Magic-angle spinning and polarization transfer in proton-enhanced NMR

Abstract

High-speed spinning at the magic angle can significantly modify the rate of polarization transfer from abundant to rare spins in proton-enhanced NMR experiments on solids, if the spinning speed is greater than or comparable to the static dipole-dipole interaction among abundant spins in the rotating frame. In adamantane, this fact can be strikingly demonstrated experimentally. When the spinning speed is much less than dipolar coupling among abundant spins, the effect of spinning on polarization transfer is not dramatic, regardless of the nature of the static dipolar coupling between rare and abundant spins. A semiquantitative theoretical analysis is presented which describes the principal features of these phenomena in terms of the amplitude and frequency modulation of the dipolar interactions between and among the two spin systems. This analysis of the effect of coherent motion (spinning) on polarization transfer also provides some qualitative insight into how incoherent random molecular motion affects transfer in cross-polarization experiments.