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CHAPTER 9

CHAPTER 9. Spectroscopy: the study of the interaction of energy with matter Energy applied to matter can be absorbed, emitted, cause a chemical change, or be transmitted Spectroscopy can be used to elucidate the structure of a molecule. Electromagnetic radiation.

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CHAPTER 9

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  1. CHAPTER 9 Spectroscopy: the study of the interaction of energy with matter • Energy applied to matter can be absorbed, emitted, cause a chemical change, or be transmitted • Spectroscopy can be used to elucidate the structure of a molecule

  2. Electromagnetic radiation • Electromagnetic radiation is the energy that is transmitted through space in the form of waves. • Types of electromagnetic radiation: Radio waves, Ultraviolet (UV), Infrared(IR), Visible (vis).

  3. Characterization of waves • Waves are characterized by: 1- Wavelength (λ): the distance from the crest of one wave to the crest of the next wave. λ = nm,um, cm

  4. 2- the waves are also characterized by Frequency (v)= number of complete cycles per second (cps), also called Hertz (Hz).

  5. Wavelength and frequency are inversely proportional. • In IR ,frequency is expressed as wavenumbers Wavenumbers have units of reciprocal cm (cm-1)

  6. The relationship between wavelength (or frequency) and energy (E) is well defined • 1- Wavelength and frequency are inversely proportional . • 2- The higher the frequency, the greater the energy of the wave. • 3- The shorter the wavelength, the greater the energy of the wave.

  7. UV VIS IR RADIO waves Increasing wavelength- Decreasing frequency Decreasing Energy Absorption of UV Result of promotion of electron to a higher energy level Absorption of IR  Result in increase of vibration of bonds

  8. Features of a spectrum An infrared spectrum of a compound is a plot of percent transmission (%T) versus either wavelength of frequency changing. %T = (Intensity/original intensity) x 100

  9. Absorption of Infrared Radiation • Cause increase the vibration of bonded atoms • Different type of bonds ( C-H, C-C, C-O, C=O, O-H) absorb IR at different λ. • Type of vibration absorb at different λ. Types of vibrations 1- stretching 2- bending

  10. The relative amount of absorbed energy depends on the change of bond moment • Non-polar bonds  weak absorption • Polar bonds  strong absorption

  11. The infrared spectrum The instrument used to measure absorption of infrared radiation  infrared spectrophotometer

  12. Interpretation of IR spectrum Correlation Chart

  13. A. c-c Bonds • C-C single bond  weak absorption (not useful) • C=C (sp2) 1600- 1700 cm-1 • C=C( aryl, sp2)  1450-1600cm-1

  14. C-H Bonds • (sp3) C-H  2800-3000cm-1 • (sp2) C-H (=C-H)  3000-3300 cm-1

  15. Aromatic Compounds • Aromatic Compounds The C-C bond stretching gives a set of characteristic sharp peaks between 1450-1600 cm -1

  16. Haloalkanes: C-X  500-1430cm-1

  17. Ether:C-O 1050-1260 cm-1 (strong)

  18. Alcohol:O-H 3000-3600cm-1 (strong) + C-O

  19. Hydrogen bonding O-H broadNo H-bonding  O-H sharp

  20. Amines: RNH23000-3600cm-1 (medium or weak double peaks) + C-N (900-1300 CM-1)

  21. Amines: R2NH3000-3600cm-1 (medium or weak one peak) + C-N (900-1300cm-1)

  22. Amines: R3Nno N-H peak only C-N at 900-1300cm-1

  23. Carbonyl Functional Groups Generally the carbonyl group gives a strong peak which occurs at 1630-1780 cm-1 The exact location depends on the actual functional group present

  24. Ketones: C=O  1680-1750 cm-1 (strong)

  25. Aldehydes: Carbonyl (C=O) 1720-1740cm-1 Also must show aldehyde C-H bend Two peaks 1) 2820-2900cm-1 2) 2700-2780 cm-1

  26. Carboxylic acid: carbonyl (C=O)1700-1725 cm-1 (strong) • Also must show O-H stretching  very broad from 3330- 2500 cm-1

  27. Esters: C=O  1735-1760 cm-1 • Also shows C-O  1100-1300 cm-1

  28. Conclusion • IR is used for functional groups identification. • Not all the peaks can be analyzed. • Example

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