Spectroscopic AnalysisPart 4 – Molecular Energy Levels

and IR Spectroscopy

Chulalongkorn University, Bangkok, Thailand January 2012

Dr Ron Beckett

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

Email: Ron.Beckett@monash.eduWaterStudiesCentre

1

E2

E2

E = hE2

E1

E = h

Frequency

Intensity

Frequency

Intensity

Absorbance Emission

2

Molecular Energy Levels

Molecules can have the following types of energyKinetic (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

3

Molecular Energy Levels

Rotational Energy Levels

Vibrational Energy Levels

Ground Electronic State

Excited Electronic State

4

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

5

Molecular Energy Levels

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

Rotational Energy Levels

Vibrational Energy Levels

Ground Electronic State

Excited Electronic State

Rotational Transition

Vibrational Transition

Electronic Transition 6

Molecular Energy Levels

Rotational Energy Levels

Vibrational Energy Levels

Ground Electronic State

Excited Electronic State

Rotational Transition

1-20 cm-1

Microwave

Vibrational Transition

2000-4000 cm-1

Infrared

Electronic Transition

10000-50000 cm-1

UV-Visible 7

Rotational Energy of a Diatomc Molecule

8

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 moleculese.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 J = 1

12B

6B

2B0

2B 4B 6B ?

Rotational Microwave Spectrum

9

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

10

Exchange of PE and KE during vibration

Allowed vibrational energy levels

Evib = (v + ½)h0 J

V = 0, 1, 2, …

Vibrational Energy of Diatomic Molecules

11

Vibrational Energy of Diatomic Molecules

Allowed vibrational energy levels

Evib = (v + ½)0 cm-1

V = 0, 1, 2, …

Allowed transitions

v = 1

Thus expect only one vibrational peak in the IR spectrum

-

12

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.

13

Vibrational Energy of Diatomic Molecules

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

14

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

15

IR Spectrum of Carbon Monoxide (CO)

Fundamental Peak

First Overtone

16

Fundamental vibration peak in the IR spectrum and the force constants for some diatomic

moleculesNote the expected correlation with k and (refer to equations)

17

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 18

Vibrational Spectrum of Carbon Dioxide

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

19

Vibrational Spectrum of Carbon DioxideThe bending mode of vibration gives a peak in the IR spectrum

20

Vibrational Spectrum of Carbon Dioxide

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

21

Vibrational Modes for Water

22

Fundamental IR Bands for Water

23

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

24

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.

25

26

27