Introduction and principle of ir spectrophotometry

Introduction and
Principle of IR
Spectrophotometry
Presented by:
Anuradha gupta

Contents
 Introduction of Spectroscopy
 Introduction of IR spectrophotometry
 Principle of IR spectrophotometry
 Molecular vibrations

Spectroscopy
Method of “Seeing the unseeable”
 using electromagnetic radiation to
obtain information about atoms and
molecules that are too small to see.
Atoms
molecules

•Spectroscopy is an instrumentally
aided study of the interactions
between matter (sample being
analyzed) and energy (any portion of
the electromagnetic spectrum)

IR spectrophotometry
• Energy of molecule = Electronic energy+
Vibrational energy + Rotational energy
• IR spectroscopy is concerned with the
study of absorption of infrared radiation,
which causes vibrational transition in the
molecule.
• Hence, IR spectroscopy also known as
Vibrational spectroscopy.
• IR spectra mainly used in structure
elucidation to determine the functional
groups.

Introduction and principle of ir spectrophotometry

Principle of IR spectroscopy
• Molecules are made up of atoms linked by
chemical bonds. The movement of atoms
and the chemical bonds like like spring and
balls (vibration)
Vibration of diatomic molecule
Approximates an oscillation spring
• This characteristic vibration are called
Natural frequency of vibration.

When energy in the form of infrared
radiation is applied then it causes the
vibration between the atoms of the
molecules and when, Applied infrared
frequency = Natural frequency of vibration
Then, Absorption of IR radiation takes place
and a peak is observed. Different functional
groups absorb characteristic frequencies of
IR radiation. Hence gives the characteristic
peak value. Therefore, IR spectrum of a
chemical substance is a finger print of a
molecule for its identification.

1. Correct wavelength of
radiation
2. Change in dipole
moment
Criteria for a compound to
absorb IR radiation

1. Correct wavelength of
radiation:
•A molecule to absorb IR
radiation, the natural
frequency of vibrations of
some part of a molecule is
the same as the frequency
of incident radiation.

2. Change in dipole
moment
•A molecule can only absorb IR
radiation when its absorption
cause a change in its electric
dipole
• A molecule is said to have an
electric dipole when there is a
slight positive and a slight
negative charge on its
component of atoms.

Introduction and principle of ir spectrophotometry

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