Analyses of the one-dimensional B-11 NMR spectra from static and MAS experiments of a polycrystalline ReB2 sample have extracted parameters characterizing both the electric field gradient and the shielding at the boron site. In contrast to that previously assumed for AlB2, the principal axis systems for these two interactions are not coincident in ReB2. A brief discussion of the literature regarding spin lattice relaxation for half-integer quadrupolar nuclei and the mathematical functions used for the extraction of relaxation time constants is presented. With radiofrequency conditions chosen to excite the central and satellite transitions, the saturation recovery spin lattice relaxation rate of ReB2 is better described by a single exponential rather than the Andrew-Tunstall multiexponential model for I = 3/2. In addition, relaxation measurements for B-11 in ReB2 have been extended to temperatures above ambient and show a deviation from the Korringa relationship.

The structure of tungsten diboride, WB2, differs from those of metal diborides with AlB2- or ReB2-type structures that have been previously studied by B-11 NMR spectroscopy. As opposed to the single B-11 resonance reported for the metal diborides having those AlB2- or ReB2-type structures, four groups of B-11 resonances are found for WB2. There is also B-11 NMR spectral evidence to support the proposal by Kiesling in 1947 of an additional boron atom at the center of some of the six-membered boron rings. The B-11 quadrupolar frequencies of all four groups of resonances in WB2 are all quite small (<20 kHz), in keeping with the trend observed for B-11 in metal diborides having the AlB2-structure as the metal is taken from further down the groups (families) in the periodic table.

Numerous nuclear magnetic resonance (NMR) investigations of the KH2PO4 family of hydrogen-bonded compounds that display ferroelectric behavior have reported results related to structure, molecular motion, and phase transitions. The current study looks at how high magnetic field and high-resolution solid-state techniques affect the measured NMR parameters for ammonium dihydrogen phosphate (NH4H2PO4). Under magic-angle spinning (MAS), the ammonium and acid protons do not achieve a common spin temperature as previously stated in the literature [i.e, the spin-lattice relaxation times (T-1) are different]. While the relaxation behavior in the static sample can be well-described with a single exponential at ambient temperature and below, the behavior becomes increasingly nonexponential as the sample is taken above ambient temperature. This behavior may arise from the motion of the ammonium moiety or from ionic conduction. The three principal values of the H-1 shielding tensors for the different species of protons in both NH4H2PO4 and KH2PO4 were measured with slow spinning H-1 combined rotational and multiple pulse spectroscopy experiments. The behavior of spinning sidebands under the multipulse sequence is briefly discussed.

Elemental boron typically exists in either of two states: crystalline or amorphous. In the synthesis of boron-based superhard materials, such as WB4, elemental boron is in some instances a side product that is difficult to separate from the desired superhard material. In the present study, both crystalline and amorphous boron are characterized by 10B and 11B nuclear magnetic resonance spectroscopy as a prelude for the study of boron-based superhard materials. The 11B spectrum of a static sample reflects both bulk magnetic susceptibility and second-order quadrupolar line shapes of quadrupolar frequencies ranging from 0 to 680 kHz. The 10B spectrum of a static sample shows quadrupolar frequencies ranging from 0 to 142 kHz. In contrast to the previous literature indicating relaxation of quadrupolar origin, the variable temperature spin–lattice relaxation data indicate that the 11B relaxation at 248 K and below is dominated by spin diffusion from paramagnetic centers. Above 248 K, relaxation is dominated by a thermally activated interaction with the conduction charge carriers originating from the boron vacancies. Relaxation in amorphous elemental boron shows an additional insulating component with a comparatively long time constant of 44 s.

Beneath the surface of a topological insulator is the bulk region (represented by its band structure), whose conductance should be kept minimal. New reproducible tools are needed which do not interfere with the surface states. Such non-invasive measurements of the bulk electronic properties can be provided by NMR spectroscopy, as shown by L.-S. Bouchard and co-workers on page 1519, which probes the resonant coupling of nuclear spins to the electrons near the Fermi level. Cover credit goes to Charlotte Gomez.

A model describing the production, distribution, and decay of 14>sC led to a proposal that the 14>sC content of an organic sample could be used as an index of age. Willard F. Libby conducted the critical experiments which established the basic validity of the method during 1947-9. Since that time all of the 6 basic physical assumptions of the method have been the subject of continuing scrutiny. Archaeologists who rely on 14>sC values to construct their chron-ologies are especially concerned with errors that may be introduced as a result of violations of lte 1 of the fundamental assumptions. Error reduction strategies are best implemented by close interdisciplinary collaboration and cooperation between radiocarbon specialists and those engaged in archaeological research studies. See also AATA 16-1172.

