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Infrared Spectrometry
Principle
Instrument
Qualitative Analysis
Quantitative Analysis
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Infrared Light
Energy of IR photon insufficient to cause electronic
excitation but can cause vibrational or rotational excitation
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Selection Rule of Infrared Spectrum
Molecule must have change in dipole
moment due to vibration or rotation to
absorb IR radiation.
Homonuclear diatomic molecules will have
no IR spectrum.
Molecule dipole moment interacts with IR
photon electric field.
Absorption causes increase in vibration
amplitude/rotation frequency.
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Molecules with permanent dipole
moments () are IR active
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Types of Molecular VibrationsStretchchange in bond length
symmetric stretching
asymmetric stretching
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Types of Molecular Vibrations
Bendchange in bond angle
scissoring
wagging
rocking
twisting/torsion
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Vibrational Modes:
To determine the vibration of polymolecule we need to consider:
-motion of the entire molecule in space
-rotational motion of the molecule around its centre of gravity
-Motion of each atom (individual vibrations).
Molecule containing N atoms have 3N degree of freedom
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Normal Modes of Vibration
Linear molecule of N atoms: normal
modes = 3N - 5
Nonlinear molecule of N atoms: normal
modes = 3N - 6
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Only some modes may be IR active
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The three fundamental vibrations
for sulfur dioxide
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How many vibrational modes?
2 atoms (H2) - 1 vibration
3 atoms (H2O) - 3 vibrations
3 atoms (CO2) - 4 vibrations
4 atoms (H2CO) - 6 vibrations
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Vibrational coupling :
In addition to the vibrations mentioned above, interactionbetween vibrations can occur (coupling) if the vibrating bonds
are joined to a single, central atom. Vibrational coupling is
influenced by a number of factors;
Strong coupling of stretching vibrations occurs when there is a
common atom between the two vibrating bonds
Coupling of bending vibrations occurs when there is a common
bond between vibrating groups
Coupling between a stretching vibration and a bending vibration
occurs if the stretching bond is one side of an angle varied by
bending vibration
Coupling is greatest when the coupled groups have
approximately equal energies
No coupling is seen between groups separated by two or more
bonds
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Instrumentation: Sources
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Instrumentation: Transducers
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Fourier Transform Instruments
Michelson interferometer
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Fourier Transform
FrequencyTimeFT
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FTIR advantages
High S/N ratios
Rapid (
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Qualitative and Quantitative Analysis
Gases-fill gas cell
(a) transparent windows (NaCl/KBr)
(b) long pathlength (10 cm) - few molecules
Liquids-fill liquid cell
(a) solute in transparent solvent - not water
(b) short pathlength (0.015-1 mm)Solids-make semi-transparent pellet with
KBr
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Qualitative Analysis
Step One: Identify
functional groups
Step Two:
Compare withstandard spectra
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Quantitative Analysis
IR more difficult than UV-VIS
complex spectra
weak incident beam low sensitivity
solvent absorption
IR mostly used for rapid qualitative but notquantitative analysis