Intensity of IR Absorptions

The intensity of an absorption band in an IR spectrum is directly related to the magnitude of the change in the molecule's dipole moment during the vibration. In simpler terms, if a vibration causes a significant shift in the distribution of electrons within the molecule, it will result in a strong absorption. Conversely, if the electron distribution remains relatively unchanged, the absorption will be weak or even non-existent.

Here's a breakdown of the key concepts:

• Dipole Moment:
  • A dipole moment arises from the separation of positive and negative charges within a molecule.
  • Polar bonds, such as C=O, have a significant dipole moment due to the difference in electronegativity between the atoms.
  • Nonpolar bonds, such as C-C in alkanes, have little or no dipole moment.
• Change in Dipole Moment (dμ/dQ):
  • During a vibration, the bond lengths and angles change, which can alter the dipole moment.
  • The intensity of an IR absorption is proportional to the square of the change in dipole moment with respect to the normal coordinate of the vibration (dμ/dQ)².
  • A large change in dipole moment leads to a strong absorption, while a small change leads to a weak absorption.
• Factors Affecting Intensity:
  • Bond Polarity: Highly polar bonds, such as C=O, generally produce strong absorptions because stretching or bending these bonds results in a large change in dipole moment.
  • Symmetry:
    • Symmetrical molecules, such as diatomic molecules like O₂ or N₂, do not exhibit IR absorptions because their vibrations do not result in a change in dipole moment.
    • Symmetrical vibrations in polyatomic molecules may also be IR-inactive. For example, a symmetrically substituted alkene may have a very weak or absent C=C stretch.
  • Concentration: The intensity of an absorption band is also proportional to the concentration of the absorbing species. Higher concentrations result in stronger absorptions.
  • Path Length: The path length of the IR beam through the sample also affects intensity. Longer path lengths result in stronger absorptions.
• Examples:
  • C=O Stretch: The carbonyl group (C=O) is highly polar, and stretching this bond leads to a substantial change in dipole moment, resulting in a very strong absorption band.
  • O-H Stretch: The O-H bond in alcohols and carboxylic acids is also polar, leading to strong absorptions. Hydrogen bonding further broadens and intensifies these peaks.
  • C-H Stretch: The C-H bond is relatively nonpolar, so C-H stretches are typically of medium intensity. However, the intensity can vary depending on the hybridization of the carbon atom.
  • C=C Stretch: In symmetrical alkenes, the C=C stretch may be very weak or absent. In unsymmetrical alkenes, the change in dipole moment is greater, resulting in a more intense absorption.

Significance of Intensity Information:

• Intensity information can help distinguish between different functional groups. For example, a strong absorption near 1700 cm⁻¹ strongly suggests the presence of a carbonyl group.
• Variations in intensity can provide clues about the symmetry and substitution patterns of molecules.
• It can be used in quantitative analysis. The intensity of an absorption band is proportional to the concentration of the substance.

By understanding the factors that influence the intensity of IR absorptions, you can extract more detailed information about the structure and properties of molecules from their IR spectra.