Hu Yan-Yan, Levin E M, Schmidt-Rohr Klaus
Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA.
J Am Chem Soc. 2009 Jun 24;131(24):8390-1. doi: 10.1021/ja903334p.
High-resolution magic-angle spinning NMR of high-Z spin-1/2 nuclei such as (125)Te, (207)Pb, (119)Sn, (113)Cd, and (195)Pt is often hampered by large (>1000 ppm) chemical-shift anisotropies, which result in strong spinning sidebands that can obscure the centerbands of interest. In various tellurides with applications as thermoelectrics and as phase-change materials for data storage, even 22-kHz magic-angle spinning cannot resolve the center- and sidebands broadened by chemical-shift dispersion, which precludes peak identification or quantification. For sideband suppression over the necessary wide spectral range (up to 200 kHz), radio frequency pulse sequences with few, short pulses are required. We have identified Gan's two-dimensional magic-angle-turning (MAT) experiment with five 90 degrees pulses as a promising broadband technique for obtaining spectra without sidebands. We have adapted it to broad spectra and fast magic-angle spinning by accounting for long pulses (comparable to the dwell time in t(1)) and short rotation periods. Spectral distortions are small and residual sidebands negligible even for spectra with signals covering a range of 1.5 gammaB(1), due to a favorable disposition of the narrow ranges containing the signals of interest in the spectral plane. The method is demonstrated on various technologically interesting tellurides with spectra spanning up to 170 kHz, at 22 kHz MAS.
高Z自旋1/2核（如（125）Te、（207）Pb、（119）Sn、（113）Cd和（195）Pt）的高分辨率魔角旋转核磁共振常常受到大的（>1000 ppm）化学位移各向异性的阻碍，这会导致很强的旋转边带，从而可能掩盖感兴趣的中心带。在各种用作热电材料和数据存储相变材料的碲化物中，即使22 kHz的魔角旋转也无法分辨因化学位移色散而变宽的中心带和边带，这使得峰的识别或定量变得不可能。为了在必要的宽光谱范围内（高达200 kHz）抑制边带，需要使用具有少量短脉冲的射频脉冲序列。我们已确定甘氏的具有五个90度脉冲的二维魔角转向（MAT）实验是一种有前景的宽带技术，可用于获取无边带的光谱。我们通过考虑长脉冲（与t(1)中的驻留时间相当）和短旋转周期，将其应用于宽光谱和快速魔角旋转。即使对于信号覆盖范围达1.5γB(1)的光谱，由于光谱平面中包含感兴趣信号的窄范围的有利分布，光谱畸变也很小，残余边带可忽略不计。该方法在各种具有技术意义的碲化物上得到了验证，在22 kHz MAS下光谱范围可达170 kHz。
