Messung der rückstoßfreien Resonanz-Absorption am 6,2 keV-Niveau in181Ta

A strong recoilless resonance absorption was observed for the extraordinarily narrow 6.2 keV level (T _1/2=6.8 μs) of¹⁸¹Ta. Using sources of¹⁸¹W in W-metal and Ta-metal absorbers, a linewidth of about 9 times the natural width was obtained, with an enormous isomer shift of +(0.85±0.02) mm/sec, which is equivalent to a shift by 260 times the natural linewidth. Recoilless resonance spectra of cold worked and of oxygen doped absorbers showed a high sensitivity of the linewidth against lattice imperfections due to inhomogeneous quadrupole interactions and isomer shifts. Measurements of the residual resistivity ratio and of the lattice constant of the impurity doped absorbers yielded a linear correlation between linewidth and isomer shift.     

P and Al NMR study of Al1−xGaxPO4

Mixed phosphates Al_1−xGa_xPO₄ (0.0≤x≤1.0), with orthorhombic structure, have been prepared by co-precipitation method followed by annealing at high temperature (T≈1350 °C) and investigated by using powder X-ray diffraction and NMR spectroscopy. A systematic decrease observed for the unit cell parameters with increasing Ga³⁺ contents, suggests the formation of a solid solution between AlPO₄ and GaPO₄. ³¹P MAS NMR spectra of both AlPO₄ and GaPO₄ show a single peak at δ=−24.6 and −8.5 ppm, respectively. For mixed phosphates, multi-component ³¹P NMR spectra characterized by varying values of chemical shifts, have been observed due to the existence of different types of ³¹P structural configurations formed by varying number of Al³⁺ and Ga³⁺ as its next nearest neighbours. The intensity analysis of these component peaks suggested a perfectly random distribution of Al³⁺ and Ga³⁺ cations in these mixed phosphates. Chemical shift anisotropy parameters for the different structural configurations of ³¹P have been evaluated from the intensity analysis of the spinning side bands corresponding to different isotropic peaks. ²⁷Al NMR spectra of mixed phosphate samples exhibited a slightly asymmetric broad peak at δ≈42.0 ppm, which is characteristic of the tetrahedral configuration of Al³⁺ ions with significant covalent bonding between the metal and oxygen ions.     

Magic Angle-Oriented Sample Spinning (MAOSS): A New Approach toward Biomembrane Studies

The application of magic angle sample spinning (MAS) NMR to uniformly aligned biomembrane samples is demonstrated as a new general approach toward structural studies of membrane proteins, peptides, and lipids. The spectral linewidth from a multilamellar lipid dispersion is dominated, in the case of protons, by the dipolar coupling. For low-γ or dilute spins, however, the chemical shift anisotropy dominates the spectral linewidth, which is reduced by the two-dimensional order in a uniformly aligned lipid membrane. The remaining line broadening, which is due to orientational defects (“mosaic spread”) can be easily removed at low spinning speeds. This orientational order in the sample also allows the anisotropic intermolecular motions of membrane components (such as rotational diffusion, τ_c= 10⁻¹⁰s) for averaging dipolar interactions to be utilized, e.g., by placing the membrane normal parallel to the rotor axis. The dramatic resolution improvement for protons which are achieved in a lipid sample at only 220 Hz spinning speed in a 9.4 T field is slightly better than any data published to date using ultra-high fields (up to 17.6 T) and high-speed spinning (14 kHz). Additionally, the analysis of spinning sidebands provides valuable orientational information. We present the first¹H,³¹P, and¹³C MAS spectra of uniformly aligned dimyristoylphosphatidylcholine (DMPC) bilayers. Also,¹H resolution enhancement for the aromatic region of the M13 coat protein reconstituted into DMPC bilayers is presented. This new method combines the high resolution usually achieved by MAS with the advantages of orientational constraints obtained by working with macroscopically oriented samples. We describe the general potential and possible perspectives of this technique.     

Multiple-quantum MAS NMR of quadrupolar nuclei. Do five-, seven- and nine-quantum experiments yield higher resolution than the three-quantum experiment?

