Rotor-assisted population transfer

Signal enhancement of the CT can also be obtained using rotor-assisted population transfer (RAPT), introduced in 2000 by Yao et al. [65], It is based on the same... 

H.-T. Kwak, S. Prasad, T. Clark and P. J. Grandinetti, Enhancing sensitivity of quadrupolar nuclei in solid-state NMR with multiple rotor assisted population transfers. Solid State Nucl. Magn. Reson., 2003,24,71-77. 

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. 

The rotor assisted population transfer (RAPT) sequence has been used to enhance the sensitivity of the RIACT(II) experiment for spin-3/2 quadrupolar nuclei. A detailed theoretical analysis of the polarizations that contribute to different types of MQ MAS experiments has been provided. In particular, two polarization pathways have been distinguished for the creation of triplequantum coherence. The existence of these pathways has been experimentally demonstrated by comparing the sensitivities of different sequences with and without RAPT preparation. 

Rubidium ( Rb ) (/ = 3/2). The structure of Rb2[Pd(Se4)2] Se8 was studied by Rb MAS and static NMR and Se MAS NMR using their nuclear spin interaction parameters. The dependence of the Rotor Assisted Population Transfer (RAPT) enhancement on offset frequency for nuclei experiencing different quadrupolar couplings was exploited and applied to the Rb spectrum of Rb2S04. Rb-loaded LTA-type zeolite was studied in comparison with K-loaded one by Rb NMR. It was found that the mass center of the Rb spectrum shifts clearly to higher frequency side <10 K and the existence of Fermi contact interaction between a electron and Rb nuclei was suggested. ... 

Rotor-assisted population transfer has been developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarisation associated with satellite transitions to the central m = 1/2 -1/2 transi-... 

In the rotor-assisted population transfer (RAPT) experiment, a series of pulses is applied with alternating phases. This leads to a saturation of the energy levels for the STs, and to an enhancement of the CT. For spin-3/2 nuclei, the CT is enhanced by approximately a factor of 2 the maximum enhancement for saturation of the STs is 7-1-1/2. Madhu etal. obtained enhancements of approximately 2.5 (compared to a maximum possible enhancement of 3.0) when they applied RAPT to two 7 = 5/2 isotopes and Al. In this case, the mi = 5/2 — mi = 3/2 and... 

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