Transcript File

INFRARED SPECTROSCOPY
Pramod K Singh
School of Basic Sciences &Research
Sharda University, Greater Noida, INDIA
What is Infrared?
IR radiation lies betn the visible and microwave portions of the em
spectrum.
*Infrared waves have wavelengths longer than visible and shorter
than microwaves, and have frequencies which are lower than visible
and higher than microwaves.
•
The IR region is divided into: near, mid and far-infrared.
– Near-infrared refers to the part of the infrared spectrum that is closest to
visible light
– far-infrared refers to the part that is closer to the microwave region.
– Mid-infrared is the region between these two.
What is Infrared?
• The primary source of infrared radiation is thermal radiation. (heat)
• It is the radiation produced by the motion of atoms and molecules in an
object.
• The higher the temperature, the more the atoms and molecules move and
the more infrared radiation they produce.
• Any object radiates in the infrared. Even an ice cube, emits infrared.
*The IR region of the em spectrum covers a wide range of
wavelengths, from about 800 nm (end of the visible region) to
about 0.2 mm (beginning of the microwave region).
*The units of wavelength commonly employed in the infrared
region is wave number,  = 1/, in reciprocal cm (cm-1).
Infrared radiations
• IR does not have sufficient energy to cause the
excitation of e, however it causes atoms and group of
atoms of organic compounds to vibrate faster about
the covalent bonds which connect them.
• The vibrations are quantised as they occur, the
compound absorbs infrared energy in particular
regions of the spectrum.
What is a vibration in a molecule?
Any change in shape of the moleculestretching of bonds, bending of bonds, or
internal rotation around single bonds
Molecular Vibrations
Covalent bonds vibrate at only certain
allowable frequencies.
Calculation of vibrational frequencies :
Let us consider a chemical bond to be a stretched spring. Then
according to the Hooke’s law the frequency of vibration depends
upon the strength of the bond.
Stronger the bond more is the amount of energy required to
stretch it.
Wave numbers are proportional to energy. K is force constant
Conditions for I R activity (Selection Rule)
The necessary conditions for a molecule to be IR active are:
*The frequency of IR radiation must be equal to the vibrating frequency
of a particular group or bond in the molecule
*The molecule must have finite dipole moment.
* If it does not have dipole moment,Some of its vibrations must produce
an induced dipole moment.
Asymmetrical stretching/bending and internal
rotation change the dipole moment of a molecule.
Asymmetrical stretching/bending are IR active.
Symmetrical stretching/bending does not and hence Not
IR active
IR-Active and Inactive
• A polar bond is usually IR-active.
• A nonpolar bond in a symmetrical
molecule will absorb weakly or not at all.
Infrared spectrometer
double – beam infrared spectrometer. Its essential features
are described briefly as follows:
(i) Radiation source :
*Small ceramic rod, heated electrically in the range 1100 – 18000C and is made
of either silicon carbide (Glowbar) or Nernst filament (a high – resistance,
brittle element composed of mixture of sintered oxides of zirconium,
thorium and cerium held together by a binding material).
The radiation is divided into two beams, one of which passes through the
sample while the other function as a reference beam. The reference and
the sample beams are then passed alternately into a monochromator at
very short intervals by means of a rotating mirror.
(ii) Absorption cells and sample preparation : The cells generally employed
are made up of rock salt or potassium bromide
*(glass and quartz cells are unsuitable as these materials themselves absorb IR
radiation).
(iii) Monochromator :
The pulse beam enters the monochromator through an entrance slit and is
dispersed by a grating or by a Littrowmount prism.
In the monochromator the emergent beams are sorted out into individual wave
lengths by means of sodium chloride prism which is transparent to infrared
radiation throughout the range 4000 – 550 cm-1.
(iv) Detector, Amplifier and Recorder : The pulsating single beam now
emerging through the exit slit is a narrow band consisting of only a very few
frequencies
After this sorting out (dispersion), the beams are focused alternately at each particular
wave length throughout the spectral range, by means of a mirror system on to the
detector, usually a sensitive fast thermocouple.
The signals from this are amplified electronically and by means of a
mechanical arrangement, the spectrum is recorded on a special graph
paper mounted on a rotating drum.
FT-IR Spectrometer
• Fourier transform infrared spectroscopy is a technique
for obtaining high quality infrared spectra by
mathematical conversion of an interference pattern into
a spectrum.
• Uses an interferometer.
• Has better sensitivity.
• Less energy is needed from source.
• Completes a scan in 1-2 seconds.
• Takes several scans and averages them.
• Has a laser beam that keeps the instrument accurately
calibrated.
FTIR seminar
FT Optical System Diagram
Light
source
He-Ne gas laser
(ceramic)
Beam splitter
Movable mirror
Sample chamber
(DLATGS)
Fixed mirror
Interferometer
Detector
How the IR spectra of solid, liquid and gaseous compounds
recorded?
1. The spectrum of a solid sample is determined with an alkali
halide pellet. About 1-3 mg of substance and 100-200 mg of
alkali halide are ground together, dried to remove moisture and
pressed at room temperature under high pressure into a small
disc. KBr does not absorb IR radiation in the region 4000-650
cm-1 a complete spectrum of the solid is obtained.
2. For free flowing liquids a neat spectrum may be recorded.
3. Spectrum of a substance in a solution can also be done by
using cells of 0.1 mm with 10% solution are used. CCl4, CS2
and
CHCl3
are
the
solvents
to
be
used.
4. The spectra of gases or low boiling liquids can be obtained by
expansion of the sample into an evacuated cell.
An Alcohol IR
Spectrum
An Amine
IR Spectrum
Carbonyl Stretching
• The C=O bond of simple ketones,
aldehydes, and carboxylic acids absorb
around 1710 cm-1.
• Usually, it’s the strongest IR signal.
• Carboxylic acids will have O-H also.
• Aldehydes have two C-H signals around
2700 and 2800 cm-1.
A Ketone
IR Spectrum
An Aldehyde
IR Spectrum
O-H Stretch of a
Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cm-1,
due to strong hydrogen bonding.
Thank you