IR-Spectroscopy.pptx.pdf if you want then take it

• IR spectroscopy is the study of interaction between
infrared radiations and matter.
• IR radiations refers broadly to that part of
electromagnetic spectrum between visible and
microwave region.
• The principle of IR spectroscopy is related to the
vibrational and rotational energy of a molecule.
• Absorption of IR radiation causes an excitation of
molecule from a lower to the higher vibrational level.
• Each vibrational level is associated with a number of
closely placed rotational level.
• Therefore the IR spectroscopy is also called as
“vibrational-rotational spectroscopy”
Introduction

• All the bonds in a molecule are not capable of absorbing
IR energy but those bonds which are accompanied by a
change in dipole moment will absorb in the IR region and
such transitions are called IR active transitions.
• The transitions which are not accompanied by a change
in dipole moment of the molecule are considered as IR
inactive.
• When the frequency of the IR radiation is equal to the
natural frequency of vibration, the molecule absorb IR
radiation.
Criteria for a compound to be IR Active

Instrumentation
Radiation Source
• IR radiation is usually produced by electrically
heating a Nernst filament (mainly composed of
oxides of zirconium, thorium and cerium) or a
globar (rod of silicon carbide) to 1000-1800°C.

• The infrared radiation of successively increasing
wavelength is used. The radiation from the source
is divided into sample and reference beams of
equal intensity by beam divider.
Sample and Reference Cells
• Reference and sample beams pass through the
reference cell and sample cell, respectively.
• Alkali metal halide such as NaCl, NaBr, KCl and
KBr are most commonly used as these are
transparent to most of the IR region.
Attenuator and Comb (Photometer)
• The reference beam passes through the
attenuator and the sample beam through the
comb.

• Then the two beams can be alternately reflected out of
the optical system and to the entrance slit of the
monochromator with the help of several mirrors.
• Thus, the photometer combines the reference and
sample beams into a single beam of alternating
segments. The comb allows balancing of the two beams.
Monochromator
• The combined beam passes through the prism or
grating of the monochromator which disperses the
beam into various frequencies.
• The prism or grating rotates slowly, it sends
individual frequency bands to the detector, thus
allowing a scan of frequency bands.

• Gratings give better resolutions than prisms consist of
a series of parallel and straight thin lines on a smooth
reflecting surface
• The spacing between lines is of the order of few
angstrom (Ä) depending on the desired wavelength
range.
Detector and Amplifier
• The detector is a thermocouple which measures
radiant energy by means of its heating effect that
produces current.
• Due to difference in the intensity of the two beams
falling on the detector, an alternating current starts
flowing from the detector to the amplifier where it is
amplified and relayed to the recorder.

MOLECULAR VIBRATIONS
There are 2 types of vibrations
1. Stretching vibrations
2. Bending vibrations
1. Stretching vibrations
• Vibration or oscillation along the line of bond
• Change in bond length
• Occurs at higher frequency: 4000-1250 cm-1
• 2 types:
a) Symmetrical stretching
b) Asymmetrical stretching

a) Symmetrical stretching. In this mode of vibration,
the movement of atoms with respect to the
common (or central) atom is simultaneously in the
same direction along the same bond axis.
b) Asymmetrical Stretching. In this vibration, one
atom approaches the common atom while the
other departs from it.

2. Bending Vibrations
• In such vibrations, the positions of the atoms
change with respect to their original bond axes.
• Bending vibrations are of four types:
(a) Scissoring. In this mode of vibration, the
movement of atoms is in the opposite direction with
change in their bond axes as well as in the bond angle
they form with the central atom.

(b) Rocking: In this vibration, the movement of
atoms takes place in the same direction with change
in their bond axes .
Scissoring and rocking are in-plane bending's.
(c) Wagging: In this vibration, two atoms
simultaneously move above and below the plane
with respect to the common atom.

