First-principles calculations based on cluster models have been performed to investigate the ground state and the optically excited states of the [AlO(4)](0) hole in alpha-quartz and in the siliceous zeolite ZSM-5. The structure and spectroscopic properties of this defect have been studied using the recently developed Becke88-Becke95 one-parameter model for kinetics (BB1K) functional of Zhao et al., [J. Phys. Chem. A 108, 2715 (2004)]. Our results show that the BB1K method is significantly more reliable and more accurate than the standard density-functional theory (DFT) functionals at reproducing the localized spin density on one oxygen atom and the hyperfine coupling constants associated with the hole. Furthermore, we find that the BB1K results are in close agreement with experiments, and with the self-interaction-free unrestricted Hartree-Fock (UHF) and unrestricted second-order Møller-Plesset perturbation theory (UMP2) calculations. For the first time, we present results of the ground-state paramagnetic properties of the Al defect in ZSM-5. Similar to the theoretical work for defective alpha-quartz, we find that the BB1K, UHF, UHFLee-Yang-Parr, and UMP2 calculations show a localized hole on one oxygen neighboring the Al, while even the best to date thermochemically derived hybrid generalized gradient approximation density-functional, B97-2, predicts a different model where the hole is distributed over two oxygen. We have further considered the optical transitions of the [AlO(4)](0) center in alpha-quartz and ZSM-5. In both systems, our BB1K time-dependent density-functional theory (TDDFT) and configuration interaction singles (CIS) calculations predict that the most likely transition involves electron transfer from the hole-bearing oxygen to other neighboring oxygen ions. This reinforces the experimental conclusions obtained for defective alpha-quartz. Notably, the two lowest, most dominant excitation energies calculated by BB1K-TDDFT (1.99 and 3.03 eV) show excellent agreement with experiment (1.96 and 2.85 eV [B. K. Meyer, J.M. Spaeth, and J.A. Weil, J. Phys. C: Solid State Phys. 17, L31 (1987)]) clearly outperforming the CIS method and other DFT calculations available in the literature.

We report a series of computations on the active site in Ti-substituted zeolites, specifically TS-1. Hybrid QM/MM methods based on density functional calculations using the BB1K functional and a valence force field are used to study the processes of hydrolysis of Ti-O-Si linkages and inversion of the TiO 4 tetrahedra. The structural features of the resulting series of tetra-and tripodal Ti moieties are in good agreement with data from EXAFS studies. The suggestion is made that the tripodal species will dominate in hydrous conditions, and that this is likely to be the chemically active form. We have made extensive use of the massively parallel HPCx computer system for these investigations and outline some of the technical developments to the ChemShell software that were needed to support the study.

An experiment has been constructed to determine the rovibrational states populated in the formation of H 2 on the surface of cosmic dust under conditions approaching those of the interstellar medium (ISM). During the experiment, a beam of atomic hydrogen of controlled temperature is incident upon a target which is an analogue of cosmic dust. Molecular hydrogen desorbing from the target's surface is ionized using (2 + 1) resonance enhanced multiphoton ionization and detected using time-of-flight mass spectrometry. The experiment allows the rovibrational populations of the H 2 molecules desorbing from the cosmic dust targets to be determined providing information on the energy budget of the H 2 formation process in the ISM. Preliminary results from the experiment, to prove its viability, show that H 2 molecules formed on an highly oriented pyrolytic graphite surface have a measurable population of excited vibrational and rotational states.

Following the discovery of the titanosilicalite zeolite TS-1 and its remarkable catalytic properties in selective oxidation reactions with aqueous H 2 O 2 , [1] there have been several reports on the possible use of other heteroatom-substituted zeolites. Of particular interest are tin and zirconium silicalites, which are efficient catalysts in the hydroxylation of phenol with aqueous H 2 O 2 .[2] Germanium-containing silicalites, as well as a number of Ti-Ge-Si zeolites, showing catalytic activity toward oxidation with H 2 O 2 , have also been successfully synthesized. [3] Other important examples are leadcontaining zeolites with framework Pb II species; these compounds are promising photocatalysts in denitrification (de-NO x ) reactions. [4,5] Despite the considerable effort that has been devoted to the synthesis and characterization of these interesting materials, there is still no well-defined model to explain the observed catalytic properties. As the heteroatoms are postulated to be the active sites for the oxidation reactions, accurate information on their coordination environment and electronic structure is of fundamental importance for understanding the catalytic behavior of the heteroatom-substituted zeolites and for designing new materials with predetermined properties.Herein, we propose a new model for the formation of active sites in silicalite, which is based on the hydrolysis and inversion of tetrahedral sites in the zeolitic framework. We demonstrate that the inversion mechanism can help to stabilize the active sites, as well as increase their accessibility to guest molecules in the zeolite pores. This model is in agreement with early experimental studies on TS-1, [6] which suggested that hydrolysis of a TiÀOÀSi bridge forces the titanium atom to move away from its tetrahedral configuration to a new more "external" relaxed position. In contrast, current models make a strong distinction between the modes of incorporation of titanium atoms: in zeolites they occupy tetrapodal sites within the framework, while in mesoporous materials they are grafted onto the silica surface and have a tripodal coordination environment.[7] Our calculations show that both configurations are realized within zeolites and that the hydrolysis-plus-inversion mechanism is the missing link between them.The characteristic features of this model are closely related to a number of structures in silica. For example, it has been postulated that the hydrolysis step during silica synthesis occurs through an S N 2 mechanism with inversion of a silicon tetrahedron.[8] A similar mechanism was reported for the formation of positively charged oxygen-vacancy defects (E' centers) in a-quartz and amorphous silica.[9] It was suggested that the E' center undergoes a distortion to a stable puckered configuration, which is accompanied by a large relaxation of the silicon atoms adjacent to the oxygen vacancy. Recently, Sokol et al. advanced the hydrolysis-plusinversion model for the stabilization of intrinsic defects, for example, vicinal disilan...

