Pruski M, Wiench J W, Amoureux J P
Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA.
J Magn Reson. 2000 Dec;147(2):286-95. doi: 10.1006/jmre.2000.2190.
A systematic experimental and numerical evaluation of several basic approaches to multiple-quantum magic angle spinning (MQMAS) NMR is presented for spin-32 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 nu(RF), the sample rotation speed nu(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 nu(RF) is small compared to the quadrupole frequency nu(Q). The experiments performed using nutation, RIACT, and FAM-1 methods yield similar resolution in the isotropic dimension, regardless of nu(RF).
本文针对自旋为3/2的原子核，对多量子魔角旋转（MQMAS）核磁共振的几种基本方法进行了系统的实验和数值评估。这些方法通过高功率单射频脉冲进行相同的多量子激发，并采用三种类型的方法将其转换为可观测的相干性：（a）强连续波脉冲作用下的章动；（b）旋转诱导绝热相干转移（RIACT），以及（c）快速幅度调制（FAM-1）。在广泛的实验参数范围内，使用几种相干转移方案研究了RbNO₃和LiRbSO₄中⁸⁷Rb的优化策略和可实现的最大MQMAS效率。这些参数包括射频磁场强度ν(RF)、样品旋转速度ν(R)、转换周期长度以及FAM-1中的调制频率。这些数据为这些技术中涉及的自旋动力学提供了新的见解，并为实现最佳灵敏度提供了实验指导。转换最大效率所需的射频条件取决于所使用的方法。一般来说，就效率和偏离共振行为而言，FAM-1比章动和RIACT方法表现更好，特别是当ν(RF)与四极频率ν(Q)相比较小时。无论ν(RF)如何，使用章动、RIACT和FAM-1方法进行的实验在各向同性维度上产生的分辨率相似。
