| Abstract
| - Experimental vibrational data (FTIR) concerning the interaction ofethylene, acetylene, and methylacetylenewith the acidic hydroxyls of amorphous silica (SiOH) and H-ZSM5 zeolite(Si(OH)Al) are compared withSCF/DZP and MP2/DZP results concerning the same molecules interactingwith H3SiOH (SIL) andH3Si(OH)AlH3 (BRO), the minimal-size clusters mimicking thesurface species. The interaction of dimethylacetylene with SIL and BRO has also been studied. Computedharmonic shifts in the O−H stretchingfrequency Δνh(OH) are in semiquantitativeagreement with experiment, in that the experimental dataaresystematically underestimated by a factor of≈/3 at best: the main reason is the lessacidic character of SILand BRO with respect to SiOH and Si(OH)Al, which also causesa systematic underevaluation of the bindingenergies (BE). All this notwithstanding, calculated BE andΔνh(OH) values are good descriptors oftheinteractions, as a number of correlations may be drawn: (i) between theΔνh(OH) values on SIL and BRO,respectively (showing a constant ratio, identical with the ratiobetween experimental data); (ii) between BEson SIL and BRO (showing the same ratio as strong bases on SiOH andSi(OH)Al); (iii) between BE and(Δνh(OH))1/2, in agreement withexperimental data on ΔH° of interaction. Thepossible causes for the lessacidic nature of SIL and BRO with respect to SiOH andSi(OH)Al are discussed. The nature of theinteractionis predominantly electrostatic, in that SCF interaction energies are insemiquantitative agreement with thosecomputed via a classical Coulombic expression usingelectrostatic-potential derived charges. However, asmall electron transfer takes place from the base molecule to theacidic species, which parallels the strengthof interaction and, within the same family of molecules, the ionizationenergy of the base molecules. Thecomparison between calculated and experimental values of C−H modessuggests that a secondary interactiontakes place between the acidic hydrogen in acetylene andmethylacetylene and a basic oxygen atom close tothe active site.
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