Basics of Infrared Spectroscopy

1. MINAKSHI RATHEE
M.PHARMACY
PDM UNIVERSITY
FACULTY OF P.CEUTICAL SCIENCES

2. CONTENT
 SPECTROSCOPY.
 IR SPECTROSCOPY.
 MOLECULAR VIBRATIONS.
 EXPERIMENTAL SETUP.
 FUTURE DIRECTIONS.

3. SPECTROSCOPY???
 Method of “Seeing the unseeable”
 Using electromagnetic radiation to obtain information about
atoms and molecules that are too small to see.
Atoms
Molecules
E
M
R

4. Spectroscopy is an instrumentally aided study of the
interactions between matter (sample being analyzed) and
energy (any portion of the electromagnetic spectrum)
EMR ANALYTE SPECTROPHOTOGRAPH
1.UV-Visible radiations--------excitation of electrons---------UV-visible spectrum
2.IR-radiations------------------vibration changes in electrons--------IR spectrum
3.Radio frequency---------------spin rotational changes-------------N.M.R spectrum
Conc. should be lower

5. IR SPECTROPHOTOMETRY
Energy of molecule = Electronic energy+ Vibrational energy +
Rotational energy
 Concerned with the study of absorption of infrared radiation,
which causes vibrational transition in the molecule.
 Thus known as Vibrational spectroscopy.
 Mainly used in structure elucidation to determine the functional
groups.

6. Most of the analytical applications are confined to the middle IR region because
absorption of organic molecules are high in this region.
IR region: 0.8
µm (800nm) to
1000 µm (1mm)
Near IR: 0.8-2
µm
Middle IR: 2-15
µm
Far IR: 15-1000
µm

7. PRINCIPLE :
 Molecules are made up of atoms linked by chemical bonds. The movement of
atoms and the chemical bonds look like spring and balls (vibration).
 This characteristic vibration are called
Natural frequency of vibration.
 Applied infrared frequency = Natural frequency of vibration
 Change in dipole moment is required

8. MOLECULAR VIBRATIONS
Bending vibrationsStretching vibrations
 Vibration or oscillation
along the line of bond
 Change in bond length
 Occurs at higher energy:
4000-1250 cm-1
a) Symmetrical stretching
b) Asymmetrical stretching
•Vibration not along the
line of bond
•Bond angle is altered
•Occurs at low energy:
1400-666 cm-1
a) In plane bending
b) Out plane bending

9. A) SYMMETRICAL STRETCHING:
2 bonds increase or decrease in length simultaneously.
H
H
C
STRECHING VIBRATIONS

10. B) ASYMMETRICAL STRETCHING
 in this, one bond length is increased and other is decreased.
H
H
C

11. A) IN PLANE BENDING
Scissoring:
• 2 atoms approach each other
• Bond angles are decrease
H
H
CC

12. Rocking:
 Movement of atoms take place in the same direction.
H
H
CC

13. B) OUT PLANE BENDING
i. Wagging:
 2 atoms move to one side of the plane. They move up and down the plane.
ii. Twisting:
 One atom moves above the plane and another atom moves below the plane.
H
H
CC
H
H
CC

14. FACTORS RESPONSIBLE FOR SHIFTING THE VIBRATIONAL
FREQUENCIES FROM THEIR NORMAL VALUES
 Coupled vibrations
 Fermi resonance
 Electronic effects
 Hydrogen bonding

15. COUPLED VIBRATIONS
 An isolated C-H bond has only one stretching vibrational frequency
where as methylene group shows two stretching vibrations,
symmetrical and asymmetrical.
 Because of mechanical coupling or interaction between C-H stretching
vibrations in the CH2 group.
 Assymetric vibrations occur at higher frequencies or wave numbers
than symmetric stretching vibrations.
 These are known as coupled vibrations because these vibrations
occur at different frequencies than that required for an isolated C-H
stretching.
 A strong vibrational coupling is present in carboxylic acid anhydrides
in which symmetrical and asymmetrical stretching vibrations appear
in the region 1720 – 1825 cm-1.

16. o The interaction is very effective probably because of the partial double
bond character in the carbonyl oxygen bonds due to resonance which also
keeps the system planar for effective coupling.
o Asymmetrical stretching band in acyclic anhydride is more intense where as
symmetrical stretching band is more intense in cyclic anhydrides.