The question of whether or not higher-order (five-, seven- and nine-quantum) multiple-quantum magic angle spinning (MQMAS) experiments yield isotropic NMR spectra of half-integer quadrupolar nuclei with higher resolution than the basic three-quantum MAS experiment is examined. The frequency dispersion is shown theoretically to be greatly increased in higher-order MQMAS spectra, but it is argued that whether or not this translates into an increase in resolution depends upon the ratio of the homogeneous to inhomogeneous contributions to the isotropic linewidth. Experimentally, it is demonstrated using three-, five- and seven-quantum 45Sc MAS NMR and three- and five-quantum 27Al MAS NMR of crystalline samples that higher-order MQMAS experiments can yield a real and useful increase in resolution but that, owing to the presence of inhomogeneous broadening in the isotropic spectra, this increase is less than the theoretically predicted value. A number of practical issues relating to resolution in MQMAS NMR are also pointed out.     

31P nmr study of I–IV group polycrystalline phosphates

Polycrystalline phosphates of group I–IV elements have been studied using solid state ³¹P NMR. The relationship between the spectral parameters (the principal components of the chemical shielding tensor and obtained from MAS values of the isotropic shifts of ³¹P) and the environment of the phosphorus atoms in these compounds has been considered. The shape of the lines in stationary ³¹P NMR spectra reflects the near neighbour environment of a phosphorus atom and the degree of distortion of the PO₄ tetrahedron. The type of cation, as well as the presence of the water of crystallization in the structure affect considerably an isotropic shift of ³¹P in phosphates. The high sensitivity of the solid state ³¹P NMR to even slight variations in the structure of the phosphate anions is shown.     

On the drastic temperature broadening of hard mode Raman lines of ferroelectric KDP type crystals near Tc

The drastic broadening of a few A₁ Raman lines of ferroelectric KH₂PO₄(KDP) and KH₂AsO₄(KDA) crystals have been measured at various temperatures near T_c. The results in KDP are in fair agreement with two different simplified expressions of linewidths, which are based on pseudospin-phonon interaction arguments. No such agreement is found between the results and a simplified linewidth expression derived from a theory based on soft mode-hard mode interaction arguments. The linewidths and the reciprocal of the proton spin-lattice relaxation time in KDP exhibit a very similar temperature dependence below T_c but behave differently above T_c. This phenomena is presented in detail, and discussed.     

The evaluation of different MAS techniques at low spinning rates in aqueous samples and in the presence of magnetic susceptibility gradients

It was recently demonstrated that the nuclear magnetic resonance (NMR) linewidths for stationary biological samples are dictated mainly by magnetic susceptibility gradients, and that phase-altered spinning sideband (PASS) and phase-corrected magic angle turning (PHORMAT) solid-state NMR techniques employing slow and ultra-slow magic angle spinning (MAS) frequencies can be used to overcome the static susceptibility broadening to yield high-resolution, spinning sideband (SSB)-free 1H NMR spectra [Magn. Reson. Med. 46 (2001) 213; 47 (2002) 829]. An additional concern is that molecular diffusion in the presence of the susceptibility gradients may limit the minimum useful MAS frequency by broadening the lines and reducing SSB suppression at low spinning frequencies. In this article the performance of PASS, PHORMAT, total sideband suppression (TOSS), and standard MAS techniques were evaluated as a function of spinning frequency. To this end, 300MHz (7.05T) 1H NMR spectra were acquired via PASS, TOSS, PHORMAT, and standard MAS NMR techniques for a 230-microm-diameter spherical glass bead pack saturated with water. The resulting strong magnetic susceptibility gradients result in a static linewidth of about 3.7kHz that is larger than observed for a natural biological sample, constituting a worst-case scenario for examination of susceptibility broadening effects. RESULTS: (I) TOSS produces a distorted centerband and fails in suppressing the SSBs at a spinning rate below approximately 1kHz. (II) Standard MAS requires spinning speeds above a few hundred Hz to separate the centerband from the SSBs. (III) PASS produces nearly SSB-free spectra at spinning speeds as low as 30Hz, and is only limited by T(2)-induced signal losses. (IV) With PHORMAT, a SSB-free isotropic projection is obtained at any spinning rate, even at an ultra-slow spinning rate as slow as 1Hz. (V) It is found empirically that the width of the isotropic peak is proportional to F(-x), where F is the spinning frequency, and x=2 for MAS, 0.84 for PASS, and 0.5 for PHORMAT.     

