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Factors affecting the frequency of infrared peaks
1. Resonance, symmetry and conjugation
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Effect of resonance, symmetry and conjugation on infrared frequencies
N
O
O
+N
O
O
+-
-
C
O
O
C
O
O
-
-
What about the effect of conjugation?
C
O
C
NR2
O -
+NR2
O O-
+
Do the facts support this interaction?
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C
O
O
C
O
O
-
-
Carbonyl frequency: 1720-1680 (acid)
1700 cm-1
Ether frequency = 1000 to 1400 cm-1
1200 cm-1
Average of the two fundamentals = 1450 cm-1
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C
O
O
C
O
O
-
-
1410 cm-11560 cm-1
(1560+ 1410)/2 = 1485 cm-1
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Wavenumbers, cm-1
5001000150020002500300035004000
% T
rans
mitt
ance
0
20
40
60
80
100
Figure IR-42. Sodium benzoate, KBr pellet:
1550 1400
Resonance
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Sodium benzoate: (1550, 1400 cm-1 ); Average : 1475 cm-1
Sodium acetate: (1560, 1410 cm-1 ); Average : 1485 cm-1
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R NR2
O
R
O-
NR2+
How important is resonance in amides?
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NCH3
CH3
CO
CH3
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R NR2
O
R
O-
NR2+
How important is resonance in amides?
In the barrier to rotation of the two
CH3 groups is approximately 18 kcal/mol
(A C-C
bond is worth about 60 kcal/mol)
H N
O
CH3
CH3
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Table 4. The effect of conjugation on carbonyl frequencies. Non-conjugated Compound
Frequency cm-1
Conjugated Compound
Frequency cm-1
Frequency cm-1
butanal 1725 2-butenal 1691 benzaldehyde 1702
2-butanone 1717 methyl vinyl ketone
1700, 1681
acetophenone 1685
propanoic acid 1715 propenoic acid 1702
benzoic acid 1688
ethyl propionate 1740 ethyl acrylate 1727
ethyl benzoate 1718
butanoic anhydride 1819, 1750
2-butenoic anhydride
1782, 1722
benzoic anhydride
1786, 1726
cis-cyclohexane- 1,2-dicarboxylic anhydride
1857, 1786
1-cyclohexene- 1,2-dicarboxylic anhydride
1844, 1767
phthalic anhydride
1852, 1762
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Wavenumbers, cm-1
5001000150020002500300035004000
% T
rans
mitt
ance
0
20
40
60
80
100
Other effects of conjugation on carbonyl frequencies
Figure IR-45. Ethyl vinyl ketone, neat liquid: CH3 CH2 COCH=CH2
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Wavenumbers, cm-1
5001000150020002500300035004000
% T
rans
mitt
ance
0
20
40
60
80
100
Figure IR-44. 3-Nonen-2-one, 95%; neat liquid, thin film:
Why the extra carbonyl peaks? CH3 CH=CH(CH2)4CH3
O
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OO
Factors affecting the intensities : Extent of interaction (dipole moment change)
Concentration of each conformer
O O-
+
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Overtones in terminal olefins
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What’s this?
1804 cm-1
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1796 cm-1
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Factors affecting the frequency of infrared peaks
1. Resonance and conjugation
2. Ring strain:
A: on carbonyl frequencies
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1720 cm-1
1750 cm-1
1775 cm-1
cyclopropanone
1800 cm-1
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Table. The Effect of Ring Strain on the Carbonyl Frequencies of Some Cyclic Molecules
Ring Size ketone: cm-1 lactone: cm-1 lactam: cm-1
3 cyclopropanone: 1800
4 cyclobutanone: 1775 -propiolactone: 1840
5 cyclopentanone: 1751 -butyrolactone: 1750 -butyrolactam: 1690
6 cyclohexanone: 1715 -valerolactone: 1740 -valerolactam: 1668
7 cycloheptanone: 1702 caprolactone: 1730 caprolactam: 1658
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Factors affecting the frequency of infrared peaks
1. Resonance and conjugation
2. Ring strain:
A: on carbonyl frequencies
B. on C-H stretching frequencies
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What is the hybridization of a C-H bond in cyclopropane?
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CH2OH
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CH2OH
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Factors affecting the frequency of infrared peaks
1. Resonance and conjugation
2. Ring strain:
A: on carbonyl frequencies
B. on C-H stretching frequencies
3. Halogens
A: on carbonyl frequencies
B. on C-H stretching frequencies
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1760
3011
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1754,1783 cm-1
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Factors affecting the frequency of infrared peaks
1. Resonance and conjugation
2. Ring strain:
A: on carbonyl frequencies
B. on C-H stretching frequencies
3. Halogens
A: on carbonyl frequencies
B. on C-H stretching frequencies
4. Chirality
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Why the big differences between D-serine and DL-serine?
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d l
d
d
l
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Some Examples of Conglomerates
Asparagine
Threonine
Glutamic Acid
Serine Anhydride
N-Acetylproline
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Factors affecting the frequency of infrared peaks
1. Resonance and conjugation
2. Ring strain:
A: on carbonyl frequencies
B. on C-H stretching frequencies
3. Halogens
A: on carbonyl frequencies
B. on C-H stretching frequencies
4. Chirality
5. Phase: solid, liquid and gas (fundamentals in the gas phase are shifted to higher frequencies) ie. solvent or solute interactions lead to weakening of force constants; effects of H-bonding.
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Wavenumbers, cm-1
1000150020002500300035004000
% T
rans
mitt
ance
0
20
40
60
80
100
120Vapor phase spectrum(135 °C)
liquid film
Figure IR-25. The liquid and vapor spectra of phenol. OH
Effects of H-bonding
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Gas Phase FT-IR spectrum of 2,4-pentanedione, Aldrich Chemical Co.
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Liquid film
Effect of Phase
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CCl4 solution
Effect of Phase
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KBr
Effect of Phase
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2000 1000 cm-1
Effects of H-bonding
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FT IR
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ATR: attenuated total reflectance; when a beam of electromagnetic radiation is reflected off an object, it actually penetrates of the order of one wavelength.
In IR, this is of the order of microns which is sufficient to obtain a spectrum of the material provided it is poly- dispersed.
The depth of penetration is wavelength dependent, it is therefore necessary to compensate for this dependency.
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2
3
4
5
6
7
8
9
10
11Si
ngle
Bea
m
1000 2000 3000 4000 Wavenumbers (cm-1)
Infrared spectrum of ATR and of the background CO2 and H2 O vapor.
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