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Chemistry LibreTexts

Vibrational Modes

  • Page ID
    • Combination Bands, Overtones and Fermi Resonances
      Combination bands, overtones, and Fermi resonances are used to help explain and assign peaks in vibrational spectra that do not correspond with known fundamental vibrations. Combination bands and overtones generally have lower intensities than the fundamentals, and Fermi resonance causes a spilt and shift in intensity of peaks with similar energies and identical symmetries. Hot bands will also be briefly addressed.
    • Introduction to Vibrations
      IR spectroscopy which has become so useful in identification, estimation, and structure determination of compounds draws its strength from being able to identify the various vibrational modes of a molecule. A complete description of these vibrational normal modes, their properties and their relationship with the molecular structure is the subject of this article.
    • Isotope Effects in Vibrational Spectroscopy
      This page provides an overview of how an isotope can affect the frequencies of the vibrational modes of a molecule. Isotopic substitution is a useful technique due to the fact that the normal modes of an isotopically substituted molecule are different than the normal modes of an unsubstituted molecule, leading to different corresponding vibrational frequencies for the substituted atoms.
    • Mode Analysis
    • Normal Modes
      Normal modes are used to describe the different vibrational motions in molecules. Each mode can be characterized by a different type of motion and each mode has a certain symmetry associated with it. Group theory is a useful tool in order to determine what symmetries the normal modes contain and predict if these modes are IR and/or Raman active. Consequently, IR and Raman spectroscopy is often used for vibrational spectra.
    • Number of Vibrational Modes in a Molecule
      All atoms in a molecule are constantly in motion while the entire molecule experiences constant translational and rotational motion. A diatomic molecule contains only a single motion. Polyatomic molecules have more than one type of vibration, known as normal modes.
    • Symmetry Adapted Linear Combinations
      The construction of linear combinations of the basis of atomic movements allows the vibrations belonging to irreducible representations to be investigated. The wavefunction of these symmetry equivalent orbitals is referred to as Symmetry Adapted Linear Combinations, or SALCs.