Broad NMR Spectra Research Articles

Automated detection of broad NMR spectra: 19F NMR of paramagnetic UF4 and 195Pt NMR of supported Pt catalysts

The automated detection of broad NMR spectra via the controlled stepwise motion of the NMR probe along the field axis of the superconducting solenoid is demonstrated for the detection of 19F NMR of paramagnetic UF4 and 195Pt NMR of supported metal catalysts. The sensitivity advantages of performing these measurements at 2.3 T are discussed with reference to applying this methodology to room temperature in situ electrochemical 195Pt NMR.

Intrinsic Proton NMR Studies of Mg(OH)2 and Ca(OH)2

We studied the short proton free induction decay signals and the broad ^1H NMR spectra of Mg(OH)2 and Ca(OH)2 powders at 77-355 K and 42 MHz using pulsed NMR techniques. Using a Gaussian-type back extrapolation procedure for the obscured data of the proton free induction decay signals, we obtained more precise values of the second moments of the Fourier-transformed broad NMR spectra than those in a previous report [Y. Itoh and M. Isobe, J. Phys. Soc. Jpn. Vol. 84, 113601 (2015)] and compared with the theoretical second moments. The decrease in the second moment could not account for the large decrease in the magnitude of the intrinsic proton spin-lattice relaxation rate 1/T_1 from Mg(OH)2 to Ca(OH)2. The analysis of 1/T_1 $propto$ exp(-E_g/k_BT) with E_g~0.01 eV points to a local hopping mechanism, and that of 1/T_1 $propto$ T^n with n~0.5 points to an anharmonic rattling mechanism.

Field-stepped broadband NMR in pulsed magnets and application to SrCu2(BO3)2 at 54 T

Pulsed magnets generate the highest magnetic fields as brief transients during which the observation of NMR is difficult, however, this is the only route to unique insight into material properties up to the regime of 100T. Here, it is shown how rather broad NMR spectra can be assembled in a pulsed magnet during a single field pulse by using the inherent time dependence of the field for the recording of field-stepped free induction decays that cover a broad frequency range. The technique is then applied to 11B NMR of the spin-dimer system SrCu2(BO3)2, a magnetic insulator known to undergo a series of field-driven changes of the magnetic ground state. At peak fields of about 54T at the Dresden High Magnetic Field Laboratory, 11B NMR spectra spanning a total of about 9MHz width are reconstructed. The results are in good accordance with a change from a high-temperature paramagnetic state to a low-temperature commensurate superstructure of field-induced spin-dimer triplets.

Hydroxyl Motion in Mg(OH)2

We report on pulsed 1H NMR studies of the hydroxyl OH groups in magnesium hydroxide Mg(OH)2 at 77–355 K at 42.5772 MHz. The Fourier-transformed NMR spectra show the superposition of broad and narrow components. The broad NMR spectrum is assigned to dipole-coupled protons on a rigid lattice in the bulk Mg(OH)2, while the narrow NMR spectrum is assigned to extrinsic protons, e.g., conduction protons facilitated by lattice defects. We found a monotonically decreasing linewidth of the broad NMR spectrum on heating. The monotonic decrease in the linewidth is associated with hopping protons around a threefold axis (rotational hydroxyl protons).

Conformational Properties of β-PrP

Prion propagation involves a conformational transition of the cellular form of prion protein (PrPC) to a disease-specific isomer (PrPSc), shifting from a predominantly alpha-helical conformation to one dominated by beta-sheet structure. This conformational transition is of critical importance in understanding the molecular basis for prion disease. Here, we elucidate the conformational properties of a disulfide-reduced fragment of human PrP spanning residues 91-231 under acidic conditions, using a combination of heteronuclear NMR, analytical ultracentrifugation, and circular dichroism. We find that this form of the protein, which similarly to PrPSc, is a potent inhibitor of the 26 S proteasome, assembles into soluble oligomers that have significant beta-sheet content. The monomeric precursor to these oligomers exhibits many of the characteristics of a molten globule intermediate with some helical character in regions that form helices I and III in the PrPC conformation, whereas helix II exhibits little evidence for adopting a helical conformation, suggesting that this region is a likely source of interaction within the initial phases of the transformation to a beta-rich conformation. This precursor state is almost as compact as the folded PrPC structure and, as it assembles, only residues 126-227 are immobilized within the oligomeric structure, leaving the remainder in a mobile, random-coil state.

