两位置交换问题魔角旋转核磁共振时域信号的准确解(英文)

AN EXACT SOLUTION OF A MAS FID IN TWO-SITE EXCHANGE PROBLEM

已知两位置交换分子运动速率对于静态13CNMR谱的影响是由一简单代数关系决定，它已成为分析固体中分子运动的常规方法．慢速的魔角旋转可以保留边带，所产生的边带型原则上也可用来分析分子运动，只是缺乏象静态谱那样的简单的关系，以至于在实际应用上是不可行的．本文给出作者所发现的存在分子运动时魔角旋转核磁共振自由诱导衰减信号的解析表达式的理论推导，提供分析固体分子运动的理论谱计算公式．我们选择二甲矾（dimethylsulfone）作为模型化合物，记录其不同温度下13CNMR谱，又计算了两个甲基做两位置交换运动的理论谱．理论谱与实验谱定性一致．定量上，边带强度存在一些差别，但边带宽度符合很好．通过理论谱与实验谱的比较，得到交换速率与温度的对应关系．虽然本文限于化学位移各向异性相互作用两位置交换问题，但容易推广到偶极作用多位置交换问题．

An exact solution of a magic-angle-spinning free-induction-decay under two-siteexchange has been derived. The expression has been confirmed by comparing the calculated spinningsideband patterns for powdered crystalline dimethyl sulfone at different exchange rates with experimentally measured CP/MAS 13C NMR spectra at various temperatures. The information of themode and the site exchange rate constants of molecular motion can be provided by spectral line fittingbetween calculated and experimental spectra. The numerical calculations with this algorithm showthat the exchange effect results in a slight narrowing of the centerband and a slight broadening ofsidebands compared to the equal, linewidtbs without exchange.