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which allows structural information to be deduced from a measured vibrational spectrum. Strong hydrogen bonds with short 0-0 distances correlate with long, weakened 0 -H covalent bonds and low hydrogen stretching vibrations. If the 0-0 distance is increased. the length ofthe covalent bond approaches its unperturbed value approximately exponentially; the 0 -H stretching vibration increases almost linearly as the length of the covalent bond decreases. Aluminum-rich zeolites differ from their aluminum-poor counterparts both in their IR spectra and in their rea~tivity. '~. 16] In particular, the highest peak at 3500 cm-' seems to vanish as the Si:AI ratio approaches its lower limit of 1 ; 1. Therefore we investigated the interaction of methanol with two acid sites simultaneously. We find that the hydrogen bond between the methanol proton H, and an oxygen adjacent to another aluminum atom is shortened by about 5% and is therefore stronger than the hydrogen bond formed with an isolated acid site. Preliminary molecular dynamics simulations of I ps reveal a downward shift of the H,-0 vibration (located at 3500cm-' for the methanol hydroxyl group adsorbed on an isolated acid site) of roughly 400cm-', which could explain the disappearance of the former absorption band. However, it should be noted that aluminum-rich zeolites exhibit a substantially richer variety of structures for acid-site centers and adsorption complexes than aluminum-poor zeolites, a fact that has not yet been explored exhaustively.
We studied sodalite to unravel processes that should also be relevant for the more complex zeolites actually used in technologically relevant processes. Therefore we should discuss to what extent our results apply to zeolites other than sodalite. We expect that the essential features of structure A are rather independent of the zeolite, as the tetrahedral angle on the Si and A1 atoms is largely conserved. Structure B, however, depends strongly on the local structure. Since it depends on the proximity of distant oxygen atoms, this structure can only occur in sufficiently small or highly deformed larger rings. Rings consisting of four atoms with tetrahedral coordination cannot be bridged, because the acid-site protons point outward. Since the structures A and B have similar vibrational patterns, we expect only quantitative changes in the IR spectra for zeolites that cannot form structure B. The largest deviations in the IR spectra will occur in the lower part (see Fig. ), because this part is most sensitive to the strength of bond of the acid-site proton.
Our computer experiments predicted adsorption structures of methanol in zeolites which, for the first time, are consistent with the available experimental data. This information forms the basis for the understanding of a large class of zeolite-catalyzed reactions. We have applied a theoretical approach t o zeolite chemistry, which allows us to study with a high level of accuracy dynamic behavior of molecules in zeolites at finite temperatures without the limitation of small zeolite fragments. Real-time simulations of chemical reactions in zeolites are currently in progress.