These distances alone, however, cannot provide an explanation why IFR shows extensive RT dynamic disorder, whereas STF, F@Si3 and CHA do not, as the shortest F•••Si distances are similar in these systems. A marked difference between STF, F@Si3 and IFR concerns the distance to the third-nearest Si atom, which is much shorter in IFR. This indicates that the propensity towards dynamic disorder does not depend exclusively on the distance to the secondary Si site, but that cooperative interactions with several Si atoms can come into play. The importance of such cooperative effects can be especially well illustrated when looking at STT: Here, the fluoride anion moves from the Si7 to the Si12 site (Figure ), despite an F•••Si7 distance of 2.98 Å. As there are three other Si atoms that are closer, with Si4 and Si5 being within 2.7 Å, it is straightforward to infer that the interactions with these sites lead to an increased displacement of fluoride from its equilibrium position and a weakening of the primary F-Si bond, thereby enabling a dynamic jump towards the Si12 site (F@Si12 is much lower in energy than both F@Si4 and F@Si5). This weakening of the F-Si bond is even detectable in the DFT-optimized structure, where the F-Si12 bond length is somewhat longer than in the other zeolites.