Protonation of an H2O dimer by a zeolitic Bronsted acid site

The potential energy surface for the interaction of a water dimer with the Bronsted acid site in a zeolite represented by a Si4AlO4H13 cluster is exam ined using the B3LYP density functional method. Local energy minima corresp onding to both neutral and ion-pair adsorption structures were located, as well as the transition state for proton transfer to the dimer. The neutral complex is more stable than the ion-pair structure by 2.9 kcal/mol.at the h ighest level of calculation. In all structures both ends of the adsorbed sp ecies form hydrogen bonds (H . . .O) to the zeolitic cluster. The zero poin t energy corrections cause the energy of the ion-pair structure to rise abo ve that of the transition state, indicating that the ion-pair structure is not a true local energy minimum on the potential energy surface. These resu lts reveal that, like the protonated water monomer complex, the protonated water dimer complex is a transition state for proton exchange between adjac ent framework oxygen atoms in our cluster model of the zeolite. However, si nce the energy differences between the three structures investigated here a re so small, the protonated water dimer might possibly be a true equilibriu m structure for a particular zeolite framework. The calculated vibrational frequencies for the adsorbed complexes are compared with experimental infra red spectra. This comparison suggests that experimental spectra for zeolite -water systems with loadings of two or more water molecules per acid site a re a superposition of features from both neutral and ion-pair-water complex es. This interpretation is consistent with the calculated energies of the t wo complexes.