1. Infrared Spectroscopy

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
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3. Electromagnetic Spectrum
Examples: X rays, microwaves, radio waves, visible light, IR, and UV
Frequency and wavelength are inversely proportional
c = ln, where c is the speed of light
Energy per photon = hn, where h is Planck’s constant
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4. The Electromagnetic Spectrum
High
Frequency
Energy
Low
Micro
wave
X-ray UV IR
Radio uv
NMR
visible
Vibrational IR
200 nm nm nm 2.5 m m 1m m
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5. Energy Transitions Table 1. Types of energy transitions of
the electromagnetic spectrum
Region of spectrum Energy transition
X-rays Bond breaking
UV/Visible Electronic
IR Vibrational
Microwave Rotational
Radiofrequencies Nuclear spin (NMR)
Electron spin (ESR)
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6. The Spectrum and Molecular Effects
=>
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7. The IR Region Just below red in the visible region
Wavelengths usually mm
More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters ( cm-1)
Wavenumbers are proportional to frequency and energy
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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
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9. IR Spectrum
Baseline
Absorbance/Peak
No two molecules will give exactly the same IR spectrum (except enantiomers)
Simple stretching: cm-1
Complex vibrations: cm-1, called the “fingerprint region”
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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
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11. Carbon-Carbon Bond Stretching
Stronger bonds absorb at higher frequencies:
C-C cm-1
C=C cm-1
CC cm-1 (weak or absent if internal)
Conjugation lowers the frequency:
isolated C=C cm-1
conjugated C=C cm-1
aromatic C=C approx cm-1
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12. Carbon-Hydrogen Stretching
Bonds with more s character absorb at a higher frequency
sp3 C-H, just below 3000 cm-1 (to the right)
sp2 C-H, just above 3000 cm-1 (to the left)
sp C-H, at 3300 cm-1
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13. An Alkane IR Spectrum
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14. An Alkene IR Spectrum
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15. An Alkyne IR Spectrum
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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 (R2NH), broad with one sharp spike
Primary amine (RNH2), broad with two sharp spikes
No signal for a tertiary amine (R3N)
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17. An Alcohol IR Spectrum
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18. An Amine IR Spectrum
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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
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20. A Ketone IR Spectrum
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21. An Aldehyde IR Spectrum
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22. O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, cm-1, due to strong hydrogen bonding
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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, cm-1
The C=O of an ester absorbs at a higher frequency, ~ cm-1
Carbonyl groups in small rings (5 C’s or less) absorb at an even higher frequency
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24. An Amide IR Spectrum
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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
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26. A Nitrile IR Spectrum
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27. Summary of IR Absorptions
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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
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29. Infrared Regions Region wavelength wavenumber frequency
(mm) (cm-1) (Hz)
Near ~ ~ x 1014~ 1.2 x 1014
Middle 2.5 ~ ~ x 1014~ 6.0 x 1012
Far 50 ~ ~ x 1012~ 3.0 x 1011
Most used 2.5 ~ ~ x 1014~ 2.0 x 1013
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30. 1. Using photometers or spectrophotometer similar to UV spectrometry
IR Application
Near IR: ~ 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.
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31. Used largely for qualitative organic analysis
IR Application
Mid-IR:
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.
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32. IR Sources
An inert solid heated btw 1500 ~ 2000k, The max radiation ~ 5900 cm-1 (1.7 mM); at long wavelength, 1% of max ~ 670 cm-1 (15 mM).
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 mm.
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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
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34. IR Absorption Region ν(Hz) =  c = c (cm/sec) /  (cm)
λ= 2.5 ~ 15 mm (10 -4 cm = 1 mm = 104 Ao).
(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) = / wavelength (mm)
wavelength (mm) = 104 /(cm-1)
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35. Theoretical Introduction
ΔE = h .ν = h c /λ = h c 
C : velocity of light, 3 x 1010 cm/sec
h : Planck’s constant
ν : frequency (1/sec)
λ : wavelength (mm)
 : wavenumber (cm-1)
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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. H2, Cl2)
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37. IR Application
Infrared spectrum can be a fingerprint used for identification.
Infrared spectrum gives the structural information about a molecule.
