1. Infrared Spectroscopy
Lokanathan Arcot
Department of Forest Products Technology
School of Chemical Technology
Aalto University

2. Basis of Infrared Spectroscopy
Atoms
Molecules
Bond
(e– density transfer)
Dipole Moment 𝛿0 𝛿+ 𝛿– + –
Non-Polar
’0’
Dipole moment
Highly Polar
’High’
The two atoms shown in zoomed in version of molecule is in fact NaCl
Delta EN (difference in electronegativity) Measure of Polarity
Dr. Lokanathan Arcot

3. Vibrational Spectroscopy: Infrared
Examples of Molecular Vibrations
Others
Rocking
Wagging
Twisting
In-plane Scissoring
Asymmetric
Stretching
Symmetric
Stretching
Effect of Vibrations
- Monopoles of dipole vibrate at a Freq.
- Oscillating elec. Field of same Freq.
Absorption of Light of wavelength λ or Frequency
Absorption occurs if the incident light wave has the same frequency as oscillating electric field of a molecule vibrating in a ’non-zero’ dipole moment mode
Dr. Lokanathan Arcot

4. Vibrational Properties of Bonds
Vibrational frequencies of a bond between two atoms (1 and 2)
Energy of Vibration
Frequency of Vibration
En is the energy of the nth vibrational level
n is an integer
h is Planck’s constant
 is the frequency of the vibration
k is the force constant of the bond
µ is the reduced mass
m1 and m2 the mass of the vibrating atoms 1&2
Reduced Mass
Dr. Lokanathan Arcot

5. Vibrational Properties of Bonds
Vibrational frequencies of a bond between two atoms (1 and 2)
Energy of Vibration
Frequency of Vibration
Reduced Mass
Main implications:
Vibrational frequencies increase () with increasing bond strength (k)
Vibrational frequencies increase with decreasing mass of the vibrating atoms
Dr. Lokanathan Arcot

6. How a IR spectrum is recorded
Source of Light
A range of λ
Intensity IO
Sample
Molecules
Absorption
Transmitted I
Detector
IO - I
Dr. Lokanathan Arcot

7. What an IR spectrum looks like
spectrum is plotted as a function of either absorbance or transmittance
IR spectrum of clay
100 %
0,02
0,04
0,06
0,08
0,10
0,12
0,14
0,16
0,18
0,20
0,22
0,24
0,26
0,28
500
1000
1500
2000
2500
3000
3500
4000
Wavenumbers (cm-1) 55 60 65 70 75 80 85 90 95
500
1000
1500
2000
2500
3000
3500
4000
Wavenumbers (cm-1)
Absorbance
% Transmittance
I =Intensity measured with
a sample in the beam
Io= Intensity measured with no sample in the beam
Course 3130, Dr. Lokanathan Arcot

8. Unit Wavenumber (cm –1) instead of nm or Hz
A typical IR Spectrum
Example: 2-pentanone
Unit Wavenumber (cm –1) instead of nm or Hz
Dr. Lokanathan Arcot

9. Why use Wavenumber (cm –1) instead of m or Hz
Mid IR region is the most useful region for spectroscopy
Microns – 2.5µm to 25µm
Hertz – 120 THz to 12 THz
Wavenumber – 4000 cm –1 to 400 cm –1
c = λ* 
 ∝ 1/ λ
Example: 2.5µm = 2.5*10-4cm
Wavenumber = 1/Wavelength (cm)
For 2.5µm we get 4000 cm –1
Where
C – velocity, λ – wavelength,  – Frequency of light
Course 3130, Dr. Lokanathan Arcot

10. What causes the absorption?
IR radiation induces vibrations in molecules and/or functional groups
each vibration by a functional group is induced at a distinct wavelength
(or wavenumber 1/)
Example: CH2 -group
Asymmetrical stretching
(as CH2) ~ 2926 cm –1
Symmetrical stretching
(s CH2) ~ 2853 cm –1
Course 3130, Dr. Lokanathan Arcot

