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APPLICATIONS OF INFRA-RED SPECTROSCOPY

By,Joan Vijetha.R

M.Pharm (Pharmaceutics)

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CONTENT• Introduction• Application of IR

Qualitative analysisQuantitative analysis

• Summary

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INTRODUCTIONSufficient information about the structure.Large no. of absorption band from which a

wealthy information about the structure can be derived.

Radiation causes the molecules to stretch or bend with respect to one other.

0.8-2.4µ/12500-4000cm-1-Near Infra Red2.5-15µ/4000-667cm-1- Mid Infra Red15-200µ/667-50cm-1-Far Infra RedPrinciple-Excitation of molecule from lower

to higher vibrational level

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Qualitative analysisUsed for identifying organic, inorganic, and

biological species. The time required to perform a structural

determination was reduced by a factor of ten, one hundred, or even one thousand (by FTIR modern instruments).

Identification of an organic compound is a two-step process.

The first step involves determining what functional groups are most likely present by examining the group frequency region.

The second step then involves a detailed comparison of the spectrum of the unknown with the spectra of pure compounds that contain all of the functional groups found in the first step.

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The fingerprint region, from 1200 to 700 cm -1 is particularly useful because small differences in the structure and constitution of a molecule result in significant changes in the appearance and distribution of absorption peaks in this region.

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Identification of functional group

A large number of compound can be estimated by knowing the functional group.

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frequency, cm–1 bond functional group

3640–3610 (s, sh) O–H stretch, free hydroxyl alcohols, phenols

3500–3200 (s,b) O–H stretch, H–bonded alcohols, phenols

3400–3250 (m) N–H stretch primary, secondary amines, amides

3300–2500 (m) O–H stretch carboxylic acids

3330–3270 (n, s) –C(triple bond)C–H: C–H stretch

alkynes (terminal)

3100–3000 (s) C–H stretch aromatics

3100–3000 (m) =C–H stretch alkenes

3000–2850 (m) C–H stretch alkanes

2830–2695 (m) H–C=O: C–H stretch aldehydes

2260–2210 (v) C(triple bond)N stretch nitriles

2260–2100 (w) –C(triple bond)C– stretch alkynes

1760–1665 (s C=O stretch carbonyls (general)

1760–1690 (s) C=O stretch carboxylic acids

1750–1735 (s) C=O stretch esters, saturated aliphatic

1740–1720 (s) C=O stretch aldehydes, saturated aliphatic

1730–1715 (s) C=O stretch alpha,beta–unsaturated esters

1715 (s) C=O stretch ketones, saturated aliphatic

1710–1665 (s) C=O stretch alpha,beta–unsaturated aldehydes, ketones

m=medium, w=weak, s=strong, n=narrow, b=broad, sh=sharp

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1680–1640 (m) –C=C– stretch alkenes

1650–1580 (m) N–H bend primary amines

1600–1585 (m) C–C stretch (in–ring) aromatics

1550–1475 (s) N–O asymmetric stretch nitro compounds

1500–1400 (m) C–C stretch (in–ring) aromatics

1470–1450 (m) C–H bend alkanes

1370–1350 (m) C–H rock alkanes

1360–1290 (m) N–O symmetric stretch nitro compounds

1335–1250 (s) C–N stretch aromatic amines

1320–1000 (s) C–O stretch alcohols, carboxylic acids, esters, ethers

1300–1150 (m) C–H wag (–CH2X) alkyl halides

1300–1150 (m) C–H wag (–CH2X) alkyl halides

1250–1020 (m) C–N stretch aliphatic amines

1000–650 (s) =C–H bend alkenes

950–910 (m) O–H bend carboxylic acids

910–665 (s, b) N–H wag primary, secondary amines

900–675 (s) C–H "oop" aromatics

850–550 (m) C–Cl stretch alkyl halides

725–720 (m) C–H rock alkanes

700–610 (b, s) –C(triple bond)C–H: C–H bend

alkynes

690–515 (m) C–Br stretch alkyl halides

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Methyl salicylate

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Study of keto-enol tautomerism

Diketones and ketoester exhibit keto-enol tautomerism provided they have α-H atom in them.

