Chemistry and the material world
123.102
Lecture 7
Matthias Lein
Molecular Orbital Theory vs Valence Bonding
Comparison of Lewis Structures and their corresponding VB orbitals with
Molecular Orbitals.
Resonance structures in VB vs delocalization in MO.
σ bonds in VB and σ bonds in MO.
π system of benzene
Photoelectron spectroscopy and MO theory
Introduction to spectroscopy
The electromagnetic spectrum
Photoelectron spectroscopy and MO theory
Example: Nitrate: NO3Lewis Structure:
Valence Bond Orbitals:
σ framework
π bond
Example: Nitrate: NO3Molecular Orbitals:
Example: Water: H2O
Valence Bond Orbitals
Molecular Orbitals
Example: Benzene: C6H6
σ framework
Lewis resonance structures
Example: Benzene: C6H6
Molecular π orbitals
The electromagnetic spectrum
Energy levels and photoelectron spectra
absorption spectrum
emission spectrum
Characteristic flame colour can be used to identify elements
A sample of an inorganic salt is prepared
on the tip of an alumina or metal rod.
The sample is then brought into the
flame of a Bunsen burner.
A sample of an inorganic salt is
dissolved in an alcohol that burns
'cleanly'.
The dish with the alcohol is then lit up.
The thermal energy induces electronic
transitions which give each metal their
characteristic colour.
Photoelectron spectra – shining light on things
In photoelectron spectroscopy (UPS, XPS, ESCA)
light of a defined energy (wavelength) is used to
ionize a material.
The energy of the emitted electron is then
measured.
By calculating the difference of the energies of
the photon and the electron, the binding energy of
the electron is determined.
The binding energy of the electron is equated with
the orbital energy in the material.
The Photoelectron
spectrum of N2
2π
energy
4σ
2p
2p
3σ
1π
3σ
1π
15.6 eV
16.7 eV
18.8 eV
2σ
2σ
2s
2s
1σ
Today we covered
Comparison of Lewis Structures and their corresponding VB orbitals with
Molecular Orbitals.
Resonance structures in VB vs delocalization in MO.
σ bonds in VB and σ bonds in MO.
π system of benzene.
Photoelectron spectroscopy and MO theory.
Spectroscopy in analogy to potential energy level differences.
The electromagnetic spectrum.
Photoelectron spectroscopy as a means to 'measure' orbital energies.
The breakdown of VB for determining electronic spectra.
Tom Lehrer
1928, United States