The interaction of benzene with Cu+ sites in zeolites IR studies and DFT quantum chemical calculations

As Cu+ ions in zeolites catalyse several reactions of organic molecules, we undertook IR and DFT studies of the interaction of organic molecules with Cu+. Our previous studies had shown that Cu+ was able to activate strongly the double and triple bonds in alkenes and acetylene, and less strongly the C=O bond in acetone, which resulted in a distinct multiple bond weakening. The DFT calculations provided evidence that this was realized by pi back donation of d electrons of Cu+ to pi antibonding orbitals of the molecules. The present study concerns the activation of the benzene molecule, which is different from previously studied ones. It is a good electron acceptor, but the LUMO orbital of benzene has only a small antibonding character, therefore the interaction with Cu+ was expected to result in a strong adsorption but only slight activation of the C-C bond. Our IR studies have shown, that the C-C bond weakening was indeed very small (Delta v(C-C) = 13 cm(-1), a value 10 times less than that for alkenes). DFT calculations showed that the molecule acquired a negative charge (-0.06) indicating that pi back-donation prevailed over pi donation. Both IR and DFT revealed that benzene adsorption on Cu+ ions was relatively strong, which resulted in a withdrawing Cu+ from the oxygen ring. DFT provided information on the geometry of the Cu+-benzene adduct and evidenced that the interaction of the benzene molecule with Cu+ resulted in non-symmetric adsorption. The distances Cu+-C were in the range: 0.203-0.303 nm. The shorter the Cu+-C distance, the more important the bond activation. The benzene molecule interacting with Cu+ was no longer flat, some hydrogens were directed towards Cu+ and others in the opposite direction. (c) 2006 Elsevier B.V. All rights reserved.