Analysis of a Photoelectron Spectrum (Worksheet)

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A photoelectron spectrum is obtained by measuring the kinetic energy, E_K, of electrons emitted from a molecule by ionization with a high-energy source with energy \(h\nu _0\). The ionization energy or potential, I_E, of these electrons is given by

\[I_E = h\nu _0 - E_K \tag {10-77}\]

Koopmans’ Theorem or Approximation says that an orbital wave function, \(\varphi _k\), and orbital energy, obtained by a SCF calculation doesn’t change if an electron is added or removed from this orbital. Consequently, the peaks or bands appearing in a photoelectron spectrum can be assigned to ionization from particular molecular orbitals in a molecule by comparing the SCF orbital energies with the ionization energies determined from the photoelectron spectrum.

In this activity, Hyperchem (or the software provided by your instructor) will be used to calculate orbital energies to provide the information needed to analyze the photoelectron spectrum of formamide. Formamide is the simplest manifestation of the amide bond. The amide bond, also known as the peptide linkage, joins amino acids to form peptides and proteins.

Use Hyperchem to perform a 6-31G SCF calculation on formamide.
Use the data to identify the orbitals being ionized to produce the peaks labeled 1 through 6 in the formamide photoelectron spectrum given below.
Characterize these orbitals by specifying their atomic orbital parentage and their bonding, antibonding, and nonbonding properties.
Experiment with other Basis Sets and Configuration Interaction if you have time.

Figure 10.8: Photoelectron Spectrum of formamide H₂NCHO. (from Kimura, permission pending)