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Photoelectron Spectroscopy

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

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Photoelectron Spectroscopy

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  1. Photoelectron Spectroscopy • Lecture 9: Core Ionizations • Information from core ionization data • Separating charge and overlap effects • Jolly’s LOIP Model

  2. 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.

  3. 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…

  4. 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

  5. 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

  6. 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

  7. F2O compared to H2O 12.62 eV O 2p 13.25 eV 3.8 eV antibonding interaction 0.8*5.33= 4.4 eV 17.0 eV LOIP 539.80 eV O 1s  = 5.33 eV H2O 545.33 eV F2O

  8. Fe(CO)4(C2H4): A More Complicated Example • This molecule does not have nonbonding lone pairs: LOIPs can be estimated from the ionization potentials of reference molecules. • Use C2H4 and Fe(CO)5 as reference molecules. • Also, have to calculate “shifted” LOIPs for Fe(CO)4(C2H4). • “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 C2H4 C=C p ionization. • Calculate the “shifted” LOIP for Fe(CO)4(C2H4) from the XPS and UPS data.

  9. Fe(CO)4(C2H4): A More Complicated Example • The Fe 2p3/2 ionization for Fe(CO)5 and Fe(CO)4(C2H4) are 715.79 and 715.40 eV, respectively. • Therefore, the “shifted” LOIP for the Fe(CO)4(C2H4) (dxy, dx2-y2) orbitals should be -0.3 eV* (-0.39 x 0.8) lower than the LOIP of Fe(CO)5. • Since there is a only a small shift between the LOIP and IP of Fe(CO)4(C2H4), the equatorial C2H4 affects the (dxy, dx2-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 * Difference value is negative since XPS data is destabilized with respect to reference compound.

  10. Fe(CO)4(C2H4): A More Complicated Example • Comparison of the IPs and LOIPs for Fe(CO)4(C2H4) show that: • The equatorial C2H4 affects the (dxy, dx2-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. a: Table taken from Jolly, W. L. Acc. Chem. Res.1983, 16, 370-376

  11. 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

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