STRUCTURE AND VIBRATIONAL FEATURES OF COMPLEXES BETWEEN UNSATURATED-HYDROCARBONS AND ACIDIC SITES IN SILICA AND ZEOLITES - AN AB-INITIO STUDY

Experimental vibrational data (FTIR) concerning the interaction of eth ylene, acetylene, and methylacetylene with the acidic hydroxyls of amo rphous silica (SiOH) and H-ZSM5 zeolite (SI(OH)Al) are compared with S CF/DZP and MP2/DZP results concerning the same molecules interacting w ith H3SiOH (SIL) and H3Si(OH)AlH3 (BRO), the minimal-size clusters mim icking the surface species. The interaction of dimethylacetylene with SIL and BRO has also been studied. Computed harmonic shifts in the O-H stretching frequency Delta nu(h)(OH) are in semiquantitative agreemen t with experiment, in that the experimental data are systematically un derestimated by a factor of approximate to 1/3 at best: the main reaso n is the less acidic character of SIL and BRO with respect to SiOH and Si(OH)AI, which also causes a systematic underevaluation of the bindi ng energies (BE). All this notwithstanding, calculated BE and Delta nu (h)(OH) values are good descriptors of the interactions, as a number o f correlations may be drawn: (i) between the Delta nu(h)(OH) values on SIL and BRO, respectively (showing a constant ratio, identical with t he ratio between experimental data); (ii) between BEs on SIL and BRO ( showing the same ratio as strong bases on SiOH and Si(OH)AI); (iii) be tween BE and (Delta nu(h)(OH))(1/2) in agreement with experimental dat a on Delta H-o of interaction. The possible causes for the less acidic nature of SIL and BRO with respect to SiOH and Si(OH)AI are discussed . The nature of the interaction is predominantly electrostatic, in tha t SCF interaction energies are in semiquantitative agreement with thos e computed via a classical Coulombic expression using electrostatic-po tential derived charges. However, a small electron transfer takes plac e from the base molecule to the acidic species, which parallels the st rength of interaction and, within the same family of molecules, the io nization energy of the base molecules. The comparison between calculat ed and experimental values of C-H modes suggests that a secondary inte raction takes place between the acidic hydrogen in acetylene and methy lacetylene and a basic oxygen atom close to the active site.