Chapter 3 Infrared Spectroscopy

1
Chapter 3 Infrared Spectroscopy

• Each interatomic bond may vibrate in several
  different motions (stretching or bending) -
  vibrational, rotational energy level
• Absorption of radiation by a typical organic
  molecule results in the excitation of
  vibrational, rotational and bending modes would
  be between 0.75 200 µ, or infrared region

2
IR Theory Principles

• IR energy lower than visible

3
Infrared Region
Near IR 0.75 2.5 µ (12820 4000 cm-1)
reciprocal centimeters, cycle
per cm,
wavenumber IR 2.5 25 µ (4000 400 cm-1
) Useful region for organic analysis Far IR
25 1000 µ (400 10 cm-1 )
4
(No Transcript)
5

• Infrared energy absorbed as vibrations of polar
  molecules (thermal energy) when sample is placed
  in IR beam -

6

• Vibration frequencies are quantized and depend on
  atom masses, bond strengths, and change in
  dipole.

7
Twisting Wagging Rocking
VibrationMotions
Stretching Bending
can be
Symmetric Asymmetric
can be
8
(No Transcript)
9

• Spectrum is transmission of IR light plotted
  against frequency in wavenumbers (cm-1).

10
Structural Information from IR

• Good for confirming certain functional groups -
  especially
• -O-H
• CO
• -C?N and C?C
• Not good for carbon backbone.
• Intensities vary so not normally quantitative.
• electronegativity, vibration mode,
  conjugation, etc
• NOT GREAT FOR FIGURING OUT STRUCTURE
• USED FOR VERIFYING OR ELIMINATING POSSIBLE
  STRUCTURES.

11
IR Interpretation Chart
12
(No Transcript)
13
IR Interpretation Chart
14
2.2 Structural Group Analysis

• Extensive correlations exist between absorption
  peak positions and structural units of organic
  molecules
• stretching (?) and bending (d, ?)

15
Alkanes (1)

• The most prominent peaks in IR spectra of
  saturated hydrocarbons are due to C-H stretching
  and bending.
• 1. ? C-H appears in 2975 to 2845 cm-1 regions,
  its due to symmetric and asymmetric stretching
  vibration of CH3, CH2 and CH groups. They can be
  applied to identify alkanes (lt 3000 cm-1 ) from
  unsaturated compounds

16
Alkane C-H Stretch
17
Alkanes (2)
2. dC-H appears in 1460 and 1380 cm-1. The
first one is attribute to das of CH3 and CH2, the
later is due to d s of C-H of CH3. d s is an
evidence of the existence of CH3 in unknown
compound. The exact positions of these as
well as the stretching frequencies depend upon
the nature of adjacent atoms.
18
Alkane Bend and Rock
19
Alkanes (3)

• ? c-c are within 1250 800 cm-1, and are weak
  peaks. Usually they are not specific for
  compounds and can not be used in IR analysis.

20
Alkenes (1)

• The big difference of IR spectra between alkanes
  and alkenes are due to the vibration of C-H and
  C-C. The specific peaks of Olefin are
• 1. ? CC-H generally appear in the region 3100 to
  3000 cm-1, thus differentiating saturated and
  unsaturated hydrocarbons.

21
Alkene, Alkyne Aromatic C-H Stretch
22
Alkenes (2)

• 2. The out-of-plane bending vibrations in the
  1000 to 650 cm-1 region ?CC-H are often useful
  in predicting the substitution pattern of the
  double bond.

