In their experimental study, Datt et al. presented IR spectra of three FAU samples loaded with 5-FU. Since spectra of zeolites with different Si/Al ratios were very similar, Figure visualises the IR spectrum of the most Si-rich sample (dubbed HY-60 in that work, Si/Al ratio = 30). Datt et all. identified four bands in the wavenumber range from 1800 to 1200 cm -1 , which were attributed to C=O stretching (ν(C2=O2) at 1756 cm -1 , ν(C4=O4) at 1717 cm -1 ), C=C stretching (ν(C5=C6) at 1690 cm -1 ), and C-F stretching (ν(C5-F) at 1247 cm -1 ). These experimental values are shown as vertical lines in Figure . The figure also shows DFT-calculated IR spectra obtained for the two lowest-energy 5-FU@FAU_fwAloct_2Al configurations discussed above and to a spectrum computed for a system where 5-FU interacts with the framework proton of a protonic zeolite (5-FU@H-FAU_1H, newly optimised in the present work). Starting with the spectrum of 5-FU@H-FAU_1H, it is apparent that several features do not match the experimental observations: Most notably, there is only one major C=O stretching vibration at 1743 cm -1 . The second C=O stretching band is red-shifted to 1673 cm -1 due to the formation of a C4=O4•••Hfw hydrogen bond and its intensity is reduced considerably. The formation of this hydrogen bond also gives rise to a relatively intense band related to Ofw-Hfw •••O4 angle bending at about 1500 cm -1 , which is not observed in the experimental spectrum at all.