17. FERMI RESONANCE
 Resonance
- A vibration of large amplitude produced by a relatively small
vibration.
 Coupling of two fundamental vibration modes produces two new
modes of vibration ,with frequencies higher and lower than that
observed in absence of interaction. Interaction can also take place
between fundamental vibrations and overtones or combination tone
vibrations and such interactions are known as Fermi Resonance.

18.  If two different vibrational levels, belonging to the same species, have
nearly the same energy.
 Shifting of one towards lower and other towards higher frequency
occur.
 A substantial increase in the intensity of the respective bands occur.

19.  For e.g. symmetrical stretching vibration of CO2 in Raman spectrum shows
band at 1337 cm-1.The two bending vibrations are equivalent and absorb
at the same frequency of 667.3cm-1.
 The first overtone of this is 2 X 667.3 = 1334.6 cm-1..
 Fermi resonance occurs
 There is mixing of 1337cm-1 and 1334.6 cm-1to give two bands at 1285.5
cm-1 and at 1388.3 cm-1 with intensity ratio 1 : 0.9 respectively.
Asymmetric stretching
Symmetric stretching

20. HYDROGEN BONDING
 It occurs in any system containing a proton donor group(X-H) and a proton
acceptor. if the s-orbital of the proton can effectively overlap the P or π
orbital of the acceptor group.
 The stronger the hydrogen bond, the longer the O-H bond, the lower the
vibration frequency and broader and more intense will be the absorption
band.
 The N-H stretching frequencies of amines are also affected by hydrogen
bonding as that of the hydroxyl group but frequency shifts for amines are
lesser than that for hydroxyl compounds.
 Because nitrogen is less electronegative than oxygen so the hydrogen
bonding in amines is weaker than that in hydroxy compounds.

21.  Intermolecular hydrogen bonds gives rise to broad bands, while
intramolecular hydrogen bonds give sharp and well defined bands.
 The inter and intramolecular hydrogen bonding can be distinguished
by dilution.
 Intramolecular hydrogen bonding remains unaffected on dilution and
as a result the absorption band also remains unaffected where as in
intermolecular, bonds are broken on dilution and as a result there is a
decrease in the bonded O-H absorption .

22. The strength of hydrogen bonding is also affected by :
 Ring strain
 Molecular geometry
 Relative acidity and basicity of the proton donor and acceptor groups

23. ELECTRONIC EFFECT
 Changes in the absorption frequencies for a particular group take
place when the substituents in the neighbourhood of that particular
group are changed.
It includes :
 Inductive effect
 Mesomeric effect
 Field effect

24. INDUCTIVE EFFECT
 The introduction of alkyl group causes +I effect which results in the
lengthening or the weakening of the bond
 Hence the force constant is lowered and wave number of absorption
decreases.
 Let us compare the wave numbers of v (C=O) absorptions for the following
compounds :
 Formaldehyde (HCHO) 1750 cm-1.
 Acetaldehyde (CH3CHO) 1745 cm-1.
 Acetone (CH3COCH3) 1715 cm-1.
 Introduction of an electronegative atom or group causes –I effect which
results in the bond order to increase.
 Hence the force constant increases and the wave number of absorption
rises.

25. MESOMERIC EFFECT :
 It causes lengthening or the weakening of a bond leading in the lowering of
absorption frequency.
 As nitrogen atom is less electronegative than oxygen atom, the electron pair on nitrogen atom
in amide is more labile and participates more in conjugation.
 Due to this greater degree of conjugation, the C=O absorption frequency is much less in
amides as compared to that in esters.

26. FIELD EFFECT :
 In ortho substituted compounds, the lone pair of electrons on two
atoms influence each other through space interactions and change
the vibrational frequencies of both the groups.
 This effect is called field effect.

27. EXPERIMENTAL SETUP

28. COMPONENTS
1. Source
2. Fore optics
3. Monochromator
4. Detector
5. Recorder

29. SOURCE
•Ideal => black body radiator
•COMMONLY USED => GLOBER FILAMENT &NERNST GLOWER
•Globar- resistance rod of silicon carbide
- for longer wavelengths
•Nernst glower- a spindle of rare earth oxide(thorium, zirconium , etc)
- for shorter wavelengths

30. FORE OPTICS
•CONSISTS OF:
1. SOURCE
2. MIRRORS - M1, M2 and a rotating mirror M
 M1,M2 – divides the beam
 M - alternately allows the sample beam and
reference beam to pass through

31. MONOCHROMATOR
 Splits the polychromatic radiation to component wavelengths.
 Make use of prisms or grating or both.
 Resolution depends on slit width and quality of mirrors.
 Rock salt prism is generally used in the range of 650-4000 cm-
1

32. DETECTOR
• Measure the radiant energy by its heating effect.
• Thermopiles bolometer and golay cells are generally used
• Photoconductivity is also used.
• Radiation is allowed to fall on photo conducting material and the conductivity of
the material is measured continuously by a bridge network.
• Once the sample absorbs radiation, there will be inequality between the two
radiations and signal will be produced.