(d) Twisting: In this mode of vibration, one of the atom
moves up and the other moves down the plane with
respect to the common atom.
Number of Fundamental Vibrations
• The IR spectra of polyatomic molecules may exhibit
more than one vibrational absorption bands.
• The number of these bands corresponds to the
number of fundamental vibrations in the molecule
which can be calculated from the degrees of
freedom of the molecule.

• The degrees of freedom of a molecule are equal to
the total degrees of freedom of its individual atoms.
• Each atom has three degrees of freedom
corresponding to the three Cartesian Coordinates (x,
y and z) necessary to describe its position relative to
other atoms in the molecule.
• A molecule having n atoms will have 3n degrees of
freedom.
• In case of a nonlinear molecule, three of the degrees
of freedom describe rotation and three describe
translation.
• The remaining (3n - 3 - 3) = 3n - 6 degrees of
freedom are its vibrational degrees of freedom or
fundamental vibrations

• In case of a linear molecule, only two degrees of
freedom describe rotation and three describe
translation.
• Thus, the remaining (3n - 2 - 3) = 3n – 5 degrees of
freedom are vibrational degrees of freedom or
fundamental vibrations.
Vibrational degrees of freedom for CO2
Number of atoms (n) = 3
Total degrees of freedom (3n) = 3 x 3 = 9
Rotational degrees of freedom = 2
Translational degrees of freedom = 3
Therefore, vibrational degrees of freedom = 9 - 2 - 3 = 4
CO2
molecule have four theoretical fundamental bands

• The carbon dioxide molecule is linear and has four
fundamental vibrations (3 x 3)- 5 = 4.
• Thus, four theoretical fundamental bands are expected
but actually it shows only two.
• The symmetrical Stretching vibration in carbon dioxide
is IR inactive because it produces no change in the
dipole moment of the molecule.
• The two bending vibrations are equivalent and absorb
at the same wavenumber (667.3 cm-1
).

Calculation of Vibrational Frequencies
• The stretching vibrations of two bonded atoms may be
regarded as the vibration of two balls connected by a
spring, a situation for which Hooke's law applies.
k = 5 X 105
dynes/cm for single
bonds and approximately twice
and thrice of this value for
double and triple bonds

Factors Affecting Vibrational Frequencies
1. Coupled vibration
2. Electronic effects
3. Hydrogen bonding
4. Bond angle
• 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.
• There are 4 factors that affect the vibrational frequencies
of bonds:

1. Coupled vibration
• For an isolated C-H bond, only one stretching vibrational
frequency is expected but a methylene group shows two
absorptions corresponding to symmetrical and
asymmetrical stretchings
• This is because there is mechanical coupling or
interaction between the C-H stretching vibrations in the
CH2
group.
• Asymmetric stretching vibrations occur at higher
frequencies or wavenumbers than the symmetric
stretching vibrations.
• These are called coupled vibrations since these
vibrations occur at different frequencies compared to
CH2 group.

• 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 (due to symmetric and
asymmetric stretching)
• The interaction is very effective because of the partial
double band 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.

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.

2. Electronic Effects
• Changes in the absorption frequencies for a
particular group take place when the
substituents in the neighborhood of that
particular group are changed.
• The frequency shifts are due to the electronic
effects which include:-
a) Inductive effect
b) Mesomeric effect
• Strength (force constants) of a particular bond is
changed by these effects and hence its stretching
frequency is also changed with respect to the
normal values.

• 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.
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.
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
a) Inductive effect

b) 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.
• Amides show ʋc=o
band at a lower frequency than that of
esters. Due to lesser electronegativity of nitrogen than
oxygen, its lone pair of electrons are more readily involved in
resonance than that of oxygen.
• 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.

3. 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
a) Intermolecular Hydrogen Bonding
b) Intra-molecular Hydrogen Bonding
• 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.

• In case of amines, the N-H stretching is 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.
4. Bond angle
• The carbonyl stretching frequency in cyclic ketones having
ring strain is shifted to a higher value.
• The C-CO-C bond angle in strained rings is reduced below
the normal value of 120° (acyclic and six-membered cyclic
ketones have the normal C-CO-C angle of 120°).
• This Ieads to an increase in s character in the sp2 orbital
of carbon involved in the C=O bond. Hence, the C=O bond
is shortened (strengthened) resulting in an increase in the
Vc=o frequency.

