IR Interpretation

IR Interpretation

1. Infrared Spectroscopy
Interpretation
5/22/2020 1
Prepared By :
Mahendra G S
M-Pharm,Pharmaceutical
Chemistry
JSSCP, MYSURU

2. Interferogram
Theinterferogramattheright
displaystheinterferencepattern
andcontainsallofthespectrum
information.
5/22/2020 2
AFouriertransformconvertsthe
timedomaintothefrequency
domainwithabsorptionasafunction
offrequency.

3. Identificationoffunctionalgroupsona
molecule–thisisaveryimportanttoolin
organicchemistry
Spectralmatchingcanbedonebycomputer
softwareandlibraryspectra
SinceabsorbancefollowsBeer’sLaw,cando
quantitativeanalysis
5/22/2020 3
Use of IR spectra

4. •AnIRspectrumisaplotofpercent
transmittance(orabsorbance) against
wavenumber(frequencyorwavelength).A
typicalinfraredspectrumisshownbelow.
5/22/2020 4
•A100percenttransmittanceinthespectrumimpliesno
absorptionofIRradiation.WhenacompoundabsorbsIR
radiation,theintensityoftransmittedradiation
decreases.Thisresultsinadecreaseofpercent
transmittanceandhenceadipinthespectrum.Thedipis
oftencalledanabsorptionpeakorabsorptionband.
FEATURES OF AN IR SPECTRUM
•Differenttypesofgroupsofatoms(C-H,O-H,N-H,
etc…)absorbinfraredradiationatdifferent
characteristicwavenumbers.

5. •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”
5/22/2020 5
Baseline
Absorbance/
Peak
IR Spectrum

6. Infrared Spectrum of Propanal (CH
3CH
2CHO)4600 3800 3000 2200 1800 1400
0
10
20
30
40
50
60
70
80
90
100
1000 800 600 400
C=O
P
e
r
c
e
n
t
t
r
a
n
s
m
it
t
a
n
c
e
Wavenumber(cm)
-1
5/22/2020 6

7. Describing IR Absorptions
IR absorptions are described by their frequency
and appearance.
•Frequency(n) is given in wavenumbers (cm
-1
)
•Appearanceis qualitative: intensity and shape
•conventional abbreviations:
vs very strong
s strong
m medium
w weak
br broad
sh sharp OR shoulder
5/22/2020 7

8. Ingeneral,theIRspectrumcanbesplitinto
fourregionsforinterpretation:
40002500cm
-1
:Absorptionofsinglebonds
formedbyhydrogenandotherelements
e.g.OH,NH,CH
25002000cm
-1
:Absorptionoftriplebonds
e.g.C≡C,C≡N
20001500cm
-1
:Absorptionofdoublebonds
e.g.C=C,C=O
1500400cm
-1
:Thisregionoftenconsistsof
manydifferent,complicatedbands.Thispartofthe
spectrumisuniquetoeachcompoundandisoften
calledthefingerprintregion.Itisrarelyusedfor
identificationofparticularfunctionalgroups.
5/22/2020 8

9. Summary of IR Absorptions
5/22/2020 9

10. BASE VALUES
(+/-10 cm
-1
)
Theseare
theminimum
numberof
valuesto
memorize.
O-H 3600
N-H 3400
C-H 3000
C N 2250
C C 2150
C=O 1715
C=C 1650
C O ~1100
large range
5/22/2020 10

11. O-H STRETCH
5/22/2020 11

12. Typical Infrared Absorption
Regions
O-H
2.5 4 5 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*
5/22/2020 12

13. •O-H 3600 cm
-1
(alcohol, free)
•O-H 3300cm
-1
(alcohols&acids,H-bonding)
5/22/2020 13
3600 3300
H-BONDEDFREE
broadens
shifts
The O-H stretching region