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38. Absorption Transmittance: T = I/Io Absorbance: A = Log Io/I
UV example
Transmittance: T = I/Io
Absorbance: A = Log Io/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
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39. An Infrared Spectrum
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40. IR Spectrum
Baseline
Absorbance/Peak
No two molecules will give exactly the same IR spectrum (except enantiomers)
Simple stretching: cm-1
Complex vibrations: cm-1, called the “fingerprint region”
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41. Vibration Modes Symmetric Stretch Asymmetric Stretch Symmetric Bend
        
Symmetric Stretch
Asymmetric Stretch
Symmetric Bend
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42. Vibration Modes of CH2 Group
Gas Phase IR Spectrum of Formaldehyde, H2C=O
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43. Vibration Modes Scissoring wagging Sym. Stretching (~ 1250) (~ 2853)
(~ 1450)
Asym. Stretching
Rocking
Twisting
(~ 720)
(~ 2926)
(~ 1250)
in plane
Out of plane
Stretching vibrations
Bending Vibrations
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44. Basic Principle Eosc  h νosc Applying Hooke’s Law K = - k .Δr
As for any harmonic oscillator, the energy of bond vibration:
Eosc  h νosc
Applying Hooke’s Law
K = - k .Δr
K Restoring force
-   Arising force opposed to extension
k Elasticity constant (unit: N / m = kg . m / sec2 / m)
( ~ bond strength between two atoms)
Δr extension
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45. Basic Principle νosc = ½  . √k / m (1) E = (n + ½) ½  h. √k / m (2)
The natural frequency of the oscillation
νosc = ½  . √k / m (1)
Wave equation of quantum mechanics
E = (n + ½) ½  h. √k / m (2)
h : Planck’s const.
n : vibration quantum number
Take (1) into (2):
Evib = h νosc  (n + ½ )
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46. Basic Principle From SchrÖdiger Equation: Evib = h νosc  (n + 1/2)
vibration quantum number, n = 0, 1, 2,
Ground state (n = 0): E0 = ½ hνosc
First excited state (n = 1): E1 = 3/2 hνosc
ΔEvib = E1 – E0
= 3/2 hνosc - ½ hνosc = hνosc
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47. Basic Principle The frequency of radiation, ν
 vibrational frequency, νosc
E radiation = h ν = ΔEvib = hνosc
= ½  h . √k / m or
ν = νosc = ½  . √k / m
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48. Basic Principle νosc = 1/2  . √k / m
m = m1. m2 / (m1 + m2) = reduced mass of atoms
in AMU (unit: kg)
ΔE vib = h . νosc = h / 2  . √k / m
ν(cm-1) = E vib / h c = 1 / 2  c .√k / m
= 5.3 x 10 –12 sec/cm . √k / m
[ unit of √k / m : sec -1 ]
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49. Example (cm-1) = 5.3 x 10 –12 s/cm . √1 x 10 3 N /m /2.7 x 10 –26 kg
Calculate the wavenumber and wavelength of the fundamental
frequency peak due to the stretching vibration of C=O group.
Solution:
m1 = 12 x kg / mol / 6.0 x atoms / mol
= x 10 –26 kg
m2 = 16 x kg / mol / 6.0 x atoms / mol
= x 10 –26 kg
m = m1. m2 / (m1 + m2) = 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 103 cm-1
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50. Example (cm-1) = E vib / h c = 1 / 2  c .√k / m
= 7.76 x 1011/2  c √k / m’
(taking 6.02 x 1023 out of root square)
= 4.12√k / m’
‘ = M1M2/(M1+M2); both are atomic weights.
K : force constant in dynes/cm.
1 N = 1 kg  m/sec2
1 Dyne = 1 g  cm/sec2
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51. k (Chemical bond force constant) :
Force Constants
k (Chemical bond force constant) :
single bond 5 x 102 N/m = 5 x 105 dynes/cm
double bond 1.0 x 103 N/m = 106 dynes/cm
triple bond 1.5 x 103 N/m = 1.5 x 106 dynes/cm
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52. Force Constants C=C bond:  = 4.12√k / m K = 106 dynes/cm
m = 12x12/(12+12) = 6
= 4.12√106/6
= 1682 cm-1 (calculated)
= 1650 cm-1 (experimental)
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53. Force Constants C-H Bond: = 4.12√k / m K = 5 x 105 dynes/cm
m = 12x(1)/(12+1) = 0.923
= 4.12√5 x 105/0.923
= 3032 cm-1 (calculated)
= 3000 cm-1 (experimental)
C-D Bond:
 = 2206 cm-1 (experimental)
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54. Frequencies of vibration
CC C=C C-C
2150 cm cm cm-1
increasing K
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55. Frequencies of vibration
As the atom bonded to carbon increases in mass, the
Quantity m increases, the frequency of vibration goes down.
