Solid-state 87Rb NMR of RbVO3. A comparison of experiments for retrieving chemical shielding and quadrupole coupling tensorial interactions.

Solid-state 87Rb NMR spectra for a powder and single crystal of RbVO3 have been acquired for the central transition at two magnetic field strengths (9.4 and 14.1 T) and using two single-crystal NMR probes of different design. The powder spectra have been obtained using spin-echo techniques without sample spinning because the widths of the spectra are in the range 100-150 kHz. The spectra are analyzed in terms of the chemical shielding and quadrupole coupling interactions and the parameters are compared in an evaluation of the precision for the techniques. Parameters of high precision including the relative orientation for the two tensors are obtained from the single-crystal spectra at 14.1 T. Finally, the orientations of the two tensors in the crystal frame are deduced from the crystal symmetry and an XRD analysis.

Separation of (2)H MAS NMR spectra by two-dimensional spectroscopy.

New methods for optimum separation of (2)H MAS NMR spectra are presented. The approach is based on hypercomplex spectroscopy that is useful for sign discrimination and phase separation. A new theoretical formalism is developed for the description of hypercomplex experiments. This exploits the properties of Lie algebras and hypercomplex numbers to obtain a solution to the Liouville-von Neumann equation. The solution is expressed in terms of coherence transfer functions that describe the allowed coherence transfer pathways in the system. The theoretical formalism is essential in order to understand all the features of hypercomplex experiments. The method is applied to the development of two-dimensional quadrupole-resolved (2)H MAS NMR spectroscopy. The important features of this technique are discussed and two different versions are presented with widely different characteristics. An improved version of two-dimensional double-quantum (2)H MAS NMR spectroscopy is developed. The conditions under which the double-quantum experiment is useful are discussed and its performance is compared with that observed for the quadrupole-resolved experiments. A general method is presented for evaluating the optimum pulse sequence parameters consistent with maximum sensitivity and resolution. This approach improves the performance of the experiments and is essential for any further development of the techniques. The effects of finite pulse width and hypercomplex data processing may lead to both intensity and phase distortions in the spectra. These effects are analyzed and general correction procedures are suggested. The techniques are applied to polycrystalline malonic-acid-(2)H(4) for which the spinning sideband manifolds from the carboxyl and methylene deuterons are separated. The spinning sideband manifolds are simulated to determine the quadrupole parameters. The values are consistent with previous results, indicating that the techniques are both accurate and reliable.

Efficient spectral simulations in NMR of rotating solids. The gamma-COMPUTE algorithm.

We explore the time-translational relation between one of the powder angles (gamma) and the sample rotation angle (omegart) in NMR spectroscopy of rotating solids. Averaging over the gamma powder angle is shown to be generally equivalent to a cross correlation of two periodic functions. This leads to a fundamental relation concerning the phases of NMR spectra of rotating solids as well as improved strategies for efficient simulation of experimental spectra. Using these results in combination with the frequency-domain simulation procedure COMPUTE (M. Edén et al., J. Magn. Reson. A 120, 56 (1996)), it proves possible to reduce the computation time for spectral simulations by typically a factor 10-30 relative to the state-of-the-art calculations using the original COMPUTE algorithm. The advantage and the general applicability of the new simulation procedure, referred to as gamma-COMPUTE, are demonstrated by simulation of single- and multiple-pulse MAS NMR spectra of 31P-31P and 1H-1H spin pairs influenced by anisotropic chemical shielding and homonuclear dipolar interactions.

Improved hardware and software for single-crystal NMR spectroscopy.

Design of state-of-the-art instrumentation and software for acquisition and analysis of single-crystal NMR spectra is presented. The design involves highly accurate rotation of a goniometer, and the acquisition of all the spectra for each rotation axis is automatically controlled by the host computer of the spectrometer using a homebuilt interface between the computer and the single-crystal probe. Moreover, a software package (ASICS) for fast and routine assignment/analysis of complex single-crystal spectra has been developed. Employing this equipment, the acquisition and complete analysis of single-crystal NMR spectra may be performed in about the same time as required for powder methods (spinning or static). The hardware and software are compared to recent alternative approaches within single-crystal NMR. Finally, it has been observed that single-crystal NMR techniques may provide the desired data for samples where powder methods fail.

Line shapes and widths of MAS sidebands for 27Al satellite transitions. multinuclear MAS NMR of tugtupite Na8Al2Be2Si8O24Cl2.

A multinuclear 9Be, 23Na, 27Al, and 29Si magic-angle spinning (MAS) NMR study has been performed for the mineral tugtupite (Na8Al2Be2Si8O24Cl2). The extremely well-resolved spectra allow observation of separate spinning sidebands (ssb's) from the inner (+/- 1/2, +/- 3/2) and outer (+/- 3/2, +/- 5/2) 27Al satellite transitions, and are utilized in a detailed analysis of the line shapes and widths of the individual ssb's from simulations. The line widths of the ssb's from the inner and outer 27Al satellite transitions are found to decrease systematically with increasing order of the ssb's across the spectrum. Accurate values for the 9Be, 23Na, and 27Al quadrupole coupling parameters and isotropic chemical shifts are obtained from simulations of the manifolds of ssb's from the satellite transitions. MAS NMR of the 9Be satellite transitions for tugtupite, BeO, and beryl(Al2Be3Si6O18) shows that these transitions are particularly useful for determination of 9Be quadrupole couplings because of the small 9Be quadrupole moment. The 29Si shielding anisotropy of delta sigma = 48 ppm in tugtupite is the largest determined so far for a framework SiO4 tetrahedron. Finally, the crystal structure of the tugtupite sample has been refined by single-crystal X-ray diffraction, and correlations between the multinuclear NMR parameters and structural data are reported.