Decoupling 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... 

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. 

A theoretical analysis of rotor-synchronized homonuclear dipolar decoupling schemes that cause a z-rotation of the spins has been presented by... 

To monitor the effective aluminium-fluorine dipolar couplings in cryolite, as a function of temperature, amplified 2D separated local field (SLF) NMR experiments under the action of fast MAS (which is in turn desirable for the simple elimination of the homonuclear couplings) were recorded by Kotecha et al [52]. In such SLF MAS experiments rotor-synchronized pulses were applied to achieve a net heteronuclear dipolar evolution with variable amplification factors xN of the Al- interaction along the indirect domain (x2 SLF, x4 SLF, and x8 SLF), followed by observation of aluminium s central-transition F]-decoupled evolution along the direct domain [53]. 

The dynamics of the rotor speed can be conveniently analysed and adjusted by feeding a signal from the rotor motion monitor (via an optical fibre) to a console ADC (Fig. 7). The corresponding spectra are shown in Fig. 8. The faster sweep clearly reduces K12, and to a lesser extent K13. Both direct complementary peaks (1-Kmn) and relayed peaks depend on the product of complementary transfer coefficients, and correlate well with the expected influence of the sweep rate variation. The observed relay process can be shorted by proton-driven spin diffusion. Consequently, efficient rotation-speed independent (or carefully synchronized) decoupling is required during the entire mixing period. 

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