• This increase in the s character of the outside sp2 orbital is
there because it gives more p character to the sp2 orbitals of
the ring bonds which relieves some of the strain, as the
preferred bond angle of p orbitals is 90°.
• In ketones where C-CO-C angle is greater than the normal
angle (120°), an opposite effect operates and they have
lower Vc=o frequency. For example, in di-t-butyl ketone,
where the C-CO-C angle is pushed outward above 120°, has
very low Vc=O frequency (1698 cm-1).

Interpretation of Infrared Spectra
(i) 3200-3650 cm-1
• The appearance of medium to strong absorption bands in
this region shows the presence of hydroxyl or amino
groups.
• These bands arise from ʋO-H
or ʋN-H
vibrations. The
position, intensity and width of the bands indicate
whether the group is free or intermolecularly hydrogen
bonded or intramolecularly hydrogen bonded.
• A medium band due to =C-H stretching also appears near
3300 cm-1.
(ii) 3000-3200 cm-1
• Absorption bands due to =C-H stretching and aromatic
C-H stretching appear in this region. These bands are of
medium intensity.

(iii) 2700-3000 cm-1
• In this region, usually a complex band or bands appear
near 2850 cm-1 due to stretching vibrations of C-H
bonds of saturated groups, i.e. -CH3, -CH2 or CH- .
• The appearance of weak but sharp bands near
2700-2900 cm-1 due to ʋc-H
indicates the presence of
aldehyde, methoxyl or N-methyl groups.
• A broad ʋO-H
band present in the 2700-3000 cm– 1
region
is characteristic of hydrogen bonded -COOH groups.
(iv) 2000-2700 cm-1
• Groups of the type XΞY, X=Y=X, etc. absorb in this
region and exhibit bands of variable intensities. For
example, bands due to C Ξ C Stretching appear in the
region 2100-2260 cm- 1 and that due to CΞN Stretching
appear in the region 2200-2260 cm-1 .

• Isocyanates absorb in the region 2240-2275 cm-1 due to N
= C = 0 stretching.
• Besides these, ʋo-H
, ʋN-H
and ʋs-H
bands of carboxylic acid
dimers, amine salts and thiols (or thiophenols),
respectively also appear in this region (2000-2700 cm-1).
(v) 1600-1900 cm-1
• Strong absorption bands in the upper part of this region
are due to C=O stretching. Aldehydes, ketones,
carboxylic acids, esters, amides, acid anhydrides, acyl
halides, etc. absorb strongly in this region due to C=O
stretching.

FINGERPRINT REGION
• In IR, the region below 1500 cm-1 is rich in many absorption
bands and the region is known as fingerprint region.
• Here the number of bending vibrations are usually more
than the number of stretching vibrations.
• In this region, small difference in the structure and
constitution of a molecule results significant changes in the
absorption bands.
• Many compounds show unique absorption bands in this
region and which is very useful for the identification of the
compound.
• Fingerprint region can be sub-divided into three
i. 1500-1350 cm-1
Here doublet near 1380 cm-1 and 1365 cm-1 shows the
presence of tertiary butyl group in the compound.

ii. 1350-1000 cm-1
All classes of compounds having groups like alcohols,
esters , lactones, acid anhydrates show characteristic
absorptions (s) due to C – O stretching.
iii. Below 1000 cm-1
Distinguishes between cis and trans alkenes and mono
and disubstitutions at ortho, meta, para

CHARACTERISTIC INFRARED ABSORPTION BANDS OF
FUNCTIONAL GROUPS

Applications of Infrared Spectroscopy
1. Detection of Functional groups
2. Confirmation of the identity of Compounds
3. Estimation of the Purity of Samples
4. Study of Hydrogen Bonding
5. Calculation of Force Constants
6. Orientations in aromatic Compounds
7. Study of the Progress of Reactions

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