Application of ir
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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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THANK YOU