Transcript Document

Spectroscopic Analysis
Part 4 – Molecular Energy Levels
and IR Spectroscopy
Chulalongkorn University, Bangkok, Thailand January 2012
Dr Ron Beckett
Water Studies Centre & School of Chemistry
Monash University, Melbourne, Australia
Email: [email protected]
Water
Studies
Centre
1
Absorbance
DE = hn
Emission
E2
E2
E2
E1
DE = hn
Intensity
Intensity
n
Frequency
n
Frequency
2
Molecular Energy Levels
Molecules can have the following types of energy
Kinetic (due to motion)
Electronic (PE and KE of electrons)
Vibrational (oscillation of atoms in bonds)
Rotational
All except the KE are quantized
Emolecule = Erotational + Evibrational + Eelectronic
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Molecular Energy Levels
Excited
Electronic
State
Rotational
Energy Levels
Vibrational
Energy Levels
Ground
Electronic
State
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Molecular Energy Levels
The relative energy of the spacings between energy
levels for various types of transitions in a molecule are
in the order:
Rotational
Transition
1-20 cm-1
<<
Vibrational
Transition
2000-4000 cm-1
<<
Electronic
Transition
10000-50000 cm-1
Thus the various types of energy transitions occur in
different regions of the EMR spectrum and do not
overlap
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Molecular Energy Levels
Radiation can be absorbed or emitted if the molecule
changes any of its energy states
Excited
Electronic
State
Vibrational
Energy Levels
Rotational
Energy Levels
Ground
Electronic
State
Rotational
Transition
Vibrational
Transition
Electronic
Transition
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Molecular Energy Levels
Excited
Electronic
State
Vibrational
Energy Levels
Rotational
Energy Levels
Ground
Electronic
State
Rotational
Transition
Vibrational
Transition
Electronic
Transition
1-20 cm-1
2000-4000 cm-1
10000-50000 cm-1
Microwave
Infrared
UV-Visible
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Rotational Energy of a Diatomc Molecule
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Rotational Energy of a Diatomc Molecule
• Rotational energy is quantized
E = J(J + 1)B J=0,1,2,...
• EMR will only be absorbed by polar molecules
e.g. HCl & CO absorb EMR but not H2 and N2
• The electrical molecular dipole interacts with the
fluctuating electric field of the EMR wave
• Only certain transition are allowed
12B
DJ = 1
Rotational Microwave Spectrum
6B
2B
0
2B
4B
6B
?
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Vibrational Energy of Diatomic Molecules
• The bonds between atoms behave like springs
• The atoms vibrate approximately like an harmonic
oscillator obeying Hooke’s Law:
F = -k(r – req)
k is the force constant
EPE = ½k(r – req)2
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Vibrational Energy of Diatomic Molecules
Exchange of PE and KE
during vibration
Allowed vibrational
energy levels
Evib = (v + ½)hw0 J
V = 0, 1, 2, …
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Vibrational Energy of Diatomic Molecules
Allowed vibrational
energy levels
- cm-1
Evib = (v + ½)w
0
V = 0, 1, 2, …
Allowed transitions
Dv = 1
Thus expect only one
vibrational peak in
the IR spectrum
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Vibrational Spectrum of Diatomic Molecules
Interaction between EMR and the vibrational energy of
molecules can only occur if the bond is polar and a
change of dipole moment occurs during oscillation.
Thus only polar bonds generate peaks in the infrared
spectrum of molecules.
Thus HCl, CO and HF absorb EMR and have an IR
spectrum but H2 and N2 do not.
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Vibrational Energy of Diatomic Molecules
Deviations in the energy
profile of a real molecule
undergoing anharmonic
vibration.
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Vibrational Energy of Diatomic Molecules
Additional allowed
transitions and peaks
for a real molecule.
The first peak is called
the fundamental and
the additional peaks
are the overtones
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IR Spectrum of Carbon Monoxide (CO)
Fundamental
Peak
First
Overtone
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Fundamental vibration peak in the IR spectrum and
the force constants for some diatomic molecules
Note the expected correlation with k and m (refer to equations)
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Vibrational Spectrum of Carbon Dioxide
CO2 molecule
This stretching mode results in no peak because the dipole
moment is zero does not change during vibration
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Vibrational Spectrum of Carbon Dioxide
Asymmetric stretching results in a change in dipole moment
during vibration and produces a peak in the IR spectrum.
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Vibrational Spectrum of Carbon Dioxide
The bending mode of vibration gives a peak in the IR spectrum
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Vibrational Spectrum of Carbon Dioxide
Two fundamental peaks
are expected plus
overtones, combination
and difference bands
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Vibrational Modes for Water
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Fundamental IR Bands for Water
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IR Spectrum of Complex Molecules
There are many possible vibrational modes giving rise
to complicated spectra with many peaks.
IR spectra are mainly used to identify unknown
compounds
Peak positions can demonstrate what functional
groups are present in the molecule.
The peak positions and intensities of an unknown can
be compared with the spectrum of known suspects
in the same manner that police use fingerprints
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IR Spectrum of Complex Molecules
Two types of vibrational modes are possible:
1. Skeletal vibrations where all the atoms in the molecule move
about to some extent.
These vibrations give rise to absorption peaks in the range
700 – 1400 cm-1 which is called the fingerprint region.
2. Functional group vibrations in which only the atoms in that
functional group vibrate appreciably.
Each functional group gives rise to an absorption peak at a
characteristic frequency, no matter what the rest of the
molecule contains. These peaks can be used to identify the
functional groups present in the molecules.
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