INFRARED
SPECTROSCOPY
10m RADIATION
SPECTROSCOPY: INTERACTION OF
LIGHT AND MATTER
electronic
transitions
ionization
Interaction with nuclear
magnetic moment in a
magnetic field
molecules
rotate
bonds
vibrate
c = 
c: speed of light (m/s)
(lamda): wavelength (m)
 (nu): frequency (Hz)
energy per photon
E = h
WREK
FM91.1
h is Planck’s constant
1
Pavia 2.1-2.6
MOLECULAR VIBRATIONAL MODES
A nonlinear molecule with n atoms usually has 3n - 6 fundamental
vibrational modes.
symmetric stretching asymmetric stretching
(sym. str.)
(asym. str.)
bending
Frequency of Fundamental Vibrations
4.12
To observe a bond vibration by infrared spectroscopy there must be a
change in dipole moment upon vibration




+
–
+
–
- Polar bonds are usually IR-active
- Nonpolar bonds in a symmetrical molecule absorb weakly or not at all
e.g., R–CC–R
2
X-axis: Wavelength or wavenumber?
3
4
wavelength (nm)
5
6
7
8
9 10 12 15
20
4000-2000 and
2000-500 cm-1 on
different linear scales
Transmittance (%)
100
50
Transmittance (%)
0
4000
100
4000-500 cm-1
on linear scale
3000
2000
1500
1000
500
Spectral Database for Organic Compounds (SDBS)
2.5
50
0
4000
3000
2000
Wavenumber (cm-1)
1000
Y-axis: Transmittance or Absorbance
Percent transmittance, %T
%T = 100
It
Io
Relating absorbance (A) to %T
( )
100
%T
= 2 – log (%T)
Absorbance
A = log
3
Other vibrations
Spectra include a number of other peaks. In addition to fundamental stretching
and bending absorptions….
Overtones: lower intensity vibration at an integral times the fundamental
frequency
– overtone = n  – fundamental
Combination bands: the sum of two interacting vibrational frequencies, but only
certain combinations are allowed
–combination = –1 + –2
Difference bands: similar to combination bands, but the difference of two
interacting vibrations
– difference = –1 – – 2
Fermi resonance: when a fundamental absorption couples with an overtone or
combination band, most often observed for C=O
Selected infrared absorptions for bond stretches
Functional Group
Range cm-1
Intensity and shape
sp3C─H
2850-2960
medium to strong; sharp
sp2C─H
spC─H
3010-3190
about 3300
medium to strong; sharp
medium to strong; sharp
C═C
1620-1660
weak to medium; sharp
C≡C
2100-2260
weak to medium; sharp
N─H
O─H (very dilute)
3300-3500
about 3600
medium; broad
medium; sharp
O─H (H-bonded)
O─H (carboxylic acid)
3200-3550
2500-3000
strong; broad
medium; very broad
C─O
1050-1150
medium to strong; sharp
C═O
~1700
C≡N
2200-2260
Pavia 2.7-2.9
very strong; sharp (see later)
medium; sharp
4
Pavia
Appendix 1
Transmittance (%)
100
CC
CN
C=C
C=N
C=O
C—C
C—N
C—O
50
H—C
H—N
H—O
0
4000
3000
2000
1500
1000
500
Wavenumber (cm-1)
5
Textbook
organization
Lecture/Notes
organization
Hydrocarbons Pavia 2.10
Arenes Pavia 2.11
Alcohols and Phenols Pavia 2.12
Ethers Pavia 2.13
Carbonyl compounds Pavia 2.14
Amines Pavia 2.15
Nitriles Pavia 2.16
more…. Pavia 2.17-21
C-H and C-C bonds
O-H and N-H bonds
C-O bonds
C=O bonds
C-N, C=N, CN bonds
Problems
C-H and C-C bonds
alkanes
alkenes
alkynes
arenes
6
Pavia 2.10
C-H, C–C, C=C and CC Bonds: Alkanes,
Alkenes and Alkynes
Transmittance (%)
100
Spectral Database for Organic Compounds (SDBS)
hexane
“long chain band”
CH3 C–H bend
50
CH2 C–H bend
C–H str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
1000
500
CH2 bond vibrations
asymmetric
stretch
~2929 cm-1
scissoring
~1450 cm-1
twisting
~1250 cm-1
symmetric
stretch
~2853 cm-1
rocking
720 cm-1
wagging
~1250 cm-1
7
Transmittance (%)
100
50
CH2 C–H bend
C–H str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
1000
500
Spectral Database for Organic Compounds (SDBS)
cyclohexane
Transmittance (%)
100
overtone
of 910 cm-1
50
3079 cm-1
sp2 C–H str.
