关键词: 自旋扩散, 交叉极化, 魔角旋转核磁共振, 退极化

Abstract: 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 decays 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 700μs, 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

Key words: Spin diflusion, CP MAS NMR, Depolarization