M. Roos, P. Micke, K. Saalwächter, G. Hempel.
MAS enhances local 1H spin exchange and spin diffusion.
J. Magn. Reson. 260, 28-37 (2015).
Proton NMR spin-diffusion experiments are often combined with magic-angle spinning (MAS) to achieve
higher spectral resolution of solid samples. Here we show that local proton spin diffusion can indeed
become faster at low (<10 kHz) spinning rates as compared to static conditions. Spin diffusion under
static conditions can thus be slower than the often referred value of 0.8 nm²/ms, which was determined
using slow MAS (Clauss et al., 1993). The enhancement of spin diffusion by slow MAS relies on the
modulation of the orientation-dependent dipolar couplings during sample rotation and goes along with
transient level crossings in combination with dipolar truncation. The experimental finding and its
explanation is supported by density matrix simulations, and also emphasizes the sensitivity of spin
diffusion to the local coupling topology. The amplification of spin diffusion by slow MAS cannot be
explained by any model based on independent spin pairs; at least three spins have to be considered.