanning ca. 600 ppm for the C-Se-Se-C dihedral angle range of -180° to +180°. Our calculations show that around the minimum energy structure with a C-Se-Se-C dihedral angle of ca. -90°, the energy costs to alter the dihedral angle in the range from -120° to -60° are within only 2.5 kcal/mol. This makes it possible to realize these conformations in a protein or crystal environment. (77)Se NMR was found to be a sensitive probe to such changes and has an isotropic chemical shift range of 272 ± 30 ppm for this energetically favorable conformation range. The energy-minimized structures exhibited calculated isotropic shifts that lay within 3-9% of those reported in previous solution NMR studies. The experimental solid-state NMR isotropic chemical shift is near the lower bound of this calculated range for these readily accessible conformations. These results suggest that the dihedral information may be deduced for a protein with appropriate structural models. These first-time experimental and theoretical results will facilitate future NMR studies of selenium-containing compounds and proteins.

Local Spin Susceptibility of α-D2I3(D = bis(ethylendithio)tetraselenafulvalene (BETS) and bis(ethylendithio)dithiadiselenafulvalene (BEDT-STF)) Studied by77Se NMR

We performed static susceptibility and 77 Se NMR measurements on α-(BETS) 2 I 3 and α-(BEDT-STF) 2 I 3 which are analogues of the “zero gap” system, α-(BEDT-TTF) 2 I 3 to systematically compare the electronic properties of the α-type I 3 systems, where BETS, BEDT-STF, and BEDT-TTF are bis(ethylendithio)tetraselenafulvalene, bis(ethylendithio)dithiadiselenafulvalene, and bis(ethylendithio)tetrathiafulvalene, respectively. From the analysis of the NMR lineshape for a polycrystalline sample at high temperatures, which are explained as an admixture of several powder patterns, we found that the local spin susceptibility is not uniform but at least one of the molecular sites has very small susceptibility. It was confirmed from single crystal data that there are three kinds of molecules with different local susceptibilities in a unit cell. The analysis of the angular dependence on a single crystal clarified that the relative magnitude of the local susceptibilities of A, B, and C molecules, χ loc A , χ loc B , and χ loc C , are similar to those of the BEDT-TTF salt; χ loc C > χ loc A > χ loc B . On the contrary, the temperature dependence of spin susceptibility and relaxation rate suggested a gap opening well above the metal–insulator transition, whereas no indication of symmetry breaking of the lattice was detected.

RF Surface Impedance and77Se-NMR Studies of FeSe: Effect of the Sample Quality

We report the results of RF surface impedance measurements and 77 Se-NMR measurements on improved-quality FeSe. The linewidth of 77 Se-NMR spectrum for the improved FeSe is one third of that for FeSe 0.92 , evidencing the homogeneity of the sample. From the field dependence of the R F surface impedance measurements, the upper critical field H c2 (0) is estimated to be ≈26 T. We also measured nuclear spin–lattice relaxation rate 1/ T 1 in the temperature range of 190–2 K under external magnetic fields of 2, 7, and 16 T. In the normal state below 100 K, 1/ T 1 T shows a Curie–Weiss like temperature dependence, suggesting antiferromagnetic spin fluctuations develop below the structural transition. In the superconducting state, 1/ T 1 S measured at H =2, 7, and 16 T exhibit marked decrease with decreasing temperature due to an opening of the superconducting gap.

Coexistence of localized and itinerant electronic states in the multiband iron-based superconductorFeSe0.42Te0.58

We report X-band EPR and $^{125}$Te and $^{77}$Se NMR measurements on single-crystalline superconducting FeSe$_{0.42}$Te$_{0.58}$ ($T_c$ = 11.5(1) K). The data provide evidence for the coexistence of intrinsic localized and itinerant electronic states. In the normal state, localized moments couple to itinerant electrons in the Fe(Se,Te) layers and affect the local spin susceptibility and spin fluctuations. Below $T_c$, spin fluctuations become rapidly suppressed and an unconventional superconducting state emerges in which $1/T_1$ is reduced at a much faster rate than expected for conventional $s$- or $s_\pm$-wave symmetry. We suggest that the localized states arise from the strong electronic correlations within one of the Fe-derived bands. The multiband electronic structure together with the electronic correlations thus determine the normal and superconducting states of the FeSe$_{1-x}$Te$_x$ family, which appears much closer to other high-$T_c$ superconductors than previously anticipated.

A solid-state NMR investigation of single-source precursors for group 12 metal selenides; M[N(iPr2PSe)2]2(M = Zn, Cd, Hg)

