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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: Ron.Beckett@monash.edu. Water Studies Centre. E 2.

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slide1

Spectroscopic AnalysisPart 4 – Molecular Energy Levelsand IR Spectroscopy

Chulalongkorn University, Bangkok, Thailand January 2012

Dr Ron Beckett

Water Studies Centre & School of ChemistryMonash University, Melbourne, Australia

Email: Ron.Beckett@monash.edu

Water

Studies

Centre

slide2

E2

E2

DE = hn

DE = hn

E1

E2

Intensity

Intensity

n

n

Frequency

Frequency

Absorbance

Emission

molecular energy levels
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
molecular energy levels1

Excited Electronic State

Vibrational Energy Levels

Rotational Energy Levels

Ground Electronic State

Molecular Energy Levels
molecular energy levels2
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

molecular energy levels3

Excited Electronic State

Vibrational Energy Levels

Rotational Energy Levels

Ground Electronic State

Rotational Transition

Vibrational Transition

Electronic Transition

Molecular Energy Levels

Radiation can be absorbed or emitted if the molecule changes any of its energy states

molecular energy levels4

Excited Electronic State

Vibrational Energy Levels

Rotational Energy Levels

Ground Electronic State

Molecular Energy Levels

Rotational Transition

1-20 cm-1

Microwave

Vibrational Transition

2000-4000 cm-1

Infrared

Electronic Transition

10000-50000 cm-1

UV-Visible

rotational energy of a diatomc molecule

12B

6B

2B

4B

6B

?

2B

0

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 DJ = 1

Rotational Microwave Spectrum

vibrational energy of diatomic molecules
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
slide11

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, …

slide12

Vibrational Energy of Diatomic Molecules

Allowed vibrational energy levels

Evib = (v + ½)w0 cm-1

V = 0, 1, 2, …

Allowed transitions

Dv = 1

Thus expect only one vibrational peak in the IR spectrum

-

vibrational spectrum of diatomic molecules
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.

slide14

Vibrational Energy of Diatomic Molecules

Deviations in the energy profile of a real molecule undergoing anharmonic vibration.

slide15

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

slide16

IR Spectrum of Carbon Monoxide (CO)

Fundamental Peak

First Overtone

slide17

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)

slide18

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

slide19

Vibrational Spectrum of Carbon Dioxide

Asymmetric stretching results in a change in dipole moment during vibration and produces a peak in the IR spectrum.

slide20

Vibrational Spectrum of Carbon Dioxide

The bending mode of vibration gives a peak in the IR spectrum

slide21

Vibrational Spectrum of Carbon Dioxide

Two fundamental peaks are expected plus overtones, combination and difference bands

ir spectrum of complex molecules
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

ir spectrum of complex molecules1
IR Spectrum of Complex Molecules
  • Two types of vibrational modes are possible:
  • 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.
  • 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.