IR Spectroscopy with detailed introduction

IR Spectroscopy detailed introduction

1. Seminar on
Infrared Spectroscopy
Introduction, Principle & Theory
Presented by :
NIVEDITHA G
1
st
semMpharm
Dept of pharmaceutics
NARGUND COLLEGE OF PHARMACY

2. Contents
Introduction of spectroscopy
IR introduction
Principle
Theory
2

3. Introduction
Spectroscopy is an analytical technique
which helps determine structure
It destroys little or no sample
The amount of light absorbed by the
sample is measured as wavelength is
varied
3

4. Types of Spectroscopy
Infrared (IR) spectroscopy
–measures the bond vibration frequencies in a molecule and is
used to determine the functional group
Mass spectrometry (MS)
–fragments the molecule and measures the masses
Nuclear magnetic resonance (NMR) spectroscopy
–detects signals from hydrogen atoms and can be used to
distinguish isomers
Ultraviolet (UV) spectroscopy
–uses electron transitions to determine bonding patterns
4

5. IR INTRODUCTION
IR-spectroscopyisalsocalledasVibrational
spectroscopy.
ItisconcernedwiththestudyofAbsorptionofIR
radiation,whichresultsinVibrationaltransitions.
IRspectroscopyismainlyusedtoStructure
elucidationtodtmtheFunctionalgroups.
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6. Different Regions of IR
TheabsorptionofIRradiationcanbeexpressedeitherintermsof
Wavelength(λ)/inWaveno.
InIRspectraofanorg-compthegraphisplottedas
%Transmittancev/sWaveno.
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)
6

7. The Relationship b/w Wavelength & Wave number
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
7

8. Criteria for a Molecule to show IR Absorption
Spectrum
1.Dipolemovement:AmoleculecanabsorbIRradiationonly
whenit’sabsorptioncausesachangeinit’sDipole
movement.
2.CorrectWavelengthofRadiation:AmoleculecanabsorbIR
radiationonlywhentheappliedInfraredfrequencyisequal
totheNaturalfrequencyofvibrationofthatmolecule
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9. Principle
Thetechniqueisbasedonthesimplefactthatachemicalsubstanceshows
markedabsorptionintheIRregionoftheelectromagneticspectrum.
AfterabsorptionoftheIRradiation,themoleculeofachemicalsubstance
VibratesatmanyratesofvibrationgivingrisetoclosepackedBandscalled
theIRabsorptionspectra.
VariousbandswillbepresentintheIRspectrawhichwillcorrespondsto
thecharacteristicFunctionalgroups&bondspresentinthechemical
substance.
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10. THEORY
AbsorptionintheIRregionisduetothechangesinthe
Vibrational&Rotationallevels
InIRspectroscopytheabsorbedenergybringsabout
predominantchangesintheVibrationalenergywhichdepends
on:
1.Massesoftheatomspresentinthemolecule
2.Strengthofthebond
3.Thearrangementofatomswithinthemolecule.
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11. TYPES OF VIBRATIONS
VIBRATIONS
STRETCHING
SYMMETRICAL
ASYMMETRICAL
BENDING
INPLANE
SCISSORING
ROCKING
OUT-OF-PLANE
WAGGING
TWISTING
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12. Stretching vibrations :
Symmetric
Asymmetric
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13. Bending Vibrations
IN-PLANE
Scissoring
Rocking
OUT-OF-
PLANE
Wagging
Twisting
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14. Vibrational Frequency
ThevalueoftheStretchingVibrationalfrequencyofaBondcanbe
calculatedbyusingHooke’sLaw:
V = 1 / 2πc [ K / m
1. m
2 / (m
1+m
2)]
1/2
= 1/2πc√[k/μ]
Where….c=velocityofradiation(2.998×10
10
cm/sec)
μ=reducedmass
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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15. Degree of Freedom
Polyatomicmoleculesexhibitsmorethanonefundamental
Vibrationalabsorptionbandsthesebandsisrelatedto
degreesoffreedominamolecule.
Eachatomhas3-degreesoffreedomcorrespondstothe3
co-ordinates(X,Y,Z)whichisnecessarytodescribethe
positionrelativetootheratominamolecule.
Whenaatomcombinetoformamolecule,nodegreeof
freedomislost,i.e.,thetotalno.ofdegreeoffreedomofa
moleculewillbeEqualto3nwhere…n=no.ofatomsina
molecule.
15

16. 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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17. 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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18. Ex:forNon-Linearmolecule:C
6H
6
Noofatoms=12
Totaldegreeoffreedom=3×12=36
Translationaldegreeoffreedom=3
Rotationaldegreeoffreedom=3
HenceVibrationaldegreeoffreedom=36–3–3=30
HencetheoreticallyC
6H
6shouldhave30fundamental
bands.
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19. Factors Influencing Vibrational
Frequencies
Coupled vibrations &
Fermi Resonance
Electronic Effect
Hydrogen Bonding
Bond Angles
19

