IR introduction Introduction, Principle & Theory

Introduction, Principle & Theory

1. INFRAREDSPECTROSCOPY
Introduction, Principle & Theory
THE ELECTROMAGNETIC SPECTRUM
INFRA RED
GAMMA RAYS X RAYS UV
VISIBLE
Presented by :
NIVEDITHA G
1
st
sem Mpharm
Dept of pharmaceutics
NARGUND COLLEGE OF PHARMACY

2. INTRODUCTION
IR-spectroscopyisalsocalledasVibrational
spectroscopy.
ItisconcernedwiththestudyofAbsorptionofIR
radiation,whichresultsinVibrationaltransitions.
IRspectroscopyismainlyusedtoStructureelucidation
todtmtheFunctionalgroups.
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3. DIFFERENTREGIONSOFIR
Near IR /Overtone region
(0.8 –2.5µ) (12,500 -4000/cm)
Mid IR / Vib-Rot region
(2.5 -15µ) (4000 –667/cm)
Far IR / Rotation region
(15 -200µ) (667 –50/cm)
TheabsorptionofIRradiationcanbeexpressedeitherintermsof
Wavelength(λ)/inWaveno.
InIRspectraofanorg-compthegraphisplottedas
%Transmittancev/sWaveno.
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4. The Relationship b/w Wavelength & Wave number
w.k.t… Wave no = reciprocal of wavelength (1 / λ)
Wave no = 1 / λ= 1 / λin cm (1μ= 10
-4
/cm)
if λ= 2.5μthen ….. Wave no = 1 / 2.5 ×10
-4
cm
= 4000/cm
if λ= 15μthen….. Wave no = 1/ 15 ×10
-4
cm
= 667/cm
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5. CRITERIAFORAMOLECULETOSHOWIR
ABSORPTIONSPECTRUM
1.Dipolemovement:AmoleculecanabsorbIRradiationonly
whenit’sabsorptioncausesachangeinit’sDipolemovement.
2.CorrectWavelengthofRadiation:AmoleculecanabsorbIR
radiationonlywhentheappliedInfraredfrequencyisequalto
theNaturalfrequencyofvibrationofthatmolecule
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6. PRINCIPLE
Thetechniqueisbasedonthesimplefactthatachemicalsubstance
showsmarkedabsorptionintheIRregionoftheelectromagnetic
spectrum.
AfterabsorptionoftheIRradiation,themoleculeofachemical
substanceVibratesatmanyratesofvibrationgivingrisetoclosepacked
BandscalledtheIRabsorptionspectra.
VariousbandswillbepresentintheIRspectrawhichwillcorrespondsto
thecharacteristicFunctionalgroups&bondspresentinthechemical
substance.
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7. THEORY
AbsorptionintheIRregionisduetothechangesinthe
Vibrational&Rotationallevels
InIRspectroscopytheabsorbedenergybringsaboutpredominant
changesintheVibrationalenergywhichdependson:
1.Massesoftheatomspresentinthemolecule
2.Strengthofthebond
3.Thearrangementofatomswithinthemolecule.
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8. TYPES OF VIBRATIONS
VIBRATIONS
STRETCHING
SYMMETRICAL
ASYMMETRICAL
BENDING
INPLANE
SCISSORING
ROCKING
OUT-OF-PLANE
WAGGING
TWISTING
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9. STRETCHINGVIBRATIONS:
Symmetric
Asymmetric
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10. BENDINGVIBRATIONS
IN-PLANE
Scissoring
Rocking
OUT-OF-
PLANE
Wagging
Twisting
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11. VIBRATIONALFREQUENCY:
ThevalueoftheStretchingVibrationalfrequencyofaBondcanbe
calculatedbyusingHooke’sLaw:
V = 1 / 2C [ K(m
1+m
2) / m
1. m
2 ]
1/2
V = 1 / 2C [ K / m
1. m
2 / (m
1+m
2)]
1/2
Where….C=velocityofradiation(2.998×10
10
cm/sec)
K=forceconstant=strengthofthebond
i.e.…forsinglebond=5×10
5
g/sec
2
fordoublebond=10×10
5
g/sec
2
fortriplebond=15×10
5
g/sec
2
m1&m2=Mol.Wt.ofatomsm1&m2
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12. DEGREEOFFREEDOM
Polyatomicmoleculesexhibitsmorethanonefundamental
Vibrationalabsorptionbandsthesebandsisrelatedto
degreesoffreedominamolecule.
