Spin-echo Spectra Research Articles

Real figure of two-dimensional spin-echo NMR spectra for a homonuclear two-spin system in rotating solids

Real figure of two-dimensional spin-echo NMR spectra for a homonuclear two-spin system in rotating solids

Single-crystal like NMR spectra of CeCu 5 Au powder samples

Single-crystal like nuclear magnetic resonance spin-echo spectra are obtained for powder samples of the antiferromagnet CeCu5Au in the paramagnetic phase. Line shifts and quadrupolar splittings are analyzed for 63Cu and 65Cu. The influence of the extreme magnetic anisotropy of the CeCu6-xAux compounds on nuclear spin relaxation is discussed.

Electron spin relaxation of vibronic Cu(H2O)6 complexes inK2Zn(SO4)2·6H2O single crystals

The low temperature (6 K) EPR spectrum of Cu2+ inK2Zn(SO4)2·6H2O is characteristic for the ground statea|x2-y2⟩-b|z2⟩ with essentially the |x2-y2⟩ stateand 4.5% admixture of the |z2⟩ state due to the zero-point motions.Temperature variations of the g and A parameters are characteristic fortwo-well vibronic dynamics and the vibronic averaging of the g-factor iswell described with the simple Silver-Getz model above 60 K with energydifference δ12 = 68 cm-1 between the wells. However, thismodel does not reproduce the A(T) dependence, which seems to be influencedby temperature induced changes in the unpaired electron delocalization ontoligands. Electron spin relaxation was measured at low temperatures up to 55 Kwhere the electron spin echo signal was detectable. At low temperatures, inthe static Jahn-Teller limit, the Cu(H2O)6 complexes are stronglylocalized in the deepest potential well and the slow vibronic dynamics isoverdominated by two-phonon Raman processes in electron spin-latticerelaxation. The relaxation rate is described by 1/T1 = aT9I8(ΘD/T)with transport integral I8 and the Debye temperature ΘD = 170 K.Electron spin echo decay is strongly modulated by dipolar coupling with 1Hand 39K nuclei and the decay function is V(2τ) = V0exp (-aτ-mτ2). The quadratic term dominates in the staticJahn-Teller limit (below 18 K) and describes the decay produced by thenuclear spectral diffusion. For higher temperatures the exp (-aτ) termdominates and describes an effect of the intrawell excitations with phaserelaxation rate 1/TM = a + bexp (-Δ/kT) with Δ = 67 cm-1being the energy of the first excited vibronic level. The excitations producea clear broadening of the Fourier transform electron spin echo (FT-ESE)spectra (at 18 K), where peaks from potassium and hydrogen nuclei have beenidentified.

Anisotropy of manganese hyperfine interaction in the intermetallic compound GdMn 6Ge 6

The zero-field 55Mn nuclear magnetic resonance (NMR) spin-echo spectra are analyzed in GdMn 6Ge 6 intermetallic compounds. By varying the excitation conditions of the NMR signal in the low-temperature flat-spiral spin structure of GdMn 6Ge 6 the anisotropy of electric and magnetic hyperfine interaction is derived for powder samples.

Local magnetic fields in antiferromagnetic Bi 2CuO 4: as seen from 63,65Cu and 209Bi nuclear resonance

A complex spin-echo spectrum of 63,65Cu and 209Bi has been observed in antiferromagnetic Bi 2CuO 4 at 4.2 K. The spectrum consists of 63,65Cu AFNR and 209Bi NQR. The Cu AFNR is characterized by the internal field H int =99 kOe at Cu nuclei and by the 63ν Q=10 MHz . It is argued that the low-limit of the internal field at the 209Bi nuclei is 4.3 kOe. The occurrence of H hf at 209Bi nuclei shows that Bi 3+ anions take part in the superexchange interaction between Cu atoms along the Cu–O–Bi–O–Cu bonds.