11. What causes the absorption?
Other fundamental vibrations in CH2 group, induced by IR radiation:
In-plane bending or scissoring
(δs CH2)
~ 1465 cm –1
Out-of-plane bending or wagging
(ωs CH2)
~ 2926 cm –1
Out-of-plane bending or twisting
(t CH2)
~ cm –1
in-plane bending or rocking
(r CH2)
~ 720 cm –1
Course 3130, Dr. Lokanathan Arcot

12. What causes the absorption?
Example:
IR spectrum of cyclohexane
(contains only CH2 groups)
Fundamental vibrations in
CH2 induced by IR radiation
Asymmetrical stretching
(as CH2)
~ 2926 cm –1
Symmetrical stretching
(s CH2)
~ 2853 cm –1
Course 3130, Dr. Lokanathan Arcot

13. What causes the absorption?
Example 2: Water
Fundamental vibrations
symmetrical stretching at 3652 cm-1 has no change in dipole moment
IR requires the vibration be such that it changes the dipole moment
hydrogen bonding shifts the absorption to lower wavenumbers
Course 3130, Dr. Lokanathan Arcot

14. What causes the absorption?
Example 2: Water
Fundamental vibrations
although water has only 2 IR bands in IR spectrum, they are very broad
 water usually disturbs the IR spectrum (samples are measured without water)
s – symmetric
as - asymmetric
r – scissoring
w - wagging
Course 3130, Dr. Lokanathan Arcot

15. IR active vibrations
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16. Carbon Dioxide Bending Dipole moment change Symmetric Stretching
Asymmetric Stretching
No Dipole moment change
Dipole moment change
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17. Carbon Dioxide – IR Spectrum
Course 3130, Dr. Lokanathan Arcot

18. CHARACTERISTIC GROUP ABSORPTIONS OF ORGANIC MOLECULES
Course 3130, Dr. Lokanathan Arcot

19. Example: dodecane CH3(CH2)10CH3 s – symmetric as - asymmetric
C-H bend:
s CH2: 1467 cm-1
asCH3: 1450 cm-1
s CH3: 1378 cm-1
C-H stretch:
as CH3: 2962 cm-1
s CH3: 2872 cm-1
as CH2: 2924 cm-1
s CH2: 2853 cm-1
CH2 rock:
 CH2: 721 cm-1
s – symmetric
as - asymmetric
r – scissoring
w - wagging
Course 3130, Dr. Lokanathan Arcot

20. Example: tert-butyl alcohol
1040 cm-1
C-O stretch
C-H bend
C-H stretch
O-H stretch
Neat sample
Course 3130, Dr. Lokanathan Arcot

21. Example: phenol Neat sample Overtone – multiple of given frequency
1224 cm-1
C-O stretch
810 cm-1
752 cm-1
Out-of-plane
C-H bend
1360 cm-1
In-plane
O-H bend
C=C ring
stretch
Overtone or
combination
bands
3008 cm-1
Aromatic
C-H stretch
O-H stretch
690 cm-1
Out-of-plane C=C bend
Neat sample
Overtone – multiple of given frequency
Course 3130, Dr. Lokanathan Arcot

22. Example: 2-pentanone 1366 cm-1 s CH3 of CH3O unit C-H bend 1171 cm-1
C-CO-C
stretch and bend
1717 cm-1
C=O stretch
for ketones
C-H stretch

23. Example: hexanoic acid
1413 cm-1
C-O-H
in-plane bend
1285 cm-1
C-O stretch
with C-O-H interaction
939 cm-1
O-H
out-of-plane bend
cm-1
Broad O-H stretch
1711 cm-1
C=O stretch for
carboxylic acids
C-H stretch
Neat sample

24. Example: octylamine Diluted sample 1073 cm-1 C-N stretch 1617 cm-1
N-H bend
(scissoring)
1467 cm-1
 CH2 of
(scissoring)
~780 cm-1
N-H wag
C-H stretch
Diluted sample

