Theory of zero-field NMR spectra. Effects of motion

Zero-field NMR spectra of tunnelling methyl group, classically reorienting methyl group and of a pair of spins 1/2 jumping between two perpendicular orientations are calculated. A formula for the second moment of a rigid lattice is derived from a formal analogy between zero-field and high-field spectra. Fast reorientation about a perpendicular axis reduces the intramolecular second moment to 1/4 of its rigid value. The zerofield method seems to be a promising tool for the determination of tunnelling splitting or of jumping rates.     

Near-zero-field nuclear magnetic resonance

We investigate nuclear magnetic resonance (NMR) in near zero field, where the Zeeman interaction can be treated as a perturbation to the electron mediated scalar interaction (J?coupling). This is in stark contrast to the high-field case, where heteronuclear J?couplings are normally treated as a small perturbation. We show that the presence of very small magnetic fields results in splitting of the zero-field NMR lines, imparting considerable additional information to the pure zero-field spectra. Experimental results are in good agreement with first-order perturbation theory and with full numerical simulation when perturbation theory breaks down. We present simple rules for understanding the splitting patterns in near-zero-field NMR, which can be applied to molecules with nontrivial spectra.     

75As, 63Cu NMR and NQR characterization of selected arsenic minerals

The direct measurement and identification of solid state arsenic phases using ⁷⁵As NMR is made difficult by the simultaneous conditions of large quadrupole moment and low coordination symmetry in many compounds. However, specific arsenic minerals can efficiently be detected and discriminated via nuclear quadrupolar resonance (NQR). We report on the first NMR and NQR measurements in the natural minerals enargite (Cu₃AsS₄), niccolite (NiAs), arsenopyrite (FeAsS) and loellingite (FeAs₂). The NQR frequencies have been determined from both high-field NMR powder patterns and via zero-field frequency sweeps. Density functional theory (DFT) based ab initio calculations support the experimental results. The compounds studied here are common in terms of the known set of As-containing minerals. They are sometimes encountered in the context of base metal or gold mining. The study represents a significant addition to the list of arsenic minerals that can now be detected with NQR techniques.     

Non-secular part of nuclear dipolar broadening detected by zero-field spin relaxation of positive muon

The zero-field spin relaxation function of μ⁺ observed in ZrH₂ has revealed that the second moment of the nuclear dipolar broadening is five times larger than the high-field value. This experiment clearly demonstrates the recovery of the non-secular part of dipolar interaction between unlike spins. A general expression of zero-field relaxation function is presented to account for a slow modulation of random fields on μ⁺ found at room temperature.     

Polarization and energy barriers in ferroelectric pyridinium tetrafluoroborate

Measurements of the ¹H and ¹⁹F nuclear magnetic resonance (NMR) second moments were performed for a polycrystalline sample of (PyH)BF₄, whereas the shape of the ²H NMR line was analysed for a polycrystalline sample of (d₅PyH)BF₄. Asymmetry parameter δ has been calculated for four models of pyridinium cation reorientation among inequivalent potential energy minima, using the experimental value of the ¹H NMR second moment as well as the ²H NMR line width. From knowledge of the potential shape and the population of its minima, the temperature dependence of polarization for all the models has been found. From the comparison of the experimentally determined polarization with the calculated polarization, the most appropriate model of the pyridinium cation reorientation has been chosen.     

Proton nuclear magnetic resonance in paramagnetic CoCl2.6 H2O

The present paper deals with a study of nuclear magnetic resonance (NMR) of protons of the crystal water of paramagnetic CoCl₂.6 H₂O. The measurements were carried out on powdered samples at room temperature, for values of the external magnetic field ranging from 0.3 to 1.0 T.The NMR signals of protons of the crystal water exhibit asymmetric shape which changes with the applied external magnetic field. We found that the second moment of the resonance line shows a linear dependence on the square of the induction of the externally applied magnetic field.Both the cause of the asymmetry of the NMR line of protons of the crystal water and the dependence of the second moment of the resonance line on the induction of external magnetic field are interpreted in the paper.     

