Download Figure IR-13.

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
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
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
In the condensed phase collision broadening also obcures the n=1, J =0 null
A summary of the principle infrared bands and their
assignments. R is an aliphatic group.
Funct. Type
Group
C-H sp3 hybridized
sp2 hybridized
sp hybridized
aldehyde C-H
N-H primary amine, amide
secondary amine, amide
tertiary amine, amide
O-H alcohols, phenols
CN
CC
carboxylic acids
nitriles
acetylenes
R3C-H
=CR-H
C-H
H-(C=O)R
RN-H2, RCONH2
RNR-H, RCONHR
RN(R3), RCONR2
free O-H
hydrogen bonded
R(C=O)O-H
RCN
R-CC-R
R-CC-H
Frequencies
cm-1
2850-3000
3000-3250
3300
2750, 2850
3300, 3340
3300-3500
none
3620-3580
3600-3650
3500-2400
2280-2200
2260-2180
2160-2100
Peak
Intensity
M(sh)
M(sh)
M-S(sh)
M(sh)
S,S(br)
S(br)
W(sh)
S(br)
S(br)
S(sh)
W(sh)
M(sh)
Examples
Figure No.
6, 18, 22
7, 13, 42
13
14, 15
18, 19
20, 21
22, 23
17, 24, 25
24, 25, 28
26, 27, 29, 30
31
32
13
sp3 hybridized CH
100
% Transmittance
80
60
(CH2)4
rock
40
CH3, CH2 bending
vibrations
CH3, CH2 symmetric
and asymmetric stretches
20
0
4000
3500
3000
2500
2000
Wavenumbers, cm-1
Figure IR-6. N-Decane, neat liquid, thin film:
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
1500
1000
500
sp3 hybridized CH
100
90
70
100
60
50
40
30
20
10
NH2 bend
90
80
% Transmittance
% Transmittance
80
symmetric
and
70
asymmetric NH2
60
stretches
NH2 bend
50
symmetric and
asymmetric NH2
stretches
40
30
4000 20
3500
3000
10
4000
2500
2000
1500
1000
500
Wavenumbers, cm-1
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-18. Butylamine, neat liquid; thin film:
CH3CH2CH2CH2NH2
sp3 hybridized CH
100
% Transmittance
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-22. Triethylamine, neat liquid; thin film: (CH3CH2)3N
A summary of the principle infrared bands and their
assignments. R is an aliphatic group.
Funct. Type
Group
C-H sp3 hybridized
sp2 hybridized
sp hybridized
aldehyde C-H
N-H primary amine, amide
secondary amine, amide
tertiary amine, amide
O-H alcohols, phenols
CN
CC
carboxylic acids
nitriles
acetylenes
R3C-H
=CR-H
C-H
H-(C=O)R
RN-H2, RCONH2
RNR-H, RCONHR
RN(R3), RCONR2
free O-H
hydrogen bonded
R(C=O)O-H
RCN
R-CC-R
R-CC-H
Frequencies
cm-1
2850-3000
3000-3250
3300
2750, 2850
3300, 3340
3300-3500
none
3620-3580
3600-3650
3500-2400
2280-2200
2260-2180
2160-2100
Peak
Intensity
M(sh)
M(sh)
M-S(sh)
M(sh)
S,S(br)
S(br)
W(sh)
S(br)
S(br)
S(sh)
W(sh)
M(sh)
Examples
Figure No.
6, 18, 22
7, 13, 42
13
14, 15
18, 19
20, 21
22, 23
17, 24, 25
24, 25, 28
26, 27, 29, 30
31
32
13
sp2 hybridized CH
100
% Transmittance
80
60
1410
40
1020
860
20
0
4000
3500
3000
2500
2000
Wavenumbers, cm-1
Figure IR-8. Indene; neat; 0.05 mm cell:
1500
1000
500
sp and sp2 hybridized CH
100
% Transmittance
80
60
carbon-carbon
triple bond stretch
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-13. Phenylacetylene, neat liquid; thin
film (note that R here is an aromatic group):
H
sp2 hybridized CH
100
% Transmittance
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-42. Sodium benzoate, KBr pellet:
CO2Na
Can the frequency dependence on hybridization be rationalized?
Table 2.
Carbon Hydrogen Bond Strengths as a Function of Hybridization
Type of C-H bond
sp3 hybridized C-H
sp2 hybridized C-H
sp hybridized C-H
CH3CH2CH2-H
CH2=CH-H
HCC-H
h
E =
2
Bond Strength
kcal/mol
99
108
128
IR Frequency
cm-1
<3000
>3000
3300
k
1
(n  )

2
The bond strength seems dependent of the amount of s
character in the bond
1
Potential Energy
k1 >k2
k2
0
k1
0
1
2
Internuclear separation
3
What would happen to
k (force constant ) if
stretching a bond
resulted in severe
steric interactions?
