These chapters discuss UV-visible spectrophotometry and its applications. They explain molecular orbital diagrams and how molecules absorb photons to transition to excited singlet states, labeled S1, S2, etc. depending on the energy level. Beer's law is also covered, noting accuracy decreases for absorbances above 2 due to less than 1% of light exiting the sample. A monochromator is illustrated to separate wavelengths of light.

1. Chapter 13+14
UV-visible spectrophotometry
and its applications

2. • Molecular Orbital (MO) diagram.
• The energies of orbitals are represented by a set of horizontal lines, with energy increasing vertically.
When a molecule is in its ground state (having the lowest amount of electronic energy possible), then
electrons occupy the lowest available MO’s (diagram - electrons are filled up to the HOMO).
• In most molecules, there are even numbers of electrons, and the spins of all the electrons are paired. The
molecule is said to be in a singlet state because a magnetic field does not cause any splitting of the
energy state. The ground singlet state of a molecule is labeled S0
• Energy Molecular orbital diagram of a molecule in the S0 state.
• If a molecule absorbs a photon of light of sufficient energy, then an electron is promoted to a higher
energy MO. Since the electron spin does not change during absorption of a photon, the molecule is still in
a singlet state, but now it is in an excited state. The lowest excited state is designated S1 ; an electron
resides in the LUMO. Higher excited states are S2 , S3 , and so on.

3. • * notes on Beers Law:
• For absorbances above 2, less than 1% of the incident light is
exiting the sample. This has significant consequences in
absorption spectrometers; accuracy decreases and noise
increases in absorption measurements.

8. Illustration of monochromator