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Infrared Spectroscopy-IR
Common Applications
Identification of compounds by matching spectrum ofunknown compound with reference spectrum
(fingerprinting)
Identification of functional groups in unknown substances
Identification of reaction components and kinetic studiesof reactions
Identification of molecular orientation in polymer films
Detection of molecular impurities or additives present in
amounts of 1% and in some cases aslow as 0.01% Identification of polymers, plastics, and resins
Analysis of formulations such as insecticides and
copolymers
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Complementary or Related Techniques
Nuclear magnetic resonance providesadditional information on detailed molecular
structure
Mass spectrometry provides molecular mass
information and additional structural
information
Raman spectroscopy provides complementary
information on molecular vibration. (Somevibrational modes of motion are IR-inactive
but Raman-active and vice versa.)- NOT
INCLUDED IN THIS COURSE
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Introduction
Infrared (IR) spectroscopy is one of the
most common spectroscopic techniques
used by organic and inorganic chemists
it is the absorption measurement of
different IR frequencies by a samplepositioned in the path of an IR beam.
The main goal of IR analysis is to
determine the chemical functional groupsin the sample.
Different functional groups absorb
characteristic frequencies of IR radiation.
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IR Frequency Range
Region range (m) Wave number range,
cm-1
Frequency Range
Near 0.78-2.5 12, 000 to 4000 3.8x1014- 1.2x1014
Middle 2.5-50 4000 to 200 1.2x1014- 6.0x1012
Far 50-1000 200 to 10 6.0x1012- 3.0x1011
Most used 2.5-15 4000 to 670 1.2x1014- 2.0x1013
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FT-IR Instrument
1. The Source: Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which
controls the amount of energy presented to the sample(and, ultimately, to the detector).
2. The Interferometer: The beam enters the interferometerwhere the spectral encoding takes place. The resultinginterferogram signal then exits the interferometer.
3. The Sample: The beam enters the sample compartmentwhere it is transmitted through or reflected off of thesurface of the sample, depending on the type of analysisbeing accomplished. This is where specific frequencies ofenergy, which are uniquely characteristic of the sample, areabsorbed.
4. The Detector: The beam finally passes to the detector forfinal measurement. The detectors used are speciallydesigned to measure the special interferogram signal.
5. The Computer: The measured signal is digitized and sent tothe computer where the Fourier transformation takesplace. The final infrared spectrum is then presented to the
user for interpretation and any further manipulation.
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FT-IR Instrument- Schematic
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Michaelson Interferometer
Source
Stationary mirror
Moving mirror
Sample
Detector
Beam Splitter
PMT
HeNe laser
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Michaelson Interferometer
Interferometers employ a beamsplitter which takes the incoming infrared beam and divides it
into two optical beams.
One beam reflects off of a flat mirror which is fixed in place. The other beam reflects off of a flat
mirror which is on a mechanism which allows this mirror to move a very short distance(typically a few millimeters) away from the beamsplitter.
The two beams reflect off of their respective mirrors and are recombined when they meet back
at the beamsplitter.
Because the path that one beam travels is a fixed length and the other is constantly changing as
its mirror moves, the signal which exits the interferometer is the result of these two beams
interfering with each other. The resulting signal is called an interferogram which has the unique property that every data
point (a function of the moving mirror position) which makes up the signal has information
about every infrared frequency which comes from the source.
This means that as the interferogram is measured, all frequencies are being measured
simultaneously.
Thus, the use of the interferometer results in extremely fast measurements.
Because the analyst requires a frequency spectrum (a plot of the intensity at each individual
frequency) in order to make an identification, the measured interferogram signal can not be
interpreted directly.
A means of decoding the individual frequencies is required. This can be accomplished via a well-
known mathematical technique called the Fourier transformation. This transformation is
performed by the computer which then presents the user with the desired spectral informationfor analysis.
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IR Spectrum
IR absorption information is generally presented in the form of a spectrum
with wavelength or wavenumber as the x-axis and absorption intensity or
percent transmittance as the y-axis
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Theory of Infrared
Dipole changes During Vibrations and Rotations
IR absorption - a molecule must undergo a net change in dipole momentdue to vibrational or rotational motion.
Homonuclear species such as O2, N2or Cl2no net change in dipolemoment occurs during vibration or rotation.
Eg. The charge distribution around a molecule of HCl is not symetricbecause the Cl has a higher electron density than the hydrogen.
