INFRARED SPECTROSCOPY

-By,Revti Lohey.

OVERVIEW Introduction to IR Spectroscopy Basic principle of IR Spectroscopy Instrumentation Modes of vibration Stretching vibration Bending vibration Absorption bands Application

INTRODUCTION Infrared spectroscopy (IR

spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light.

 As with all spectroscopic techniques, it can be used to identify and study chemicals.

The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum.

REGION WAVELENGTH RANGE (MICRO M.)

WAVELENGTH NO. (1/cm)

NEAR 0.78-2.5 12800-4000

MIDDLE 2.5-50 4000-200

FAR 50-1000 200-10

BASIC PRINCIPLE IR radiation does not have enough

energy to induce electronic transitions as seen with UV. 

Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states.

For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule

The alternating electrical field of the radiation interacts with fluctuations in the dipole moment of the molecule.

If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.

INSTRUMENTATION

IR SPECTROMETER

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Frequencies absorbed by the molecules depends upon the characteristic of their structure.

 These absorptions are called resonant frequencies, i.e. the frequency of the absorbed radiation matches the transition energy of the bond or group that vibrates.

Vibration falls in two main categories:1. Stretching vibration- Change in inter-

atomic distance along bond axis.2. Bending vibration- Change in angle

between two bonds. They can be further categorized as:

symmetric and anti-symmetric stretching (in stretching vibration) & scissoring, rocking, wagging and twisting (in bending vibration)

STRETCHING VIBRATION Symmetrical stretching.

Anti-symmetrical stretching.

BENDING VIBRATION Rocking.

Scissoring.

Wagging.

Twisting.

ABSORPTION BANDS

APPLICATIONS Widely used in organic chemistry. Used for the analysis of complex bio-

molecules such as proteins, lipids and nucleic acids.

It can be used to determine molecular structure.

It studies the progress of reaction (disappearance of characteristic absorption band)

Analysis of multilayered polymeric film. Identification of functional group.

Used to study isomerism in organic chemistry

In industry, for quality control check, to determine the percent of required product.

Detection of impurities.Ex. Ketone impurity in alcohol. Used for the characterization of the

heterogeneous catalyst. Used for analysis of petroleum, oil &

grease. Examination of old paintings and

artifacts.

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