Infrared Spectroscopy and Molecular Vibrations

Infrared Light and Molecular Bonds

• Infrared Light Interaction: Infrared light can cause molecular bonds to absorb energy, leading to stretching vibrations.
• Stretching Vibration: This is similar to the oscillation of a spring, where the bond can be modeled like a spring between two masses.

Modeling Bonds as Springs

• Example: Carbon-Hydrogen bond modeled as a spring.
  • Energy input can stretch the bond.
  • Oscillation happens as the bond stretches and contracts.

Infrared Spectrum Analysis

• One Octyne: Used as an example to understand IR spectrum.
• Transmittance Analysis:
  • 100% transmittance: No absorption, light passes through.
  • <100% transmittance: Some light absorbed, indicating bond stretching.

Wave Number and Frequency

• Wave Number Definition: Reciprocal of the wavelength (in cm).
  • Example Calculation: Wavelength of 0.002 cm gives a wave number of 500 cm⁻¹.
• Relation to Frequency:
  • Frequency (ν) = Wave number × Speed of light (c).
  • Frequency and wave number are directly proportional.

Interpreting Infrared Spectrum

• Diagnostic Region:
  • Left of 1500 cm⁻¹, indicates functional groups.
  • Example: A signal at 2100 cm⁻¹ indicates a triple bond.
• Fingerprint Region:
  • Right of 1500 cm⁻¹, unique to each molecule, more complex.

Important Considerations

• Signal Interpretation:
  • Focus on location, intensity, and shape of signals.
  • Wave number scale might vary; focus on relative positioning of signals.

Future Topics

• Further Exploration: In upcoming lectures, the concept of bonds as springs will be developed using classical physics principles.