IR, Infrared spectroscopy

1. Infrared Spectroscopy
Presented By:
Lok Raj Bhandari
Asst. Professor
M.Pharma,MBA
KU

2. Contents
• Introduction
• Theory of IR
• Modes of Vibration
• Terms
• Interpretations
• Factors affecting Vibrational frequency
• Instrumentation
• Sampling
• Adv and Disadv
• Applications

3. Introduction
• Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is
the measurement of the interaction of infrared radiation with matter
by absorption, emission, or reflection.
• It is used to study and identify chemical substances or functional
groups in solid, liquid, or gaseous forms.
• Infrared radiation is that part of the electromagnetic spectrum
between the visible and microwave regions.
• Infrared spectroscopy is used in identification of functional groups in
pure compounds.

4. • Infra-red (IR) does not have sufficient energy to induce electronic
transition as seen in UV spectroscopy.
• When molecule absorbed electromagnetic radiation in IR region,
undergoes vibrational or a rotational transitions which causes net
change in the dipole moment in the molecule (IR active, for example
HCl, CO etc), if dipole moment does not change in molecules then
they are IR inactive (for example: O2, H2, N2 etc.) means they does
not absorb IR radiation.
• IR region ranges from 4000-400 cm-1

5. • The three Infra red regions of interest in the electromagnetic
spectrum in terms of wavelengths the three regions in micrometers
(µm) are the following:
 Near Infrared Spectroscopy (NIRS): (0.7 µm to 2.5 µm)
 Mid Infrared Spectroscopy (MIRS): (2.5 µm to 25 µm)
 Far Infrared Spectroscopy (FIRS): (25 µm to300 µm)
In Another Wavenumbers the three Regions in cm-1 are:
 Near Infrared Spectroscopy (NIRS) : (14000-4000 cm-1)
 Mid Infrared Spectroscopy (MIRS) : (4000-400 cm-1)
 Far Infrared Spectroscopy (FIRS) : (400-10 cm-1).

6. Principle
• IR radiation in the range from about 10000 -100cm-1 is absorbed and
converted by an organic molecule into energy of molecular vibration.
• This absorption is also quantized, but vibration spectra appear as
bands rather than as lines because a single vibrational energy change
is accompanied by a number of rotational energy changes.
• It is with these vibrational rotational band, particularly those
occurring between 4000 and 400cm-1. The frequency or wavelength
of absorption depends on the relative masses of the atoms, the force
constants of the bonds, and the geometry of the atoms.
• If the frequency of IR radiation matched with the vibrational
frequency of molecule, then molecule absorb radiation.

7. • IR spectroscopy based on Hooke’s law, suppose two atoms or masses
are connected through spring (bond), then frequency of vibration can
be represented by following equation:
Where, is force constant of the bond, ῡ is wave-number (cm-1 ), ν is the frequency, c is speed of light and is reduce mass
𝜅 𝜇
(m1 and m2 are the masses of atoms)
• Stronger the bond, greater the value of force constant ( ), higher the
𝜅
frequency vibration or wave-number (cm-1 ). For example,

8. Molecular Vibration or or Mode of vibration
• At room temperature, organic molecules are always in motion, as
their bonds stretch, bend, and twist.
• These complex vibrations can be broken down mathematically into
individual vibrational modes.
• As a result molecule experiences constant translational and rotational
motion.
• Polyatomic molecules have more than one type of vibration, known
as normal modes. The frequency of the periodic motion is known as a
vibration frequency.

9. Mode of Vibration

10. • These vibrations are arising when molecule promoted from ground
state to lower excited state. The fundamental vibrations for linear and
non-linear molecules are determined by following way:
• The vibrations discuss below are fundamental vibrations.
a) Stretching vibration: Distance between two atom increase and decrease but
bond angle remains constant.
Types of stretching vibrations
i) Symmetric stretching vibration: In this case both the atoms stretched or
compressed in same direction.

11. ii) Asymmetric stretching vibration: In this vibration one atom
undergoes stretching and other atom undergoes compression and vice
versa.
b) Bending vibrations: Distance between two atom remains constant but bond
angle changes. These vibrations can occur either in plane or out of plane.
Types of bending vibrations
1) In plane bending vibrations:
i) Scissoring: both the atom move towards each other just like scissor.

12. ii) Rocking: both the atoms move in same direction, either in left side or
right side.
2) Out of plane bending vibrations:
i) Wagging: both the atom move up and down with respect to central
atom.
ii) Twisting: one atom move up and other atom move down with
respect to central atom.

