Chapter 7
Infrared Spectrometry
‰ Spectroscopy is the study of the interaction between radiant energy
and matter.
‰ Energy applied to matter can be absorbed, emitted, cause a
chemical change, or be transmitted.
‰ The wavelengths at which an organic compound absorbs radiant
energy are dependent upon the structure of the compound.
‰ Spectroscopic techniques are used to determine the structures of
unknown compounds.
1
Electromagnetic Radiation
‰ Electromagnetic radiation is the energy that is transmitted
through space in the form of waves.
2
‰ Each type of electromagnetic radiation is characterized
by its wavelength (λ)
‰ Wavelength (λ) is the distance from the crest of one
wave to the crest of the next wave.
3
‰ Radiation may also be characterized by its frequency (ν) which is the
number of complete cycles per second (cps), also called Hertz (Hz).
‰ In IR, frequency is expressed as wavenumbers: the number of
cycles per centimeter
‰ Wavenumbers have units of reciprocal centimeters (cm-1)
4
‰The relationship between wavelength (or
frequency) and energy (E) is well defined
ƒ Wavelength and frequency are inversely proportional .
ƒ The higher the frequency, the greater the energy of the wave.
ƒ The shorter the wavelength, the greater the energy of the wave.
ultraviolet
5
visible
infrared
radio
increasing λ (or decreasing ν) means decreased energy
Types of Energy Transitions in Each Region
of the Electromagnetic Spectrum
REGION
ENERGY TRANSITIONS
X-ray
Bond-breaking
UV/Visible
Electronic
Infrared
Vibrational
Microwave
Rotational
Radio Frequency
Nuclear and
Electronic Spin
(NMR)
6
Features of a Spectrum
‰ When a sample absorbs photons from a beam of transmitted radiation,
the intensity of the transmitted radiation decreases.
¾ It is this change in intensity that is measured in absorption
spectroscopy.
‰ An infrared spectrum of a compound is a plot of percent transmission
(%T) versus either wavelength or frequency changing.
7
%T = (Intensity/original intensity) x 100
Absorption of Infrared Radiation
‰ IR spectroscopy is a method for functional group analysis.
‰ Nuclei of atoms bonded by covalent bonds undergo vibrations or oscillations.
‰ When molecules absorb IR radiation, the absorbed energy causes an
increase in the amplitude of the vibrations of the bonded atoms.
‰ Different type of bonds ( C-H, C-C, C-O, C=O, O-H) absorb IR radiation at
different λ.
‰ A bond within a molecule may absorb energy at more than one wavelength.
O
bending R
O
H
R
H
at 3330 cm-1
stretching
1250 cm-1
‰ The relative amount of absorbed energy depends on the change of bond
moment:
ƒ Non-polar bonds (e.g., C-C, C-H bonds) Î weak absorption
ƒ Polar bonds (e.g., O-H, C-O, C=O bonds) Î strong absorption
8
9
INFRARED SPECTROSCOPY
* * * * *
ν = frequency of absorption
c = speed of light
k = force constant
m 1m
m* = reduced mass =
k
m*
υ= 1
2π c
2.5 µ
3
m* for: CH = 12/13 =0.92
CD = 24/14 =1.7
CC =144/24 = 6.0
CCl = 450/49.5 = 9.1::::725 cm-1
2
m 1 +m
2
4
5
6
7
C C
H C
3100
H
H2N
2850
2750
C C
C C
9
8
10
-1
460 N.m
1090
1630
12
C H
N H
O H
15
O
C N
1630 - 1690
2200
3300
Aromatic, C=C
1450 - 1600
~3200 - 3700
(4 peaks)
H O
H C
O
2900
20
C C (w)
C O
C N
X
1000 - 1200
H O C
680 - 900
C O
3000
3500
2500
2000
-1
cm
Frequency
Wavenumber
%T =
10
25
For alkenes:
(=CH; =CH2
bending)
690-990 weak
Y
1640 - 1820
4000
580 N.m-1
710
840
shoulder
(sh)
1600 - 1700
D C
C C
C N
C
H C
HN
H O
k for C C
more polar the bond - stronger the peak
HN
%Transmittance
* * * * *
,υ =
C
λ
,υ =
cps
or Hz
1
1
=
× 10
λ cm
λ µ
Intensity
Original Intensity
×100 =
4
cm
I
×100
IO
−1
1800
1600
1400
1200
1000
800
600
400
Fingerprint region
ketone 1715
aldehyde 1725
ester
1735
(CH bending)
X
acid: dimer
1710
monosubstituted: ~700; ~750
monomer 1760
(two peaks)
Y disubstituted:
amide: amide I band 1690-1630 (C=O)
ortho: ~750 (one peak)
amide II band 1515-1670 (NH)
para: ~830 (one peak)
acid halide, RCOCl 1785-1815
meta: ~700; 780; 880
acid anhydride 1740-1840 (usually two peaks)
(three peaks)
carboxylate
1590-1600
For CHEM 202, 303 & higher level courses ..........Designed, drawn and compiled by Prof SA ALI
Preparation of the sample
‰ Compounds that are subjected to IR analysis should be pure.
