High Resolution Correlation NMR Techniques

In an S 1 REDOR experiment dipolar recoupling is accomplished by applying 7t-pulses on the I channel in the middle of the rotor period. Although REDOR has successfully been applied to systems where one or other of the dipolar-coupled nuclei is quadrupolar (S or I), 

To be exhaustive about ID double resonance NMR experiments, we have to mention the TEDOR (transferred echo double resonance) experiment, which involves transferring magnetization between two heteronuclear spins [45]. Nevertheless, this experiment has been applied only once to Al-based fluoride, on AIPO4-CJ2, a microporous 

It is also possible to combine REDOR with MQMAS. Two approaches, MQ-t2-REDOR [49] and MQ-ti-REDOR [50], were presented and ascertained on AIPO4-CHA. An easier experiment, termed MQMAS with dipolar dephasing (DDMQMAS), was also proposed [51] and tested on the same compound. Finally, as previously mentioned, the combination of MQMAS and TEDOR is also possible (in an analogous manner to that of MQMAS and CP) as recently shown, by Amoureux et al, on AIPO4-CJ2 [46]. 

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]. 

Heteronuclear correlation (HETCOR) solid-state NMR spectroscopy [54] has been widely used to provide information on the spatial proximity of different nuclei in complex spin systems. When more than one distinct / and/or S spin species is present, a 2D correlation experiment enables the detection of dipolar couplings between specific distinct IS pairs. Typically, the experiment consists of a 90° pulse that creates transverse I spin magnetization, which evolves for a time tj before it is transferred to spin S, usually via CP. The S spin FID is then detected in t2- 2D Fourier transform yields a 2D spectrum, with the appearance of cross peaks between individual I and S resonances from spins which are dipolar coupled. 

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