<Superscript>1</Superscript>H and <Superscript>19</Superscript>F NMR relaxation time studies in (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>ZrF<Subscript>6</Subscript> superionic conductor

¹H and ¹⁹F spin-lattice relaxation times in polycrystalline diammonium hexafluorozirconate have been measured in the temperature range of 10–400 K to elucidate the molecular motion of both cation and anion. Interesting features such as translational diffusion at higher temperatures, molecular reorientational motion of both cation and anion groups at intermediate temperatures and quantum rotational tunneling of the ammonium group at lower temperatures have been observed. Nuclear magnetic resonance (NMR) relaxation time results correlate well with the NMR second moment and conductivity studies reported earlier.     

Nuclear magnetic resonance of extremely dilute CrFe

We report nuclear orientation and pulsed NMR on radioactive ⁵¹Cr$\text{Fe}$ samples. The zero-field resonance frequency of 13.543(18) MHz from polycrystalline foil samples yields a hyperfine field B_hf = ?6.655(35) T in the dilute, low-temperature limit for Cr$\text{Fe}$.     

Sensitivity enhancement in solid state (15)N NMR by indirect detection with high-speed magic angle spinning

Enhancement of sensitivity in solid state (15)N NMR by indirect detection through (1)H NMR signals under high-speed magic angle spinning and high-field conditions is demonstrated experimentally on two (15)N-labeled peptides, polycrystalline AlaGlyGly and the helix-forming, 17-residue peptide MB(i + 4)EK in lyophilized form. Sensitivity enhancement factors ranging from 2.0 to 3.2 are observed experimentally, depending on the (15)N and (1)H linewidths and polarization transfer efficiencies. The (1)H-detected two-dimensional (1)H/(15)N correlation spectrum of AlaGlyGly illustrates the possibility of increased spectral resolution and resonance assignments in indirectly detected experiments, in addition to the sensitivity enhancement.     

Nuclear resonance studies of95NbFe

Dilute alloys of 35-day⁹⁵Nb in polycrystalline iron have been studied by nuclear magnetic resonance on oriented nuclei (NMR/ON) using frequency modulation and fast passage techniques. Comparison of resonance frequencies with nuclear orientation results confirms the spin 9/2 for the⁹⁵Nb ground state. The zero-field resonance frequency is 275.29(9) MHz, leading to ag-factor of 1.3653(25) if the hyperfine field from spin-echo measurements on⁹³NbFe is used. Relaxation times, line shapes, and the frequencyversus field behavior are discussed. A summary of data for CoFe alloys, used here for calibration purposes, is also given.A preliminary report of this work was given at the International Conference on Low Temperature Physics (LT 14), Helsinki, 1975. The work was supported in part by the Deutsche Forschungsgemeinschaft (SFB 161).     

FeSiF66H2O晶体的基态能级和零场分裂参量

由单晶的中子衍射方法得到FeS_iF₆6H₂O的晶体结构,这种晶体结构可以用S_iF^6-和Fe(H₂O)⁺⁺两个离子来描述,而局域三角对称的Fe(H₂O)⁺⁺离子反映了这种晶体的主要光谱性质利用不可约张量的理论,构成了晶体场和自旋轨道相互作用哈密顿完全对角化矩阵因此,由完全对角化的晶体场和自旋轨道相互作用哈密顿矩阵和电子顺磁共振的理论公式来求出晶体FeS_iF₆6H₂O中Fe²⁺离子的电子顺磁共振零场分裂参量D和Fa并研究了低自旋³L态对电子顺磁共振(EPR)零场分裂参量的贡献结果显示低自旋³L态对电子顺磁共振的零场分裂参量的贡献是较强的理论计算的结果与实验值是相符的.     

NMR line broadening in solids by slowing down of spin fluctuations

The¹⁰⁹Ag nuclear magnetic resonance line in a sample of polycrystalline AgF is observed to broaden substantially when the¹⁹F spins are irradiated near the magic angle in their rotating frame. This is due to the reduction of¹⁹F—¹⁹F dipolar coupling, which normally causes fluctuations in the¹⁹F—¹⁰⁹Ag interactions (Abragam and Winter), inducing an exchange narrowing analogous to classical motional narrowing. The¹⁰⁹Ag linewidths obtained over the entire motional range at different¹⁹F frequencies are compared with those calculated exactly from the ratio of second to fourth moment.     