Potential Energy
1
0
0
1
2
Internuclear separation
3
Me
Me
Me
C
C
H
C
Me
Me
Me
Me
Me
Me
Hindered sp3 C-H
110
% Transmission
100
90
3017.18
80
1478.5
70
2952.5
ATR of tri-t-butylmethane
60
-4000
-3500
-3000
-2500
-2000
Wavenumber cm-1
-1500
-1000
-500
A summary of the principle infrared bands and their
assignments. R is an aliphatic group.
Funct. Type
Group
C-H sp3 hybridized
sp2 hybridized
sp hybridized
aldehyde C-H
N-H primary amine, amide
secondary amine, amide
tertiary amine, amide
O-H alcohols, phenols
CN
CC
carboxylic acids
nitriles
acetylenes
R3C-H
=CR-H
C-H
H-(C=O)R
RN-H2, RCONH2
RNR-H, RCONHR
RN(R3), RCONR2
free O-H
hydrogen bonded
R(C=O)O-H
RCN
R-CC-R
R-CC-H
Frequencies
cm-1
2850-3000
3000-3250
3300
2750, 2850
3300, 3340
3300-3500
none
3620-3580
3600-3650
3500-2400
2280-2200
2260-2180
2160-2100
Peak
Intensity
M(sh)
M(sh)
M-S(sh)
M(sh)
S,S(br)
S(br)
W(sh)
S(br)
S(br)
S(sh)
W(sh)
M(sh)
Examples
Figure No.
6, 18, 22
7, 13, 42
13
14, 15
18, 19
20, 21
22, 23
17, 24, 25
24, 25, 28
26, 27, 29, 30
31
32
13
aldehyde C-H
100
90
% Transmittance
80
70
60
50
H-C=O
Impurity peak,
probably O-H
40
30
20
10
4000
3500
3000
2500
2000
1500
1000
Wavenumbers, cm-1
Figure IR-14. Butanal, neat liquid, thin film:
CH3CH2CH2CHO
500
aldehyde C-H
100
% Transmittance
80
60
40
C-H stretch of -CHO
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
O
CH
Figure IR-15. Benzaldehyde, neat, thin film
(R here is an aromatic group):
Focusing on –NH2
A summary of the principle infrared bands and their
assignments. R is an aliphatic group.
Funct. Type
Group
C-H sp3 hybridized
sp2 hybridized
sp hybridized
aldehyde C-H
N-H primary amine, amide
secondary amine, amide
tertiary amine, amide
O-H alcohols, phenols
CN
CC
carboxylic acids
nitriles
acetylenes
R3C-H
=CR-H
C-H
H-(C=O)R
RN-H2, RCONH2
RNR-H, RCONHR
RN(R3), RCONR2
free O-H
hydrogen bonded
R(C=O)O-H
RCN
R-CC-R
R-CC-H
Frequencies
cm-1
2850-3000
3000-3250
3300
2750, 2850
3300, 3340
3300-3500
none
3620-3580
3600-3650
3500-2400
2280-2200
2260-2180
2160-2100
Peak
Intensity
M(sh)
M(sh)
M-S(sh)
M(sh)
S,S(br)
S(br)
W(sh)
S(br)
S(br)
S(sh)
W(sh)
M(sh)
Examples
Figure No.
6, 18, 22
7, 13, 42
13
14, 15
18, 19
20, 21
22, 23
17, 24, 25
24, 25, 28
26, 27, 29, 30
31
32
13
NH2- Stretching Frequencies
90
100
80
90
70
60
50
40
30
20
80
% Transmittance
% Transmittance
100
70
NH bend
2
NH2 bend
60
50
symmetric
40
symmetric
and and
asymmetric NH2
asymmetric NH2
30
stretches
stretches
20
10
10
4000
4000
3500
3500
3000
3000
2500
2000
1500
Wavenumbers,
cm-1
2500
2000
1000
1500
500
1000
500
Wavenumbers, cm-1
Figure IR-18. Butylamine, neat liquid; thin film:
CH3CH2CH2CH2NH2
NH2- Stretching Frequencies
120
% Transmittance
100
80
60
40
symmetric and
asymmetric NH2
stretches
20
0
4000
3500
3000
2500
2000
1500
1000
Wavenumber, cm-1
Figure IR-19. Benzamide, KBr pellet
(R here is an aromatic group):
O
CNH2
500
Focusing on –NHR
A summary of the principle infrared bands and their
assignments. R is an aliphatic group.