14. O
H
R
O
H
R
RO
H
R
OH
H
RO
R H
O HYDROGEN-BONDED HYDROXYL
ManykindsofOH
bondsofdifferent
lengthsandstrengths
Thisleadstoabroad
absorption.
Longer bonds are
weaker and lead to
lower frequency.
Hydrogen bonding occurs in concentrated solutions
( for instance, undiluted alcohol ).
“Neat” solution.
5/22/2020 14

15. “FREE” HYDROXYLRO
H
CCl4
CCl4
CCl4
CCl4
CCl4
Distinctbondhasawell-
definedlengthandstrength.
Occurs in dilute solutions of alcohol in an “inert” solvent like CCl
4.
Solvent molecules
surround but do not
hydrogen bond.
The “free” hydroxyl vibrates without interference from any other
molecule.
5/22/2020 15

16. Cyclohexanol
5/22/2020 16OH
O-H
H-bond
C-H
C-O
CH
2
ALCOHOL
neat solution
,

17. Butanoic Acid
5/22/2020 17CH
3
CH
2
CH
2
COH
O
O-H
H-bond
C-H C=O
CH
2
C-O
CARBOXYLIC ACID
neat solution

18. C
O
OH
R
C
O
OH
R CARBOXYLIC ACID DIMER
Strong hydrogen bonding in the dimer weakens the OH
bond and leads to a broad peak at lower frequency.
5/22/2020 18

19. N-H STRETCH
5/22/2020 19

20. Typical Infrared Absorption Regions
N-H
2.5 4 5 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*
5/22/2020 20

21. The N-H stretching region
•Primary amines give twopeaks
•Secondary amines give onepeak
•Tertiary amines give no peak
5/22/2020 21N
H
H
N
H
H
symmetric asymmetric
N-H3300 -3400 cm
-1

22. CH
3
CH
2
CH
2
CH
2
NH
2 NH
2
NH
2
scissor
CH
2
CH
3
PRIMARY AMINE
aliphatic
1-Butanamine
5/22/2020 22

23. NH
2
CH
3 NH
2
Ar-H
-CH
3
benzene
Ar-H
PRIMARY AMINE
aromatic
3-Methylbenzenamine
5/22/2020 23

24. NHCH
2
CH
3 NH
benzene
Ar-H
CH
3
SECONDARY AMINE
N -Ethylbenzenamine
5/22/2020 24

25. N
CH
3
CH
3 no N-H
benzene
CH
3
Ar-H
Ar-H
-CH
3
TERTIARY AMINE
N,N -Dimethylaniline
5/22/2020 25

26. C-H STRETCH
5/22/2020 26

27. Typical Infrared Absorption Regions
C-H
2.5 4 5 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*
We will look at
this area first
5/22/2020 27

28. •C-H aldehyde, two peaks (both weak)
~ 2850 and 2750 cm
-1
3000 divides
UNSATURATED
SATURATED
•C-H spstretch ~ 3300 cm
-1
•C-H sp
2
stretch > 3000 cm
-1
•C-H sp
3
stretch < 3000 cm
-1
The C-H stretching region
BASE VALUE = 3000 cm
-1
5/22/2020 28

29. 3000
-C-H=C-H
31003300
=C-H=
2900 2850 2750
-CH=O
(weak)
increasing CH Bond Strength
sp
3
-1ssp
2
-1ssp-1s
increasing frequency (cm
-1
)
aldehyde
increasing s character in bond
increasing force constant K
STRONGERBONDSHAVELARGERFORCECONSTANTS
ANDABSORBATHIGHERFREQUENCIES
CH BASE VALUE = 3000 cm
-1
5/22/2020 29

30. Hexane
5/22/2020 30CH
3
CH
2
CH
2
CH
2
CH
2
CH
3
CH
stretching
vibrations
ALKANE
includes
CH
3
sym and asym
CH
2
sym and asym
CH bending vibrations
discussed shortly

