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 Water Studies Centre 1

E2E2 E2E2  E = h E2E2 E1E1 Frequency Intensity Frequency Intensity Absorbance Emission 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 E molecule = E rotational + E vibrational + E electronic 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 cm -1 Electronic Transition 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 cm -1 Infrared Electronic Transition cm -1 UV-Visible 7

Rotational Energy of a Diatomc Molecule 8

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 H 2 and N 2 The electrical molecular dipole interacts with the fluctuating electric field of the EMR wave Only certain transition are allowed  J = 1 12B 6B 2B 0 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 – r eq ) k is the force constant E PE = ½k(r – r eq ) 2  10

Exchange of PE and KE during vibration Allowed vibrational energy levels E vib = (v + ½)h  0 J V = 0, 1, 2, … Vibrational Energy of Diatomic Molecules 11

Vibrational Energy of Diatomic Molecules Allowed vibrational energy levels E vib = (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 H 2 and N 2 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 molecules Note the expected correlation with k and  (refer to equations) 17

Vibrational Spectrum of Carbon Dioxide CO 2 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 Dioxide The 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

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