2. Introduction
• The infrared region of the spectrum encompasses radiation with
wavenumbers ranging from about 12,800 to 10 cm-1 or wavelengths
from 0.78 to 1000 m.
• The infrared spectrum is usually subdivided into near-, mid-, and far-
infrared radiation
3. IR photons have low energy. The only transitions that have comparable
energy differences are molecular vibrations and rotations.
4. Theory of Infrared Absorption Spectroscopy
1.There must be a change in the dipole moment of the molecule as a result of a molecular
vibration (or rotation). The change (or oscillation) in the dipole moment allows interaction
with the alternating electrical component of the IR radiation wave. Symmetric molecules
(or bonds) do not absorb IR radiation since there is no dipole moment.
2.If the frequency of the radiation matches the natural frequency of the vibration (or
rotation), the IR photon is absorbed and the amplitude of the vibration increases.
IR absorption, emission, and reflection spectra for molecular species can be rationalized by
assuming that all rises from various changes in energy brought about by transitions of
molecules from one vibrational or rotational energy state to another. In order for IR
absorbance to occur two conditions must be met:
5.
6. There are three types of molecular transitions that occur in IR
i) Rotational transitions
When an asymmetric molecule rotates about its center of mass, the dipole moment
seems to fluctuate.
∆E for these transitions correspond to ν < 100 cm-1
Quite low energy, show up as sharp lines that subdivide vibrational peaks in gas
phase spectra.
ii) Vibrational-rotational transitions
complex transitions that arise from changes in the molecular dipole moment due
to the combination of a bond vibration and molecular rotation.
iii)Vibrational transitions
The most important transitions observed in qualitative mid-IR spectroscopy.
ν = 13,000 – 675 cm-1 (0.78 – 15 mM)
7. Dipole moment changes during Vibrations and Rotations
• IR radiation is not energetic enough to bring about the kinds of electronic transition
as compared to UV and visible radiaiton. Absorption of IR radiation
• Thus confined to molecular species that have small energy differences between
various vibrational and rotational states.
• To absorb IR radiation, a molecule must undergo a net change in dipole moment as it
vibrates or rotates.
• The dipole moments is determined by the magnitude of the charge difference and the
distance between the two centers of charge.
• No net change in dipole moment occurs during the vibration or rotation of
homonuclear species such as O2, N2, or Cl2; consequently, such compounds cannot
absorb in the infrared
8. Types of Molecular Vibrations
Vibrations fall into the basic categories of stretching and bending.
●Stretching vibration involves a continuous change in the interatomic
distance along the axis of the bond between two atoms.
● Bending vibrations are characterized by a change in the angle between two
bonds and are of four types: scissoring, rocking, wagging, and twisting.
The relative position of atoms in a molecule are not fixed but instead fluctuate
continuously as a sequence of a multitude of different types of vibrations and
rotations about the bonds in the molecule.
9.
10. Mechanical Model of Stretching Vibrations
Simple harmonic oscillator.
• Hooke’s Law (restoring force of a spring is proportional to the
displacement)
F = -ky
Where: F = Force
k = Force Constant
(stiffness of spring)
y = Displacement
• Natural oscillation frequency of a mechanical oscillator depends on:
a) mass of the object
b) force constant of the spring (bond)
• The oscillation frequency is independent of the amount of energy
imparted to the spring.
11. Anharmonic oscillators
• In reality, bonds act as anharmonic oscillators because as atoms get close, they repel
one another, and at some point a stretched bond will break.
12. The natural frequency of the oscillation is
m = natural frequency
m = mass of the attached body
k = force constant of the spring
m
k
m
1
2
Vibrational Frequency
13. The equation may be modified to describe the behavior of a system consisting of
two masses m1 and m2 connected by a spring. Here, it is only necessary to
substitute the reduced mass for the single mass m where
Thus, the vibrational frequency for such a system is given by
m m
m m
1 2
1 2
continued…
m
k k m m
m m
1
2
1
2
1 2
1 2
( )
14. • Quantum Treatment of Vibrations
The radiation in wavenumbers,
E h
h k
m
2
E h E h
h k
radiation m
2
1
2
53 10
c
k k
.
