Organic Spectra Photoelectron Spectroscopy H. D. Roth

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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

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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

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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)

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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

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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.