CHEM 515 Spectroscopy Vibrational Spectroscopy I

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Transcript of CHEM 515 Spectroscopy Vibrational Spectroscopy I

  • Slide 1
  • CHEM 515 Spectroscopy Vibrational Spectroscopy I
  • Slide 2
  • 2 Rotational, Vibrational and Electronic Levels
  • Slide 3
  • 3 Molecular Vibrations of CO 2
  • Slide 4
  • 4 Harmonic Oscillator Approximation Selection rule v = 1
  • Slide 5
  • 5 Harmonic Oscillator Approximation At lower energies, the harmonic oscillator model determines the quantum levels quite well. Deviations become more significant at higher energy levels.
  • Slide 6
  • 6 Types of Potential Function Curves VV RR DissociatveNon-dissociatve 1330 cm -1 667 cm -1
  • Slide 7
  • 7 Force Constant The force constant is a measure of the strength of the spring (or chemical bond) connecting two particles. The force constants is proportional to the bond order.
  • Slide 8
  • 8 Anharmonicity Deviations due to anharmonicity become more clear at higher energy levels (v), and larger x = r r e values that correspond to dissociation.
  • Slide 9
  • 9 Anharmonicity Electrical anharmonicity: (electrical properties, dipole moment and polarizability). Mechanical anharmonicity: (nature of molecular vibration). Selection rule because of the effect of anharmonicity: v = 1, 2, 3,
  • Slide 10
  • 10 Types of Vibrational Transitions The intensity of v= 1 transitions is stronger than that for v= 2, 3, transitions. Both electrical and mechanical anharmonicity contribute to the intensities of v= 2, 3, transitions.
  • Slide 11
  • 11 Vibrational Spectrum of HCl (cm -1 ) v Vibrational spectrum of HCl is based on the harmonic oscillator model with e = 2989 cm -1.
  • Slide 12
  • 12 Vibrational Spectrum of HCl
  • Slide 13
  • 13 Vibrational Spectrum of HCl
  • Slide 14
  • 14 Vibrational Spectrum of HCl
  • Slide 15
  • 15 Morse Potential
  • Slide 16
  • 16 Morse Potential It is a better approximation for the vibrational structure of the molecule than the quantum harmonic oscillator because it explicitly includes the effects of bond breaking, such as the existence of unbound states.
  • Slide 17
  • 17 Morse Potential It also accounts for the anharmonicity of real bonds and the non-zero transition probability for overtones and combinations.
  • Slide 18
  • 18 Morse Potential Morse function is not well behaved where r 0 or x r e. Although V(x) becomes large but is doesnt go to infinity.
  • Slide 19
  • 19 Dissociation Energy from Spectroscopic Data
  • Slide 20
  • 20 Birge-Sponer Diagram
  • Slide 21
  • 21 Birge-Sponer Diagram
  • Slide 22
  • 22 Vibration-Rotation Spectra Energy increases
  • Slide 23
  • 23 Vibration-Rotation Spectra Infrared spectrum J = 1 Raman spectrum J = 0, 2
  • Slide 24
  • 24 Vibration-Rotation Infrared Spectrum of HCl vib is different for H 35 Cl and H 37 Cl molecules due to the slight difference in their reduced masses. au
  • Slide 25
  • 25 Vibration-Rotation Infrared Spectrum of HCl The lines due to H 35 Cl transitions are more intense because the isotopic abundance ration of H 35 Cl to H 37 Cl molecules is 3:1.
  • Slide 26
  • 26 Vibration-Rotation Infrared Spectrum of HCl Band center H 35 Cl Band center H 37 Cl
  • Slide 27
  • 27 Vibration-Rotation Infrared Spectrum of HCl The rotational constant B slightly decreases as going to higher vibrational levels. This results in decrease of the gaps between transition lines as one goes to higher frequencies.
  • Slide 28
  • 28 Vibration-Rotation Infrared Spectrum of HCl The rotational constant B slightly decreases as going to higher vibrational levels. This results in decrease of the gaps between transition lines as one goes to higher frequencies.
  • Slide 29
  • 29 Vibration-Rotation Infrared Spectrum of HCl Approximation of B values
  • Slide 30
  • 30 Vib-Rot Infrared Spectrum of Nitric Oxide Exceptions to the infrared J 0 selection rule are found for some diatomic molecules such as NO. Q-branch P-branch R-branch
  • Slide 31
  • 31 Vib-Rot Infrared Spectrum of the DCl Molecule vib (HCl) > vib (DCl) because of the differences in force constants and reduced massed between the two molecules. B 0 = 5.392263 cm -1 B 1 = 5.279890 cm -1
  • Slide 32
  • 32 Raman Stokes and Anti-Stokes Transitions v v v
  • Slide 33
  • 33 Rot-Vib Raman Spectrum of Carbon Oxide Selection rule for Raman transitions in diatomic molecules is J = 0, 2.
  • Slide 34
  • 34 Gross Selection Rule of Infrared Vibrational Spectroscopy The gross selection rule for infrared vibrational spectroscopy states that electric dipole moment of the molecule must change when the atoms are displaced. The molecule need NOT to have permanent dipole moment in order to be infrared active.