25. Note: effect of hydrogen bonding
IR spectra of alcohols, carboxylic acids, amines etc. are severely
affected by their surrounding medium during the measurement.
In gas phase or
diluted in a solvent
Narrow O-H stretch
Neat sample
Broad O-H stretch

26. Absorption regions of some organic functional groups
Course 3130, Dr. Lokanathan Arcot

27. Example - Cellulose
although a relatively simple molecule, cellulose is more complex than
cyclohexane or water
IR spectrum of cellulose comprises of ~ 60 different bands
qualitative analysis based only on IR is difficult
often IR is used as complementary technique (especially with NMR)
Course 3130, Dr. Lokanathan Arcot

28. Example - Cellulose
IR is reliable with pure compounds when spectral libraries are used
IR is also a handy tool for quick detection of certain functional groups
Course 3130, Dr. Lokanathan Arcot

29. cellulose vs. trimethylsilyl cellulose (TMSC)
Example – Modified Cellulose
cellulose vs. trimethylsilyl cellulose (TMSC)
Course 3130, Dr. Lokanathan Arcot

30. Example of Nanoparticle Characterization using IR Spectroscopy
Cationic
How do we follow this reaction ?
STEP 1: Look at all the bonds in reactants
STEP 2: Reactant specific bonds
Difference between Cellulose (CNC) and cationic molecule (C18)
CH2 groups – 1 in each glucose molecule
16 in each cationic molecule
C-N group – only in cationic
From Thesis:
Course 3130, Dr. Lokanathan Arcot

31. How do we follow a reaction ?
STEP 1: Look at all the bonds in reactants
STEP 2: Reactant specific bonds (CH2 , C-N)
STEP 3: Check if the bonds are IR or Raman active
Raman and IR comparison
IR bands
We need a IR/Raman database
Course 3130, Dr. Lokanathan Arcot

32. Example of Nanoparticle Characterization using IR Spectroscopy
SymCH2
assymCH2
C=O
Course 3130, Dr. Lokanathan Arcot

33. Example of Polymer Characterization using IR Spectroscopy
Alternating Co-polymer
Random Co-polymer
Ethylene
Propene
What is the difference between the two polymers?
Course 3130, Dr. Lokanathan Arcot

34. Polyethylene-propylene co-polymer+
Ethylene
(C 2)
Propene
(C 3)
Random Copolymer
Course 3130, Dr. Lokanathan Arcot

35. IR spectroscopy of mixture PE, PP
X- axis on top is in wavenumber units and blow it is microns
PE- Polyethylene, PP- Polypropylene
Course 3130, Dr. Lokanathan Arcot

36. Pyrolysis IR spectroscopy Out of plane C-H olefinic (C=C) bending
of mixture PE, PP
450 °C
Pyrolysis PE
909 cm-1
Vinyl groups
Out of plane C-H olefinic (C=C) bending
450 °C
Pyrolysis PP
889 cm-1
Vinylidene groups
PE- Polyethylene, PP- Polypropylene
Course 3130, Dr. Lokanathan Arcot

37. Pyrolysis IR spectroscopy
of mixture PE, PP
Different IR spectra after pyrolysis of mixtures of PP and PE
Increasing PP %
Note: Polyethylene (PE) - Vinyl group – 909 cm-1
Polypropylene (C3) Vinylidene – 889 cm-1
Course 3130, Dr. Lokanathan Arcot

38. Pyrolysis IR spectroscopy
of mixture PE, PP
Increasing PP %
Note: Polyethylene (PE) - Vinyl group – 909 cm-1
Polypropylene (C3) Vinylidene – 889 cm-1
Peak ratio at 909cm-1 and 889 cm-1 gives a quantitative estimate of relative amount of Propylene-Ethylene copolymer ratio
Course 3130, Dr. Lokanathan Arcot

