When turning to the calculated spectra of the two 5-FAU@FAU_fwAloct_2Al systems, both common features and differences are apparent, especially in the wavenumber range of 1750 to 1600 cm -1 . Both spectra show a moderately intense band at higher wavenumbers, which stems from C=O stretching vibrations associated with the 5-FU oxygen atom that is not bonded to the Al atom, and a more intense band at lower wavenumbers that is associated with vibrations of the O atom that is coordinated to Aloct atom. However, the separation between the two bands is rather different, amounting to 111 cm -1 for configuration 1_O4, but only 45 cm -1 for 2_O2. In both cases, the ν(C5=C6) vibration makes only a slight contribution to the spectrum, appearing as a shoulder on the low-wavenumber side of the intense band. With the exception of the ν(C5-F) band, all bands below 1600 cm -1 are very weak, agreeing with the lack of distinctive bands in the experimental spectrum in this wavenumber range. Altogether, neither of the two individual spectra shows a one-to-one correspondence with the experimental spectrum. Based on the good agreement of individual signals in the wavenumber range of 1750 to 1600 cm -1 , it appears, however, plausible to assume that the experimental spectrum arises from vibrations of Al-coordinated 5-FU molecules in different local environments. While only one type of framework-associated Aloct site and two positions and orientations of 5-FU were considered here, a real sample is likely to possess a variety of different environments of octahedral Al atoms (different parts of the pore, different degrees of association with the framework, etc.), which may, in turn, favour different coordination modes of 5-FU. For illustrative purposes, a composite IR spectrum was prepared, in which the relative contribution of the two configurations was set to a value that optimises the agreement with experiment (20% 1_O4, 80% 2_O2). This composite IR spectrum, shown in Figure , reproduces all key features of the experimental spectrum in the wavenumber range of 1750 to 1600 cm -1 , even without applying any particular adjustment of the full width at half maximum (arbitrarily chosen as 10 cm -1 ).