Study Notes on UV Spectroscopy

UV Spectroscopy Basics

• UV spectroscopy detects the presence of conjugated systems.

• Conjugated systems: alternating double and single bonds.

Conjugated Systems

• Characteristics:

  • Involves sp² hybridized carbons with unhybridized p orbitals.

  • Sideways overlapping of p orbitals forms pi bonds.

Molecular Orbital Theory

• Electrons behave as both waves and particles.

• Four possible arrangements of electrons (as waves):

  • Sigma bonding molecular orbital (lowest energy, all electrons in phase).

  • Pi bonding molecular orbital (one node).

  • Anti pi bonding molecular orbital (two nodes).

  • Sigma star antibonding molecular orbital (three nodes).

Energy Levels

• Energy ordering (increasing energy):

  1. Sigma bonding

  2. Pi bonding

  3. Anti pi bonding

  4. Sigma star antibonding

HOMO and LUMO

• HOMO (Highest Occupied Molecular Orbital): last occupied level.

• LUMO (Lowest Unoccupied Molecular Orbital): first unoccupied level.

UV Light Interaction

• Non-conjugated systems let UV light pass unabsorbed.

• Conjugated systems absorb UV light, leading to electron transitions from HOMO to LUMO.

• Energy of light relates to wavelength for UV light (190-300 nm).

Absorption and Data Interpretation

• Absorption spectra:

  • Non-conjugated: flat line (no absorption).

  • Conjugated: peaks at lambda max (wavelength of maximum absorption).

Beer Lambert Law

• Formula: A = ε * c * L

  • A = absorbance, ε = molar absorptivity, c = concentration, L = path length.

• Understand the components for general knowledge; not necessary to memorize for exams.