HCl has a significant dipole moment and known as polar molecule
Dipole moment: determined by the magnitude of the charge differenceand the distance between the two centre of centre of charge.
as a HCl vibrates, a regular fluctuation in dipole moment occurs, an afield is established that can interact with the electrical field associated
with radiation. If the frequency of the radiation exactly matches a naturalvibrational frequency of the molecule, a net transfer of energy takesplace that results in a change in the amplitude of the molecular vibration,absorption of the radiation is the consequence. Similarly, the rotation ofasymmetric molecules around their centres of mass result in a periodicdipole fluctuation that can interact with radiation.
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Theory of Infrared
Rotational Transitions
The energy required to cause a change in rotational is minute (small)and corresponds to radiation of 100 cm-1or less (>100m)
Rotational levels are quantized;
i) Absorption by gases in the far IR region is characterized bydiscrete, well-defined line
ii) Absorption by liquids or solids intramolecular collisions andinteractions cause broadening of the lines in to a continuum.
Vibrational/Rotational Transitions
Vibrational energy levels are also quantized for most molecules theenergy differences between quantum states correspond to the mid-IRregion.
The IR spectrum of a gas usually consists of closely spaced lines,because there are several rotational energy state for each vibrationalstate.
On the other hand, rotation is highly restricted in liquids and solids, insuch sample, discrete vibrational/rotational lines disappear, leaving only
somewhat broadening vibrational peaks.
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Types of molecular vibration
i) Stretching:involves a continues changein the inter atomic distance along the
axis of the bond between two atoms. 2
types: Symetric stretching and asymetric
stretching
ii) Bending: characterized by a change in
the angle between two bonds. 4 types of
bending vibrations: scissoring(bending),
rocking, wagging and twisting.
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Stretching and Bending
Symmetric Asymmetric
In plane rocking In plane scissoring
Out of plane wagging Out of plane twisting
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The IR spectrum ( Dudley Williams pg 30)
A complex molecule-many vibrate modes involve in the
whole molecule
However, these molecular vib. are largely associated with thevib. of individual bonds and called localized vibrations.
These localized vibrations are useful for the identification offunctional groups, such as the stretching vibs of:
Single bond (O-H & N-H) and all kinds of triple and doublebonds, all of which occur with frequency greater than1500cm-1
The stretching vib. of other single bonds, most bendingvibrations and the soggier vibration of the molecule as awhole give rise to a series of absorption bands at lowerenergy, below 1500cm-1, the position of which arecharacteristic of that molecule (Fingerprint region).
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Full spectrum of cortisone acetate , pg 31
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Full spectrum of cortisone acetate , pg 31
Strong absorption from the stretchingvibrations above 1500cm-1, showing that
the presence of each of the functional
groups:
O-H group
3 diff C=O groups
The weaker absorption of the C=C
double bond
finger print region below 1500cm-1
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Identification of IR spectrum (organic comp)
Two step processes:
Determine what functional groups are present-examining thegroup frequency region (36001200cm-1)
Detailed comparison of the spectrum of the unknown with
the spectra of pure compounds that contain all of thefunctional groups found in the first step. The fingerprintregion (1200-600cm-1) is useful due to small differences inthe structure and constitution of a molecule result insignificant changes in the appearance and distribution ofabsorption peaks in this region. Consequently, a close match
between two spectra in the fingerprint region (as well asothers) constitutes almost certain evidence for the identityof the compound yielding the spectra.
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Table of group frequency for organic group
Bond Type of compound Frequency range,
cm-1
Intensity
C-H Alkanes 2850-2970
1340-1470
Strong
StrongC-H Alkenes (C=C) 3010-3095
675-995
Medium
Strong
C-H Alkynes (CC) 3300 Strong
C-H Aromatic ring 3010-3100
690-900
Medium
Strong
O-H Monomeric alcohols, phenols
Hydrogen-bonded alcohols, phenols
Monomeric carboxylic acids
H-bonded carboxylic acids
3590-3650
3200-3600
3500-3650
2500-2700
Variable
Variable
(sometimes broad)
Medium
Broad
N-H Amines, amides 3300-3500 Medium
C=C Alkenes 1610-1680 Variable
C=C Aromatic rings 1500-1600 Variable
CC Alkynes 2100-2260 Variable
C-N Amines, amides 1180-1360 Strong
CN Nitriles 2210-2280 Strong
C-O Alcohols, ethers, carboxylic acids, esters 1050-1300 Strong
C=O Aldehydes, ketone,-COOH, esters 1690-1760 Strong
NO2 Nitro compounds 1500-15701300-1370
StrongStrong
IR S t T t i l H t l IR t
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IR Spectroscopy Tutorial: How to analyze IR spectra
The distinctive bands of the common functional bands:
3500-3300 cm-1
N
H stretch amines
3500-3200 cm-1 OH stretchalcohols, a broad, strong
band
3100-3000 cm-1 CH stretch alkenes
3000-2850 cm-1 CH stretch alkanes
1760-1665 cm-1 C=O stretchketones, aldehydes,
esters
1680-1640 cm-1 C=C stretch alkenes
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i) Begin by looking in the region from 4000-1300. Look at the CH
stretching bands around 3000:
ii) Look for a carbonyl in the region 1760-1690. If there is such a band:
Indicates:
Is an OH band also present? a carboxylic acid group
Is a CO band also present? an ester
Is an aldehydic CH band also present? an aldehyde
Is an NH band also present? an amide
Are none of the above present? a ketone
Indicates:
Are any or all to the rightof 3000?alkyl groups (present in most organicmolecules)
Are any or all to the leftof 3000?a C=C bond or aromatic group in the
molecule
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Look for a broad OH band in the region 3500-3200 cm-1. If there is such a band:
Indicates:
Is an OH bandpresent? an alcoholor phenol
Look for a single or double sharp NH band in the region 3400-3250 cm-1. If
there is such a band:
Indicates:
Are there two bands? a primary amine
Is there only one band? a secondary amine
Other structural features to check for:
Indicates:
Are there C
O stretches?
