IR Spectroscopy - Sudheerkumar Kamarapu, M. Pharmacy Lecture pdf

INFRARED
SPECTROSCOPY
By
Sudheerkumar Kamarapu
pharmaceutical analysis
Sri Shivani College of Pharmacy

Classification of analytical techniques

1.Separation techniques →Chromatography
2.Spectrophotometric → SPECTROSCOPY
3. Electro analytical → Potentiometry,conductometry
4. Titrimetric analysis→ Titrations

Spectroscopy
“seeing the unseeable”.

Spectroscopy is the branch of science deals with 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→SPECTROPHOTOGRAPH
↓
concentration should be lower

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 λ 1/υ
E = hυ E υ
E = hc/λ E 1/λ

λ = wave length
υ = frequency
c = speed of light λ
E = kinetic energy
c
h = Planck’s constant

Two oscillators will strongly interact when their energies are equal.

E1 = E2
λ1 = λ2
υ1 = υ2

If the energies are different, they will not strongly interact!
We can use electromagnetic radiation to probe atoms and molecules to
find what energies they contain.

some electromagnetic radiation ranges
Approx. freq. range Approx. wavelengths
Hz (cycle/sec) meters

Radio waves 104 - 1012 3x104 - 3x10-4
Infrared (heat) 1011 - 3.8x1014 3x10-3 - 8x10-7
Visible light 3.8x1014 - 7.5x1014 8x10-7 - 4x10-7
Ultraviolet 7.5x1014 - 3x1017 4x10-7 - 10-9
X rays 3x1017 - 3x1019 10-9 - 10-11
Gamma rays > 3x1019 < 10-11

IR SPECTROSCOPY
INTRODUCTION
 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.
 Infrared region lies between visible and microwave
region.

The infrared region constitutes 3 parts
a) The near IR (0.8 -2.5µm) (12,500-4000cm-1)
b) The middle IR (2.5 -15µm) (4000-667cm-1)
i) Group frequency Region (4000-1500cm-1)
ii) Finger print Region (1500-667cm-1)
c) The far IR (15-200µm) (667-50cm-1)
 most of the analytical applications are confined to the middle IR region
because absorption of organic molecules are high in this region.
Wave number is mostly used measure in IR absorption because wave
numbers are larger values & easy to handle than wave length which are
measured in µm.
 E = hν = hc/λ = hcν¯
It gives sufficient information about the structure of a compound.

PRINCIPLE
 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

MOLECULAR VIBRATIONS
There are 2 types of vibrations.
1) Stretching vibrations
2) Bending vibrations
• 1)Stretching vibrations: in this bond length is altered.
• They are of 2 types
• a) symmetrical stretching: 2 bonds increase or decrease in length.

b) Asymmetrical stretching: in this one bond length is
increased and other is decreased.

2)Bending vibrations:
•These are also called as deformations.
•In this bond angle is altered.
•These are of 2 types
•a) in plane bending→ scissoring, rocking
•b) out plane bending→ wagging, twisting

Scissoring:
This is an in plane bending.
In this bond angles are decreased.2 atoms approach each other.

Rocking:
•In this movement of atoms takes place in same direction.

Wagging:
It is an out of plane bending.
In this 2 atoms move to one side of the plane. They move up and down the
plane.

Twisting:
•In this one atom moves above the plane and the other atom moves below
the plane.

NUMBER OF VIBRATIONAL MODES

 A molecule can vibrate in many ways, and each way is
called a vibrational mode.
 If a molecule contains ‘N’ atoms, total number of
vibrational modes
 For linear molecule it is (3N-5)
 For non linear molecule it is (3N-6)

Eg: H2O, a non-linear molecule, will have 3 × 3 – 6 = 3
degrees of vibrational freedom, or modes.