39. Summary of Part I Basics of IR spectroscopy
Conditions for IR absorption
Frequency of vibration – bond characteristics
A typical IR spectrum
Vibrational modes
IR active- inactive (Water and Carbon dioxide)
Simple examples – Alkane, OH, COOH, NH, H-bond
General molecular bond-IR absorption bands
Cellulose and silylated cellulose
Example I - Nanoparticle
Cellulose Nanocrystal
Example II – Polymer – Pyrolysis IR spectroscopy
Polyethylene-porpylene copolymer
Course 3130, Dr. Lokanathan Arcot

40. Short Break
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41. INSTRUMENTATION
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42. IR instrument Radiation source: Globar thermal radiators or
Nernst rod
thermal radiators
provide high intensity in IR region
Course 3130, Dr. Lokanathan Arcot

43. IR instrument Detectors Photoelectric detectors:
based on changes in electrical
conductivity caused by radiation
in semiconductors
Thermal detectors:
based on the changes in material
upon thermal energy of radiation
Course 3130, Dr. Lokanathan Arcot

44. Dispersive Vs Fourier Transform
Spectra Apparatus
Dispersive Vs Fourier Transform
Dispersive : One wavelength of IR radiation at a time
FTIR : Simultaneously measuring several wavelengths
Course 3130, Dr. Lokanathan Arcot

45. FT-IR instrument
all modern commercial IR instruments are Fourier Transform
Infrared (FT-IR) Spectroscopes
two beams: one fixed length,
the other variable length
the beam length is varied
by moving mirror
occasionally the difference
in wavelengths hits an
integer
 constructive interference
occasionally, the difference in
wavelengths hits an odd integer of
one quarter of the wavelength
 destructive interference
Course 3130, Dr. Lokanathan Arcot

46. FT-IR instrument Result: INTERFEROGRAM IR SPECTRUM Fourier
transformation
Course 3130, Dr. Lokanathan Arcot

47. FT-IR instrument
FT-IR allows the detection of all the wavenumbers simultaneously
before FT-IR, each wavenumber had to be measured separately
 measuring one spectrum took several hours, maybe days
introduction of FT-IR in 1960s revolutionised the IR technique
advances in computer technology in 1980s (PC) made FT-IR
a common instrument in all chemical laboratories
Course 3130, Dr. Lokanathan Arcot

48. Types of Samples (Physical)
SOLIDS
LIQUIDS
Powder
Solid Surface
Rough Smooth
Course 3130, Dr. Lokanathan Arcot

49. Sample Preparation Remember: Water interferes with IR spectroscopy
Samples should be dry
Compare this with Sample Preparation for Raman Spectroscopy
Dispersion of sample inside a IR transparent matrix
Pellet
Mull
Neat
II. Direct measurement (no sample preparation required)
Attenuation Total Reflectance
Diffuse Reflectance
Photoacoustic Spectroscopy
Course 3130, Dr. Lokanathan Arcot

50. Dispersion of sample inside a IR transparent matrix
Pellet - for powders
Mix KBr with sample
Alkali Halides become IR transparent pellets upon applying high pressure
Example: KBr
Load it up in the pressing machine
psi
Absorbance measurement
The formed Pellet can be used for IR absorption spectroscopy
Course 3130, Dr. Lokanathan Arcot

51. Sample between two IR transparent plates
Mull - for powders: Sample is mixed with a liquid and placed in-between two NaCl plates for IR absorbance meaurement. (Why not KBr? )
Nujol is brand of mineral oil most commonly used to make mull ( Nujol Mull)
The oil or liquid used must be transparent in the IR region of interest, non-volatile
Mix sample with liquid (mineral oil)
Spread it over one of the pair of NaCl plates
Make a sandwitch of sample between two plates
KBr is more hygroscopic than NaCl
But it is easier to polish
Course 3130, Dr. Lokanathan Arcot