20. Coupled vibrations & Fermi
Resonance
IncaseofisolatedC-Hbondtherewillbeonestretchingvibrationis
observed,
But…incaseofMethylene(-CH
2-)twoabsorptionoccur,i.e.,oneis
Symmetric&anotherisAsymmetric.
Insuchcasesasymmetricvibrationsalwaysoccurathigherwaveno
comparedwiththesymmetricvibrations.
ThesearecalledCoupledVibrationssincetheseoccuratdifferent
frequenciesthanthatrqforanisolatedC-Hstretching.
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21. Similarlyinmethyl(-CH
3-)groupcoupledvibrationstakesplaceat
differentfrequenciescomparedto-CH
2-groups
sometimestheremaybetwodifferentvibrationswhichhavenearly
sameenergy.Ifsuchvibrationsbelongstosamespeciesthena
mutualperturbationofenergymayoccur,
Thisresultingshiftofonetowardslowerfrequencyandanother
towardshigherfrequency.Hencetherewillbeincreasingintensity
ofrespectiveband.
Sometimesitmayhappenthattheenergyofovertonelevelmay
coincidewithfundamentalmodeofdifferentvibrations.Hence
theremaybearesonanceoccurs,thistypeofresonanceiscalled
Fermiresonance.
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22. Here,amoleculetransferitsenergyfromfundamentaltoovertone
andbackagain.
Henceresonancepushesthetwolevelsapartandmixestheir
charactersothateachlevelbecomespartlyfundamentaland
overtoneincharacter.
Thistypeofresonancegivesrisetoapairoftransitionsofequal
intensity
ForEg:n-butylvinylether……theovertoneofthefundamental
vibrationsisat810/cm,theirmaybechancetocoincidewiththe
bandat1640/cm.
Hencetheirwillbemixingofthetwobandsinaccordancewith
Fermiresonance&gives2-bandsofequalintensityat1640/cm&
1630/cm
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23. Electronic Effect
The frequency shifts are due to the electronic effect which includes
Undertheinfluenceoftheseeffects,theforceconstant/thebondstrength
changes&it’sabsorptionfrequencyshiftsfromnormalvalue.
ForEx:Acetone(CH
3COCH
3)=1715/cm
Chloroacetone(CH
3COCH
2Cl)=1725/cm
Dichloroacetone(CH
3COCHCl
2)=1740/cm
Theintroductionofelectronegativegroupcauses–Ieffectwhichresultsin
bondordertoincrease,thustheforceconstincreases&hencethewavenoof
absorptionincreases.
Inductive effect
Mesomeric effect
Field effect
23

24. Introduction of alkyl group causes +I lengthening / weakening
of the bond absorption at lower wave no.
In most of cases , Mesomericeffect 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.
Mesomericeffect causes lengthening / the weakening of a
bond leading in the lowering of absorption of frequency
24

25. Hydrogen Bonding
Hydrogenbondingbringsaboutremarkabledownwardfrequency
shift.
StrongertheHydrogenbonding,greateristheabsorptionshift
towardslowerwavenothanthenormalvalue.
InIRtechniqueHydrogenbondingareof2types:
1.IntermolecularH
2bonding:giverisetobroadbands.
2.IntramolecularH
2bonding:giverisetosharp&welldefined
bands.
INTERMOLECULARH
2
BONDING:
Theseareconc.dependent.Ondilution,theintensityofsuch
bandsareindependentofconc.
Theabsorptionfrequencydifferenceb/wfree&associated
moleculeishigher
25

26. INTRAMOLECULAR H
2BONDING:
Theseareobservedindilsolnofdi-&polyhydroxy
compoundsinCCl
4.WherenointermolecularH
2bondsare
formed.
Insuchconditionitwasobservedthatanoofcyclic&
acyclicdiolshave2bands&otherhavesinglebandinthe
O-Hstretchingmoderegion.
ThespectrumofglycolindilCCl
4shows2-bandsatwave
no’s:3644/cm&at3612/cm.
Herethebandat3644/cmisduetofreeO-Hbond…..
whereasthebandat3612/cmisduetoO-H
…
Obonding
26

27. Bond Angles
In cyclobutanonesthe 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 stroccurs 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
27

28. References…..
1.Elementary Organic Spectroscopy by
Y.R.Sharma.
2.Text book of Pharmaceutical analysis by
Dr.S.Ravishankar
3.Internet source
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29. Thank you
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