12
continued…
Where ν is the wavenumber of an absorption maximum (cm-1) , k is the force constant for the bond in
newtons per meter (N/m), c is the velocity of light (cm s-1) and µ is the reduced mass (kg). k has been
found to lie in the range between 3 x 102 and 8 x 102 N/m for most single bonds, with 5 x 102 serving as a
reasonable average value. Double and triple bonds are found by this same means to have force constants of
about two and three times this value (1 x 103 and 1.5 x 103 , respectively).
15. Number of possible modes
●Nonlinear molecule: 3N – 6
● Linear molecule: 3N – 5
-3 degrees of freedom – i.e., 3 coordinates in space
-3 translations and 3 rotations account for 6 motions of molecule
-Rotation about center bond in linear molecule is indistinguishable
-Remaining degrees of motion represent vibrational motion (i.e., number of vibrations
within the molecule)
Vibrational Modes
16. Factors Influence the Normal Modes
Four factors tend to produce fewer experimental peaks than would be expected
from the theoretical number of normal modes.
(1) the symmetry of the molecules is such that no change in dipole results from
a particular vibration;
(2) the energies of two or more vibrations are identical or nearly identical;
(3) the absorption intensity is so low as to be undetectable by ordinary means; or
(4) the vibrational energy is in a wavelength region beyond the range of the
instrument.
17. Factors Influence the Normal Modes
●Sometimes more peaks are found than are expected based upon the number
of normal modes.
● The occurrence of overtone peaks that occur at two or three times the
frequency of a fundamental peak.
● In addition combination bands are sometimes encountered when a photon
excites two vibrational modes simultaneously.
● The frequency of the combination band is approximately the sum or
difference of the two fundamental frequencies.
18. The energy of a vibration, and thus the wavelength of the corresponding absorption
maximum, may be influenced by ( or coupled with) other vibrations in the molecule.
A number of factors influence the extent of such coupling
1. Strong coupling between stretching vibrations occurs only when there is an atom
common to the two vibrations.
2. Interaction between bending vibrations requires a common bond between the
vibrating groups.
3. Coupling between a stretching and a bending vibration can occur if the stretching
bond forms one side of the angle that varies in the bending vibrations.
4. Interaction is greatest when the coupled groups have individual energies that are
nearly equal.
5. Little or no interaction is observed between groups separated by two or more bonds.
6. Coupling requires that the vibrations be of the same symmertry species.
Vibrational coupling
19. Stretching
Bending
Example: Carbon dioxide is a linear molecule and thus has (3× 3 ) − 5 = 4 normal
modes. Two stretching vibration are possible, furthermore ,interaction between the two
can occur because the bonds involved are associated with a common carbon atom.
20. Contd..
• Symmetric vibration causes no change in dipole moment, because the two oxygen
atoms simultaneously move away from or toward the central atom. Thus, the
symmetric vibration is IR inactive.
• In asymmetric vibration, one oxygen moves away from the carbon atom as the
carbon atom moves towards the other oxygen. As a consequence, a net change in
distribution occurs periodically, producing a change in dipole moment, so absorption
at 2350 cm-1 results.
• The remaining two vibrational modes of carbon dioxide involve scissoring. These
two bending vibrations are the resolved components at 90oC to one another of the
bending and thus produce a single absorption band at 667 cm-1.
22. H2O molecule
• Triatomic molecule such as water, sulfur dioxide, or nitrogen dioxide have
3 x 3 – 6 = 3 vibrational modes.
• The central atom is not in line with the other two, a symmetric stretching
vibration will produce a change in dipole and will thus be responsible for
infrared absorption.
• Stretching peaks at 3650 and 3760 cm-1 appear in the infrared spectrum for
the symmetric and asymmetric vibrations of the water molecule.
• There is only one component to the scissoring vibration for this nonlinear
molecule.
• For water, the bending vibration cause absorption at 1595 cm-1.
