Chapter 12 - Structure Determination: Mass Spectrometry And Infrared Spectroscopy Page 13
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44Overtone, combination and difference bands, and Fermi
resonance
Apart from fundamental absorptions due to excitation from ground state to the lowest-energy excited state,
the spectrum is usually complicated by the presence of weak overtone, combination, and difference bands.
Overtone Bands
Excitation from ground state to higher energy states, which corresponds integral multiples of the frequency
of the fundamental (). Weak overtone bands could occur at 2, 3, ….
Combination bands
and
Two vibrational frequencies (
) in a molecule couple to give rise to a new infrared active frequency.
1
2
=
+
This band is the sum of the two interacting bands (
).
combination
1
2
Difference bands
A new infrared active frequency arising due to the difference between the two interacting bands (
=
difference
-
).
1
2
Fermi resonance
When a fundamental vibration couples with an overtone or combination band, the coupled vibration is called
Fermi resonance. Fermi resonance is often observed in carbonyl compounds.
These additional weak bands can be calculated directly by multiplication, addition, and subtraction
processes.
13
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