Two‐dimensional nutation echo 75As NQR spectroscopy in antiferroelectric NH4H2AsO4 using the maximum entropy method

Two-dimensional nutation FT/MEM 35Cl nuclear quadrupole resonance spectroscopy and hydrogen bonding in 2,6-dichlorophenol

Rb2ZnBr4 and its isomorphs are known to exhibit displacively modulated structures in their incommensurate phases and ferrophases. Correspondingly the electrical field gradient (EFG) tensor at the site of quadrupole nucleus is also modulated. This EFG tensor can be calculated assuming the validity of the point charge model. Using the experimental data for the amplitude of the EFG tensor modulation in the incommensurate phase we have calculated the displacements of the atoms in the incommensurate structure. The results are compared to the experimentaly determined ferrophase structures of K2ZnCl4 and Rb2ZnCl4.

The electric field gradient (EFG) tensors of 7Li and 23Na in a zektzerite, NaLiZrSi6O15 single crystal have been determined by the nuclear magnetic resonance technique. The quadrupole coupling constants, e2qQ/h and the asymmetry parameters, η, for 7Li are 119 ± 5 kHz, 0.05 and for 23Na 2.42 ± 0.15 MHz, 0.86 respectively. In both cases, the EFG tensors show the two-fold site-symmetries at the Li- and Na-sites. For 7Li, the magnitude of the quadrupole coupling constant and the principal axis of the EFG tensor reflect the large angular distortion of the [LiO4] tetrahedron. The large quadrupole coupling constant of 23Na indicates a high degree of distortion of the Na coordination polyhedron.

Theoretical investigation of the effects of diverse hydrogen-bonding characteristics on the 17O chemical shielding and electric field gradient tensors within the active sites of MraYAA bound to nucleoside antibiotics capuramycin, carbacaprazamycin, 3′-Hydroxymureidomycin A, and muraymycin D2

Natural-abundance (43)Ca solid-state NMR spectroscopy at 21.1 T and gauge-including projector-augmented-wave (GIPAW) DFT calculations are developed as tools to provide insight into calcium binding environments, with special emphasis on the calcium chemical shift (CS) tensor. The first complete analysis of a (43)Ca solid-state NMR spectrum, including the relative orientation of the CS and electric field gradient (EFG) tensors, is reported for calcite. GIPAW calculations of the (43)Ca CS and EFG tensors for a series of small molecules are shown to reproduce experimental trends; for example, the trend in available solid-state chemical shifts is reproduced with a correlation coefficient of 0.983. The results strongly suggest the utility of the calcium CS tensor as a novel probe of calcium binding environments in a range of calcium-containing materials. For example, for three polymorphs of CaCO3 the CS tensor span ranges from 8 to 70 ppm and the symmetry around calcium is manifested differently in the CS tensor as compared with the EFG tensor. The advantages of characterizing the CS tensor are particularly evident in very high magnetic fields where the effect of calcium CS anisotropy is augmented in hertz while the effect of second-order quadrupolar broadening is often obscured for (43)Ca because of its small quadrupole moment. Finally, as an application of the combined experimental-theoretical approach, the solid-state structure of the vaterite polymorph of calcium carbonate is probed and we conclude that the hexagonal P6(3)/mmc space group provides a better representation of the structure than does the orthorhombic Pbnm space group, thereby demonstrating the utility of (43)Ca solid-state NMR as a complementary tool to X-ray crystallographic methods.

We report solid-state 17O NMR determinations of the oxygen chemical shift (CS) and electric field gradient (EFG) tensors for a series of 17O-enriched organic compounds containing various functional groups. In several cases, analysis of the n O magic-angle-spinning (MAS) and static NMR spectra yields both the magnitude and relative orientations of the 17O CS and EFG tensors. We also demonstrate the feasibility of solid-state 17O NMR as a potentially useful technique for studying molecular structure and hydrogen bonding.