24. Q. Caffeine molecules absorb infrared radiation at 1656 cm−1. Calculate the following:
(i) wavelength of this radiation;
(ii) frequency of this radiation;
(iii) energy change associated with this absorption.
We know that,
where = wavenumber (cm-1) , λ = wavelength, ν = frequency and c = velocity of light
(2.997 925 × 108 m s−1)
And ,
∆E = hν
Where, h = planck constant
(h = 6.626 × 10−34 J s)
25. Q. How many vibrational degrees of freedom does a chloroform (CHCl3)
molecule possess?
27. IR Instrumentation
Three types of instruments for infrared absorption measurements are
available from commercial sources:
(1) Multiplex instruments, employing the Fourier transform that are suited
to both qualitative and quantitative infrared measurements, and
(2) Dispersive grating spectrophotometers that are used primarily for
qualitative work
(3) Nondispersive photometers that have been developed for quantitative
determination of a variety of organic species in the atmosphere by
absorption, emission , and reflectance spectroscopy.
28. Overall Instrument Design
-Need chopper to discriminate source light
from background IR radiation
-Monochromator after sample cell
-Not done in UV-Vis since letting in all hν to sample
may cause photodegradation (too much energy)
-IR lower energy
-Advantage that allows
monochromator to be used to screen
out more background IR light
-Problems:
-Source weak , need long scans
-Detector response slow – rounded
peaks
29. Fourier Transfer IR (FTIR) – alternative to Normal IR
Principle:
1) light from source is split by central mirror into 2
beams of equal intensity
2) beams go to two other mirrors, reflected by
central mirror, recombine and pass through
sample to detector
3) two side mirrors. One fixed and other movable
a) move second mirror, light in two-paths travel
different distances before recombined
b) constructive & destructive interference
c) as mirror is moved, get a change in signal
30. Destructive Interference can be created when two waves from the same source
travel different paths to get to a point.
This may cause a difference in the phase between the two waves.
• If the paths differ by an integer multiple of a wavelength, the waves will also be in
phase.
• If the waves differ by an odd multiple of half a wave then the waves will be 180
degrees out of phase and cancel out.
Contd..
31.
32. Advantages of FTIR compared to Normal IR:
1) much faster, seconds vs. minutes
2) use signal averaging to increase signal-to-noise (S/N)
3) higher inherent S/N – no slits, less optical equipment, higher light intensity
4) high resolution (<0.1 cm-1)
Disadvantages of FTIR compared to Normal IR:
1) single-beam, requires collecting blank
2) can’t use thermal detectors – too slow
In normal IR, scan through frequency range. In FTIR collect all frequencies at
once
.
/
increase S N number scans
33. Dispersive Instruments
Dispersive IR spectrophotometer are
Generally double-beam, recording
instruments, which use reflection gratings
for dispersing radiation
Generally, dispersive IR spectrophotometers
Incorporate a low frequency chopper
( five to thirty cylcle per second) that permits
The detector to discriminate between the
Signal from the source and signals from
extranaeous radiation, such as IR radiation
From various bodies surrounding the
transdurcer.
35. INFRARED SOURCES AND TRANSDUCERS
• Infrared sources consist of an inert solid that is heated electrically to
a temperature between 1500 and 2200 K.
• Continuum radiation approximating that of a blackbody results.
• The maximum radiant intensity at these temperatures occurs between
5000 to 5900 cm-1.
• At longer wavelengths, the intensity falls off until it is about 1% of
the maximum at 670 cm-1 .
• On the short wavelength side, the decrease is much rapid, and a
similar reduction in intensity is observed at about 10,000 cm-1
Sources
36. The Nernst Glower:
• The Nernst glower is composed of rare earth oxides formed into a cylinder
having a diameter of 1 to 3 mm and a length of perhaps 2 to 5 cm.
• The Nernst glower is an obsolete device for providing a continuous source
of (near) infrared radiation for use in spectroscopy
• Platinum leads are sealed to the ends of the cylinder to permit electrical
connection to what amounts to a resistive heating element.
