Andersen Morten Daugaard, Jakobsen Hans J, Skibsted Jørgen
Department of Chemistry, Instrument Centre for Solid-State NMR Spectroscopy, University of Aarhus, DK-8000 Aarhus C, Denmark.
J Magn Reson. 2005 Apr;173(2):209-17. doi: 10.1016/j.jmr.2004.12.002.
Asymmetries in the manifold of spinning sidebands (ssbs) from the satellite transitions have been observed in variable-temperature 27Al MAS NMR spectra of alum (KAl(SO4)2.12H2O), recorded in the temperature range from -76 to 92 degrees C. The asymmetries decrease with increasing temperature and reflect the fact that the ssbs exhibit systematically different linewidths for different spectral regions of the manifold. From spin-echo 27Al NMR experiments on a single-crystal of alum, it is demonstrated that these variations in linewidth originate from differences in transverse (T2) relaxation times for the two inner (m=1/2<-->m=3/2 and m=-1/2<-->m=-3/2) and correspondingly for the two outer (m=3/2<-->m=5/2 and m=-3/2<-->m=-5/2) satellite transitions. T2 relaxation times in the range 0.5-3.5 ms are observed for the individual satellite transitions at -50 degrees C and 7.05 T, whereas the corresponding T1 relaxation times, determined from similar saturation-recovery 27Al NMR experiments, are almost constant (T1=0.07-0.10 s) for the individual satellite transitions. The variation in T2 values for the individual 27Al satellite transitions for alum is justified by a simple theoretical approach which considers the cross-correlation of the local fluctuating fields from the quadrupolar coupling and the heteronuclear (27Al-1H) dipolar interaction on the T2 relaxation times for the individual transitions. This approach and the observed differences in T2 values indicate that a single random motional process modulates both the quadrupolar and heteronuclear dipolar interactions for 27Al in alum at low temperatures.
在温度范围为-76至92摄氏度记录的明矾（KAl(SO4)2·12H2O）变温27Al MAS NMR谱中，观察到来自卫星跃迁的自旋边带（ssbs）流形中的不对称性。这些不对称性随温度升高而减小，反映出ssbs在流形的不同光谱区域表现出系统不同的线宽这一事实。通过对明矾单晶进行自旋回波27Al NMR实验表明，线宽的这些变化源于两个内层（m = 1/2 ↔ m = 3/2和m = -1/2 ↔ m = -3/2）以及相应的两个外层（m = 3/2 ↔ m = 5/2和m = -3/2 ↔ m = -5/2）卫星跃迁的横向（T2）弛豫时间的差异。在-50摄氏度和7.05 T下，各个卫星跃迁的T2弛豫时间在0.5 - 3.5 ms范围内，而通过类似的饱和恢复27Al NMR实验确定的相应T1弛豫时间对于各个卫星跃迁几乎是恒定的（T1 = 0.07 - 0.10 s）。明矾中各个27Al卫星跃迁的T2值变化可以通过一种简单的理论方法来解释，该方法考虑了四极耦合的局部波动场与异核（27Al - 1H）偶极相互作用对各个跃迁的T2弛豫时间的交叉相关性。这种方法以及观察到的T2值差异表明，在低温下，单个随机运动过程调制了明矾中27Al的四极和异核偶极相互作用。
