Infrared Spectroscopy

Professor, Department of Pharmaceutical Analysis,
Santhiram College of Pharmacy, Nandyal, Kurnool Dist. AP. India.
drbmdishaq@gmail.com
AICTE PRERANA Virtual GPAT Class for
B. Pharmacy students
http://srcpnandyal.com/

All the information and views shared in this presentation belongs
solely to me and not necessarily to my employer, organization,
committee or other group or individual. This presentation is
delivered with the whole and sole educational purpose of students
and not involved any commercial benefits. Thus the presenter or
his employer neither claim for any copyright nor responsible for
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DISCLAIMER

Santhiram College of pharmacy, Nandyal, Kurnool Dist. AP. 518501.
Classification of analytical techniques
1.Separation techniques → Chromatography
2. Molecular structure det. → SPECTROSCOPY
3. Electro analytical → Potentiometry, Conductometry
4. Titrimetric analysis → Titrations

Spectroscopy
“Seeing the unseen”. Molecular Eye
Spectroscopy is the branch of science that deals the study of
interaction of electromagnetic radiation with matter.
Electromagnetic radiation is a type of energy that is
transmitted through space at enormous velocities.
EMR Analyte (low conc.) Spectra

 Using electromagnetic radiation as a probe to obtain information
about atoms and molecules that are too small to see.
 Electromagnetic radiation is propagated at the speed of light
through a vacuum as an oscillating wave.
Electromagnetic relationships:
λυ = c
E = hυ
E = hc/λ
λ  1/υ
E  υ
E  1/λ
λ = wave length
υ = frequency
c = speed of light
E = kinetic energy
h = Planck’s constant
6.626 × 10−34 joule second.

 Infrared spectroscopy (IR) measures the bond vibration
frequencies in a molecule and is used to determine the
functional groups.
 The infrared region of the spectrum encompasses radiation with
wave numbers ranging from about 12,500 to 50cm-1 (or) wave
lengths from 0.8 to 200µ.
IR SPECTROSCOPY
The infrared region constitutes 3 parts
a) The near IR (0.8 -2.5µm) (12,500-4000 cm-1)
b) The middle IR (2.5 -15µm) (4000-667 cm-1)
i) Group frequency Region (4000-1500 cm-1)
ii)Finger print Region (1500-400 cm-1)
c) The far IR (15-200µm) (400-40 cm-1)

 In any molecule it is known that atoms or groups of atoms are
connected by bonds.
 These bonds are analogous to springs and not rigid in
nature.
 Because of the continuous motion of the molecule they
maintain some vibrations with some frequency
characteristic to every portion of the molecule. This is called
the natural frequency of vibration.
 When energy in the form of infrared radiation is applied and
when, Applied infrared frequency= Natural frequency of
vibration, a signal corresponding to the energy transferred is
obtained.
PRINCIPLE

 For a molecule to be IR active there must be a change in dipole
moment as a result of the vibration that occurs when IR radiation is
absorbed.
Dipoles need not be permanent. A small change in dipole is
sufficient to absorb IR radiation.
Homonuclear diatomic molecules such as N2 and O2 do not have
dipole moments. If such molecules undergoes a stretching
vibration, there is no change in the dipole moment during the
vibrational motion, therefore N2 and O2 do not absorb infrared
radiation.

There are 2 types of vibrations.
Stretching vibrations
Bending vibrations
• 1) Stretching vibrations: in this bond length is altered.
• They are of 2 types
• a) symmetrical stretching and b) Asymmetrical stretching
MOLECULARVIBRATIONS

2)Bending vibrations:
•These are also called as deformations.
•These are of 2 types
•a) In plane bending → Scissoring Rocking
•b) Out plane bending→ Wagging Twisting

Molecular vibrations and Hooke’s Law
The stretching frequency of a bond can be approximated by
Hooke’s Law.
Hooke’s law describes the relationship of frequency to mass and
bond length.
The frequency of bond vibration can be derived from Hooke’s law,
which describes the motion of a vibrating spring:
• The force constant (f) is the strength of the bond (or spring). The
larger the value of f, the stronger the bond, and the higher the υ of
vibrations.
•The mass (m) is the mass of atoms. The smaller the value of m, the
higher the υ of vibration.

