Spectroscopic Techniques in Chemistry.ppt

Spectroscopy

1. Spectroscopy
Infrared Spectroscopy

2. 114/03/10 2
Introduction
Spectroscopy is an analytical technique
which helps determine structure
It destroys little or no sample
The amount of light absorbed by the sample
is measured as wavelength is varied

3. 114/03/10 3
Electromagnetic Spectrum
Examples: X rays, microwaves, radio waves,
visible light, IR, and UV
Frequency and wavelength are inversely
proportional
c = , where c is the speed of light
Energy per photon = h, where h is Planck’s
constant

4. 114/03/10 4
X-ray
uv visible Vibrational IR
IRMicro
wave
Radio
NMR
Frequency
Energy
Low
Low
High
High
200 nm 400 nm 800 nm 2.5  15  1m 5m
The Electromagnetic Spectrum
UV

5. 114/03/10 5
Table 1. Types of energy transitions of
the electromagnetic spectrum
Region of spectrumEnergy transition
X-rays Bond breaking
UV/Visible Electronic
IR Vibrational
Microwave Rotational
RadiofrequenciesNuclear spin (NMR)
Electron spin (ESR)
Energy Transitions

6. 114/03/10 6
The Spectrum and Molecular Effects
=>

7. 114/03/10 7
The IR Region
Just below red in the visible region
Wavelengths usually 2.5-25 m
More common units are wavenumbers, or cm
-1
, the
reciprocal of the wavelength in centimeters (4000 -
400 cm
-1
)
Wavenumbers are proportional to frequency
and energy

8. 114/03/10 8
Molecular Vibrations
Light is absorbed when radiation frequency =
frequency of vibration in molecule
Covalent bonds vibrate at only certain allowable
frequencies
Associated with types of bonds and movement of
atoms
Vibrations include stretching and bending

9. 114/03/10 9
IR Spectrum
No two molecules will give exactly the same IR
spectrum (except enantiomers)
Simple stretching: 1600-3500 cm
-1
Complex vibrations: 400-1400 cm
-1
, called the
“fingerprint region”
Baseline
Absorbance/
Peak

10. 114/03/10 10
Interpretation
Looking for presence/absence of functional
groups
Correlation tables
A polar bond is usually IR-active
A nonpolar bond in a symmetrical molecule
will absorb weakly or not at all

11. 114/03/10 11
Carbon-Carbon Bond Stretching
Stronger bonds absorb at higher frequencies:
C-C 1200 cm
-1
C=C 1660 cm
-1
CC 2200 cm
-1
(weak or absent if internal)
Conjugation lowers the frequency:
isolated C=C 1640-1680 cm
-1
conjugated C=C 1620-1640 cm
-1
aromatic C=C approx. 1600 cm
-1

12. 114/03/10 12
Carbon-Hydrogen Stretching
Bonds with more s character absorb at a
higher frequency
sp
3
C-H, just below 3000 cm
-1
(to the right)
sp
2
C-H, just above 3000 cm
-1
(to the left)
sp C-H, at 3300 cm
-1

13. 114/03/10 13
An Alkane IR Spectrum

14. 114/03/10 14
An Alkene IR Spectrum

15. 114/03/10 15
An Alkyne IR Spectrum

16. 114/03/10 16
O-H and N-H Stretching
Both of these occur around 3300 cm
-1
, but
they look different
Alcohol O-H, broad with rounded tip
Secondary amine (R
2NH), broad with one
sharp spike
Primary amine (RNH
2), broad with two sharp
spikes
No signal for a tertiary amine (R
3
N)

17. 114/03/10 17
An Alcohol IR Spectrum

18. 114/03/10 18
An Amine IR Spectrum

19. 114/03/10 19
Carbonyl Stretching
The C=O bond of simple ketones, aldehydes,
and carboxylic acids absorb around 1710 cm
-1
Usually, it’s the strongest IR signal
Carboxylic acids will have O-H also
Aldehydes have two C-H signals around 2700
and 2800 cm
-1

20. 114/03/10 20
A Ketone IR Spectrum

21. 114/03/10 21
An Aldehyde IR Spectrum

22. 114/03/10 22
O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cm
-1
,
due to strong hydrogen bonding

23. 114/03/10 23
Variations in C=O Absorption
Conjugation of C=O with C=C lowers the
stretching frequency to ~1680 cm
-1
The C=O group of an amide absorbs at an even
lower frequency, 1640-1680 cm
-1
The C=O of an ester absorbs at a higher
frequency, ~1730-1740 cm
-1
Carbonyl groups in small rings (5 C’s or less)
absorb at an even higher frequency