• As current is passed through the device, temperature between 1200 K and
2200 K result.
Continued…
37. The Globar Source:
• A Globar is a silicon carbide rod, usually about 5 cm in length and 5 mm in
diameter.
• It also is electrically heated (1300 to 1500 K).
• Spectral energies of the Globar and the Nernst glower are comparable except
in the region below 5m, where the Globar provides a significantly greater
output.
Incandescent Wire Source:
• A source of somewhat lower intensity but longer life than the Globar or Nernst
glower is a tightly wound spiral of nichrome wire heated to about 1100 K by
an electrical current.
Continued…
38. Incandescent Wire Source:
• A source of somewhat lower intensity but longer life than the
Globar or Nernst glower is a tightly wound spiral of nichrome
wire heated to about 1100 K by an electrical current.
• A rhodium wire heater sealed in a ceramic cylinder has
similar properties, although it is more expensive.
• Nichrome wire source are less intense than many IR sources.
However, the incandescent wire source requires no cooling
and is nearly maintenance free. For this reason, the nichrome
wire source is often used.
39. The Mercury Arc:
• For the far-infrared region of the spectrum ( > 50 m), none of
the thermal sources just described provides sufficient radiant
power for convenient detection.
• Here, a high-pressure mercury arc is used. This device consists of
a quartz-jacketed tube containing mercury vapor at a pressure
greater than one atmosphere.
• Passage of electricity through the vapor forms an internal plasma
source that provides continuum radiation in the far-infrared region.
Continued…
40. The Tungsten Filament Lamp: An ordinary tungsten filament
lamp is a convenient source for the near-infrared region of 4000 to
12,800 cm-1.
The Carbon Dioxide Laser Source:
• A tunable carbon dioxide laser is used as an infrared source for
monitoring the concentrations of certain atmospheric pollutants
and for determining absorbing species in aqueous solutions.
• A carbon dioxide laser produces a band of radiation in the 900 to
1100 cm-1 range.
Continued…
42. Thermal Transducers:
• Thermal transducers, whose responses depend upon the heating
effect of radiation, are employed for detection of all but the shortest
infrared wavelengths.
• With these devices, the radiation is absorbed by a small blackbody
and the resultant temperature rise is measured.
• The problem of measuring IR radiation by thermal means is
compounded by thermal noise from the surroundings. For this
reason, thermal transducers are housed in a vaccum and are
carefully shielded from thermal radiation emitted by other nearby
objects.
43. Thermocouples: In its simplest form, a thermocouple
consists of a pair of junctions formed when two pieces of a
metal such as bismuth are fused to either end of a dissimilar
metal such as antimony. A potential develops between the two
junctions that varies with their difference in temperature.
Continued…
44. Pyroelectric Transducers:
• Pyroelectric transducers are constructed from single crystalline wafers of
pyroelectic materials, which are insulators (dielectric materials) with very
special thermal and electrical properties.
• Triglycine sulfate, (NH2CH2COOH)3. H2SO4 is the most important
pyroelectric material used in the construction of infrared detection systems.
• When electric field applied across any dielectric material, polarization takes
place, with the magnitude of the polarization being a function of the dielectric
constant of the material.
Continued…
45. Photoconducting Transducers:
• Infrared photoconducting transducers consist of a thin film of a semiconductor
material, such as lead sulfide, mercury/cadmium telluride, or indium antimonide,
deposited on a nonconducting glass surface and sealed into an evacuated envelope
to protect the semiconductor from the atmosphere.
• Absorption of radiation by these materials promotes nonconducting valence
electrons to a higher energy conducting state, thus decreasing the electrical
resistance of the semiconductor.
• Typically, a photoconductor is placed in series with a voltage source and load
resistor, and the voltage drop across the load resistor serves as a measure of the
power of the beam of radiation.
Continued…
48. IR spectroscopy is a versatile tools that is applied to the qualitative and
quantitative determination of molecular species of all types.