33. THE RECORDER
• The amplified signal is used to move an attenuator which cuts down
the radiation coming out of the reference beam until energy balance is
restored.
• This is achieved by a motor which drives the comb into the reference
beam when an absorbing band is encountered and out of the beam
when the band is passed over.
• The recorder pen is also coupled to this motor so that the comb
movement is followed exactly by the pen.

34. APPLICATIONS OF IR SPECTROSCOPY
 QUALITATIVE ANALYSIS
 1. Identification of Substances
 To compare spectrums.
 No two samples will have identical IR spectrum.
 Criteria: Sample and reference must be tested in identical conditions,
like physical state, temperature, solvent, etc.
 Disadvantages: Enantiomers cannot be distinguished (spectrum are
identical).

35. The “Fingerprint” Region (1200 to 700 cm-1) :
 Small differences in structure & constitution of molecule  result in
significant changes in the peaks in this region.
 Hence this region helps to identify an unknown compound.

36. Computer Search Systems:
 Newer IR instruments offer computer search systems to identify
compounds from stored infrared spectral data.
 The position and magnitudes of peaks in the spectrum is compared
with profiles of pure compounds stored.
 Computer then matches profiles similar to that of the analyte and
result is displayed.

37. 2. DETERMINATION OF MOLECULAR STRUCTURE
 Used along with other spectroscopic techniques.
 Identification is done based on position of absorption bands in the
spectrum.
 Eg.: C=O at 1717 cm-1.
 Absence of band of a particular group indicates absence of that group
in the compd.

38. 4. DETECTION OF IMPURITIES
 Determined by comparing sample spectrum with the spectrum of pure
reference compound.
 Eg.: ketone impurity in alcohols.
 Detection is favoured when impurity possess a strong band in IR
region where the main substance do not possess a band.

39. 5. ISOMERISM IN ORGANIC CHEMISTRY
(i) Geometrical Isomerism:
 trans isomers give a simpler spectrum than cis due to symmetry.
(ii) Conformers (Rotational Isomers):
Identified with the help of high resolution IR spectrometers
(iii) Tautomerism:
Existence of 2 or more chemical compds capable of intercovertion ,
usually by exchanging a hydrogen atom between the 2 atoms.
e.g.: Thiocarboxylic acid

40. 3. STUDYING PROGRESS OF REACTIONS
 Observing rate of disappearance of characteristic absorption band in
reactants; or
 Rate of increasing absorption bands in products of a particular
product.
 Eg.: O—H = 3600-3650 cm-1
C=O = 1680-1760 cm-1

41. QUANTITATIVE ANALYSIS
 Based on the determination of one of the functional groups.
E.g.: concn of hexanol in hexane-hexanol mixture.
A = -log I1/I0 = abc (Beer-Lambert’s law)
A = Absorbance
I0 = Intensity of radiation before entering the sample
I1 = Intensity of radiation after leaving the sample
a = Absorptivity of the solution
b = Initial path length of the sample cell
c = concn. of the solution
If ‘b’ & ‘a’ are const., then ‘A’ α ‘c’

42. 2 methods to determine ‘A’ and conc. ‘c’:
1. Cell-in cell-out Method:
Std. calibration curve method
2. Baseline Method:
 selection of suitable absorption band
 P0 & P are measured
 Abs, log (P0/P) plotted against conc; determine unknown

43. Using KBr Pellets (Disk Technique):
 Uniform pellets of similar weight & thickness
 Known wts. of KBR + known qty of test
 Calibration curve plotted
 Disks are weighed and thickness measured
Using Internal Std. (pot. thiocyanate):
 Dried, ground with KBr to make a conc of 0.2% by wt of thiocyanate.
 Calibration curve plotted.
 Ratio of thiocyanate absorption at 2125 cm-1 to a chosen band of test
is plotted vs conc.

44. OTHER APPLICATIONS
1. Determination of unknown contaminants in industry using FTIR.
2. Determination of cell walls of mutant & wild type plant varieties using
FTIR.
3. Biomedical studies of human hair to identify disease states (recent
approach).
4. Identify odour & taste components of food.
5. Determine atmospheric pollutants from atmosphere itself.