By Sudheerkumar Kamarapu pharmaceutical analysis Sri Shivani College of Pharmacy INFRARED SPECTROSCOPY
Jun 19, 2015
By
Sudheerkumar Kamarapu
pharmaceutical analysis
Sri Shivani College of Pharmacy
INFRARED SPECTROSCOPY
INFRARED SPECTROPHOTOMETER
1.Separation techniques →Chromatography
2.Spectrophotometric → SPECTROSCOPY
3. Electro analytical → Potentiometry,conductometry
4. Titrimetric analysis→ Titrations
Classification of analytical techniques
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
Spectroscopy
“seeing the unseeable”.
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
E = kinetic energy
h = Planck’s constant
λ
c
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
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.
IR SPECTROSCOPY
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.
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
PRINCIPLE
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.
MOLECULAR VIBRATIONS
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.
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
VIBRATIONAL FREQUENCY
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
Factors influencing vibrational frequencies
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.
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.
TYPES OF INSTRUMENTATION
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
To separate IR light, a grating is used.
Grating
Light source
Detector
Sample
Slit
To select the specified IR light,
A slit is used.
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.
Fixed CCM
B.S.
Moving CCM
IR Light source
Sample
Detector
An interferogram is first made
by the interferometer using IR
light.
The interferogram is calculated and transformed
into a spectrum using a Fourier Transform (FT).
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
FTIR seminar
Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light
source
(ceramic)
Detector
(DLATGS)
Beam splitter
FT Optical System Diagram
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 – Incandescent lamp
– Nernst Glower
– Globar Source
– 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
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I R Radiation Sources
Incandescent Lamps
• ordinary lamp used
• glass enclosed
Disadv.
• fails in far infrared
• low spectral emissivity
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Nernst Glower
Composed of rare earth oxides such as Zirconia, Yttria & Thoria
Non conducting at room temperature
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
WO
RK
ING
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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
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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
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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
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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.
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• 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.
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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
double beam spectrometer. sudheerkumar kamarapu 2/5/2013 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
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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
• Unsuccessful for polymer which are difficult to grind with KBr.
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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.
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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.
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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 sudheerkumar kamarapu 2/5/2013 45
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.
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• 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. sudheerkumar kamarapu 2/5/2013 47
• 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
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DETECTORS
They convert the radiation into electrical signal. Two Types Of Detectors
Thermocouple Bolometers Thermistors Golay Detectors Pyroelectric Detectors
Photon Detectors Thermal Detectors
Semiconductors Photovoltaic Intrinsic
Detectors Photoconductive
Intrinsic Detectors
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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
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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
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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.
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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
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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.
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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
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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.
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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.
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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.
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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.
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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.
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COMPARISON
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Identification of organic and inorganic compounds
by IR Spectroscopy (Interpretation of Spectra)
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IR source sample prism detector
graph of % transmission vs. frequency
=> IR spectrum
4000 3000 2000 1500 1000 500
v (cm-1)
100
%T
0
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toluene
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Intensity: Transmittance (T) or %T
T = I
I0
Absorbance (A)
