Deals with the factors affecting vibrational frequency in IR spectroscopy and it's applications in pharmaceutical analysis

1. By:
Kasturi Banerjee
M. Pharm- Pharmaceutics (1st Year)
KLE Society’s College of Pharmacy, Bengaluru

2. Factors Affecting Vibrational
Frequency in IR Spectroscopy
 The value of absorption frequency is shifted if the force
constant of a bond changes with its electronic
structure.
 Frequency shifts also takes place on working with the
same substance in different states (solids, liquids and
vapour).
 A substance usually absorbs at higher frequency in a
vapour state as compared to liquid and solid states.

3. Factors Responsible for Shifting the
Vibrational Frequencies from their
Normal Values
There are 4 factors:-
1. Coupled vibration and Fermi Resonance
2. Electronic effects
3. Hydrogen bonding
4. Bond angle

4. Coupled Vibrations
 An isolated C-H bond has only one stretching vibrational
frequency whereas methylene group shows two stretching
vibrations, symmetrical and asymmetrical.
 In such cases, asymmetric vibrations always occur at higher
wave number compared to symmetric stretching vibration.
 These are called coupled vibrations since these vibrations
occur at different frequencies compared to -CH2- group.

5.  In case of acid anhydrides, two C=O stretching absorptions
between 1850-1800cm-1 and 1790-1745cm-1 with a difference
of about 65cm-1 was found. This is due to symmetric and
asymmetric stretching.
 In case of amide (-CONH2), a strong peak is observed which
lies between 1563-1515cm-1.
 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.
 Asymmetric stretching in acyclic anhydride is more intense
whereas symmetrical stretching band is more intense in cyclic
anhydrides.

6. Requirements:
 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.

7. Fermi Resonance
 Resonance
A vibration of large amplitude produced by a relatively
small vibration.
 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.
 This phenomenon was first observed by Enrico Fermi in case
of CO2.

8.  If two different vibrational levels, belonging to the same
species, have nearly the same energy, a mutual perturbation
of energy may occur.
 Shifting of one towards lower and other towards higher
frequency occurs.
 A substantial increase in the intensity of the respective bands
occurs.
 In this, a molecule transfers its energy from fundamental
vibrational level to overtone or combination tone level and
back.
 Resonance pushes the two levels apart and mixes their
character, consequently each level has partly fundamental
and partly overtone or combination tone character.

9. Example:
 CO2 (triatomic) is linear and four fundamental vibrations
are expected for it. Out of these symmetric stretching
vibration is infrared inactive since it produces no change
in the dipole moment of the molecules.
 In case of aldehyde (-CHO), the stretching absorption
usually appears as a doublet due to interaction between
C-H stretching (fundamental) and the overtone of C-H
bending which is found to be at 2820cm-1 and in case of
ketone (-C=O), no such doublet spectrum is found.
 In cyclopentanone, the absorption due to carbonyl group
occurs at 1746cm-1 and 1750cm-1.

10. Electronic Effects
Changes in the absorption frequencies for a particular
group take place when the substituents in the
neighbourhood of that particular group are changed.
The frequency shifts are due to the electronic effects
which include:-
I. Inductive effect
II. Mesomeric effect
III.Field effect

11. Inductive Effect
The introduction of alkyl group cause +I effect which results in the lengthening or the
weakening of the bond and hence the force constant is lowered and the wave number of
absorption decreases.
Example:
Formaldehyde (HCHO) = 1750cm-1
Acetaldehyde (CH3CHO) =1745cm-1
Acetone (CH3COCH3) = 1715cm-1
The introduction of an electronegative atom or group causes -I effect which results in the
bond order to increase. Thus, the force constant increases and hence, the wave number of
absorption increases.
Example:
Acetone (CH3COCH3) = 1715cm-1
Chloroacetone (CH3 CO CH2Cl) = 1725cm-1
Dichloro acetone (CH3 CO CHCl2) = 1740cm-1
Tetrachloro acetone (Cl2 CH2 CO CHCl2) = 1750, 1778cm-1

12. Mesomeric Effect
 They cause lengthening or the weakening of a bond leading in the
lowering of the absorption frequency. It is found in conjugated
systems.
 More will be the conjugation, less will be the bond strength and
lower will be the wave number.
Methyl vinyl ketone Acetophenone
Stretch C=O: 1706cm-1 Stretch C=O: 1693cm-1
 In some cases, where the lone pair of electrons present on an atom
is in conjugation with the double bond of a group, the mobility of a
lone pair of electron matters.