Specific heat and NMR studies on the pyrite-type superconductors CuS 2 and CuSe 2

We have investigated the superconducting and magnetic properties of the pyrite-type CuS 2 and CuSe 2 synthesized under high pressures by measurements of specific heat, magnetization and NMR. The first measurements of the specific heat gave T S=1.50 (2.47) K and Δ C/ γT S=1.34(1.47) for CuS 2 (CuSe 2 ). The broad NMR spectra for 63,65Cu in CuS 2 measured at 1.3 K in various fields can be explained well by assuming the incommensurate CDW transition occurs at 150 K. The magnetization mesurements in CuS 2 (type B) showed the weak ferromagnetism with T C =23 K .

Comparative study of motions in dimethylsulfone by noise excitation and solid echo spectroscopy

2H NMR spectra of dimethylsulfone were measured with noise excitation and solid echo NMR spectroscopy in the temperature range from 125 to 355 K. Besides the known fact that broad NMR spectra can be measured with both methods, in comparable times it is shown that for noise excitation, the signal loss is negligible compared to echo spectroscopy in the regime when the correlation times of the motions are of the order of magnitude of the echo pulse spacing. For simulating the dynamic NMR spectra acquired with noise excitation, only the motional process must be taken into account and relaxation can be neglected. Furthermore, the problem of restricted acquisition bandwidth in noise NMR spectroscopy is discussed.

Ferromagnetism in the insulating cuprate La4Ba2Cu2O10

The 139La and 63,65Cu resonance in the broad NMR spectrum of ferromagnetic La4Ba2Cu2O10 was investigated below Tc = 5.0 K. The electric field gradient and the anisotropic hyperfine field of Cu can be described by a hole localized in the dx2−y2 orbital. The ferromagnetic coupling results from the orthogonal Cu orbitals in the c-plane of the tetragonal structure. From the temperature dependence of the spontaneous magnetization and the large Suhl-Nakamura interaction we conclude that the system is a two-dimensional ferromagnet with an exchange constant J = 13.5(10) K and an anisotropy field BA = 120(20) mT perpendicular to the ferromagnetic planes. The large transferred hyperfine field for La (Bhf = 6.15(5)) indicates a considerable ferromagnetic coupling along the c-axis.

Ammonium Sulfate as a Standard for 33S-NMR Spectra

Ammonium sulfate was found possibly useful as a standard for the chemical shift and line-width of sulfur-33 NMR. 2 M aqueous solution gave a signal with S/N larger than 4 in 1013 s and, thus show-ing it useful for probe tuning and matching as well as deciding on optimum pulse conditions in a short time. A line-width relative to that of ammonium sulfate or a reference was essentially constant for all pulse conditions. Thus, by measuring the line-width of signals at any experimental pulse condition linewidth can be expressed for any of the following standard pulse conditions : spectral width of 1500 Hz, data points of 4 K, pulse width π/2, pulse delay time of 0.1 s, and a line broadening factor of 5 Hz, by which ammonium sulfate of 1 M solution gives a width of 8.5 Hz. The present method is applicable to all other insensitive and broad NMR spectra.

Spin echo defocusing in Y 2Co 17

The influence of the small pulses of magnetic field on 59Co spin echo intensity has been investigated in Y 2Co 17 and (Y 0.9Gd 0.1) 2Co 17. Such pulses acting between r.f. pulses modulate the angular frequency of the rotating nuclear moments due to an anisotropic component of the hyperfine field (ACHF). This effect leads to the defocusing of the nuclear moments in the rotating frame during the formation of the spin echo signal. The drop of the spin echo intensity reflects the magnitude of ACHF. The strong influence of Gd ions on ACHF in (Y 0.9Gd 0.1) 2Co 17 has been detected in narrow range of the broad NMR spectrum. The main advantage of the described method over conventional spin echo technique is its higher resolution.

