APPLICATIONS OF IR SPECTROSCOPY

Presented by:Muhammed Fahad

1st M.Pharm

QUALITATIVE ANALYSIS

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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).

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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.

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.

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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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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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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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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Contd…

• E.g.: Ethanol normal OH – 3636 cm-1 weak band – 3622 cm-1

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(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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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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7. APPLICATIONS OF IR SPECTROSCOPY TO INORGANIC COMPLEXES

Difficulties:1. High modes of vibration:

Since they contain more than 5 atoms; hence min. 10 modes of vibrations.

2. Lower symmetry of Complexes:Due to formation of ligand & polynuclear complexes.

3. Formation of chelates:Large no. of new bands appear.

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APPLICATIONS OF IR SPECTROSCOPY TO INORGANIC COMPLEXES

Assumptions (not always true):1. Complex formation only affects the vibrations of

the ligand slightly.2. Vibrations do not undergo coupling with other

vibrations of another ligand.3. When ligand is coordinated with another atom,

it will not change the symmetry of the ligand.

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Geometrical Isomerism:Eg: Bipyridyl cobalt (III) chloride

[Co(Bipy)2(Cl2)Cl]

2 isomers – trans isomer has more symmetry than cis isomer. Hence complex spectrum is expected for cis isomer.

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APPLICATIONS OF IR SPECTROSCOPY TO INORGANIC COMPLEXES

8. 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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9. 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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10. 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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11. 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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12. 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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13. In Industry1. 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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14. 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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15. 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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16. 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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17. 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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Other Applications1. 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.28

QUANTITATIVE ANALYSIS

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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 = AbsorbanceI0 = Intensity of radiation before entering the sampleI1 = Intensity of radiation after leaving the samplea = Absorptivity of the solutionb = Initial path length of the sample cellc = concn. of the solution

If ‘b’ & ‘a’ are const., then ‘A’ α ‘c’30

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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Baseline Method:

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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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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.34

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THANK YOU