VIBRATIONAL FREQUENCY
 occurs when atoms in a molecule are in periodic motion while the molecule as a
whole has constant translational and rotational motion.
 The frequency of the periodic motion is known as a vibration frequency.
 The value of stretching vibrational frequency of a bond can be calculated by the
application of hooke’s law.
ν/c = ν¯ = 1/2пc[k/m1m2/m1+m2]1/2
= 1/2пc√k/µ
Where, µ→reduced mass
m1&m2 →masses of the atoms
k →force constant
c →velocity of radiation

Factors influencing vibrational
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 vibrational shifts

1) Vibrational coupling:
• it is observed in compounds containing –CH2 &
-CH3.
EG. Carboxylic acid anhydrides
amides
aldehydes

2) 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
•hydrogen bonding in O-H and N-H compounds deserve special
attention.
•Eg: alcohols&phenols
enols & chelates

3) Electronic effects:

In this the frequency shifts are due to electronic effects
which include conjugation, mesomeric effect, inductive
effect.
a) conjugation: conjugation lowers the absorption
frequency of C=O stretching whether the conjugation is
due to α,β- unsaturation or due to an aromatic ring.
b) mesomeric effect: a molecule can be represented by
2or more structures that differ only in the arrangement of
electrons.
c) inductive effect: depends upon the intrinsic tendency
of a substituent to either release or withdraw electrons.

TYPES OF INSTRUMENTATION
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)
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.

DISPERSIVE INSTRUMENTS
 These are often double-beam recording instruments,
employing diffraction gratings for dispersion of radiation.
 These 2 beams are reflected to a chopper which consists
of rotating mirror.
 It sends individual frequencies to the detector
thermopile.
 Detector will receive alternately an intense beam & a
weak beam.
 This alternate current flows from detector to amplifier.

INTERFEROMETRIC INSTRUMENTS
THE MICHELSON INTERFEROMETER:

 It is used to produce a new signal of a much lower frequency
which contains the same information as the original IR
signal.
 The output from the interferometer is an interferogram.
 Radiation leaves the source and is split.
 Half is reflected to a stationary mirror and then back to the
splitter.
 The other half of the radiation from the source passes
through the splitter and is reflected back by a movable
mirror. Therefore, the path length of this beam is variable.
The two reflected beams recombine at the splitter, and they
interfere .
 interference alternates between constructive and
destructive phases.
 The accuracy of this measurement system means that the
IR frequency scale is accurate and precise.

FOURIER TRANSFORM IR
SPECTROMETER
 In the FT-IR instrument, the sample is placed between the output of the
interferometer and the detector. The sample absorbs radiation of particular
wavelengths.
 An interferogram of a reference is needed to obtain the spectrum of the
sample.
 After an interferogram has been collected, a computer performs a Fast Fourier
Transform, which results in a frequency domain trace (i.e intensity vs
wavenumber).
 The detector used in an FT-IR instrument must respond quickly because
intensity changes are rapid .
 Pyroelectric detectors or liquid nitrogen cooled photon detectors must be used.
Thermal detectors are too slow.
 To achieve a good signal to noise ratio, many interferograms are obtained and
then averaged. This can be done in less time than it would take a dispersive
instrument to record one scan.

Advantages of Fourier transform IR over
dispersive IR
Improved frequency resolution
Improved frequency reproducibility (older
dispersive instruments must be recalibrated for
each session of use)
Faster operation
Computer based (allowing storage of spectra and
facilities for processing spectra)
Easily adapted for remote use (such as diverting the
beam to pass through an external cell and detector,
as in GC - FT-IR)

 Opaque or cloudy samples

 Energy limiting accessories such as diffuse reflectance or FT-IR
microscopes

 High resolution experiments (as high as 0.001 cm -1 resolution)

 Trace analysis of raw materials or finished products
 Depth profiling and microscopic mapping of samples

 Kinetics reactions on the microsecond time-scale
 Analysis of chromatographic and thermogravimetric sample
fractions

Comparison Beetween Dispersion Spectrometer
and FTIR
To separate IR light, a grating is used.
Detector Dispersion
Grating
Slit Spectrometer
In order to measure an IR spectrum,
the dispersion Spectrometer takes
Sample several minutes.
Also the detector receives only
a few % of the energy of
original light source.
To select the specified IR light,
Light source A slit is used.

Fixed CCM An interferogram is first made
by the interferometer using IR FTIR
light. In order to measure an IR spectrum,
Detector FTIR takes only a few seconds.
Moreover, the detector receives
up to 50% of the energy of original
B.S. light source.
(much larger than the dispersion
spectrometer.)
Sample

Moving CCM
The interferogram is calculated and transformed
IR Light source into a spectrum using a Fourier Transform (FT).

