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 are proportional to energy. 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 E vib = (n+1/2)h(k/  ).5 /2 π where  = m 1 m 2 /(m 1 +m 2 ) E rot = J(J+1)h 2 /8 π2I; where I = moment of inertia E vib-rot = (n+1/2)h(k/  ).5 /2 π + J(J+1)h 2 /(8 π 2 I); 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 E vib-rot = (n+ 1/2 )h(k/  ).5 /2 π + J(J+1)h 2 /(8 π 2 I); 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. Figure IR-6. N-Decane, neat liquid, thin film: CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 sp 3 hybridized CH

7. Figure IR-18. Butylamine, neat liquid; thin film: CH 3 CH 2 CH 2 CH 2 NH 2 sp 3 hybridized CH

8. FigureIR-22. Triethylamine, neat liquid; thin film: (CH 3 CH 2 ) 3 N sp 3 hybridized CH

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

10. sp 2 hybridized CH Figure IR-8. Indene; neat; 0.05 mm cell:

11. Figure IR-13. Phenylacetylene, neat liquid; thin film (note that R here is an aromatic group): sp and sp 2 hybridized CH

12. Figure IR-42. Sodium benzoate, KBr pellet: sp 2 hybridized CH

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

14. k 1 >k 2 k1k1 k2k2

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

17. ATR of tri-t-butylmethane 3017.18 2952.5 1478.5 Hindered sp 3 C-H

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

19. Figure IR-14. Butanal, neat liquid, thin film: CH 3 CH 2 CH 2 CHO aldehyde C-H

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

21. Focusing on –NH 2

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

23. Figure IR-18. Butylamine, neat liquid; thin film: CH 3 CH 2 CH 2 CH 2 NH 2 NH 2 - Stretching Frequencies

24. Figure IR-19. Benzamide, KBr pellet (R here is an aromatic group): NH 2 - Stretching Frequencies

25. Focusing on –NHR

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

27. Figure IR-20. Diethylamine, neat liquid; thin film: CH 3 CH 2 NHCH 2 CH 3 NH- Stretching Frequencies

28. Figure IR-21. N-Methyl acetamide, neat liquid; thin film: CH 3 CONHCH 3 NH- Stretching Frequencies

29. Focusing on –NR 2

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

31. FigureIR-22. Triethylamine, neat liquid; thin film: (CH 3 CH 2 ) 3 N Tertiary Amines

32. Figure IR-23. N,N-Dimethylacetamide, neat liquid; thin film: CH 3 CON(CH 3 ) 2 Tertiary Amides

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

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

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

36. Figure IR-26. The liquid and vapor spectra of hexanoic acid: CH 3 (CH 2 ) 4 CO 2 H

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

38. Figure IR-27. Decanoic acid, neat liquid, thin film: CH 3 (CH 2 ) 8 CO 2 H Hydrogen Bonded H-O-R

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

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

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

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

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

44. The –CN triple bond

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

47. Figure IR-31. trans-2-Phenyl-1-cyanoethene, in CHCl 3 solution: Ph-CH=CH-CN -C  N 

48. ClCH 2 C  N

50. The C-C triple bond in terminal acetylenes

51. Figure IR-13. Phenylacetylene, neat liquid; thin film: C  C Triple Bond Stretch 

52.  2120 cm -1

53. The C  C triple bond in symmetrical acetylenes

54. Figure IR-32. Diethyl acetylenedicarboxylate; neat liquid: C 2 H 5 OCO-C  C-CO 2 CH 2 CH 3 C  C Triple Bond Stretch Where is it?

55. The C  C triple bond in internal acetylenes

57. The C  C triple bond when next to a heteroatom

58. Spectrum in hexanes (50%)