Funct. Type
Group
C-H sp3 hybridized
sp2 hybridized
sp hybridized
aldehyde C-H
N-H primary amine, amide
secondary amine, amide
tertiary amine, amide
O-H alcohols, phenols
CN
CC
carboxylic acids
nitriles
acetylenes
R3C-H
=CR-H
C-H
H-(C=O)R
RN-H2, RCONH2
RNR-H, RCONHR
RN(R3), RCONR2
free O-H
hydrogen bonded
R(C=O)O-H
RCN
R-CC-R
R-CC-H
Frequencies
cm-1
2850-3000
3000-3250
3300
2750, 2850
3300, 3340
3300-3500
none
3620-3580
3600-3650
3500-2400
2280-2200
2260-2180
2160-2100
Peak
Intensity
M(sh)
M(sh)
M-S(sh)
M(sh)
S,S(br)
S(br)
W(sh)
S(br)
S(br)
S(sh)
W(sh)
M(sh)
Examples
Figure No.
6, 18, 22
7, 13, 42
13
14, 15
18, 19
20, 21
22, 23
17, 24, 25
24, 25, 28
26, 27, 29, 30
31
32
13
NH- Stretching Frequencies
120
% Transmittance
100
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
Wavenumbers, cm-1
Figure IR-20. Diethylamine, neat liquid; thin film:
CH3CH2NHCH2CH3
500
NH- Stretching Frequencies
100
90
% Transmittance
80
70
60
50
40
30
Amide I
20
Amide II
10
4000
3500
3000
2500
2000
1500
1000
500
Wavnumbers, cm-1
Figure IR-21. N-Methyl acetamide, neat liquid; thin film:
CH3CONHCH3
Focusing on –NR2
A summary of the principle infrared bands and their
assignments. R is an aliphatic group.
Funct. Type
Group
C-H sp3 hybridized
sp2 hybridized
sp hybridized
aldehyde C-H
N-H primary amine, amide
secondary amine, amide
tertiary amine, amide
O-H alcohols, phenols
CN
CC
carboxylic acids
nitriles
acetylenes
R3C-H
=CR-H
C-H
H-(C=O)R
RN-H2, RCONH2
RNR-H, RCONHR
RN(R3), RCONR2
free O-H
hydrogen bonded
R(C=O)O-H
RCN
R-CC-R
R-CC-H
Frequencies
cm-1
2850-3000
3000-3250
3300
2750, 2850
3300, 3340
3300-3500
none
3620-3580
3600-3650
3500-2400
2280-2200
2260-2180
2160-2100
Peak
Intensity
M(sh)
M(sh)
M-S(sh)
M(sh)
S,S(br)
S(br)
W(sh)
S(br)
S(br)
S(sh)
W(sh)
M(sh)
Examples
Figure No.
6, 18, 22
7, 13, 42
13
14, 15
18, 19
20, 21
22, 23
17, 24, 25
24, 25, 28
26, 27, 29, 30
31
32
13
Tertiary Amines
100
% Transmittance
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-22. Triethylamine, neat liquid; thin film:
(CH3CH2)3N
Tertiary Amides
100
% Transmittance
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-23. N,N-Dimethylacetamide, neat liquid; thin
film: CH3CON(CH3)2
A summary of the principle infrared bands and their
assignments. R is an aliphatic group.
Funct. Type
Group
C-H sp3 hybridized
sp2 hybridized
sp hybridized
aldehyde C-H
N-H primary amine, amide
secondary amine, amide
tertiary amine, amide
O-H alcohols, phenols
CN
CC
carboxylic acids
nitriles
acetylenes
R3C-H
=CR-H
C-H
H-(C=O)R
RN-H2, RCONH2
RNR-H, RCONHR
RN(R3), RCONR2
free O-H
hydrogen bonded
R(C=O)O-H
RCN
R-CC-R
R-CC-H
Frequencies
cm-1
2850-3000
3000-3250
3300
2750, 2850
3300, 3340
3300-3500
none
3620-3580
3600-3650
3500-2400
2280-2200
2260-2180
2160-2100
Peak
Intensity
M(sh)
M(sh)
M-S(sh)
M(sh)
S,S(br)
S(br)
W(sh)
S(br)
S(br)
S(sh)
W(sh)
M(sh)
Examples
Figure No.
6, 18, 22
7, 13, 42
13
14, 15
18, 19
20, 21
22, 23
17, 24, 25
24, 25, 28
26, 27, 29, 30
31
32
13
Free and Hydrogen Bonded H-O-R
100
vapor 102 °C
% Transmittance
80
liquid film
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-24. The liquid and vapor spectra of n-hexanol.
Free and Hydrogen Bonded H-O-R
120
Vapor phase spectrum(135 °C)
% Transmittance
100
80
liquid film
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
Wavenumbers, cm-1
Figure IR-25. The liquid and vapor spectra of phenol.
