CHEMICAL-SHIFT ANISOTROPY IN POWDERED SOLIDS STUDIED BY 2D FT NMR WITH FLIPPING OF THE SPINNING AXIS
CHEMICAL-SHIFT ANISOTROPY IN POWDERED SOLIDS STUDIED BY 2D FT NMR WITH FLIPPING OF THE SPINNING AXIS
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DOI：
10.1016/0022-2364(83)90134-8
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发表时间：
1983-01-01
影响因子：
2.2
通讯作者：
MACIEL, GE
中科院分区：
文献类型：
--
作者：
BAX, A;SZEVERENYI, NM;MACIEL, GE
关于化学位移各向异性的信息通常很难从非旋转粉末样品的光谱中提取出来，因为分子中化学性质不同的位点的粉末图案通常有很大的重叠。已经提出了几种类型的实验来促进化学位移各向异性的测量（1-9）。所有这些都具有共同的特征，即通过快速样品旋转（1-7，9）或通过以三个离散步骤（8）旋转样品，使样品围绕魔角轴（1-8）或非常接近魔角的轴（9）旋转。我们提出了一种新的二维方法获得的各向异性信息。在这个新的实验中，样品的旋转轴在演化和检测期间从90“翻转到54.7”。该实验具有广泛的适用性，对复杂样品的研究具有很大的前景。实验方案如图1所示。在我们的情况下，13 C核的交叉极化是在样品绕与静磁场成90”角的轴旋转时进行的。它可以表明（IO），粉末各向异性图案，在这些条件下获得的是反向和塌陷的一半的静态非自旋情况下的宽度，但保持相同的形状。在演化周期（ti）结束时，横向13 C磁化的x分量通过90”13 C脉冲沿平行于静磁场的z轴沿着存储。然后将样品的旋转轴的方向改变为魔角。与各向异性图案的宽度相比，样品快速旋转，使得旋转边带具有可忽略的强度。最后的90 ° 13 C脉冲将z存储的13 C磁化旋转回到横向平面中，在横向平面中，其在时域tZ中进动，具有相应的各向同性化学位移频率。第一个90 ° 13 C脉冲的相位交替地沿沿着+ y和-y循环，加上和减去所采集的数据，用于消除寄生信号。通过相对于t2的傅立叶变换获得的检测到的各向同性频谱S（t1，F2）在幅度上用在演化周期ti期间存在的频率进行调制。因此，粉末各向异性信息和各向同性化学位移将出现在F1维中。由于振幅调制，可以通过计算余弦傅里叶变换P（F1，F2）来获得纯2D吸收光谱。
Information about chemical shift anisotropy is usually difficult to extract from the spectrum of a nonspinning powdered sample because of the usually extensive overlap of the powder patterns from the chemically different sites in the molecule. Several types of experiments have been proposed to facilitate the measurement of chemical shift anisotropy (1-9). All have the common feature that the sample is rotated about the magic-angle axis (1-8) or an axis very close to the magic angle (9), either by rapid sample spinning (1-7, 9) or by rotating the sample in three discrete steps (8). We propose a new two-dimensional approach for obtaining the anisotropy information. In this new experiment, the spinning axis of the sample is flipped from 90 to 54.7” between the evolution and detection periods. The experiment appears to be widely applicable and has great promise for the study of complex samples. The experimental scheme is set out in Fig. 1. Cross polarization of, in our case, 13C nuclei is performed while the sample is spun about an axis that makes an angle of 90” with the static magnetic field. It can be shown (IO) that the powder anisotropy pattern that obtains under these conditions is reversed and collapsed to half the width of the static nonspinning case, but keeps the same shape. At the end of the evolution period (t,), the x component of the transverse 13C magnetization is stored along the z axis, parallel to the static magnetic field, by means of a 90,” 13C pulse. The orientation of the spinning axis of the sample is then changed to the magic angle. The sample is spun fast compared with the width of the anisotropy patterns, so that spinning sidebands have negligible intensities. A final 90 o 13C pulse rotates the z-stored 13C magnetization back into the transverse plane, where it precesses in the time domain, tZ, with the corresponding isotropic chemical shift frequencies. Cycling of the phase of the first 90 o 13C pulse alternately along+ y and-y, together with adding and subtracting of the acquired data, is used to eliminate spurious signals. The detected isotropic spectrum, S (tl, F2) obtained by Fourier transformation with respect to t2, is modulated in amplitude with the frequencies existing during the evolution period, t,. Hence, the powder anisotropy information and the isotropic chemical shifts will appear in the F, dimension. Because of the amplitude modulation, a pure 2D absorption spectrum can be obtained by calculating the cosine Fourier transform, P (F,, FJ (II, 12).