CHEM261 Lecture Notes - Infrared Spectroscopy, Wavenumber, Transmittance

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19 Nov 2013
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- Infrared spectroscopy involves studying the interaction of infrared radiation on atoms
or molecules.
- When a sample of an organic compound is subjected to infrared radiation the
compounds absorbs some of this energy.
- As energy at certain wavelengths is absorbed a detector can be used to record the
decrease in intensity of the radiation as it passes through the sample.
- An absorption spectrum is produced showing the percentage transmittance versus the
wavenumber.
- A 95% transmittance means that most radiation at a particular frequency passed
straight through the sample.
- A 5% transmittance means that nearly all the radiation at a particular frequency was
absorbed by the sample.
- The amount of absorption or transmittance of infrared radiation provides information
about the bonds in the sample.
- If a particular frequency is being absorbed as it passes through a sample then it must
mean that its energy is being transferred to the compound.
- The energies specific to infrared radiation correspond to the energies involved in bond
vibrations such as bond stretching or bond bending.
- When the frequency of infrared radiation is the same as the natural frequency of an
interatomic bond, the molecule will absorb energy and the amplitude of the bond
vibration will increase.
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