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• Infrared Radiation


  Causes vibrational and rotational transitions in molecules
• Frequency of IR radiation

  Must correspond to the frequency of the vibration in the molecule for absorption to occur
• Transmittance


  Low transmittance = High absorption of that particular wavelength
• IR Spectrum


  • Provides information on functional groups present
  • Allows precise identification (if data available)
• Degrees of Freedom


  A molecule consisting of n atoms has a total of 3n degrees of freedom, corresponding to the Cartesian coordinates of each atom
• Linear Molecule


  • 2 rotational degrees, 3 translational degrees, the rest (3n - 5) are fundamental vibrations
• Non-Linear Molecule


  • 3 rotational degrees, 3 translational degrees, the rest (3n - 6) are fundamental vibrations
• Among the fundamental vibrations, only those producing a change in the molecule dipole will be IR active</b>
• Homonuclear diatomic molecules (like H2, N2, Cl2,etc) have (3*2 - 5) = 1 fundamental vibration, that doesn't change the molecule dipole, so they will be IR inactive
• Linear Triatomic Molecule (CO2)

  • 4 fundamental vibrations: Asymmetric stretching, Symmetric stretching, Vertical bending, Horizontal bending
• Non-Linear Triatomic Molecule (H2O)


  • 3 fundamental vibrations
• Vibrations in -CH2- Fragment

  • Asymmetrical stretching
  • Symmetrical stretching
  • Rocking (bending)
  • Scissoring (bending)
  • Twisting (bending)
  • Wagging (bending)
• Harmonic Oscillator Model


  Used to approximate the stretching frequency of a bond
• The actual absorption values will be shifted and/or will give bands (instead of discrete lines) due to presence of rotational motions coupled with vibrational motions, overtones, and isotopic effect
• Fingerprint Region


  1500 to 500 cm-1, very complicated due to co-presence of many absorptions that will affect each other
• If IR results are reported WITHOUT the spectrum image, it is useful to remember the position of main organic groups
• The importance of the "fingerprinting" region is that it allows discriminating between different compounds even if they have similar functional groups