13. Terms in IR
Q)OVERTONES?
When molecule absorbed electromagnetic radiation in IR region, and
then molecule promoted from ground state to second, third or even
fourth vibrational excited state.These bands are known as Overtones.
The intensity of these bands is very weak.
It is helpful in characterization of aromatic compounds.

14. Q) Combination Band??
When two fundamental vibrational frequencies (ν1 + ν2) in a molecule
couple to give rise to a new vibrational frequency within the molecule,
it is known as combination band.
Q) COUPLED VIBRATIONS???
The coupled vibrations are observed in group like –CH2, NH2 etc. In
these groups same atoms are attached to the central atom. When –CH2
undergoes vibration by the absorption of IR radiation, due to internal
perturbation, energy of one C-H bond is transfer to neighboring C-H
bond which enhance its vibrational frequency.Therefore two stretching
vibrational frequencies for –CH2 group is observed at 2950 cm-1
(asymmetric stretching) and 2860 cm-1
(symmetric stretching).

15. Q)FERMI RESONANCE????
When fundamental vibration coupled with overtones or combination
band, the coupled vibration is called Fermi resonance or when
molecule absorb IR radiation then it transfers its energy or intensity
from fundamental vibration to overtones,then Fermi resonance is
observed.
As we know that the intensity of overtones band is very weak as
compare to fundamental vibrations. But, due to transfer of energy,
the strong band is observed for overtones along with the fundamental
frequency.
Fermi resonance is generally observed in carbonyl groups.

16. • For example, in benzoyl chloride –C=O stretching vibration observed
at 1790 cm-1
and 1745 cm-1
• The lower frequency band at 1745 cm-1
is observed due to
combination of overtones of CH bending vibration at 875 cm-1
with
the fundamental vibration of C=O stretching.

17. FINGERPRINT REGION
• The region from 1500-600 cm-1
in IR spectrum is known as Fingerprint
region. In this region number of bending vibration is more than the
number of stretching vibration.
• Number of molecules contains same functional group & show similar
peak above 1500 cm-1 but they show different peak in finger print
region. Therefore we can say that each and every molecule have
unique peak or band which is observed in fingerprint region, it is just
like the finger print of human.

20. Factor affecting vibrational frequency
a) Conjugation: As the conjugation increase, stretching frequency
decreases,because force content decrease due to conjugation.
b) Inductive effect and resonance effect:
Oxygen is more electronegative than nitrogen, therefore nitrogen easily
donate electron or lone pair of nitrogen undergoes delocalization with C=O bond.
Due to delocalization double bond of C=O changes into partial double bond
therefore force constant decreases which decrease the C=O stretching frequency.

21. c) Hydrogen bonding: Intermolecular hydrogen bonding weakens the O-H
bond, thereby shifting the band to lower frequency. For example, in neat
solution O-H stretching vibration of phenol observed in the range from 3400-
3300 cm-1. When solution is dilute then O-H frequency shifted towards higher
frequency at 3600 cm-1.

22. • Whereas in case of methyl salisilate, intramolecular hydrogen bonding
lower down the stretching frequency of O-H at 3200 cm-1.
Intramolecular hydrogen bonding does not change its frequency even
in very dilute solution because upon dilution structure of compound
does not change.

23. d) Ring strain: As the size of the ring decrease, vibrational frequency of
C=O increase. For example.

24. Instrumentation
The main parts of the IR spectrometer are as follows:
Radiation source
Sample cells and sampling of substances
Monochromators
Detectors
Recorder

25. A. IR radiation sources
• In common with other types of absorption spectrometers, infrared instruments require a source
of radiant energy which provides a means for isolation narrow frequency bands. The radiation
source must emit IR radiation which must be
Intensive enough for detection
Steady
Extend over the desired wavelength
• Although these radiations are continuous, only selected frequencies will be absorbed by the
sample. The various popular sources of IR radiations are:
Nernst glower
Incandescent lamp
Mercury arc
Tungsten lamp
Glober source
Nichrome wire

26. a. Incandescent lamp: An everyday incandescent light is typically utilized in near-
infrared devices.Unfortunately, because to its low spectral emissivity and glass
enclosure, this fails in the far infrared.
b.Nernst Glower: It consists of a hollow rod which is about 2mm in diameter and 30mm
in length. The glower composed of rare earth oxides as zirconia, yttria and thoria.
Nernst glower is non- conducting at room temperature and must be heated by external
means to bring it to a conducting state. Glower is generally heated temperature
between 1000 to 18000C. It provides maximum radiation at about 7100 cm-1 (1.4µ).
The main disadvantage Nernst glower is that is emit IR radiation over wide wavelength
range; the intensity of radiation remains steady and constant over long periods of time.
One main disadvantage of Nernst is its frequent mechanical failure.
Another disadvantage is that its energy also concentrated in the near infrared regions
of the spectra.

