Infrared spectroscopy involves analyzing how infrared light interacts with molecules. It can be used to analyze organic and inorganic samples in liquid, solid, and gas phases. IR spectroscopy identifies functional groups and structures by detecting the vibrational and rotational frequencies of covalent bonds as they absorb infrared radiation. This technique is commonly employed to determine molecular structure, identify substances, study reaction progress, detect impurities, and perform quantitative analysis.

1. infrared Spectroscopy (IR)
It is the analysis of infrared light interacting
with a molecule.
This can be analyzed in three ways by
measuring absorption, emission and
reflection. The main use of this technique is in
organic and inorganic chemistry.
infrared region most useful for analysis of
organic compounds is wavelength range from
2,500 to 16,000 nm, with a corresponding
frequency range from 1.9*1013 to 1.2*1014 Hz.

2. Photon energies associated with this part of the
infrared (from 1 to 15 kcal/mole) are not large
enough to excite electrons, but may induce
vibrational excitation of covalently bonded atoms
and groups.
 covalent bonds in molecules are more like stiff
springs that can be stretched and bent.
all organic compounds will absorb infrared radiation
that corresponds in energy to these vibrations.
Thus a sample that did not absorb at all would
record a horizontal line at 100% transmittance (top of
the chart).

3. The frequency scale at the bottom of the chart is given in
units of reciprocal centimeters (cm-1) rather than Hz,
because the numbers are more manageable.
The reciprocal centimeter is the number of wave cycles in
one centimeter;
whereas, frequency in cycles per second or Hz is equal to
the number of wave cycles in 3*1010 cm (the distance
covered by light in one second).
 Infrared spectra may be obtained from samples in all
phases (liquid, solid and gaseous).
Liquids are usually examined as a thin film sandwiched
between two polished salt plates (note that glass absorbs
infrared radiation, whereas NaCl is transparent).

4. If solvents are used to dissolve solids, care must
be taken to avoid obscuring important spectral
regions by solvent absorption. Perchlorinated
solvents such as carbon tetrachloride, chloroform
and tetrachloroethene are commonly used.
 Alternatively, solids may either be incorporated
in a thin KBr disk, prepared under high pressure, or
mixed with a little non-volatile liquid and ground to
a paste (or mull) that is smeared between salt
plates.

5. IR radiation causes the excitation of the vibrations
of covalent bonds within that molecule. These
vibrations include the stretching and bending
modes.
Number of vibrational modes-
In order for a vibrational mode in a sample to be
"IR active", it must be associated with changes in
the dipole moment. A permanent dipole is not
necessary, as the rule requires only a change in
dipole moment.
A molecule can vibrate in many ways, and each way
is called a vibrational mode.

6. For molecules with N number of atoms, linear
molecules have 3N – 5 degrees of vibrational modes,
whereas nonlinear molecules have 3N – 6 degrees
of vibrational modes (also called vibrational degrees
of freedom).
As an example H2O, a non-linear molecule, will
have 3 × 3 – 6 = 3 degrees of vibrational freedom, or
modes

7. Simple diatomic molecules have only one bond
and only one vibrational band.
If the molecule is symmetrical, e.g. N2, the band
is not observed in the IR spectrum, but only in the
Raman spectrum.
Asymmetrical diatomic molecules, e.g. CO,
absorb in the IR spectrum.
More complex molecules have many bonds, and
their vibrational spectra are correspondingly more
complex, i.e. big molecules have many peaks in
their IR spectra. atom, can vibrate in nine different
ways.

8. symmetrical stretching
Asymmetrical stretching
Scissoring rocking
Wagging
Twisting

9. in water, the rocking, wagging, and twisting
modes do not exist because these types of motions
of the H represent simple rotation of the whole
molecule rather than vibrations within it.
An IR spectrum show the energy absorptions as
one 'scans' the IR region of the EM spectrum. As
an example, the IR spectrum of butanal is shown
below.
In general terms it is convenient to split an IR
spectrum into two approximate regions:
4000-1000 cm-1 known as the functional group
region, and
< 1000 cm-1 known as the fingerprint region

10. Most of the information that is used to interpret
an IR spectrum is obtained from the functional
group region.
In practice, it is the polar covalent bonds than are
IR "active" and whose excitation can be observed
in an IR spectrum.
In organic molecules these polar covalent bonds
represent the functional groups.
Hence, the most useful information obtained
from an IR spectrum is what functional groups
are present within the molecul

11. some functional groups can be "viewed"
as combinations of different bond
types. For example, an ester, CO2R
contains both C=O and C-O bonds, and
both are typically seen in an IR spectrum
of an ester.
In the fingerprint region, the spectra
tend to be more complex and much
harder to assign.