23
Olefinic C-H out-of-plane bending frequencies
24
Alkene C-H Bend and Twist
25
Alkenes (3)

• 3. The CC stretching frequency ?CC in the 1600
  to 1675 cm-1 region also varies with substitution
  but to a lesser degree

26
Alkene, Alkyne and Aromatic Carbon-Carbon Stretch
27
IR Interpretation Chart
28
Alkynes (1)

• Its not difficult to distinguish alkynes from IR
  spectra based on the following specific band
• 1. The C-H stretching vibration of terminal
  acetyenes (?CC-H )generally appears at 3300 to
  3100 cm-1 as a strong sharp band.
• The peak of ?N-H is also in this region, but
  can be recognized with its broad band

29
Alkene, Alkyne Aromatic C-H Stretch
30
Alkynes (2)

• 2. The CC stretching band is found in the region
  2150 to 2100 cm-1 if the alkyne is
  monosubstituted and at 2270 to 2150 cm-1 if
  disubstituted. The latter are usually quite weak
  absorptions

31
Alkene, Alkyne and Aromatic Carbon-Carbon Stretch
32
Alkynes (3)

• The CC-H out-of-plane bending vibration (?CC-H
  ) is found in the region 680 to 610 cm-1

33
Aromatic compounds (1)

• 1. Aromatic C-H stretching absorption appears in
  the region 3100-3000 cm-1, close to that of
  olefinic C-H stretching
• 2. Aromatic C-H out-of-plane bending bands (
  ?CC-H)in the 900 to 690 cm-1 region are
  reasonably well determined by the substitution
  pattern of the benzene ring as indicated. These
  strong, usually sharp bands can be used in
  distinguishing positional isomers of substituted
  benzenes.

34
Aromatic ?CC-H Frequencies
35
Alkene, Alkyne Aromatic C-H Stretch
36
Aromatic compounds (2)

• 3. Sharp peaks at 1600 and 1500 cm-1 are very
  characteristic of all benzenoid compounds a band
  at 1580 cm-1 appears when the ring is conjugated
  with a substituent.

37
Alkene, Alkyne and Aromatic Carbon-Carbon Stretch
38
(No Transcript)
39
Alcohols and phenols (1)

• 1. The very characteristic infrared band due to
  O-H stretching appears at 3650 to 3600 cm-1 in
  dilute solution. In spectra of neat liquids or
  solids intermolecular hydrogen banding broadens
  the band and shifts its position to lower
  frequency (3500 3200 cm-1 )

40
O-H and N-H Stretch
41
Alcohols and phenols (2)

• Strong bands due to O-H bending and C-O
  stretching are observed at 1500 to 1300 cm-1 and
  1220 to 1000 cm-1 , respectively. In simple
  alcohols and phenols the exact position of the
  latter is useful in classification of the
  hydroxyl group.
• Phenol 1230 cm-1 tertiary 1200 cm-1
• secondary 1125 cm-1 primary 1050 cm-1

42
C-O Stretch
43
IR Interpretation Chart
44
Ether

• The asymmetric C-O stretching absorption of
  ethers appears in the region 1280 to 1050 cm-1 .
  As in alcohols, the exact position of this strong
  peak is dependent on the nature of the attached
  groups. Phenol and enol ethers generally absorb
  at 1275 to 1200 cm-1 , dialkyl ethers at 1150 to
  1050 cm-1 . Alcohols, acids, esters may
  interfere the identification.

45
IR Interpretation Chart
46
Aldehydes and Ketones

• The CO stretching frequencies of saturated
  aldehydes and acyclic ketones are observed at
  1735 to 1710 cm-1 and 1720 to 1700 cm-1 ,
  respectively. Aldehydes are also recognizable by
  the C-H stretching vibration which appears as two
  peaks in the 2850 to 2700 cm-1 region.

47
CO Stretching in Aldehydes, Ketones, Acids and
Esters
48
IR Interpretation Chart
49
Carboxylic acids

• The most characteristic absorption of carboxylic
  acids is a broad peak extending from 3300 to 2500
  cm-1 due largely to hydrogen bonded O-H
  stretching. The C-H stretching vibrations appear
  as small peaks on top of this band.
• The CO stretching band appears from 1725 to 1700
  cm-1 and is shifted to 16901680 cm-1 by adjacent
  unsaturation. Its a strong band.

50
Carboxylic acids

• C-O stretching band appears from 14401395 cm-1 ,
  and usually weak peak.

