Types of Molecular Vibrations & Factors Influencing Vibrational Frequency in I.R. Spectroscopy

Types of Molecular Vibrations & Factors Influencing Vibrational Frequency in I.R. Spectroscopy

1. Types of Molecular Vibrations & Factors
Influencing Vibrational Frequencyin
I.R. Spectroscopy
Presented by,
Mr. Ranjit Thavare
Roll No. 04
Class: First Year of M. Pharmacy
Department: Pharmaceutical Chemistry
Dattakala College of Pharmacy,
Chincholi, Daund.
INFRARED SPECTROSCOPY
01

2. CONTENTS
_____________________________________________________________________________________________________________________________ ___________________________________________________
_____________________________________________________________________________________________________________________________ ___________________________________________________
01. Types of Molecular Vibrations
02. Stretcing and Bending Vibrations
03. Stretching Vibrations
04. Bending Vibrations
05. Hook’s Law for Frequency Calculation
06. Factors Influencing Vibrational Frequency
07. References
02

3. 01.Types of Molecular Vibrations
Molecular
Vibrations
Fundamental
Vibrations
Stretching
Vibrations
Asymmetric Symmetric
Bending
Vibrations
In Plane Bending
Scissoring Rocking
Out of Plane
Bending
Wagging Twisting
Non Fundamental
Vibrations
Overtones, Fermi
Resonance
Combination
Tones
03

4. Stretching
Vibrations
Stretching vibration involves a
continuous change in the inter-
atomic distance along the axis
of the bond between two atoms.
In requires more energy so
appear at shorter wavelength
Bending
Vibrations
Bending vibrations are
characterized by a change in
the angle between two bonds
It requires less energy so
appear at longer
wavelength
02. Stretchingand Bending Vibrations Vibrations
04
04

5. Stretching
Vibrations
Asymmetric
Inter atomic
distance between
two atoms is
opposite
alternate/opposite
Symmetric
Inter atomic
distance between
two atoms is
increase/decreases
03. StretchingVibrations Vibrations
05

6. 04. Bending Vibrations
Bending
Vibrations
In Plane
Bending
Scissoring Rocking
Out of
Plane
Bending
Wagging Twisting
06

7. 04. Bending Vibrations Cont.
❖These are characterized by continuously changing bond angle and axis with common atom.
❖These are of various types as (+veindicates movement out of plane above and -veindicates
movement back of plane)
❑Scissoring vibrationsoccurwhentwo atoms moveback andforth towards eachother.
❑Rocking vibration occurs due to oscillation of atoms and forth out of equilibrium plane.
07

8. 04. Bending Vibrations Cont.
❑Wagging vibrations result when the unit oscillates in equilibrium plane-formed by the atoms.
❑Twisting vibrations occur when structural unit rotates around the bond which joins to the molecule
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9. 05. Hook’s Law for Frequency Calculation.
❑The probable frequency of absorption can be calculated by the Hook's law.
❑Hook’s law: -
??????=
1
2????????????
??????
??????�??????�/??????�+??????�
Where,
??????= The vibrational frequency (cm
-1
)
??????= Velocity of light (cm/s)
??????= force constant of the bond (dyne/cm)
??????
�&??????
�= mass (g) of atom �and atom �respectively.
❑However, it has been observed that the calculated value of frequency of absorption is not exactly
equal to the experimental value.
❑There are many factors which are responsible for shifts in vibrational frequencies are as follows;
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10. 06. Factors Influencing Vibrational Frequency.
❑Coupled vibrations
❑Fermi-resonance
❑Hydrogenbonding
❑Electronic effects
❑Coupled vibrations:
✓AnisolatedC-Hbondhasonlyonestretchingvibrationalfrequencywhereas
methylenegroupshowstwostretchingvibrations,symmetricalandasymmetrical.
✓BecauseofmechanicalcouplingorinteractionbetweenC-Hstretchingvibrationsin
theCH2group.
✓Asymmetricvibrationsoccurathigherfrequenciesorwavenumbersthansymmetric
stretchingvibrations.
✓Theseareknownascoupledvibrationsbecausethesevibrationsoccuratdifferent
frequenciesthanthatrequiredforanisolatedC-Hstretching.
✓Astrongvibrationalcouplingispresentincarboxylicacidanhydridesinwhich
symmetricalandasymmetricalstretchingvibrationsappearintheregion1720-
1825cm-1. 10