1H NMR study of the α phase of Mg2NiHx system

Hydrogen behavior in the α phase of Mg₂NiH_x system was studied by ¹H NMR. ¹H NMR spectra and spin-lattice relaxation times, T₁ and T_1ρ, of Mg₂NiH_0.22 were measured in the temperature range between 100 and 480 K. The drastic change in the linewidth is observed between 170 and 340 K, and ¹H rigid lattice is observed below 170 K, from which it is deduced that the hydrogen atoms are randomly distributed in α-Mg₂NiH_x. The relaxation mechanism for t₁ is the paramagnetic one, while the T_1ρ value is determined partially by hydrogen diffusion. The hydrogen diffusion rate has been determined from the linewidth and the T_1ρ value. The paramagnetic relaxations observed in T₁ and T_1ρ have been discussed relating to the hydrogen diffusion.     

Optorelaxers: Achieving real-time control of NMR relaxation

We present an approach to increase the detection sensitivity of NMR by shortening the spin-lattice relaxation time using transient paramagnetic species created by light irradiation of “optorelaxer” molecules. In the ultimate implementation of this concept, not yet realized here, these transient species are absent during the detection period, thereby avoiding the loss of spectral resolution caused by inhomogeneous broadening from paramagnetic species. Real-time control of NMR relaxation by visible light is demonstrated with Fe(II)(ptz)₆(BF₄)₂, (ptz = 1-propyltetrazole), abbreviated FePTZ. Illumination of FePTZ at 30 K results in a decrease of the ¹H NMR spin-lattice relaxation time T₁ due to formation of a high spin photoexcited state. The ¹H NMR of polystyrene containing a low concentration of FePTZ molecules shows a similar reduction in T₁, establishing that FePTZ can act as an optorelaxer for the protons of a matrix. Numerical modeling of the spin-diffusion processes from the protons in a FePTZ core to those in a shell of polystyrene accounts for the observed T₁ effects under both dark and light conditions. Additionally, ¹H MAS (magic-angle spinning) NMR results for pure FePTZ provide information on the isotropic and anisotropic portions of the electron-nuclear hyperfine interactions.     

Refocussing of chemical and paramagnetic shift anisotropies in 2H NMR using the quadrupolar-echo experiment

A simple two-pulse spin-echo experiment is shown to refocus inhomogeneous broadening arising from both chemical and/or paramagnetic shift anisotropy and a first-order I=1 quadrupolar interaction. The method is shown to yield ²H NMR spectra of a paramagnetic solid (CuCl₂ · 2D₂O) and of a non-paramagnetic solid (D₂C₂O₄ · 2D₂O) that are significantly less distorted than those provided by the conventional quadrupolar-echo method. The technique will thus prove useful in studies of motion and dynamics where detailed analysis of the ²H lineshape is performed.     

Effect of alkali-earth ions on the local structure of LaAlO3-La0.67A0.33MnO3 (A = Ca, Sr, Ba) diluted solid solutions: 27Al NMR studies

²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) was studied for diluted alkali earth metal-doped lanthanum manganite solid solutions in the lanthanum aluminate (1 − y)LaAlO₃-yLa_0.67 A _0.33MnO₃ (A = Ca, Sr, Ba) with y =0, 2, 3, and 5 mol %. The spectra depended on the dopant species and showed higher substitutional ordering for the Ba-containing mixed crystals. Magnetically shifted lines were observed in all solid solutions and were attributed to Al in the octahedral oxygen environment near manganese trivalent ions. Nonlinear dependences of their intensity were referred to the manganese-rich cluster formation. An additional MAS NMR line corresponding to aluminum at sites different from the octahedral site in pure LaAlO₃ was observed only in solutions doped with Ba. 3Q MAS NMR revealed that the broadening of this line is governed mainly by quadrupole coupling and made it possible to calculate the isotropic chemical shift. The article was submitted by the authors in English.     