The active oxidizing species in the H2O2/TS-1 catalytic system is investigated using a hybrid quantum mechanical/molecular mechanical approach. In this computational technique, the site of interest is described with the density functional theory using the BB1K exchange and correlation functional, and the remainder of the system is treated with a valence force field. We have examined the formation of dioxygen adsorbate structures with η and η Ti-peroxo configurations, including radical species, which arise from the attack of hydrogen peroxide on a tetrahedral TiIV site along with effects of water co-adsorption. Our results show that hydrogen peroxide physisorbed on tetra- and tripodal Ti sites, as well as coadsorbed H2O2/H2O, is energetically favorable and involves minor structural modifications in the framework, as Ti atoms change their coordination from four to six. Furthermore, our calculations of various Ti-peroxo complexes formed from tripodal sites and H2O2 strongly suggest that Ti-η2(OOH), Ti-η1(OOH), and Ti-η1(O(H)OH) complexes will all form in the pores of TS-1 under reactions with H2O2 with a preferential coordination of six in the presence of water. In particular, both six-coordinate Ti-η1(OOH) and Ti-η2(OOH) compare favorably with EXAFS data obtained from the mesoporous materials with the isomorphous Ti active sites. These results indicate that water is not just a medium for transporting reactants and products at the catalytic sites, but has an active role in stabilizing the peroxo species present on the working catalyst. The Ti−O bond distances are also close to those in the five-coordinated complexes reported for TS-1 (also based on EXAFS data). Although, we find that both anhydrous and hydrated η2(O2·) type species are unlikely to be the predominant oxygen-donating species, we have demonstrated that our calculated g-tensors for the short-lived radical species are in line with EPR data, which supports our assertion of the tripodal Ti site as the main active site in TS-1 for partial oxidation catalysis.

The interaction of water molecules with Ti sites in titanium silicalite has been studied using a hybrid quantum mechanical/molecular mechanical approach. We have examined the structure and stability of a number of structurally different sites, many of which are based on the hydrolysis and inversion of tetrahedral sites in the zeolitic framework. The hydration of all considered Ti centers is found to be exothermic, which is in good agreement with experiment and previous theoretical work. Under such conditions, the most stable configuration considered is the bis(aquo) tripodal model, indicating that hydrolysis and inversion of tetrahedral sites also helps to increase the reactivity of the Ti site toward water. Our results further show that the insertion of one water molecule increases the coordination of the Ti center from four to five in the tetrapodal, tripodal, bipodal, and titanyl configurations and from five to six in the 2MR structure. In the presence of two water molecules, however, only one of the molecules binds directly to the Ti center; thus, the coordination remains the same as the mono(aquo) complex. In the bis(aquo) complexes, there is also a strong hydrogen-bonding network between the hydrogen of the first water molecule and the oxygen of the second water molecule. We find that the local structural features of the resulting series of tetra- and tripodal Ti moieties are in good agreement with data from EXAFS studies.

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Following the discovery of the titanosilicalite zeolite TS-1 and its remarkable catalytic properties in selective oxidation reactions with aqueous H 2 O 2 , [1] there have been several reports on the possible use of other heteroatom-substituted zeolites. Of particular interest are tin and zirconium silicalites, which are efficient catalysts in the hydroxylation of phenol with aqueous H 2 O 2 .[2] Germanium-containing silicalites, as well as a number of Ti-Ge-Si zeolites, showing catalytic activity toward oxidation with H 2 O 2 , have also been successfully synthesized. [3] Other important examples are leadcontaining zeolites with framework Pb II species; these compounds are promising photocatalysts in denitrification (de-NO x ) reactions. [4,5] Despite the considerable effort that has been devoted to the synthesis and characterization of these interesting materials, there is still no well-defined model to explain the observed catalytic properties. As the heteroatoms are postulated to be the active sites for the oxidation reactions, accurate information on their coordination environment and electronic structure is of fundamental importance for understanding the catalytic behavior of the heteroatom-substituted zeolites and for designing new materials with predetermined properties.Herein, we propose a new model for the formation of active sites in silicalite, which is based on the hydrolysis and inversion of tetrahedral sites in the zeolitic framework. We demonstrate that the inversion mechanism can help to stabilize the active sites, as well as increase their accessibility to guest molecules in the zeolite pores. This model is in agreement with early experimental studies on TS-1, [6] which suggested that hydrolysis of a TiÀOÀSi bridge forces the titanium atom to move away from its tetrahedral configuration to a new more "external" relaxed position. In contrast, current models make a strong distinction between the modes of incorporation of titanium atoms: in zeolites they occupy tetrapodal sites within the framework, while in mesoporous materials they are grafted onto the silica surface and have a tripodal coordination environment.[7] Our calculations show that both configurations are realized within zeolites and that the hydrolysis-plus-inversion mechanism is the missing link between them.The characteristic features of this model are closely related to a number of structures in silica. For example, it has been postulated that the hydrolysis step during silica synthesis occurs through an S N 2 mechanism with inversion of a silicon tetrahedron.[8] A similar mechanism was reported for the formation of positively charged oxygen-vacancy defects (E' centers) in a-quartz and amorphous silica.[9] It was suggested that the E' center undergoes a distortion to a stable puckered configuration, which is accompanied by a large relaxation of the silicon atoms adjacent to the oxygen vacancy. Recently, Sokol et al. advanced the hydrolysis-plusinversion model for the stabilization of intrinsic defects, for example, vicinal disilan...