The first solid-state NMR investigation of dichalcogenoimidodiphosphinato complexes, M[N(R(2)PE)(2)](n), is presented. The single-source precursors for metal-selenide materials, M[N((i)Pr(2)PSe)(2)](2) (M = Zn, Cd, Hg), were studied by solid-state (31)P, (77)Se, (113)Cd, and (199)Hg NMR at 4.7, 7.0, and 11.7 T, representing the only (77)Se NMR measurements, and in the case of Cd[N((i)Pr(2)PSe)(2)](2)(113)Cd NMR measurements, to have been performed on these complexes. Residual dipolar coupling between (14)N and (31)P was observed in solid-state (31)P NMR spectra at 4.7 and 7.0 T yielding average values of R((31)P,(14)N)(eff) = 880 Hz, C(Q)((14)N) = 3.0 MHz, (1)J((31)P,(14)N)(iso) = 15 Hz, alpha = 90 degrees , beta = 26 degrees . The solid-state NMR spectra obtained were used to determine the respective phosphorus, selenium, cadmium, and mercury chemical shift tensors along with the indirect spin-spin coupling constants: (1)J((77)Se,(31)P)(iso), (1)J((111/113)Cd,(77)Se)(iso), (1)J((199)Hg,(77)Se)(iso), and (2)J((199)Hg,(31)P)(iso). Density functional theory magnetic shielding tensor calculations were performed yielding the orientations of the corresponding chemical shift tensors. For this series of complexes the phosphorus magnetic shielding tensors are essentially identical, the selenium magnetic shielding tensors are also very similar with respect to each other, and the magnetic shielding tensors of the central metals, cadmium and mercury, display near axial symmetry demonstrating an expected deviation from local S(4) symmetry.

NMR studies of the exotic members of the Bechgaard salts NO3 and FSO3 salts

1H, 19 F and Se NMR measurements were performed to investigate the properties of Bechgaard salts, (TMTSF) 2 X, with noncentrosymmetric anions, X=NO 3 and FSO 3 . Single crystal Se NMR signal for NO 3 salt detects the spin density wave transition. Preliminary analyses of the angular dependence of 1 H NMR spectra were also carried out. Spin density wave in the NO 3 salt was confirmed to have an incommensurate wave number and relatively small arnplitude. 19 F and 77 Se NMR signals in the FSO 3 salt could detect drastic anomaly at the anion ordering temperature. The insulating state was confirmed to be nonmagnetic with a large spin gap of 420 K.

Metal–insulator transitions in magnetic conductor λ-(BEDT-TSF) 2FeCl 4 studied by [formula omitted] and [formula omitted] NMR

We have performed 1 H and 77 Se NMR measurements on a π–d organic metal λ-(BEDT-TSF) 2FeCl 4 containing paramagnetic ions Fe 3+. It is found from 1 H NMR measurements that the resonance line starts to split around T FM =70 K , just a temperature where a ferroelectric-like transition occurs, and with further decreasing temperature, the spectrum becomes significantly broad near the metal–insulator transition T MI =8.3 K . From 77 Se NMR measurements, near the phase boundary between the forced-ferromagnetic metallic state and the canted antiferromagnetic insulating state at 1.6 K and 11.67 T, a single peak becomes split with two new peaks emerging at the both sides of the zero-shift.

Electronic properties of novel donor–acceptor type charge transfer complexes, (BETS) 2(X 2TCNQ) ( X=Cl, Br): 1H, 77Se and 13C-NMR

13 C and 77 Se NMR measurements were performed to investigate the electronic properties of (BETS) 2 (X 2 TCNQ) (X=Cl, Br). The susceptibility was decomposed into the contributions of BETS and X 2 TCNQ site. The line broadening of the 1 H/ 77 Se NMR spectra were observed in the Br 2 TCNQ complex at the metal insulator transition temperature, 12 K. Non magnetic spin singlet formation on the Br 2 TCNQ site was observed at 80 K by 13 C NMR. Line broadening of the 1 H NMR spectra were also observed in the Cl 2 TCNQ complex at the metal insulator transition temperature. The insulating state of the Br and Cl 2 TCNQ complexes under pressure are both magnetic. The holes on the BETS layers are relevant to the magnetic ordering.

Synthesis and Structural Characterization of Monomeric Selenolato Complexes of Zinc, Cadmium, and Mercury.

The synthesis and characterization of homoleptic zinc(II), cadmium(II), and mercury(II) selenolates incorporating the intramolecularly chelating oxazoline ligand are described. The derivatives, M[Se(Ox)](2) [M = Zn (1), Cd (2), Hg (3); Ox = 2-(4,4-dimethyl-2-oxazolinyl)benzene], were prepared in good yield via the metathesis reactions of MCl(2) with a lithium areneselenolate, OxSe(-)Li(+). The mercury complex 3 was also synthesized by treating the corresponding diselenide with elemental mercury. The complexes are quite stable and highly soluble in common nonpolar organic solvents. X-ray diffraction results show that the complexes are monomeric in the solid state. The geometry around the metal ion in all complexes is found to be distorted tetrahedral. The crystal structure of Zn[Se(Ox)](2) (1) shows that the complex is "helically" chiral and enantiomerically pure. The spontaneous splitting of the racemates indicates the solubility differences between the racemates and pure enantiomers. (1)H, (13)C, and (77)Se NMR measurements indicate that complex 1 retains its "helical" structure in solution. Crystal data (Mo Kalpha; 293(2) K) are as follows: 1, monoclinic space group P2(1), a = 9.3900(12) Å, b = 11.618(2) Å, c = 10.8822(14) Å, beta = 98.245(8) degrees, Z = 2; 2, orthorhombic space group Pbca, a = 12.777(6) Å, b = 17.841(10) Å, c = 21.010(8) Å, Z = 8; 3, monoclinic space group P2(1)/c, a = 9.087(2) Å, b = 11.889(2) Å, c = 22.456(4) Å, beta = 98.780(13) degrees, Z = 4.