C-H C-C C-O
3000 cm cm cm-1
C-Cl C-Br C-I
800 cm cm-1 ~ 500 cm-1
Increasing m
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56. Frequencies of vibration
Bending motion easier than stretching motions
(The force constant K is smaller)
C-H stretching C-H bending
~ 3000 cm-1 ~ 1340 cm-1
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57. Frequencies of vibration
Hybridization affects the K values
sp sp2 sp3
C-H =C-H -C-H
cm-1
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58. Example
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59. Frequencies of Vibration
C  C C = C CC
2150 cm cm cm-1
Increasing K
CX stretching
CH CC CO CCl CBr CI
~500 cm-1
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60. Frequencies of Vibration
Resonance:
C=O
1715 cm-1
1675 ~ 1680 cm-1
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61. Substituent Effects e nm Hyperchromic Hypsochromic (blue shift)
Bathochromic
(red shift) e
Hypochromic nm
UV Profiles
400
800
blue
red
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62. Substitution Effects on IR Peaks
Electronic effects
Intramolecular factors
Inductive effects
R-COR C= cm ; R-COH C= cm -1 ；
R-COCl C= cm ; R-COF C= cm ;
F-COF C= cm ; R-CONH2 C= cm ;
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63. Conjugated Effects
cm -1
cm -1
cm -1
cm -1
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64. C H 1 5 7 6 c m 4 8 Space Effects Steric Hindrance Ring Strain- 4 8 2 2 2 2
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65. 2．Intermolecular Factors
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66. Example
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67. Symbols of Vibrations s = symmetric as = asymmetirc
ν = stretching vibrations (bonding vibrations)
δ = deformation vibrations (bending vibrations)
γ = out-of-plane deformation vibrations
τ = torsional vibrations
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68. Characterization of IR Spectra
Fingerprint Region: 1450 to 600 cm-1 region
Group Frequency Region: 4000 to 1450 cm-1 region
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69. Degree of Freedom (DF) 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
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70. Degree of Freedom (DF)
DF of Linear CO2:
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71. Degree of Freedom (DF) Three fundamental vibrations of
nonlinear triatomic water molecule:
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72. Vibration Modes CH2 group A. Stretching vibrations
Change of bond length
B.  Deformation vibrations (two types: planar or non-planar)
Change of bond angle
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73. Fingerprint Regions 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.
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74. Group Frequency Regions
Absorption bands in the 4000 to 1450 cm-1 region
are usually due to stretching vibrations of diatomic units
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75. Some General Trends
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).
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76. Characterization of IR Spectra
Carbonyl/ethylene groups
C=O 1850 ~1630 cm-1, sharp & intensity
C=C 1680 ~ 1620 cm-1, generally weak
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77. Characterization of IR Spectra
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.
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78. Characterization of IR Spectra
N-H & O-H regions usually overlap
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.
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79. Characterization of IR Spectra
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.
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80. Characterization of IR Spectra
Intra- H-bond is stronger when a six-membered ring is formed.
H-bond is strongest when the bonded structures is stabilized by resonance.
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81. Characterization of IR Spectra
Stretching frequencies in hydrogen bonding
Intermolecular bonding intramolecular bonding
Frequency reduction(cm-1)
Frequency reduction(cm-1)
X— H --- Y
ν OH ν CO comp’ds
ν OH ν CO comp’ds
Weak R-OH, PhOH < ,2-diols
intermol OH to CO most b-OH ketones
Medium ~
Strong > RCOOH dimers >
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82. 1820 ~ 1660 cm-1 : the strongest peak
Analysis of Spectra
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
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83. Alcohol/phenol: check for OH - broad near 3600~3300
Analysis of Spectra
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
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84. 3. Double bonds/or Aromatic rings
Analysis of Spectra
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
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85. - two strong peaks at 1600-1500 & 1390-1300
Analysis of Spectra
5. Nitro group
- two strong peaks at &
6. Hydrocarbon
-none of above found
-major peaks of CH near 3000
-very simple spectrum, others at 1450 & 1375
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86. Summary of IR Absorptions
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87. Characterization of IR Spectra
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88. Problem Set
 1. Analysis of C5H10O
IR Spectrum                          
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89. Problem Set
 2. Analysis of C8H8O
IR Spectrum
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90. Problem Set
