3. 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.
• Disadvt: Enantiomers cannot be distinguished
(spectrum are identical).
3
4. 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.
5.
6. 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.
7.
8. 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.
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9. 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
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10. 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.
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11. 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.
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12. Contd…
• E.g.: Ethanol
normal OH – 3636 cm-1
weak band – 3622 cm-1
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13. (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
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14. 6. Functional Group Isomerism
• Isomerism shown by compounds having same
molecular formula but different functional
groups.
Eg: CH3–O–CH3 and CH3–CH2–OH
(Diethyl ether) (Ethanol)
OH = 3500-3100 cm-1
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15. 7. Shape of Symmetry of a Molecule
• E.g.: Nitrogen dioxide, NO2
If linear --> only 2 bands should be present.
If bent --> 3 bands should be present.
Actual spectrum shows 3 peaks at 750, 1323 and
1616 cm-1.
• Similarly, IR spectrum was used to determine
structures of XeF2, XeF4 & XeF6
linear, square
planar and octahedral resp.:
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16. 8. Identification of Functional Groups
Due to the presence of functional group region.
E.g.:
(3500-3100 cm-1) (1700 cm-1)
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17. 9. Presence of Water in Sample
• If lattice water is present, spectra will contain 3
characteristic bands at 3600-3200 cm-1, 1650
cm-1 and 600-300 cm-1.
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18. 10. Measurement of Paints &
Varnishes
• Measured by ‘reflectance analysis’
• Advt: Measure IR absorbance of paints on
appliances or automobiles without destroying the
surface.
• Make and year of car can be determined from IR
spectral analysis.
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19. 11. Examination of Old Paintings &
Artifacts
• Help to determine fake “masterpieces”.
• Varnish & paints from old items (statues, canvas,
etc.) are analysed by IR spectroscopy.
• Presence of new paint traces implies the
“masterpiece” is fake.
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20. 12. In Industry
1. Determine impurities in raw materials (to ensure
quality products).
2. For Quality Control checks; to determine the %
of required product.
3. Identification of materials made in industrial
research labs,
or materials of competitors.
E.g.: Impurity in bees wax (with petroleum wax)
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21. 13. Analysis of Petroleum HCs, Oil &
Grease contents
• These contain C–H bonds.
Absorption at 3100-2700 cm-1.
• ‘Freons’—Fluorocarbon-113; do not contain C–H
bond.
• Thus, quantity of HCs, oil & grease in freons is
determined by measuring C–H absorption at
2930 cm-1.
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22. 14. Quantitative Analysis of
Multicomponent Mixtures of Sulfur-oxygen
Anions by ATR Spectroscopy
• FTIR-ATR help to determine sulfur-oxygen anions in
aqueous solutions.
S–O stretching band at 1350-750 cm-1.
• ATR uses water resistant cells,
have short & reproducible effective path length.
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23. 15. Characterization of Heterogenous
Catalysts by Diffuse Reflectance
Spectroscopy
• Diffuse Reflectance Spectroscopy help to determine
nature of molecules attached to catalyst surfaces.
E.g.: characterization of olefin polymerization
catalysis with silica gel; diff. types of Si–OH bonds are
determined.
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24. 16. Analysis of Multilayered Polymeric
Film using FTIR Spectroscopy
• Determine identities of polymer materials in
multilayered film.
• FTIR helps in quick characterization.
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25. 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.
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27. 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’
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28. 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
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30. Advantages:
1. Common possible errors are eliminated.
2. Same cell is used for all determinations.
3. All measurements are done on points defined by
the spectrum; hence no dependence on λ
intensity.
4. Eliminate changes in instrument sensitivity and
source intensity.
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31. 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.
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