FTIR seminar

FT Optical System Diagram

Light He-Ne gas laser
source

(ceramic)

Beam splitter
Movable mirror

Sample chamber

(DLATGS)
Fixed mirror
Detector
Interferometer

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.

PARTS OF INSTRUMENTATION
• I R Radiation Source Monochromators
– Incandescent lamp
– Nernst Glower
– Globar Source
– Mercury Arc
• Detectors
• Sample Cells & Sampling – Bolometers
Substances – Thermocouple
– Sampling of solids – Thermistors
• Solids run solution – Golay Cells
• Solid films
– Photoconductivity cell
• Mull technique
• Pressed pellet technique
– Semiconductor
– Pyroelectric detectors
– Sampling of Liquids
– Sampling of Gases

2/5/2013 sudheerkumar kamarapu 32

I R Radiation Sources
Incandescent Lamps
• ordinary lamp used
• glass enclosed

Disadv.
• fails in far infrared
• low spectral emissivity


Nernst Glower
Composed of rare earth oxides such as Zirconia, Yttria & Thoria
Non conducting at room temperature
WORKING
Heating
Conducting state
Provides radiation of about 7100 cm-1

Disadv.
Emitts I R radiation over wide wavelength range
Frequent mechanical failure
Energy concentrated in visible & near I R region of spectrum


Globar Source
• Self starting, Controlled conveniently with variable
transformer

Works at wavelength longer than 650 cm-1 (0.15µ)
5200 cm-1 radiation given at 1300 – 1700 OC

Disadv.
Less intense source than Nernst Glower


Mercury Arc

Special high pressure mercury lamps are used in far I R

Beckman devised the Quartz Mercury Lamps in unique manner

shorter wavelength ------- heated quartz envelope provides radiation
longer wavelength -------- mercury plasma provides radiation


MONOCHROMATORS

• They convert polychromatic light into mono chromatic
light.
• They must be constructed of materials which transmit
the IR.
• They are of 3 types.
• a) metal halide prisms
• b) NaCl prisms
• c) gratings


 a) metal halide prisms:
• prisms which are made up of KBr are used in the
wavelength region from 12-25µm.
• LiF prisms are used in the wavelength region from
0.2-6µm.
• CeBr prisms used in wavelength region from 15-38µm.
 b) NaCl prisms:
• Used in the whole wave length region from 4000-
650cm-1.
• they have to be protected above 20•c because of
hygroscopic nature.
 c) gratings:
• They offer better resolution at low frequency than prisms. 38
2/5/2013 sudheerkumar kamarapu

• Sample cells made up of alkali halides like NaCl or KBr are
used.
• Aqueous solvents cannot be used as they dissolve alkali halides.
• Only organic solvents like chloroform is used.
• IR spectroscopy has been used for the characterization of solid,
liquid, gas samples.


Solids run in solution
Solids dissolved in a aqueous solvent

Placed over the alkali metal disk

Solvent is allowed to evaporate

Thin film of solute formed

Entire solution is run in one of the cells for liquids
Note :
This method not used because suitable number of solvents available are
less.
Absorption due to solvent has to be compensated by keeping the solvent
in a cell of same thickness as that containing the reference beam of
2/5/2013
double beam spectrometer.sudheerkumar kamarapu 40

Solid Films
• Technique used for Amorphous sample.
• Deposited on the KBr / NaCl cell by evaporation of solution.
• Only useful for rapid qualitative analysis.

Mull Technique
• Finely ground solid sample is used.
• Mixed with Nujol (mineral oil)
• Thick paste is made.
• Spread between I R transmitting windows
• Mounted in path of I R beam &
• The spectrum is run.
Disadv. Nujol has the absorption maxima at 2915, 1462, 1376 & 719 cm-1

Pressed Pellet Technique
• Finely ground sample used
• Potassium Bromide is mixed (100 times more)
• Passed through a high pressure press
• Small pellet formed (1-2 mm thick, 2cm diameter)
• The pellet is transparent to I R radiation & is run as such
Adv.
• Pellet can be stored for long period of times.
• Concentration of sample can be adjusted in KBr pellet hence used
for quantitative analysis.
• Resolution of spectrum is superior.
Disadv.
• Always has a band at 3450 cm-1 (moisture OH-)
• At high pressure polymorphic changes occur
•2/5/2013
Unsuccessful for polymersudheerkumar kamarapu
which are difficult to grind with KBr. 42

