Unit III Spectroscopic Techniques

Infra red spectroscopy

( Vibrational Spectroscopy)

Department of Chemistry
Thiagarajar College of Engineering
Madurai
 Introduction

Infrared (IR) spectroscopy is one of the most common and widely used
spectroscopic techniques in analyzing the organic and inorganic molecule due to its
usefulness in determining structures of compounds and identifying them.
Infrared (IR) spectroscopy measures the change in dipole moment of molecules
due to irradiating them with light at frequencies that trigger transitions between
vibrational energy levels.
 IR spectroscopy which is short for infrared spectroscopy deals with the infrared
region of the electromagnetic spectrum, that is light with a longer wavelength and
lower frequency than visible light. (4000 cm-1to 400 cm-1).
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 Principle
The IR spectroscopy theory utilizes the concept that molecules tend to absorb specific

frequencies of light that are characteristic of the corresponding structure of the molecules.

The energies are reliant on the shape of the molecular surfaces, the associated vibronic

coupling, and the mass corresponding to the atoms.

For instance, the molecule can absorb the energy contained in the incident light and the

result is a faster rotation or a more pronounced vibration.

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 Selection rule
Molecules such as O2, N2, Br2, do not have a changing dipole moment when
they undergo rotational and vibrational motions, as a result, they cannot absorb
IR radiation.
Δv = ± 1
The selection rule says, that
vibrations are only IR active (or
allowed), if the molecular dipole
moment changes during the
vibration. A diatomic molecule with
the same atoms cannot be excited to
vibrate because no dipole moment is
present.

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 iii.) Types of Molecular Vibrations

There are two kinds of fundamental vibrations

in the molecule.
1. Stretching vibration: During stretching
vibration, the distance between two atoms
decreases or increases, but bond angle remain
unaltered.
2. Bending or deformation vibration: During
bending vibration, the distance between two
atoms remains same, but bond angle
increases or decreases.

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 Number of Vibrational Modes:
- for non-linear molecules, number of types of vibrations: 3N-6
- for linear molecules, number of types of vibrations: 3N-5

Examples:
1) HCl: 3(2)-5 = 1 mode

2) CO2: 3(3)-5 = 4 modes

- + -

moving in-out of plane

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 Transmission Vs Absorption
When a chemical sample is exposed to the action of IR LIGHT, it can
absorb some frequencies and transmit the rest. Some of the light can
also be reflected back to the source.
Transmitted light
IR Chemical Detector
source sample

From all the frequencies it receives, the chemical sample

can absorb (retain) specific frequencies and allow the
rest to pass through it (transmitted light).

The detector detects the transmitted frequencies, and by doing so also reveals the
values of the absorbed frequencies.
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 Infrared active bonds
 Not all covalent bonds display bands in the IR spectrum. Only polar bonds
do so. These are referred to as IR active.

 The intensity of the bands depends on the magnitude of the dipole moment
associated with the bond in question.
 Strongly polar bonds such as carbonyl groups (C=O) produce strong bands.
 Medium polarity bonds and asymmetric bonds produce medium bands.
 Weakly polar bond and symmetric bonds produce weak or non observable
bands.

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 Information obtained from IR spectra
 IR is most useful in providing information about the presence or absence of
specific functional groups.

 IR can provide a molecular fingerprint that can be used when comparing

samples. If two pure samples display the same IR spectrum it can be argued
that they are the same compound.

 IR does not provide detailed information or proof of molecular formula or

structure. It provides information on molecular fragments, specifically
functional groups.

 Therefore it is very limited in scope, and must be used in conjunction with

other techniques to provide a more complete picture of the molecular
structure.
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 IR Spectrum
ABSORPTION MODE TRANSMISSION MODE

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 IR Absorption range
The typical IR absorption range for covalent bonds is 600 - 4000 cm-1.
The graph shows the regions of the spectrum where the following types of bonds
normally absorb. For example a sharp band around 2200-2400 cm-1 would indicate
the possible presence of a C-N or a C-C triple bond.

