On the Conversion of Triple- to Single-Quantum Coherences in MQMAS NMR

✍ Scribed by M Pruski; J.W Wiench; J.-P Amoureux

• Publisher: Elsevier Science
• Year: 2000
• Tongue: English
• Weight: 170 KB
• Volume: 147
• Category: Article
• ISSN: 1090-7807
• DOI: 10.1006/jmre.2000.2190

✦ Synopsis

A systematic experimental and numerical evaluation of several basic approaches to multiple-quantum magic angle spinning (MQ-MAS) NMR is presented for spin-3 2 nuclei. The approaches use identical MQ excitation, via a single RF pulse of high power, and three types of methods for conversion to observable coherence: (a) nutation by strong continuous wave pulse; (b) rotation-induced adiabatic coherence transfer (RIACT), and (c) fast amplitude modulation (FAM-1). The optimization strategies and maximum achievable MQMAS efficiencies of 87 Rb in RbNO 3 and LiRbSO 4 are investigated using several coherence transfer schemes under a wide range of experimental parameters. These parameters include the strength of the RF magnetic field RF , the sample rotation speed R , the length of the conversion period, and the modulation frequency in FAM-1. The data provide new insights into the spin dynamics involved in these techniques and the experimental guidelines for achieving the best sensitivity. The RF requirements for maximum efficiency of conversion depend on the method to be used. In general, FAM-1 performs better than the nutation and RIACT methods in terms of efficiency and off-resonance behavior, especially when RF is small compared to the quadrupole fre- quency Q . The experiments performed using nutation, RIACT, and FAM-1 methods yield similar resolution in the isotropic dimension, regardless of RF .