FUNDAMENTAL VIBRATIONALMODES
 A molecule can vibrate in many ways, and each way is called a
vibrational mode.
 The number of possible vibrations for a molecule is determined by
the degrees of freedom of the molecule.
 If a molecule contains ‘N’ atoms, will have 3N degrees of
freedom.
 For linear molecule it is (3N-5)
 For non linear molecule it is (3N-6)

Factors influencing Vibration Frequencies
Calculated value of frequency of absorption for a particular
bond is never exactly equal to its experimental value.
There are many factors which are responsible for vibration shifts
1. Vibration coupling
2. Hydrogen bonding
3. Electronic effect (Mesomeric effect and Inductive effect)
4. Overtone & Fermi-Resonance

Vibration coupling
An isolated C-H Bond has only one stretching
vibrational frequency, whereas –CH2 (Methylene) group
shows two stretching they are: Symmetrical and
asymmetrical.
Because of mechanical coupling or interaction between
C-H stretching vibrations in the CH2 group.
Assymetric vibration require more energy to take place
so, it will occur at higher frequencies or wave numbers
than symmetrical vibrations.
As these vibration occur at different frequencies than
the required for an isolated C-H stretching, these types of
vibrations are called coupled vibrations.

Hydrogen bonding
Hydrogen bonding brings about remarkable downward
frequency shifts.
Stronger the hydrogen bonding, greater is the absorption shift
towards lower wave length than the normal value.
There is 2 types of hydrogen bonding
a) inter molecular→broad bands
b) intra molecular → sharp bands
 The inter and intra molecular hydrogen bonding can be
distinguished by dilution.
 Intramolecular hydrogen bonding remains unaffected on dilution
and as a result absorption band also remains unaffected
Inter molecular, bonds are broken on dilution and as a result there
is a decrease in the bonded O-H absorption.

Electronic effects
Inductive 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.
Mesomeric effects

Overtone & Fermi-Resonance
Fermi Resonance Coupling of a
fundamental vibration with an
overtone

There are 2 types of infrared spectrophotometer, characterized
by the manner in which the ir frequencies are handled.
1) dispersive type (IR)
2) Interferometric type (FTIR)
TYPES OF INSTRUMENTATION
 In dispersive type the infrared light is separated into individual
frequencies by dispersion, using a grating monochromator.
In interferometric type the IR frequencies are allowed to interact
to produce an interference pattern and this pattern is then analyzed,
to determine individual frequencies and their intensities.

Dispersion Spectrometer
In order to measure an IR
spectrum, the dispersion
Spectrometer takes several
minutes.
Also the detector receives only
a few % of the energy of
original light source.
FTIR
In order to measure an IR
spectrum, FTIR takes only a few
seconds.
Moreover, the detector receives
up to 50% of the energy of
original light source.
(much larger than the
dispersion spectrometer.)
Comparison Beetween Dispersion Spectrometer and FTIR

PARTS OF INSTRUMENTATION
• I R Radiation Source
– Incandescent lamp
– Nernst Glower (Composed of rare earth oxides
such as Zirconia, Yttria & Thoria)
– Globar Source (silicon carbide)
– Mercury Arc
• Sample Cells & Sampling Substances
– Sampling of solids
• Solids run solution
• Solid films
• Mull technique
• Pressed pellet technique
– Sampling of Liquids
– Sampling of Gases
• Detectors
– Bolometers
– Thermocouple
– Thermistors
– Golay Cells
– Photoconductivity cell
– Semiconductor
– Pyroelectric detectors
•Monochromators

