Reversal of radio-frequency-driven spin diffusion by reorientation of the sample spinning axis

The dipolar Hamiltonian in a rapidly rotating sample is scaled by the second Legendre polynomial of the cosine of the angle between the rotation axis and the static magnetic field. It is, therefore, possible to refocus the spatial polarization-transfer process, often termed spin diffusion, in extended spin systems by reorienting the rotor axis such that the dipolar interaction Hamiltonian changes sign. We present experimental results which demonstrate that a rapid mechanical sample reorientation leads to a time reversal of the "radio-frequency-driven" spin diffusion among C-13 spins. (C) 1998 American Institute of Physics. [S0021-9606(98)03503-X].