13. IR spectroscopy
the infrared spectrum of a sample is recorded by passing a beam of
infrared light through the sample. When the frequency of the IR is
the same as the vibrational frequency of a bond or collection of
bonds, absorption occurs. Examination of the transmitted light
reveals how much energy was absorbed at each frequency (or
wavelength). This measurement can be achieved by scanning the
wavelength range using a monochromator. Alternatively, the entire
wavelength range is measured using a Fourier
transform instrument and then a transmittance or
absorbance spectrum is generated using a dedicated procedure.
This technique is commonly used for analyzing samples with
covalent bonds. Simple spectra are obtained from samples with
few IR active bonds and high levels of purity. More complex
molecular structures lead to more absorption bands and more
complex spectra.

14. Sample preparation-
Gaseous samples require a sample cell with a long path
length to compensate for the diluteness. The path length of
the sample cell depends on the concentration of the
compound of interest. A simple glass tube with length of 5
to 10 cm equipped with infrared-transparent windows at
the both ends of the tube can be used for concentrations
down to several hundred ppm.
Liquid samples can be sandwiched between two plates of
a salt (commonly sodium chloride, or common salt,
although a number of other salts such as potassium
bromide or calcium fluoride are also used). The plates are
transparent to the infrared light and do not introduce any
lines onto the spectra.

15. Solid samples can be prepared in a variety of ways.
One common method is to crush the sample with
an oily mulling agent (usually mineral oil Nujol). A
thin film of the mull is applied onto salt plates and
measured. The second method is to grind a
quantity of the sample with a specially purified salt
(usually potassium bromide) finely (to remove
scattering effects from large crystals). This powder
mixture is then pressed in a mechanical press to
form a translucent pellet through which the beam
of the spectrometer can pass

16. Comparing to a reference
It is typical to record spectrum of both the sample
and a "reference". This step controls for a number
of variables, e.g.IR detector, which may affect the
spectrum. The reference measurement makes it
possible to eliminate the instrument influence. The
simplest reference measurement is to simply
remove the sample (replacing it by air). However,
sometimes a different reference is more useful. For
example, if the sample is a dilute solute dissolved
in water in a beaker, then a good reference
measurement might be to measure pure water in
the same beaker.

17. Absorption bands
IR spectroscopy is often used to identify structures
because functional groups give rise to characteristic bands
both in terms of intensity and position (frequency). The
positions of these bands are summarized in correlation
tables as shown below.

18. IR Instrumentation
The main parts of IR spectrometer are as follows:
radiation source
sample cells and sampling of substances
Monochromators
detectors
Recorder
IR radiation sources--IR instruments require a source of radiant
energy which emit IR radiation which must be steady, intense
enough for detection and extend over the desired wavelength.
Various sources of IR radiations are as follows.
a) Nernst glower
b) Incandescent lamp
c) Mercury arc
d) Tungsten lamp
e) Glober source
f) Nichrome wire

19. sample cells and sampling of substances-IR spectroscopy
has been used for the characterization of solid, liquid or
gas samples.
Monochromators – Various types of monochromators are
prism, gratings and filters. Prisms are made of Potassium
bromide, Sodium chloride or Caesium iodide. Filters are
made up of Lithium Fluoride and Diffraction gratings are
made up of alkali halides.
Detectors – Detectors are used to measure the intensity of
unabsorbed infrared radiation. Detectors like
thermocouples, Bolometers, thermisters, Golay cell, and
pyro-electric detectors are used.

21. APPLICATIONS OF IR SPECTROSCOPY
1.Identification of functional group and structure
elucidation
2. Identification of substances
IR spectroscopy is used to establish whether a
given sample of an organic substance is identical
with another or not. This is because large number
of absorption bands is observed in the IR spectra
of organic molecules and the probability that any
two compounds will produce identical spectra is
almost zero. So if two compounds have identical IR
spectra then both of them must be samples of the
same substances.

22. 3. Studying the progress of the reaction
Progress of chemical reaction can be determined by
examining the small portion of the reaction mixture
withdrawn from time to time. The rate of disappearance of a
characteristic absorption band of the reactant group and/or
the rate of appearance of the characteristic absorption band
of the product group due to formation of product is
observed.
4. Detection of impurities
IR spectrum of the test sample to be determined is
compared with the standard compound. If any additional
peaks are observed in the IR spectrum, then it is due to
impurities present in the compound.

23. 5. Quantitative analysis
The quantity of the substance can be determined
either in pure form or as a mixture of two or more
compounds. In this, characteristic peak
corresponding to the drug substance is chosen and
log I0/It of peaks for standard and test sample is
compared. This is called base line technique to
determine the quantity of the substance.