Zeolitic imidazolate frameworks (ZIFs) constitute a sub-family of MOFs. They somewhat bridge the gap between zeolites and MOFs. The structural topologies of ZIFs are similar to zeolite or zeolite-like topologies [25]. ZIFs have tetrahedral frameworks where transition metals (Zn, Co, etc.) are linked by imidazolate ligands, the angle formed by imidazolates when bridging transition metals being close to the Si-O-Si bond angle in zeolites (^145°). These new materials exhibit exceptional chemical and thermal stability.

Although ZIFs can have large cavities, the windows apertures whereby mass — transport proceeds have rather small diameters. In ZIF-8, the linker between Zn atoms is 2-methylimidazolate, and the diameter of the window is approximately

1.4 A [25]. The diffusivities computed from MD, even for H2, have been found to be quite sensitive to the mobility of the linker [26]. This is illustrated in Fig. 2.9, where a comparison between experimental and calculated self-diffusivities shows that an agreement is only reached for a flexible framework. As detailed in Sect. 2.3.2, quantum corrections were included via the Feynman-Hibbs approach [26]. The influence of flexibility on computed diffusivities in ZIF-3, which has a larger window diameter (4.6 A), was found to be less pronounced, the agreement with QENS values being not as good as in ZIF-8 [26].