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Organic Spectra Photoelectron Spectroscopy H. D. Roth
1
THEORY and INTERPRETATION of ORGANIC SPECTRA H. D. Roth
Photoelectron Spectroscopy UV-PES is an analytical technique based on the ionization of
molecules with high-energy photons of known energy (Ehν), typically the He(I)α line (21.21 eV). The high excitation energy causes electrons to be
ejected from essentially all levels of the target molecule. By measuring the energy of the ejected electrons (Ekin) PES provides information about the energy required to remove the electron, i.e., how strongly it is bound (Iv).
The experiment has three phases:
1) The substrate is ionized by radiation with photons of known energy;
2) The kinetic energy (Ekin) of the emitted electrons is measured;
3) The vertical ionization potential (Iv) of the ejected electron can be calculated from Iv = Ehν – Ekin.
The energy of the emitted electrons is determined in a variable-
strength magnetic field (charges are deflected in a magnetic field); lower-
energy electrons are deflected more readily than higher-energy electrons.
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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The orbital energies of a given molecule may change with subtle
changes in structure; systematic comparisons within series of related
compounds can help in the assignment of individual PES bands to specific
orbitals (Koopmans theorem). Molecular orbital calculations allow the
assignment of individual transitions to individual orbitals; some transitions
can be assigned based on general principles. The PES spectrum of glyoxal (C2H2O2), a molecule of six atoms and
30 electrons, shall serve to illustrate some features this technique.
The spectrum has five clearly discernible bands, at 10.6, 12.2, 13.85,
15.5 and 16.8 eV. We will focus on the first two bands; the lowest-energy
band at 10.6 eV identifies the least strongly bonded electron. Elementary
considerations suggest that the 8 non-bonding (n-) electrons on O are highest
in energy and most easily ionized: the first band is assigned to ejection of an
electron from that level. Ionization of lower-lying orbitals requires
increasingly higher energies; in order they are the orbitals of the 4 π electrons,
the 4 C–O and 2 C–C σ electrons, the 4 C–H σ electrons, and the 8 1s
electrons, 2 each at the C and O atoms.
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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Glyoxal, C2H2O2
CCO
H
H
O
4 n
2 π C=O
2 σ C–O
1 σ C–C
2 σ C–H
2 x 1s2 O
2 x 1s2 COccupied orbitals
Whereas the first band is broad and “featureless”, the second and
third bands have fine structure. This feature, previously seen in UV-Vis
spectroscopy, is due to vibrationally excited levels in the “product” of the
spectroscopic transition. In our case, ejection of an electron from a C=O
bond generates a radical cation with an electron missing in that π bond. The
resulting species lies 12.2 eV higher in energy than the neutral molecule
(the figure is schematic). The radical ion may be populated not only in the
ground vibrational state but also in several vibrationally excited states. The
vibrational spacing can be determined from the separation of the PES lines.
It is important to realize that the ionization is vertical; therefore, the PES
reflects the structure (and orbital energies) of the parent molecule, not the
equilibrium (relaxed) structure of the radical cation (molecular ion). Still, PES
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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data are quite valuable for considering radical cation structures; they identify
the parent molecule’s highest occupied molecular orbitals (HOMOs); they
identify the bond(s) most likely to be weakened upon ionization; they provide
a good starting point for the radical ion structure(s) to be identified.
Electronic Transitions in Radical Ions The lowest-energy electronic excitation of a molecule occurs from the
highest occupied molecular orbital (HOMO) to the lowest unoccupied (LU)
MO (blue curved arrow, center). Because the highest occupied MOs of radical
cations are singly occupied (SOMOs), they have a new transition from the
next lower MO to the SOMO (red curved arrow, left). Its ΔE is lower (red vs.
blue arrows), shifting the band to longer wavelengths. For that reason, many
radical cations of colorless compounds absorb in the visible; this feature
helped in the early (19th century) recognition of radical ions.
E
ΔE• –
Parent MoleculeRadical Cation Radical Anion
ΔE ΔEΔE
ΔE• +
bonding
anti-bonding
HOMO
LUMO
SOMO
SOMO
A similar relationship exists for radical anions: their highest occupied
MO is the LUMO of the parent, i. e., an antibonding SOMO. Therefore, a new
transition is possible (fuchsia curved arrow). Because the energy differences
of antibonding orbitals in essence are the “mirror image” of the bonding
orbitals, the ES of radical anions (lower ΔE) are shifted into the visible.
