(available only for the four main modes shown in Figure ). The root mean square error of the DFT frequencies from the present work with respect to experimental results for 5-FU in an Ar matrix amounts to 17 cm -1 (including only the modes falling between 1200 and 1800 cm -1 listed in Table ). This agreement appears satisfactory, especially when considering that the DFT frequencies were directly taken from the calculations, without the application of any scaling factor. Scaling of vibrational frequencies computed with electronic structure methods by a method-dependent scaling factor is often done to minimise systematic deviations from experimental frequencies. Upon adsorption in SiO2-FAU, the most significant change in the IR spectrum is a slight redshift of the two ν(C=O) modes, correlated with a modest elongation of the C=O bonds. In the higher-frequency region (not included in the figure), the ν(N1-H1) mode is red-shifted due to the presence of the H1•••Ofw hydrogen bond (Figure ). Much more pronounced changes are visible in the IR spectrum of 5-FU@H-FAU_1H: The deprotonation of the framework by 5-FU results in the emergence of a new ν(O4-H) stretching mode at 2000 cm -1 , which is very prominently visible. The splitting of the ν(C=O) modes increases drastically, with the ν(C4=O4) mode being red-shifted due to the bond elongation upon protonation of O4, whereas the ν(C2=O2) mode is (slightly) blue-shifted. The ν(C5=C6) mode is red-shifted, and the O4-H bending mode appears as a fairly intense signal at 1360 cm -1 . A closer look at Table shows that several of the bond distances in the 5-FU ring change rather significantly upon protonation.