Copper and Silver Atoms in the β‐Cage of a Zeolite: Model Calculations

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Abstract
The electronic structures of the 4‐4 SBU, the β‐cage, and the β‐cage with two 4‐4 SBU's 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 O‐atoms. Reversible color changes of Cu+1 and Ag+1 zeolites observed upon hydration‐dehydration experiments can be understood as charge‐transfer transitions from the HOMO concentrated on the zeolite O‐atoms to the metal cations. As soon as the Cu+1 or Ag+1 are partially hydrated, the ns* and np* states are shifted to higher energies. The luminescence observed with dehydrated Cu+1‐zeolites X is caused by a 4p*←HOMO absorption, followed by spontaneous 4s*←4p* emission. After a detailed study of a Cu+1 in the 6‐6 SBU, we discuss the electronic structure of a β‐cage filled with 1,2,4,8, and 9 Cu+1. In each case, the β‐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 [Ag3(H2O)3]3+ in a β‐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 the Cu+1 and the Ag+1 zeolites, the ligand‐field picture is found to be insufficient to explain the electronic structure, when the metal is coordinated to the zeolite oxygen framework.
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