24. 114/03/10 24
An Amide IR Spectrum

25. 114/03/10 25
Carbon - Nitrogen Stretching
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 absorbs strongly just above 2200 cm
-1
.
The alkyne C  C signal is much weaker and
is just below 2200 cm
-1

26. 114/03/10 26
A Nitrile IR Spectrum

27. 114/03/10 27
Summary of IR Absorptions

28. 114/03/10 28
Strengths and Limitations
IR alone cannot determine a structure
Some signals may be ambiguous
The functional group is usually indicated
The absence of a signal is definite proof that
the functional group is absent
Correspondence with a known sample’s IR
spectrum confirms the identity of the compound

29. 114/03/10 29

Region wavelength wavenumber frequency
(m) (cm
-1
) (Hz)
Near 0.78 ~ 2.5 12800 ~ 40003.8 x 10
14
~ 1.2 x 10
14
Middle 2.5 ~ 50 4000 ~ 200 1.2 x 10
14
~ 6.0 x 10
12
Far 50 ~ 1000 200 ~ 10 6.0 x 10
12
~ 3.0 x 10
11
Most used2.5 ~ 15 4000 ~ 670 1.2 x 10
14
~ 2.0 x 10
13

Infrared Regions

30. 114/03/10 30
Near IR: 770 ~ 2500 nm
1. Using photometers or spectrophotometer
similar to UV spectrometry
2. Application: routine quantitative determination
of species, e.g. moisture, proteins, CHs, fats in
agricultural, food, and chemical industries.
IR Application

31. 114/03/10 31
Mid-IR:
1.Used largely for qualitative organic analysis
and structural determination based on
absorption spectra.
2. FT-IR has been used for quantitative analysis
of complex gaseous, liquid, or solid mixtures.
Far-IR:
Used for qualitative analysis of pure inorganic
or metal organic species.
IR Application

32. 114/03/10 32
IR Sources
An inert solid heated btw 1500 ~ 2000k, The
max radiation ~ 5900 cm
-1
(1.7 M); at long
wavelength, 1% of max ~ 670 cm
-1
(15 M).
Types of Sources
►The Nernst Glower: rare earth oxides,
negative temp coeff of resistance, heated to
red heat
► The Globar Source: a silicon carbide rod,
positive temp coeff, greater output at > 5 m.

33. 114/03/10 33
Types of Instruments for IR
Dispersive grating photometers: used for
qualitative work
Fourier transform photometers: used for both
qualitative and quantitative works; speedy,
reliable, and convenient
Nondispersive photometers: for quantitative
use

34. IR Absorption Region
114/03/10 34
λ= 2.5 ~ 15 m (10
-4
cm = 1 m = 10
4
A
o
).
(4000 cm
-1
~ 666 cm
-1
)
wavenumber:

(cm
-1
, reciprocal centimeters)
  (cm
-1
) = 1 /  (cm)
ν(Hz) =

c = c (cm/sec) /  (cm)
wavenumber (cm
-1
) = 10
4
/ wavelength (m)
wavelength (m) = 10
4
/(cm
-1
)

35. Theoretical Introduction
114/03/10 35
ΔE = h
.
ν = h c /λ = h c

C : velocity of light, 3 x 10
10
cm/sec
h : Planck’s constant
ν : frequency (1/sec)
λ : wavelength (mm)


: wavenumber (cm
-1
)

36. IR Absorption Process
A Quantized Process:
Only selected frequencies (energies) of
infrared
radiation will be absorbed by a molecule
Bonds have dipole moment, Yes !
Symmetric bonds, No ! (e.g. H
2
, Cl
2
)
114/03/10 36

37. IR Application
Infrared spectrum can be a fingerprint used
for identification.
Infrared spectrum gives the structural
information about a molecule.
114/03/10 37

38. Absorption
114/03/10 38
Transmittance: T = I/I
o
Absorbance: A = Log I
o/I
if no absorption, T = 1; A = 0
Most spectrometers display absorbance on vertical axis,
and the commonly observed range from 0 (100% T)
to 2 (1% T)

max: The wavelength of maximum absorbance
UV example

39. An Infrared Spectrum
114/03/10 39

40. 114/03/10 40
IR Spectrum
No two molecules will give exactly the same IR spectrum
(except enantiomers)
Simple stretching: 1600-3500 cm
-1
Complex vibrations: 400-1400 cm
-1
, called the
“fingerprint region”
Baseline
Absorbance/
Peak

41. Vibration Modes
114/03/10 41
  
      
  
      
  
      
Symmetric StretchAsymmetric StretchSymmetric Bend

42. 114/03/10 42
Vibration Modes of CH
2
Group
Gas Phase IR Spectrum of Formaldehyde, H
2C=O