31. C-H BENDING
5/22/2020 31

32. •CH
2 bending ~ 1465 cm
-1
•CH
3bending (asym) appears near
the CH
2value ~ 1460 cm
-1
•CH
3bending (sym) ~ 1375 cm
-1
5/22/2020 32
THE C-H BENDING REGION

33. C
H
H
C
H
H
C
H
H
C
H
H
C
HH
C
HH Scissoring Wagging
Rocking Twisting
Bending
Vibrations
~1465 cm
-1
~720 cm
-1
~1250 cm
-1
~1250 cm
-1
in-plane out-of-plane
METHYLENE GROUP BENDING VIBRATIONS
5/22/2020 33

34. CH
3CH
2
14651460 1375
asym sym
METHYLENE AND METHYL BENDING VIBRATIONS
these two peaks
frequently overlap
and are not resolved
C-H Bending, look near
1465 and 1375 cm
-1C
H
H
H
5/22/2020 34

35. CH
3CH
2
14651460 1375
asym sym
13701380
13701390C
CH3
CH
3 CCH
3
CH
3
CH
3 CCH3
METHYLENE AND METHYL BENDING VIBRATIONS
geminal dimethyl
t-butyl
(isopropyl)
two peaks
two peaks
The symmethyl peak
splits when you have
more than one CH
3
attached to a carbon.
ADDITIONAL DETAILS FOR SYM CH
3
one peak
5/22/2020 35

36. Hexane
5/22/2020 36CH
3
CH
2
CH
2
CH
2
CH
2
CH
3
CH
stretch
CH
2
bend
CH
3
bend
CH
2
rocking
ALKANE

37. 1-Hexene
5/22/2020 37CH
2
CHCH
2
CH
2
CH
2
CH
3
=CH
CH
C=C CH
2
CH
3
bend
CH
ALKENE

38. Toluene
5/22/2020 38CH
3
C=C
benzene
CH
3
Ar-H
Ar-H
AROMATIC

39. 1-Hexyne
5/22/2020 39CH CCH
2
CH
2
CH
2
CH
3
C=C=
=C-H=
C-H
CH
2, CH
3
ALKYNE

40. C
10H
22
C
12H
26
Similar
But Not
Identical
Fingerprinting
5/22/2020 40

41. C N AND C C STRETCH
5/22/2020 41

42. Typical Infrared Absorption
Regions
C=N
C=C
=
=
2.5 4 5 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*
5/22/2020 42

43. The triple bond stretching region
•C N2250 cm
-1
•C C2150 cm
-1
5/22/2020 43
=
=
The cyano group often gives a strong, sharp peak
due to its large dipole moment.
The carbon-carbon triple bond gives a sharp peak,
but it is often weak due to a lack of a dipole. This is
especially true if it is at the center of a symmetric
molecule. RCCR

44. Propanenitrile
5/22/2020 44CH
3
CH
2
CN
C=N=
NITRILE
BASE = 2250

45. 1-Hexyne
5/22/2020 45CH CCH
2
CH
2
CH
2
CH
3
C=C=
=C-H=
ALKYNE
BASE = 2150

46. C=O STRETCHING
5/22/2020 46

47. Typical Infrared Absorption
Regions
C=O
2.5 4 5 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*
5/22/2020 47

48. This region stretches from about 1800 to 1650
cm
-1
-RIGHT IN THE MIDDLE OF THE SPECTRUM
The base value is 1715 cm
-1
(ketone)
The bands are very strong !!! due to the large
C=O dipole moment.
5/22/2020 48
C=O is often one of the strongest peaks in
the spectrum
THE CARBONYL STRETCHING REGION

49. 2-ButanoneCH
3
CCH
2
CH
3
O
KETONE
C=O
C-H
overtone
2x C=O
CH bend
BASE = 1715
1715
C=O
5/22/2020 49

