We combined nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulation to study xylene behavior in MOF-5, probing the effects of adsorbate geometry in a weakly interacting model isotropic metalorganic framework (MOF) system. We employed NMR diffusometry and relaxometry techniques at low field (13 MHz) to quantify the self-diffusion coefficients (D s ) and the longitudinal relaxation times (T 1 ) of xylenes in MOF-5 as a function of temperature at saturated loading for each xylene. These experiments reveal the translational motion activation energies to be 15.3, 19.7, and 21.2 kJ mol −1 and the rotational activation energies to be 47.26, 12.88, and 11.55 for the (p-,m-,o-) xylene isomers respectively. Paraxylene exhibits faster translational motion, yet shows four times the activation energy barrier for rotational motion vis-à-vis the other isomers. MD simulations performed on these model systems corroborate the findings for paraxylene and suggest that paraxylene has the a lower free energy barrier for hopping away from its binding sites. These simulations show that paraxylene has the slowest rotational motion in the plane of the xylene molecule while it actually has the fastest out-of-plane rotational motion.
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