 3. Analysis of C7H8O
IR Spectrum
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91. Problem Set
 4. Analysis of C8H7N
IR Spectrum
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92. Problem Set
5. Analysis: C7H6O
IR Spectrum
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93. Problem Set
6. Analysis: C3H7NO
IR Spectrum
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94. Problem Set
7. Analysis: C4H8O2
IR Spectrum
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95. Problem Set
8. Analysis: C7H5OCl
IR Spectrum
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96. Problem Set
9. Analysis: C6H6S
IR Spectrum
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97. Problem Set
10. Analysis: C4H6
IR Spectrum
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98. Solutions of Problem Set
No. 2:             
acetophenone
No. 3:            
benzyl alcohol
No. 1:          
3-pentanone
No. 4:             
benzyl nitrile
No. 5:           
benzaldehyde
No. 6:            
N-methylacetamide
No. 8:           
benzoyl chloride
No. 7:          
ethyl acetate
No.9:          
thiophenol
No.10:             
1,3-butadiene
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99. A Survey of the Important Functional Groups With Examples
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100. Characterization of IR Spectra
9. Hydrocarbons: Alkanes, Alkenes, and Alkynes
A. Alkanes
C-H stretch ~ cm-1
CH2 ~ 1450
CH3 ~ 1375
C-C stretch : not useful for interpretation
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101. Characterization of IR Spectra
B. Alkene
=C-H stretch
=C-H out-of-plane (oop) bending,
C=C (w)
symmetrical type, no absorb
symmetrical (cis), stronger
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102. Characterization of IR Spectra
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
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103. Characterization of IR Spectra
C. Alkynes
C-H stretch ~ 3300 (3.0 m)
-CC stretch ~ 2150 (4.65 m)
disubstituted or symmetrical:
no or weak absorption
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104. An Alkyne IR Spectrum
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105. Characterization of IR Spectra
Table 2. Physical constants for sp, sp2, sp3
Hybridized carbon and the resulting C-H values
Bond C-H =C-H -C-H
Type sp- 1s sp2- 1s sp2-1s
Length 1.08Å 1.10Å 1.12Å
Strength 121Kcal 106Kcal 101Kcal
IR freq ~3100 ~2900
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106. Characterization of IR Spectra
C-H Stretch Region
3.03m 3.22m 3.33m 3.51m 3.64m
acetylenic vinyl =C-H aliph. 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
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107. Characterization of IR Spectra
Table 3. The stretching vibrations for various
sp3 hybridized C-H bonds
Stretching vibration
group
asym.
sym.
Methyl CH3-
Methylene –CH2-
Methine –C-H
2962
2926
2872
2853
2890 very weak
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108. Characterization of IR Spectra
C-H Bending Vibrations
CH2
CH3
Scissoring
Bending (as.) bending (sy.)
Lone Me g’p
Gem-dimethyl
Sometimes overlap
CH2, ~ 720 rocking band
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109. Characterization of IR Spectra
C=C Stretching Vibrations
Conjugated effects.
cm-1
Ring size effects.
The frequency of (endo) double bonds in
cyclic compounds is sensitive to ring size.
Higher freq.
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110. Characterization of IR Spectra
Angle > 90
Angle < 90
~ 1611
Figure 3. C=C Stretching vibrations in
endocyclic Systems
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111. Characterization of IR Spectra
External (exo) double bonds
H2C=C=CH2
Allene
Strain moves the peak to the left
Ring fusion moves the absorption
to the left
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112. Characterization of IR Spectra
C-H Bending Vibrations for Alkenes
In-plane scissoring: ~ 1415
Out-of-plane region: 1000 ~ 650
Valuable to indicate the substitution patterns
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113. Characterization of IR Spectram s s
Monosubst.
cis-1,2
trans-1,2
1,1-disubst.
trisubst.
tetrasubst. s s s m
cm-1
Figure. The C-H out-of-plane bending vibrations for
substituted alkenes
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114. 11 12 13 14 15 m s s s m s s s m s s m s m 900 800 700 cm-1 Monosubst.
Ortho
Meta
Para
1,2,4-
1,2,3-
1,3,5- s m s s s m s s m s m
cm-1
C-H oop
C=C oop
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115. Characterization of IR Spectra
D. Alcohols and Phenols
O-H “free” O-H sharp, 3650~3600 if no H-bonding.
H-bonded O-H, broad at 3500~3200
,usually in neat (pure) liquids.
C-O 1250~1000(s)
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116. An Alcohol IR Spectrum
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117. Characterization of IR Spectra
Table. The C-O and O-H stretching vibrations in
Alcohols and Phenol
Compound C-O stretch O-H stretch
Phenols
3  - OH
2  - OH
1  - OH
1100    1017
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118. Characterization of IR Spectra
E. Ethers
C-O stretch, 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)
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119. Characterization of IR Spectra
F. Carbonyl compounds
acid ester ketone amide
chloride
Anhydride anhydride aldehyde carboxylic
(band 1) (band 2) acid
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120. Characterization of IR Spectra
F-1. Factors affect C=O vibration
Conjugated effects.
a,b-unsaturated, 30 cm-1 to lower freq.
