3) IR spectra and molecular structure of adsorbed 5-FU molecules Diffraction methods are often unable to resolve the locations of guest molecules in zeolite pores due to their disorder over different possible positions and orientations. In such cases, spectroscopic methods can be very useful to gain insights into host-guest interactions and adsorption-induced changes in the molecular structure. As an IR-spectroscopic characterisation of FAU samples loaded with 5-FU has been reported, DFT-computed IR spectra were generated for free 5-FU and three representative models visualised in Figure , namely 5-FU@SiO2-FAU, Config2 (weak H1 … Ofw hydrogen bond), 5-FU@H-FAU_1H