Magic Angle Spinning Conditions Research Articles

The stop-and-go spinning technique in MAS experiments

High-resolution solid-state NMR experiments are described in which detection occurs under magic-angle spinning conditions, but other portions of the experimental sequence are carried out with the sample static. This is achieved by starting and stopping the spinner during the sequence. In a 2D FT experiment designed to provide a chemical-shift tensor pattern for each isotropic chemical-shift tensor average, the sample is static during the evolution period and the magnetization evolves according to the complete chemical-shift tensor during that period. In another experiment, I to S cross polarization is carried out under static-sample conditions, avoiding the well-known modulation patterns in the Hartmann-Hahn match condition for systems with weakly coupled I spins. Examples of the application of these techniques are presented and prospects for such techniques are discussed.

2D NMR nutation spectroscopy in solids

The quadrupole interaction parameters of half-integer spin nuclei are accessible from the dependence of NMR central transition magnetization on the RF excitation pulse length. These nutation spectra are obtained from spectral analysis of the magnetization amplitude and consist of up to 2 I major lines. The lines correspond to central transition magnetization components, precessing in the plane perpendicular to the rotating magnetic field created by the RF excitation pulse. The magnetization components may be associated with single-quantum coherences in rotating magnetic field tilted by 90° from the strong stationary magnetic field. The precession direction differs for different transitions in the tilted frame and can be determined by an additional attenuated RF pulse, acting selectively on the central transition only. The RF field used must be as strong as possible. Increasing RF field strength leads to less complicated nutation spectra and facilitates their interpretation even under rapid magic-angle sample spinning conditions.

Spectral spin diffusion in polycrystalline solids under magic-angle spinning

Spectral spin diffusion in 13C NMR of double 13C-labelled sodium acetate trihydrate (SAC), and in 31P NMR of zinc(II) bis(O,O′-diethyldithiophosphate)(ZNP) has been studied under magic-angle spinning conditions. Spin-diffusion time constants, TSD, were determined from the intensities of the spinning sidebands in experiments using rotation-synchronized DANTE pulse sequences, at several different spinning frequencies. The theory of Suter and Ernst, developed for spectral spin diffusion in single crystals, was extended to the case of polycrystalline samples rotating under magic-angle spinning conditions. We considered two mechanisms for the spin diffusion, i.e. dipolar interaction and J-coupling. The spin-diffusion time constants, TSD, were related to the zero-quantum lineshape functions in a manner similar to the theory of Suter and Ernst. The zero-quantum lineshape functions were estimated from the observed single-quantum lineshape functions. In the present studies the dependence of the experimental values for TSD on the rotational frequency vr are in good agreement with those calculated from the theory based on the dipolar interaction mechanism. The values of TSD for SAC showed a deep minimum at Δω≈ 2ωr, and a shallow minimum at Δω≈ 3ωr. This phenomenon is rotational relaxation resonance.

Variable Temperature15N CPMAS NMR Studies of Dye Tautomerism in Crystalline and Amorphous Environments

AbstractThis paper deals with the question of how symmetric double minimum potentials of isolated bistable molecules are affected in the ordered or disordered solid state, in a timescale of slow molecular motion. It is shown that high resolution solid state NMR under conditions of magic angle spinning (MAS) and cross‐polarization (CP) can contribute to answering this question. In particular, variable temperature high resolution solid state15N CPMAS NMR spectra (CP = cross polarization, MAS = magic angle spinning) of15N enriched tetraaza[14]annulene dyes in the crystalline state and imbedded in disordered glassy polystyrene are presented. These dyes interconvert fast between different tautomeric states whose gasphase degeneracy is lifted in the solid. By contrast to the crystalline state where all dye molecules experience the same solid state perturbation i.e. the same equilibrium constants of tautomerism, molecules dissolved in the glass exist in a multitude of different inequivalent sites with different solid state perturbations. This effect leads to a broad distribution of equilibrium constants of dye tautomerism. This interpretation is confirmed by NMR lineshape simulations and two‐dimensional15N CPMAS NMR experiments by which the temperature dependent distribution can be characterized. High temperature experiments in the region of the glass transition give insights into the process leading to motionally averaged symmetric double minimum potential of tautomerism within the NMR timescale. Thus, solid solution NMR studies provide information about elementary steps of molecular rearrangements in a timescale of slow solvent reorientation. In addition, dyes such as tetraaza[14]annulenes can be used as novel NMR probes for microscopic order and motion in glasses.

Effects of quadrupolar nuclei on NMR spectra of [formula omitted] nuclei in magic-angle spinning experiments

Nuclear magnetic resonance spectra for I = 1 2 nuclei of solids which are observed under magic-angle spinning (MAS) conditions show a characteristic deformation of the lineshape, mostly observed as a line broadening, for signals of those nuclei which are directly coupled to a quadrupole moment bearing nucleus ( S > 1 2 ). For a quantitative interpretation of this effect the theory of VanderHart et al. ( J. Chem. Phys. 49, 261 (1968)) was extended to MAS experiments. A computer program was developed to calculate numerically the solid state spectra of the I = 1 2 nuclei and their second moments due to magnetic dipolar coupling with nuclei S > 1 2 in static and MAS experiments. As a characteristic parameter for the influence of S > 1 2 nuclei, the ratio of their quadrupole resonance ( ω SQ) and Zeeman NMR ( ω SZ) frequencies has been introduced. The formulas are valid for arbitrary values of the spin S > 1 2 . Results are presented for S = 1, 3 2 , 5 2 , and compared with literature and experimentally studied solid state MAS spectra.

Slow molecular motion detected in the NMR spectra of rotating solids

We present an approach for detecting and analyzing slow molecular motions in solids based on changes in the zero order sideband of an inhomogeneously broadened spin system under magic angle spinning conditions. The effect is discussed theoretically from the standpoint of second order perturbation theory and the stochastic Liouville equation of Kubo, both for the case where the chemical shift is the main interaction. Experimental investigations of decamethylferrocene (resulting in Ea=3.3 kcal/mole) and hexamethyl benzene support the predicted trends. A five site jump model is used to interpret the results for decamethyl ferrocene, while the interpretation of the hexamethyl- benzene data is obfuscated by the presence of a chemical shift inequivalency presumably caused by a distortion of the aromatic ring above the phase transition (Tc=116 K).