Study of Guest Molecules in Metal–Organic Frameworks by Powder X-ray Diffraction: Analysis of Difference Envelope Density

The structural characterization of metal–organic frameworks (MOFs) by powder X-ray diffraction can be challenging. Even more difficult are studies of guest solvent or gas molecules inside the MOF pores. Hence, recently we successfully designed several new approaches for structural investigations of porous MOFs. These methods use structure envelopes, which can be easily generated from the structure factors of a few (1–10) of the most intense low index reflections. However, the most interesting results have been found by using difference envelope density (DED) analysis. DED can be produced by taking the difference between observed and calculated structure envelope densities. The generation and analysis of DED maps are straightforward but allow studying guest molecules in the pores of MOFs by using routine powder X-ray diffraction data. Examples of DED used for studies of solvent molecule location, porosity activation, and gas loading are presented herein. We show that DED analysis is an important technique in the study of host–guest properties in MOFs by providing position, shape, and approximate occupancy of molecules in the MOF pores.