Cabrita Eurico J, Berger Stefan, Bräuer Peter, Kärger Jörg
Institut für Analytische Chemie, Universität Leipzig, Johannisallee 29, D-04103, Leipzig, Germany.
J Magn Reson. 2002 Jul;157(1):124-31. doi: 10.1006/jmre.2002.2574.
The effect of chemical exchange in the diffusion-ordered (DOSY) spectra of a two-site system in the slow-exchange limit with respect to the chemical shift is studied. The problem is addressed both theoretically and experimentally. The relationship between diffusion time (t) and mean lifetimes (tau) is studied by the simulation of the magnetization attenuations as a function of the gradient strength, under PFG conditions. The influence of the difference in populations and diffusion coefficients of the two sites is also considered. In analogy to the limiting cases of fast- and slow-exchange with respect to the chemical shift, limiting cases with respect to the diffusion dimension are defined. The slow-exchange limit in diffusion corresponds to the relation of t and tau that allows us to observe the two spins in exchange associated with the individual diffusion coefficients of the two sites when no exchange is present. The fast-exchange limit in diffusion is reached when the relation of t and tau is such that the two spins present the same apparent diffusion coefficient. By using a model system consisting of water/t-butanol it is shown that by recording several DOSY experiments with increasing diffusion times it is possible to estimate the value of the exchange rate.
研究了在化学位移的慢交换极限下，双位点体系的扩散排序（DOSY）谱中的化学交换效应。从理论和实验两方面探讨了该问题。在脉冲场梯度（PFG）条件下，通过模拟作为梯度强度函数的磁化衰减，研究了扩散时间（t）与平均寿命（τ）之间的关系。还考虑了两个位点的丰度和扩散系数差异的影响。类似于化学位移的快交换和慢交换极限情况，定义了扩散维度的极限情况。扩散中的慢交换极限对应于t和τ的关系，当不存在交换时，该关系使我们能够观察到与两个位点各自扩散系数相关的交换中的两个自旋。当t和τ的关系使得两个自旋呈现相同的表观扩散系数时，达到扩散中的快交换极限。通过使用由水/叔丁醇组成的模型体系表明，通过记录几个扩散时间增加的DOSY实验，可以估计交换速率的值。
