STMAS spinning

Satellite transition magic angle spinning (STMAS) Selection of coherences... 

In the following section, we explain the basic protocols used for removing the second-order quadrupolar broadening based on the refocusing of the second-order quadrupolar interaction. These protocols rely on mechanical reorientation of the rotor axis (DAS) or use a combination of sample spinning and rf manipulation of the spins evolution (MQMAS and STMAS). Experimental aspects of these methods, as well as methods for data processing and analysis, are described in Sects. 5.3 and 5.4. 

The quadrupolar effects of order higher than two (7) are usually assumed to be negligible, especially at high magnetic fields. However, once the first- and second-order effects are removed, the measurement of third-order contributions becomes realistic. It can be easily shown that, similar to the first-order case, the CT and all symmetric MQ transitions (q = 0) are free of the third-order contribution, which thus can be safely ignored in DAS, DOR, and MQMAS experiments [161,162]. This is not the case for transitions between non-symmetric spin states, such as the STs. Indeed, numerical simulations of the third-order effect have explained the spectral features that have been observed in 27A1 STMAS spectra of andalusite mineral [161]. 

S. E. Ashbrook and S. Wimperis, Fligh-resolution NMR of quadrupolar nuclei in solids the satellite-transition magic angle spinning (STMAS) experiment. Prog. Nucl. Magn. Reson. Spectrosc., 2004, 45,53-108. 

SateUite Transition MAS (SATRAS or STMAS), developed by Gan in 2000 [25], is an alternative approach to MQMAS for the acquisihon of high-resolution NMR spectra of quadrupolar nuclei. The principal advantage of SATRAS over MQMAS is that it is not dependent upon an efficient transfer of multiple-quantum coherences. like MQMAS, SATRAS is a 2D experiment performed under MAS conditions. The technique involves exciting the sateUite transitions in the spin manifold of quadmpolar nuclei using short radio frequency (rf) pulses. The second-order... 

In this review, we shall introduce some basic concepts about sohd-state NMR of half-integer quadrupolar nuclei and discuss the most useful and promising methods presently available to study them. These include older methods, such as Double Rotation (DOR) and Dynamic Angle Spinning (DAS), and novel techniques including MQMAS, Quadrupolar Phase Adjusted Spinning Sidebands QPASS, SateUite Transition (ST) MAS and Inverse-STMAS NMR, and Fast Amplitude Modulation (FAM). We also discuss several techniques based on dipolar interactions between quadrupolar and spin-1/2 nuclei, such as Cross-Polarization (CP) MQMAS, MQ Heteronuclear Correlation Spectroscopy... 

To obtain a high-resolution spectrum free from the unwanted CT CT peak, the third method proposed uses half-rotor synchronisation, instead of full-rotor period synchronisation, during [40]. The increment in (or kt for split-ti experiments) is set to half the rotor period, thereby doubling the isotropic spectral width (Fi=2Vj., where Vj. is the MAS rate). This means that two sets of STn CT resonances will be observed, a centreband and a spinning sideband. In principle, the latter will be well resolved from the CT CT diagonal peaks, which in general do not display any sidebands in Compared with a full-rotor synchronisation, the S/N ratio is reduced by a factor of two and the experimental time is doubled. As an example. Fig. 13a shows the STMAS spectrum of zeolite scolecite. 

Trebosc et al. introduced a frequency-selective (FS) REDOR approach to a multi-spin system Sl y where S is a quadrupolar nucleus. FS-REDOR may be used for accurate trough-space distance measurements in spin pairs that involve quadrupolar nuclei [106]. The experiment reveals heteronuclear dipolar and scalar couplings, which can be reintroduced selectively, site after site. Importantly, FS-REDOR may also be used under high-resolution provided by MQMAS, STMAS or I-STMAS. 

In the following, we show how an MQMAS echo is formed. The quadmpolar Hamiltonian under MAS will be derived as a perturbation to the strong static magnetic field Bq. The evolution of the density matrix under this Hamiltonian will be then shown to form an echo under a suitable selection of symmetric coherences. The manipulation of spin coherences by RF pulses will be assumed to be ideal here. The effects of nonidealities were discussed by the group of Vega. From the expression for the elements of the quadmpolar Hamiltonian, line narrowing by DOR experiment and echo formation by two alternative experiments, DAS and STMAS, will also be demonstrated and briefly discussed. Finally, some additional line narrowing schemes will be mentioned. 

In the STMAS experiment,like in MQMAS, spin coherences are manipulated rather than the spinning angle. However, instead of creating symmetric MQCs, the singlequantum satellite transition coherences. In — 1/2 >< n -f l/2l, are now excited. After an evolution period tj, these are then refocused with the symmetric CT SQCs evolving during t2- The relevant accumulated phase thus becomes... 

STMAS, as indicated in Section 3, correlates the single-qiianttirn satellite transitions to single-quanmm CT leading to an echo of the Fourier transformation which gives rise to high-resolution spectra of half-integer quadrupole spins. A possible correlation is between the l >< i satellite transitions (ST ) in the fi period of a 2D experiment and the CT in the t2 period. Such a correlation leads to an echo free of the second-order... 

Finally, it should be noted that STMAS [75] is another spin-manipulating approach for removing the second-order quadrupole interactions however, there are no reports of ST MAS studies on biomolecules. 

S.E. Ashbrook and S. Wimperis, High-Resolution NMR of Quadm-polar Nuclei in Solids The Satellite-Transition Magic Angle Spinning (STMAS) Experiment , p. 53... 

Rocha et al. have reviewed recent advances in NMR spectroscopy of quadrupolar nuclei with half-integer spins. In particular, B, O, Na, A1, Ga, Nb MQ MAS applications have been considered. A range of related techniques, such as satellite transition (ST) MAS, inverse-STMAS NMR, fast amplitude modulation, and techniques based on the dipolar interactions between quadrupolar and spin-1/2 nuclei (CP MQ MAS and MQ HETCOR) and the recently introduced /-coupling based experiments (/-HMQC), have also been reviewed. 

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