SPECTROSCOPY
Atoms and molecules interact with electromagnetic radiation (EMR) in a wide variety of ways.
Atoms and molecules may absorb and/or emit EMR.
Absorption of EMR stimulates different types of motion in atoms and/or molecules.
The patterns of absorption or emission are called ‘spectra’.
The field of spectroscopy is concerned with the interpretation of spectra in terms of atomic and molecular structure (and environment).
Monday, November 2, 2009
INFRARED SPECTROSCOPY
INFRARED SPECTROSCOPY
Infrared radiation stimulates molecular vibrations.
Infrared spectra are traditionally displayed as %T (percent transmittance) versus wave number.
Useful in identifying presence or absence of functional groups.
¨IR range- 10cm- - 12800cm-
¨Sub regions
¨Near IR – 12500 – 4000cm-
¨Middle IR – 4000 – 667cm-
¨Far IR – 667 – 50 cm-
Finger print region – 900 – 1400cm- (identify the compound)
Water is a
-bent- non linear – triatomic molecule
Formula used for finding vibrations in non-linear molecule – (3N-6)
Carbon-di-oxide is a
– Linear – triatomic molecule
Formula used for finding vibrations in non-linear molecule - (3N-5)
Infrared radiation stimulates molecular vibrations.
Infrared spectra are traditionally displayed as %T (percent transmittance) versus wave number.
Useful in identifying presence or absence of functional groups.
¨IR range- 10cm- - 12800cm-
¨Sub regions
¨Near IR – 12500 – 4000cm-
¨Middle IR – 4000 – 667cm-
¨Far IR – 667 – 50 cm-
Finger print region – 900 – 1400cm- (identify the compound)
Water is a
-bent- non linear – triatomic molecule
Formula used for finding vibrations in non-linear molecule – (3N-6)
Carbon-di-oxide is a
– Linear – triatomic molecule
Formula used for finding vibrations in non-linear molecule - (3N-5)
INFRARED ABSORPTION BANDS
INFRARED ABSORPTION BANDS
POSITION -REDUCED MASS- BOND STRENGTH (STIFFNESS)
LIGHT ATOMS HIGH FREQUENCY
STRONG BONDS HIGH FREQUENCY
STRENGTH -CHANGE IN ‘POLARITY’
STRONGLY POLAR BONDS GIVE INTENSE BANDS
WIDTH-HYDROGEN BONDING
STRONG HYDROGEN BONDING GIVES BROAD BANDS
POSITION -REDUCED MASS- BOND STRENGTH (STIFFNESS)
LIGHT ATOMS HIGH FREQUENCY
STRONG BONDS HIGH FREQUENCY
STRENGTH -CHANGE IN ‘POLARITY’
STRONGLY POLAR BONDS GIVE INTENSE BANDS
WIDTH-HYDROGEN BONDING
STRONG HYDROGEN BONDING GIVES BROAD BANDS
INFRARED (VIBRATIONAL) SPECTROSCOPY
POLARITY
Only vibrations that cause a change in ‘polarity’ give rise to bands in IR spectra.
Which of the vibrations for CO2 are infrared active?
Symmetric stretch
Asymmetric stretch
Bending (doubly degenerate)
Two types of stretching vibrations
Symmetric stretch
Asymmetric stretch
Four types of Bending Vibrations
¨Scissoring
¨Rocking
¨Twisting
¨Wagging
Only vibrations that cause a change in ‘polarity’ give rise to bands in IR spectra.
Which of the vibrations for CO2 are infrared active?
Symmetric stretch
Asymmetric stretch
Bending (doubly degenerate)
Two types of stretching vibrations
Symmetric stretch
Asymmetric stretch
Four types of Bending Vibrations
¨Scissoring
¨Rocking
¨Twisting
¨Wagging
Sunday, November 1, 2009
IR INSTRUMENTATION
Source- Incandescent material
Other sources – Nernst glower , Globar , Carbon arc, Heated Nichrome
Monochromator – Quartz – prism
Detectors – Photo conductivity cell, Thermocouple, Pyroelectric detectors
APPLICATIONS OF IR SPECTROSCOPY
—Identification of a substance (penicillin and peptides)
—Study of geometrical isomerism
—Characteristic of organic substances
—Study of photosynthesis and reparative in plants
—Assessment of purity of a sample
—Ideal and rapid method
—Study of geometrical isomerism
—Characteristic of organic substances
—Study of photosynthesis and reparative in plants
—Assessment of purity of a sample
—Ideal and rapid method
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