0
4000
1627 cm-1
C=C str.
3000
2000
1500
Wavenumber (cm-1)
Spectral Database for Organic Compounds (SDBS)
1-hexene
out of plane (oop)
=C–H bend.
1000
500
8
Transmittance (%)
100
50
0
4000
cis =C–H
oop bend
1695/1666 cm-1
C=C str.
3008 cm-1
sp2 C–H str.
3000
2000
1500
Wavenumber (cm-1)
Spectral Database for Organic Compounds (SDBS)
cis-3-hexene
1000
500
Transmittance (%)
100
~3028 cm-1
sp2 C–H str.
50
Spectral Database for Organic Compounds (SDBS)
trans-3-hexene
trans =C–H
oop bend
0
4000
3000
2000
1500
Wavenumber (cm-1)
1000
500
9
Pavia
Table 2.22
Characteristic C-H oop bending peaks for alkenes
Transmittance (%)
100
50
2120 cm-1
CC str.
3315 cm-1
sp2 C–H str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
Spectral Database for Organic Compounds (SDBS)
1-octyne
≡C–H bend
1000
500
10
Transmittance (%)
100
50
0
4000
3000
2000
1500
Wavenumber (cm-1)
1000
500
Spectral Database for Organic Compounds (SDBS)
4-octyne
Effect of Hybridization on C–H stretching
~2900
~3100
~3300 cm -1
C s character , – , peak 
11
Effect of force constant on carbon-carbon bond stretching
often weak
~2150 cm -1
~1630
fingerprint region
CC bond order , – , peak 
Effect of Alkyl Substituents on C=C stretching of alkenes
RCH=CH2
R2C=CH2
~1640
~1650
RCH=CHR
R2C=CHR R2C=CR2
cis ~1658 trans ~1670
–
# substituents ,  , peak 
~1670 cm-1
what does this imply about bond strength?
Relating this effect to something you have seen before…heats of hydrogenation
C4H8 + H2
Pt
or Pd
DHo
<0
C4H10
+ H2
DHo = -127
kJ/mol
DHo = -120
kJ/mol
+ H2
+ H2
DHo = -115
kJ/mol
12
+H2
+H2
+H2
DH
Relative Stability
–
# substituents , C=C bond strength ,  , peak 
Hyperconjugation accounts for the enhanced stability of cations, radicals and
alkenes with higher degrees of substitution.
H
C
C
H
carbocation
Relative
Stability
Stability
explains
H
H
H
C
C
C
H
radical
more subst. > less subst.
3° > 2° > 1° > Me
mass spectra
fragmentation patterns
C
H
C
H
H
alkene
more subst. > less subst.
trans > cis
C=C bond
stretching frequency
13
Effect of Resonance on C=C stretching of alkenes
~1640 cm-1
~1600 cm-1
- Electron donating groups
- Electron withdrawing groups
Pavia 2.11
Aromatic Molecules
Transmittance (%)
100
50
aromatic
C–H str.
0
4000
aromatic
C–H
out-ofplane
(oop)
bending
aromatic
CC str.
3000
2000
1500
Wavenumber (cm-1)
1000
500
Spectral Database for Organic Compounds (SDBS)
benzene
14
Transmittance (%)
100
50
Aromatic
C–H str.
0
4000
3000
aromatic
C–H
out-ofplane
(oop)
bending
CC str.
