Datt et al. also presented Al NMR spectra of their zeolite samples before and after the adsorption of 5-FU. Calculated Al NMR chemical shifts are compiled in Table for several of the FAU models considered here. For tetrahedral Al atoms, chemical shifts on the order of 56 to 59 ppm are predicted, and there is no evident effect of the presence of 5-FU on the position, in agreement with experiment (~60 ppm). For the octahedral Al atoms, a chemical shift of 1 to 3 ppm is predicted for systems without 5-FU. This shift moves to about -3 to -8 ppm in systems where 5-FU is coordinated to the Al atom. The experimental chemical shift of about -2 ppm seems to be less affected by 5-FU adsorption, however, the considerable broadening of the corresponding peak in the two more Si-rich samples renders it difficult to determine the chemical shift precisely. This broadening may, at least partially, be due to the In order to probe the stability of the Al-coordinated 5-FU molecules at finite temperature, AIMD simulations were performed for a temperature of 298 K, starting from the two lowest-energy configurations visualised in Figure . Figure shows the evolution of the Al-O distances during the simulation time of 25 ps for the three Al-coordinated species (5-FU and two H2O molecules). It is evident that one of the Al-O(H2O) bonds dissociates during the simulation, in analogy to the observations made above for the FAU_fwAloct_2Al system at elevated temperatures in the absence of additional non-coordinated H2O molecules. A re-optimisation of snapshots shows a further lowering of the total energy by up to 26 kJ mol -1 after breaking of the bond in the 2_O2 configuration, whereas the total energy of the 1_O4 configuration remains rather close to its initial value (Figure ). The energetically most favourable arrangement obtained from these optimisations is visualised in Figure . In this structure, the water molecule that has been removed from the framework-associated Al atom acts as hydrogen bond acceptor for the other H2O molecule and for 5-FU, as well as forming a hydrogen bond to the framework. The fact that no breaking of the Al-O(5-FU) bond is observed in either of the simulations indicates that Al-coordinated 5-FU is more stable than Alcoordinated H2O, in line with the negative replacement energies reported in Table .