57Fe NMR study of a spatially modulated magnetic structure in BiFeO3

The 57 Fe spin echo spectra were studied in local magnetic fields of a BiFeO3 ferroelectric antiferromagnet over the temperature range 77–304 K. The line shape analysis confirmed the presence of a spatially modulated, incommensurate cycloidal spin structure in BiFeO3 and allowed the actual spin distribution to be reproduced throughout the cycloid length. The distribution was found to be essentially anharmonic. The modulated structure is stable over the whole temperature range studied. The cycloid wave becomes more harmonic with temperature elevation.

Nuclear magnetic resonance study of magnetic phase segregation in La1−xCaxMnO3

A Mn55 nuclear magnetic resonance (NMR) study of the La1−xCaxMnO3 system, 0<x<1, is reported. For x<0.3 and x>0.5 the spin echo spectra contain resonance lines corresponding to both the microscopic ferromagnetic and antiferromagnetic regions. A resonance line near 380 MHz, which is similar to that of the spectrum of the ferromagnetic and metallic compounds (0.2<x<0.5) is observed for all x. This indicates the presence of clusters in which the Mn magnetic moments are double exchange coupled in the whole range of doping. Their hyperfine field exhibits a much weaker temperature dependence than that of the lines of the host. This reveals the greater strength of the magnetic coupling within the clusters than in the antiferromagnetic regions and indicates a weak magnetic coupling of the clusters to the host matrix. A comparison with the NMR results on the hole doped nonstoichiometric compounds, LaMnO3+δ is made. The temperature dependence of the line intensities corresponding to the double exchange clusters and to the host matrix is attributed to the evolution of the magnetic phases in the compounds.

Magnetic structures of GdMn6Ge6 - a nuclear magnetic resonance analysis

The origin of the interesting magnetic properties of GdMn6Ge6 in the low-temperature, low-field phase is a spiral arrangement of the magnetic moments. This has been established by pulsed nuclear magnetic resonance experiments. Spin-echo spectra taken in an external magnetic field, and the observed enhancement of the radio-frequency field, suggest the presence of a spiral structure. This information is crucial for a quantitative analysis of the complicated 55Mn spectra. The anisotropy of the hyperfine field at the 55Mn site is about 10% and the enhancement has been found to be highly anisotropic, as well. Comparing the temperature dependencies of the Zeeman splitting at the individual lattice sites, Mn has been identified as the source of the transferred hyperfine fields at the 73Ge nuclei. An analysis of the 73Ge and 155/157Gd spectra yields the size and relative orientation of the hyperfine fields and electric field gradients at the nuclei in agreement with 155Gd Mössbauer spectroscopy results.

Characterization of phosphorus chemical shielding tensors in a phosphole tetramer: a combined experimental and theoretical study

Phosphorus-31 1D NMR spectra of a stationary powder sample of a phosphole tetramer containing two phosphorus spin pairs have been obtained at 4.7 T and 9.4 T. In order to separate 31P-31P spin-spin coupling from anisotropic chemical shielding, 2D spin-echo NMR spectra have been acquired. Phosphorus-31 CPMAS NMR experiments indicate that the two spin pairs of the tetramer are equivalent and each may be treated as an isolated spin pair. Within a given spin pair, the difference between the isotropic chemical shifts of two directly bonded phosphorus nuclei is 1.7 ppm. As well, they are spin-spin coupled by both the indirect and direct interactions, 1J(31P, 31P) = -362 Hz and RDD = 1.80 kHz, respectively. The principal components and relative orientation of the two phosphorus chemical shielding tensors have been determined using the dipolar-chemical shift technique; however, since the dipolar tensor is axially symmetric, ambiguities in the chemical shielding tensor orientation relative to the molecular framework result. Using ab initio calculations and simulations of the 2D spin-echo spectra, many of these ambiguities have been resolved. The spans and skews of the phosphorus shielding tensors for all four three-coordinate phosphorus nuclei are the same within experimental error, 115 ppm and 0.70, respectively. Combined experimental and theoretical results indicate that the phosphorus shielding tensor orientations are dictated by the local environment. For both shielding tensors, the most shielded component, δ33, is approximately 78° from the P-P bond and in the phosphole ring plane. The relative orientation of the δ33 components is described by a dihedral angle of 82°, similar to the dihedral angle of approximately 76° defining the twist of the phosphole rings about the bridging P-P bond.Key words: solid-state 31P NMR, phosphorus chemical shielding tensors, phosphole tetramer, 31P—31P spin pairs, ab initio calculations.