1710  1680
1715  1690
1725  1700
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121. Characterization of IR Spectra
2. Ring size effects
1715  1745
1715  1780
1735  1770
1690  1705
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122. Characterization of IR Spectra
3. Alpha-substitution effects
Axial Cl
~ 1725
Equatorial Cl
~ 1750
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123. Characterization of IR Spectra
4. Hydrogen Bonding Effects
1680
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124. Characterization of IR Spectra
5. Cyclic ketones
Ring strain
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125. Characterization of IR Spectra
6. Carboxylic Acids
O-H stretch, very broad (strongly H-bond)
3400 ~ 2400
C=O stretch, broad, 1730 ~ 1700
C-O stretch, 1320 ~ 1210 (m)
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126. Characterization of IR Spectra
7. Esters (R-C(=O)-O-R’)
C=O stretch, 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-O stretch, two or more bands,
one stronger and broader, ~ 1000
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127. Characterization of IR Spectra
Table. The general effects of a,b-unsaturation or aryl
Substitution and conjugation with oxygen on the C=O vibrations
1770
1735
1720
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128. Characterization of IR Spectra
Table. The Effects of Ring Size, a,b-unsaturation, and
Conjugation with Oxygen in the C=O Vibrations in Lactones
1760
1735
1720
1770
1750
1800
1820
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129. Characterization of IR Spectra
8. Amides
C=O stretch, ~ 1640
N-H stretch (1 and 2), 3500, 3100
N-H bending, ~ 1550
~ ~ ~ 1745
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130. Characterization of IR Spectra
9. Acid Chlorides
C=O stretch at ~ 1800
conjugation moves to the right
10. Anhydrides
C=O stretch, two bands, 1830~1800 & 1775~1740
ring strain moves to the left
C-O stretch, ~ 900
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131. Characterization of IR Spectra
G. Amines
N-H stretch, 3500~3300
1-, 2 bands; 2-, 1 band
weak, aliphatic amines
stronger, aromatic amines
N-H bend, 1640~ 1560
2-, near 1500
N-H oop bending, near 800
C-N stretch, 1350 ~ 1000
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132. Characterization of IR Spectra
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
CC give much weak band at ~ 2250 region.
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133. Characterization of IR Spectra
J. Nitro Compounds
N=O stretch, two strong bands,
1600 ~ 1500 & ~ 1300
Nitroalkanes: ~ 1550, 1380
Nitroarenes: ~ 1530, 1350
Nitroso (R-N=O) one band, ~ 1500
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134. Characterization of IR Spectra
J-1. Carboxylate Salts, RC(=O)-O-
asym ~1600 (s)
sym ~1400 (s)
J-2. Amine Salts
N-H stretch (broad), 3300 ~ 2600
N-H bend, ~ 1500 (s)
1 (two bands), 1610, 1500
2 (one band), 1610 ~ 1550
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135. Characterization of IR Spectra
K. Sulfur Compounds
Mercaptans S-H stretch, ~ 2550 (w)
Sulfides, R-S-R
Sulfoxides, R-S(=O)-R
S=O stretch, ~1050 (s)
Sulfones, RSO2R
S=O (s), (s)
Sulfates RSO3R
S=O 1375(s), 1200(s)
S-O ~750
Sulfonamides RSO2NH2, RSO2NHR
S=O (s), (s)
N-H , 3250 (1 )
3250 (2 )
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136. Characterization of IR Spectra
L. Alkyl and Aryl Halides
Fluorides
C-F stretch, 1400~1000 (s)
Chlorides
C-Cl stretch, 800~ 600 (s)
CH2-Cl bend (wagging), ~ 1200
Bromides/Iodides
C-Br/C-I stretch, ~ 660 out of range
CH2-Br/CH2-I
bend (wagging), ~ 1150
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137. Infrared Sources and Transducers
An inert solid with continuous radiation at
heated temperature: 1500 ~ 2000 K.
Range of radiation intensity:
5000 ~ 670 cm-1 (2 ~ 15 mM)
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138. electrically heated (1300 ~ 1500 K)
The Nernst Glower
temperature: 1200 ~ 2200 K
2. The Globar Source
a silicon carbide rod
electrically heated (1300 ~ 1500 K)
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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. CS2, CCl4, CHCl3, dioxane, cyclohexane, benzene.
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