A = log I
I0
Intensity in IR
IR : Plot of %IR that passes through a sample (transmittance) vs Wavelenght
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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
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Infrared: Low frequency spectra of window materials
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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
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Calculating stretching frequencies Hooke’s law :
n = 1
2pc K m
n : Frequency in cm-1
c : Velocity of light => 3 * 1010 cm/s
K : Force constant => dynes /cm
m:masses of atoms in grams
m=m1 m2
m1 + m2 =
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
CC K = 15* 105 dynes/cm
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Calculating stretching frequencies
C=C K = 10* 105 dynes/cm n = 4.12 K m
m=M1 M2
M1 + M2 =
(12)(12)
12 + 12 =6 n= 4.12 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
m=M1 M2
M1 + M2 =
(12)(1)
12 + 1 =0.923
n Experimental 3000 cm-1
C—D K = 5* 105 dynes/cm n= 4.12 5* 105
.923= 2228 cm-1
m=M1 M2
M1 + M2 =
(12)(2)
12 + 2 =1.71
n Experimental 2206 cm-1 2/5/2013 sudheerkumar kamarapu 70
Vibrations www.cem.msu.edu/~reusch/Virtual/Text/Spectrpy/InfraRed/infrared.htm
Modes of vibration
C—H Stretching Bending C
O
H
H
H
Symmetrical 2853 cm-1
H
H
Asymmetrical 2926 cm-1
H
H
H
H
Scissoring 1450 cm-1
Rocking 720 cm-1
H
H
H
H
Wagging 1350 cm-1
Twisting 1250 cm-1
Stretching frequency
Bending frequency
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Vibrations www.cem.msu.edu/~reusch/Virtual/Text/Spectrpy/InfraRed/infrared.htm
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
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Structural Information from Vibration Spectra
• 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
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:
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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) => Direct measure of intensity 2/5/2013 sudheerkumar kamarapu 74
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
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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
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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)
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n-pentane
CH3CH2CH2CH2CH3
3000 cm-1
1470 &1375 cm-1
2850-2960 cm-1
sat’d C-H
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CH3CH2CH2CH2CH2CH3
n-hexane
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2-methylbutane (isopentane)
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2,3-dimethylbutane
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cyclohexane
no 1375 cm-1
no –CH3
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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)
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1-decene
910-920 &
990-1000
RCH=CH2
C=C 1640-1680
unsat’d
C-H
3020-
3080
cm-1
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4-methyl-1-pentene
910-920 &
990-1000
RCH=CH2
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2-methyl-1-butene
880-900
R2C=CH2
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2,3-dimethyl-1-butene
880-900
R2C=CH2
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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
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ethylbenzene
690-710,
730-770
mono-
1500 & 1600
Benzene ring
3000-3100 cm-1
Unsat’d C-H
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o-xylene
735-770
ortho
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p-xylene
810-840(m)
para
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m-xylene
meta
690-710,
750-810(m)
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styrene
no sat’d C-H
910-920 &
990-1000
RCH=CH2
mono
1640
C=C
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2-phenylpropene
mono 880-900
R2C=CH2
Sat’d C-H
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p-methylstyrene
para
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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)
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1-butanol
CH3CH2CH2CH2-OH
C-O 1o
3200-3640 (b) O-H
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2-butanol
C-O 2o
O-H
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tert-butyl alcohol
C-O 3o O-H
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methyl n-propyl ether
no O--H
C-O ether
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2-butanone
C=O
~1700 (s)
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C9H12
C-H unsat’d &
sat’d
1500 & 1600
benzene
mono
C9H12 – C6H5 = -C3H7
isopropylbenzene
n-propylbenzene? 2/5/2013 sudheerkumar kamarapu 102
n-propylbenzene
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isopropyl split 1370 + 1385
isopropylbenzene
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C8H6
C-H
unsat’d 1500, 1600
benzene
mono
C8H6 – C6H5 = C2H
phenylacetylene
3300
C-H
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C4H8
1640-
1680
C=C
880-900
R2C=CH2
isobutylene CH3
CH3C=CH2
Unst’d
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Which compound is this? a) 2-pentanone b) 1-pentanol c) 1-bromopentane d) 2-methylpentane
1-pentanol
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What is the compound? a) 1-bromopentane b) 1-pentanol c) 2-pentanone d) 2-methylpentane
2-pentanone
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H2C C
HCH2
CH3
CH3CH3CH2CH2CH2CH3
H2C
H2C
CH2CH2CH2CH3
biphenyl allylbenzene 1,2-diphenylethane
o-xylene n-pentane n-butylbenzene
A
B
C
D
E
F
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.
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1
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2
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3
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4
2/5/2013 sudheerkumar kamarapu 114
5
2/5/2013 sudheerkumar kamarapu 115
6
2/5/2013 sudheerkumar kamarapu 116
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
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