Diffuse Reflectance
• Sometimes referred to as DRIFTS (diffuse reflectance infrared
Fourier transform spectroscopy)
• Involves irradiation of the powdered sample by an infrared
beam.
• The incident radiation undergoes absorption, reflection, and
diffraction by the particles of the sample.
• Only the incident radiation that undergoes diffuse reflectance
contains absorptivity information about the sample.


Microspectroscopy
• The ultimate sampling technique, since only one particle
is required for analysis.
• Particles of interest must be greater in size than 10 X
10 μm.
• Sample placed on an IR optical window and the slide is
placed onto the microscope stage and visually
inspected
• Once the sample of interest is in focus, the field of view
is apertured down to the sample.
• Depending on sample morphology, thickness, and
transmittance properties, a reflectance and/or
transmittance IR spectrum may be acquired by the IR
microscope accessory.

Attenuated Total Reflectance

• The basic premise of the technique involves
placing the sample in contact with an infrared
transmitting crystal with a high refractive index.
• The infrared beam is directed through the
crystal, penetrating the surface of the sample,
and displaying spectral information of that
surface.
• Advantage of this technique is that it requires
very little sample preparation,
• Simply place the sample in contact with the
crystal

Photoacoustic

• The PAS phenomenon involves the selective absorption
of modulated IR radiation by the sample.
• Once absorbed, the IR radiation is converted to heat
and subsequently escapes from the solid sample and
heats a boundary layer of gas.
• The increase in temperature produces pressure changes
in the surrounding gas.
• The pressure changes in the coupling gas occur at the
frequency of the modulated light, as well as the acoustic
wave.
• This acoustical wave is detected by a very sensitive
microphone and the subsequent electrical signal is
Fourier processed and a spectrum produced.

• Liquid samples taken.
• Put it into rectangular cells of KBr, NaCl etc.
• I R spectra obtained.

• Sample thickness … such that transmittance lies
between 15 – 20 % i.e., 0.015 – 0.05 mm in thickness.

• For double beam, matched cells are generally employed
• One cell contains sample while other has solvent used
in sample.
• Matched cells should be of same thickness, protect
from moisture.

• Small size particles hence the cells are large.
• 10 cm to 1m long
• Multiple reflections can be used to make the effective path
length as long as 40 cm
• Lacks sensitivity


DETECTORS
They convert the radiation into electrical signal.
Two Types Of Detectors
Thermal Detectors Photon Detectors
 Thermocouple  Semiconductors
 Bolometers  Photovoltaic Intrinsic
 Thermistors Detectors
 Golay Detectors  Photoconductive
 Pyroelectric Detectors Intrinsic Detectors


Thermocouples
Based upon the fact that
an electrical current will flow when two dissimilar metal wires
are connected together at both ends and a temperature
differential exists between the two ends

Example : Bismuth & Antimony

Bolometer
• It consists of thin metallic conductor, its resistance changes
due to increase in temperature when IR radiation falls on it.
• It is a electrical resistance thermometer which can detect
and measure feeble thermal radiation.
• The electrical resistance increases approx 0.4% for every
celsius degree increase of temperature .
• The degree of change in resistance is regarded as the
measure of the amount of IR radiation falling on it.
• A bolometer is made of two platium strips, covered with
lamp black, one strip is sheilded from radiation and one
exposed to it. The strips formed two branches of
wheatstone bridge


Working – The circuit thus effectively operating as resistance
temperature detector. When IR radiations falling on the exposed
strip would heat it, and change the resistance, this causes current
to flow, the amount of current flowing is a measure of intensity of
IR radiation
The response time is 4 secs.


Golay Cells
• It consist of a small metal detector closed by a rigid
blackened metal plate (2 mm), flexible silvered
diaphragm at the other end filled with Xenon gas.

