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Organic Chemistry

Organic Chemistry. William H. Brown Christopher S. Foote Brent L. Iverson. Infrared Spectroscopy . Chapter 12. Electromagnetic Radiation. Electromagnetic radiation: light and other forms of radiant energy Wavelength (  ): the distance between consecutive peaks on a wave

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Organic Chemistry

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  1. Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson

  2. Infrared Spectroscopy Chapter 12

  3. Electromagnetic Radiation • Electromagnetic radiation:light and other forms of radiant energy • Wavelength ():the distance between consecutive peaks on a wave • Frequency ():the number of full cycles of a wave that pass a given point in a second • Hertz (Hz):the unit in which radiation frequency is reported; s-1 (read “per second”)

  4. Electromagnetic Radiation • Common units used to express wavelength

  5. Molecular Spectroscopy • Molecular spectroscopy:the study of which frequencies of electromagnetic radiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure • we study three types of molecular spectroscopy

  6. -2 -1 10 m•cm -2 -1 10 m•cm -1 -1 = 400 cm n = = 4000 cm -5 2.5 x 10 m -6 2.5 x 10 m Infrared Spectroscopy • The vibrational IR extends from 2.5 x 10-6 m (2.5 m) to 2.5 x 10-5 m (25 m) • the frequency of IR radiation is commonly expressed in wavenumbers • wavenumber :the number of waves per centimeter, with units cm-1 (read reciprocal centimeters) • expressed in wavenumbers, the vibrational IR extends from 4000 cm-1 to 400 cm -1 n =

  7. Infrared Spectroscopy • IR spectrum of 3-methyl-2-butanone

  8. Molecular Vibrations • atoms joined by covalent bonds undergo continual vibrations relative to each other • the energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed • the energies associated with transitions between vibrational energy levels correspond to frequencies in the infrared region, 4000 to 400 cm-1

  9. H C C H 3 3 H C - C C - C H 3 3 H C C H 3 3 Molecular Vibrations • For a molecule to absorb IR radiation • the bond undergoing vibration must be polar and • its vibration must cause a periodic change in the bond dipole moment • Covalent bonds which do not meet these criteria are said to be IR inactive • the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, are IR inactive because they are not polar bonds C C 2,3-Dimethyl-2-butene 2-Butyne

  10. Molecular Vibrations • For a nonlinear molecule containing n atoms, there are 3n - 6 allowed fundamental vibrations • For even a relatively small molecule, a large number of vibrational energy levels exist and patterns of IR absorption can be very complex • The simplest vibrational motions are bending and stretching

  11. Molecular vibrations • Fundamental stretching and bending vibrations for a methylene group

  12. Molecular Vibrations • Consider two covalently bonded atoms as two vibrating masses connected by a spring • the total energy is proportional to the frequency of vibration • the frequency of a stretching vibration is given by an equation derived from Hooke’s law for a vibrating spring K = a force constant, which is a measure of the bonds’ strength; force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 105 dynes/cm m = reduced mass of the two atoms, (m1m2)/(m1 + m2), where m is the mass of the atoms in grams

  13. Molecular Vibrations • From this equation, we see that the position of a stretching vibration • is proportional to the strength of the vibrating bond • is inversely proportional the masses of the atoms connected by the bond • The intensity of absorption depends primarily on the polarity of the vibrating bond

  14. Correlation Tables • Table 12.4Characteristic IR absorptions for the types of bonds and functional groups we deal with most often

  15. Hydrocarbons-Table 12.5

  16. Alkanes • IR spectrum of decane (Fig 12.4)

  17. Alkenes • IR spectrum of cyclohexene (Fig 12.5)

  18. Alkynes • IR spectrum of 1-octyne (Fig 12.6)

  19. Aromatics • IR spectrum of toluene (Fig 12.7)

  20. Alcohols • IR spectrum of 1-hexanol (Fig 12.8)

  21. Ethers • IR spectrum of dibutyl ether (Fig 12.9)

  22. Ethers • IR spectrum of anisole (Fig 12.10)

  23. Amines • IR spectrum of 1-butanamine (Fig 12.11)

  24. IR of Molecules with C=O Groups

  25. IR of Molecules with C=O Groups

  26. Aldehydes and Ketones • IR spectrum of menthone (Fig 12.12)

  27. -1 -1 -1 -1 1715 cm 1745 cm 1780 cm 1850 cm -1 -1 -1 1717 cm 1690 cm 1700 cm Carbonyl groups • The position of C=O stretching vibration is sensitive to its molecular environment • as ring size decreases and angle strain increases, absorption shifts to a higher frequency • conjugation shifts the C=O absorption to lower frequency O O O O O O O H

  28. Carboxylic acids • IR spectrum of pentanoic acid (Fig 12.13)

  29. Esters • IR of ethyl butanoate (Fig 12.14)

  30. Infrared Spectroscopy End Chapter 12

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