NMR relaxation studies and spectroscopic measurements of zeolitic imidazolate framework-8 (ZIF-8) are reported. The dominant nuclear spin–lattice relaxation (T1) mechanism for ZIF-8 in air arises from atmospheric paramagnetic molecular oxygen. The 13C T1 measurements indicate that the oxygen interacts primarily with the imidazolate ring rather than the methyl substituent. Similar relaxation behavior was also observed in a ZIF with an unsubstituted ring, ZIF-4. Single-crystal X-ray diffraction was used to provide data for the study of the thermal ellipsoids of ZIF-8 at variable temperatures from 100 to 298 K, which further confirmed the rigid nature of this ZIF framework. These results highlight a rigid ZIF framework and are in contrast with dynamic metal–organic frameworks based on benzenedicarboxylate linking groups, for which the relaxation reflects the dynamics of the benzenedicarboxylate moiety.

The framework motions in IRMOF-3 (Zn4O(BDC-NH2)3), where BDC-NH2 represents 2-amino-1,4-benzenedicarboxylate, have been investigated with 1H NMR relaxation measurements. Isotopic enrichment of the 2-amino group with 15N was critical in elucidating the lattice dynamics and enhancing spectral resolution. These results indicate a low energy process associated with rotation of the amino group, with an activation energy of 1.8±0.6kcal/mol, and full 180° rotation of the phenylene group in the BDC-NH2 moiety with an activation energy of 5.0±0.2kcal/mol. A relatively low pre-exponential factor for amine rotation (1.3×107s−1) is tentatively associated with the need to break a hydrogen bond as the rate-limiting step. Both amine rotation and the aromatic ring flip occur at frequencies that provide an effective relaxation mechanism for the 99.6% natural abundance quadrupolar 14N in the amino group. Dipolar coupling of the 14N to adjacent spin-½ nuclei (both 1H and 13C) occurs not only in the static sample but also in the MAS experiments at the 7T magnetic field used in this study. As a result, the spin dynamics and the cross-polarization dynamics are affected, resulting in spectral broadening. In the MAS experiments, isotopic replacement of the natural abundance 14N with 15N significantly improves resolution of the 15N spectra as well as in the 1H and 13C spectra.

The scientific literature contains reports of external magnetic fields affecting the fundamental properties and structure of water, including a report of the rotational motion of water molecules being slowed due to increased hydrogen bonding resulting from magnetic treatment. To investigate such a change in molecular motion, 1H spin–lattice relaxation times of water were measured at increasing magnetic field strengths. Strong radiation damping was observed at each magnetic field strength when using inversion-recovery experiments. To measure the relaxation times in the presence of radiation damping, several experimental methods of saturation recovery to measure spin–lattice relaxation were applied to a 90% H2O sample to obtain results as a function of field strength for proton frequencies from 300 to 800MHz.

Rotary biomolecular machines rely on highly symmetric supramolecular structures with rotating units that operate within a densely packed frame of reference, stator, embedded within relatively rigid membranes. The most notable examples are the enzyme FoF1 ATP synthase and the bacterial flagellum, which undergo rotation in steps determined by the symmetries of their rotators and rotating units. Speculating that a precise control of rotational dynamics in rigid environments will be essential for the development of artificial molecular machines, we analyzed the relation between rotational symmetry order and equilibrium rotational dynamics in a set of crystalline molecular gyroscopes with rotators having axial symmetry that ranges from two- to fivefold. The site exchange frequency for these molecules in their closely related crystals at ambient temperature varies by several orders of magnitude, up to ca. 4.46 x 10(8) s(-1).

The principal elements of the 199Hg chemical-shift (CS) tensors of the mercuric halides (HgX2, X=F, Cl, Br, and I) and the mercurous halides (Hg2X2, X=F and Cl) were determined from spectra of static polycrystalline powders and from magic-angle spinning (MAS) spectra. The CS tensors of both HgCl2 and Hg2Cl2 are axially symmetric (η=0) within experimental error, differing from literature reports of η=0.12 and η=0.14, respectively. The principal elements of the axially symmetric CS tensor in HgBr2 were also measured using a static sample, and the wideline spectra of HgF2 and HgI2 (red polymorph) give chemical-shift tensors that suggest, within experimental error, that the mercury sits in sites of cubic symmetry. The 199Hg CS tensor for Hg2F2 is asymmetric. Experiments with static polycrystalline samples may allow the determination of the elements of the 199Hg CS tensors even when MAS fails to completely average the dipolar coupling of the spin-½ 199Hg and the quadrupolar halide nucleus.