51
CO Stretching in Aldehydes, Ketones, Acids and
Esters
52
Carboxylic esters

• Saturated ester carbonyl stretching is observed
  at 1740 to 1720 cm-1. Unsaturation adjacent to
  the carbonyl group lowers the frequency by 10 to
  15 cm-1 .
• The C-O-C stretching of esters appears as two
  bands in the 1280 to 1050 cm-1 region. The
  asymmetric stretching peak at 1289 to 1150 cm-1
  is usually strong.

53
CO Stretching in Aldehydes, Ketones, Acids and
Esters
54
Anhydrieds

• Acid anhydrides are readily recognized by the
  presence of two high frequency (1830 to 1800 cm-1
  and 1775 to 1740 cm-1 carbonyl absorptions. As
  with other carbonyl stretching vibrations, the
  frequency is increased by incorporating the group
  in a ring and decreased by adjacent unsaturation.
  Cylic anhydrides differ from acyclic anhydrides
  also in that the lower frequency band is stronger
  in the former, while the reverse is true of the
  latter.

55
Amines (1)

• Primary and secondary amines show N-H stretching
  vibrations in the 3500 to 3300 cm-1 region.
  Primary amines generally have two bands
  approximately 70 cm-1 apart due to asymmetric and
  symmetric stretching modes. Secondary amines
  show only one band. Inter- or intramolecular
  hydrogen bonding broadens the absorptions and
  lowers the frequency.

56
Amines (2)

• In general the intensities of N-H bands are less
  than of O-H bands. The N-H bending and C-N
  stretching absorptions are not as strong as the
  corresponding alcohol bands and occur at
  approximately 100 cm-1 higher frequencies

57
Amines (3)

• In addition, NH2 groups give an additional broad
  band at 900 to 700 cm-1 due to out-of-plane
  bending

58
O-H and N-H Stretch
59
Amides

• Amide carbonyl stretching is observed in the 1670
  to 1640 cm-1 region. In contrast to other
  carbonyl groups, both adjacent unsaturation and
  ring formation cause the absorption to shift to
  higher frequencies. Primary and secondary amides
  also show N-H stretching at 3500 to 3100 cm-1 and
  N-H bending at 1640 to 1550 cm-1

60
Analysis procedure

• Qualitative analysis
• Compare with standard
• Database
• Sadtler, Catalog of Infrared Standard Spectra
• Documentation of Molecular Spectroscopy
• American Petroleum Institute Infrared Spectral
  Data
• IR Data Committee

61
Degree of unsaturation
62
Interpretation of IR spectra
63
2.3 Interpretation of IR Spectra
64
2.3 Interpretation of IR Spectra

• The 7 important region of IR spectra
• Most organic compounds are composed of C, H, O
  and N.
• Most chemical groups have specific absorption
  with 4000 670 cm-1 . This region can be
  divided into 7 regions as

65
7 important region
66
Interpretation (part 1)O-H, N-H stretching
(3750-3000 cm-1 )

• ?O-H at 3700-3200 cm-1 is an important evidence
  of the existence of OH
• Free OH group (dilute CCl4 solution) shows sharp
  peak. H-bond and intermolecular association
  broad and lower the wavenumbers (3450-3200 cm-1 )

67
(No Transcript)
68
Interpretation 1

• The peak numbers of ?N-H depend on the number of
  substituents of N atom. Primary amines and
  primary amides have two peaks with almost the
  same strength. Amide can be distinguished by the
  strong ?CO peak around 1600 cm-1

69
(No Transcript)
70
Interpretation 1

• Secondary amines and amides have only one weak
  ?N-H peak at 3500 3100 cm-1 . Secondary
  aromatic amines have strong absorption at 3450
  3490 cm-1 .
• No peak for tertiary amines and amides in the
  corresponding ?N-H region.
• IR is a simple way to distinguish primary,
  secondary and tertiary amines and amides

71
Interpretation (part 2)C-H stretching (3300-2700)

• Distinguish saturated hydrocarbons from
  unsaturated
• Alkynes,
• aromatic (3030 cm-1 , weak and sharp)

72
(No Transcript)
73
Interpretation (2)

• Besides the CO stretching frequencies of
  saturated aldehydes at 1735 to 1710
  cm-1,Aldehydes are also recognizable by the C-H
  stretching vibration which appears as two peaks
  in the 2850 to 2700 cm-1 region.

