Photoelectron Spectroscopy

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Photoelectron Spectroscopy

• Lecture 10 Gas phase vs solid/solution phase
  measurements
• Gas phase vs solid phase UPS
• Gas phase ionization vs solution phase oxidation

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Why gas phase spectroscopy?

• Allows us to study isolated molecules.
• This is the way we usually visualize electronic
  structure of molecules.
• We of course know that solution, environment is a
  factor, but how big a factor?
• Need to consider relative strength of
  environmental factors compared to intramolecular
  forces.

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Solvation Effects
Gas phase
Condensed phase
M
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Measuring the donor strength of phosphines
Solution-phase IR (?CO) of Ni(CO)3L1
and the pKa of the conjugate acid (PR3H) in
aqueous solution.2
PMe3 is a better donor than PPh3
(1) Tolman, T.A. J. Am. Chem. Soc. 1970, 92,
2953. (2) Shaw, B.L. J. Chem. Soc., Chem. Comm.
1979, 104.
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Measuring the donor strength of phosphines
Gas-phase proton affinities1.
and gas-phase PES studies.2
- 0.74 eV
PPh3 is a better donor than PMe3
Trend in donor strength is reversed with
different phases.
(1) Ikuta, S., et al. J. Am. Chem. Soc. 1982,
104, 5899. (2) Bancroft, G.M., et al. Inorg.
Chem. 1986, 25, 3675.
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Preparation of thin films
Condensed phase samples were prepared by vapor
deposition
Mo(CO)4dppe
with an optimum film thickness of 50-150 A.
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Photoionization of a Solid What has to happen
for a photoelectron to escape?
3) Penetration through the surface
2) Transport to the surface

• Photoexitation
• of the electron

?EVB
Evac
?
EF
?EVB
Valence Band
vacuum
bulk
surface
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Escape Depths The Universal Curve
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Comparison of dmpe and dppe substitution in the
gas and condensed phase
Gas phase
1.1 eV
Condensed phase
dmpe
0.2 eV
dppe
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8
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10
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Ionization Energy (eV)
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Electrochemistry

• Oxidation also removal of an electron
• M ? M e-
• Solvation effects.
• Timescale is much slower than photoionization,
  allows geometry changes to occur.
• Measure an adiabatic value, not vertical.
• Reference is reference electrode, or internal
  reference such as ferrocene oxidation.
• Different definition of second ionization/oxidatio
  n.

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Relationship Between Ionization and Oxidation
Fc Fc e- E(V)
Fc Fc e-
DG
DG -nFE
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Correlation of Ionization and Oxidation Energies
for Substituted Anthracenes
Masnovi et al., Can. J. Chem. 1984, 62, 2552.
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Correlation of Ionization and Oxidation Energies
for 1,2-Dithiins
E-C Mechanism
R. S. Glass et al. JACS 2000, 122, 5052-5064
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Summary

• Important to think about environmental effects.
• Gas phase and solid/solution phase measurements
  do not always show the same trends.