Presentation on theme: "Photoelectron Spectroscopy Lecture 9: Core Ionizations –Information from core ionization data –Separating charge and overlap effects Jolly’s LOIP Model."— Presentation transcript:
Photoelectron Spectroscopy Lecture 9: Core Ionizations –Information from core ionization data –Separating charge and overlap effects Jolly’s LOIP Model
What information do we get from core ionizations (XPS)? Qualitative and quantitative information on the elements present in a sample –Electron Spectroscopy for Chemical Analysis (ESCA) Sensitivity is on the order of parts-per-thousand Oxidation states of the elements present Chemical environment around the elemental centers, which influence charge potential of the atom.
What Influences MO Ionization Energies? Core ionizations: –Ionization energies of atomic orbitals –Oxidation state, formal charge, charge potential Valence ionizations: –Same two contributions as for core ionizations –Plus bonding, antibonding interactions Overlap So, seems that we should be able to separate and measure only bonding/overlap interactions by comparing core and valence ionization energies…
The Jolly Model for comparing core and valence ionizations Core ionizations are perturbed by changes in charge effects, whereas valence ionizations are affected by charge effects and chemical bonding. Jolly Model: explains the relationship between core and valence ionization energy changes –quantifies the bonding or antibonding character of molecular orbitals. When comparing the ionizations of related molecules, strictly non-bonding valence orbital ionizations will shift 80% as much as core orbital ionizations. Ionization Energy = Ionization Potential Jolly, W. L. Acc. Chem. Res. 1983, 16, 370-376
Logic of applying the Jolly Model If we have a molecule in which there is a non-bonding valence orbital from a particular atom, we can define a “localized orbital ionization potential” LOIP for that atom. –Need to know the valence and core ionization potentials We then can measure the core ionization potential of a different molecule containing the same atom. Calculate the difference in core ionization potentials, –80% of this is the expected difference in valence ionization potentials for a non-bonding orbital gives the LOIP of the second molecule Measure the actual shift in the valence ionization potential –Any difference between the expected LOIP and the actual valence ionization potential must be caused by bonding or anti-bonding
Example: an O 2p LOIP The HOMO of water is a purely non-bonding 2p orbital. Ionization potential of 12.62 eV can be defined as the O 2p LOIP
F 2 O compared to H 2 O H2OH2O O 1s O 2p 539.80 eV 12.62 eV F2OF2O 545.33 eV = 5.33 eV 0.8*5.33= 4.4 eV 17.0 eV LOIP 13.25 eV 3.8 eV antibonding interaction
Fe(CO) 4 (C 2 H 4 ): A More Complicated Example This molecule does not have nonbonding lone pairs: LOIPs can be estimated from the ionization potentials of reference molecules. Use C 2 H 4 and Fe(CO) 5 as reference molecules. Also, have to calculate “shifted” LOIPs for Fe(CO) 4 (C 2 H 4 ). “Shifted” LOIPs are based on a reference molecule’s ionizations rather than a strictly nonbonding electron pair. The LOIP of the metal ionizations of the reference compound Fe(CO) 5 are based on the IP data. Do the same for C 2 H 4 C=C ionization. Calculate the “shifted” LOIP for Fe(CO) 4 (C 2 H 4 ) from the XPS and UPS data.
The Fe 2p 3/2 ionization for Fe(CO) 5 and Fe(CO) 4 (C 2 H 4 ) are 715.79 and 715.40 eV, respectively. Therefore, the “shifted” LOIP for the Fe(CO) 4 (C 2 H 4 ) (d xy, d x2-y2 ) orbitals should be -0.3 eV* (-0.39 x 0.8) lower than the LOIP of Fe(CO) 5. Fe(CO) 4 (C 2 H 4 ): A More Complicated Example * Difference value is negative since XPS data is destabilized with respect to reference compound. Orbital a Fe(CO) 5 Fe(CO) 4 (C 2 H 4 ) (d xy, d x2-y2 ) IP8.68.38 (d xy, d x2-y2 ) LOIP(8.6)8.3 Since there is a only a small shift between the LOIP and IP of Fe(CO) 4 (C 2 H 4 ), the equatorial C 2 H 4 affects the (d xy, d x2-y2 ) orbitals to the same extent as an equatorial CO ligand. a: Table taken from Jolly, W. L. Acc. Chem. Res. 1983, 16, 370-376
Orbital a Fe(CO) 5 C2H4C2H4 Fe(CO) 4 (C 2 H 4 ) (d xy, d x2-y2 ) IP8.68.38 (d xy, d x2-y2 ) LOIP(8.6)8.3 (d xz, d yz ) IP9.99.23 (d xz, d yz ) LOIP(9.9)9.6 (C=C ) IP 10.5110.56 (C=C ) LOIP (10.51)10.06 Fe(CO) 4 (C 2 H 4 ): A More Complicated Example a: Table taken from Jolly, W. L. Acc. Chem. Res. 1983, 16, 370-376 Comparison of the IPs and LOIPs for Fe(CO) 4 (C 2 H 4 ) show that: The equatorial C 2 H 4 affects the (d xy, d x2-y2 ) orbitals to the same extent as an equatorial CO ligand. The (dxz, dyz) orbitals are destabilized due to loss of backbonding to CO. The (C=C ) is stabilized due to the interaction between the ethylene and the iron atom.
Summary Core ionizations give qualitative and quantitative information on elemental analyses When comparing ionization energies of related molecules, consider that core ionizations shift due to charge effects, valence ionizations shift due to both charge and overlap effects