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Infrared Spectroscopy

Infrared Spectroscopy. Chapter 12. Energy. Table 12.1, p.472. Final Exam Schedule, Thursday, May 22, 10:30 AM. Infrared spectroscopy causes molecules to vibrate.

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Infrared Spectroscopy

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  1. Infrared Spectroscopy Chapter 12

  2. Energy Table 12.1, p.472

  3. Final Exam Schedule, Thursday, May 22, 10:30 AM

  4. Infrared spectroscopy causes molecules to vibrate

  5. A non-linear molecule having n atoms may have many different vibrations. Each atom can move in three directions: 3n. Need to subtract 3 for translational motion and 3 for rotations # vibrations = 3 n – 6 (n = number of atoms in non-linear molecule) Infrared radiation does not cause all possible vibrations to vibrate. For a vibration to be caused by infrared radiation (infrared active) requires that the vibration causes a change in the dipole moment of the molecule. (Does the moving of the atoms in the vibration causes the dipole to change. Yes: should appear in spectrum. No: should not appear.) Consider C=C bond stretch… ethylene 1,1 difluoro ethylene What about 1,2 difluoro ethylene?

  6. Different bonds have different resistances to stretching, different frequencies of vibration Table 12.4, p.478

  7. Typical Infra-red spectrum. wavelength Frequency, measured in “reciprocal centimeters”, the number of waves in 1 cm distance. Energy. Figure 12.2, p.475

  8. “fingerprint region”, complex vibrations of the entire molecule. C=O C-H Vibrations characteristic of individual groups. Figure 12.2, p.475

  9. BDE of C-H 414 464 556 472 Table 12.5, p.480

  10. BDE and CC stretch 376 727 966 Table 12.5, p.480

  11. Alkane bands Figure 12.4, p.480

  12. Recognition of Groups: Alkenes (cyclohexene). Compare these two C-H stretches Sometimes weak if symmetric

  13. Recognition of Groups: Alkynes (oct-1-yne) • This is a terminal alkyne and we expect to see • Alkyne C-H • Alkyne triple bond stretch (asymmetric)

  14. Recognition of Groups: Arenes. (methylbenzene, toluene) Out-of-plane bend; strong

  15. Recognition of Groups: Alcohols The O-H stretch depends on whether there is hydrogen bonding present Compare –O-H vs -O-H….O Hydrogen bonding makes it easier to move the H with H bonding as it is being pulled in both directions; lower frequency

  16. Recognition of Groups: Alcohols

  17. Recognition of Groups: Ethers No O-H bond stretch present but have C-O in same area as for alcohol.

  18. C-O stretch in assymetric ethers

  19. Recognition of Groups: Amines Easiest to recognize is N-H bond stretch: 3300 – 3500 cm-1. Same area as alcohols. Note tertiary amines, NR3, do not have hydrogen bonding. Hydrogen bonding can shift to lower frequency

  20. Esters One C=O stretch and two C-O stretches.

  21. Recognition of Groups: Carbonyl • C=O stretch can be recognized reliably in area of 1630 – 1820 cm-1 • Aldehydes will also have C(O)-H stretch • Esters will also have C-O stretch • carboxylic acid will have O-H stretch • Amide will frequently have N-H stretch • Ketones have nothing extra

  22. What to check for in an IR spectrum C-H vibrations about 3000 cm-1 can detect vinyl and terminal alkyne hydrogens. O-H vibrations about 3500 cm-1 C=O vibrations about 1630 – 1820 cm-1 C-O vibrations about 1000-1250 cm-1

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