Identification of organic
compounds by IR Spectroscopy
(Interpretation of Spectra)

Group Frequencies and Analysis
A. Introduction
1. When approaching any IR spectrum be sure to use the larger-to-smaller
region approach- do not immediately focus on any one single peak
(even –OH or C=O)
2. From the Hooke’s Law derivation we are using we find that the IR can be
conveniently be divided into four major regions:
Bonds to H Triple bonds Double bonds Single Bonds
O-H
N-H
C-H
C≡C
C≡N
C=O
C=N
C=C
C-C
C-N
C-O
C-X
“Fingerprint
Region”
4000 cm-1 2700 cm-1 2000 cm-1 1600 cm-1 400 cm-1

Before we begin – Each functional group will be described as follows:
Group
General – What is most recognizable? What makes it different from similar
groups?
Group Frequencies (cm-1):
Bond
observed
n in cm-1 type of vibration Exceptions and things to watch
Scale on bottom summarizes band positions and strengths
Strong - Medium - Weak -

The Hydrocarbons
Alkanes
General – due to the small electronegativity difference between C and H,
hydrocarbon bands are of medium intensity at best and give simple
spectra
C-H 3000-2800 Stretch Strained ring systems may have
higher n
-CH2- ~1465 Methylene bend (scissor)
-CH3 ~1375 Methyl bend (sym)
-(CH2)4- ~720 Rocking motion 4 or more
–CH2- (long chain band)
C-C Not interpretively useful,
small weak peaks

The Hydrocarbons
Alkanes – Dodecane – C12H26

The Hydrocarbons
Alkanes – Cyclopentane – C5H10

The Hydrocarbons
Alkenes
General – slightly more complex than alkanes; asymmetric C=C is observed as
well as the sp2-C-H stretch. Still, bands are weak to medium in intensity
=C-H 3095-3010 Stretch - Diagnostic for unsaturation- may be
aromatic as well
=C-H 1000-650 Out-of-plane (oop) bend - Can be used to determine degree
of substitution
C=C 1660-1600 Stretch - Can be reduced by resonance
- Symmetrical C=C do not absorb
- trans- weaker than cis-

The Hydrocarbons
Alkenes – 1-octene – C8H16
Note – you still have alkane present!

The Hydrocarbons
Alkenes – trans-4-octene – C8H16
Note – absence of C=C band, shouldering of C-H band

The Hydrocarbons
Alkynes
General – can be symmetric, psuedo-symmetric or internal – greatly reducing
the number of observed bands
C-H ~3300 Stretch - Diagnostic for terminal alkyne
CC ~2150 Stretch - Can be reduced by resonance
-Symmetrical and psuedo-sym. CC
do not absorb
C-H 900-700 Bend (Text does not list)
Possible not to observe any bands
for the CC system

The Hydrocarbons
Alkynes – 1-hexyne – C6H10
Nice terminal, asymmetric, well behaved alkyne
C
HC

The Hydrocarbons
Alkynes – 3-hexyne – C6H10
A not-so-nice, internal, symmetrical alkyne C
C

sp3 Oxygen – Alcohols, phenols and ethers
Alcohols
General – the best recognized group on carefully selected spectra, but H-
bonding effects can drastically change the position, intensity and shape of
the O-H band
O-H
(free)
3650-3600 Stretch Seen in dilute solution or gas
phase spectra
O-H
(H-bond)
3400-3300 Stretch The “classic” H-bonded band,
seen in addition to the free band
in solution
C-O-H 1440-1220 Bend Often obscured by -CH3 bend
C-O 1260-1000 Stretch Can be used to determine 1o, 2o,
3o or phenolic structure

Alcohols – 1-octanol
Neat liquid sample gives classic spectrum HO

Ethers
General – like alkynes, the simplicity of the spectra may allow them to pass
unnoticed – deduce from molecular formula if one should be present
C-O 1300-1000 Stretch (asymm.) Absence of C=O and O-H will
confirm it is not ester or alcohol
Simple alkyl ethers usually one
band at 1120, aryl alkyl ethers
give two bands – 1250 & 1040