43. Vibration Modes
114/03/10 43
HH
HH
HH
HH
H
H
H
H
Asym. Stretching
Sym. Stretching
Scissoring wagging
Rocking Twisting
in planeOut of plane
Stretching vibrations Bending Vibrations
(~ 2853)
(~ 2926)
(~ 1450)
(~ 1250)
(~ 720)
(~ 1250)

44. Basic Principle
114/03/10 44
As for any harmonic oscillator, the energy of bond vibration:
E
osc  h ν
osc
Applying Hooke’s Law
K = - k
.
Δr
K Restoring force
- 
  
Arising force opposed to extension
k Elasticity constant (unit: N / m = kg
.
m / sec
2
/ m)
( ~ bond strength between two atoms)
Δr extension
m1 m2
m2m1
k
r
ox
1
x
2

45. Basic Principle
114/03/10 45
The natural frequency of the oscillation
ν
osc = ½ 
.
√k / 
Wave equation of quantum mechanics
E = (n + ½) ½  h
.
√k / 
h : Planck’s const.
n : vibration quantum number
Take (1) into (2):
E
vib
= h ν
osc
 (n + ½ )

46. Basic Principle
114/03/10 46
From SchrÖdiger Equation:
E
vib
= h ν
osc
 (n + 1/2)
vibration quantum number, n = 0, 1, 2, 3 ----
Ground state (n = 0): E
0 = ½ hν
osc
First excited state (n = 1): E
1 = 3/2 hν
osc
ΔE
vib
= E
1
– E
0
= 3/2 hν
osc
- ½ hν
osc
= hν
osc

47. Basic Principle
114/03/10 47
The frequency of radiation, ν
 vibrational frequency, ν
osc
E
radiation
= h ν = ΔE
vib
= hν
osc
= ½  h
.
√k / 
ν = νosc = ½  . √k / 
or

48. Basic Principle
114/03/10 48
ν
osc
= 1/2 
.
√k /
 = m
1
.
m
2
/ (m
1
+ m
2
) = reduced mass of atoms
in AMU (unit: kg)
ΔE
vib
= h
.
ν
osc
= h / 2 
.
√k /
ν(cm
-1
) = E
vib
/ h c = 1 / 2  c
.
√k /
= 5.3 x 10
–12
sec/cm
.
√k /
[ unit of √k /sec
-1
]

49. Example
114/03/10 49
Calculate the wavenumber and wavelength of the fundamental
frequency peak due to the stretching vibration of C=O group.
Solution:
m
1
= 12 x 10
-3
kg / mol / 6.0 x 10
23
atoms / mol
= 2.0 x 10
–26
kg
m
2
= 16 x 10
-3
kg / mol / 6.0 x 10
23
atoms / mol
= 2.7 x 10
–26
kg
 = m
1
.
m
2
/ (m
1
+ m
2
) = 1.1 x 10
–26
kg
(cm
-1
) = 5.3 x 10
–12
s/cm
.
√1 x 10
3
N /m /2.7 x 10
–26
kg
= 1.6 x 10
3
cm
-1

50. Example
114/03/10 50
(cm
-1
) = E vib / h c = 1 / 2  c .√k / 
7.76 x 10
11
/2  c √k / ’
(taking 6.02 x 10
23
out of root square)
= 4.12√k / ’
‘ = M
1M
2/(M
1+M
2); both are atomic weights.
K : force constant in dynes/cm.
1 N = 1 kg  m/sec
2
1 Dyne = 1 g  cm/sec
2

51. 114/03/10 51
k (Chemical bond force constant) :
single bond 5 x 10
2
N/m = 5 x 10
5
dynes/cm
double bond 1.0 x 10
3
N/m = 10
6
dynes/cm
triple bond 1.5 x 10
3
N/m = 1.5 x 10
6
dynes/cm
Force Constants

52. 114/03/10 52
C=C bond:


= 4.12√k / 
K = 10
6
dynes/cm
m = 12x12/(12+12) = 6


= 4.12√10
6
/6
= 1682 cm
-1
(calculated)
= 1650 cm
-1
(experimental)
Force Constants

53. 114/03/10 53
C-H Bond:
= 4.12√k / m
K = 5 x 10
5
dynes/cm
m = 12x(1)/(12+1) = 0.923
= 4.12√5 x 10
5
/0.923
= 3032 cm
-1
(calculated)
= 3000 cm
-1
(experimental)
C-D Bond:


= 2206 cm
-1
(experimental)
Force Constants

54. 114/03/10 54
Frequencies of vibration
CC C=C C-C
2150 cm
-1
1650 cm
-1
1200cm
-1
increasing K

55. 114/03/10 55
As the atom bonded to carbon increases in mass, the
Quantity  increases, the frequency of vibration goes down.
C-H C-C C-O
3000 cm
-1
1200 cm
-1
1100 cm
-1
C-Cl C-Br C-I
800 cm
-1
550 cm
-1
~ 500 cm
-1
Increasing 
Frequencies of vibration