ZnSiF66H2O∶Fe2+晶体光谱结构的杨特勒效应和电子顺磁共振g因子

研究了过渡金属络合物ZnSiF66H2O∶Fe2+晶体光谱结构的杨特勒效应和电子顺磁共振g因子。由单晶的中子衍射方法得到ZnSiF66H2O∶Fe2+的晶体结构,这种结构可以用SiF2-6和Zn(H2O)++∶Fe2+两个离子来描述。而局域三角对称的Zn(H2O)++∶Fe2+离子反映了这种晶体的主要光谱性质。利用不可约张量的理论构成了晶体场和自旋轨道相互作用哈密顿矩阵和电子顺磁共振理论公式,求出了晶体ZnSiF66H2O∶Fe2+中Fe2+离子的电子顺磁共振零场分裂参量(D,F-a)及g因子,并研究了低自旋3L态对电子顺磁共振零场分裂参量的贡献是不能忽略的,而对g因子的贡献是非常小的,并理论计算了它的晶体结构,证实了杨特勒效应的存在,理论计算的结果与实验值是相符的。     

Characterization of local environments in crystalline borophosphates using single and double resonance NMR

(11)B and (31)P magic-angle spinning as well as (11)B{(31)P} and (31)P{(11)B} rotational echo double resonance (REDOR) NMR have been applied to characterize the local environments in the crystalline borophosphates K(3)[BP(3)O(9)(OH)(3)], NH(4)[ZnBP(2)O(8)] and Rb(3)[B(2)P(3)O(11)(OH)(2)]. Dipolar second moment values extracted from the REDOR curves at short evolution times (DeltaS/S(0) < or = 0.2) are in reasonable agreement with those calculated from the internuclear distances in the corresponding crystal structures. In particular, the method is found to be useful for distinguishing between boron and phosphorus local environments with different numbers of B-O-P connectivities, making REDOR a well-suited tool for medium-range order investigations in glasses.     

Experimental observation of the spin screening effect in superconductor/ferromagnet thin film heterostructures

When studying the nuclear magnetic resonance (NMR) of ⁵¹V nuclei in superconductor/ferromagnet thin film heterostructures, anomalous behavior of the NMR signal was observed. This effect was manifested as a systematic distortion of the high-field wing of the resonance line. We consider this to be the first experimental evidence for penetration of ferromagnetism into a superconductor.     

The Evaluation of Dielectric Resonators Containing H2O or D2O as RF Coils for High-Field MR Imaging and Spectroscopy

Electromagnetic resonators consisting of low-loss dielectric material and/or metallic boundaries are widely used in microwave technologies. These dielectric resonators usually have high Q factors and well-defined field distributions. Magnetic resonance imaging was shown as a way of visualizing the magnetic field distribution of the resonant modes of these resonators, if the dielectric body contains NMR sensitive nuclei. Dielectric resonators have also been proposed as RF coils for magnetic resonance experiments. The feasibility of this idea in high-field MR is discussed here. Specifically, the dielectric resonances of cylindrical water columns were characterized at 170.7 MHz (4 T¹H Larmor frequency), and evaluated as NMR transmit and receive coils. The dielectric resonance of a cylindrical volume of D₂O was used to image a hand at 170.7 MHz. This study demonstrated that MRI is an effective way of visualizing the magnetic field in dielectric structures such as a water cylinder, and can potentially be generalized to solid-state dielectric devices. The possible applications of dielectric resonators other than simple cylindrical volumes in MRI and MR solution spectroscopy at high field strengths are also discussed.     

