Molecular Mechanism of Swing Effect in Zeolitic Imidazolate Framework ZIF-8: Continuous Deformation upon Adsorption

Zeolitic imidazolate framework ZIF-8 displays flexibility of its structure by rotation of its imidazolate linker. This "swing effect" has been widely described in the literature, both experimentally and theoretically, as a bistable system where the linker oscillates between two structures: "open window" and "closed window". By using quantum chemistry calculations and first-principles molecular dynamics simulations, it is shown that the deformation upon adsorption is in fact continuous upon pore loading, with thermodynamics of packing effects being the reason behind stepped adsorption isotherms experimentally observed. Finally, we study a variant of ZIF-8 with a different linker, highlighting the influence of the linker and the balance of microscopic interactions on the framework's flexibility.