A model potential in two forms (harmonic and anharmonic) is proposed to be used in molecular dynamics simulations of silicate frameworks. This model is applied to the calculation of structural and vibrational properties of the anhydrous phase of Linde 2Mite 4 A. Our system is formed by a cubic box corresponding to one crystallographic cell containing 662 atoms without constraints. The results are compared with experimental data. The proposed models satisfactorily reproduce the main features of the aluminosilicate framework structure and dynamics.

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The physico-chemical properties of the beta-diketonate diamine Cu(ii) compound with hfa (1,1,1,5,5,5-hexafluoro-2-4-pentanedionate) and TMEDA (N,N,N',N' tetramethylethylenediamine), Cu(hfa)(2).TMEDA, have been thoroughly investigated via an integrated multi-technique experimental-computational approach. In the newly found orthorhombic polymorph, as revealed by low temperature single-crystal X-ray studies, the complex is present as a monomer with a distorted octahedral geometry at the Cu(ii) centre. The compound sublimates, without premature side decompositions, at 343 K and 10(-3) Torr. The structural, vibrational, electronic and thermal behavior of the neutral Cu(hfa)(2).TMEDA complex has been investigated along with its fragmentation pathways, initiated by the release of an anionic hfa ligand with formation of a positive Cu(hfa).TMEDA(+) ion. Joint experimental and theoretical analyses led to the rationalization of the first fragmentation steps in terms of the Cu(ii)-ligands bonding properties and Jahn-Teller distortion. The present study suggests applications of Cu(hfa)(2).TMEDA as a precursor for copper and copper oxide materials by Chemical Vapor Deposition.

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First principles studies on periodic TS-1 models at Ti content corresponding to 1.35% and 2.7% in weight of TiO(2) are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi(95)O(192)] and adopting for the first time a periodic DFT approach, thus providing an energy scale for Ti in the different crystallographic sites in nondefective TS-1. The structure with Ti in site T3 is the most stable, followed by T4 (+0.3 kcal/mol); the less stable structure, corresponding to Ti in T1, is 5.6 kcal/mol higher in energy. The work has been extended to investigate models with two Ti's per unit cell [Ti(2)Si(94)O(192)] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO(4) tetrahedra, is discussed. By using cluster models cut from the optimized periodic DFT structures, both vibrational (DFT) and electronic excitation spectra (TDDFT) have been calculated and favorably compared with the experimental data available on TS-1. Interesting features emerged from excitation spectra: (i) Isolated tetrahedral Ti sites show a Beer-Lambert behavior, with absorption intensity proportional to concentration. Such a behavior is gradually lost when two Ti's occupy sites close to each other. (ii) The UV-vis absorption in the 200-250 nm region can be associated with transitions from occupied states delocalized on the framework oxygens to empty d states localized on Ti. Such extended-states-to-local-states transitions may help the interpretation of the photovoltaic activity recently detected in Ti zeolites.

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The ethylene epoxidation cycle in a H2O2/H2O-loaded Ti zeolite has been simulated by a Car-Parrinello approach. Results indicate a process where the zeolitic framework is the active oxygen mediator. The dissociative chemisorption of H2O2 leads, via a transient Ti-hydroperoxo species, to H2O and a Ti-peroxo zeolite intermediate. Transfer of active oxygen to ethylene follows, giving the epoxide and recovering the catalyst. A thorough theoretical characterization indicates that the active oxidizing species is an asymmetric eta2-Ti-peroxo, absorbing in the visible range. The lability of the intermediate is found related to eta2 <--> eta1 interconversions of the Ti-peroxo structure. The interconversions, triggered by water molecules, could account for the experimentally found reduced catalytic activity in aged TS-1 catalysts. The results provide a microscopic picture of the reactivity and dehydration/aging processes of the catalyst fully consistent with experiments and highlight the fundamental role of the Lewis acid character of Ti in the formation, reactivity, and degradation of the active oxidizing species.