Double-rotation (DOR) NMR spectroscopy: Progress and perspectives

Double-rotation (DOR) solid-state NMR spectroscopy is a high-resolution technique developed in the late 1980s. Although multiple-quantum magic-angle spinning (MQMAS) became the most widely used high-resolution method for half-integer spin quadrupoles after 1995, development and application of DOR NMR to a variety of chemical and materials science problems has endured. This Trend article recapitulates the development of DOR NMR, discusses various applications, and describes possible future directions. The main technical limitations specific to DOR NMR are simply related to the size of the double rotor system. The relatively large outer rotor (and thus coil) used for most applications over the past 35 years translates into relatively low rotor spinning frequencies, a low filling factor, and weak radiofrequency powers available for excitation and for proton decoupling. Ongoing developments in NMR instrumentation, including ever-shrinking MAS rotors and spherical NMR rotors, could solve many of these problems and may augur a renaissance for DOR NMR.

Cesium Oxo-fluoro-aluminates in the CsF-Al2O3 System: Synthesis and Structural Characterization

In an experiment combining various approaches, a precise examination of a portion of the phase diagram of a CsF-Al2O3 system was carried out up to 40 mol% Al2O3. CsF-Al2O3 solidified mixtures have been investigated using high-field solid-state NMR (133Cs, 27Al, and 19F) spectroscopy and X-ray powder diffraction over a broad range of compositions with synchrotron powder diffraction and Rietveld analysis. A new cesium oxo-fluoro-aluminate, Cs2Al2O3F2, was discovered, prepared, and structurally analyzed by synchrotron diffraction analysis. In addition to Cs2Al2O3F2, we have synthesized the following pure compounds in order to aid in the interpretation of NMR spectra of the solidified samples: CsAlF4, Cs3AlF6, and CsAlO2.

Separation of isotropic chemical and second-order quadrupolar shifts by multiple-quantum double rotation NMR

Using a two-dimensional multiple-quantum (MQ) double rotation (DOR) experiment the contributions of the chemical shift and quadrupolar interaction to isotropic resonance shifts can be completely separated. Spectra were acquired using a three-pulse triple-quantum z-filtered pulse sequence and subsequently sheared along both the nu(1) and nu(2) dimensions. The application of this method is demonstrated for both crystalline (RbNO(3)) and amorphous samples (vitreous B(2)O(3)). The existence of the two rubidium isotopes ((85)Rb and (87)Rb) allows comparison of results for two nuclei with different spins (I=3/2 and 5/2), as well as different dipole and quadrupole moments in a single chemical compound. Being only limited by homogeneous line broadening and sample crystallinity, linewidths of approximately 0.1 and 0.2 ppm can be measured for (87)Rb in the quadrupolar and chemical shift dimensions, enabling highly accurate determination of the isotropic chemical shift and the quadrupolar product, P(Q). For vitreous B(2)O(3), the use of MQDOR allows the chemical shift and electric field gradient distributions to be directly determined-information that is difficult to obtain otherwise due to the presence of second-order quadrupolar broadening.

Determination of NMR interaction parameters from double rotation NMR

It is shown that the anisotropic NMR parameters for half-integer quadrupolar nuclei can be determined using double rotation (DOR) NMR at a single magnetic field with comparable accuracy to multi-field static and MAS experiments. The (17)O nuclei in isotopically enriched l-alanine and OPPh(3) are used as illustrations. The anisotropic NMR parameters are obtained from spectral simulation of the DOR spinning sideband intensities using a computer program written with the GAMMA spin-simulation libraries. Contributions due to the quadrupolar interaction, chemical shift anisotropy, dipolar coupling and J coupling are included in the simulations. In l-alanine the oxygen chemical shift span is 455 +/- 20 ppm and 350 +/- 20 ppm for the O1 and O2 sites, respectively, and the Euler angles are determined to an accuracy of +/- 5-10 degrees . For cases where effects due to heteronuclear J and dipolar coupling are observed, it is possible to determine the angle between the internuclear vector and the principal axis of the electric field gradient (EFG). Thus, the orientation of the major components of both the EFG and chemical shift tensors (i.e., V(33) and delta(33)) in the molecular frame may be obtained from the relative intensity of the split DOR peaks. For OPPh(3) the principal axis of the (17)O EFG is found to be close to the O-P bond, and the (17)O-(31)P one-bond J coupling ((1)J(OP)=161 +/- 2 Hz) is determined to a much higher accuracy than previously.