2D MQMAS

Nimieroiis schemes exist that are used to obtain 2D MQMAS spectra [24]. The simplest fomi of the experiment is when the MQ transition is excited by a single, high-power RF pulse, after which the MQ coherence is allowed to evolve for a time (figure B 1.12.12). After the evolution time, a second pulse is applied which converts the MQ coherence into ap = coherence which is observed during /2- The signal is then acquired inunediately after the second pulse and the echo will fomi at a time /2 = Both pulses are... 

Fig. 2. (a) 2D MQMAS spectrum of a sample of RbNOs at a magnetic field of T with a... 

The essential requirements of a 2D MQMAS pulse scheme are a pulse or a set of pulses that excite MQCs, followed by an evolution under 7Yduring U, another set of pulses to convert the MQC into SQCs, and an acquisition period t2, during which an echo is formed. 

Two cases need attention Improving the excitation and conversion efficiency of MQC in 2D MQMAS experiments and implementing appropriate modulation techniques to... 

After a 2D Fourier transformation the resonances will show up as ridges lying along the quadrupole anisotropy (QA) axis. The isotropic spectrum can be obtained by projection of the entire 2D spectrum on a line through the origin (v j = V2 = 0) perpendicular to the QA axis, figure B1.12.12 shows some of the many different pulse sequences and their coherence pathways that can be used for the 2D MQMAS experiment. 

The corresponding TQMAS spectra are shown in Figs 11 and 12. These preliminary studies show that the presence of chemical shift anisotropy could severely deteriorate the intensity of the multiple-quantum coherence. Since most Co systems have substantial chemical shift anisotropies, the general utility of such experiment in this area remains doubtful at the present stage. Nevertheless, in view of the powerful simulation protocol proposed for 2D MQMAS spectra and the launching of commercial MAS probes capable of spinning speed in excess of 30 kHz, more cobalt complexes should soon become accessible by MQMAS at low Bq field. 

In 1995, Frydman and Harwood presented a 2D MQMAS experiment, which, by means of the formation of an echo corresponding to the refocusing of the fourth-rank second-order quadrupolar broadening, yields 2D spectra in which anisotropi-cally broadened ridges are resolved on the basis of their different isotropic shifts [168]. The resolution of the three distinct Rb sites in RbN03 in an isotropic MQMAS spectrum is demonstrated in Fig. 9.34c. It is to be noted that the experiment is only applicable for odd MQ orders (e. g. 3Q or 5Q), for which there is no first-order quadrupolar broadening. Moreover, as compared to the spin 1=112 MQ methods described earlier in this article, MQC can be excited for a single isolated nucleus. 

Another illustration of how powerful structural NMR tools can be is found in the multiple quantum MAS (MQMAS) NMR study of a G-quadruplex channel. A guanine nucleoside derivative forms tetramers, G-quar-tets. which, in the presence of monovalent cations, assemble to result in a G-quadruplex channel consisting of the stacking of four 6-quartets, coordinating in the channel four cations in this case, three Na" and one While the ID Na-MAS NMR spectrum exhibits only a broad, umesolved signal the 2D MQMAS spectrum gives a clear and neat separation for the three crystallographically distinct Na sites (Fig. 2). 

A Na solid-state NMR study of the bound sodium cations in a G-quad-ruplex formed by Oxytricha nova telomere DNA was shown. Using a 2D MQMAS Na NMR method, three sodium cations residing inside the quad-ruplex channel were observed. For comparison, solid-state N MAS NMR... 

Two Dimensional Multiple Quantum MAS. For non-integer spin quadrupolar nuclei a two dimensional (2D) Multiple Quantum Magic Angle Spiiming (2D MQMAS) technique emerged in 1995 that has had a profound effect on their study, and has meant that much higher resolution spectra can be produced from these nuclei in sol-gel materials. This experiment correlates the (m, -m) multiple quantum transition to the (1/2, -1/2) transition (Fig. 3-2(c)). The resolution enhancement stems from the fact that the quadrupole frequencies for both transitions are correlated. At specific times, the anisotropic parts of the quadrupole interaction are refocused and an echo forms. The amplitude of the echo is determined onlyhy isotropic interactions, so that the experiment is repeated at different echo times which forms the 2D data set where the echo amplitude is modulated only by isotropic effects. FT of the echo amplitude then produces an isotropic spectrum that... 

Fig. 13 87Rb NMR spectra of polycrystalline RbN03 (a) sheared z-filter 2D triple-quantum MQMAS spectrum, (b) MAS spectrum, (c) MQMAS isotropic projection, and (d-f) cross-sections of the 2D spectmm showing the three different crystallographic sites...

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). 

Several basic 2D schemes have been applied in these experiments, based on (1) the use of the SQ evolutions during t and f2, in analogy to the NOESY experiment, (2) the MQ-SQ experiment, in which the evolutions of MQ coherences during t are correlated with the CTs of recoupled spins to enhance the resolution in analogy to MQMAS, and (3) the DQCT-SQ protocol similar to that used in DQMAS NMR of spin-1/2 nuclei. 

Fig. 15. Comparison between the static, MAS and MQMAS 23Na NMR spectra of multisite sodium salts. Contour levels in the 2D plots were taken at 70%, 35%, 16%, 8%, and 4% of the maximum spectral intensities asterisks indicate the spinning sidebands. (Reproduced, with permission, from Medek et a/.209)...

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... 

Figure 6.6. 2D O MQMAS NMR spectrum of ferrierite showing in the vertical direction (the projection along the y-axis) the MQMAS scaled isotropic spectrum and in the horizontal direction the anisotropic lineshapes for the five resolved peaks with their simulations (light broken lines) from which the 0 NMR interaction parameters were derived. From Bull el at. (2000) by permission of the American...

Figure 6.24. O MQMAS NMR spectrum of hydrated zeolite Na-A showing at right anisotropic slices of the 2D spectrum with their corresponding 8iso values. The MAS spectrum at lower right was fitted in accordance with the values derived from the 2D data. From Pingel et al. (1998) by...

---