Pulsed and cw-ESR study of E′ and Al hole centers in quartz produced by irradiations and crushing

E′ centers in quartz irradiated by alpha-rays (He⁺-beam), neutrons, and those produced by gamma-irradiation after crushing as well as in gamma-irradiated silica glass were investigated using cw/pulsed electron spin resonance. Lineshape of E′ centers produced by alpha-rays was similar to that of centers produced by neutrons but different from that of centers produced by gamma-irradiation in silica glass. E′ centers in crushed quartz were localized in amorphous region presumably around dislocations. The obtained phase relaxation times,T ₂, are 6.5±0.4 μs (by neutrons), 7.3±0.2 μs (by crushing and gamma-rays) and 18.5±02. μs (in glass) and are compared with the linewidths and the ratios of homogeneous to inhomogeneous broadening obtained from the microwave power dependence.     

Manifestation of a series of phase transitions in IR spectra of a multiferroic TbMnO3

We have studied IR transmittance spectra of a multiferroic TbMnO₃. Three phase transitions (T _N = 43 K, T _FE = 28 K, and T _Tb = 7 K), which manifest themselves in a shift of the absorption band edge of manganese and in changes in the spectral parameters of lines of f-f transitions of terbium, have been detected. An incommensurate character of the magnetic structure leads to nonequivalence of terbium positions and to an additional inhomogeneous broadening. The spontaneous electric polarization, which arises at T < T _FE, significantly affects the crystal field that acts on terbium and noticeably shifts its energy levels. The phase transition with ordering of terbium (7 K) is accompanied by a lowering of the energy of the ground state of the Tb³⁺ ion.     

Biomolecular solid state NMR with magic-angle spinning at 25K

A magic-angle spinning (MAS) probe has been constructed which allows the sample to be cooled with helium, while the MAS bearing and drive gases are nitrogen. The sample can be cooled to 25 K using roughly 3 L/h of liquid helium, while the 4-mm diameter rotor spins at 6.7 kHz with good stability (±5 Hz) for many hours. Proton decoupling fields up to at least 130 kHz can be applied. This helium-cooled MAS probe enables a variety of one-dimensional and two-dimensional NMR experiments on biomolecular solids and other materials at low temperatures, with signal-to-noise proportional to 1/T. We show examples of low-temperature ¹³C NMR data for two biomolecular samples, namely the peptide Aβ_14–23 in the form of amyloid fibrils and the protein HP35 in frozen glycerol/water solution. Issues related to temperature calibration, spin–lattice relaxation at low temperatures, paramagnetic doping of frozen solutions, and ¹³C MAS NMR linewidths are discussed.     

Vanadium phosphates on mesoporous supports: model catalysts for solid-state NMR studies of the selective oxidation of n-butane

SBA-15 was utilized as mesoporous support for the dispersion of vanadium phosphate (VPO) compounds. Loading of SBA-15 with VPO compounds was found to be accompanied by decreasing ²⁹Si MAS NMR signals of Q² (Si(2Si,2OH)) and Q³ (Si(3Si,1OH)) silicon species, which indicates coverage of the mesoporous support by the guest compounds. The ⁵¹V MAS NNR spectra of the activated VPO/SBA-15 catalysts consist of patterns typical for the α_II- and β-phases of vanadyl orthophosphate. In the ³¹P MAS NMR spectra of the activated VPO/SBA-15 catalysts, signals of β-, δ-, and α_II-VOPO₄ phases could be identified. Upon conversion of n-butane-¹³C₄, a strong decrease of the ³¹P MAS NMR signals characteristic for the δ-VOPO₄ phase occurred, while by ¹³C MAS NMR spectroscopy the formation of maleic anhydride, carbon monoxide, and carbon dioxide was observed. This finding supports the active role of the δ-VOPO₄ phase in the selective oxidation of n-butane on VPO/SBA-15 catalysts.     

