C566lecture24_000

C566lecture24_000 - C566 Master Lecture Notes Lecture 24...

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1 C566 Master Lecture Notes Lecture 24 Which vibrational modes are “active” in electronic spectra (active mode = progressions for that vibrational mode are observed in the spectrum) Bottom Line For totally symmetric vibrational modes , in an electronic transition,  = 0, 1, 2, For non-totally symmetric modes ,  = 0, 2, 4, … But, the fact that something is allowed does not mean you’ll see it! Overlap integrals for non-totally symmetric modes are generally very small (there Q for non-totally symmetric modes is ZERO unless the molecule in the different electronic states belongs to different point groups!, so the  = 0 FCF is close to 1, and the  = 2 will be very small), Analysis of Vibrational Structure in S 1 S 0 transitions Assign observed vibronic lines to quantum changes in particular vibrational modes. Learn about vibrational frequencies in S 1 state- back out information about bonding, relate to electronic structure change in S 1 S 0 transition Set up studies of excited state dynamics (how does energy flow in the excited state?) Tools available: Helpful to know vibrational frequencies on the ground state (S 0 ) o Raman spectroscopy o IR Single vibrational level fluorescence Supersonic jets- low temperatures simplify vibrational structure by depopulating excited vibrational levels in  . High-resolution band contours- distinguish between x , y , z , get vibrational symmetry, structural parameters.
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2 Easiest to look at “case studies.” Example: Oxalyl halide spectroscopy, J. Mol. Spec. 260 , 124 (2010). The g u A X A A 1 1 ~ ~ transition is studied in this paper. In this transition, electron density is shifted from an in-plane a g orbital to an out-of-plane a u orbital. (non-bonding orbital to a C-O antibonding orbital). Is this an allowed transition? A u = ( z ) so yes, it is allowed. There are five totally symmetric modes in this molecule, all of which could be active. This is the spectrum from the paper, pieced together. O H C C O H z 0 0 0 5 0 1 8 1 0 7 1 1 7 2 2 7 2 2 8 1 0 7 1 1 8 1 0 4 0 1
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3 Character table for D 6h point group E 2C 6 2C 3 C 2 3C' 2 3C'' 2 i 2S 3 2S 6 σ h 3 σ d 3 σ v Linear, rotations Quadratic A 1g 1 1 1 1 1 1 1 1 1 1 1 1 x 2 +y 2 , z 2 A 2g 1 1 1 1 -1 -1 1 1 1 1 -1 -1 R z B 1g 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 B 2g 1 -1 1 -1 -1 1 1 -1 1 -1 -1 1 E 1g 2 1 -1 -2 0 0 2 1 -1 -2 0 0 (R x , R y ) (xz, yz) E 2g 2 -1 -1 2 0 0 2 -1 -1 2 0 0 (x 2 -y 2 , xy) A 1u 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 A 2u 1 1 1 1 -1 -1 -1 -1 -1 -1 1 1 z B 1u 1 -1 1 -1 1 -1 -1
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This note was uploaded on 01/18/2012 for the course C 566 taught by Professor Carolinechickjarroll during the Spring '11 term at Indiana.

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C566lecture24_000 - C566 Master Lecture Notes Lecture 24...

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