COORDINATION CHEMISTRY OF CO AND NH3 ON CUCL(111) - AN EXPERIMENTAL AND THEORETICAL-STUDY OF THE CO AND NH3 BONDING TO A D(10) ION

A detailed investigation of the electronic structure of CO chemisorbed on CuCl has been carried out by coupling diffuse reflectance infrared (IR) spectroscopy to the local-density-functional molecular-cluster approach. IR measurements indicate a red-shift of the C-O stretching mode on passing from the free molecule to the chemisorbed one, proving a slight weakening of the C-O bond upon chemisorption. Theoretical results, obtained for CO on CuCl(111) by using the Cu22Cl24-CO model cluster, reproduce such a slight bond weakening giving for the C-O stretching a red-shift of 38 cm-1 with respect to the free molecule. The bonding of CO to CuCl(111) implies a donation from the CO 5sigma HOMO into the empty levels of the coordinatively unsaturated Cu surface ions assisted by a significant backdonation from the fully occupied 3d orbitasls of the Lewis acid site into the CO 2 pi* LUMO. The interaction of NH3 to CuCl(111.) has been also investigated by employing the Cu22Cl24-NH3 model cluster. The bonding interaction is here limited to a donation from the NH3 3a1 HOMO into the empty levels of the unsaturated Cu sites. Any backbonding from the 3d orbitals of the Lewis acid site is prevented by the high energy of the NH3 2e LUMO. At variance to NH3 on ZnO(0001), the low value of the Lewis acid site effective charge makes negligible the electrostatic interaction with the permanent dipole moment of NH3.