Bräuniger Thomas, Pike Kevin J, Harris Robin K, Madhu P K
Department of Chemistry, University of Durham, DH1 3LE Durham, UK.
J Magn Reson. 2003 Jul;163(1):64-72. doi: 10.1016/s1090-7807(03)00124-1.
We report here an efficient multiple-quantum magic-angle spinning (MQMAS) pulse sequence involving fast amplitude-modulated (FAM) radio-frequency pulses for excitation and conversion of five-quantum (5Q) coherences of spin-5/2 nuclei. The use of a FAM-I type pulse train for the conversion of 5Q into 1Q coherences proves to be easier to implement experimentally than the earlier suggested use of a FAM-II type sequence [J. Magn. Reson. 154 (2002) 280], while delivering at least equal signal enhancement. Results of numerical simulations and experimental 27Al 5QMAS spectra of aluminium acetylacetonate for different excitation and conversion schemes are compared to substantiate these claims. We also demonstrate the feasibility of acquiring 5QMAS spectra of spin-5/2 systems using cogwheel phase cycling [J. Magn. Reson. 155 (2002) 300] to select the desired coherence pathways. A cogwheel phase cycle of only 57 steps is shown to be as effective as the minimum conventional nested 77-step phase cycle.
我们在此报告一种高效的多量子魔角旋转（MQMAS）脉冲序列，该序列涉及快速幅度调制（FAM）射频脉冲，用于激发和转换自旋 - 5/2 核的五量子（5Q）相干性。事实证明，使用FAM - I型脉冲序列将5Q转换为1Q相干性在实验上比早期建议的FAM - II型序列[《磁共振杂志》154 (2002) 280]更易于实现，同时至少能提供相同的信号增强效果。对乙酰丙酮铝在不同激发和转换方案下的数值模拟结果以及实验27Al 5QMAS谱进行了比较，以证实这些说法。我们还展示了使用齿轮相位循环[《磁共振杂志》155 (2002) 300]获取自旋 - 5/2 系统的5QMAS谱以选择所需相干路径的可行性。结果表明，仅57步的齿轮相位循环与传统的最少77步嵌套相位循环一样有效。