56. 114/03/10 56
Bending motion easier than stretching motions
(The force constant K is smaller)
C-H stretching C-H bending
~ 3000 cm
-1
~ 1340 cm
-1
Frequencies of vibration

57. 114/03/10 57
Hybridization affects the K values
sp sp
2
sp
3
C-H =C-H -C-H
3300 3100 2900 cm
-1
Frequencies of vibration

58. 114/03/10 58
Example

59. Frequencies of Vibration
114/03/10 59
C  C C = C CC
2150 cm
-1
1650 cm
-1
1200 cm
-1
Increasing K
CH CC CO CCl CBr CI
3000 1200 1100 800 550 ~500 cm
-1

CX stretching

60. Frequencies of Vibration
114/03/10 60
Resonance:
O
O
C
+

C=O
1715 cm
-1 1675 ~ 1680 cm
-1

61. Substituent Effects
114/03/10 61
Hyperchromic
Bathochromic
(red shift)
Hypsochromic
(blue shift)
Hypochromic

nm800400
blue red
UV Profiles

62. Substitution Effects on IR Peaks
Electronic effects
Intramolecular factors

Inductive effects
114/03/10 62
R-COR 
C=0
1715cm
-1
; R-COH 
C=0
1730cm
-1
；
R-COCl 
C=0
1800cm
-1
; R-COF 
C=0
1920cm
-1
;
F-COF 
C=0
1920cm
-1
; R-CONH
2

C=0
1928cm
-1
;

63. Conjugated Effects
114/03/10 63
C
O
H
3C CH
3
C
O
CH
3 C
O
CH
3 C
O
1715 1685 1685 1660
cm
-1
cm
-1
cm
-1
cm
-1

64. 114/03/10 64
Space Effects
Steric Hindrance
Ring Strain
CH
CH
CH
CH
1576cm
-1
1611cm
-1
1644cm-1
1781cm
-1
1678cm
-1
1657cm
-1
1651cm
-1
3060-3030 cm
-1
2900-2800 cm
-1
2
2
2
2

65. 114/03/10 65
2．Intermolecular Factors
OCH
3
O
C
O
H
3C
H
HO
3705-31252835O-HÉìËõ
R
HN
O
R
NH
O
H
H
C=O N-H N-H
ÉìËõ ÉìËõ ±äÐÎ
ÓÎÀë
Çâ¼ü
1690 3500
1650 3400
1620-1590
1650-1620

66. Example
114/03/10 66

67. Symbols of Vibrations
114/03/10 67
s = symmetric
as = asymmetirc
ν = stretching vibrations (bonding vibrations)
δ = deformation vibrations (bending vibrations)
γ = out-of-plane deformation vibrations
τ = torsional vibrations

68. Characterization of IR Spectra
114/03/10 68
Fingerprint Region: 1450 to 600 cm
-1
region
Group Frequency Region: 4000 to 1450 cm
-1
region

69. Degree of Freedom (DF)
114/03/10 69
According to Cartesian to describe positions:
Each atom has 3 degrees of freedom, (x, y, z).
A molecule of n atoms has 3n DF.
For nonlinear molecules: 3n-6 are fundamental vibration of DF.
For linear molecules: 3n-5 fundamental vibration of DF.
fundamental vibration involves no change in the center of
gravity of molecules

70. Degree of Freedom (DF)
114/03/10 70
OO Cb
OO Cb
OO Cb
-+-
OO Cb
(1) Symmetric
stretching (vs CO
2
)
1340 cm
-1
(2) Asymmetric
stretching (vs CO
2
)
2350 cm
-1
(3) Scissoring (bending)
(ds CO
2
), 666 cm-1 (4) Scissoring (bending)
(ds CO
2
), 666 cm-1
DF of Linear CO
2
:

71. Degree of Freedom (DF)
114/03/10 71
H
O
H
Symmetrical
stretching (vs OH)
3652 cm
-1
H
O
H
Asymmetrical
stretching (vs OH)
3756 cm
-1
H
O
H
Scissoring
stretching (ds HOH)
1596 cm
-1
Three fundamental vibrations of
nonlinear triatomic water molecule:

72. Vibration Modes
114/03/10 72
A. 

Stretching vibrations
Change of bond length
 
 
 
B. 