Nuclear magnetic resonance (NMR) and magnetic order in Y 6Mn 23H x hydrides

Investigations of 55 Mn NMR spin echo in the Y 6Mn 23H x ( x=0; 1; 9.8; 18, 29) hydrides are presented. The spin-echo spectra and relaxation times T 1 and T 2 were measured as a function of temperature. The 55 Mn resonance lines were attributed to the Mn structural sites. The different temperature dependencies of the resonance frequencies for the Mn sites have been observed. From the fitted dependencies on T 3/2 an estimate of the spin-wave stiffness coefficient D was made. The temperature dependencies of the relaxation rates were also found to be related to the spin-wave excitations. For x=9.8 an anomaly in the temperature dependence of the T 2 relaxation rate has been observed at 35 K. Together with the AC susceptibility data this indicates a change of magnetic structure at this temperature.

NMR spectroscopy in mixed valence manganites

A survey is given of the nuclear magnetic resonance (NMR) experiments in the manganese perovskites of the form RE 1 − x A x MnO 3 and REMnO 3 + δ (RE = rare earth, A = Ca, Sr, Ba). The 55Mn, 139La and 87Sr spin-echo spectra in the frequency range 5–650 MHz are discussed and the microscopic information on the Mn valence states and the nature of magnetic interactions is analysed as a function of the hole doping.

Precipitation in an Al-3 wt% Cu alloy studied by nuclear magnetic resonance measurements

63Cu nuclear magnetic resonance (NMR) has been applied to a study of the precipitation process in an Al-3 wt% Cu alloy. Structural changes of the precipitate through ageing are reflected in the line shape of the NMR spectra. An intense narrow resonance line appears in the spin-echo spectrum from the solid solution, while the NMR spectrum from the CuAl2 phase of C16-type structure exhibits a characteristic resonance line broadened owing to secondorder quadrupolar effects. The spectra from the Al-3 wt% Cu alloys which contain dispersed precipitates can be fitted by a superposition of the sharp line for the solid solution and a broad line for the precipitate. The breadth of the broad line differs for various precipitates, reflecting the difference in the local environment around the probe atoms. From the intensity ratio of the broad line to the sharp line, the proportion of Cu atoms dissolved in the precipitates is determined. The NMR parameters, such as the Knight shift K and the quadrupolar coupling constant e2qQ/h, are evaluated for the theta and theta precipitates.

Photo-induced electron-transfer processes in (anthracene–) quinquethiophene–fullerene diads and triads

We have investigated novel model systems for photo-induced intramolecular electron-transfer processes. The molecules presented here are of the structure T5–C60 or A–T5–C60, i.e., fullerene C60 is used as acceptor, and quinquethiophene (T5) or anthracene (A) can act as a donor. From theoretical considerations we learn that a charge transfer should occur after excitation of the quinquethiophene or anthracene part, whereby an energy transfer is still not excluded. Optical absorption spectra show the electronic independence of the fullerene from the substituent and hint at isolated molecules in solutions. After laser-flash excitation of frozen solutions of the molecules, we used pulsed electron spin resonance (ESR) to detect the radical signals of the charge-separated state. In both the diad and the triad, we observe at low temperatures a free induction decay (FID) and a broad spin-echo spectrum with the same lifetime of 1–2 μs. The signals can be assigned to the C60− anion (FID) and the T5+cation (spin echo) by the g factor and the linewidth.

