AN AB-INITIO STUDY OF TERMINAL SIOH AND BRIDGING SI(OH)AL GROUPS IN ZEOLITES AND THEIR INTERACTION WITH CARBON-MONOXIDE

Ab initio calculations have been performed at both levels SCF and seco nd-order Moller-Plesset theory, using double-zeta plus polarization ba sis set, to investigate the complexes formed between CO and both termi nal and bridging OH groups in zeolites. These are mimicked by H-satura ted minimal-size clusters, H3SiOH(SIL) and H3SiOHAlH3(BRO), respective ly. 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 thos e O-bound. Semi-quantitative agreement with experiment is instead reac hed for the vibrational features. Electron correlation considerably st abilizes bonding via the C-end and yields vibrational frequencies in b etter agreement with experiment. Complexes with BRO are much more stab le than with SIL: accordingly, the calculated frequency shifts for OH and CO stretching vibrations are much larger for complexes with BRO th an for SIL. The in-plane and out-of-plane O-H bending vibrations are s ensitive to complex formation, as is the O-H stretching vibration. Thi s latter has been studied both in the harmonic and anharmonic approxim ations, by numerically solving the related Schrodinger equation, for b oth free and GO-interacting SIL and BRO species. Unpublished experimen tal data are reported concerning the change in anharmonicity brought a bout in =SiOH and =Si(OH)Al= by CO complexation. In full agreement wit h experiment, interaction does not cause any change of anharmonicity w ith SIL, and a moderate increase with BRO.