Lecture Notes on Molecules and Light Absorption

Introduction

• Molecules absorb different wavelengths of light.
• Ultraviolet (UV) or visible light absorption can be measured with a UV/Vis spectrophotometer.
• Wavelengths range from 200 to 800 nanometers.

Absorption Spectrum

• Shining light through a sample yields an absorption spectrum.
• Example: 1,3-Butadiene
  • Strongest absorption at 217 nm (lambda max).
  • Absorbs in the UV region, hence colorless.

Molecular Orbital Theory

• 4 carbons in 1,3-Butadiene, each sp2 hybridized, with 4 p orbitals.
• 4 atomic orbitals form 4 molecular orbitals: 2 bonding and 2 antibonding.

Energy and Molecular Orbitals

• Bonding orbitals are lower in energy than antibonding orbitals.
• Total of 4 pi electrons in the ground state are in bonding orbitals.
• Transition upon light absorption:
  • Pi electron promoted from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO).

Light Absorption and Energy

• Energy required for electron promotion related to light frequency and wavelength.
• Energy (E) = h (Planck's constant) x c (speed of light) / lambda (wavelength).
• Energy and wavelength are inversely proportional.

Broad Absorption Bands

• Broad range of wavelengths due to molecular vibrations and rotations.

Ethanal Absorption Transitions

• 2 pi electrons in bonding molecular orbitals.
• Possible transitions:
  • Pi to pi* (star) transition at ~180 nm.
  • Non-bonding (n) to pi* transition at ~290 nm, smaller energy difference and longer wavelength.

Energy-Wavelength Relationship

• Smaller energy differences lead to longer wavelength absorption.
• Concept crucial for understanding color in subsequent discussions.