2000
1500
Wavenumber (cm-1)
1000
500
Spectral Database for Organic Compounds (SDBS)
toluene
Transmittance (%)
100
50
aromatic
C–H
out-ofplane
(oop)
bending
Aromatic
C–H str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
1000
500
Spectral Database for Organic Compounds (SDBS)
p-xylene
15
Practice: three isomers of C10H14
Overtones
C-H out-of-plane bending
1,2,4
4000
3000
2000
1500
1000
500
Mono
Meta
4000
3000
2000
1500
1000
500
4000
3000
2000
1500
1000
500
O-H and N-H bonds
alcohols
phenols
carboxylic acids
amines
amides
16
Pavia 2.12,
2.14D, 2.15
O-H and N-H bonds: Alcohols, Carboxylic Acids,
Amines and Amides
Transmittance (%)
100
50
C–O str.
~3300 cm-1
broad
O–H str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
1000
500
Spectral Database for Organic Compounds (SDBS)
1-hexanol
Spectral Database for Organic Compounds (SDBS)
The effect of hydrogen bonding on O-H str.: Phenols
See Chapter II of Spectrometric
Identification of Organic
Compounds 7th Ed.
4000
3000
2000
1500
1000
500 cm-1
17
Transmittance (%)
100
Spectral Database for Organic Compounds (SDBS)
1-hexanoic acid
O–H oop bend
50
3500-2500 cm-1
very broad
O–H str.
0
4000
3000
C=O
2000
1500
Wavenumber (cm-1)
1000
500
Carboxylic acid are present as H-bonded
dimers when neat or in concentrated solution
Primary and Secondary Amines
hexylamine
2 peaks
sym, asym
N–H str.
N-ethylbutylamine
Spectral Database for Organic Compounds (SDBS)
1 peak
N–H str.
N,N-dimethylbutylamine
4000
3000
2000
1500
1000
500 cm-1
18
Primary, Secondary and Tertiary Amides
propionamide
2 peaks
sym, asym
N–H str.
N-methylpropionamide
C=O
Spectral Database for Organic Compounds (SDBS)
1 peak
N–H str.
N,N-dimethylpriopionamide
4000
3000
2000
1500
1000
500 cm-1
Summary: O–H, N–H stretches
O–H of alcohols and N–H of amine both have stretching vibrations
at ~3300 cm-1.
- Alcohol O-H stretching peaks are broad with rounded tip
- Secondary amines (R2NH) are often broad with a sharp spike.
- Primary amine (RNH2) are broad with two sharp spikes
- H-bonding important to appearance
The O–H of carboxylic acids appears as a very broad peak, between 3800 and
2600 cm-1, often with a W-shape.
19
C-O bonds
alcohols
carboxylic acids
esters
Pavia 2.12,
2.13
C–O bonds: Alcohols and Ethers
Spectral Database for Organic Compounds (SDBS)
butyl methyl ether
C–O asym., sym.
str.
O–H
C–O str.
4000
3000
2000
1500
1000
1-hexanol
500 cm-1
20
Silverstein
Table 2.5
Silverstein
Table 2.5
4000
3000
2000
1500
1000
500 cm-1
Effect of Resonance on C–O str.
~1120 cm-1
~1220 cm-1
21
C–O bonds: Esters and Carboxylic Acids
hexanoic acid
O–H
C=O
C–O str.
Spectral Database for Organic Compounds (SDBS)
C–O str.
methyl propionate
C=O bonds
Aldehydes
Ketones
Carboxylic acids
Carboxylic acid derivatives
(anyhydrides, acyl chlorides, esters, amides)
C-N, C=N, CN bonds
22
Pavia 2.14B-C
C=O Bonds: Aldehydes and Ketones
Transmittance (%)
100
50
C=O str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
R–C(O)–R
bend
1000
500
1000
500
Spectral Database for Organic Compounds (SDBS)
2-hexanone
Transmittance (%)
100
C(O)–H
str.
50
C=O str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
Spectral Database for Organic Compounds (SDBS)
hexanal
23
REVIEW RESONANCE, NOW!