Deformation vibrations (two types: planar or non-planar)
Change of bond angle
H
C
H
Symmetric Asymmetric
H
C
H
H
C
H
in plane
H
C
H
H
C
H
C
H
H
+
_
+
+
out-of-plane
bending rocking twist wagging
CH
2
group

73. 114/03/10 73
Localized vibrations:
spectra below 1500 cm
-1
are
difficult to interpretation, which, we call it
“fingerprint region”, are characteristic for the molecule
as a whole. Most of them are derived from overtone or
combination vibrations.
Fingerprint Regions

74. 114/03/10 74
Group Frequency Regions
Absorption bands in the 4000 to 1450 cm
-1
region
are usually due to stretching vibrations of diatomic units

75. 114/03/10 75
Some General Trends
i)Stretching frequencies are higher than corresponding
bending frequencies. (It is easier to bend a bond than
to stretch or compress it.)
ii)
 
Bonds to hydrogen have higher stretching frequencies
than those to heavier atoms.
iii)
 
Triple bonds have higher stretching frequencies than
corresponding double bonds, which in turn have higher
frequencies than single bonds.
 
      (
Except for bonds to hydrogen).

76. 114/03/10 76
C=O 1850 ~1630 cm
-1
, sharp & intensity
C=C 1680 ~ 1620 cm
-1
, generally weak
Characterization of IR Spectra
Carbonyl/ethylene groups

77. 114/03/10 77
Important infrared frequencies:
The IR spectra of complicated molecules have many peaks,
only a few of which can be easily interpreted.
Here are the places to look at:
1500 – 1800 cm
-1
: A strong signal for (C=O) or an imine.
2000 – 2200 cm
-1
: stretch vibrations of triple bonds and cumulenes
(-C≡N, -C≡C , -N=C=O, -C=C=O).
2900 – 3000 cm
-1
: stretch vibrations of aliphatic C – H bond.
3000 – 3100 cm
-1
: vinyl C – H bond stretches.
3100 – 3600 cm
-1
: O – H bond stretches (broad if inter-molecularly at
H-bonded). Be careful, since water appears in this region.
Characterization of IR Spectra

78. 114/03/10 78
O-H 3650 ~ 3200 cm
-1
, broad peak
N-H 3500 ~ 3300 cm
-1
,
1 or 2 sharp absorption bands of low intensity,
O-H in this region usually gives broad peak.
N-H & O-H regions usually overlap
Characterization of IR Spectra

79. 114/03/10 79
Hydrogen Bonding
X— H --- Y
: s-orbital of proton overlapping with p- or -orbital of the acceptors.
Common proton donor groups: -COOH, -OH, Amines, or amides.
Common proton acceptors: O, N, halogens
X— H : move to lower frequency with increased intensity.
The acceptors, Y ( e.g. C=O), also move to lower frequency
but to a less degree.
Intermolecular H-bonding: usually in dimers (e.g. RCOOH), or
in neat or concentrated solutions of R-OH.
Temperature, concentration affect the inter or intra- H-bonding.
Characterization of IR Spectra

80. 114/03/10 80
Intra- H-bond is stronger when a six-membered ring is formed.
H-bond is strongest when the bonded structures is stabilized by resonance.
O
O
H
RO
Characterization of IR Spectra

81. 114/03/10 81
Stretching frequencies in hydrogen bonding
Intermolecular bondingintramolecular bonding
X— H --- Y ν
OH
ν
CO
comp’ds ν
OH ν
CO
comp’ds
Weak 300 15R-OH, PhOH < 10010 1,2-diols
intermol OH to CO most -OH ketones
Medium 100 ~ 30050
Strong >500 50 RCOOH dimers > 300 100
Frequency reduction(cm
-1
) Frequency reduction(cm
-1
)

Characterization of IR Spectra

82. 114/03/10 82
Analysis of Spectra
1.If C=O
1820 ~ 1660 cm
-1
: the strongest peak
Acid: is OH present? Broad near 3400~2400
Amide: is NH present? Medium peak ~3500
Ester: is C-O present? Strong at 1300~1000
Anhydride: two C=O ~ at 1810 & 1760
Aldehyde: is C-H present? Two peaks ~ 2850 & 2750
Ketone: the above 5 choices were eliminated

83. 114/03/10 83
2. If C=O absent
Alcohol/phenol: check for OH
- broad near 3600~3300
- confirm by C-O near 1300~1000
Amine: check for NH
-medium peak near 3500
Ether: check for C-O near 1300~1000
Analysis of Spectra

84. 114/03/10 84
3. Double bonds/or Aromatic rings
Alkene:-C=C is weak near 1650
Aromatic: -medium to strong at 1650~1450
Aromatic & vinyl CH:weak peak at 3000
4. Triple bond
-CN: medium, sharp near 2250
-CC: weak, sharp near 2150
check also for acetylenic CH near 3300
Analysis of Spectra