• Its response time is 20 msec, hence faster than other
thermal detectors

• It is suitable for wavelengths greater then 15 u


Working
• The radiation falls on the blackened metal plate, this heats
the gas which lead to deformation of flexible silvered
diaphragm.
• The light from a lamp inside the detector is made to fall on
the diaphragm which reflects the light on to a phototube.
• The signal seen by the phototube / photocell is modulated
in accordance with the power of the radiation beams
incident on the gas cell.


Thermistors
• It is made up of metal oxides.
• It functions by changing resistance when heated.
• It consists of two closely placed thermistor flakes, one of the 10
um is an active detector, while the other acts as the
compensating / reference detector.
• A steady voltage is applied, due to the temperature increase
there is change in resistance which is measured and this gives
the intensity of the IR radiation


Pyroelectric Detectors
• It consist of a thin dielectric flake on the face of which
an electrostatic charge appears. When the
temperature of the flake changes upon exposure to IR
radiations, electrodes attached to the flake collect the
charge creating a voltage.
• The most common is TGS (Triglycine Sulfate) however
its response deteriorates above 45 C and is lost above
the 49 C
• Detureated triglycine Sulfated are available and can
be used at room temperature.


PHOTON DETECTORS

• These detectors convert photons directly into free
current carriers by photo exciting electrons across the
energy band gap of the semiconductor to the
conduction band. This produces a resistance change
in the detectors.

• This photon excitation is caused by radiation
interacting directly with the lattice sites.


Semiconductors

• These act as insulators but when radiation fall on them, they
become conductors.
• Exposure to radiation causes a rapid response to the IR signal.
• Working – An IR photon displaces an electron in the detector
which excites electrons to move from the valence band to the
higher energy conduction band.
• Semiconductor materials are Telluride, Indium, Antimonide &
Germanium.


Photovoltaic intrinsic detectors

• Under IR radiation, the potential barrier of the P N junction leads
to the photovoltaic effect. An incident photon with the energy
greater the energy band gap of the junction generates electron
hole pairs and the photocurrent is excited.
• The amount of the photon excited current is denoted by
photocurrent.
• The highest performance PV detectors are fabricated from Si, Ge,
As, In & Sb.


Photoconductive Intrinsic Detectors

• This is non thermal detector of greater sensitivity.
• It consists of a thin layer of lead sulfide supported on gas
envelope. When IR radiation is focused on the lead sulfide its
conductance increases and causes more current to flow.
• It has high sensitivity and good speed of about 0.5 msec
• Upon drastic cooling the range can be broadened.
• PC detectors include Germanium and Silicon detectors.


COMPARISON


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


IR source  sample  prism  detector

graph of % transmission vs. frequency
=> IR spectrum

100

%T

0
4000 3000 2000 1500 1000 500

v (cm-1)


toluene


Intensity in IR
Intensity: Transmittance (T) or %T

I
T=
I0

Absorbance (A)

I
A = log
I0

IR : Plot of %IR that passes through a sample (transmittance)
vs Wavelenght


Infrared

• Position, Intensity and Shape of bands gives
clues on Structure of molecules
• Modern IR uses Michelson Interferometer
=> involves computer, and Fourier Transform
Sampling => plates, polished windows, Films …
Must be transparent in IR

NaCl, KCl : Cheap, easy to polish
NaCl transparent to 4000 - 650 cm-1
KCl transparent to 4000 - 500 cm-1
KBr transparent to 400 cm-1


Infrared: Low frequency spectra of window
materials


Bond length and strength
vs
Stretching frequency

Bond C-H =C-H -C-H
Length 1.08 1.10 1.12

Strenght 506 kJ 444 kJ 422 kJ

IR freq. 3300 cm-1 3100 cm-1 2900 cm-1


Calculating stretching frequencies
Hooke’s law :
n : Frequency in cm -1

n
1

= K c : Velocity of light => 3 * 1010 cm/s
2pc m
K : Force constant => dynes /cm

m:masses of atoms in grams

m1 m2 M1 M2
m= =
m1 + m2 M1 + M2 (6.02 * 1023)

n = 4.12
 K
m
C—C K = 5* 105 dynes/cm
C=C K = 10* 105 dynes/cm
CC K = 15* 105 dynes/cm


