2. What is Infrared?
An Electromagnetic Radiation
Lies between the visible and microwave portions of the
electromagnetic spectrum
The Infrared region is divided into: near, mid and far-infrared.
3. IR ABSORPTIONS
Atoms within a molecule are never
still. They vibrate in a variety of ways
(modes).
Atoms may be considered as
weights connected by springs.
Absorption of IR radiation
excitation of vibrational and rotational
energy levels
not sufficient energy for electronic
excitation
Vibrational energy states are of
greatest importance for IR
spectroscopy
symmetric
stretch
asymmetric
stretch
bending
CO2
4. Correct wavelength of radiation
Vibrational mode involves a
change in dipole moment
An induced dipole moment is
observed in polar molecules
– Magnitude of dipole moment
depends on overlap of
electron densities in molecule
– Vibration results in change in
internuclear distance and
therefore fluctuations in
magnitude of dipole
Homonuclear diatomic
molecules (e.g. H2, N2, O2) don’t
absorb IR radiation, b/c of no
dynamic dipole moment
symmetric
stretch
asymmetric
stretch
bending
no dipole
no dipole
change in dipole - IR active
change in dipole - IR active
5. CO2 IR spectra
The bigger the
change in dipole,
the more intense
the absorption
The symmetric stretch is not IR active
(no change in dipole)
Wavenumber /cm-1
Stretching higher energy than bending
2800 2400 2000 1600 1200 800 400
0
100
Transmittance
/%
6. Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light
source
(ceramic)
Detector
Beam splitter
FT Optical System Diagram
8. Four regions in the spectrum:
4000 3500 3000 2500 2000 1500 1000
O-H
N-H
C-H
stretching
C C
C N
X Y Z
stretching
C C
C O
C N
stretching
N-H
bending
N O
other stretching,
bending and
combination
bands:
fingerprint
region
Wavenumber / cm-1
9. INFRA RED SPECTRUM
Group frequency region:
Simple stretching: 1600-3500 cm-1
Can be calculated from Hooke’s law
Factor influencing:
Coupling
Electronic effects
Hydrogen bonding
Fingerprint region:
Sensitive to structure
Bending vibrations: 600-1500 cm-1
Stretching vibrations :C-C,C-O and C-N
Molecules containing the same functional group shows different
spectra in this region
11. Qualitative Analysis
Step One : Identify functional groups (group
frequency region)
Step Two : Compare with standard spectra
containing these functional groups
– fingerprint region play important role
21. Quantitative Analysis
Use Beer- Lambert law
Convert transmittance value to corresponding
value
Plot calibration graph of absorbance against
concentration
Multi-component sample mixture can be analyse
22. Strengths and Limitations
IR alone cannot determine a structure.
Some signals may be ambiguous.
The functional group is usually indicated.
The absence of a signal is definite proof that
the functional group is absent.
Correspondence with a known sample’s IR
spectrum confirms the identity of the
compound.
23. Applications of Infrared Analysis
Identification and quantification of organic solid, liquid or gas
samples.
Structure Determination: probable structure can be predicted if
some chemical data is available
Qualitative analysis of functional groups
Study of a chemical reaction
Study of Keto-Enol tautomerism
Geometrical isomerism
Detection of impurity in a compound
Analysis of powders, solids, gels, emulsions, pastes, pure
liquids and solutions, polymers, pure and mixed gases.
Infrared used for research, methods development, quality
control and quality assurance applications.
24. References
Koulis, Cynthia, et. al. Comparison of Transmission and
Internal Reflection Infrared Spectra of Cocaine. Journal
of Forensic Sciences, 2001.
Skoog, Holler, and Nieman. Principles of Instrumental
Analysis. 5th edition, 1998
Engel and Reid. Physical Chemistry. Pearson Education,
2006.
http://wwwchem.csustan.edu/Tutorials/INFRARED.HTM
http://www.micromemanalytical.com/ATR_Ken/ATR.htm