The recently reported direct enhancement of integer spin magnetization (DEISM) methodology for signal enhancement in solid-state NMR of integer spins has been used to obtain static (14)N powder patterns from alpha-glycine, L-leucine and L-proline in relatively short experimental times at 9.4 T, allowing accurate determination of the quadrupolar parameters. Proton decoupling and deuteration of the nitrogen sites were used to reduce the (1)H-(14)N dipolar contribution to the transverse relaxation time allowing more echoes to be acquired per scan. In addition, ab initio calculations using molecular clusters (Gaussian 03) and the full crystal lattice (CASTEP) have been employed to confirm these results, to obtain the orientation of the electric field gradient (EFG) tensors in the molecular frame, and also to correctly assign the two sets of parameters for L-leucine. The (14)N EFG tensor is shown to be highly sensitive to the surrounding environment, particularly to nearby hydrogen bonding.

PEANUT experiment in NQR spectroscopy for I=3/2

The electric field gradient (EFG) tensor at the site of the deuteron in the hydrogen bond of K${\mathrm{D}}_{2}$P${\mathrm{O}}_{4}$ and the relaxation times of the deuteron have been studied. The measured quadrupole coupling constant is 119 kc/sec, the asymmetry parameter is 0.049, and the major principal axis of the tensor is along the bond direction. None of these features of the EFG tensor change appreciably with temperature over the entire range studied, which range includes the ferroelectric Curie point ${T}_{c}$. An interesting additional splitting in the spectral lines below ${T}_{c}$ is observed however. The thermal relaxation time of the deuteron has been studied carefully only at room temperature. One obtains ${T}_{1}=6.5$ minutes at a particular orientation of the crystal decreasing by a factor of 2 at other particular orientations. ${T}_{2}$ is of the order 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ second. All deuterons have the same relaxation as a function of crystal orientation although falling into distinct categories from the point of view of relative orientation of electric field gradient tensor and applied magnetic field. This interdependence of deuterons in different environments and the possible causes of relaxation are discussed.

Solid-state 25Mg QCPMG NMR of bis(cyclopentadienyl)magnesium

Hyperfine interactions measurements on 181Ta probes in HfV 2H x compounds: (0⩽ x≲4)

The EFG tensors at the Fe atoms in myoglobin-02 and -CO are determined by Mzssbauer ex- periments on single crystals. In the MbOz compound the quadrupole interaction (e&Vzz) is negative and the largest principal axis of the EFG lies nearly in the heme plane, while in the MbCO compound eQVzZ is positive and the axis of the EFG is nearly perpendicular to the heme plane. Mzssbauer spectroscopy has provided the most direct knowledge of the iron-oxygen binding in myo- globin (Mb) and hemoglobin (Hb). Although both the 02 and CO compounds of Mb and Hb are diamagnetic, the quadrupole splittings in the 02-form are quite different from those in the CO-form, and the pre- sence of large asymmetric charge transfer is sug- gested in the 02-form /I/. In order to further dis- cuss the bonding structure of ligands to heme iron, not only the principal values of the electric field gradient (EFG) tensor, but also its orientation is strongly required to be determined for these com- pounds. Single crystals were grown from a solution of sperm whale met%, enriched with 57~e, and vi- sually selected perfect crystals were converted into VELOCITY lrnrn/s1 -3 -2 -1 0 1 2 3,-3 -2 -1 0 1 2 3

The Burg algorithm for maximum entropy power spectral density estimation is applied to a time series of data obtained from a Michelson interferometer and compared with a standard fast Fourier transform (FFT) estimate for resolution capability. The propane transmittance spectrum was estimated by use of the FFT with a 2(18)-data sample interferogram, giving a maximum unapodized resolution of 0.06 cm(-1). This estimate was then interpolated by zero filling an additional 2(18) points, and the final resolution was taken to be 0.06 cm(-1). Comparison of the maximum entropy method (MEM) estimate with the FFT was made over a 45-cm(-1) region of the spectrum for several increasing record lengths of interferogram data beginning at 2(10). It is found that over this region of 45 cm(-1), the MEM estimate with 2(16) data samples is in close agreement with the FFT estimate using 2(18) samples.

Density functional calculations of oxygen, nitrogen and hydrogen electric field gradient and chemical shielding tensors to study hydrogen bonding properties of peptide group (O [dbnd]C–NH) in crystalline acetamide

Computational studies on boron nitride and boron phosphide nanotubes: Density functional calculations of boron-11 electric field gradient tensors