Calculating stretching frequencies
C=C K = 10* 105 dynes/cm
n = 4.12
 K
m

n= 4.12
M1 M2 (12)(12)
m=
M1 + M2
=
12 + 12
=6
 10* 105
6
= 1682 cm-1

n Experimental  1650 cm-1

C—H K = 5* 105 dynes/cm
n= 4.12
 5* 105
.923
= 3032 cm-1
M1 M2 (12)(1)
m= = =0.923

n
M1 + M2 12 + 1
Experimental  3000 cm-1

C—D K = 5* 105 dynes/cm
n= 4.12
 5* 105
.923
= 2228 cm-1
M1 M2 (12)(2)
m= = =1.71

n
M1 + M2 12 + 2
Experimental  2206 cm-1

www.cem.msu.edu/~reusch/Virtual/Text/Spectrpy/InfraRed/infrared.htm
Vibrations

Stretching
frequency

Bending
frequency

O
Modes of vibration
Bending C H
Stretching C—H H
H H Wagging
Scissoring
H H 1350 cm-1
1450 cm-1
H

H H H
H
H
Symmetrical Asymmetrical Rocking Twisting
2853 cm-1 720 cm-1 1250 cm-1
2926 cm-1 H

www.cem.msu.edu/~reusch/Virtual/Text/Spectrpy/InfraRed/infrared.htm
Vibrations

General trends:
•Stretching frequencies are higher than bending frequencies
(it is easier to bend a bond than stretching or compresing them)
•Bond involving Hydrogen are higher in freq. than with heavier atoms
•Triple bond have higher freq than double bond which has higher freq than single bond


Structural Information from Vibration Spectra
The symmetry of a molecule determines the number of bands expected
Number of bands can be used to decide on symmetry of a molecule

Tha task of assignment is complicated by presence of low intensity bands and
presence of forbidden overtone and combination bands.
There are different levels at which information from IR can be analyzed to allow
identification of samples:

• Spectrum can be treated as finger print to recognize the
product of a reaction as a known compound. (require
access to a file of standard spectra)
• At another extreme , different bands observed can be
used to deduce the symmetry of the molecule and force
constants corresponding to vibrations.
• At intermediate levels, deductions may be drawn about
the presence/absence of specific groups

Methods of analyzing an IR spectrum
The effect of isotopic substitution on the observed spectrum
Can give valuable information about the atoms involved in a particular vibration

1. Comparison with standard spectra : traditional approach
2. Detection and Identification of impurities
if the compound have been characterized before, any bands that are
not found in the pure sample can be assigned to the impurity
(provided that the 2 spectrum are recorded with identical conditions: Phase,
Temperature, Concentration)

3. Quantitative Analysis of mixture
Transmittance spectra = I/I0 x 100 => peak height is not
lineraly related to intensity of absorption
In Absorbance A=ln (Io/I) sudheerkumar=> Direct measure of intensity 74
2/5/2013 kamarapu

Analyzing an IR spectrum
In practice, there are similarities between frequencies of molecules containing similar groups.
Group - frequency correlations have been extensively developed for organic compounds and
some have also been developed for inorganics


Some characteristic infrared absorption frequencies
BOND COMPOUND TYPE FREQUENCY RANGE, cm-1
C-H alkanes 2850-2960 and 1350-1470
alkenes 3020-3080 (m) and
RCH=CH2 910-920 and 990-1000
R2C=CH2 880-900
cis-RCH=CHR 675-730 (v)
trans-RCH=CHR 965-975
aromatic rings 3000-3100 (m) and
monosubst. 690-710 and 730-770
ortho-disubst. 735-770
meta-disubst. 690-710 and 750-810 (m)
para-disubst. 810-840 (m)
alkynes 3300

O-H alcohols or phenols 3200-3640 (b)

C=C alkenes 1640-1680 (v)
aromatic rings 1500 and 1600 (v)

C≡C alkynes 2100-2260 (v)

