Rotor synchronization

Type of motor (cage, wound-rotor, synchronous, ordc). 

The novelty of (9) is that the net homonuclear dipolar dephasing can be controlled by a systematic variation of the number of the Ho, Hi, and Hi blocks. This technique has the acronym of PITHIRDS-CT [55] and has the virtues that the effect of T2 is identical for all data points and that the rf field of all the pulses is only 1.67 times the spinning frequency. The only experimental concern is that very stable spinning or active rotor synchronization may be required for the implementation of PITHIRDS-CT. 

Fig. 6 (a) SQ and (b) DQ rotor-synchronized 2H MAS NMR spectra of sodium tetrathionate dihydrate-d4 (solid lines). The dashed line in (a) represents the exact numerical simulation of the SQ spectrum for random molecular motion with the rate constant k given in the figure, (c) The corresponding experimental and simulated static 2H quadrupolar-echo spectra, (d) Simulated SQ (solid line) and DQ (dashed line) linewidths as functions of k. (Reproduced with permission from [88])... 

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...

Radio-frequency driven recoupling (RFDR) [58] uses rotor-synchronized 180°-pulses to prevent the averaging of the homonuclear dipolar coupling by the MAS rotation. A single 180°-pulse is placed in the middle of each rotor period (Fig. 11.6a), often using an XY-8 phase cycle [66]. The efficiency of the recoupling depends on the isotropic chemical-shift difference of the two spins and the size and relative orientation of their CSA tensors. 

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-... 

Mono(hydride) zirconium species grafted on a silica surface show a high reactivity toward alkanes. This system has been studied using modern 2D NMR techniques, such as double quantum (DQ) rotor synchronized 2D H MAS and C- H HETCOR, to investigate the mechanism of formation of a zirconium bis(hydride) species and the simultaneous generation of silicon mono- and bis(hydride) species [111]. Figure 11.4 shows the H MAS NMR spectrum and DQ rotor-synchronized... 

H MAS NMR clearly shows five signals at 0.8, 1.9, 4.4, 10.1 and 12.1 ppm. The signals at 0.8, 1.9 and 4.4 are assigned to alkyl fragments bonded to silicon atoms, residual silanol groups on the silica surface and to silicon hydride species, respectively. The two downfield signals at 10.1 and 12.1 ppm are an indication of the presence of two types of zirconium hydride surface species [111]. DQ rotor-synchronized 2D H MAS NMR was used to discriminate these zirconium hydride... 

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. 

Song et al.261 showed that rotor-synchronized 2D PASS experiment can also be used to find the orientation distribution function of chemical shift tensors. Because it permits the use of MAS, the sensitivity is higher than when static or slow-turning samples are used. 

Computation times for 14N FIDs employing parameter set P3 and A —10n Hz varied between 2.41 min (two-site jump, QCPMG) and 12.6 min (six-site jump, QCPMG) for the calculation of a static QCPMG FIDs. The computation time for an MAS FID with ideal excitation was 14 min for a two-site jump and 83 min for a six-site jump. In these calculations, the density operator was set to — Iy when acquisition begins. Calculations of the rotor-synchronized 14N SQ- and DQ-FIDs corresponding to parameter set P7 required 20 and 32 min, respectively. 

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