PRINCIPLES AND APPLICATIONS OF IR

SPECTROSCOPY IN QUALITY
CONTROLE
Introduction
➢ IR is the most powerful technique which offers the possibilities of
chemical identification
➢This technique based upon the simple fact that the chemical
substance shows marked selective region
INFRA RED REGION
Near infra red( Overtone region) 0.8µm to 2µm
Middle infra red Fundamental region 2µm to 8µm
 region( fundamenta Fingerprint region 8µm to 15µm
l region)
Far infra red region( rotational region) 15µm to 1000µm

Types of energy transitions in each region of the electromagnetic region

 Region of the Energy Transitions
Spectrum
Hukels law
X- Ray Bond Breaking

UV- rays Electronic

I can discues about
Infra red Vibrational Vibrational Transitions

Microwave Rotational

Radio frequencies Nuclear spin(NMR)

How to identify
Electronic Spin(ESR)
the Functional
groups
• 2.2 USES OF THE INFRARED SPECTRUM
• Since every type of bond has a different natural frequency of vibration
• since two of the same type of bond in two different compounds are in two slightly different
environments, no two molecules of different structure have exactly the same infrared
absorption pattern, or infrared spectrum.
• Although some of the frequencies absorbed in the two cases might be the same, in no case
of two different molecules will their infrared spectra (the patterns of absorption) be
identical. Thus, the infrared spectrum can be used for molecules much as a fingerprint can
be used for humans. By comparing the infrared spectra of two sub-stances thought to be
identical, you can establish whether they are, in fact, identical.
• If their infrared spec tra coincide peak for peak (absorption for absorption), in most cases
the two substances will be identical. A second and more important use of the infrared
spectrum is to determine structural information about a molecule. The absorptions of each
type of bond (N-H, C-Ң, О-Н. С-Х. С-O, C-O, C-C, C-C CC, C = N and so on) are
regularly found only in certain small portions of the vibrational infrared region.
• A small range of absorption can be defined for each type of bond. Outside this range,
absorptions are normally due to some other type of bond.
• For instance, any absorption in the range 3000 ±150 cm is almost always due to the
presence of a C-H bond in the molecule; an absorption in the range 1715± 100cm is
normally due to the presence of a c = o bond (carbonyl group) in the molecul
• 2.5 THE INFRARED SPECTROMETER
• The instrument that determines the absorption spectrum for a compound is called
an infrared spectrometer or, more precisely, a spectrophotometer. Two types of
infrared spectrometers are in common use in the organic laboratory: dispersive and
Fourier transform (FT) instruments.
• Both of these types of instruments provide spectra of compounds in the common
range of 4000 to 400 cm-1. Although the two provide nearly identical spectra for a
given compound, FT infrared spectrometers provide the infrared spectrum much
more rapidly than the dispersive instruments.
• We can briefly discuess about
➢FTIR
➢DIR
Dispersive Infrared Spectrometers.
• The instrument produces a beam of infrared radiation from a hot wire and, by
means of mirrors, divides it into two parallel beams of equal-intensity radiation.
The sample is placed in one beam, and the other beam is used as a reference. The
 beams then pass into the monochromator, which disperses each into a continuous
spectrum of frequencies of infrared light.
• The monochromator consists of a rapidly rotating sector (beam chopper) that passes
the two beams alternately to a diffraction grating (a prism in older instruments).
• The slowly rotating diffraction grating varies the frequency or wavelength of
radiation reaching the thermocouple detector.
• The detector senses the ratio between the intensities of the reference and sample
beams. In this way the detector determines which frequencies have been absorbed
by the sample and which frequencies are unaffected by the light passing through the
sample.
• After the signal from the detector is amplified, the recorder draws the resulting
spectrum of the sample on a chart.
• It is important to realize that the spectrum is recorded as the frequency of infrared
radiation changes by rotation of the diffraction grating. Dispersive instruments are
said to record a spectrum in the frequency domain.
• Fourier Transform Spectrometers
• The most modern infrared spectrometers (spectrophotometers) operate on a different
principle.
• The design of the optical pathway produces a pattern called an interferogram.
• The interferogram is a complex signal, but its wave-like pattern contains all the frequencies
that make up the infrared spectrum.
• An interferogram is essentially a plot of intensity versus time (a time-domain spectrum).
However, a chemist is more interested in a spectrum that is a plot of intensity versus
frequency (a frequency-domain spectrum).
• A mathematical operation known as a Fourier transform (FT) can separate the individual
absorption frequencies from the interferogram, producing a spectrum virtually identical to
that obtained with a dispersive spectrometer.
• This type of instrument is known as a Fourier transform infrared spectrometer, or FT-IR.
The advantage of an FT-IR instrument is that it acquires the interferogram in less than a
second.
• It is thus possible to collect dozens of interferograms of the same sample and accumulate
them in the memory of a computer. When a Fourier transform is performed on the sum of
the accumulated interferograms, a spectrum with a better signal-to-noise ratio can be
plotted.