The 199Hg chemical-shift tensor of solid HgCl2 was determined from spectra of polycrystalline materials, using static and magic-angle spinning (MAS) techniques at multiple spinning frequencies and field strengths. The chemical-shift tensor of solid HgCl2 is axially symmetric (η=0) within experimental error. The 199Hg chemical-shift anisotropy (CSA) of HgCl2 in a frozen solution in dimethylsulfoxide (DMSO) is significantly smaller than that of the solid, implying that the local electronic structure in the solid is different from that of the material in solution. The experimental chemical-shift results (solution and solid state) are compared with those predicted by density functional theory (DFT) calculations using the zeroth-order regular approximation (ZORA) to account for relativistic effects.199Hg spin–lattice relaxation of HgCl2 dissolved in DMSO is dominated by a CSA mechanism, but a second contribution to relaxation arises from ligand exchange. Relaxation in the solid state is independent of temperature, suggesting relaxation by paramagnetic impurities or defects.

Proton NMR spin–lattice relaxation times in the laboratory frame (T1) and in the rotating frame (T1ρ) were measured as a function of temperature for a static sample of α-glycine. Both T1 and T1ρ data can be fit quantitatively by a single thermally-activated motion (the modulation of the dipolar coupling by random hopping about the threefold axis of the –NH3 group), with no addition of other mechanisms at any temperature between 173 and 415K. An activation energy of 21.7±1kJ/mol was extracted and is compared with previously reported values for both α- and γ-glycine. Such comparisons allow the correction of glycine polymorphs misidentified in the literature. The minimum in T1 at 325K corresponds to a correlation time of 0.53ns. Chemical shifts as a function of temperature were measured by 1H CRAMPS and by 13C and 15N CP/MAS experiments. These results are discussed relative to a previous report of anomalous electrical behavior in α-glycine within this temperature range.

The interaction of water with cellulose and its influence on the nuclear spin dynamics in Gossypium barbadense (Pima) cotton were investigated by 1H and 13C solid-state NMR techniques. 1H spin diffusion results from a Goldman-Shen experiment indicate that the water is multilayered. 1H MAS experiments provide evidence of a range of correlation times for the water, indicative of molecular motion ranging from restricted to relatively mobile. The 1H spin-lattice relaxation time varies with water content and is different for static and MAS conditions. By coupling the Goldman-Shen sequence with 13C CP/MAS, cross-polarization from the molecularly mobile water protons distributes magnetization throughout the cellulose (as opposed to enhancing 13C resonances from only the crystalline or the amorphous domains or from only the surface of the cellulose). However, spatial localization of the combined Goldman-Shen-13C CP/MAS experiment using both short mixing and contact times yields a spectrum consistent with predominantly the Iβ polymorph of cellulose. Longer mixing times and the same, short contact time yield a spectrum that is indicative of an increased Iα polymorph content in the crystallite interiors relative to the smaller values found with short mixing times.

We observe non-monotonic development of the 13C magnetization in polycrystalline samples of glycine, sucrose, and adamantine during cross-polarization. We demonstrate, by fitting the time dependence, that the development quantitatively results from dipolar oscillations. To fit the data quantitatively requires one to assume two types of spin-diffusion behavior.

F and Hg high-resolution solution NMR spectra were acquired for cyclic trimeric perfluoro-ortho-phenylenemercury. Even with the high superconducting magnetic fields currently available, the F spectrum was not interpretable with a simple first-order analysis. Spectroscopic parameters for chemical shift and scalar coupling interactions in the NMR spectra were extracted from fourand five-spin simulations. Differential line widths in the F spectrum result from scalar coupling to the Hg.

Proton NMR spin–lattice relaxation times T1 were measured for urea as a function of temperature. An activation energy of 46.3±4.7kJ/mol was extracted and compared with the range of 38–65kJ/mol previously reported in the literature as measured by different magnetic resonance techniques. In addition, proton NMR spin–lattice relaxation times in the rotating frame T1ρ were measured as a function of temperature. These measurements provide acquisition conditions for the 13C and 15N CP/MAS spectra of pure urea in the crystalline phase.

19F and 199Hg high-resolution solution NMR spectra were acquired for cyclic trimeric perfluoro-ortho-phenylenemercury. Even with the high superconducting magnetic fields currently available, the 19F spectrum was not interpretable with a simple first-order analysis. Spectroscopic parameters for chemical shift and scalar coupling interactions in the NMR spectra were extracted from four- and five-spin simulations. Differential line widths in the 19F spectrum result from scalar coupling to the 199Hg.