Résumé
A combination of experimental (quasi-elastic neutron scattering and H-2 NMR) and computational (molecular dynamics) tools was used to uncover the molecular mobility of benzene trapped inside the flexible channel-type MIL-53 (Cr3+) MOF. This material was shown to undergo a contraction of the structure upon benzene adsorption with the formation of a narrow pore phase with a smaller aperture. This confinement was found to strongly influence the dynamics of the guest: benzene diffuses in a region centered in the middle of the pore by a 1D-jump translational mechanism along the tunnel ruled by the presence of the mu(2)-OH groups present at the MOF pore wall. This translational diffusion is combined with a fast uniaxial rotational motion around the C6-axis. Any other rotational motion that involves the tumbling of the phenyl rings about the channel axis is much less probable due to a high activation energy barrier (49 kJ mol(-1)). In this way benzene can be pictured as a rotating disc that diffuses rapidly through the central part of the channel by short jumps between neighboring low energy basins located in the vicinity of the mu(2)-OH groups of the MIL-53 channels.