Scaling and labeling the high-resolution isotropic axis of two-dimensional multiple-quantum magic-angle-spinning spectra of half-integer quadrupole spins
Scaling and labeling the high-resolution isotropic axis of two-dimensional multiple-quantum magic-angle-spinning spectra of half-integer quadrupole spins
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半整数四极自旋二维多量子魔角自旋谱的高分辨率各向同性轴的缩放和标记
DOI：
10.1103/physrevb.58.2764
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发表时间：
1998
影响因子：
3.7
通讯作者：
P. Man
中科院分区：
文献类型：
--
作者：
P. Man
（观察）！二维中 MQ 过滤的中心跃迁峰的重心使我们能够确定吸收线的真实各向同性化学位移，这与化合物中的平均键角相关。在自旋系统自由进动过程中，仅考虑以高自旋速率以魔角旋转的样品的各向同性化学位移和二阶四极相互作用。另一方面，在脉冲期间仅考虑静态样品的一阶四极相互作用。采用超复合检测方法获得纯二维吸收光谱。描述了脉冲程序和数据处理的连续阶段。为简单起见，仅详细计算 w 相第一个脉冲结束时自旋 I5 3 的密度矩阵，这使我们能够推断出选择性检测第一个脉冲生成的 63 量子相干性的脉冲序列的相位循环。对于四个半整数四极自旋，推导了回波和反回波相对于第二个脉冲的位置，以及 MQ 滤波的中心跃迁峰相对于载波频率 (v 0) 沿 F1 维度的位置。 v 0 相对于外部水溶液的频率偏移在F1维度与F2维度线性相关。剪切变换的主要目的是将采集周期的开始从第二个脉冲的末尾移动到回波位置，并沿 F1 维度产生高分辨率频谱，对此进行了详细介绍。在文献中，使用两种约定来定义演化周期。约定Cz将实验演化周期作为演化周期，而约定Ck将相对于第一脉冲的回波位置视为演化周期。真正的各向同性化学位移、峰重心的二阶四极杆位移以及将四极耦合常数与不对称参数相关的参数CQh是d G1 (obs)和d G2 (obs)的函数；它们的关系取决于惯例。事实上，约定 Ck 引入了一个比例因子~取决于自旋和相干阶！这不仅改变了光谱宽度和线宽，还改变了 F1 维度中旋转边带的位置。建议使用约定 Cz 进行线宽比较。使用广泛研究的 RbNO3 粉末中的 87 Rb (I5 3 2 ) 和两个载波频率（98.2 和 163.6 MHz）检查分析表达式。我们的数据与文献中的数据一致。
(obs) ! of the center of gravity of an MQ-filtered central-transition peak in the two dimensions allows us to determine the true isotropic chemical shift of an absorption line, which is related to the mean bond angle in a compound. Only the isotropic chemical shift and the second-order quadrupole interaction for a sample rotating at the magic angle at a high spinning rate are considered during the free precession of the spin system. On the other hand, only the first-order quadrupole interaction for a static sample is considered during the pulses. The hypercomplex detection method is used to obtain a pure 2D absorption spectrum. The pulse program and the successive stages of data processing are described. For simplicity, only the density matrix for a spin I5 3 at the end of the first pulse of phase w is calculated in detail, which allows us to deduce the phase cycling of the pulse sequence that selectively detects the 63-quantum coherences generated by the first pulse. The positions of the echo and antiecho relative to the second pulse, and that of the MQ-filtered central-transition peak relative to the carrier frequency (v 0) along the F1 dimension are derived for the four half-integer quadrupole spins. The frequency offset of v 0 relative to an external aqueous solution in the F1 dimension is linearly related to that in the F2 dimension. The shearing transformation, whose main interest is to shift the beginning of the acquisition period from the end of the second pulse to the echo position and to yield a high-resolution spectrum along the F1 dimension, is presented in great detail. In the literature, two conventions are used to define the evolution period. Convention Cz takes the experimental evolution period as the evolution period, whereas convention Ck considers the echo position relative to the first pulse as the evolution period. The true isotropic chemical shift, the second-order quadrupole shift of the center of gravity of a peak, and the parameter CQh relating the quadrupole coupling constant with the asymmetry parameter are functions of d G1 (obs) and d G2 (obs) ; their relationships depend on the conventions. In fact, convention Ck introduces a scaling factor ~depending on the spin and the coherence order! that modifies not only the spectral width and the linewidth, but also the positions of the spinning sidebands in the F1 dimension. Convention Cz is recommended for linewidth comparison. The analytical expressions are checked using 87 Rb (I5 3 2 ) in the extensively studied RbNO3 powder with two carrier frequencies: 98.2 and 163.6 MHz. Our data are in agreement with those of the literature.