85. 114/03/10 85
5. Nitro group
- two strong peaks at 1600-1500 & 1390-1300
6. Hydrocarbon
-none of above found
-major peaks of CH near 3000
-very simple spectrum, others at 1450 & 1375
Analysis of Spectra

86. 114/03/10 86
Summary of IR Absorptions

87. 114/03/10 87
Characterization of IR Spectra

88. 114/03/10 88
IR Spectrum
 
                        
 1. Analysis of C
5
H
10
O
Problem Set

89. 114/03/10 89
IR Spectrum
 2. Analysis of C
8H
8O
Problem Set

90. 114/03/10 90
IR Spectrum
 3. Analysis of C
7
H
8
O
Problem Set

91. 114/03/10 91
IR Spectrum
 4. Analysis of C
8H
7N
Problem Set

92. 114/03/10 92
5. Analysis: C
7
H
6
O
IR Spectrum
Problem Set

93. 114/03/10 93
6. Analysis: C
3
H
7
NO
IR Spectrum
Problem Set

94. 114/03/10 94
7. Analysis: C
4
H
8
O
2

IR Spectrum
Problem Set

95. 114/03/10 95
8. Analysis: C
7H
5OCl
IR Spectrum
Problem Set

96. 114/03/10 96
9. Analysis: C
6
H
6
S
IR Spectrum
Problem Set

97. 114/03/10 97
10. Analysis: C
4
H
6

IR Spectrum
Problem Set

98. 114/03/10 98
No.9:   
       
thiophenol
No. 8:   
        
benzoyl chloride
No. 7:   
       
ethyl acetate
No. 6:   
         
N-methylacetamide
No. 5:   
        
benzaldehyde
No. 4:   
          
benzyl nitrile
No. 3:   
         
benzyl alcohol
No. 2:   
          
acetophenone
No. 1:
         
3-pentanone
No.10:   
          
1,3-butadiene
Solutions of Problem Set

99. 114/03/10 99
A Survey of the Important Functional Groups
With Examples

100. 114/03/10 100
9. Hydrocarbons: Alkanes, Alkenes, and Alkynes
A. Alkanes
C-H stretch ~2980 - 2800 cm-1
CH
2
~ 1450
CH
3
~ 1375
C-C stretch : not useful for interpretation
Characterization of IR Spectra

101. 114/03/10 101
B. Alkene
=C-H stretch 3100-3000
=C-H out-of-plane (oop) bending, 1000-650
C=C 1660-1600 (w)
symmetrical type, no absorb
symmetrical (cis), stronger
Characterization of IR Spectra

102. 114/03/10 102
Aromatic rings
=C-H stretch, 3100 ~ 3000
=C-H out-of-plane (oop) bending, 900 ~ 690
great utility to assign the ring substituted pattern
C=C ring stretch, occurs in pair at 1600 & 1475
overtone/combination bands, 2000 ~ 1667
used to assign the ring substituted patterns
Characterization of IR Spectra

103. 114/03/10 103
C. Alkynes
C-H stretch ~ 3300 (3.0)
-CC stretch ~ 2150 (4.65 
disubstituted or symmetrical:
no or weak absorption
Characterization of IR Spectra

104. 114/03/10 104
An Alkyne IR Spectrum

105. 114/03/10 105
Table 2. Physical constants for sp, sp
2
, sp
3
Hybridized carbon and the resulting C-H values
Bond C-H =C-H -C-H
Type sp- 1s sp
2
- 1ssp
2
-1s
Length 1.08Å 1.10Å 1.12Å
Strength121Kcal106Kcal101Kcal
IR freq.3300 ~3100 ~2900
Characterization of IR Spectra

106. 114/03/10 106
3300 3100 3000 2850 2750
3.03 3.22 3.33 3.51 3.64
acetylenicvinyl =C-Haliph. C-H aldehyde
C-H arom. =C-H
cyclopropyl -C-H -CH=O
sp sp2 sp3
Strain moves absorption to left
Increasing s character moves to left
C-H Stretch Region
Characterization of IR Spectra

107. 114/03/10 107
Table 3. The stretching vibrations for various
sp
3
hybridized C-H bonds
Stretching vibration
asym.
2962
2926
2872
2853
2890 very weak
group
Methyl CH
3
-
Methylene –CH
2-
Methine –C-H
Characterization of IR Spectra
sym.

