8-15 Orbital Energies and Ionization Energies
Chapter 8 The Simple H ¨uckel Method and Applications8-15 Orbital Energies and Ionization Energies
Suppose that monochromatic light is beamed into a gaseous sample of a compound.
If the light is of sufficient energy, electrons will be “knocked out” of the molecules. The kinetic energy of such a photoelectron will be equal to the kinetic energy of the incident photon (hν) minus the energy needed to remove the electron from the molecule, that is, the ionization energy. Measurement of the kinetic energies of photoelectrons emitted in this manner is known as “photo-electron spectroscopy” [12].
In measuring the kinetic energies of photoelectrons from, say, benzene, it is found that a large number are near a particular energy value, another large number of electrons are near a different value, and so on for several kinetic energy values.
Because the photoelectrons tend to clump near several kinetic energy values, it follows that we are, in effect, measuring several ionization energies. It is reasonable to associate these with removal of electrons from different MOs of the molecule.13 A correlation plot (Fig. 8-22) between HMO orbital energies and experimentally measured ionization energies for a number of alternant and nonalternant hydrocarbons has been produced by Brogli and Heilbronner [13]. Their best fit was achieved using α = −6.553 ± 0.340 eV, β = −2.734 ± 0.333 eV, where the limits define a range for the predicted ionization energy (IE) that will include the experimental value nine
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HMO values for IE (eV)
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Experimental IE (eV)
Figure 8-22
Experimental ionization energies for alternant and nonalternant hydrocarbons versus HMO orbital energies using α = −6.553 eV, β = −2.734 eV.
13Note that each ionization energy is associated with removal of an electron from a different MO of the neutral molecule. This differs from the first, second, etc. ionization energies produced by successive ionization.