1 / 27

Spectroscopy 1: Rotational and Vibrational Spectra CHAPTER 13

Spectroscopy 1: Rotational and Vibrational Spectra CHAPTER 13. Vibrations of Diatomic Molecules. Gross selection rule : Electric dipole moment of molecule must change when atoms are displaced relative to each other. Specific selection rule : Δ v = ±1.

Download Presentation

Spectroscopy 1: Rotational and Vibrational Spectra CHAPTER 13

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Spectroscopy 1: Rotational and Vibrational Spectra CHAPTER 13

  2. Vibrations of Diatomic Molecules • Gross selection rule: Electric dipole moment of • molecule must change when atoms • are displaced relative to each other. • Specific selection rule: Δv = ±1

  3. Fig 13.34 High resolution vibration-rotation spectrum of HCl for a v + 1 ← v transition ΔJ =0 ΔJ =-1 ΔJ =+1 Combined vib-rot terms, S: S(v, J) = G(v) + F(J) = (v+½) ṽ + BJ(J+1)

  4. Vibrational Raman Spectra of Diatomic Molecules • Gross selection rule: Polarizability should change • as molecule vibrates • Specific selection rule: ΔJ = 0, ±2

  5. Fig 13.37 Formation of O, Q, and S branches in vib-rot Raman spectrum ΔJ =0 ΔJ =-2 ΔJ =+2

  6. Fig 13.37 Relative intensities in O, Q, and S branches of a Raman vib-rot spectrum ΔJ =-2 ΔJ =0 ΔJ =+2

  7. Fig 13.38 Structure of a vibrational line in vib-Raman spectrum of CO

  8. Table 13.2 Properties of diatomic molecules

  9. Vibrations of Polyatomic Molecules

  10. Vibrational Normal Modes • Approach: • Each atom in a molecule can be located • with three coordinates (degrees of freedom) • A molecule with N atoms then has 3N DOF • Translational motion defined by center-of-mass coordinates (COM)

  11. Linear Molecules • 3 DOF to define translation • 2 DOF to define rotation • 3N – 5 ≡ number of vibrational modes • Nonlinear Molecules • 3 DOF to define translation • 3 DOF to define rotation • 3N – 6 ≡ number of vibrational modes

  12. Examples N2 H20 CO2

  13. Fig 13.40(a) Description of the vibrations of CO2 using νL and νR. Stretching modes are not independent

  14. Fig 13.40(b) Alternative description of the vibrations of CO2 using linear combination of νL and νR. Symmetric and asymmetric stretching modes are independent and therefore are normal modes

  15. Fig 13.40(c) Alternative description of the vibrations of CO2 using linear combination of νL and νR. The two scissoring modes are also normal modes

  16. Fig 13.41 The three normal modes of H2O

  17. Vibrations of Polyatomic Molecules • Gross selection rule: Motion corresponding to a • normal mode (q) should be accompanied by a • change in dipole moment • e.g., IR-inactive • IR-active • Specific selection rule: Δvq = ±1 • In condensed phases, the rotational structure • is always blurred due to random collisions

  18. Vibrations of CO2 } No dipole change Dipole change Dipole change

  19. Vibrations of H2O

  20. Fig 13.42 Intensity of IR radiation lost from earth: In absence of greenhouse gases N2 and O2 are not IR active In presence of greenhouse gases

  21. Vibrational Raman spectra of polyatomic molecules IR active? Yes, if electric dipole moment changes. Raman active? Yes, if polarizability changes. • Exclusion rule: • If a molecule has a center of symmetry, • then no modes can be both IR and Raman active. • A mode may be inactive in both

  22. Examples Raman active? molecule IR active? N2 no yes CO yes yes yes all modes H2O yes yes for ν2 and ν3 yes for ν1 CO2 no for ν2 and ν3 no for ν1

  23. Vibrational resonance Raman spectra • Use incident radiation that nearly coincides • with the frequency of an electronic transition

  24. Fig 13.45 Conventional vs. resonance Raman spectroscopy Virtual states Real states

  25. Vibrational resonance Raman spectra • Use incident radiation that nearly coincides • with the frequency of an electronic transition • Characterized by much greater scattering intensity • Because only a few modes contribute to scattering, • spectrum is simplified • Used to study biological molecules that absorb • strongly in the UV-vis

  26. Fig 13.46 Resonance Raman spectra of a protein complex responsible for e– transfer in photosynthesis chlorophyll and β-carotene • Laser excitation spectrum • at 407 nm β-carotene • Laser excitation spectrum • at 488 nm

More Related