108. 114/03/10 108
C-H Bending Vibrations
HH
H
C'H
H
H
C'H
H
ScissoringBending (as.) bending (sy.)
1465 1450 1375
Sometimes overlap
Lone Me g’p
1380 1370 Gem-dimethyl
CH
2
CH
3
CH
2
, ~ 720 rocking band
Characterization of IR Spectra

109. 114/03/10 109
C=C Stretching Vibrations
Conjugated effects.
1660-1640 cm
-1
Ring size effects.
The frequency of (endo) double bonds in
cyclic compounds is sensitive to ring size.
C''
C'
a
b
c
C''
C'
C'
Higher freq.
Characterization of IR Spectra

110. 114/03/10 110
Figure 3. C=C Stretching vibrations in
endocyclic Systems
1650 1646 1611 1566 1641
~ 1611
Angle > 90 Angle < 90
Characterization of IR Spectra

111. 114/03/10 111
External (exo) double bonds
H
2
C=C=CH
2
Allene
1950 1780 1678 16571655 1651
(a)Strain moves the peak to the left
(b)Ring fusion moves the absorption
to the left
Characterization of IR Spectra

112. 114/03/10 112
C-H Bending Vibrations for Alkenes
In-plane scissoring: ~ 1415
Out-of-plane region: 1000 ~ 650
Valuable to indicate the substitution patterns
C
1
H
C
2
H
Characterization of IR Spectra

113. 114/03/10 113
10 11 12 13 14 15 
1000 900 800 700 cm
-1
Monosubst.
cis-1,2
trans-1,2
1,1-disubst.
trisubst.
tetrasubst.
Figure. The C-H out-of-plane bending vibrations for
substituted alkenes
m
s
s
s s
s
Characterization of IR Spectra

114. 114/03/10 114
11 12 13 14 15 
900 800 700 cm
-1
m
s
m
m
s
s
s
Monosubst.
Ortho
Meta
Para
1,2,4-
1,2,3-
1,3,5-
s
s
s
s
m
s
C=C oopC-H oop

115. 114/03/10 115
D. Alcohols and Phenols
O-H “free” O-Hsharp, 3650~3600 if no H-bonding.
H-bonded O-H, broad at 3500~3200
,usually in neat (pure) liquids.
C-O 1250~1000(s)
Characterization of IR Spectra

116. 114/03/10 116
An Alcohol IR Spectrum

117. 114/03/10 117
Table. The C-O and O-H stretching vibrations in
Alcohols and Phenol
Compound C-O stretch O-H stretch
Phenols 1220 3610
3  - OH 1150 3620
2  - OH 1100 3630
1  - OH 1050 3640
OH
OH
CH
2
OH
1100  1070 1100  1070 1050  1017
Characterization of IR Spectra

118. 114/03/10 118
E. Ethers
C-Ostretch, 1300 ~ 1000
phenyl & vinyl ethers, shift to higher frequency
(increase of double bond character)
1250 (asy), 1040 (sy).
Epoxides, 3 bands (1250, 950~815, 850~750)
Characterization of IR Spectra

119. 114/03/10 119
F. Carbonyl compounds
18101800176017351725171517101690
acid ester ketone amide
chloride
Anhydride anhydride aldehydecarboxylic
(band 1) (band 2) acid
Characterization of IR Spectra

120. 114/03/10 120
F-1. Factors affect C=O vibration
1.Conjugated effects.
-unsaturated, 30 cm
-1
to lower freq.
H
O
OH
O
CH
3
O
1715  1690 1725  1700
1710  1680
C
2
C
1
O
C
+
C
1
O
Characterization of IR Spectra

121. 114/03/10 121
O
O
O
O
NH
O
2. Ring size effects
1715  17451715  17801735  17701690  1705
Characterization of IR Spectra

122. 114/03/10 122
3. Alpha-substitution effects
C
1
X
O
O
Cl
H
O
H
Cl
Axial Cl
~ 1725
Equatorial Cl
~ 1750
Characterization of IR Spectra

123. 114/03/10 123
4. Hydrogen Bonding Effects
O
O
H
OMe
1680
Characterization of IR Spectra

124. 114/03/10 124
5. Cyclic ketones
O
OO
O
O
O
RR
Ring strain
1815 1780 1745 1715 1715 1705
Characterization of IR Spectra

125. 114/03/10 125
6. Carboxylic Acids
O-Hstretch, very broad (strongly H-bond)
3400 ~ 2400
C=Ostretch, broad, 1730 ~ 1700
C-Ostretch, 1320 ~ 1210 (m)
Characterization of IR Spectra

126. 114/03/10 126
7. Esters (R-C(=O)-O-R’)
C=Ostretch, 1735
a. conjugation at the R part: ν shift to the right
b. conjugation with O at R’ ν part: shift to the left
c. ring strain (lactones): ν shift to the left
C-Ostretch, two or more bands,
one stronger and broader, 1300 ~ 1000
Characterization of IR Spectra

127. 114/03/10 127
RO
O
C
H
CH
2 RO
O
R C
H
CH
2
O
O
R
RO
O
O
O
R
normal
Table. The general effects of -unsaturation or aryl
Substitution and conjugation with oxygen on the C=O vibrations
1770 1735 1720
Characterization of IR Spectra

