IR Spectroscopy 11/5

Introduction to IR Spectroscopy

  • IR spectroscopy is a technique used for analyzing compounds by looking at their interaction with infrared light.
    • Key focus on photon energy and molecular vibrations.

Basic Principles of IR Spectroscopy

  • Photon Energy:

    • Energy of a photon can be calculated based on its frequency or wavelength.
    • The relationship can be expressed as:
    • E=h<br/>νE = h <br />\nu
    • Where:
      • E = Energy per photon
      • h = Planck's constant
      • ν = frequency of the photon

  • For chemists, energy considerations often use units like kilocalories (kcal), kilojoules (kJ), or moles.

    • The conversion example given:
    • Typical IR light frequency at 3,000 wave numbers corresponds to approximately 86 kcal per mole.
    • This energy correlates with bond stretching and vibrations.

Significance of Bond Vibrations

  • IR spectroscopy reveals information about bond stretching and vibrations within a molecule.
  • Molecules are in a more stable state before irradiation with light.
  • Photon Interaction:
    • If the energy of a photon matches the energy gap corresponding to a molecular vibration, absorption occurs.

Observation with Deuterium

  • When using deuterium instead of regular hydrogen, a peak is observed at 7.26 ppm which corresponds to residual chloroquine hydrogen.
    • Important note on deuterium isotopic effects:
    • Deuterium gives a peak due to the residual amount of hydrogen which is never 100% replaced.
    • In this case, 99.6% is deuterium, leaving 0.4% regular hydrogen detectable as a solid peak.

Understanding Molecular Structures through NMR

  • NMR (Nuclear Magnetic Resonance) spectroscopy is used to deduce molecular structures.

  • Hydrogen (H) Signals:

    • If a methyl group (–CH₃) is present, it will generate three hydrogen (H) signals.
    • For two equivalent –CH₃ groups, it results in six hydrogen signals:
    • e.g., 3 H from CH₃ and 3 H from the second CH₃.
    • This can be determined by integration in the spectrum, which corresponds to the number of protons.

  • Ratios in observed signals may vary, but the principle remains:

    • Example ratios can include:
    • 2:3 or 4:6, depending on the molecular environment.

Predicting Peaks in the Spectrum

  • The assignment of peaks in the IR spectrum involves understanding functional group presence and their corresponding peaks.
  • Using a reference table can simplify peak predictions based on functional groups present in a molecule.
  • Example of functional groups:
    • A double bond, for instance, has a specific range in the spectrum that can be identified using a standard table.

Conclusion

  • Understanding IR spectroscopy involves integrating concepts of energy, molecular vibrations, and the structural identification through advanced techniques like NMR, providing profound insights into chemical structure and behavior.