Bond length calculations with the extended Hiickel molecular orbital (EHMO) approximation can be improved by adding a two-body repulsive energy term and a distance-dependent Wolfsberg-Helmholz constant K = k exp (SR). As demonstrated in the case of simple homonuclear two-level interactions, such a distance-dependent K leads to problems, however. This drawback of the otherwise very useful improvement of the EHMO method can be overcome by applying = 1 + exp (-5(7?

UV irradiation of the gadsolid interface of microcrystalline Ti02 in the presence of water and carbon dioxide in the gas phase at 343 K leads to photoreduction of carbon dioxide to form carbon monoxide, hydrogen and methane.

Self-sensitisation of photo-oxygen evolution occurs in aqueous dispersions of silver zeolites. In presence of C1-, chlorine is the photoproduct in acidic medium, and the same type of self-sensitisation occurs. Self-sensitisation means that systems which are first insensitive to light of a certain wavelength become photo-active after they have been illuminated by light of higher energy. For a better understanding of silver zeolites, we have carried out EH-MO calculations on the 6-6 subunit (SBU) of a zeolite, on the 6-6 SBU with an Ag-atom in the center, on the 6-6 SBU with one Ag-atom in the center and one outside on top of the hexagon, and finally on another with one Ag-atom in the center and two Ag-atoms outside, each on top of a hexagon. The Ago in the cage of the 6-6 SBU is significantly polarized by the 6-6 SBU environment. The energy barrier to escape the 6-6 SBU is 0.8 eV for Ago and 0.5 eV for Ag' . The HOMO of the Ag(6-6 SBU) is a totally symmetric 5s* orbital and the LUMO is a 5pt type. 5pf+5s* electronic excitation reduces the energy barrier and allows an (Ago)* to exit the 6-6 SBU, provided the excited-state lifetime is long enough. The MO picture predicts low-energy charge-transfer transitions from the zeolite framework to the 5s* orbital. The highest occupied orbitals of the zeolite framework are localized on the 0-atoms. Interactions between an Ag-atom in the 6-6 SBU and one or two external Ag-atoms are discussed.

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The electronic structures ofthe 4-4 SBU, the 6-6 SBU, the8-cage, and the 8-cage with two 4-4 SBUs attached to it have been studied by means of EH-MO calculations. No indication of the formation of a band structure has been found. The HOMO region consists of many closely spaced, localized states, 98.6% of them concentrated on the 0-atoms. Reversible color changes of Cu+' and Ag" zeolites observed upon hydration-dehydration experiments can be understood as charge-transfer transitions from the HOMO concentrated on the zeolite 0-atoms to the metal cations. As soon as the Cu" or Ag" are partially hydrated, the ns* and np* states are shifted to higher energies. The luminescence observed with dehydrated Cu+'-zeolites X is caused by a 4p*+HOMO absorption, followed by spontaneous 4s*t4p* emission. After a detailed study of a Cu+' in the 6-6 SBU, we discuss the electronic structure o f a 8-cage filled with I , 2,4, 8, and 9 Cu". In each case, the B-cage is found to be too small to allow the formation of a band structure. The levels caused by the added copper are distinctly quantized. Calculations on [Ag,(H20)3]3+ in a 8-cage are reported. The direct interaction between the Ag-atoms is significant. As a consequence, the states formed by Ag 5s and 5p atomic orbitals are delocalized over the three Ag-centers. In both thcCu+' and the Ag+' zeolites, the ligand-field picture is found to bc insufficient to explain the electronic structure, when the metal is coordinated to the zcolite oxygen framework.