The changes in the spectra of the acidic group in chabazite are studied by quantum chemical calculations. The zeolite is modeled by two clusters consisting of eight tetrahedral atoms arranged in a ring and seven tetrahedral atoms coordinated around the zeolite OH group. The potential energy and dipole surfaces were constructed from the zeolite OH stretch, in-plane and out-of-plane bending coordinates, and the intermolecular stretch coordinate that corresponds to a movement of the water molecule as a whole. Both the anharmonicities of the potential energy and dipole were taken into account by calculation of the frequencies and intensities. The matrix elements of the vibrational Hamiltonian were calculated within the discrete variable representation basis set. We have assigned the experimentally observed frequencies at ϳ2900, ϳ2400, and ϳ1700 cm Ϫ1 to the strongly perturbed zeolite OH vibrations caused by the hydrogen bonding with the water molecule. The ABC triplet is a Fermi resonance of the zeolite OH stretch mode with the overtone of the in-plane bending ͑the A band͒ and the overtone of the out-of-plane bending ͑the C band͒. In the B band the stretch is also coupled with the second overtone of the out-of-plane bending. The frequencies at ϳ3700 and ϳ3550 cm Ϫ1 we have assigned to the OH stretch frequencies of a slightly perturbed water molecule.
Hybrid B3LYP and gradient-corrected PW91 functionals were used for studying methanol adsorption on a zeolite cluster consisting of an 8T ring of chabazite. The comparison of the results obtained with PW91 with periodic calculations has shown that the adopted ring is an adequate approximation for the Brønsted sides in chabazite. Both physisorbed and chemisorbed methanol were found to be a minimum on the potential energy surface, with an energy difference up to 10 kJ/mol in favor of the hydrogen-bonded complex. It has been shown that compared to B3LYP, the PW91 functional overestimates the hydroxyl bond distance and underestimates the hydrogen bond distance. In the physisorbed mode, the methanol oxygen atom is strongly bonded to the zeolite proton, whereas the distance between the methanol proton and the framework oxygen atoms is 1.912−2.090 Å. We have calculated for hydrogen bonded methanol hydroxyl stretch frequencies in the intervals 3677−3582 and 2358−2187 cm-1 for the methanol and the zeolite OH bonds, respectively.
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