Eachatomhas3-degreesoffreedomcorrespondstothe3co-
ordinates(X,Y,Z)whichisnecessarytodescribetheposition
relativetootheratominamolecule.
Whenaatomcombinetoformamolecule,nodegreeof
freedomislost,i.e.,thetotalno.ofdegreeoffreedomofa
moleculewillbeEqualto3nwhere…n=no.ofatomsina
molecule.
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13. AmoleculeoffinitedimensionismadeupofRotational,
Vibrational&Translationaldegreeoffreedom.So…
3ndegreeoffreedom=Translational+Rotational+Vibrational
ForLINEARmolecules:ofnatoms…..
Totaldegreeoffreedom=3n
Translationdegreeoffreedom=3
Rotationaldegreeoffreedom=2
HenceVibrationdegreeoffreedom=3n–3–2=3n–5
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14. EachVibdegreeoffreedomcorrespondstothefundamental
modeofVib&eachfundamentalmodecorrespondstothe
band.
Hencetheoreticallyforlinearmoleculestheirwillbe3n-5
fundamentalbands
Ex:forLinearmolecule:CO
2
Noofatoms=3
Totaldegreeoffreedom=3×3=9
Translationaldegreeoffreedom=3
Rotationaldegreeoffreedom=2
HenceVibrationaldegreeoffreedom=9–3–2=4
HencetheoreticallyCO
2shouldhave4fundamentalbands.
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15. ForNON-LINEARmolecules:ofnatoms…..
Innon-linearmolecules,therearethreedegreeofrotationas
therotationaboutallthreeaxes(X,Y,Z)willresultinchange
inposition.Henceitcanbecalculatedasfollows:
Totaldegreeoffreedom=3n
Translationdegreeoffreedom=3
Rotationaldegreeoffreedom=3
HenceVibrationdegreeoffreedom=3n–3–3=3n–6
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16. Ex:forNon-Linearmolecule:C
6H
6
Noofatoms=12
Totaldegreeoffreedom=3×12=36
Translationaldegreeoffreedom=3
Rotationaldegreeoffreedom=3
HenceVibrationaldegreeoffreedom=36–3–3=30
HencetheoreticallyC
6H
6shouldhave30fundamentalbands.
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17. FACTORSINFLUENCINGVIBRATIONAL
FREQUENCIES
Coupled vibrations &
Fermi Resonance
Electronic Effect
Hydrogen Bonding
Bond Angles
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18. COUPLEDVIBRATIONS& FERMIRESONANCE
IncaseofisolatedC-Hbondtherewillbeonestretchingvibrationis
observed,
But…incaseofMethylene(-CH
2-)twoabsorptionoccur,i.e.,oneis
Symmetric&anotherisAsymmetric.
Insuchcasesasymmetricvibrationsalwaysoccurathigherwaveno
comparedwiththesymmetricvibrations.
ThesearecalledCoupledVibrationssincetheseoccuratdifferent
frequenciesthanthatrqforanisolatedC-Hstretching.
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19. Similarlyinmethyl(-CH
3-)groupcoupledvibrationstakesplaceat
differentfrequenciescomparedto-CH
2-groups
sometimestheremaybetwodifferentvibrationswhichhavenearly
sameenergy.Ifsuchvibrationsbelongstosamespeciesthena
mutualperturbationofenergymayoccur,
Thisresultingshiftofonetowardslowerfrequencyandanother
towardshigherfrequency.Hencetherewillbeincreasingintensity
ofrespectiveband.