‰ Solids should be samples from recrystallization or from chromatography.
‰ Liquids should be samples from distillation or from gas chromatographic
separation.
‰ Liquids can be examined as a thin film (neat) between two salts (KBr)
plates or can be dissolved in a nonpolar organic solvent (e.g., CCl4).
‰ FTIR instruments can subtract the solvent spectrum from the spectrum of
a sample in that solvent.
‰ For solid compounds, make pellet with KBr powder (1 mg solid to 100 mg
KBr).
‰ Using only dry equipment and dry chemicals will minimize the
appearance of water at 3000-3800 cm-1 (O-H stretching) and 1520-1750
cm-1 (O-H bending).
11
The Infrared Spectrum
‰ The instrument used to measure absorption of infrared radiation is
called infrared spectrophotometer
12
Interpretation of IR Spectrum
‰ The region from 1400-4000 cm-1 (absorption from stretching) is
especially useful for identification of the various functional groups.
‰ The region to the right of 1400 cm-1 is often quite complex
(absorption from stretching and bending): fingerprint region.
13
14
A. Carbon-Carbon and Carbon-Hydrogen Bonds
‰ C-C single bonds give rise to weak absorption bands in the IR spectrum:
not very useful for structure determination.
‰ Almost all organic compounds contain C-H bonds
‰ The C-H stretching peaks are often useful in determining the hybridization
of the carbon atom.
15
Alkane (octane)
16
ALKANE
Hexane
bending
CH2
bend
CH3
bend
CH
stretch
CH3 CH2 CH2 CH2 CH2 CH3
17
stretching
stretching
18
bending
Cyclohexene
1648 cm-1
720 cm-1
19
ALKYNE
1-Hexyne
C≡C
CH2, CH3
≡C-H
20
C-H
HC
Symmetrical alkynes do not show
a peak at this range.
C
CH2 CH2 CH2 CH3
1-Heptyne
2120 cm-1
21
NITRILE
Propanenitrile
=
C=N
CH3 CH2 C
22
N
B. Substituted Benzenes
_
‰ The aryl C_H stretching gives weak peaks near 3030 cm -1
_
‰ The aryl C_C stretching gives four peaks between 1450-1600 cm -1
-1
stretching at ~3030 cm (weak)
1450-1600 cm-1 (four peaks)
H
Y
‰ The C-H Bending Absorption of Substituted Benzenes cm-1 is as follows:
monosubstituted
two peaks: 730-770 and 690-710 cm-1
ortho-disubstituted
one peak: 735-770 cm-1
para-disubstituted
one peak: 790-840 cm-1
meta-disubstituted
three peaks: 860-900, 750-810, and 680-730 cm-1
23
‰ Substituted benzene rings have distinctive
patterns at 1667-2000 cm-1
‰ These bands are due to overtones of aromatic
bands.
‰ Such use of this region to identify substitution
patterns can be difficult since these bands are
weak, and strong bands, such as the carbonyl
bands, can obscure the overtone patterns.
24
Toluene
‰ monosubstituted
C-H Bending two peaks : 730-770 and 690-710 cm-1
‰ The aryl C_C stretching: four peaks between 1450-1600 cm -1
25
26
chlorobenzene
‰ monosubstituted
C-H Bending two peaks : 730-770 and 690-710 cm-1
‰ The aryl C_C stretching: four peaks between 1450-1600 cm -1
27
o-chlorotoluene
‰ ortho-disubstituted one peak: 735-770 cm-1
‰ The aryl C_C stretching: four peaks between 1450-1600 cm -1
28
m-chlorotoluene
‰ meta-disubstituted three peaks: 860-900, 750-810, and 680-730 cm-1
‰ The aryl C_C stretching: four peaks between 1450-1600 cm -1
29
p-chlorotoluene
‰ para-disubstituted
30‰
one peak: 790-840 cm-1
The aryl C_C stretching: four peaks between 1450-1600 cm -1
¾ Nitrobenzene
‰ Nitro group shows a coupled pair of IR bands at 1343 and
1519 cm-1 in the IR spectrum.
‰ The C-N stretching for the nitroaromatic is at 860 cm-1
31
C. Haloalkanes: C-X Î 500-1430cm-1 (fingerprint region)
32
Carbon Tetrachloride
Cl
Cl
C
Cl
C-Cl
Cl
33
Often used as a solvent for IR spectra.
When it is used, spectra show C-Cl absorptions.
D. Alcohols and Amines
‰ OH or NH: 3000-3700 cm-1 (strong for OH; medium or weak for NH)
‰ C_O or C_N: 900-1300 cm-1 (fingerprint region)
‰ Hydrogen bonding changes the position and appearance of an infrared
absorption band.
