various factor affecting vibrational frequency in IR, • Coupled vibrations • Fermi resonance • Electronic effects • Hydrogen bonding and their examples

1. various factor affecting vibrational frequency in IR
Coupledvibrations
Fermi resonance
Electronic effects
Hydrogenbonding
 COUPLED VIBRATIONS
 An isolated C-H bondhas only one stretching vibrationalfrequency
where as methylenegroup showstwo stretching vibrations, symmetrical
and asymmetrical.
 Becauseofmechanicalcouplingor interaction betweenC-H stretching
vibrations in the CH2 group.
 Assymetric vibrations occurat higherfrequenciesor wavenumbers than
symmetric stretching vibrations.
 Theseare known as coupledvibrations becausethesevibrations occurat
different frequenciesthan that required for an isolated C-H stretching.
 A strong vibrationalcoupling is presentin carboxylic acid anhydridesin
which symmetricaland asymmetrical stretching vibrations appearin the
region 1720 – 1825 cm-1.

2.  The interaction is very effective probablybecauseofthe partial double
bondcharacterin the carbonyloxygenbondsdueto resonancewhichalso
keepsthe system planarfor effective coupling.
 For interaction to occur, the vibrations must be of samesymmetry species.
 There mustbe a commonatom betweenthe groups for strong coupling
betweenstretching vibrations.
 For couplingof bendingvibrations , a common bondis necessary.
 Interaction is greatestwhen coupledgroupsabsorb, individually, nearthe
same frequency.
 Coupling is negligible whengroups are separatedby one or more carbon
atoms and the vibrations are mutually perpendicular.
 FERMI RESONANCE
 Resonance
 A vibration of large amplitude producedby a relatively small vibration.
 Coupling of two fundamentalvibration modesproducestwo new modes
of vibration ,with frequencieshigherandlower than that observedin
absenceofinteraction. Interaction canalso take place between
fundamentalvibrations andovertonesor combination tone vibrations and
suchinteractions are knownas FermiResonance.
 In this , a moleculetransfers its energyfrom fundamentalvibrational level
to overtoneor combination tone leveland back.
 Resonancepushesthetwo levels apart andmixes their character,
consequentlyeachlevelhas partly fundamentaland partly overtoneor
combinat

3.  For eg. symmetricalstretching vibration of CO2 in Ramanspectrum shows
bandat 1337 cm-1.Thetwo bendingvibrations are equivalentand absorbat
the same frequencyof 667.3cm-1.
 The first overtoneof this is 2 X 667.3= 1334.6cm-1..
 Fermi resonance occurs
 There is mixing of 1337cm-1and 1334.6cm-1to give two bandsat1285.5
cm-1 andat 1388.3cm-1 with intensity ratio 1 : 0.9 respectively.
Hydrogen bonding
 It occursin any systemcontaininga proton donorgroup(X-H) and a
proton acceptor.if the s-orbital of the proton can effectively overlap the P
or π orbital of the acceptorgroup.
 The strongerthe hydrogenbond, the longerthe O-H bond,the lowerthe
vibration frequencyandbroaderand more intensewill be the absorption
band.

4.  The N-H stretching frequenciesof amines are also affected by hydrogen
bondingas that of the hydroxylgroup butfrequencyshifts for aminesare
lesserthan that for hydroxylcompounds.
 Becausenitrogenis less electronegativethan oxygenso the hydrogen
bondingin amines is weakerthan that in hydroxylcompounds.
 Intermolecular hydrogenbondsgivesrise to broadbands,while
intramolecular hydrogenbondsgivesharp and well defined bands.
 The inter and intramolecularhydrogenbondingcan be distinguishedby
dilution.
 Intramolecular hydrogenbondingremains unaffectedon dilution and as a
result the absorption bandalso remains unaffectedwhere as in
intermolecular, bondsare brokenon dilution and as a result there is a
decreasein the bondedO-H absorption.
The strength of hydrogenbondingis also affected by :
 Ring strain
 Moleculargeometry
 Relative acidity andbasicity of the proton donorand acceptor
groups
Electroniceffect
Changesin the absorptionfrequenciesfor a particular group take placewhen the
substituentsin the neighbourhoodofthat particular group are changed.
It includes :
1. .Inductive effect
2. .Mesomeric effect
3. Field effect

5.  INDUCTIVE EFFECT
 The introduction of alkyl groupcauses+I effect which results in the
lengtheningor the weakeningof the bond
 Hencethe force constantis lowered and wave numberof absorption
decreases.
 Let us comparethe wave numbersof v (C=O) absorptionsfor the following
compounds:
Formaldehyde(HCHO) 1750cm-1.
Acetaldehyde(CH3CHO) 1745 cm-1.
Acetone(CH3COCH3) 1715cm-1.
 Introduction of an electronegativeatom or group causes –Ieffect which
results in the bondorder to increase.
 Hencethe force constantincreasesand the wave numberof absorptionrises.
 Mesomeric effect :
 It causeslengtheningorthe weakeningof a bond leadingin the lowering of
absorptionfrequency.

6.  As nitrogen atom is less electronegativethan oxygenatom, the electron pair
on nitrogen atom in amide is more labile and participatesmore in
conjugation.
 Due to this greaterdegreeof conjugation,the C=O absorption frequencyis
much less in amides as comparedto that in esters.
 FIELD EFFECT
 In ortho substitutedcompounds,thelone pair of electrons on two atoms
influence eachother throughspaceinteractions and changethe vibrational
frequenciesof both the groups.Thiseffect is called field effect