在CP MAS NMR中测量质子自旋扩散速率的方法(英文)

A METHOD FOR MEASURING PROTON SPIN DIFFUSION RATE IN CP MAS NMR

自旋扩散在固体核磁共振的许多现象中都起着非常重要的作用。现有几种理论方案以估算扩散系数。然而在实践中这种估算既不实际也不可靠。本文提出了确定自旋扩散速率的新方案,它利用的是CP MAS NMR中的稀核退极化规律。带质子的稀核磁化矢量在退极化中表现出两个阶段,慢衰减的第二阶段是单一指数过程,它提供了自旋扩散速率的直接度量。自旋扩散实质上是极化转移的一种宏观表现形式,这种转移通过一系列成对自旋的flip-flop进行,可以用一维随机走步模型描述。从退磁过程半对数曲线的斜率可以求得平均flip-flop时间。自旋扩散系数可以由此估算。在一些典型的刚性有机固体和结晶高分子聚合物中,求得平均flip-flop的时间是700微秒左右。它比偶极相关时间大一个数量级。这意味着,自旋扩散时间常数与自旋—自旋弛豫时间常数是很不相同的,虽然这两个相应过程虽密切相关的。由质子线宽估计自旋扩散系数是不可靠的。

Spin diffuson is of great significance to many phenomena in solid state NMR. There have been several theoretical schemes for evaluating the diffusion coefficient. In fact, however, this kind of evaluation is not only unpractical but also unreliable. This paper presents a new scheme for determining spin diffusion rate by using the depolarization of the rare spins in CP MAS NMR, where the magnetization of rigid protonated rare spins dccays in two-stages, and the slow second stage which is a single -exponential process offers a direct measurement of spin diffusion rate. Spin diffusion is actually a macroscopic manifestation of polarization transfer via flip-flop of successive pairs of spins, which can be reasonably described by a one-dimensional random walk model. The average flip-flop time can be deduced from the slope of the semilogarithm curve of depolarization, and the spin diffusion coefficient can thus be estimated. In some typical rigid organic solids and crystallized polymers, the average flip -flop time is found to be about 700s, which is of an order of magnitude longer than the dipolar correlation time. It means that the spin diffusion time constant is quite different from the spin-spin relaxation time constant, although the two processes are closely related. Consequently, the estimation of spin diffusion coefficient from proton line width is quite questionable