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.