(XPS)-2 - Washington State University

Chemical shift arises in the initial state from the displacement of the electroniccharge from the atom towards its ligands, reducing the electrostatic potential at theatom. There is a final state shift due to the polarization of the ligand by the core onthe central atom.Core electron BE in molecular systems exhibits chemical shifts which are simplyrelated to various quantitative measures of covalency. Greater the electronegativityof the ligands, the greater the BE of the core electron of the ligated atom.Basic concept: The core electrons feel an alteration inthe chemical environment when a change in thepotential (charge distribution) of the valence shelloccurs.For example: let’s assume that the core electrons areinside a hollow spherical charged shell. Each coreelectron then sees a potential V. A change in Q by ΔQgives a change in V.where ΔVisthechemical shiftΔV=R14πε0ΔQRCoreelectronsValence chargeQ