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Infrared Molecular Vibrations

Infrared Molecular Vibrations. By Yenling Yang and Julia Hunter. Definition of IR Molecular Vibrations.

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Infrared Molecular Vibrations

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  1. Infrared Molecular Vibrations By Yenling Yang and Julia Hunter

  2. Definition of IR Molecular Vibrations A molecular vibration is any change of shape in molecule due to bending, stretching, or rotating bonds and occurs when atoms in a molecule are in motion while the whole molecule is in constant motion. Molecular vibrations are excited when the molecule gains energy. IR radiation causes vibrations in molecules with wavelengths of 2500 to 25,000 nm.

  3. Concepts of IR Molecular Vibrations • Six Different Ways to Vibrate • Factors Influencing Vibration • IR Molecular Vibrations Applications • IR Spectroscopy

  4. Six Different Ways to Vibrate • Stretching • Symmetrical stretch • Asymmetrical stretch • Bending • Scissoring • Rocking • Wagging • Twisting • http://www.pslc.ws/macrog/irabs.htm

  5. Factors that Influence Molecular Vibration • Molecular potential energy surfaces • Masses of the atoms • Vibrational Coupling

  6. Vibrational Coupling • Vibrations can interact (couple) if the vibrating bonds are joined to a single, central atom. • Factors influencing vibrational coupling— • Strong coupling of stretching vibrations occurs when there is a central atom between the two vibrating bonds • Coupling of bending vibrations occurs when there is a common bond between vibrating groups • Coupling between a stretching vibration and a bending vibration occurs if the stretching bond overlaps a bending bond. • Coupling is greatest when the coupled groups have equal energies • No coupling occurs when groups are separated by two or more bonds

  7. Involvement in Organic Chemistry Organic molecules absorb infrared radiation when the frequency of IR radiation is synchronized with a natural vibration frequency of the molecule. When IR radiation is absorbed, the molecule begins to vibrate with a greater amplitude (but with the same frequency), and thus the molecule has gained energy.

  8. Other Uses of IR Spectroscopy • Forensic analysis (of plastics and proteins) • Examines the quality of tea leaves

  9. Infrared Spectroscopy • IR Spectroscopy uses the specific frequencies where molecules vibrate that correspond to energy levels on the IR spectrum. • The IR spectrum of a sample is collected by shining a beam of infrared light through a sample and a reference (substance the sample is dissolved in). The light reveals the energy absorbed at each wavelength.

  10. Bibliography http://www.prenhall.com/settle/chapters/ch15.pdf http://www.pslc.ws/macrog/irabs.htm http://www.yale.edu/ynhti/curriculum/units/1999/5/99.05.07.x.html#b http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/irspec1.htm http://en.wikipedia.org/wiki/Infrared_spectroscopy

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