Polymer conformation and NMR chemical shifts, 9. Poly(α‐methylene‐γ‐butyrolactone)

AbstractThe conformational energies of poly(α‐methylene‐γ‐butyrolactone) are calculated and compared with those of poly(methyl methacrylate). In spite of the structural resemblance of these two polymers, the patterns of the energy contour maps are clearly distinguishable from each other; the energy barriers between rotational isomeric states are appreciably higher in the former than in the latter polymer. The calculation indicates large non‐bonded interactions between the protons in one lactone ring and those in the adjacent lactone rings. The broad NMR spectrum of poly(α‐methylene‐γ‐butyrolactone) apparently reflects its rigid conformational structure. 1H and 13C NMR chemical shifts are calculated by theoretical shielding calculations based on conformational analysis. Much lower magnetic field resonances of the OCH2 and α‐CH2 carbons in poly(α‐methylene‐γ‐butyrolactone) as compared with those of the OCH3 and α‐CH3 carbons in poly(methyl methacrylate) are well reproduced by the calculation. The shift to lower magnetic field is mainly attributed to paramagnetic shielding derived from the interaction between OCH2 carbon and α‐CH2 carbon. Tacticity‐ and conformation‐dependent 1H and 13C NMR chemical shifts of poly(α‐methylene‐γ‐butyrolactone) are well interpreted on the basis of the conformational analysis.

Some trimethylphosphineoxide complexes of lanthanide and indium tris-(bis-trimethylsilylamides)

The reactions of the 3-coordinated metal tris-(bis-trimethylsilylamides) M[N(SiMe 3) 2] 3 (ML 3; where M = La, Pr, Eu, Gd and In) with trimethylphosphineoxide Me 3PO(L′) have been studied. All of these metals gave 1:1 complexes ML 3(L′) which dissociated on heating in vacuo. The Pr and Eu complexes gave interesting pseudo-contact shifted 1H NMR spectra which are qualitatively in accord with the expected molecular structure. Variable temperature NMR measurements proved the restricted rotation of the silylamide (L) ligands occurs around the M–N axes. Evidence is also presented for unstable 5-coordinated complexes ML 3(L′) 2. The gadolinium 1:1 complex gave a very broad NMR spectrum and its ESR spectrum is being investigated.

Dipolar Broadening of the NMR Spectrum of 19F in UF4

AbstractThe influence of internuclear dipole–dipole and electron–nuclear dipolar interactions on the NMR spectrum shape was examined for UF4. It is shown that only the dipolar interaction of the 19F nuclei with the magnetic moments of the U4+ ions leads to such a broad NMR spectrum in polycrystalline UF4 that there is no ground for separating it into several components corresponding to the seven nonequivalent fluorine nuclei in the UF4 structure. A certain additional broadening of the experimental spectrum may be accounted for by anisotropic hyperfine interaction, due to the transfer of the spin density from U4+ to fluorine.

Molecular Structure of Iodine Heptafluoride

Electron diffraction photographs taken in 1951 are here reanalyzed. All measurements and intensity estimates had been made visually; in the computations, allowance is made for the relative phase change of the scattering amplitudes between the iodine and fluorine atoms. Within a spread of 0.06 A, the I—F distances are approximately equal; the average value is 1.825±0.015 A. The molecular symmetry is almost D5h, in agreement with spectroscopic and crystal structure results. However, the electron diffraction patterns clearly indicate that the five fluorine atoms in the girdle are not coplanar and possibly are not equally spaced from the central iodine. The disorder reported in the crystalline state and the unusually broad NMR spectra of the liquid are readily accounted for by a sequential puckering about the polar axis and the possible out-of-phase extension from the axis of the girdle atoms. Revised values for the thermodynamic functions of the ideal gas are included.