These considerations already indicate that the competitive adsorption of water should result in a displacement of 5-FU away from the framework protons. AIMD simulations were employed to investigate the co-adsorption of both species in more detail. First, preliminary FF-based simulations were used to add 16 H2O molecules to the lowest-energy configurations computed for 5-FU in H-FAU_1H, H-FAU_2H_1_3, and H-FAU_2H_1_4. AIMD simulations with a total duration of 25 ps were then run for each of these (5-FU+16H2O)@FAU systems. In the following, key observations for each trajectory are presented, focussing on representative snapshots from different stages of the simulation and selected RDFs (including time-resolved RDFs for different time intervals where relevant). In the case of (5-FU+16H2O)@H-FAU_2H_1_3, the following chain of events can be identified based on representative snapshots shown in Figure (after 0.5, 2.5, 25 ps): At the very beginning of the simulation, the framework proton that participated in a hydrogen bond to the O4 atom of 5-FU in the water-free structure (Figure ) is removed from the framework, protonating the O4 atom. While the other framework proton is still bonded to the framework in the first snapshot shown in Figure , it is removed shortly afterwards, forming a covalent O2-Hfw bond. At approximately the same time, the O4 atom is deprotonated again, with the proton being transferred to a water molecule. This is the situation after 2.5 ps. After about 6 ps, the O2 atom is also deprotonated. At the end of the simulation, the vicinity of the Al atoms is occupied by the cluster consisting of H2O molecules and two hydronium ions, and the 5-FU molecule is displaced considerably from its initial location. The time-resolved RDFs shown in Figure further illustrate these findings. Three representative snapshots of (5-FU+16H2O)@H-FAU_2H_1_4 are shown in Figure , again visualising the system after 0.5, 2.5, and 25 ps. In this case, one of the framework protons moves to the O2 atom of 5-FU in the very initial stages of the simulation, while the O4 atom remains hydrogen-bonded to the other framework proton after 0.5 ps. After 2.5 ps, however, a rather unexpected situation is observed: At this point, both the O2 and O4 atom of 5-FU are protonated, and the N3 atom is deprotonated. Carrying a net charge of +1, 5-FU is now in a different tautomeric form than observed in all previous calculations involving protonated 5-FU. Although this tautomer was found to be less stable than the form having a single protonated O atom in a previous computational study, it does not seem implausible to surmise that it can be stabilised, at least temporarily, through localised interactions in a specific environment. At the end of the 25 ps trajectory, the proton that was previously attached to the Although there are non-negligible differences among the individual AIMD trajectories, some common features can be identified: In all three cases, the co-adsorption of water and 5-FU results in framework deprotonation, a finding that falls in line with previous observations made for water clusters and for co-adsorbed water and triclosan in protonic zeolites. Although protonated 5-FU occurs as an intermediate species during the simulations, the protons are transferred to water molecules over the course of the 25 ps simulations. In the final snapshots, 5-FU is always in the neutral form, interacting through hydrogen bonds with the positively charged water cluster and the deprotonated zeolite framework.