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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Structures of Radical Ions The structures of radical cations are related to those of the neutral
parent molecules in a variety of ways. Three possible relationships between
the structures of molecules and their radical cations (molecular ions) are
illustrated in the figure: (left) no change in molecular structure upon
ionization; (center) vertical ionization followed by relaxation of the radical
cation to its equilibrium structure; (right) vertical ionization followed by a
major structural change (rearrangement).
If ionization causes little or no change in molecular structure (i.e., the
structures of a radical cation and its parent molecule are very similar), their
relative orbital energies are also similar. In such cases the PES transitions (of
the parent) and the ES transitions of the corresponding radical cation are
related. As the energy diagram illustrates, the radical cation has ES transitions
of energies, ΔE, that are (nearly) equal to the differences between the PES
ionization potentials of electrons in different MOs (ΔI). Note, however, that the
radical cation is higher in energy (by ΔI).
For what compounds can we expect this kind of behavior? Considering
the major types of potential donors, π-, σ-, n-, or mixed σ and π donors, it is
obvious that σ donors will undergo significant structure changes upon
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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ionization. Their HOMOs are bonding in strained ring bonds; removing one
electron from such a bond weakens (and lengthens) it substantially. Similar
reasoning applies to mixed σ and π donors. As for n donors, amines or ethers,
their bond angles are determined by valence shell electron pair repulsion
(VSEPR). Removing one electron lessens the repulsion in the radical cation
and increases its bond angles, again a significant change in structure.
N
O
π donors n donors σ donorsmixed σ + π
donors
Among π donors, the radical cations of alkenes assume slightly
twisted geometries, once again a change in structure. Finally aromatic
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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systems, particularly the larger annulated ones, have HOMOs that are
bonding in many bonds; removing one electron from such an orbital results
in only minor changes in structure (and relative MO energies). The PES and
ES spectra of anthracene clearly bear out the anticipated relationship.
In contrast, dicyclopentadiene undergoes a significant structure change
upon ionization: one of the doubly allylic C–C bonds is cleaved and the
resulting bisallylic radical cation has relative MO energies that are very
different from those of the parent molecule.
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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The PES and ES spectra shown below have no transitions satisfying the
relation ΔE ≈ ΔI.
If the structures of a radical cation and a radical anion are very similar
to the corresponding parent compound, their UV-Vis spectra should be very
similar, as is indeed observed for the positive and negative ions of tetracene.
On the other hand if the structures of a radical cation and the corresponding
radical anion are different, their UV-Vis spectra will be different, as observed
for the ions of cyclooctatetraene below.
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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ES spectra of tetracene radical anion (top) and radical cation (bottom)
ES spectra of cyclooctatetraene radical anion (top) and radical cation (bottom)
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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Comparison of Gas Phase IP and Solution Oxidation Potential In both MS and UV-PES essentially any compound that can be
introduced into the gas phase can be ionized. Many compounds also surrender
their highest lying electron in solution; this process is called oxidation; the energy required is called the oxidation potential, E0 (given in V). The solution
experiment is vastly different from that in the gas phase because both the
substrate and the resulting ion are solvated, possibly requiring “solvent
reorganization”. In contrast to the “instant” vertical ionization in the gas phase
(red arrows), oxidation in solution requires much more time, during which the
resulting ion can fully relax. Therefore, the (solution) oxidation potential
reflects the energy required for forming the relaxed ion (blue arrows).
The different nature of IP and E0 gives us the opportunity to assess
whether the radical cation structures are similar to those of the parent
molecules or very different. Even though the ionization potential, IP, of a given compound and its oxidation potential, E0, have different values and are
measured in different units, a plot of IP vs. E0 should give a straight line for a
series of compounds where the structures of radical cation and parent are very
similar; in contrast, significant deviations from linearity must be expected if
the structures of radical cation and parent are different. These considerations
Organic Spectra Photoelectron Spectroscopy H. D. Roth
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are borne out by the comparison of IPs and E0s for three groups of substrates,
aromatic compounds, bicyclic peroxides, and hydrazine derivatives.
The figure shows a straight line for an IP vs. E0 plot of fused-ring
(triangles) and alkyl-substituted aromatic compounds (inverted triangles) and
bicyclic peroxides (diamonds); apparently these compounds undergo only
minimal structure changes upon ionization. In contrast, acylhydrazines
(circles) and tetraalkylhydrazines (squares) deviate dramatically from that
line: clearly these molecules undergo significant structure changes upon
ionization.