73Ge NMR spectra in germanium single crystals with different isotopic composition

We have studied the influence of isotopic disorder on the local deformations in Ge single crystals from both experimental and calculation points of view. The nuclear magnetic resonance (NMR) spectra of⁷³Ge nuclei (the nuclear spin equals 9/2) in perfect single crystals of germanium with different isotopic content were measured at temperatures 80, 300 and 450 K. Abnormal broadening of the spectrum was found to occur when the magnetic field was aligned along the [111] axis of a crystal. The observed specific angular dependence of the quadrupole broadening was attributed to isotopic disorder among atoms of germanium sited around the⁷³Ge NMR probe. Local lattice deformations in germanium crystal lattice due to isotopic impurity atoms were calculated in the framework of the adiabatic bond charge model. The results obtained were applied to study random noncubic crystal field interactions with the nuclear quadrupole moments and corresponding effects in NMR spectra. Simulated second and fourth moments of resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions agree qualitatively with the experimental data.     

Observation of "hidden" magnesium: first-principles calculations and 25Mg solid-state NMR of enstatite

²⁵Mg NMR parameters have been determined for two polymorphs of enstatite (MgSiO₃), an important magnesium silicate phase present as a major component of the Earth's upper mantle. The crystal structures of both polymorphs contain two crystallographically distinct magnesium sites; however, only a single resonance is observed in ²⁵Mg MAS NMR spectra recorded at 14.1 and 20.0 T. First-principles calculations performed on geometry-optimised crystal structures reveal that the quadrupolar interaction for the second site is expected to be very large, resulting in extensive broadening of the spectral resonance, explaining its apparent absence in the NMR spectrum. ²⁵Mg QCPMG NMR experiments employing variable offset cumulative spectroscopy (VOCS) are used to observe the broadened site and enable measurement of NMR parameters. The large difference in quadrupolar interaction between the two crystallographic magnesium sites is rationalised qualitatively in terms of the distortion of the local coordination environment as well as longer-range effects using a simple point charge model.     

Structural, static and dynamic magnetic properties of dextran coated γ-Fe(2)O(3) nanoparticles studied by (57)Fe NMR, M?ssbauer, TEM and magnetization measurements

The structural and magnetic properties and spin dynamics of dextran coated and uncoated γ-Fe(2)O(3) (maghemite) nanoparticles have been investigated using high resolution transmission electron microscopy (HRTEM), (57)Fe nuclear magnetic resonance (NMR), M?ssbauer spectroscopy and dc magnetization measurements. The HRTEM observations indicated a well-crystallized system of ellipsoid-shaped nanoparticles, with an average size of 10 nm. The combined M?ssbauer and magnetic study suggested the existence of significant interparticle interactions not only in the uncoated but also in the dextran coated nanoparticle assemblies. The zero-field NMR spectra of the nanoparticles at low temperatures are very similar to those of the bulk material, indicating the same hyperfine field values at saturation in accord with the performed M?ssbauer measurements. The T(2) NMR spin-spin relaxation time of the nanoparticles has also been measured as a function of temperature and found to be two orders of magnitude shorter than that of the bulk material. It is shown that the thermal fluctuations in the longitudinal magnetization of the nanoparticles in the low temperature limit may account for the shortening and the temperature dependence of the T(2) relaxation time. Thus, the low temperature NMR results are in accord with the mechanism of collective magnetic excitations, due to the precession of the magnetization around the easy direction of the magnetization at an energy minimum, a mechanism originally proposed to interpret M?ssbauer experiments in magnetic nanoparticles. The effect of the surface spins on the NMR relaxation mechanisms is also discussed.     

ESR, ENDOR, and ESEEM investigations on UV-irradiated 2,4,6-tri-tert-butyl phenol crystals

Electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) measurements were carried out for UV-irradiated 2,4,6-tri-tert-butyl phenol in the polycrystalline state. The radical produced in the crystal was detected by ESR and identified to be the corresponding phenoxyl radical, which is well characterized in the chemical oxidations in solutions. ENDOR and ESEEM spectra were unambiguously analyzed in terms of the hyperfine coupling constants determined from well-resolved ESR in solutions. Radical pairs in the crystals were also ascertained, and together with the single-crystal study the analysis disclosed zero-field splitting parameters in the triplet states. ESEEM time decays gave relaxation timesT ₁ = 5.94 andT ₂ = 1.12 μs at room temperature. These appropriate values permit an easy detection of the spin echoes, and therefore this radical matrix can be used as a useful standard for pulsed ESR investigations.