H3SIOH AND F3SIOH AS MODELS FOR ISOLATED HYDROXYL-GROUPS OF AMORPHOUSSILICA - AN AB-INITIO STUDY OF THE ADDUCTS WITH DIHYDROGEN AND CARBON-MONOXIDE

Ab initio calculations have been performed at both the self-consistent held (SCF) and the second-order Moller-Plesset (MP2) levels of theory , using both double-zeta plus polarisation functions basis sets and au gmented correlation-consistent valence-polarised (aug-cc-pVDZ and aug- cc-pVTZ) ones, to compare the acidic and vibration features and the ge ometry of H3SiOH, the model usually adopted for the isolated hydroxyls of silica, with those of its fluorinated analogue, F3SiOH. Their comp lexes with H-2 and CO have also been studied. Passing from the MP2/DZP level of computation to MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ levels re sults in a considerable improvement of calculated data for H3SiOH and its complexes when compared with experimental data. H3SiOH is, however , less acidic than isolated hyroxyls of silica, In contrast, the use o f F3SiOH as a model yields an overestimation of the acidic properties; e.g., the stretching O-H mode frequency shifts caused by hydrogen-bon d interaction with the base molecules. The combined use of both models may provide guidelines for prediction of the adducts of the isolated hydroxyl of silica with small molecules. (C) 1998 Elsevier Science B.V . All rights reserved.