52. Sample inbetween two IR transparent plates
Neat - for liquid sample: Liquid sample is placed in-between two NaCl plates for IR absorbance measurement.
Place liquid on one of the pair of NaCl plates
Make a sandwitch of sample between two plates
Absorbance measurement
Course 3130, Dr. Lokanathan Arcot

53. IR Transmittance after Sample preparation
KBr Pellet /
NaCl plates I
sample mixed
with KBr
Detector
Disadvantages:
pressing KBr pellets is laborius
KBr is hygroscopic  water interferes the analysis
What about IR spectroscopy without sample preparation?
Direct measurement (no sample preparation required)
Attenuation Total Reflectance
Diffuse Reflectance
Photoacoustic Spectroscopy

54. What is Total Internal Reflection ?
Attenuated total reflectance (ATR-IR)
What is Total Internal Reflection ?
Course 3130, Dr. Lokanathan Arcot

55. Total Internal Reflection creates an Evanescent Wave
Attenuated total reflectance (ATR-IR)
Total Internal Reflection creates an Evanescent Wave
Upon internal reflection the electric and magnetic field of incident light partially propogate into the upper lower refractive index medium
Right image from:
Left Image from:
Quote from :
‘The simulation shows that an evanescent wave is reflected from the structure at the interface between a high index dielectric material and a low index material. The reflected evanescent wave couples into the upper medium and radiates its energy forming a retro-reflected wave, which appears as a sharp peak near the edge of the structure when imaging the structure in hyper-numerical-aperture solid immersion microscopy. ‘
More Explanation:
When you have ordinary TIR going on at the boundary between a higher refractive index medium, and a lower index medium; with the beam incident from the high index side of the boundary, classical EM theory says there is no energy propagation in the low index medium; but there is an EM "field" in the low index medium, that drops exponentially with distance from the interface, in a space of the order of the wavelength. There isn't supposed to be any propagating wave, as a result of this evanescent field.
Course 3130, Dr. Lokanathan Arcot

56. Total Internal Reflection creates an Evanescent Wave
Attenuated total reflectance (ATR-IR)
Total Internal Reflection creates an Evanescent Wave
Z – distance from surface
I – intensity of field
d – arbitrary distance
The intensity of field decays exponentially as a function of distance
Evanescent means vanishing
Right image from:
Left Image from:
Quote from :
‘The simulation shows that an evanescent wave is reflected from the structure at the interface between a high index dielectric material and a low index material. The reflected evanescent wave couples into the upper medium and radiates its energy forming a retro-reflected wave, which appears as a sharp peak near the edge of the structure when imaging the structure in hyper-numerical-aperture solid immersion microscopy. ‘
More Explanation:
When you have ordinary TIR going on at the boundary between a higher refractive index medium, and a lower index medium; with the beam incident from the high index side of the boundary, classical EM theory says there is no energy propagation in the low index medium; but there is an EM "field" in the low index medium, that drops exponentially with distance from the interface, in a space of the order of the wavelength. There isn't supposed to be any propagating wave, as a result of this evanescent field.
Course 3130, Dr. Lokanathan Arcot

57. Attenuated total reflectance (ATR-IR)
a beam of radiation is reflected
on the interface of two materials with
different refractive indices (n2n1)
 an evanescent wave appears in the
material with lower refractive index (n2)
Evanescent wave penetratation depth:
is the wavelength of IR radiation
is the incident angle of IR radiation
n21 is the ratio of refractive indices
(n2/n1)
Course 3130, Dr. Lokanathan Arcot

58. Attenuated total reflectance (ATR-IR)
penetration depth usually in the
order of 1-5 µm
evanescent wave is absorbed
selectively by the sample
 IR spectrum of the surface of
the sample
 ATR-IR
ATR crystal (internal reflection
element) is very small
 easy to select a location on
 mapping
Course 3130, Dr. Lokanathan Arcot