Acetoacetic ester-exists in keto enol isomers in equilibrium

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Study of complex molecules Used to establish the structure of complex

moleculesEg: Two structure of penicillin were proposed on basis of IR*The IR spectra of oxazolones shows 2 banbs

1825 and 1675 cm-1 but no such band appeared in spectrum of penicillin so, oxazolones structure was ruled out.*ẞ lactaum ring do not absorb near 1770cm-1

But ẞ lactaum when fused with thiazolidin ring exhibits a band at 1770cm-1

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Study of conformational analysis Used to determine the relative stability of various

conformational of cyclic compound. Eg: In cyclohexane both chair and boat form existThey are stable in both the form. As per IR selection rules there 18 active C-C and CH2

rocking & twisting is possible for boat form. As per IR selection there are five band for chair form. The spectral examination cyclohexane region is

near1350-700cm-1 reveals 5 band for chair form hence more stable.

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Geometrical isomerism Vibration in IR is caused by change in dipole-

movement. Also the absorb depend upon the change in

dipole-movement. This technique clearly makes a distinction

between cis and trans isomers.Eg: 1,2-dichloro ethene

In trans isomers no dipole occur so no peak But in case of cis dipole movement occur so

peak form at 1580cm-1

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Rotational isomerism IR spectroscopy helps in the detection of

skew(gauche) and trans(staggerd) conformation.

Eg:1,2 dichloro ethane.

Two bands at 1291 and 1235cm-1

Trans form predominates at low temperature. Skew at higher temperature.

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Identity by Fingerprinting Each compound has a different and unique

characteristic set of adsorption band in spectrum. 700-1400cm-1 is called fingerprint region. To identify the unknown compound one need to

compare with the standard spectra record. Substance that give same IR spectra are identical. Small differences in the structure and constitution of a

molecule result in significant changes in the distribution of absorption peaks .

As a consequence, a close match between two spectra in this fingerprint region constitutes strong evidence for the identity of compounds yielding the spectra.

Exact interpretation of spectra in this region is seldom possible because of the complexity of the spectra.

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Study of hydrogen bonding IR spectroscopy is a power full and widely used

method for studying hydrogen bonding. Hydrogen bonding alter the vibration

frequencies of OH and NH group. An inert simple alcohol exhibits a sharp

absorption at 3620-3640cm-1 and broad at 3500-3200cm-1.

As solution dilution increases sharp peak is obtained.

The sharp absorption at higher wave length is due to OH stretching.

It becomes more intense at higher dilution because association of solute molecules is less probable.

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At higher concentration they form dimers, trimers and polymers and so it gives broad peaks.

Intermolecular hydrogen bonding decrease with dilution, where as interamolecular hydrogen bonding shows no effect.

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Progress of reactionsBy this method the progress of the reaction

can be followed.By examination of the small portion of the

spectrum is sufficient to indicate if the product has been formed or not.

Eg: Oxidation of secondary alcohol by chrominum(VI) is accompanied by decreases in the absorption intensity of OH group.

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Detection of impurity

This can be done by comparing the standard and the test spectrum.

Pure sample consist of a sharp peak and impure consist of

bent one and also some additional band.

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QUANTITATIVE ANALYSISThe quantity of the substance can be

determined either in pure form or as a mixture of two or more compounds.

In this, characteristic peak corresponding to the drug substance is chosen by of peaks for standard and test sample is compared.

This is called base line technique to determine the quantity of the substance.

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There are few difficulties in using this method Deviation from Beer's law Determination of single analytic

concentration in a mixture. The absorption band of a group in a

compound is never symmetrical

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A typical band: C

A

D

B

By applying Beer’s law, the concentration can be determined

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Application of NIRHas been widely used in analysis of

agriculture, food and pharmaceutical products.

It is a rapid technique and also adapted to quality control of streams as, optical fiber, remote sampling in industrial environment is possible.

Mostly useful in determination of solid material.

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Application of FirMostly use in treatment of disease and

illness.These rays are totally visible to our naked

eyes, capable of penetrating deep into human body.

Also used to observe interstellar gases.

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Summary• There are two main application of IR

qualitative & quantitative.• The determination of the functional group,

structural determination, Progress of reactions, geometrical isomers, Study of hydrogen bonding, ect,.

• The application of near IR, mid IR & far IR.• Near IR-Agriculture, food and pharmaceutical

products.• Mid IR-Organic, inorganic, and biological

species.• Far IR- Treatment of chronic health, observe

interstellar gases.

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Reference

1) William kemp-Organic spectroscopy, page no.-55-58.

2) R.Gopalan, PS Subramaniam, K.Rengarajan, Elements of analytical chemistry, 3rd edition 2003, page no-244-249.

3) D.Kealey and PJ Haines, Analytical chemistry, 1st edition-2002, page no-245-247.

4) R Gurdeep Chatwal,K Sham Anand, Instrumental method of chemical analysis, 7th edition-2007, page no-2.73-2.74.

5) YR Sharma, Elementry organic spectroscopy, page no-89,137-140.

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