50. CR
O
H
CR
O
O
CR
O
CR
O
Cl
CR
O
OR'
CR
O
R
CR
O
NH
2
CR
O
OH 169017101715172517351800
1810 and 1760
BASE
VALUE
acid
chloride esteraldehyde
carboxylic
acid amideketone
anhydride
( two peaks )
EACH DIFFERENT KIND OF C=O COMES AT A DIFFERENT FREQUENCY
C=O IS SENSITIVE TO ITS ENVIRONMENT
THESE VALUES ARE
WORTH LEARNING
all are +/-10 cm
-1
5/22/2020 50

51. 1.225 A 1.231 A 1.235 A 1.248 A
acid
chloride
ester ketone amide
C=O BOND LENGTHS IN CARBONYL COMPOUNDS
shorter longer
1780 cm
-1
1735 cm
-1
1715 cm
-1
1680 cm
-1
5/22/2020 51

52. Ketonesare at lower frequency than Aldehydesbecause of
the second electron-donating alkyl group.
Acid chloridesare at higher frequency than ketones because
of the electron-withdrawing halide.
Estersareathigherfrequenciesthanketonesduetothe
electron-withdrawingoxygenatom.Thisismoreimportantthan
resonancewiththeelectronpairontheoxygen.
Amides areatlowerfrequenciesthanketonesdueto
resonanceinvolvingtheunsharedpaironnitrogen.Theelectron-
withdrawingeffectofnitrogenislessimportantthanthe
resonance.
SUMMARY
Notetheelectronegativitydifference,OversusN,weightsthe
twofactors(resonance/e-withdrawal)differentlyinesters
thaninamides.
Acidsare at lower frequency than ketones due to H-bonding.
5/22/2020 52

53. CH
3
CCH
2
CH
3
O KETONE
C=O
C-H
overtone
CH bend
BASE = 1715
2-Butanone
5/22/2020 53
1719 x 2 = 3438
overtone of strong C=O peak
3438

54. CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
O ALDEHYDE
C=O
CHO
CH bend
BASE = 1725
Nonanal
5/22/2020 54
3460

55. C
Cl
O
CH
3
(CH
2
)
10 ACID CHLORIDE
C=O
C-H
CH bend
BASE = 1800
Dodecanoyl Chloride
5/22/2020 55
3608

56. C
O
O
CH
2
CH
2
CH
3
CH
2
CH
3 ESTER
C=O
C-O
C-H
BASE = 1735
Ethyl Butanoate
5/22/2020 56
3482

57. C
OH
O
CHCH
3
CH
3 CARBOXYLIC ACID
C=O
O-H
C-H
C-O
BASE = 1710
2-Methylpropanoic Acid
5/22/2020 57

58. C
O
OH
R
C
O
OH
R CARBOXYLIC ACID DIMER
Strong hydrogen-bonding in the dimer weakens the O-H and
C=O bonds and leads to broad peaks at lower frequencies.
lowers
frequency
of C=O
and also
of O-H
RECALL
5/22/2020 58

59. C
NH
2
O
CH
2
CH
3 AMIDE
C=O
NH
2
C-H
CH bend
BASE = 1690two peaks
sym / asymPropanamide
5/22/2020 59

60. C=C STRETCHING
ALKENES
AROMATICS
5/22/2020 60

61. Typical Infrared Absorption
Regions
C=C
2.5 4 5 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*
5/22/2020 61

62. The C=C stretching region
C=C double bond at 1650 cm
-1
is often weak or not
even seen.
C=C benzene ring shows peak(s) near 1600 and 1400
cm
-1
, one or two at each value -CONJUGATION LOWERS
THE VALUE.
When C=C is conjugated with C=O it is stronger and
comes at a lower frequency.
5/22/2020 62

63. 1-Hexene
5/22/2020 63CH
2
CHCH
2
CH
2
CH
2
CH
3
ALKENE
oops
C=C=C-H
C-H
aliphatic
C-H
bend