principle
• Applied IR Frequency = Natural frequency of Vibration
 These bonds are undergoes to Springs

When energy in form of infra red radiation is applied molecule undergoes vibration

Absorption of IR Radiation takes place and a peak is absorbed

Every bond or functional group requires differentabsorbtion

frequency for
Any molecule
it is known that atom / groups of atoms are cvonnected by bonds
Hence, Characteristic peak is observed for every functional group
CRITERIA FOR A COMPOUND TO BE ABSORB IR RADITION
➢ Dipole Movement
✓ No Change in Dipole moment
✓ No IR spectra
 ➢ Applied IR Frequency
✓ IR frequency Should be equal to the natural frequency of radiation

➢ Bond position is an IR spectra may be expressed by wave number whoise

units is cm-1
➢ Bond position is an IR spectra may be expressed by transmittence(T) or
➢ A = log10(1/T)
➢ value of the stretching vibration frequency of a bond can be
expressed by the application of Hooks law
Types of Vibration
Type of Vibration
Stretching Symmetrical Two bends increase or decrease in length symetrically
Vibration( Bo Two bends increase or decrease in length asymetrically
Asymmetrical
nd length
aitered) In plane Scissoring Bond angle decrease
bonding Rocking Bond angle maintained but both bonds move
Bending Vi in a plane
bration( Bond Out of plane Wagging Both bonds move to one side of the plane
angle bonding Twisting One atom moves up the plane and another
altered)
atom moves down the plane
 • Degree of freedom
Degree of freedom
Type of degree of Liner Example Non- liner Example
freedom
Fundamental 3n-5 Co2 3n-6 C6H5
absorption band 3n-5 3n-6
Transitional degree of 3 3*3-5 3 3*12-6= 6
freedom =4
Rotational degree of 2 3
freedom
Vibrational degree of 3n-5 3n-6
freedom

Applications of IR Spectroscopy in Quality Controle

❑ To detection of the Functional group
▪ IR Spectroscopy is most powerful analytical technique to detect the
functional group in the compound
▪ It is because different functional groups absorbs the different
radiation & different signals in the region
❑ To detect Isotopes
❑To detect optical isomers
❑ to detect impurities
• HOW TO A ROACH THE ANALYSIS OF A SPECTRUM
• When analyzing the spectrum of an unknown, concentrate your first
efforts on determining the presence (or absence) of a few major
functional groups. The C-O, O-H. N-H. C-O, CС-С. CC, CN, and NO,
 peaks are the most conspicuous and give immediate structural
information if they are present.
• Do not try to make a detailed analysis of the C-H absorptions near 3000
cm almost all compounds have these absorptions.
• Do not worry about subtleties of the exact environment in which the
functional group is found. Following is a major checklist of the important
gross features.
• Is a carbonyl group present? The C=O group gives rise to a strong
absorption in the region 1820-1660 cm-1. The peak is often the strongest
in the spectrum and of medium width.

Acids Is OH group present

Bond absorption nearly 3400- 2400 cm-1
Amides Is NH group present
Medium absorption near 3400cm-1

Esters is CO group present strong intensity absorptions

near 1300-1000cm-1

Anhydrides Two C=O absorptions near 1800 and 1760 cm-1

Aldehydes Is aldehyde group present

Two weak absorptions near 2850 and 2750cm-1

Ketones The preceding five choices have been eliminated

• A SURVEY OF THE IMPORTANT FUNCTIONAL GROUPS, WITH

EXAMPLES
• The following sections describe the behaviors of important functional
groups toward infrared radiation. These sections are organized as
follows.1. The basic information about the functional group or type of
vibration is abstracted and placed in a Spectral Analysis Box, where it
may be consulted easily.
• 2. Examples of spectra follow the basic section. The major absorptions
of diagnostic value are indicated on each spectrum.
• 3. Following the spectral examples, a discussion section provides
details about the functional groups and other information that may be
of use in identifying organic compounds. You may choose to omit this
section on your first reading of the material.