C-O primary alcohols 1050 (b)
secondary alcohols 1100 (b)
tertiary alcohols 1150 (b)
phenols 1230 (b)
alkyl ethers 1060-1150
aryl ethers 1200-1275(b) and 1020-1075 (m)

all abs. strong unless marked: m, moderate; v, variable; b, broad

IR spectra of ALKANES
C—H bond ―saturated‖
(sp3) 2850-2960 cm-1
+ 1350-1470 cm-1

-CH2- + 1430-1470
-CH3 + ― and 1375
-CH(CH3)2 + ― and 1370, 1385
-C(CH3)3 + ― and 1370(s), 1395 (m)


n-pentane

2850-2960 cm-1
3000 cm-1
sat’d C-H

1470 &1375 cm-1

CH3CH2CH2CH2CH3

n-hexane

CH3CH2CH2CH2CH2CH3

2-methylbutane (isopentane)


2,3-dimethylbutane


cyclohexane

no 1375 cm-1
no –CH3


IR of ALKENES
=C—H bond, ―unsaturated‖ vinyl
(sp2) 3020-3080 cm-1
+ 675-1000
RCH=CH2 + 910-920 & 990-1000
R2C=CH2 + 880-900
cis-RCH=CHR + 675-730 (v)
trans-RCH=CHR + 965-975

C=C bond 1640-1680 cm-1 (v)

1-decene

unsat’d
C-H

3020-
3080
cm-1 910-920 &
C=C 1640-1680 990-1000
RCH=CH2


4-methyl-1-pentene

910-920 &
990-1000
RCH=CH2


2-methyl-1-butene

880-900
R2C=CH2


2,3-dimethyl-1-butene

880-900
R2C=CH2


IR spectra BENZENEs
=C—H bond, ―unsaturated‖ ―aryl‖
(sp2) 3000-3100 cm-1
+ 690-840
mono-substituted + 690-710, 730-770
ortho-disubstituted + 735-770
meta-disubstituted + 690-710, 750-810(m)
para-disubstituted + 810-840(m)

C=C bond 1500, 1600 cm-1


ethylbenzene

3000-
3100
cm-1
Unsat’d
1500 & 1600
C-H 690-710,
Benzene ring 730-770
mono-


o-xylene

735-770
ortho


p-xylene

810-840(m)
para


m-xylene

meta

690-710,
750-810(m)


styrene

no sat’d C-H

1640
C=C 910-920 &
990-1000
mono
RCH=CH2


2-phenylpropene

Sat’d C-H

880-900
mono
R2C=CH2


p-methylstyrene

para


IR spectra ALCOHOLS & ETHERS

C—O bond 1050-1275 (b) cm-1
1o ROH 1050
2o ROH 1100
3o ROH 1150
ethers 1060-1150

O—H bond 3200-3640 (b) 


1-butanol

3200-3640 (b) O-H

C-O 1o

CH3CH2CH2CH2-OH

2-butanol

O-H
C-O 2o


tert-butyl alcohol

O-H
C-O 3o


methyl n-propyl ether

no O--H

C-O ether


2-butanone

 C=O
~1700 (s)


C9H12

1500 & 1600
benzene

C-H unsat’d & mono
sat’d C9H12 – C6H5 = -C3H7

isopropylbenzene
n-propylbenzene?

n-propylbenzene


isopropylbenzene

isopropyl split 1370 + 1385


C8H6

C-H 1500, 1600
unsat’d benzene

3300 C 8 H6 – C 6 H5 = C 2 H
C-H mono

phenylacetylene


C4H8

Unst’d

1640- 880-900
1680
R2C=CH2
C=C

isobutylene CH3
CH3C=CH2


Which compound is this?
a) 2-pentanone 1-pentanol
b) 1-pentanol
c) 1-bromopentane
d) 2-methylpentane

What is the compound?
a) 1-bromopentane 2-pentanone
b) 1-pentanol
c) 2-pentanone
2/5/2013 2-methylpentane
d) sudheerkumar kamarapu 108

In a ―matching‖ problem, do not try to fully analyze each spectrum. Look
for differences in the possible compounds that will show up in an infrared
spectrum.

H2 H2 H2
A C C C CH2 E C C
H

biphenyl allylbenzene 1,2-diphenylethane

CH3
CH3
D CH3CH2CH2CH2CH3 F CH2CH2CH2CH3
B

o-xylene n-pentane n-butylbenzene


1


2


3


4


5


6


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


IR Spectroscopy - Sudheerkumar Kamarapu, M. Pharmacy Lecture pdf

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