Coordination chemistry of CO and NH 3 on CuCl(111): an experimental and theoretical study of the CO and NH 3 bonding to a d 10 ion

Abstract

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 Cu 22Cl 24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 5σ HOMO into the empty levels of the coordinatively unsaturated Cu surface ions assisted by a significant backdonation from the fully occupied 3d orbitals of the Lewis acid site into the CO 2π ∗ LUMO. The interaction of NH 3 to CuCl(111) has been also investigated by employing the Cu 22Cl 24NH 3 model cluster. The bonding interaction is here limited to a donation from the NH 3 3a 1 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 NH 3 2e LUMO. At variance to NH 3 on ZnO(0001), the low value of the Lewis acid site effective charge makes negligible the electrostatic interaction with the permanent dipole moment of NH 3.