MID- IR ABSORPTION SPECTROSCOPY
• Mid-IR absorption and reflection spectroscopy are major tools for
determining the structure of organic and biochemical species
49. Sample Handling:
• In UV-Visible spectroscopy absorption measurement in the optimal
range are obtained by suitably adjusting either the concentration or the
cell length. Unfortunately this approach is often not applicable to IR
spectroscopy because no good solvents are transparent throughout the
region of interest. Hence, sample preparation is one of the tedious and
time-consuming part of an IR spectrometric analysis.
Gases:
• The spectrum of low boiling point liquid or gas can be obtained by
permitting the sample to expand into an evacuated cylindrical cell
equipped with suitable windows. For this purpose , a verity of
cylindrical cells are obtained with path lengths that range from a few
centimeters to 10 m or more.
50. Solution:
• When feasible , a convenient way of obtaining IR spectra is on
solution prepared to contain known concentration of sample like UV-
Visible spectroscopy.
• Availability of suitable solvent is critical in IR.
• Water and the alcohols are difficult to use as solvent in IR
spectrometry. Water shows several strong absorption bands in the IR
region.
• Water and alcohols also attack alkali-metal halides, the most common
materials used for cell windows. Hence water insoluble windows
materials, such as barium fluoride must be used with such solvents.
51. Figure:Solvent for IR spectroscopy. Horizontal lines indicates useful regions.
● Because of the tendency for solvents to absorb IR radiation, IR liquid cells are
ordinarily much narrower (0.01 to 1 m m) than those employed in the ultraviolet and
visible region.
● Hence, relatively high sample concentrations (0.1% to 10%) are required because of
the short path length and the low molar absorptivities of IR bands.
52. Common IR Window Materials
●Window materials are chosen on the basis of cost , range of transperancy , solubility in the solvent
and reactivity with the Sample or solvent.
● Sodium chloride and potassium bromide windows are most commonly employed and are the least
expensive.
● For sample that are wet or aqueous, calcium and barium fluoride are suitable , although neither
transmit well throughout the entire mid- IR region.
53.
54. Liquids:
• When the amount of liquid sample is small or when a suitable
solvent is unavailable , it is common practice to obtain
spectra on the pure liquid.
• In this case, a very thin film has sufficient short path length to
produce satisfactory spectra
• Commonly a drop of neat liquid is squeezed between two
rock salt plates to give a layer 0.015 mm thick or less.
55. Solid:
• Most organic compounds exhibits numerous absorption bands
through out the mid-IR region, and finding a solvent that does
not have overlapping peaks is often impossible.
• Because of this spectra are often obtained on dispersions of
the solid in a liquid or solid matrix.
• In these technique , the solid sample must be ground until its
particle size is less than the wavelength of the radiation to
avoid the effect of scattered radiation.
56. Pelleting:
• One of the most popular technique for handling solid samples
has been KBr pelleting.
• Halides salts have the property of cold flow, in which they have
glasslike transparent properties when sufficient pressure is
applied to the finely powdered materials.
• In this technique, a milligram or less of the finely ground sample
is intimately mixed with about 100 mg of dried KBr powder.
• KBr pelleting produce excellent spectra .
• Being ionic, KBr transmit thoughout most of the IR region with
lower cutoff of about 400 cm-1
57. Mulls:
• IR spectra of solid that are not soluble in an IR-transparent solvent
or are not conveniently pelleted in KBr are often obtained by
dispersing the analyte in a mineral oil or a fluorinated hydrocarbons
mull.
• Mulls are formed by grinding 2 to 5 mg of the finely powdered
sample
• (particle size <2 µm) in the presence of one or two drops of a heavy
hydrocarbon oil.
• If hydrocarbon bands are likely to interfere, Fluoroluble, a
halogenated polymer can be used instead.
58. Application of IR
Qualitative Analysis (Compound Identification)
► Use of IR, with NMR and MS, in late 1950’s revolutionized organic chemistry
► decreased the time to confirm compound identification 10-1000 fold
General Scheme
examine what functional groups are present by looking at group frequency region -
3600 cm-1 to 1200 cm-1
59. IR wavenumber of different functional groups:Key
points
• The greater the dipole moment, the greater the changes in dipole moment
when a bond stretch
• The greater the change in dipole mement, the more intense the absorption
band.