Nuclear magnetic resonance studies on Cu3Au, CuAu and CuAu3alloys

Abstract Nuclear magnetic resonance (NMR) measurements of 63 Cu nuclei have been carried out on Cu3Au, CuAu and CuAu3 alloys with different degrees of order. The spin-echo spectra obtained from the disordered and the ordered CuAu3 phases exhibit an intense sharp resonance line. On the other hand, the characteristic resonance line broadened owing to second-order quadrupole interaction appears in the spin-echo spectra from the fully ordered Cu3Au and CuAu I phases. The spectra from the partially ordered Cu3Au and CuAu I phases and the high-temperature CuAu II phase can be fitted by the superposition of the broad line for the ordered state and the sharp line for the disordered state. It is demonstrated that the order-disorder transition for these alloys occurs heterogeneously, in contrast with the result of X-ray diffraction analysis. The NMR parameters such as the Knight shift K and the quadrupole coupling constant e 2 qQ/h are evaluated for the ordered and disordered phases for three kinds of alloy of diff...

The role of N-acetylcysteine in protecting synovial fluid biomolecules against radiolytically-mediated oxidative damage: A high field proton NMR study

High field proton (1H) NMR spectroscopy has been employed to evaluate the abilities of the antioxidant thiol drug N-acetylcysteine and exogenous cysteine to protect metabolites present in intact inflammatory synovial fluid samples against oxidative damage arising from gamma-radiolysis (5.00 kGy) in the presence of atmospheric O2. Although oxidation of urate to allantoin by radiolytically-generated *OH radical was readily circumventable by pre-treatment of synovial fluids with N-acetylcystine (1.00 or 3.00 × 10-3 mol · dm-3) or cysteine (1.00, 2.00 or 5.00 × 10-3 mol · dm-3), both thiols offered only a limited protective capacity with respect to hyaluronate depolymerisation and the production of formate from carbohydrates in general. Radiolytic products generated from the added thiols (predominantly their corresponding disulphides) were simultaneously detectable in 1H Hahn spin-echo spectra of gamma-irradiated synovial fluids, permitting a quantitative evaluation of the radioprotective capacity of these agents. It is concluded that the multicomponent analytical ability of high field 1H NMR spectroscopy provides much useful molecular information regarding mechanisms associated with the radioprotectant actions of thiols in intact biofluids.

A 55Mnnuclear magnetic resonance study of mixed-valence manganites

The temperature dependence of the 55Mn nuclear magnetic resonance of some manganese perovskites of the form [La1-xAx]MnO3 (A = Ca,Sr) for x of 1/3 is reported. The 55Mn spin-echo spectra were measured in zero field and in an applied magnetic field up to 6 T for polycrystalline samples of [La0.67-yTbyCa0.33]MnO3 (y = 0,0.1,0.22), [La0.6Y0.07Ca0.33]MnO3 and [La0.7Sr0.3]MnO3. The single resonance line observed for all compounds, for temperatures between 4.2 K and 293 K, is attributed to a fast carrier motion (<10-9 s) between the Mn3+ and Mn4+ sites resulting from the double-exchange mechanism. For some of the samples the zero-field nuclear magnetic resonance signal has been detected at temperatures far above the magnetic ordering temperature, providing direct evidence of magnetic correlations whose lifetime is greater than 10-5 s. The correlations can possibly be related to magnetic polarons.

