Transcript Core Level Ionization

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
H2O
Ar
17
15
13
Ionization Energy (eV)
11
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
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.
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.
Orbitala
(dxy, dx2-y2) IP
(dxy, dx2-y2) LOIP
Fe(CO)5
Fe(CO)4(C2H4)
8.6
8.38
(8.6)
8.3
•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.
Fe(CO)4(C2H4): A More Complicated Example
Orbitala
Fe(CO)5
(dxy, dx2-y2) IP
8.6
8.38
(dxy, dx2-y2) LOIP
(8.6)
8.3
(dxz, dyz) IP
9.9
9.23
(dxz, dyz) LOIP
(9.9)
9.6
C 2H 4
Fe(CO)4(C2H4)
(C=C p) IP
10.51
10.56
(C=C p) LOIP
(10.51)
10.06
• 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 p) 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
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