11. 06. Factors Influencing Vibrational Frequency Cont.
✓Theinteractionisveryeffectiveprobablybecauseofthepartialdoublebond
characterinthecarbonyloxygenbondsduetoresonancewhichalsokeepsthe
systemplanarforeffectivecoupling.
✓Forinteractiontooccur,thevibrationsmustbeofsamesymmetryspecies.
✓Theremustbeacommonatombetweenthegroupsforstrongcouplingbetween
stretchingvibrations.
✓Forcouplingofbendingvibrations,acommonbondisnecessary.
✓Interactionisgreatestwhencoupledgroupsabsorb,individually,near
thesamefrequency.
❑ Fermi-resonance:
➢Avibrationoflargeamplitudeproducedbyarelativelysmallvibration.
➢Couplingoftwofundamentalvibrationmodesproducestwonewmodesofvibration,
withfrequencieshigherandlowerthanthatobservedinabsenceofinteraction.
Interactioncanalsotakeplacebetweenfundamentalvibrationsandovertonesor
combinationtonevibrationsandsuchinteractionsareknownasFermiResonance.11

12. 06. Factors Influencing Vibrational Frequency Cont.
✓Foreg.symmetricalstretchingvibrationofCO2inRamanspectrumshowsbandat
1337cm
-1
.
✓Thetwobendingvibrationsareequivalentandabsorbatthesamefrequencyof
667.3cm
-1
.
✓Thefirstovertoneofthisis2X667.3=1334.6cm
-1
.
✓Fermiresonanceoccurs.
✓Thereismixingof1337cm
-1
and1334.6cm
-1
togivetwobandsat1285.5cm
-1
and
at1388.3cm
-1
withintensityratio1:0.9respectively.
❑Hydrogenbonding:
oItoccursinanysystemcontainingaprotondonorgroup(X-H)andaprotonacceptor.
ifthes-orbitaloftheprotoncaneffectivelyoverlapthePor??????orbitaloftheacceptor
group.
oThestrongerthehydrogenbond,thelongertheO-Hbond,thelowerthevibration
frequencyandbroaderandmoreintensewillbetheabsorptionband.
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13. 06. Factors Influencing Vibrational Frequency Cont.
oTheN-Hstretchingfrequenciesofaminesarealsoaffectedbyhydrogenbondingas
thatofthehydroxylgroupbutfrequencyshiftsforaminesarelesserthanthatfor
hydroxylcompounds.
oBecausenitrogenislesselectronegativethanoxygensothehydrogenbondingin
aminesisweakerthanthatinhydroxylcompounds.
oIntermolecularhydrogenbondsgivesrisetobroadbands,whileintramolecular
hydrogenbondsgivesharpandwelldefinedbands.
oTheinterandintramolecularhydrogenbondingcanbedistinguishedbydilution.
oIntramolecularhydrogenbondingremainsunaffectedondilutionandasaresultthe
absorptionbandalsoremainsunaffectedwhereasinintermolecular,bondsare
brokenondilutionandasaresultthereisadecreaseinthebondedO-Habsorption.
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14. 06. Factors Influencing Vibrational Frequency Cont.
❑Electroniceffects:
•Changesintheabsorptionfrequenciesforaparticulargrouptakeplacewhenthe
substituentsintheneighborhoodofthatparticulargrouparechanged.
•Itincludes;
1.Inductiveeffect
2.Mesomericeffect
3.Fieldeffect
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15. 07. References.
1.‘PHARMACEUTICAL ANALYSIS’Volume-II‘InstrumentalMethods’by,Dr.A.V.
Kasture,etal.PageNo.207-221.
2.Sandorfy,C.,R.Buchet,andG.Lachenal."Principlesofmolecularvibrationsfor
near-infraredspectroscopy."Near-InfraredSpectroscopyinFoodScienceand
Technology;Ozaki,Y.,McClure,WF,Christy,AA,Eds(2007):11-46.
3.Czarnecki,M.A.,Morisawa,Y.,Futami,Y.andOzaki,Y.,2015.Advancesin
molecularstructureandinteractionstudiesusingnear-infraredspectroscopy.
Chemicalreviews,115(18),pp.9707-9744.
4.Wilson,E.B.,Decius,J.C.andCross,P.C.,1980.Molecularvibrations:thetheoryof
infraredandRamanvibrationalspectra.CourierCorporation.
5.Hsu,C.P.S.,1997.Infraredspectroscopy.Handbookofinstrumentaltechniquesfor
analyticalchemistry,249.
6.Noda,I.,1989.Two-dimensionalinfraredspectroscopy.JournaloftheAmerican
ChemicalSociety,111(21),pp.8116-8118.
7.Gütlich,P.,Goodwin,H.A.,Tuchagues,J.P.,Bousseksou,A.,Molnár,G.,McGarvey,
J.J.andVarret,F.,2004.Theroleofmolecularvibrationsinthespincrossover
phenomenon.SpinCrossoverinTransitionMetalCompoundsIII,pp.84-103. 15

16. THANK YOU ☺
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