  Ab initio simulations are combined with in situ infrared spectroscopy to
unveil the molecular transport of H$_2$, CO$_2$, and H$_2$O in the metal
organic framework MOF-74-Mg. Our study uncovers---at the atomistic level---the
major factors governing the transport mechanism of these small molecules. In
particular, we identify four key diffusion mechanisms and calculate the
corresponding diffusion barriers, which are nicely confirmed by time-resolved
infrared experiments. We also answer a long-standing question about the
existence of secondary adsorption sites for the guest molecules, and we show
how those sites affect the macroscopic diffusion properties. Our findings are
important to gain a fundamental understanding of the diffusion processes in
these nano-porous materials, with direct implications for the usability of MOFs
in gas sequestration and storage applications.
