Unraveling the Molecular Details of the "Gate Opening" Phenomenon in ZIF-8 with X-ray Absorption Spectroscopy

The local structural changes associated with the ZIF-8 frameworkflexibility upon nitrogen gas adsorption have beenstudied byin situX-ray absorption spectroscopy (XAS) and high-energy-resolutionfluorescence-detected X-ray absorption near-edgestructure (HERFD-XANES) spectroscopy. Different thermodynamic conditions (isobar and isotherm) have been used to explorethe so-called"gate opening"transition in which the hexagonal pore windows of the sodalite cage open, increasing the accessiblevolume for gas adsorption. To elucidate the source of the spectral changes in the XANES region of the absorption spectra observedalong the gas adsorption and through the transition from the closed to the open pore configuration,ab initiocalculations have beenperformed. Our results demonstrate that the transition from the closed to the open pore configurations involves not only therotation of the MeIM ligand but also a further bend of the Me group away from the plane defined by the IM ring. Additionally, thecontribution of the N2molecules adsorbed in the center of the 4-ring window has been included in the scattering model to fullyreproduce the main features of the X-ray absorption spectra in the open pore configuration