The adsorption of one and two water molecules on cluster models of Brønsted acid sites of zeolite catalysts has been investigated by ab initio quantum chemical methods at the Hartree-Fock self-consistent field (HF), at the second-order Møller-Plesset perturbation theory (MP2), and at the density functional theory (DFT) levels. Among the two possible structures of the 1:1 adsorption complex, the water H-bonded to the zeolitic OH group (neutral complex) and the hydroxonium ion attached to the negatively charged zeolite surface (ion pair), only the former is a minimum. The ion pair complex is a transition structure for the proton transfer from one lattice oxygen to a neighboring one via the adsorbed water. However, the energy difference between both structures is only a few kJ/mol. For the neutral 1:1 adsorption complex we predict an average shift of the three protons involved of 7-8 ppm; the observed shifts are 6-7 ppm for one water molecule per site. The vibrational frequencies calculated for the ion pair structure do not permit an interpretation of the observed infrared spectrum. For the neutral structure (MP2) we predict frequencies of 1317 and 1022 cm -1 for the zeolitic in-plane and out-of-plane modes, respectively, while the zeolitic OH stretching mode is strongly red-shifted down to 2740-2850 cm -1 . These data support a recent interpretation of the IR spectrum which explains the observed triplet of bands as a result of Fermi resonance between the strongly perturbed zeolitic OH stretch and the OH bending overtones. The MP2 calculations for the neutral complex also provide a complete assignment of the peaks observed by inelastic neutron scattering for water on H-mordenite. Inclusion of electron correlation proves crucial, and comparison of MP2 and DFT (gradient corrected functionals) methods is made. While energy differences are very similar, the DFT approach yields by far too large frequency shifts for OH donor groups in H bonds. When a second water molecule is added (2:1 complex), both the neutral and the ion pair structure prove to be local minima on the potential energy surface. The adsorption energy is found to drop by 25%, and the ion pair structure becomes the more stable one. Predictions are made on how the vibrational spectra and the 1 H NMR chemical shifts change.
Inelastic neutron scattering (INS) has been used to study the adsorption of water, at different concentrations, in H-ZSM-5. INS is the only vibrational technique where the intensities can be calculated with reasonable accuracy from atomic displacements. This feature is used here to simulate the INS spectra of the two possible structures resulting from water interaction with the Brönsted acid sites of the zeolite: hydrogen-bonded water or hydroxonium ion. The atomic displacements for the two structures are derived from recent ab initio MP2 calculations (Krossner, M.; Sauer, J. J. Phys. Chem. 1996, 100, 6199-6211). The comparison between experimental and calculated INS spectra confirms that the first water molecule is attached to the acid site via two hydrogen bonds, in agreement with the conclusion made by Krossner and Sauer. Hydroxonium ions are not found in H-ZSM-5; however, this protonated species might be present in zeolites with a different structure.
The structures, binding energies, and harmonic vibrational
frequencies of AlX3···2H2O (X =
F,Cl) complexes
have been explored for the first time at the HF, DFT, and MP2 levels
using the 6-31G*, 6-31+G*, and the
6-311G** basis sets. The optimizations were performed without
symmetry restrictions or other structural
limitations. All complexes investigated were found to be
energetically stable, regardless of the computational
method used. The calculations showed that the
DFT(B3LYP)/6-31+G* method is suitable for the
prediction
of both binding energies and vibrational frequencies for these types of
complexes. This makes possible
qualitatively accurate calculations at a relatively low computational
expense of even larger, comparable
complexes. The AlF3···3H2O
complex was therefore investigated only at this level, yielding the
basis for the
molecular interpretation of the first steps of the macroscopically
investigated hydration process of AlF3. A
comparison of the binding energies of complexes containing an
increasing number of water molecules has
been performed. Furthermore, the vibrational frequencies of all
complexes have been predicted.
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