OH
Free and Hydrogen Bonded H-O-R
120
% Transmittance
100
vapor: 134 °C
80
60
40
liquid film
20
0
4000
3500
3000
2500
2000
1500
1000
Wavenumber, cm-1
Figure IR-26. The liquid and vapor spectra of hexanoic acid:
CH3(CH2)4 CO2H
A summary of the principle infrared bands and their
assignments. R is an aliphatic group.
Funct. Type
Group
C-H sp3 hybridized
sp2 hybridized
sp hybridized
aldehyde C-H
N-H primary amine, amide
secondary amine, amide
tertiary amine, amide
O-H alcohols, phenols
CN
CC
carboxylic acids
nitriles
acetylenes
R3C-H
=CR-H
C-H
H-(C=O)R
RN-H2, RCONH2
RNR-H, RCONHR
RN(R3), RCONR2
free O-H
hydrogen bonded
R(C=O)O-H
RCN
R-CC-R
R-CC-H
Frequencies
cm-1
2850-3000
3000-3250
3300
2750, 2850
3300, 3340
3300-3500
none
3620-3580
3600-3650
3500-2400
2280-2200
2260-2180
2160-2100
Peak
Intensity
M(sh)
M(sh)
M-S(sh)
M(sh)
S,S(br)
S(br)
W(sh)
S(br)
S(br)
S(sh)
W(sh)
M(sh)
Examples
Figure No.
6, 18, 22
7, 13, 42
13
14, 15
18, 19
20, 21
22, 23
17, 24, 25
24, 25, 28
26, 27, 29, 30
31
32
13
Hydrogen Bonded H-O-R
100
% Transmittance
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-27. Decanoic acid, neat liquid, thin film: CH3(CH2)8CO2H
Hydrogen Bonded H-O-R
120
% Transmittance
100
80
60
40
20
symmetric and asymmetric
-NO2 stretch
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-28. 4-Chloro-2-nitrophenol, KBr pellet:
Cl
NO2
OH
Hydrogen Bonded H-O-R
100
% Transmittance
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
Wavenumber cm-1
Figure IR-29. Benzoic acid; KBr disk
COOH
500
100
90
% Transmittance
80
70
60
50
40
30
20
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumbers, cm
Figure IR-30. Benzoic acid, KBr; band distortions and broadening
caused by poor grinding, compare to Figure 29.
When does one see a free –OH in the condensed phase?
Free H-O-R
100
90
% Transmittance
80
70
60
50
40
30
20
10
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Me
Me
Figure IR-17. Tri-t-butylmethanol, KBr pellet:
Me
C
C
OH
C
Me
Me
Me
Me
Me
Me
The –CN triple bond
A summary of the principle infrared bands and their
assignments. R is an aliphatic group.
Funct. Type
Group
C-H sp3 hybridized
sp2 hybridized
sp hybridized
aldehyde C-H
N-H primary amine, amide
secondary amine, amide
tertiary amine, amide
O-H alcohols, phenols
CN
CC
carboxylic acids
nitriles
acetylenes
R3C-H
=CR-H
C-H
H-(C=O)R
RN-H2, RCONH2
RNR-H, RCONHR
RN(R3), RCONR2
free O-H
hydrogen bonded
R(C=O)O-H
RCN
R-CC-R
R-CC-H
Frequencies
cm-1
2850-3000
3000-3250
3300
2750, 2850
3300, 3340
3300-3500
none
3620-3580
3600-3650
3500-2400
2280-2200
2260-2180
2160-2100
Peak
Intensity
M(sh)
M(sh)
M-S(sh)
M(sh)
S,S(br)
S(br)
W(sh)
S(br)
S(br)
S(sh)
W(sh)
M(sh)
Examples
Figure No.
6, 18, 22
7, 13, 42
13
14, 15
18, 19
20, 21
22, 23
17, 24, 25
24, 25, 28
26, 27, 29, 30
31
32
13
100
% Transmittance
80
60
-CN 
40
20
Top: CHCl3 vs CHCl3
Bottom: 5% in CHCl3
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-31. trans-2-Phenyl-1-cyanoethene, in CHCl3 solution:
Ph-CH=CH-CN
ClCH2CN
The C-C triple bond in terminal acetylenes
CC Triple Bond Stretch
100
% Transmittance
80

60
carbon-carbon
triple bond stretch
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-13. Phenylacetylene, neat liquid;
thin film:
H
2120 cm-1
The CC triple bond in symmetrical acetylenes
CC Triple Bond Stretch
Where is it?
100
% Transmittance
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers, cm-1
Figure IR-32. Diethyl acetylenedicarboxylate; neat liquid:
C2H5OCO-CC-CO2CH2CH3
The CC triple bond in internal acetylenes
The CC triple bond when next to a heteroatom
Spectrum in hexanes (50%)