Protonation of O2 is observed in two other configurations (Config1 and Config5 of Table ), which are, however, about 10 kJ mol -1 less stable. This agrees with the previous finding by Wielinska et al. that protonation of O4 is energetically favoured. Configurations in which a neutral 5-FU molecule is hydrogen-bonded to the framework are even less stable. The Ofw•••Hfw and Hfw•••O4 RDFs visualised in Figure confirm that the proton remains closer to O4 than to Ofw for the majority of the simulation time. However, an overlap of the maxima in the distance range of 1.0 to 1.3 Å points to a certain degree of dynamic exchange.