64. Toluene
5/22/2020 64CH
3
AROMATIC
benzene
oops
benzene
C-H
C=C

65. Typical Infrared Absorption
Regions
C-O
2.5 4 5 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*
5/22/2020 65

66. C-OSTRETCHING
5/22/2020 66

67. The C-Ostretching region
The C-Oband appears in the range of
1300 to 1000 cm
-1
Look for one or more strong bands
appearing in this range!
Ethers, alcohols, esters and carboxylic
acids have C-Obands
5/22/2020 67

68. CH
3
CH
2
CH
2
CH
2
OCH
2
CH
2
CH
2
CH
3 ETHER
C-O
BASE = 1100
CH
2CH
3
bending
C-H
Dibutyl Ether
5/22/2020 68

69. OCH
3 AROMATIC ETHER
benzene
oops
C-O
C-H
aromatic
BASE = 1100
Anisole
5/22/2020 69

70. OH ALCOHOL
BASE = 3600
BASE = 1100
OH
C-O
CH
2
bend
C-H
Cyclohexanol
5/22/2020 70

71. C
OH
O
CHCH
3
CH
3 CARBOXYLIC ACID
OH
CH
C=O
C-O
2-Methylpropanoic Acid
5/22/2020 71

72. C
O
O
CH
2
CH
2
CH
3
CH
2
CH
3 ESTER
CH
C=O
C-O
Ethyl Butanoate
5/22/2020 72

73. N=OSTRETCHING
5/22/2020 73

74. Typical Infrared Absorption
Regions
N-O
2.5 4 5 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*
5/22/2020 74

75. The N=Ostretching region
•N=Ostretching --1550 and 1350 cm
-1
asymmetric and symmetric stretchings
•Often the 1550 cm
-1
peak is stronger than
the other one
5/22/2020 75

76. CH
3
CH
CH
3
NO
2 NITROALKANE
N=O
N=O
C-H
gem-dimethyl
2-Nitropropane
5/22/2020 76

77. Typical Infrared Absorption
Regions
C-Cl
2.5 4 5 5.56.1 6.5 15.4
4000 2500 2000 180016501550 650
FREQUENCY (cm
-1
)
WAVELENGTH (mm)
O-HC-H
N-H
C=OC=N
Very
few
bands
C=C
C-Cl
C-O
C-N
C-CX=C=Y
(C,O,N,S)
C N
C C
N=O N=O*
5/22/2020 77

78. The C-X stretching region
C-Cl785 to 540 cm
-1
,
often hard to find amongst the fingerprint bands!!
C-Brand C-I
appear outside the useful range of infrared
spectroscopy.
C-Fbonds can be found easily, but are not that
common.
5/22/2020 78

79. CCl Cl
Cl
Cl Often used as a solvent for IR spectra.
When it is used, spectra show C-Clabsorptions.
C-Cl
Carbon Tetrachloride
5/22/2020 79

80. Cl C-Cl
oops
benzene
C=C
benzene ring
combination
bands
Chlorobenzene
5/22/2020 80

81. =C-H OUT OF PLANE BENDING
5/22/2020 81

82. H
H
H
OUT-OF-PLANE BENDING
above
below
PLANE
(OOPS)
H
ALKENES
BENZENES
also with
5/22/2020 82

83. Monosubstituted
cis-1,2-
trans-1,2-
1,1-
Trisubstituted
TetrasubstitutedCC
H
R H
H
CC
H
R R
H
CC
R
H R
H
CC
R
R R
R
CC
R
R R
H
CC
R
R H
H
Disubstituted
10 11 12 13 14 15
1000 900 800 700cm
-1
s s
m
s
s
s
m
=C-H OUT OF PLANE BENDING
ALKENES
5/22/2020 83

84. 10 11 12 13 14 15
1000 900 800 700cm
-1
Monosubstituted
Disubstituted
ortho
meta
para
Trisubstituted
1,2,4
1,2,3
1,3,5
BENZENES
m
s s
s
s s
s
s
s
s
m
m
m
m
OOPS
RING H’s
combination bands
5/22/2020 84

85. 5/22/2020 85

86. 5/22/2020 86