On the existence of CuI pairs in ZSM-5--a computational study

The siting and coordination of Cu(I) pairs in zeolite ZSM-5 have been studied by means of a combined quantum mechanics/interatomic potential function technique (QM-Pot). It couples a density functional theory (DFT) description employing a hybrid functional (B3LYP) for copper ions, including their local environment, with a DFT-parameterized shell model ion-pair potential for the periodic ZSM-5 structure. A missing Cu(I)-Cu(I) interaction potential term in the force field has been parameterized on the basis of DFT results. Several distinct coordination sites for Cu(I) pairs have been identified within the ZSM-5 framework. These have been classified as open, nest, open-nest, and cage pairs, owing to the shape of their local environment in the crystal. A nest-shaped, bridged eight-membered ring constitutes the most probable site for the existence of Cu(I) ion pairs. It is energetically favored over all isolated Cu(I) sites. The excitation and emission energies for the singlet-triplet transition of the Cu(I) pairs have been studied. Earlier assignment of an emission band at 520 nm to Cu(I) pairs by others is not supported by our computed results.