Lathrop D, Franke D, Maxwell R, Tepe T, Flesher R, Zhang Z, Eckert H
Department of Chemistry, University of California, Santa Barbara 93106.
Solid State Nucl Magn Reson. 1992 Jun;1(2):73-83. doi: 10.1016/0926-2040(92)90019-6.
The ability of the 90 degrees-t1-180 degrees pulse sequence to produce accurate dipole-dipole coupling information in solids is investigated. To this end, the experimental 31P spin echo decays are measured for eighteen crystalline phosphides and phosphorus chalcogenides and compared with simulations, based on the known internuclear distances in these compounds. The experimental results are generally found accurate in compounds where the dominant contribution to the dipole-dipole coupling arises from nuclei in structurally inequivalent sites with large chemical shift anisotropies. For this situation, the quantum mechanical "flip-flop" term in the dipolar Hamiltonian is suppressed and the dipole-dipole coupling is entirely heteronuclear in character. All of those compounds that do not obey this condition show accelerated spin echo decays due to a fractional contribution of the flip-flop term and possibly incomplete refocusing of chemical shift terms on the time scale of the experiment. The results confirm on an empirical basis that the spin echo NMR technique can provide accurate dipole-dipole coupling information (and thus distance distributions) in disordered solids and glasses.
研究了90°-t1-180°脉冲序列在固体中产生准确偶极-偶极耦合信息的能力。为此，对18种结晶磷化物和硫属磷化物测量了实验性的31P自旋回波衰减，并根据这些化合物中已知的核间距与模拟结果进行比较。实验结果通常在那些偶极-偶极耦合的主要贡献来自具有大化学位移各向异性的结构不等价位点上的核的化合物中是准确的。对于这种情况，偶极哈密顿量中的量子力学“翻转”项被抑制，偶极-偶极耦合完全是异核性质的。所有不满足此条件的化合物由于翻转项的部分贡献以及在实验时间尺度上化学位移项可能不完全重聚焦而显示出自旋回波衰减加速。结果在经验基础上证实了自旋回波技术可以在无序固体和玻璃中提供准确的偶极-偶极耦合信息（从而得到距离分布）。