Ethers – diispropyl ether
Spectrum dominated by all other functionality O

sp3 Nitrogen – Amines
Amines – Once presence is determined, the substitution at nitrogen is easy
to determine; only the 3° amine may present a problem
N-H
(-NH2)
3650-3600
(2 bands)
1640-1560
Stretch (sym. and asym.)
Bend
N-H
(-NHR)
3400-3300
(1 band)
1500
Stretch
Bend
For alkyl amines, very weak –
for aromatic 2° amines, stronger
N-H ~800 Oop bend
N-N 1350-1000 Stretch Remember 3° amines have no
N-H bands

E. Carbonyls
General – Along with alcohols, the most ubiquitous group on the IR spectrum.
Although it is easy to determine if the C=O is present, deducing the exact
functionality and factors that influence the position of the band provide
the challenge
Base C=O Frequencies (cm-1):
C=O 1810 Stretch (sym.) Anhydride band 1
1800 Acid Chloride
1760 Anhydride band 2
1735 Ester
1725 Aldehyde
1715 Ketone
1710 Carboxylic Acid
1690 Amide

Carbonyls
1. Ketones – Simplest carbonyl group, for a single carbonyl compound,
implied by a lack of any other functionality except hydrocarbon
C=O 1715 Stretch (sym.) n Base, sensitive to change
conj.
w/C=C
1700-1675 nC=C reduced to 1644-1617
conj.
w/Ph
1700-1680 nring 1600-1450
C=O 1815-1705 Decreased ring size raises n
1300-1100 Bend
C
C
C
O

Carbonyls
2. Aldehydes – Presence of the unique carbonyl C-H bond differentiates
this group from ketones
conj.
w/C=C
1700-1680 nC=C reduced to 1640
conj.
w/Ph
1700-1660 nring 1600-1450
2820,
2720
Stretch Fermi doublet; Higher n band
often obscured by sp3 C-H
R
C
H
O

Carbonyls
3. Carboxylic Acids – Various H-bonding effects lead to messy spectra,
especially in the upper frequency ranges – be aware of the effects of
monomeric, dimeric and oligomeric species on the spectrum
C=O 1710 Stretch (sym.) n Base, sensitive to change;
conjugation gives reduced n
C-O 1320-1210 Stretch
O-H 3400-2400 Stretch Overlaps C-H region in most
cases; multiple “sub-peaks” can
be seen for the dimeric and
oligomeric species – simplified in
non-polar solution or gas phase
spectra

E. Carbonyls
4. Esters – Ester oxygen has an electron withdrawing effect that tends to
draw in electrons within the C=O system, strengthening it compared to
other carbonyls
conj.
C=C
1735-1715 nC=C reduced to 1640-1625
w/Ph 1735-1715 nring 1600-1450
conj. of
sp3 O
1765-1760
1850-1740 nC=O increases with smaller ring
C-O 1300-1000 Stretch, 2 bands
C
O
O

Applications of Infrared Analysis
Pharmaceutical research
Forensic investigations
Polymer analysis
Lubricant formulation and fuel additives
Foods research
Quality assurance and control
Environmental and water quality analysis methods
Biochemical and biomedical research
Coatings and surfactants
Etc.

References :
Lena Ohannesian, Antony J. Streeter; Handbook of
Pharmaceutical Analysis; Marcel Dekker, Inc.; Reprint 2002
Chatwal and Anand ; Instrumental methods of chemical analysis;
fifth edition; page no-2.43-46
Spectrometric identification of organic compounds, R M
Silverstein,T.C morril G.C. bassler Fifth edition, p.no.99-100
Internet :
www.wikipedia.com
www.answers.com
www.authorstream.com
www.slideworld.com
www.google.com

Infrared Spectroscopy

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