THEORY and INTERPRETATION of ORGANIC ccb. Organic Spectra Photoelectron Spectroscopy H. D. Roth 5...

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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

    CC O

    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 C Occupied 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 donors mixed 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.

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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 an