Unusual CO Adsorption in ZIF-7: Insight from Raman Spectroscopy and Computational Studies.

Here, we use Raman spectroscopy to investigate temperature-dependent changes in the atomic-scale structure of the zeolitic imidazolate framework ZIF-7 in a CO atmosphere and uncover the mechanism of maximal CO adsorption at 206 K. At 301 K, the Raman spectra of ZIF-7 at various CO gas pressures reveal a narrow-pore (np) to large-pore (lp) phase transition commencing at 0.1 bar as a result of adsorption of CO, as evident in the appearance of Fermi resonance bands of CO at 1272 and 1376 cm. Moreover, the Raman inactive bending mode of CO becomes active due to geometrical distortion of adsorbed CO. It further splits into two peaks due to hydrogen bonding interactions between CO and the benzene ring of the benzimidazole linker ZIF-7, as supported by our computational studies. In addition, the interaction between CO molecules plays a key role. Upon reducing the temperature at 1 bar CO gas pressure, ZIF-7 exhibits softening of the imidazole puckering mode and the Fermi resonance CO band due to interactions between CO and the framework through hydrogen bonding. At 206 K, substantial modification in the lattice mode and disappearance of the Raman inactive CO bending mode confirm the changes in the size of the pore cavity through structural rearrangements of CO.