Spectroscopic Analysis Part 4 – Molecular Energy Levels and IR Spectroscopy

1. 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

2. E2 E2 DE = hn DE = hn E1 E2 Intensity Intensity n n Frequency Frequency Absorbance Emission

3. 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

4. Excited Electronic State Vibrational Energy Levels Rotational Energy Levels Ground Electronic State Molecular Energy Levels

5. 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

6. 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

7. 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

8. Rotational Energy of a Diatomc Molecule

9. 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

10. 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

11. 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, …

12. 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 -

13. 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.

14. Vibrational Energy of Diatomic Molecules Deviations in the energy profile of a real molecule undergoing anharmonic vibration.

15. 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

16. IR Spectrum of Carbon Monoxide (CO) Fundamental Peak First Overtone

17. 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)

18. 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

19. Vibrational Spectrum of Carbon Dioxide Asymmetric stretching results in a change in dipole moment during vibration and produces a peak in the IR spectrum.

20. Vibrational Spectrum of Carbon Dioxide The bending mode of vibration gives a peak in the IR spectrum

21. Vibrational Spectrum of Carbon Dioxide Two fundamental peaks are expected plus overtones, combination and difference bands

22. Vibrational Modes for Water

23. Fundamental IR Bands for Water

24. 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

25. 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.