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Units used in IR spectroscopy

Units used in IR spectroscopy. The wavelength of light in the IR region varies from about 2.5 to 40  where 1  = 10 -4 cm. Since  cm  sec-1 = c and E = h , then  = c/ ;  is proportional to 1/ , the general convention in IR is to list frequencies proportional to energy.

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Units used in IR spectroscopy

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  1. Units used in IR spectroscopy The wavelength of light in the IR region varies from about 2.5 to 40  where 1  = 10-4 cm. Since cmsec-1 = c and E = h , then  = c/ ;  is proportional to 1/ , the general convention in IR is to list frequencies proportional to energy. Frequencies in IR are generally expressed in 1/  units; ie cm-1 since these areproportional toenergy. 1/(2.5*10-4 cm) = 4000 cm-1 ; 1/(40*10-4 cm) = 250 cm-1 To convert to true energy units, cm-1 needs to be multiplied by the speed of light, 3*10-10 cm sec-1, and Planck’s Constant, h =6.624*10-27 erg sec

  2. Infrared Spectroscopy Evib = (n+1/2)h(k/).5/2 π where  = m1m2/(m1+m2) Erot = J(J+1)h2/8 π2I; where I = moment of inertia Evib-rot = (n+1/2)h(k/).5/2 π + J(J+1)h2/(8 π2I); n, J = ± 1. In IR, the change in n is usually by +1, occasionally some + 2 also occurs (overtone). The number of molecules at room temperature that are in an n = 1 state is very small because the energy spacings are large. The rotational energy spacings are considerably smaller; J = ± 1

  3. An example of a molecule with a small moment of inertia: HBr Evib-rot = (n+1/2)h(k/).5/2 π + J(J+1)h2/(8 π2I); where I = moment of inertia n, J = ± 1. For most large molecules I is large. In this spectrum n = +1. J = ± 1 n =+1 ,J =0

  4. In the condensed phase collision broadening also obcures the n=1, J =0 null

  5. A summary of the principle infrared bands and their assignments. R is an aliphatic group.

  6. sp3 hybridized CH Figure IR-6. N-Decane, neat liquid, thin film: CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

  7. sp3 hybridized CH Figure IR-18. Butylamine, neat liquid; thin film: CH3CH2CH2CH2NH2

  8. sp3 hybridized CH Figure IR-22. Triethylamine, neat liquid; thin film: (CH3CH2)3N

  9. A summary of the principle infrared bands and their assignments. R is an aliphatic group.

  10. sp2 hybridized CH Figure IR-8. Indene; neat; 0.05 mm cell:

  11. sp and sp2 hybridized CH Figure IR-13. Phenylacetylene, neat liquid; thin film (note that R here is an aromatic group):

  12. sp2 hybridized CH Figure IR-42. Sodium benzoate, KBr pellet:

  13. Can the frequency dependence on hybridization be rationalized? The bond strength seems dependent of the amount of s character in the bond

  14. k1 >k2 k2 k1

  15. What would happen to k (force constant ) if stretching a bond resulted in severe steric interactions?

  16. Hindered sp3 C-H 3017.18 1478.5 2952.5 ATR of tri-t-butylmethane

  17. A summary of the principle infrared bands and their assignments. R is an aliphatic group.

  18. aldehyde C-H Figure IR-14. Butanal, neat liquid, thin film: CH3CH2CH2CHO

  19. aldehyde C-H Figure IR-15. Benzaldehyde, neat, thin film (R here is an aromatic group):

  20. Focusing on –NH2

  21. A summary of the principle infrared bands and their assignments. R is an aliphatic group.

  22. NH2- Stretching Frequencies Figure IR-18. Butylamine, neat liquid; thin film: CH3CH2CH2CH2NH2

  23. NH2- Stretching Frequencies Figure IR-19. Benzamide, KBr pellet (R here is an aromatic group):

  24. Focusing on –NHR

  25. A summary of the principle infrared bands and their assignments. R is an aliphatic group.

  26. NH- Stretching Frequencies Figure IR-20. Diethylamine, neat liquid; thin film: CH3CH2NHCH2CH3

  27. NH- Stretching Frequencies Figure IR-21. N-Methyl acetamide, neat liquid; thin film: CH3CONHCH3

  28. Focusing on –NR2

  29. A summary of the principle infrared bands and their assignments. R is an aliphatic group.

  30. Tertiary Amines Figure IR-22. Triethylamine, neat liquid; thin film: (CH3CH2)3N

  31. Tertiary Amides Figure IR-23. N,N-Dimethylacetamide, neat liquid; thin film: CH3CON(CH3)2

  32. A summary of the principle infrared bands and their assignments. R is an aliphatic group.

  33. Free and Hydrogen Bonded H-O-R Figure IR-24. The liquid and vapor spectra of n-hexanol.

  34. Free and Hydrogen Bonded H-O-R Figure IR-25. The liquid and vapor spectra of phenol.

  35. Free and Hydrogen Bonded H-O-R Figure IR-26. The liquid and vapor spectra of hexanoic acid: CH3(CH2)4 CO2H

  36. A summary of the principle infrared bands and their assignments. R is an aliphatic group.

  37. Hydrogen Bonded H-O-R Figure IR-27. Decanoic acid, neat liquid, thin film: CH3(CH2)8CO2H

  38. Hydrogen Bonded H-O-R Figure IR-28. 4-Chloro-2-nitrophenol, KBr pellet:

  39. Hydrogen Bonded H-O-R Figure IR-29. Benzoic acid; KBr disk

  40. Figure IR-30. Benzoic acid, KBr; band distortions and broadening caused by poor grinding, compare to Figure 29.

  41. When does one see a free –OH in the condensed phase?

  42. Free H-O-R Figure IR-17. Tri-t-butylmethanol, KBr pellet:

  43. The –CN triple bond

  44. A summary of the principle infrared bands and their assignments. R is an aliphatic group.

  45. -CN  Figure IR-31. trans-2-Phenyl-1-cyanoethene, in CHCl3 solution: Ph-CH=CH-CN

  46. ClCH2CN

  47. The C-C triple bond in terminal acetylenes

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