34
35
36
1o Amines RNH2
‰ 3000-3600cm-1(medium or weak double peaks) + C-N (900-1300 cm-1)
37
2o Amines R2NH
‰ 3000-3600cm-1(medium or weak one peak) + C-N (900-1300 cm-1)
38
3o Amines R3N
‰ No C-H peak - Only C-N peak at 900-1300 cm-1
39
40
41
E. Ethers
‰
‰
‰
42
Ether have C-O stretching band that falls in the fingerprint region at
1050-1260 cm-1
Because oxygen is electronegative, the stretching causes a large
change in bond moment, therefore C-O absorption is usually strong.
Alcohols, esters, and other compounds containing C-O single bonds
also show absorption here.
Anisole
AROMATIC ETHER
C-H
aromatic
O
CH3
benzene
C-O
43
F. Carbonyl Compounds
‰ Generally the carbonyl group gives a strong peak which occurs at
1630-1780 cm-1
‰ The exact location depends on the actual functional group present,
ring strain, conjugation, and other substituents.
44
Ketones: C=O Î 1680-1750 cm-1 (strong)
45
46
Aldehydes:
‰ Aldehyde carbonyl (C=O) shows strong absorption at 1720-1740 cm-1
‰ Also, the aldehyde C-H bond shows two characteristic stretching
bands (just to the right of the aliphatic CH band) at 2820-2900 cm-1
and 2700-2780 cm-1
‰ Both these peaks are sharp, but weak, and the peak at 2900 cm-1 may
be obscured by overlapping absorption of other CH bonds.
47
ALDEHYDE
Nonanal
CHO
CH bend
C=O
O
CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 C
48
H
Carboxylic Acid:
‰ Exhibit typical carbonyl (C=O) absorption at1700-1725 cm-1 (strong)
‰ Also shows O-H stretching Î very broad from 3330- 2500 cm-1
Strong hydrogen bonding in the dimer
weakens the OH bond and leads to a broad
peak at lower frequency
49
Esters:
‰ The C=O band is observed at 1735-1760 cm-1
‰ The C-O band is observed in the fingerprint region at 1100-1300 cm-1
50
Typical Infrared Absorption Regions
(stretching vibrations)
WAVELENGTH (µm)
2.5
4
O-H
C-H
5
6.1
C=O
C N
N-H
5.5
C C
X=C=Y
Very
few
bands
C=N
C=C
6.5
15.4
C-Cl
C-O
C-N
C-C
(C,O,N,S)
4000
2500
2000
1800 1650 1550
FREQUENCY (cm-1)
51
650
¾ Analysis of an IR Spectrum
ƒ Look for major stretching and bending peaks.
ƒ A very large peak in the 3200-3500 cm-1 range indicates the
presence of an alcohol or phenol.
ƒ Another strong peak is C=O stretching which is in the range
of 1630-1850 cm-1
ƒ Moderate bands in the 3300-3500 cm-1 range could be
indicative of N-H stretching.
ƒ A peak around 2200 cm-1 could possibly be a nitrile or alkyne
ƒ If a compound is aromatic, then aromatic peaks would
appear in the NMR spectra.
ƒ Multiple techniques can be used in identifying unknown
compounds.
52
INFRARED SPECTROSCOPY
* * * * *
ν = frequency of absorption
c = speed of light
k = force constant
m 1m
m* = reduced mass =
k
m*
υ= 1
2π c
2.5 µ
3
m* for: CH = 12/13 =0.92
CD = 24/14 =1.7
CC =144/24 = 6.0
CCl = 450/49.5 = 9.1::::725 cm-1
2
m 1 +m
2
4
5
6
7
C C
H C
3100
H
H2N
2850
2750
C C
C C
9
8
10
-1
460 N.m
1090
1630
12
C H
N H
O H
15
O
C N
1630 - 1690
2200
3300
Aromatic, C=C
1450 - 1600
~3200 - 3700
(4 peaks)
H O
H C
O
2900
20
C C (w)
C O
C N
X
1000 - 1200
H O C
680 - 900
C O
3000
3500
2500
2000
-1
cm
Frequency
Wavenumber
%T =
53
25
For alkenes:
(=CH; =CH2
bending)
690-990 weak
Y
1640 - 1820
4000
580 N.m-1
710
840
shoulder
(sh)
1600 - 1700
D C
C C
C N
C
H C
HN
H O
k for C C
more polar the bond - stronger the peak
HN
%Transmittance
* * * * *
,υ =
C
λ
,υ =
cps
or Hz
1
1
=
× 10
λ cm
λ µ
Intensity
Original Intensity
×100 =
4
cm
I
×100
IO
−1
1800
1600
1400
1200
1000
800
600
400
Fingerprint region
ketone 1715
aldehyde 1725
ester
1735
(CH bending)
X
acid: dimer
1710
monosubstituted: ~700; ~750
monomer 1760
(two peaks)
Y disubstituted:
amide: amide I band 1690-1630 (C=O)
ortho: ~750 (one peak)
amide II band 1515-1670 (NH)
para: ~830 (one peak)
acid halide, RCOCl 1785-1815
meta: ~700; 780; 880
acid anhydride 1740-1840 (usually two peaks)
(three peaks)
carboxylate
1590-1600
For CHEM 202, 303 & higher level courses ..........Designed, drawn and compiled by Prof SA ALI
END
54