MQMAS cross-polarization

In this review, we shall introduce some basic concepts about sohd-state NMR of half-integer quadrupolar nuclei and discuss the most useful and promising methods presently available to study them. These include older methods, such as Double Rotation (DOR) and Dynamic Angle Spinning (DAS), and novel techniques including MQMAS, Quadrupolar Phase Adjusted Spinning Sidebands QPASS, SateUite Transition (ST) MAS and Inverse-STMAS NMR, and Fast Amplitude Modulation (FAM). We also discuss several techniques based on dipolar interactions between quadrupolar and spin-1/2 nuclei, such as Cross-Polarization (CP) MQMAS, MQ Heteronuclear Correlation Spectroscopy... 

Recently, Asbrook and Wimperis[89] proposed a novel approach for combining SQ cross-polarization and MQMAS the SQ coherences created in the CP step are transferred directly to 3Q coherences, rather than via a population state. This results in a pulse sequence with a minimum number of coherence-transfer steps. Two types of experiments have been proposed, using either a z-filter or a reversed split-ti, where pure-phase hneshapes are recorded via acquisition of the whole-echo. In addition, the authors showed that the sensitivity of both experiments is considerably improved by incorporating FAM pulses in the SQ to MQ excitation step. 

We present a solid-state nuclear magnetic resonance (NMR) experiment that allows the observation of a high-resolution two-dimensional heteronuclear correlation (2D HETCOR) spectrum between aluminum and phosphorous in aluminophosphate molecular sieve VPI-5. The experiment uses multiple quantum magic angle spinning (MQMAS) spectroscopy to remove the second order quadrupolar broadening in Al nuclei. The magnetization is then transferred to spin-1/2 nuclei of P via cross polarization (CP) to produce for the first time isotropic resolution in both dimensions. 

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