Diffusion of Benzene in the Breathing Metal–Organic Framework MIL-53(Cr): A Joint Experimental–Computational Investigation

A combination of experimental (quasi-elastic neutron scattering and ²H 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 μ₂-OH groups present at the MOF pore wall. This translational diffusion is combined with a fast uniaxial rotational motion around the <i>C</i>₆-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 μ₂-OH groups of the MIL-53 channels