Ernst M, Verhoeven A, Meier B H
Laboratory of Physical Chemistry, University of Nijmegen, Toernooiveld 1, Nijmegen, NL-6525 ED, The Netherlands.
J Magn Reson. 1998 Feb;130(2):176-85. doi: 10.1006/jmre.1997.1311.
We have investigated the carbon line shape of solid adamantane under high-speed magic-angle sample spinning (MAS) acquired without proton decoupling. The CH-group shows a spinning-speed-dependent line broadening while the CH2-group consists of a spinning-speed-independent sharp component and a spinning-speed-dependent broader part. These phenomena can be explained by self-decoupling of the J-interaction due to proton spin diffusion. Such a self-decoupling process can be described by a magnetization exchange process between the multiplet lines. Changing the spin-diffusion rate constant by off-resonance irradiation of the protons allows us to observe the full range from slow exchange to coalescence to fast exchange of the carbon spectra. One of the multiplet components in the CH2-group corresponds to a group spin of the protons of zero and therefore does not couple to the other protons. This gives rise to the sharp central line. The magnetization exchange rate constant between the different multiplet lines can be determined from the spectra and is a measure for the spinning-speed-dependent proton spin-diffusion rate constant. Even at an MAS speed of 30 kHz, proton spin diffusion is still observable despite the relatively weak intermolecular proton dipolar-coupling network in adamantane which results in a static proton line width of only 14 kHz (full width at half height).
我们研究了在高速魔角样品旋转（MAS）且未进行质子去耦的情况下，固体金刚烷的碳谱线形状。CH基团显示出与旋转速度相关的谱线展宽，而CH₂基团由一个与旋转速度无关的尖锐成分和一个与旋转速度相关的较宽部分组成。这些现象可以通过质子自旋扩散导致的J相互作用的自去耦来解释。这样的自去耦过程可以用多重线之间的磁化交换过程来描述。通过对质子进行非共振照射来改变自旋扩散速率常数，使我们能够观察到碳谱从慢交换到合并再到快交换的整个范围。CH₂基团中的一个多重线成分对应于质子的零群自旋，因此不与其他质子耦合。这就产生了尖锐的中心线。不同多重线之间的磁化交换速率常数可以从谱图中确定，并且是与旋转速度相关的质子自旋扩散速率常数的一种度量。即使在30 kHz的MAS速度下，尽管金刚烷中分子间质子偶极耦合网络相对较弱，导致静态质子线宽仅为14 kHz（半高全宽），质子自旋扩散仍然是可观察到的。