Biochemical classification of kidney carcinoma biopsy samples using magic-angle-spinning 1H nuclear magnetic resonance spectroscopy

High resolution 1H nuclear magnetic resonance (NMR) spectra using spinning at the magic angle ( 1H MAS NMR) have been obtained on intact normal and pathological kidney tissue samples from patients undergoing surgery for renal cell carcinoma (RCC). The spectra were measured on ca. 80 mg samples and provided high resolution 1H NMR spectra in which effects of dipolar couplings, chemical shift anisotropy and magnetic susceptibility differences are minimised thus yielding high spectral resolution. Conventional one-dimensional and spin-echo spectra and two-dimensional J-resolved, TOCSY and 1H– 13C HMQC spectra were also measured on selected samples and these allowed the assignment of resonances of endogenous substances comprising both cytosolic and membrane components. The tumour tissues were characterised principally by an increased lipid content. These are the first reported results on human tumour tissues using this technique and the approach offers potential for the rapid classification of different types of tumour tissue.

Observation of 63,65Cu and 209Bi nuclear resonance in antiferromagnetic Bi 2CuO 4

A complex spin-echo spectrum of 63,65Cu and 209Bi has been observed in various ceramic and single-crystal samples of Bi 2CuO 4 in antiferromagnetic phase at a temperature of 4.2 K. The spectrum consists of 63,65Cu AFNR and 209Bi NQR with a complex structure of resonance lines due to magnetic inequivalency of Bi atoms and a large asymmetry parameter. The Cu AFNR is characterized by the internal field H int≈99 kOe at Cu nuclei and by the quadrupole frequency 63 ν Q ≈10 MHz. The obtained value of H int corresponds to 0.82 μ B for the Cu 2+ magnetic moment. It is shown that the low limit of the internal field at 209Bi nuclei is approximately 4.3 kOe and consists of the dipolar field H dip=0.85 kOe and the hyperfine field H hf which essentially exceeds H dip. The occurrence of H hf at 209Bi nuclei shows that Bi 3+ anions take part in the superexchange interaction between Cu atoms along the Cu–O–Bi–O–Cu bonds.

A nuclear magnetic resonance study of SmCo 2

A 59Co, 147Sm and 149Sm NMR study of SmCo 2 at 4.2 K is reported. The 59Co spin-echo spectrum consists of two lines centered at 44 and 59 MHz. The upper line shows a resolved quadrupole splitting of 1.15 MHz, whereas the lower one is unresolved. The 147Sm spectrum shows three septets (nuclear spin I 147 = 7 2 ), two at lower frequencies and one at higher frequencies, with different quadrupole splittings. The results are analysed and related to the presence of magnetically inequivalent sites corresponding to the easy direction of magnetisation along the 〈1 1 1〉 direction, and the rhombohedral distortion of the crystal lattice. From the Sm quadrupole splittings the lattice contribution to the electric field gradient is derived and related to the crystalline electric field coefficient A 2 0 and, thus, to the magnetocrystalline anisotropy due to the lattice distortion.

145Nd NMR study of Nd xY 2 − xCo 17 and Nd xY 2 − xCo 14B

The 145Nd Nuclear magnetic resonance (NMR) spin-echo spectra of the Nd x Y 2 − x Co 17 and Nd x Y 2 - x Co 14B compounds at 4.2K are presented. Quadrupole septets are assigned to the Nd sites 4f, 4g in Nd x Y 2 − x Co 14B and 6c in Nd x Y 2 − x Co 17. A large change of the quadrupole splitting between x = 0.1 and x = 0.2 and only slight changes of the hyperfine fields is observed in Nd x Y 2 − x Co 14B. The effect is attributed to a change of the easy magnetisation direction with Nd content from the c-plane to the c-axis. For Nd x Y 2 − x Co 17 slight changes of quadrupole splitting and hyperfine field in the whole concentration region 0.1< x<2.0 are observed only, reflecting a planar anisotropy of the compounds in the whole series. From the quadrupole splittings the values of the 4f-electron- and the lattice contributions to the electric field gradient at Nd nuclei are derived. The lattice contributions are related to the crystal electric field coefficient A 2 0 at the neodymium sites. The A 2 0 values amount to 192 Ka 0 −2 for the 4g and 57 Ka 0 −2 for the 4f site in Nd x Y 2 − x Co 14B. The directions of magnetic moments at the Nd sites are also obtained.