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 Fingerprint region
Although the entire IR spectrum can be used as a fingerprint for the
purposes of comparing molecules, the 600 - 1400 cm-1 range is called the
fingerprint region.
This is normally a complex area showing many bands, frequently
overlapping each other. This complexity limits its use to that of a fingerprint, and
should be ignored by beginners when analyzing the spectrum.

Focus your analysis on this region. This is where Fingerprint region: complex and
most stretching frequencies appear. difficult to interpret reliably.

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 IR Absorptions of common functional groups

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 Working of Infrared spectrophotometer

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 i.) Light Source:

An inert solid is electrically heated to a temperature in the range 1500-2000 K. The

heated material will then emit infrared radiation.
 The Nernst glower is a cylinder (1-2 mm diameter, approximately 20 mm long) of
rare earth oxides. The Nernst glower can reach temperatures of 2200 K.
 The Globar source is a silicon carbide rod (5mm diameter, 50mm long) which is
electrically heated to about 1500 K.
 The incandescent wire source is a tightly wound coil of nichrome wire, electrically
heated to 1100 K. It produces a lower intensity of radiation than the Nernst or Globar
sources, but has a longer working life.

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 (ii) Sample holder:
a.) Sample Cell
- must be made of IR transparent material (KBr pellets or NaCl)

Liquid Sample Holder NaCl plates

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 ii.) Detectors:
Thermal 2.Pyroelectric 3.Photoconducting
a) Thermal Detectors
1.) Thermocouple
- two pieces of dissimilar metals fused together at the ends
- when heated, metals heat at different rates
- potential difference is created between two metals that varies with their
difference in temperature
- usually made with blackened surface (to improve heat absorption)
- placed in evacuated tube with window transparent to IR (not glass or
quartz)
- IR “hits” and heats one of the two wires.
- can use several thermocouples to increase sensitivity.

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 b) Pyroelectric detectors
- Constructed from a single crystalline wafer of pyroelectric material
Ex: Triglycerine sulphate

c) Photoconducting Detectors
Photoelectric detectors such as the mercury cadmium telluride detector comprise
a filmof semiconducting material deposited on a glass surface, sealed in an
evacuated envelope. These detectors have better response characteristics than
pyroelectric detectors and are used in FT-IR instruments - particularly in GC - FT-IR.

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 Fourier-transform spectrometers
Advantages of Fourier transform IR over dispersive IR

 Improved frequency resolution

 Improved frequency reproducibility
 Higher energy throughput
 Faster operation
 Computer based (allowing storage of spectra and facilities for processing
spectra)
 Easily adapted for remote use
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 Applications of IR Spectroscopy
I. In establishing the identity of the unknown compounds by comparing the
known compounds

II)In detecting impurities in a sample

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 Applications contd ….
III. To ascertain hydrogen bonding in a molecule

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 Applications contd ….
IV. Distinguishing positional and functional isomers of a
compound

O-H O-H
( alcoholic) ( acidic)
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 Applications contd ….
V. It is used in forensic analysis
VI. Rapid quantitative analysis of mixture of compounds, in pollution detection, etc.
VII. It is used in quality control, dynamic measurement, and monitoring applications such as the long-
term unattended measurement of CO2 concentrations in greenhouses and growth chambers by
infrared gas analyzers.
VIII. In studying the progress of the reaction
IX. It can be used in determining the blood alcohol content of a suspected drunk driver.
X.IR-spectroscopy has been successfully used in analysis and identification of pigments in paintings
and other art objects

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 Applications contd ….
XI. useful way of analyzing solid samples without the need for cutting samples uses ATR or
attenuated total reflectance spectroscopy.
XII. Infrared spectroscopy is also useful in measuring the degree of polymerization in polymer
manufacture.
XIII. Another important application of Infrared Spectroscopy is in the food industry to measure the
concentration of various compounds in different food products
XIV. near infrared spectroscopy device to detect potentially illegal substances

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