In contrast to the scenario observed for H-FAU_1H, the 5-FU molecule is only hydrogenbonded to framework protons in the lowest-energy configurations for both models having two protons within one 12MR (Figure ). Among the DFT-optimised structures, only a single configuration with a protonated 5-FU molecule was found (for H-FAU_2H_1_3), which is about 25 kJ mol -1 higher in energy than the most stable configuration. In both configurations shown in Figure , the hydrogen bond involving the O2 atom is somewhat shorter than that involving the O4 atom. Repulsive interactions between the F atom and the framework are likely to prevent the formation of a very short hydrogen bond to O4. Moreover, different partial charge calculation schemes consistently deliver a more negative polarisation of the O2 atom compared to O4 (Table ). Additional hydrogen bonds occur between the H1 atom of 5-FU and framework oxygen atoms. In H-FAU_2H_1_4, this hydrogen bond is fairly short, involving an O atom that is part of the AlO4 tetrahedron, whereas a longer bond to an O atom of a Si-O-Si linkage is present in H-FAU_2H_1_3. It is plausible to infer that the presence of a shorter hydrogen bond to a more negatively polarised Ofw atom contributes to the higher stability of the former configuration. The RDFs (Figure and S5) indicate that the hydrogen bonds highlighted in Figure are stable on the timescale of the AIMD simulations. Although there appears to be some fluctuation between the hydrogen-bonded state and short-lived framework deprotonation in the vicinity of the O2 atom, the cumulative RDFs indicate that the proton is located within 1.2 Å of O2 for less than 10% of the total simulation time.