74
Interpretation (part 3)C C, C N etc.
2400-2100
75
Interpretation (part 4)CO (1900-1650 cm-1 )

• The CO stretching band (1755-1670 cm-1) is a
  very strong peak, it can be a good evidence for
  the existence of CO groups in the unknown
  compound.
• The adjacent group may affect the position of
  ?CO

76
(No Transcript)
77
Interpretation (part 5)Double bond stretching
(1690-1500 cm-1 )

• Stretching vibration of CC, CN, NN, NO and
  CC of benzene ring
• Usually weak peak. Asymmetric structure enhances
  the absorption
• Sharp peaks at 1600 and 1500 cm-1 are very
  characteristic of all benzenoid compounds

78
(No Transcript)
79
(No Transcript)
80
Interpretation (part 5)

• Besides, dNH2 of amines are also in this region.
  Primary amine 1650 1580 (m-s) with a broad peak
  at 900-650 cm-1. (maybe interfered with the
  peaks of aromatic ring). There isnt any peaks
  for secondary peaks.
• Primary amide dNH2 at 1640-1600 cm-1 is a strong
  and sharp peak(1/3-1/2 of CO). Secondary amide
  dNH2 at 1550-1530 cm-1

81
Interpretation (part 6)X-H and X-Y (1475-1000
cm-1 )

• The absorption bands include C-H in plane
  bending, stretching vibration of C-O and C-X, and
  C-C vibration

82
(No Transcript)
83
Interpretation

• 1460 cm-1 is the specific peak for CH3 and CH2
• Can be identified with the peak at 1380 cm-1
• 1380 cm-1 peak can split into 2 peaks with same
  strength.
• Example isopropyl group has two peaks at
  1389-1381 and 1372-1368 cm-1. It can be verified
  with C-C stretching of isopropyl at 1170 and 1150
  cm-1 dual peaks

84
(No Transcript)
85
Interpretation

• ds(C-H) of tert-butyl group splits into
  1401-1395 and 1374-1360 cm-1. The low frequency
  peak is stronger than the former one

86
(No Transcript)
87
Interpretation

• The strong stretching vibration peak of C-O
  appears in the spectra of alcohol, ether and
  ester as an evidence of OH group
• Alcohol 1410-1050 cm-1, reflect the structure
  of alcohols.
• Ether 1250-1100 cm-1
• Ester most of them 1200-1190 cm-1

88
Interpretation (part 7)C-H out-of-plane
bending(1000-650 cm-1 )

• Mainly of ?C-H of alkenes and aromatic compounds

89
Aromatic compound

• Two bands were applied to identify the structure
  of aromatic compound 900-650 cm-1 (?C-H ) and
  2000-1660 cm-1 (?CC)

90
(No Transcript)
91
(No Transcript)
92
Aromatic compound

• The absorption bands at 900-650 cm-1 are strong
  and specific for substitution structure of
  aromatic. They can also be used for
  quantitation.

93
Example
94
(No Transcript)
95
(No Transcript)
96
(No Transcript)
97
C8H10O
98
C12H26O
99
C9H7NO
100
(No Transcript)
101
(No Transcript)
102
(No Transcript)
103
Procedure for obtaining IR spectra

• Liquid Samples The simplest method for mounting
  the sample consists of placing a thin film of the
  liquid between two transparent windows. The most
  common material used for the windows is
  NaCl(transparent within 10000 to 650 cm-1)

104
Procedure for obtaining IR spectra

• Solid Samples As an alternative to measuring
  the spectrum of a liquid solution of a solid
  compound, a solid solution or dispersion in KBr
  is usually more convenient.
• 1-2 mg of sample is mixed with 50-100 mg of dry
  KBr powder. The mixing powder is formed into a
  transparent pellet by pressure in a small die.