31_580ln_fa08

31_580ln_fa08 - MIT OpenCourseWare http:/ocw.mit.edu 5.80...

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MIT OpenCourseWare http://ocw.mit.edu 5.80 Small-Molecule Spectroscopy and Dynamics Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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5.80 Lecture #3 1 Fall, 2008 Page 1 of 11 pages Lecture #3 1 : Electronic Spectra of Polyatomic Molecules Electronic spectra are more complex than vibration-rotation spectra. Polyatomic molecule spectra are more complex than diatomic molecule spectra * Many vibrational properties (such as Franck-Condon factors) can’t be calculated because we know too little about V Q V ′′ Q ′′ ( ) and ( ) . QUALITATIVE IDEAS LIKE NODE COUNT STILL WORK! * Inter-surface interactions are more subtle in 3N–6 dimensions than in 1 dimension (avoided crossings along high symmetry directions). * Non-radiative processes (other than predissociation and autoionization) cause spectral lines to broaden and fluorescence quantum yields to approach zero. E.g. Bixon and Jortner J. Chem. Phys. 48 , 715 (1968). This subject is so complex that only a case-by-case approach seems feasible. Some crucial background information: 1. Electronic States electronic configurations MO’s. symmetry labels qualitative effect of each occupied orbital on favored molecular shape (i.e. R AB ’s and k AB ’s and θ ABC ’s) Qualitative MO Theory: Walsh diagrams localization of orbitals onto “chromophores” concept of chromophores is an enormous simplification because: * its orbitals and electronic properties are known from other molecules **** large changes in molecular shape are confined to the region of the chromophore. 2. Electronic Transitions A single orbital promotion, e.g. π * n, gives direction of transition moment relative to body fixed a ˆ , b ˆ , c ˆ axes rotational selection rules direct product Γ φ i Γ φ f Often mixed transition type when chromophore does not lie || or to a principal axis. E.g. HCO B 2 A ′ − X 2 A ( µ AB in plane of molecule) Often axis-switching effects when a ˆ , b ˆ , c ˆ axes are not oriented identically in both electronic states because of a large change in geometry e.g. A 1 A u X 1 + g π * π in HCCH
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5.80 Lecture #3 1 3. 4. Fall, 2008 Page 2 of 11 pages Vibrational Structure Symmetry selection rules for vibrational bands in an allowed electronic transition Γ V ⊗ Γ V ⊂ Γ totally symmetric allowed to see: all v of totally symmetric mode alternate (even) v’s of non totally symmetric modes except in combination with other non-totally symmetric modes FRANCK-CONDON propensity rules v = 0 for all normal modes that are unchanged by φ′←φ″ electronic transition. Long progressions in modes exhibiting large change in
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31_580ln_fa08 - MIT OpenCourseWare http:/ocw.mit.edu 5.80...

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