Review resonance in your undergraduate organic textbook; work problems!
24
Aldehydes versus ketones
~1715 cm-1
~1725 cm-1
[and R-C(=O)-R bend, 1150 cm-1]
[and C(=O)-H str, 2690-2840 cm-1]
Effect of conjugation in aldehydes and ketones
1718 cm-1
1707 cm-1
1686 cm-1
Pavia 2.14D-H
C=O Bonds: Carboxylic Acids and Derivatives
Transmittance (%)
100
O–H
C–O
50
C=O str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
1000
500
Spectral Database for Organic Compounds (SDBS)
hexanoic acid
Carboxylic acid are present as H-bonded
dimers when neat or in concentrated solution
25
Transmittance (%)
100
50
C=O str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
C–O str.
1000
500
Spectral Database for Organic Compounds (SDBS)
methyl propionate
Transmittance (%)
100
50
C=O sym., asym. str.
0
4000
3000
2000
1500
Wavenumber (cm-1)
C–O str.
1000
500
Spectral Database for Organic Compounds (SDBS)
propionic anhydride
26
C=O str. frequencies of acid derivatives
Based on these frequencies, which of the following has the strongest bond? Why?
1815-1785 cm-1
1750-1735 cm-1
1695-1690 cm-1
C=O str.: Effect of resonance of C=O of esters…
…conjugation with C=O
1748
1732
1724 cm -1
27
…conjugation with –O–
1748
1765 cm -1
1762
C=O str.: Effect of resonance and ring size on C=O str. frequency
of lactones
1720
1750
1735
1770
1760 cm -1
1800 cm -1
28
Summary of C=O str.
The C=O str. Is often the strongest peak in the IR spectrum.
Aldehydes and ketones can be distinguished by the presence of a C(=O)–H str.
(aldehydes) and a R–C(=O)–R bend (ketones).
Conjugation of C=O of lowers the stretching frequency (corresponding to a
weaker bond.)
C=O str. frequencies of carboxylic acid derivatives can be rationalized in terms
of the electronic effects of the substituent.
Pavia 2.24F,
15, 16
Carbon – Nitrogen Stretching Vibrations
C–N absorbs around 1200 cm -1.
C=N absorbs around 1660 cm -1 and is much stronger than the C=C absorption
in the same region.
CN str.
Spectral Database for Organic Compounds (SDBS)
CN absorbs strongly just above 2200 cm-1. The alkyne C  C signal is much
weaker and is just below 2200 cm-1 .
1-heptanitrile
29
PRACTICAL CONSIDERATIONS
Sample
- in solution
- must subtract the “background” (peaks arising from the
solvent)
- need matching “sample” (sample + solvent)
and “standard” (solvent) cells
- a thin film on NaCl plates
Evaporate a drop
of solution to give
a solid film
Liquid between two plates
- in a “mull” – a mixture of a solid in an oil
must ignore the peaks of the oil (e.g., Nujol = a hydrocarbon)
- in a KBr pellet
- ATR (“Attenuated Total Reflection”) – solid sample
30
Wet sample: H–O str. of H2O
CO2 and H2O in atmosphere not cancelled out by reference beam:
Reporting IR Spectra in Papers and Progress Reports
A
B
C
D
IR (neat): 3100 (aromatic C–H str.), 2218 (CC str.), 1725 (C=O str.) cm -1.
A Form of the sample (e.g., neat, nujol mull, CCl4 solution) or method used (e.g., ATIR).
B Only list peaks that can be assigned to bonds in the structure you are describing.
You do not need to list all the other peaks that might be present in the compound
C Assign the peaks to specific bonds, and mode of vibration.
D Units!
31
Important infrared adsorptions
COOH
N-H
CC
CN
3100
sp2C–H
1600 C=C
3500, O-H
1000-1200 C–O
2900 sp3C–H
4000
1700
Wavenumber / cm-1
600
32
A
B
C
D
33
A
B
C
D
34
E
A
B
35
C
D
A
B
36
C
D
E
37