128. 114/03/10 128
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Table. The Effects of Ring Size, -unsaturation, and
Conjugation with Oxygen in the C=O Vibrations in Lactones
1735
1800
1760
1750
1720
1770
1820
Characterization of IR Spectra

129. 114/03/10 129
8. Amides
C=Ostretch, 1670 ~ 1640
N-Hstretch (1 and 2), 3500, 3100
N-Hbending, 1640 ~ 1550
NH
O
NH
O
NH
O
~ 1660 ~ 1705 ~ 1745
Characterization of IR Spectra

130. 114/03/10 130
9. Acid Chlorides
C=Ostretch at ~ 1800
conjugation moves to the right
10. Anhydrides
C=Ostretch, two bands, 1830~1800 & 1775~1740
ring strain moves to the left
C-Ostretch, 1300 ~ 900
Characterization of IR Spectra

131. 114/03/10 131
G. Amines
N-Hstretch, 3500~3300
1-, 2 bands; 2-, 1 band
weak, aliphatic amines
stronger, aromatic amines
N-Hbend, 1640~ 1560
2-, near 1500
N-Hoop bending, near 800
C-Nstretch, 1350 ~ 1000
Characterization of IR Spectra

132. 114/03/10 132
H. Nitriles, Isocyanates and Imines
-CN stretch, sharp ~ 2250
-N=C=O a broad, intense band ~ 2270
-N=C=S a broad, intense band ~ 2125
-C=N- in imine or oxime 1690-1640
CC give much weak band at ~ 2250 region.
Characterization of IR Spectra

133. 114/03/10 133
J. Nitro Compounds
N=O stretch, two strong bands,
1600 ~ 1500 & 1390 ~ 1300
Nitroalkanes: ~ 1550, 1380
Nitroarenes: ~ 1530, 1350
Nitroso (R-N=O) one band, 1600 ~ 1500
Characterization of IR Spectra

134. 114/03/10 134
J-1. Carboxylate Salts, RC(=O)-O
-
asym ~1600 (s)
sym ~1400 (s)
J-2. Amine Salts
N-Hstretch (broad), 3300 ~ 2600
N-Hbend, 1610 ~ 1500 (s)
1 (two bands), 1610, 1500
2 (one band), 1610 ~ 1550
Characterization of IR Spectra

135. 114/03/10 135
K. Sulfur Compounds
Mercaptans S-H stretch, ~ 2550 (w)
Sulfides, R-S-R
Sulfoxides, R-S(=O)-R
S=O stretch, ~1050 (s)
Sulfones, RSO
2R
S=O 1300 (s), 1150 (s)
Sulfates RSO
3
R
S=O 1375(s), 1200(s)
S-O 1000~750
SulfonamidesRSO
2NH
2, RSO
2NHR
S=O 1325 (s), 1140 (s)
N-H 3350, 3250 (1 )
3250 (2 )
Characterization of IR Spectra

136. Characterization of IR Spectra
114/03/10 136
L. Alkyl and Aryl Halides
Fluorides
C-F stretch, 1400~1000 (s)
Chlorides
C-Cl stretch, 800~ 600 (s)
CH
2
-Cl bend (wagging), 1300 ~ 1200
Bromides/Iodides
C-Br/C-I stretch, ~ 660 out of range
CH
2-Br/CH
2-I
bend (wagging), 1250 ~ 1150

137. Infrared Sources and Transducers
114/03/10 137
Infrared sources:
An inert solid with continuous radiation at
heated temperature: 1500 ~ 2000 K.
Range of radiation intensity:
5000 ~ 670 cm
-1
(2 ~ 15 M)

138. 114/03/10 138
1.The Nernst Glower
temperature: 1200 ~ 2200 K
2. The Globar Source
a silicon carbide rod
electrically heated (1300 ~ 1500 K)

139. 114/03/10 139
Sample Handling Techniques
Neat liquids:一滴樣品夾於二鹽片 (rock-salt plate)間
(厚度約為 ≦ 0.01 mm), a neat spectrum obtained.
一般僅適合用於 定性的分析.
Solids:
KBr pellet: 2 ~ 5 mg sample, mixing with 100~150 mg
of powdered KBr, pressing with pressure of 5 ~ 10 Ton
to give a transparent disc.

KBr is hygroscopic, weak OH band at 3450 cm-1
appeared
Nujol mull: grinding the sample with mineral oil (Nujol)
to create a suspension and placed between salt plates
Solutions: compound dissolved in common solvents,
ex. CS
2
, CCl
4
, CHCl
3
, dioxane, cyclohexane,
benzene.

140. 114/03/10 140

141. 114/03/10 141