An ab initio study of terminal SiOH and bridging Si(OH)Al groups in zeolites and their interaction with carbon monoxide

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Ab initio calculations have been performed at both levels SCF and second-order Møller-Plesset theory, using double-zeta plus polarization basis set, to investigate the complexes formed between CO and both terminal and bridging OH groups in zeolites. These are mimicked by H-saturated minimal-size clusters, H 3SiOH (SIL) and H 3SiOHAlH 3 (BRO), respectively. Interaction is considered through both the carbon and oxygen ends of CO. The SCF treatment yields unreliable energies of interaction: in contrast with experiment, C-bound complexes are less stable than those O-bound. Semi-quantitative agreement with experiment is instead reached for the vibrational features. Electron correlation considerably stabilizes bonding via the C-end and yields vibrational frequencies in better agreement with experiment. Complexes with BRO are much more stable than with SIL: accordingly, the calculated frequency shifts for OH and CO stretching vibrations are much larger for complexes with BRO than for SIL. The in-plane and out-of-plane O H bending vibrations are sensitive to complex formation, as is the OH stretching vibration. This latter has been studied both in the harmonic and anharmonic approximations, by numerically solving the related Schrödinger equation, for both free and CO-interacting SIL and BRO species. Unpublished experimental data are reported concerning the change in anharmonicity brought about in SiOH and Si(OH)Al by CO complexation. In full agreement with experiment, interaction does not cause any change of anharmonicity with SIL, and a moderate increase with BRO.