• The stronger the bond, the higher the frequency/ wavenumer
• The lighter the atoms, the higher the frequency/wavenumber
• The broadening of an OH absorption band is due to hydrogen bonding
• Electron delocalization can affect the wavenumber of a functional groups.
• Electron withdrawing / donating group can affect the wavenumber of a
functional groups.
60. IR wavenumber of different functional groups: Key points
O-H
N-H
C-H
Least polar
Most polar
Change in dipolemoment
Change in dipole
moment
Less intense
More intense
C
C
C
C
C
C Largest bond
Largest bond
Weakest bond
Strong bond Strong bond
Weakest bond Lower wavenumber
Higehr wavenumber
61. • The greater the s character of a hybridized carbon, the stronger the bond,
therefore the higher wavenumber
• Sp3 in bonds have absorption bands to the right of 3000 cm-1 while Sp2 and
Sp have absorption band to the left.
• Benzene rings cause absorption bands at – 1600 cm-1 and -1500 -1480 cm-1
• Aldehyde C-H bonds show absorption bands from 2900 -2600 cm-1
• NH bond show an absorption stretch at 3200- 3500 cm-1 and bend at -1600
cm-1
C H Sp
C H Sp2
C H Sp3
Longer bond
Shorter bond
Weaker bond
Stronger bond
Lower wavenumber
Higher wavenumber
62. Group frequencies:
• Frequency ( or wavenumber) at which an organic functional group, such as
C=O, C=C, C-H, C≡C or O-H, absorbs IR radiation can be calculated from the
masses of the atoms and the force constant of the bond between them. These
frequencies called group frequencies.
63.
64.
65. Bond Type of Compound Frequency Range, cm-1 Intensity
C-H Alkanes 2850-2970 Strong
C-H Alkenes 3010-3095
675-995
Medium
strong
C-H Alkynes 3300 Strong
C-H Aromatic rings 3010-3100
690-900
Medium
strong
0-H Monomeric alcohols, phenols
Hydrogen-bonded alchohols, phenols
Monomeric carboxylic acids
Hydrogen-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
Alkynes 2100-2260 Variable
C-N Amines, amides 1180-1360 Strong
Nitriles 2210-2280 Strong
C-O Alcohols, ethers,carboxylic acids, esters 1050-1300 Strong
C=O Aldehydes, ketones, carboxylic acids, esters 1690-1760 Strong
NO2 Nitro compounds 1500-1570
1300-1370
Strong
C C
H
C C H
C C
C N
Abbreviated Table of Group Frequencies for Organic Groups
66. Fingerprint Region
• Small differences in the structure and constitution of a molecule result in
significant changes in the distribution of absorption maxima in this region of the
spectrum that extend from about 1200 to 600 cm-1
• The fingerprint region is thus well suited for identifying compounds based on
spectral comparison.
• Most single bands give rise to absorption bands at these frequencies , because their
energies are about the same, strong interaction occurs between neighboring bonds.
• The absorption band composites of various interactions and depend on the overall
skeletal structure of the molecule.
• Exact interpretation of spectra in this region is seldom possible because of
complexity of the spectra.
67.
68. Computer Searches
● many modern instruments
have reference IR spectra on
file (~100,000 compounds)
● matches based on location of
strongest band, then 2nd
strongest band, and overall
skeletal structure of a
compound
● exact interpretation of this
region of spectra seldom
possible because of complexity
where, complexity
uniqueness
69. Quantitative Analysis
- not as good as UV/Vis in terms of accuracy and precision
► more complex spectra
► narrower bands (Beer’s Law deviation)
► limitations of IR instruments (lower light throughput, weaker
detectors)
► high background IR
► difficult to match reference and sample cells
► changes in e (A= εbc) common
- potential advantage is good selectivity, since so many compounds
have different IR spectra
► one common application is determination of air contaminants.
71. Example : The spectrum is for a substance with an empirical formula of C3H5N. What is the
compound?
Nitrile or
alkyne group
No aromatics
Aliphatic
hydrogens
One or more
alkane groups