an ether (or an ester if there is a
carbonyl band too)
Is there a C=C stretching band? an alkene
Are there aromatic stretching bands? an aromatic
Is there a CC band? an alkyne
Are there -NO2
bands? a nitro compound
IR S T i l Alk
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IR Spectroscopy Tutorial: Alkanes
The spectra of simple alkanes are characterized by absorptions due to CH stretching and bending (the C
C stretching and bending bands are either too weak or of too low a frequency to be detected in IR
spectroscopy). In simple alkanes, which have very few bands, each band in the spectrum can be assigned.
CH stretch from 30002850 cm-1
CH bend or scissoring from 1470-1450 cm-1
CH rock, methyl from 1370-1350 cm-1
CH rock, methyl, seen only in long chain alkanes, from 725-720 cm-1
IR SpectroscopyTutorial:Alkenes
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IR Spectroscopy Tutorial: Alkenes
Alkenes are compounds that have a carbon-carbon double bond,C=C. The stretching vibration of the
C=C bond usually gives rise to a moderate band in the region 1680-1640 cm-1.
Stretching vibrations of theC=CH bond are of higher frequency (higher wavenumber) than those of
theCCH bond in alkanes.
C=C stretch from 1680-1640 cm-1
=CH stretch from 3100-3000 cm-1
=CH bend from 1000-650 cm-1
IR Spectroscopy Tutorial: Aromatics
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IR Spectroscopy Tutorial: Aromatics
The =CH stretch in aromatics is observed at 3100-3000 cm-1. Note that this is at slightly higher frequency than is theCH
stretch in alkanes.
Aromatic hydrocarbons show absorptions in the regions 1600-1585 cm-1and 1500-1400 cm-1due to carbon-carbon stretching
vibrations in the aromatic ring.
CH stretch from 3100-3000 cm-1
overtones, weak, from 2000-1665 cm-1
CC stretch (in-ring) from 1600-1585 cm-1
CC stretch (in-ring) from 1500-1400 cm-1
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IR Spectroscopy Tutorial: Alcohols
Alcohols have characteristic IR absorptions associated with both the O-Hand the C-Ostretching vibrations.
OH stretch, hydrogen bonded 3500-3200 cm-1
CO stretch 1260-1050 cm-1(s)
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IR Spectroscopy Tutorial: Ketones
The carbonyl stretching vibration band C=Oof saturated aliphatic ketones appears at 1715 cm-1.
Conjugation of the carbonyl group with carbon-carbon double bonds or phenyl groups, as in alpha, beta-
unsaturated aldehydes and benzaldehyde, shifts this band to lower wave numbers,1685-1666 cm-1
C=O stretch:
aliphatic ketones 1715 cm-1
,-unsaturated ketones 1685-1666 cm-1
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IR Spectroscopy Tutorial: Aldehydes
The carbonyl stretch C=O of saturated aliphatic aldehydes appears from 1740-1720 cm-1. As in ketones, if the carbons
adjacent to the aldehyde group are unsaturated, this vibration is shifted to lower wavenumbers, 1710-1685 cm -1
HC=O stretch 2830-2695 cm-1
C=O stretch:
aliphatic aldehydes 1740-1720 cm-1
alpha, beta-unsaturated aldehydes 1710-1685 cm-1