Sometimesitmayhappenthattheenergyofovertonelevelmay
coincidewithfundamentalmodeofdifferentvibrations.Hencethere
maybearesonanceoccurs,thistypeofresonanceiscalledFermi
resonance.
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20. Here,amoleculetransferitsenergyfromfundamentaltoovertone
andbackagain.
Henceresonancepushesthetwolevelsapartandmixestheir
charactersothateachlevelbecomespartlyfundamentalandovertone
incharacter.
Thistypeofresonancegivesrisetoapairoftransitionsofequal
intensity
ForEg:n-butylvinylether……theovertoneofthefundamental
vibrationsisat810/cm,theirmaybechancetocoincidewiththeband
at1640/cm.
HencetheirwillbemixingofthetwobandsinaccordancewithFermi
resonance&gives2-bandsofequalintensityat1640/cm&1630/cm
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21. ELECTRONICEFFECT
The frequency shifts are due to the electronic effect which includes
Undertheinfluenceoftheseeffects,theforceconstant/thebond
strengthchanges&it’sabsorptionfrequencyshiftsfromnormalvalue.
ForEx:Acetone(CH
3COCH
3)=1715/cm
Chloroacetone(CH
3COCH
2Cl)=1725/cm
Dichloroacetone(CH
3COCHCl
2)=1740/cm
Theintroductionofelectronegativegroupcauses–Ieffectwhichresults
inbondordertoincrease,thustheforceconstincreases&hencethe
wavenoofabsorptionincreases.
Inductive effect
Mesomeric effect
Field effect
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22. Introduction of alkyl group causes +I lengthening / weakening of
the bond absorption at lower wave no.
In most of cases , Mesomeric effect works along with Inductive
effect
Conjugation lowers the absorption frequency of C=O stretching
whether the conjugation is due to α, β-unsaturation / due to an
aromatic ring.
In some cases Inductive effect dominates over Mesomeric effect.
Mesomeric effect causes lengthening / the weakening of a bond
leading in the lowering of absorption of frequency
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23. HYDROGENBONDING
Hydrogenbondingbringsaboutremarkabledownwardfrequency
shift.
StrongertheHydrogenbonding,greateristheabsorptionshift
towardslowerwavenothanthenormalvalue.
InIRtechniqueHydrogenbondingareof2types:
1.IntermolecularH
2bonding:giverisetobroadbands.
2.IntramolecularH
2bonding:giverisetosharp&welldefined
bands.
INTERMOLECULAR H
2
BONDING:
Theseareconc.dependent.Ondilution,theintensityofsuch
bandsareindependentofconc.
Theabsorptionfrequencydifferenceb/wfree&associated
moleculeishigher
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24. INTRAMOLECULAR H
2
BONDING:
Theseareobservedindilsolnofdi-&polyhydroxy
compoundsinCCl
4.WherenointermolecularH
2bondsare
formed.
Insuchconditionitwasobservedthatanoofcyclic&
acyclicdiolshave2bands&otherhavesinglebandintheO-
Hstretchingmoderegion.
ThespectrumofglycolindilCCl
4shows2-bandsatwave
no’s:3644/cm&at3612/cm.
Herethebandat3644/cmisduetofreeO-Hbond…..where
asthebandat3612/cmisduetoO-H
…
Obonding
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25. BONDANGLES
In cyclobutanones the frequency is rises due to C=O bond,
this can be explained by using Bond angles.
If the bond angle is reduce below the normal angle 120
0
C it
will leads to increase in s-character in C=O bond.
Greater s-character causes shortening of C=O bond thus C=O
str occurs at higher frequency
If the bond angle is above 120
0
C then their will be opp effect.
Due to this reason, di-tetra-butyl absorbs at (1697/cm) low
frequency
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26. References…..
1.Elementary Organic Spectroscopy by
Y.R.Sharma.
2.Text book of Pharmaceutical analysis by
Dr.S.Ravi shankar.
3.Instrumental Methods of Chemical Analysis.
G. R. Chatwal.
4.Internet source
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