basics of infrared spectroscopy

INFRARED SPECTROSCOPY

MOHAMED NIJAS VS5I.S.P

CONTENT

SPECTROSCOPY. IR SPECTROSCOPY. MOLECULAR VIBRATIONS. EXPERIMENTAL SETUP. FUTURE DIRECTIONS.

SPECTROSCOPY???

Method of “Seeing the unseeable”

Using electromagnetic radiation to obtain information about atoms and molecules that are too small to see.

AtomsMolecules

EMR

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

EMR ANALYTE SPECTROPHOTOGRAPH

1.UV-Visible radiations--------excitation of electrons---------UV-visible spectrum

2.IR-radiations------------------vibration changes in electrons--------IR spectrum

3.Radio frequency---------------spin rotational changes-------------N.M.R spectrum

Conc. should be lower

IR SPECTROPHOTOMETRY

Energy of molecule = Electronic energy+ Vibrational energy + Rotational energy

Concerned with the study of absorption of infrared radiation, which causes vibrational transition in the molecule.

Thus known as Vibrational spectroscopy. Mainly used in structure elucidation to determine

the functional groups.

Most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region.

IR region: 0.8 µm (800nm) to

1000 µm (1mm)

Near IR: 0.8-2 µm

Middle IR: 2-15 µm

Far IR: 15-1000 µm

PRINCIPLE OF IR SPECTROSCOPY Molecules are made up of atoms linked by chemical bonds.

The movement of atoms and the chemical bonds look like spring and balls (vibration).

This characteristic vibration are called

Natural frequency of vibration. Applied infrared frequency = Natural frequency of vibration Change in dipole moment is required

MOLECULAR VIBRATIONS

Bending vibrationsStretching vibrations

Vibration or oscillation along the line of bond

Change in bond length Occurs at higher energy:

4000-1250 cm-1

a) Symmetrical stretchingb) Asymmetrical stretching

•Vibration not along the line of bond•Bond angle is altered•Occurs at low energy: 1400-666 cm-1

a) In plane bendingb) Out plane bending

a) Symmetrical stretching:

2 bonds increase or decrease in length simultaneously.

H

H

C

STRECHING VIBRATIONS

b) Asymmetrical stretching in this, one bond length is increased and other is decreased.

H

H

C

a) In plane bending

i. Scissoring: 2 atoms approach each other Bond angles are decrease

H

H

CC

ii. Rocking: Movement of atoms take place in the same direction.

H

H

CC

b) Out plane bendingi. Wagging: 2 atoms move to one side of the plane. They move up and down

the plane.

ii. Twisting: One atom moves above the plane and another atom moves

below the plane.

H

H

CC

H

H

CC

EXPERIMENTAL SETUP

COMPONENTS

1. Source

2. Fore optics

3. Monochromator

4. Detector

5. Recorder

SOURCE

•Ideal => black body radiator•COMMONLY USED =>globar FILAMENT &NERNST GLOWER•Globar- resistance rod of silicon carbide•Nernst glower-a spindle of rare earth oxide(thorium,zirconium,etc)•Globar-for longer wavelengths•Nernst glower-for shorter wavelengths

FORE OPTICS

•CONSISTS OF SOURCE,MIRRORS M1,M2 AND A ROTATING MIRROR M•M1,M2 DIVIDES THE BEAM•M ALTERNATELY ALLOWS THESAMPLE BEAM AND REFERENCE BEAM TO PASS THROUGH

MONOCHROMATOR Splits the polychromatic readiation to component

wavelengths. Make use of prisms or grating or both. Resolution depends on slit width and quality of mirrors. Rock salt prism is generally use d in the range of 650-

4000cm-1

DETECTOR•Measure the radiant energy by its heating effect.•Thermopiles bolometer and golay cells are generally used•Photoconductivity is also used.•Radiation is allowed to fall on photo conducting material and the conductivity of the material is measured continuously by a bridge network.•Once the sample absorbs radiation, there will be inequality between the two radiations and signal will be produced.

THE RECORDER•The amplified signal is used to move an attenuator which cuts down the radiation coming out of the reference beam until energy balance is restored.•This is achieved by a motor which drives the comb into the reference beam when an absorbing band is encountered and out of the beam when the band is passed over.•The recorder pen is also coupled to this motor so that the comb movement is followed exactly y the pen