59. Attenuated total reflectance (ATR-IR)
Advantages:
minimal sample preparation
fast analysis
selecting locations enable mapping
Disadvantages:
problems in reproducibility:
the contact between the sample
and ATR-IR crystal (internal
reflection element) is not always
reproducible
Sample types
Powder
Solids (Pulp, paper)
Course 3130, Dr. Lokanathan Arcot

60. IR microscope Sample viewing with visible light
surface spectra with ATR-objective
Photo and instrumentation: VTT
Course 3130, Dr. Lokanathan Arcot

61. Photo and instrumentation: VTT
IR microscope
IR microscope enables
analysis of visually
intriguing spots on the
sample
for instance, a dirt speckle
on paper can be selected
by the microscope and
subjected to ATR-IR
analysis
Photo and instrumentation: VTT

62. Photoacoustic detection or photoacoustic spectroscopy (PAS)
absorption of IR radiation generates
heat in the sample
 heat waves reach the sample
surface
 heat is released to the inert gas
above the sample
 pressure changes in the gas
 pressure changes are detected
with a sensitive microphone
Sample types
Powder
Solids (Pulp, paper)
microphone
Course 3130, Dr. Lokanathan Arcot

63. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS)
Specular reflection
Diffuse reflection
When incident light penetrating a surface is scattered in all directions,
the phenomenon is called diffuse reflectance.
Applicable to: - powders
- rough surfaces
Course 3130, Dr. Lokanathan Arcot

64. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS)
Transmission-reflectance:
When light hits a particle, it can pass through or reflect
When passing through, the particle absorbs IR radiation
Transmission-reflectance event can occur many times
Finally, the outcoming IR beam is collected by a spherical mirror
Course 3130, Dr. Lokanathan Arcot

65. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS)
Three main ways to prepare a sample for DRIFTS measurement:
Powderize the sample
Scratch the sample surface and collect the detached pieces on
an abrasive paper
(3) Disperse particles (like colloids) in a volatile solvent and allow the
solvent to evaporate by placing a few drops on a substrate
 rough surface
Course 3130, Dr. Lokanathan Arcot

66. Reflection absorption infrared spectroscopy (RAIRS)
IR beam is projected on a reflective surface (substrate) which supports
an ultrathin film at grazing (very small) angle
Only those vibrations, which are perpendicular (P-polarized) to the surface,
are IR active and give rise to an observable absorption band
Course 3130, Dr. Lokanathan Arcot

67. Reflection absorption infrared spectroscopy (RAIRS)
Film roughness and thickness affect the spectrum
Measurements require an ultrahigh vacuum (UHV)
Restricted to ultrathin films on solid supports which reflect IR light
Course 3130, Dr. Lokanathan Arcot

68. Example – Alkane thiol self assembly on Au and Silver surface
Reflection absorption infrared spectroscopy (RAIRS)
Alkane thiol
(HS)
10°
30°
CH2-stretching
2855 cm-1 sym
2925 cm-1 asym Au
Silver
J. Phys. Chem. B, 1998, 102 (2),
Course 3130, Dr. Lokanathan Arcot

69. Good handbooks on IR spectroscopy
Silvestein, Bassler, Morrill Spectrometric identification of organic compounds, Wiley
Williams, Fleming Spectroscopic methods in organic chemistry, McGraw-Hill
Koenig Spectroscopy of polymers, Elsevier
(Several editions available from all titles)

70. Summary of Part II Instrumentation IR sources, Detectors
Dispersive Vs FITR
Sample preparation
KBr Pellet
Mull Nujol
Neat
Direct Measurement
Attenuated Total Internal Reflection
Photoacoustic Spectroscopy
Diffuse Reflectance
Reflection Absorption
Course 3130, Dr. Lokanathan Arcot

71. Have a nice weekend Next week Monday – Surface Plasmon Resonance
Wednesday – Quarz Crystal Microbalance
Friday – Atomic Force Microscopy
Course 3130, Dr. Lokanathan Arcot