13. Example:
Benzamide Methyl benzoate
Stretch C=O: 1693cm-1 Stretch: 1730cm-1
 As nitrogen bond is less electronegative than oxygen atom the
electron pair on nitrogen atom in amides is more liable and
participates more in conjugation.

14. Field Effect
 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.
 It is generated due to steric effect.
Example: ortho halo acetophenone

15. Hydrogen Bonding
 It occurs in any system containing a proton donor (X-H) and a
proton acceptor.
 The stronger the hydrogen bond, the longer the O-H bond,
the lower the vibration frequency and broader and more
intense will be the absorption band.
 The N-H stretching frequency 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 hydroxyl
compounds.
There are two types of hydrogen bonding:-
I. Intermolecular Hydrogen Bonding
II. Intra-molecular Hydrogen Bonding

16.  The H- bonding which is between two different
molecules is called intermolecular H-bonding.
 The H-bonding which is within the same molecules is
called intra-molecular H-bonding.
 Intermolecular H-bonding gives rise to broad bands,
while intra-molecular H-bonds give sharp and well
defined bands.
 The inter and intra-molecular bonds can be distinguished
by dilution.
 Intra-molecular H-bonding remains unaffected by
dilution and as a result the absorption band also remains
unaffected, whereas in intermolecular, bonds are broken
on dilution and as a result there is a decrease in the
bonded O-H absorption.

17.  The strength of H-bonding is also affected by:
 Ring strain
 Molecular geometry
 Relative acidity and basicity of the proton donor and acceptor
groups.
Examples:
• In case of amines, the show N-H stretching at 3500cm-1 in
dilute solutions while in condensed phase spectra, absorption
occurs at 3300cm-1.
• In aliphatic alcohols, a sharp band appears at 3650cm-1 in
dilute solutions due to free O-H group while a broad band
appears at 3350cm-1 due to H-bonded -OH group.

18. Bond Angle
 It has been found out that the highest stretch C=O
frequencies arise in the strained cyclobutanones.
 The –C-(C=O)-C- bond angle is reduced below the normal
angle of 120⁰ and this leads to increased five-character in
the C=O bond.
 Greater S- Character causes shortening of C=O bond and
thus C=O structure occurs at higher frequency.
 Bond angle will decrease, bond strength will increase,
vibrational frequency will increase and wave number will
increase.

19. Applications of IR Spectroscopy
1.Identification of Organic Compound
 The identity of an organic compound can be established
from its fingerprint region (1400-900cm-1).
 The identity of an organic compound is conformed of its
fingerprint region exactly matches with the known
spectrum of the compound.
 The compounds containing same functional group may
have similar absorption above 1500cm-1 but they differ
in the fingerprint region.

20. 2. Structural Determination
 This technique helps to establish the structure of an unknown
compound.
 All major functional groups absorbs at their characteristic wave
numbers.
Example:
I. This IR spectra of amino acids exhibits bands for ionised
carboxylic acids and amine salts (-+NH3). No band for free –NH2
and –COOH groups is observed.
+NH3-CH-COO-
I. From the IR bands of sulphanilic acid, it is solid that the
compound contains +NH3 and SO3
- and not free groups as –NH2
and –SO3H.

21. 3. Qualitative Analysis of Functional Groups
 The presence or absence of absorption bands help in predicting
the presence of certain functional groups in the compound.
Example:
I. The presence of oxygen reveals that the groups maybe –OH,
C=O, -COOR, -COOH, anhydride, etc. But the absorption band is
in between 3600-3200cm-1. The band in this region maybe due
to -O-H.
II. In case of –NH2, -NH groups , all this can be seen. –NH2 shows
two absorption bands while –NH shows only one band.
III.Its distinction from –OH structure can be made from the extent
of H-bonding which is stronger in –OH compounds and causes
lowering in wave number.

