title: Moderate MAS enhances local (1)H spin exchange and spin diffusion.
abstract: 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 nm(2)/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.
authors.affiliation: Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, Betty-Heimann-Str. 7, 06120 Halle (Saale), Germany. Electronic address: matthias.roos@physik.uni-halle.de.
authors.forename: Matthias
authors.initials: M
authors.lastname: Roos
authors.affiliation: Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, Betty-Heimann-Str. 7, 06120 Halle (Saale), Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Micke
authors.affiliation: Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, Betty-Heimann-Str. 7, 06120 Halle (Saale), Germany.
authors.forename: Kay
authors.initials: K
authors.lastname: Saalwächter
authors.affiliation: Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, Betty-Heimann-Str. 7, 06120 Halle (Saale), Germany. Electronic address: guenter.hempel@physik.uni-halle.de.
authors.forename: Günter
authors.initials: G
authors.lastname: Hempel
keywords: Dipolar truncation effect; MAS; Spin diffusion; Spin flip-flops