27. Sample cells and sampling of substances
• IR spectroscopy has been used for the characterization of solid, liquid, or gas
samples.
i. Solid – Various techniques are used for preparing solid samples such as pressed
pellet technique, solid run in solution, solid films, mull technique, etc.
ii. Liquid – Samples can be held using a liquid sample cell made of alkali halides.
Aqueous solvents cannot be used as they will dissolve alkali halides. Only organic
solvents like chloroform can be used.
iii. Gas– Sampling of gas is similar to the sampling of liquids.

28. 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.
a.PrismMonochromator:Any prism used as a dispersive element must be
constructed of materials (such as various metal halide salts) which transmit in the
infrared. While glass and quartz were utilized in the visible and ultraviolet, they
absorb and are unsatisfactory in the infrared. Because of its high dispersion in
region of 4 to 15µm, a region which is of special importance for functional group
student, sodium chloride is probably the most common prism salt. It is unfortunate
that many of these salt materials are subject to mechanical and thermal instability
and/or water solubility. Protection against damage must be continuously exercised.

29. 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.

30. a. Bolometer: A thin metal conductor is typically used in bolometers.
This conductor's temperature changeswhen radiation, like infrared
radiation, strikes it.
The amount of radiation that has reached the bolometer. A is measured
by the degree of change in resistance with temperature.
Bolometers are built inside one of the Wheatstone Bridge's arms. The
bridge's balancing arm is made of a similar metal strip. There is no IR
radiation exposure on this strip. The bridge stays balanced while the
bolometer is not exposed to radiation.

31. Sampling Method in IR
• Gaseous samples require little preparation beyond purification, but a sample cell with a
long pathlength (typically 5–10 cm) is normally needed, as gases show relatively weak
absorbances.
• Types of Sampling
1. Sampling in solids
Solid run in solution
Solid films
Mull Technique
Pressed pellet Techniques
2. Sampling in Liquids
3.Sampling in Gases

32. Solid Run in Solution

39. Advantages of IR:
• Non-invasive detection:IR can detect heat signatures without direct contact,
making it useful in applications like medical temperature checks or motion
sensing.
• Low power consumption:IR devices generally require less power compared to
other sensing technologies, leading to longer battery life.
• Short-range communication:IR is well-suited for short-distance data
transmission, like remote controls for electronics.
• Cost-effective:IR technology is often cheaper to implement compared to other
sensing methods.
• Wide application range:IR is used in various fields, including consumer
electronics, security systems, medical diagnostics, and industrial applications.

40. Disadvantages of IR
• Limited range:IR signals are easily obstructed and can't travel long
distances, making them unsuitable for large-scale applications.
• Environmental interference:Factors like dust, fog, or direct sunlight can
significantly impact IR signal reception.
• Line-of-sight requirement:For effective IR communication, a clear line
of sight between the transmitter and receiver is necessary.
• Privacy concerns:IR sensors can detect body heat, raising potential
privacy issues in certain applications.
• Accuracy limitations:IR sensors can be affected by ambient
temperature fluctuations, impacting the accuracy of their readings.

41. Applications of Infrared (IR) Spectroscopy
1. Infrared spectroscopy, being a simple as well as a reliable characterization technique, is applicable to all
aspects of organic and inorganic chemistry
2. It is also useful in research institutes and industries.
3. The purpose of adopting this characterization technique is to determine quality control, dynamic
measurement, and monitoring applications such as the long-term unattended measurement of CO2
concentrations in greenhouses, growth chambers by infrared gas analyzers, and forensic analysis.
4.Microelectronics and Semiconductors Infrared: Infrared spectroscopy has been used successfully in the field
of microelectronics, as well as semiconductors infrared, such as amorphous silicon, silicon arsenide, and other
gallium and silicon compounds. Nowadays, the durability of the instruments has made them useful for field
trials.
5. Isotope Effects: The different isotopes in a particular species may give fine details in infrared spectroscopy,
obtainable with species possessing different isotopes.
6. Linear Two-Dimensional Infrared Spectroscopy: Linear two-dimensional infrared bond spectroscopy analysis
entails the use of two-dimensional infrared bond analyses on infrared spectra