22. 4. Distinction Between Two Types of Hydrogen
Bonding
 It is known that in H-bonding the electron clouds transfer from a hydrogen atom to the
neighbouring electronegative atom.
 The strength of H-bond is maximum when the proton donor group and the axis of lone pair
orbital are collinear and varies inversely to the distance to the distance between hydrogen
and oxygen.
Example:
 The hydroxyl compounds in the solid or liquid state exist as polymeric aggregates.
 The absorption in aggregate form occurs at lower frequencies and bands formed are
relatively broad.
 But when such a substance is dissolved in non-polar solvent such as CCl4, the aggregates or
polymers break in dimers and monomers.
 Due to this, the O-H structure absorption shifts to higher frequencies and the peaks below
become sharp.
 This technique helps to distinguish between intra-molecular H-bonding.
 Ortho nitro phenol exhibits intra-molecular H-bonding. Intra-molecular H-bonded compound
doesn’t show any shift in absorption or dilution whereas intermolecular H-bonded does.

23. 5. Quantitative Analysis
The estimation of the compound of the mixture can be done by:-
 Measuring the intensities of absorption bands characteristic of each
compound.
 Knowing the optical density of the absorption band for a pure component.
Example:
 Xylene exists as a mixture of three isomers, i.e. ortho, meta and para
xylenes.
 The percentage composition of mixture can be determined by IR spectrum
of the mixture.
 Bands are formed at:
a) 740cm-1 for ortho isomers
b) 880cm-1 for meta isomers
c) 830cm-1 for para isomers
 Mixtures of known composition are recorded and the working curves are
drawn for the bands.

24. 6. Study of a Chemical Reaction
 Reduction of a standard aliphatic ketone to form a stronger
bond at about 1710cm-1 when it is subjected to reduction, it
forms butan-2-ol which absorbs at 3300cm-1due to –O-H.
 IR spectroscopy is also used to predict the products formed in a
photochemical reaction.
Example:
When verbenone is irradiated in ethanol solution, the UV
absorption maximum due to verbenone disappears and the IR
spectrum of crude verbenone appears at 1787, 1740, 1715 and
1685cm-1.
 By chromatographic separation we get chrysanthenone, ethyl
geraniate,ethyl-3,7- dimethyl octa-3,6-dienoate.

26. 7. Study of Keto- enol Tautomerism
 Diketones and keto esters exhibit keto-enol tautomerism.
 They have α-H atom in them. The IR spectrum of such
compound contains bands due to C=O, O-H, C=C bonds.
Example: Ethyl aceto acetic ester- It exists in keto-enol isomers in
equilibrium.
 The lowering of νC=O absorption in the enolic bonding form is due
intra-molecular H-bonding which is stabilised by resonance.
 The appearance of bands clearly confirms keto-enol
tautomerism in aceto acetic ester.

27. 8. Study of Complex Molecules
 This technique is also useful to establish the structure of
complex molecules.
Example:
Two structures of penicillin were prepared on the basis of IR
spectral.
β-lactam Oxazolone

28. The IR structure of oxazolone shows two characteristic bands:
a) 1825cm-1 due to νC=O
b) νC=N due to 1675cm-1
 It is found that no such band appear in the spectrum of
penicillin. Thus, oxazolone structure for penicillin is ruled out.
 It is known that β- lactams do not absorb near 1770cm-1
whereas β- lactam fused to thiazolidine ring exhibits a band at
1770cm-1. Thus, the β- lactam structure of penicillin is
confirmed.

29. 9. Conformational Analysis
 Useful for conformations of cyclic compounds cyclohexane
exists in boat form and chair form.
Chair Form Boat Form
 There are 18 IR active C-C structure and CH2 rocking and
twisting vibration for boat form (II) whereas there are only five
for the chair form (I).

30.  The spectral examination of cyclohexane in the region 1350-
700cm-1 reveals five bands expected for chair form.
 This shows the greater stability for chair conformation over
boat conformation.
 By IR spectroscopy, axial and equatorial substituents in
cyclohexane substituents in cyclohexane can be distinguished.
 The equatorial substituent usually absorbs at a higher
frequency than does the same substituent at axial position.
• This is due to steric hindrance of C-X bond with adjacent H-
atoms.

31. 10. Detection of Impurity in a Compound
 IR spectroscopy is also useful in the detection of
impurity in a compound by comparing its spectrum
with the spectrum of the authentic sample of the
compound.
 Pure sample always consists of poor bands and also
some additional bands.

32. Reference:
1. Instrumental Methods of Chemical Analysis, B. K. Sharma,
page no: 271-276.
2. Instrumental Methods of Chemical Analysis, Gurdeep R.
Chatwal, Sham K. Anand, page no: 2.55-2.59.
3. Instrumental Analysis, Douglas A. Skoog, F. James Holler,
Stanley R. Crouch, page no: 505-529.