The Shape of Absorption Bands

The shape of an IR absorption band can provide valuable information about the molecular environment and interactions.

1. Sharp Peaks

• Characteristics: Narrow, well-defined peaks.
• Interpretation:
  • Typically observed for isolated functional groups with minimal intermolecular interactions.
  • Indicates a relatively homogeneous environment for the vibrating bond.
  • Examples: C=O stretch in ketones, C≡N stretch in nitriles.
  • Also, gasses often display very sharp peaks, due to a lack of intermolecular interactions.

2. Broad Peaks

• Characteristics: Wide, often rounded peaks.
• Interpretation:
  • Strongly suggests the presence of hydrogen bonding. The variation in hydrogen bond strengths leads to a distribution of vibrational frequencies, resulting in a broadened peak.
  • Can also indicate overlapping vibrations from multiple closely spaced modes.
  • May be observed in liquids and solids due to the presence of various intermolecular interactions.
  • Examples: O–H stretch in alcohols and carboxylic acids, N–H stretch in amines and amides.
  • Carboxylic acids are well known for very broad peaks.

3. Shoulders and Splitting

• Characteristics: Peaks with a small, adjacent peak (shoulder) or peaks that appear to be split into two or more components.
• Interpretation:
  • Coupling of Vibrations: When two or more vibrational modes are close in frequency, they can couple, leading to splitting or shoulders. This is common in molecules with multiple similar functional groups.
  • Fermi Resonance: A specific type of coupling where an overtone or combination band interacts with a fundamental vibration, resulting in splitting.
  • Symmetry Breaking: In substituted molecules, particularly aromatic rings, the symmetry of the molecule can be broken, leading to splitting of degenerate vibrational modes.
  • Rotational fine structure: In gasses, rotational energy levels can be superimposed on the vibrational levels, leading to fine structure on the vibrational peaks. This fine structure can appear as shoulders or splitting.
  • Isotopes: Isotopic variants of a molecule can have slightly different vibrational frequencies, leading to splitting.
  • Crystal effects: In solids, crystal packing can lead to splitting.
• Examples:
  • Substituted aromatic rings often show characteristic splitting patterns in the C=C stretching region.
  • Anhydrides show two C=O stretches.
  • Amides can show splitting of the N-H stretch.

4. Factors Affecting Band Shape

• Temperature: Lower temperatures generally lead to sharper peaks as thermal motion is reduced.
• Solvent: The solvent can affect the shape of absorption bands, particularly through solvation and hydrogen bonding interactions.
• Concentration: High concentrations can lead to increased intermolecular interactions and broader peaks.
• Instrument Resolution: The resolution of the IR spectrometer can affect the observed shape of absorption bands.

Importance of Band Shape Information

• Band shape provides valuable clues about intermolecular interactions, particularly hydrogen bonding.
• It can help distinguish between different types of functional groups and molecular environments.
• The presence of shoulders or splitting can indicate specific structural features or interactions.

By understanding the factors that influence the shape of IR absorption bands, you can extract more detailed information from IR spectra.