Fast amplitude modulation

P. K. Madhu, A. Goldburt, L. Frydman and S. Vega, Sensitivity enhancement of the MQMAS NMR experiment by fast amplitude modulation of the pulses. Chem. Phys. Lett, 1999, 307,41-47. 

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... 

Using fast amplitude modulation pulses it is possible to redistribute the population of the spin energy levels. This is the Rotor Assisted Population Transfer (RAPT) method introduced by Yao et al. [57]. It has been shown that enhancement by a factor of 1.5-2 is achieved in a MAS experiment of spin-3/2 nuclei when RAPT is applied before the excitation pulse. It is also possible to combine RAPT with MQMAS in an experiment which uses single-quantum coherences for the excitation of multiple-quantum coherences. Madhu and Levitt [58] have shown that a combination of RAPT and RIACT-FAM gives the best performance for MQMAS experiments of spin-3/2 systems. 

Since the introduction of MQMAS, much effort has been expended on developing methods to enhance the signal of the CT, and thus to extend the utility of the technique. A common signal enhancement technique is fast amplitude modulation (FAM). Shaped pulses, including the HS pulses introduced above,as well as p)psi78.208 been used for signal enhancement in MQMAS experiments. Kwak et have combined RAPT with the... 

Brauniger, T., Madhu, P.K., Pampel, A., and Reichert, D. 2004. Application of fast amplitude-modulated pulse trains for signal enhancement in static and magic-angle-spinning Ti-NMR spectra. Solid State Nucl. Magn. Reson. 26 114—120. 

FAM fast amplitude-modulated RF pulse trains with constant... 

T. Braimiger, P.K. Madhu, Fast amplitude-modulated pulse trains with frequency sweep (SW-FAM) in solid-state NMR of spin-7/2 nuclei, J. Magn. Reson. 193 (2008) 102-109. 

The aluminium species in activated zeolites have been studied using NMR spectroscopy. Aluminium and iron in the framework of zeolites have been analysed quantitatively by Al NMR spectroscopy. The templating of high-silica zeolites by THF has been studied by C NMR spectroscopy. O NMR spectroscopy has been used to study the atomic multipoles in Fl-form aluminosilicate frameworks. A comparison of fast amplitude modulation shifted-echo pulse sequences for O 3Q- and 5Q-MAS NMR experiments has been carried out for oxygen sites in zeolites. By using a combination of the Monte-Carlo technique with Si NMR spectroscopy, cation sites in zeolites have been located. " The growth of zeolites on kaolin microspheres has been investigated by NMR spectroscopy. F1 MAS spin echo NMR spectroscopy,... 

Rocha et al. have reviewed recent advances in NMR spectroscopy of quadrupolar nuclei with half-integer spins. In particular, B, O, Na, A1, Ga, Nb MQ MAS applications have been considered. A range of related techniques, such as satellite transition (ST) MAS, inverse-STMAS NMR, fast amplitude modulation, and techniques based on the dipolar interactions between quadrupolar and spin-1/2 nuclei (CP MQ MAS and MQ HETCOR) and the recently introduced /-coupling based experiments (/-HMQC), have also been reviewed. 

Finally, Brauniger el al. have demonstrated that the use of fast amplitude-modulated RF pulse trains with constant (FAM-I) and incremented pulse durations (SW-FAM) leads to considerable sensitivity enhancement for the central-transition signal via spin population transfer from the satellite transitions) for solid-state NMR spectra of titanium, " Ti (/ = 5/2) and " Ti (/= 7/2). For the MAS spectra of Ti02 and BaTiOs, the intensity of the " Ti... 

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