FACTORS INFLUENCING VIBRATIONAL FREQUENCIES IN I R

Submitted byP.Anuradha1st M. Pharmacy

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The calculated value of wave number is never exactly equal to the experimental value, because the vibrations of each group gets influenced by the structure of the molecule in the neighbourhood of the bond/group.

Frequency shifts also take place on working with the same substance in different states (solids, liquids & vapour).

A substance usually absorbs at higher frequency in a vapour state as compared to liquid and solid states.

Factors responsible for shifting the vibrational frequencies from their normal values

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Coupled vibrations

Fermi resonance

Electronic effects

Hydrogen bonding

COUPLED VIBRATIONS

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An isolated C-H bond has only one stretching vibrational frequency where as methylene group shows two stretching vibrations, symmetrical and asymmetrical.

Because of mechanical coupling or interaction between C-H stretching vibrations in the CH2 group.

Assymetric vibrations occur at higher frequencies or wave numbers than symmetric stretching vibrations.

These are known as coupled vibrations because these vibrations occur at different frequencies than that required for an isolated C-H stretching.

Similarly ,coupled vibrations of –CH3 group take place at different frequiences compared to –CH2- group.

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• The interaction is very effective probably because of the partial double bond character in the carbonyl oxygen bonds due to resonance which also keeps the system planar for effective coupling.

•A strong vibrational coupling is present in carboxylic acid anhydrides in which symmetrical and asymmetrical stretching vibrations appear in the region 1720 – 1825 cm-1.

Requirements

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For interaction to occur, the vibrations must be of same symmetry species.There must be a common atom between the groups for strong coupling between stretching vibrations.For coupling of bending vibrations , a common bond is necessary.

Interaction is greatest when coupled groups absorb, individually, near the same frequency.

Coupling is negligible when groups are separated by one or more carbon atoms and the vibrations are mutually perpendicular.

FERMI RESONANCE

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Coupling of two fundamental vibration modes produces two new modes of vibration ,with frequencies higher and lower than that observed in absence of interaction.

Interaction can also take place between fundamental vibrations and overtones or combination tone vibrations and such interactions are known as Fermi Resonance.

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In this , a molecule transfers its energy from fundamental vibrational level to overtone or combination tone level and back again.

Resonance pushes the two levels apart and mixes their character, consequently each level has partly fundamental and partly overtone or combination tone character.

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Carbondioxide molecule(triatomic) is linear and four fundamental vibrations are expected for it. Out of these ,symmetric vibration is IR inactive since it produces no change in the D.M of the molecule

For eg. Fermi Resonance is given by aldehydes in which C-Hstretching absorption usually appears as a doublet(2820cm

-1 and 2720cm-1 )due to interaction between C-Hstretching(fundamental)and the overtone of C-H deformation(bending)

Asymmetric stretching Symmetric stretching

Hydrogen bonding

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Stronger the hydrogen bonding, greater is the absorption shift towards lower wave number than the normal value.Intermolecular hydrogen bonds give rise to broad bands and intramolecular hydrogen bonds are sharp and well defined.Intermolecular hydrogen bonds are concentration dependent on dilution,the intensities of such bands decrease and finally disappear.Intramolecular hydrogen bonds are independent of concentration. The N-H stretching frequencies of amines are also affected by hydrogen bonding as that of the hydroxyl group but frequency shifts for amines are lesser than that for hydroxyl compounds. Because nitrogen is less electronegative than oxygen so the hydrogen bonding in amines is weaker than that in hydroxy compounds.

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Mostly non associating solvents like CS2 ,CHCl3,CCl4 are used because some solvents like benzene,acetone influence O-H&N-H compounds.

For example, amines show N-H stretching at 3500cm-1 in dilute solutions and in aliphatic alcohols,a sharp band appears at 3650cm-1 due to free O-H group.

In enols&chelates,H-bonding is exceptionally strong and absorption due to O-H stretching takes place at very low values.As these bonds are not broken easily on dilution by an inet solvent, the free O-H stretching,may not be seen at low concentrations.

It is because of the fact that the bonded structure is stabilised by resonance

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The strength of hydrogen bonding is also affected by :

Ring strainMolecular geometryRelative acidity and basicity of the proton donor and acceptor groups

Electronic effect

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Changes in the absorption frequencies for a particular group take place when the substituents in the neighbourhood of that particular group are changed.

It includes :Inductive effectMesomeric effectField effect

INDUCTIVE EFFECT

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The introduction of alkyl group causes +I effect which results in the lengthening or the weakening of the bond

Hence the force constant is lowered and wave number of absorption decreases.

Let us compare the wave numbers of v (C=O) absorptions for the following compounds :

Formaldehyde (HCHO) 1750 cm-1Acetaldehyde (CH3CHO) 1745 cm-1Acetone (CH3COCH3) 1715 cm-1 Introduction of an electronegative atom or group causes –

I effect which results in the bond order to increase.Hence the force constant increases and the wave number

of absorption rises.

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Mesomeric effect : It causes lengthening or the weakening of a bond leading in the lowering of absorption frequency.

As nitrogen atom is less electronegative than oxygen atom, the electron pair on nitrogen atom in amide is more labile and participates more in conjugation. Due to this greater degree of conjugation, the C=O absorption frequency is much less in amides as compared to that in esters.It is important to note that only inductive effect is considered when the compound is meta substituted.

Field effect :

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In ortho substituted compounds, the lone pair of electrons on two atoms influence each other through space interactions and change the vibrational frequencies of both the groups.

This effect is called field effect.

Ortho halo acetophenone

REFERENCES

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Organic spectroscopy,Y.R.Sharma,pg no:77-82

Instrumental methods of chemical analysis , Gurdeep R. Chatwal, Sham K. Anand, pg no :2.55 – 2.59

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