EPR INVESTIGATION OF THE ELECTRONIC-STRUCTURE OF MONONUCLEAR COPPER(I) NITRIC-OXIDE ADDUCT FORMED UPON LOW-PRESSURE ADSORPTION OF NO ONTO CU ZSM-5 ZEOLITE/

The adsorption of NO onto copper-exchanged ZSM-5 zeolite has been stud ied by EPR spectroscopy. A distinct Cu+-NO species possessing an end-o n eta(1) bent structure has been identified. Its EPR spectrum with a w ell-defined superhyperfine coupling with (CU)-C-63,65 nuclei and hyper fine coupling with N-14 was interpreted in terms of completely anisotr opic g and hyperfine tensors with noncoincident axes (monoclinic symme try). The spin Hamiltonian parameters of this adduct have been analyze d in detail, leading to a semiquantitative molecular orbital correlati on picture of the complex. The model developed shows that the unpaired electron resides mainly on the angularly coordinated NO and the coppe r superhyperfine structure arises from delocalization of the unpaired electron density onto Cu a' orbitals (3d(zz), 3d(xz) and 4s). The tota l spin density on copper is found to be equal to 0.2 and is shared amo ng 3d(z2) (0.079), 3d(xz) (0.021), and 4s (0.1) orbitals. The remainin g part of the unpaired electron density is localized on nitrogen (0.55 ) and oxygen (0.25) atoms. The bonding interaction involves essentiall y an overlap of Cu 3d(z2) and 3d(xz) with an antibonding 2p pi and a lone pair n orbital of the NO ligand as well as a 3d(yz) overlap with a second orthogonal NO 2p pi orbital, The appropriate molecular orbit al diagram has been devised to account for the EPR data, The model is consistent with the observed magnetic properties of the investigated a dducts and satisfactorily explains the previously unrecognized complex source of hyperfine couplings, The implications of this coordination mode on the possible molecular mechanism of the catalytic decompositio n of NO over Cu/ZSM-5 catalysts are discussed.