AN EXACT SOLUTION OF A MAS FID IN TWO-SITE EXCHANGE PROBLEM

已知两位置交换分子运动速率对于静态     

Nuclear spin-lattice and nuclear spin-spin relaxation time measurements in EuB6 at low temperatures using the spin-echo technique

Experimental results on the nuclear spin-lattice and nuclear spin-spin relaxation times in the ferromagnetic EuB₆ at temperatures below 4·2 K are presented using the external magnetic field,H _ext, in the range of 0 ⩽H _ext ⩽ 10 kG. Nuclear spin-spin relaxation time computed on the basis of the Suhl-Nakamura process turns out to be 3·2μs, which compares well with the experimental value 11·1μs obtained with the 10 kG magnetic field at 1·7 K. It is found that in the ferromagnetic EuB₆,T ₁ is approximately 5 × 10³ times larger thanT ₂ at 1·7 K with the 10 kG magnetic field. Thus the effect ofT ₁ onT ₂ can be neglected. From the experimental value ofT ₂, the value of the homogeneous line broadening is found to be 14 kHz. The corresponding value obtained from the cw method is 175 kHz. This evidently shows the presence of the inhomogeneous line broadening in the cw NMR.     

Finding confined water in the hexagonal phase of Bi0.05Eu0.05Y0.90PO4·xH2O and its impact for identifying the location of luminescence quencher

¹H MAS NMR spectra of Bi_0.05Eu_0.05Y_0.90PO₄·xH₂O show chemical shift from ?0.56 ppm at 300 K to ?3.8 ppm at 215 K and another one at 5–6 ppm, which are related to the confined or interstitial water in the hexagonal structure and water molecules on the surface of the particles, respectively. Negative value of the chemical shift indicates that H of H₂O is closer to metal ions (Y^3?+? or Eu^3?+?), which is a source of luminescence quencher. H coupling and decoupling ³¹P MAS NMR spectra at 300 and 250 K show the same chemical shift (?0.4 ppm) indicating that there is no direct bond between P and H. It is concluded that the confined water is not frozen even at 215 K because of the less number of H-bonding.     

Study of synthetic diamonds by dynamic nuclear polarization-enhanced13C nuclear magnetic resonance spectroscopy

Four I_b-type synthetic diamond crystals were studied by dynamic nuclear polarization (DNP)-enhanced high resolution solid state¹³C nuclear magnetic resonance (NMR) spectroscopy. The home built DNP magic-angle-spinning (MAS) NMR spectrometer operates at a field strength of 1.9 T and the highest DNP enhancement factor of synthetic diamonds came near to 10³. Comparing with I_b-type natural diamonds, the¹³C NMR linewidths of synthetic diamonds in static spectra are broader. The¹³C spin-lattice relaxation time and DNP polarization time of synthetic diamond are shorter than those of I_b-type natural diamond. From the hyperfine structure of the DNP enhancement curve, four kinds of nitrogen-centred free radicals could be identified in synthetic diamond.     

Bulk magnetization and 1H NMR spectra of magnetically heterogeneous model systems

Bulk magnetization and ¹H static and magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of two magnetically heterogeneous model systems based on laponite (LAP) layered silicate or polystyrene (PS) with low and high proton concentration, respectively, and ferrimagnetic Fe₂O₃ nano- or micro-particles have been studied. In LAP+Fe₂O₃, a major contribution to the NMR signal broadening is due to the dipolar coupling between the magnetic moments of protons and magnetic particles. In PS+Fe₂O₃, due to the higher proton concentration in polystyrene and stronger proton–proton dipolar coupling, an additional broadening is observed, i.e. ¹H MAS NMR spectra of magnetically heterogeneous systems are sensitive to both proton–magnetic particles and proton–proton dipolar couplings. An increase of the volume magnetization by ～1 emu/cm³ affects the ¹H NMR signal width in a way that is similar to an increase of the proton concentration by ～2×10²²/cm³. ¹H MAS NMR spectra, along with bulk magnetization measurements, allow the accurate determination of the hydrogen concentration in magnetically heterogeneous systems.