STMAS

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. 

In 2000, Gan proposed the previously mentioned STMAS method, which is similar to MQMAS in that it can be performed on a standard MAS probehead, but uses a different coherence pathway [141]. The STMAS protocol, schematically described in Fig. 10, relies on excitation of the SQ ST coherences of order q (with q yk 0 and p = —1), instead of symmetric MQ coherences with q = 0. Note that such excitation inevitably involves the CT, as well. The ST coherences are converted to the CT coherence at t = r/(l + k) using suitable rf-pulse pulse(s). 

Fig. 10 Schematic representation of the STMAS experiment for I = 3/2. The unwanted CT —> CT pathway is not eliminated by this simple pulse sequence...

It follows from (63) and (68) that the isotropic frequencies in MQMAS and STMAS spectra are related by... 

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]. 

DAS), or those of the fourth rank using MAS (MQMAS, STMAS, DQF-STMAS). Until now, all these sequences have been presented in their ID versions, which entails the recording for increasing x values of a series of data points having a phase O(x) = Qx where in its general form [see (54), (59), and (64)]... 

Again, the value of X for MQMAS is given in (70), and X = — 1 for STMAS (as for the anisotropic dimension Q2). Using this convention, the apparent shifts of the quadrupolar resonances expressed in ppm are given by... 

In STARTMAS, SQ STs of order q (with q = 2 and p = —1) are initially excited as in STMAS (Fig. 14). The ST coherences then evolve during the first time period r/[2(l + k)], before being converted to a DQ transition by a selective n inversion pulse on the CT (as in DQF-STMAS [161]) and allowed to evolve during kx/(l + k). A second selective n pulse brings the system back to an SQ transition for the last part of the ST evolution. At time t = x, the phase of the detected SQ ST coherence is thus... 

The details of this analysis, which is a generalization of a previous formalism for n = 1 [236], are beyond the scope of this review. Overall, the efficiency of R-INEPT and HMQC experiments has been found to be comparable, at least in applications involving 31P and 27Al. A major advantage of R-INEPT is that it can be combined with MQMAS or STMAS into a simple 2D heteronuclear correlation experiment, MQ/ST-J-HETCOR (Sect. 7.3). 

These differences and hence the advantages are more pronounced for low-y nuclei. ° A plot of the relative sensitivity of the ST approach to the MQ approach was evaluated by numerical simulation as a function of the parameter (vi/vq) the ST experiment shows a factor of at least three better sensitivity, which is a significant advantage (Figure 2C). Although more use of STMAS has developed, it has not yet become as popular as the MQ approach, probably as a result of its reputation as an experiment... 

Figure 2 Pulse sequences and coherence pathway diagrams for the phase-modulated split-t) (A) STMAS and (B) triple-quantum MAS NMR, showing the generic sequences with the specific values for Mg being r=24/31, r —O, r"=7/31 and s=12/31, s =0, s"— 9/3 and (C) numerical simulations of the relative sensitivity of the ST and MQ experiments as a function of the ratio between the RF field strength and the quadrupolar coupling...

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. 

J. Trebosc, J.-P. Amoureux and Z. Gan, Comparison of high-resolution solid-state NMR MQMAS and STMAS methods for half-integer quadrupolar nuclei. Solid State Nucl. Magn. Reson., 2007, 31,1-9. 

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... 

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