High-resolution Solid-state NMR Experiments Research Articles

On the applicability of cosine-modulated pulses for high-resolution solid-state NMR of quadrupolar nuclei with spin > 3/2

In MQMAS-based high-resolution solid-state NMR experiments of half-integer spin quadrupolar nuclei, the high radiofrequency (RF) field requirement for the MQ excitation and conversion steps with two hard-pulses is often a sensitivity limiting factor in many practical applications. Recently, the use of two cosine-modulated (cos) low-power (lp) pulses, lasting one-rotor period each, was successfully introduced for efficient MQ excitation and conversion of spin-3/2 nuclei with a reduced RF amplitude. In this study, we extend our previous investigations of spin-3/2 nuclei to systems with higher spin values and discuss the applicability of coslp-MQ excitation and conversion in MQMAS and MQ-HETCOR experiments under slow and fast spinning conditions. For the numerical simulations and experiments we used a moderate magnetic field of 14.1 T. Two spin-5/2 nuclei (85Rb and 27Al) are mainly employed with a large variety of CQ values, but we show that the practical set up is also available for higher spin values, such as spin-9/2 with 93Nb in Cs4Nb11O30. We demonstrate for nuclei with spin value larger than 3/2 a preferential use of coslp-MQ acquisition for low-gamma nuclei and/or large CQ values with a much reduced RF-field with respect to that of hard-pulses used with conventional methods.

Covariance spectroscopy in high-resolution multi-dimensional solid-state NMR

Covariance spectroscopy (COV), a statistical method that provides increased sensitivity, can be applied to two-dimensional high-resolution solid-state NMR experiments, such as homonuclear spin-exchange spectroscopy. We the alternative States sampling scheme to the experimental time by 50%. By combining COV with other processing methods for non-uniform sampling (NUS), many different three-dimensional experiments can be performed with substantial increases in overall sensitivity. As an example, we show a three-dimensional homonuclear spin-exchange/separated-local-field (SLF) spectrum that enables the assignment of resonances and the measurement of structural restraints from a single experiment performed in a limited amount of time.

13C and 39K High-Resolution Solid-State NMR Study of the Nonferroic Phase Transition of Potassium Hydrogen Carbonate. Complementarity between NMR and Incoherent Neutron Scattering

Potassium hydrogen carbonate KHCO 3 is formed of centrosymmetric dimers (HCO 3 ) 2 linked by hydrogen bonds, the protons being disordered in an asymmetric double-well potential at ambient temperature. This compound is a model for 0-dimensional hydrogen bonds. It undergoes a nonferroic nonferroelectric phase transition at T c = 318 K, and the double-well potential becomes symmetric in the high-temperature phase. This phase transition is studied for the first time by variable-temperature 1 3 C and 3 9 K high-resolution solid-state NMR experiments. We present an original method to correlate NMR data to other local probes such as incoherent neutron scattering experiments to extract structural information from powder NMR experiments. Using this method, the eigenvalues and jump angle of the chemical shift tensor of the carbon 1 3 C and the quadrupolar interaction of the potassium 3 9 K could be extracted in the completely ordered phase, and their evolution with temperature is correlated to the proton-dynamic disorder within the hydrogen bond. This study also illustrates the complementarity between NMR and incoherent neutron-scattering experiments in the study of hydrogen bonding.

NMR characterization of Bronsted acid sites in faujasitic zeolites with use of perdeuterated trimethylphosphine oxide.

The interactions of Bronsted acid sites of H-Y (FAU) with perdeuterated trimethylphosphine oxide (TMPO-d9) are studied with a set of high-resolution solid-state NMR experiments. Double- and triple-resonance MAS NMR techniques (such as CP, TRAPDOR, and REDOR) verify that the lines in the 31P MAS NMR spectrum are indeed from TMPO interacting with Bronsted acid sites. Replacement of acidic hydrogens in the sodalite cages with sodium cations results in the disappearance of one of the peaks, leading to final assignments of the resonances.

Fd Coat Protein Structure in Membrane Environments

The membrane bound form of bacteriophage fd coat protein has a long hydrophobic membrane spanning helix and a shorter amphipathic helix in the plane of the bilayer. Residues near the N and C termini and in the turn connecting the two helices are mobile. The locations and orientations of the helical secondary structure elements and the protein backbone dynamics were characterized by combining results from multidimensional solution NMR experiments on protein samples in micelles and high resolution solid-state NMR experiments on protein samples in oriented and unoriented lipid bilayers. The coat protein is a monomer in micelles. The secondary structure of the membrane bound form of fd coat protein is very similar to that of the structural form found in the virus particles, since it is nearly all alpha helix. However, the membrane bound form of the protein differs from the structural form of the protein in virus particles in the arrangement of the secondary structure, since the membrane bound form of the protein has two distinct helical domains oriented perpendicular to each other and the structural form of the protein in the virus particles has a nearly continuous helix aligned approximately along the filament axis. In addition, there are substantial differences in the dynamics of residues in the bend between the two helices and near the C terminus, since they are mobile in the membrane bound form of the protein and not in the virus particles. Residues 1 to 5 at the N terminus are highly mobile and unstructured in both the membrane bound and structural forms of the coat protein.

The combination of synchrotron powder diffraction and high-resolution solid-state NMR experiments

Structural investigations on three zeolites, ZSM-11, ZSM-12 and ZSM-5, have been carried out by a combination of high-resolution solid-state 29Si NMR experiments and Rietveld refinement of synchrotron X-ray powder data. These techniques, being sensitive to short- and long-range order, respectively, gave a most complete picture of the structure of the solid state. Variable temperature NMR experiments of ZSM-11 revealed a displacive phase transition with a change in symmetry at about 300 K. A Rietveld refinement of a data set recorded at 373 K confirmed the proposed structure model. The structure of ZSM-12 was refined from room temperature synchrotron powder data. Space group ambiguities because of psuedo-symmetry of the silica framework of the zeolite were resolved with the aid of 29Si NMR experiments leading to C2/ c as the true space group symmetry. 29Si NMR and X-ray studies of ZSM-5 showed that the symmetry of the crystal structure depends on the temperature, the concentration of the guest molecule, and on the number of Al atoms in the framework. A full pattern refinement of silica-ZSM-5 loaded with two molecules p-dichlorobenzene per unit cell showed that the site occupied by the sorbed molecule is not at the intersection of the channels but shifted by 1 4 along b 0.

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.

Study of structural ordering in Hg 1− xCd xTe by 125Te NMR

High-resolution solid-state NMR experiments are used to study bonding in the semiconductor alloy system Hg 1− x Cd x Te over a broad range of values of x. Based on measurement of the 125Te chemical shift and its sensitivity to coordination effects, which we demonstrate for the first time, it can be concluded that Hg and Cd do not distribute themselves randomly in the alloy lattice.

High-resolution dilute spin dipolar NMR spectroscopy in the solid state

Recently developed techniques which allow heteronuclear dipolar interactions to be measured in high-resolution solid-state NMR experiments are reviewed and illustrated. The examples chosen demonstrate how dipolar and chemical-shift tensor can be obtained in molecules of chemical and biological interest.