Rotor-synchronized sequences

Fig. 21 HMQC pulse sequences for (a) 14N-13C and (b) 14N- H correlations under rotor-synchronized MAS. In (b), dipolar recoupling is usually applied during time intervals Texc and Trec. (c) Coherence transfer pathways for the observation of SQ (solid lines) and DQ (dashed lines) in the 14N dimension...

Rotational-echo double resonance (REDOR), originally introduced by Gullion and Schaefer [102], is a method to recouple heteronuclear spin pairs. The sequence relies on a train of rotor-synchronized n pulses applied to the I spin to interrupt the spatial averaging of the heteronuclear dipolar coupling under MAS to give a nonvanishing dipolar Hamiltonian over a full rotor cycle (Fig. 11.8). Typically, REDOR data are collected by col-... 

Fig. 14. The pulse sequence for recording the double-quantum 2H experiment.37 The entire experiment is conducted under magic-angle spinning. This two-dimensional experiment separates 2H spinning sideband patterns (or alternatively, static-like 2H quadrupole powder patterns) according to the 2H double-quantum chemical shift, so improving the resolution over a single-quantum experiment. In addition, the doublequantum transition frequency has no contribution from quadrupole coupling (to first order) so, the double-quantum spectrum is not complicated by spinning sidebands. Details of molecular motion are then extracted from the separated 2H spinning sideband patterns by simulation.37 All pulses in the sequence are 90° pulses with the phases shown (the first two pulses are phase cycled to select double-quantum coherence in q). The r delay is of the order 10 gs. The q period is usually rotor-synchronized.

A pulse scheme recovering the zero-quantum Hamiltonian was proposed by Baldus and Meier.142 It is weakly dependent on spectral parameters and a faithful measure of internuclear distances. This sequence is based on the former rotor-synchronized R/L-driven polarization transfer experiments.143,144 It uses the LG or FS-LG, which is used to decouple the high-7 spins, and combined MAS and RF irradiation of low-7 spins to decouple the hetero-nuclear dipolar interactions. With phase-inversion and amplitude attenuation in the rotating frame and refocusing pulses in the laboratory frame part of the pulse sequence, a zero-quantum average Hamiltonian can be obtained with optimum chemical-shift/offset independence. 

Setting up the FSLG is not straightforward. The pulse sequence is rotor synchronized and particular consideration should be taken when setting the H offset. The FSLG decoupling can be setup using the CP/MAS spectrum... 

Fig. 6 The RFDR-sequence with two 7x/2-pulses sandwiching the t evolution period. The mixing period is comprised of rotor-synchronized n-pulses. If these pulses are absent, the sequence is that for 2D-exchange spectroscopy. The third 7i/2-pulse produces transverse magnetisation...

Figure 7. Pulse sequence and coherence transfer pathway diagram for a H DQ MAS experiment using the BAB A recoupling sequence for the excitation and reconversion of DQCs. The rectangular blocks represent pulses of flip angle 90°, with the choice of the phases being described in, e.g., ref 25. If the q increment is set equal to a rotor period, a rotor-synchronized two-dimensional spectrum is obtained, while reducing q, and hence increasing the DQ spectral width, leads to the observation of a DQ MAS spinning-sideband pattern.

Rotor-synchronized H DQ MAS spectra can only deliver information about relative proton—proton proximities (except for cases where the DQ peak(s) due to a known internal or external standard are well resolved).83 The DQ MAS experiment (see Figure 7) can, however, be performed in an alternative fashion if the t increment is reduced, which corresponds to an increase in the DQ spectral width, a DQ MAS spinning-sideband pattern is observed35-36 (provided that a recoupling sequence which has an amplitude dependence on the rotor phase, e.g., BABA91 or DRAMA93, is used). 

S. Hediger, B. H. Meier and R. R. Ernst, Rotor-synchronized amplitude-modulated nuclear magnetic resonance spin-lock sequences for improved cross polarization under fast magic angle sample spinning. J. Chem. Phys., 1995, 102, 4000-4011. 

General symmetry principles for rotor-synchronized pulse sequences in MAS solid-state NMR have been presented. The synunetry theory has been extended to the case of generalized Hartmann-Hahn sequences, in which rotor-synchronized r.f. irradiation is applied simultaneously to two isotopic spin species. The symmetry theory has been used to design pulse sequences which implement heteronuclear dipolar recoupling at the same time as decoupling homonuclear spin-spin interactions, and which also suppress CSAs. Experimental demonstrations of heteronuclear 2D correlation spectroscopy, heteronuclear MQ spectroscopy, and the estimation of intemuclear dipolar couplings have been given. 

Fig. 35. Stacked plot top) and contour plot (bottom) of the two-dimensional H MAS NMR spectrum of dehydrated (673 K) sUicoaluminophosphate SAPO-5 studied by Hunger et al. [221,260], The spectrum was recorded at a resonance frequency of 300.13 MHz with a samplespinning rate of v (=2 kHz and a rotor-synchronized pulse sequence to eliminate MAS modulation in the Fl-dimension [6]...

The ID F spectrum (by direct polarization or rotor-synchronized Hahn-echo pulse sequence with MAS), 2D HETCOR [67] and relaxation times have demonstrated to be very useful for identifying correlations, interactions, amorphous contents in tablets or for investigating mixtures [68-71]. The 2D experiments that involve F are CPLG-HETCOR and f CP-DARR (which is based m... 

A typical pulse sequence for the evaluation of intemu-clear distances, the REDOR, is obtained by the application of rotor-synchronized n pulses on the dipolar coupled I spins, applied twice per rotor period, with a single S spin ir pulse applied halfway during the sequence to refocus the dipolar interaction between the two nuclei (Figure la). ... 

If more sophisticated experiments are aimed, such as those involving multiple pulse sequences, spin echo detection, rotor synchronization, or 2D methods, additional calibration steps, trial tests, and optimization procedures are required. The reader is referred to the specific literature dealing with each particular method for a proper description of the experimental setup. A general account of these methods can also be found in the monographs by MacKenzie and Smith [4] and Duer [15]. 

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