32_580ln_fa08

32_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 2 Fall, 2008 Page 1 of 9 pages Lecture #3 2 : The H 2 CO A ~ 1 A 2 X ~ 1 A 1 Transition H 2 CO was 1st (asymmetric top) polyatomic electronic transition to be rotationally analyzed. G. H. Dieke and G. B. Kistiakowsky Phys. Rev. 45 , 4 (1934). It is more complicated than linear HCCH because many values of the K a rotational quantum number can have significant thermal population in the V ′′ = 0 level. For a linear molecule in a -state, K a = 0, and = 0 in V =0. The S 1 S 0 transition in H 2 CO * is electronically forbidden in C 2v (a) [(x, y, z) = (c, b a)] * the excited state is expected and appears to be non planar - hence C 2v may not be relevant. * “quasi-planar” molecule – inversion barrier is low resulting in staggering of bending levels * 3 distinct transition mechanisms, each with its own selection rules, contribute to A ~ —X ~ system. Outline: i. classification of orbitals and normal modes ii. what do we expect (geometry and vibrational structure of S 1 ) iii. “vibronically” rather than electronically allowed system – false origin, promoter mod iv. surprise in hot band spectrum – peculiar spacings in upper state out-of-plane bend v. low barrier to inversion through planarity Next time: Vibronic Coupling (beyond Franck-Condon) Body fixed axis system: y * * 0 b a z x (c, b, a) = (x, y, z) specific correspondence inertial point group axes axis labels Molecular orbitals from atomic orbitals: IP CO 14.014eV IP H 13.595 H atom 1s orbitals lie above CO HOMO IP C 11.264 IP O 13.614 ∴σ 2s, π 2p are polarized toward O
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5.80 Lecture #3 2 C 2v E C 2 (z) σ v (xz) σ v (yz) A 1 1 1 1 1 A 2 1 1 –1 –1 B 1 1 –1 1 –1 B 2 1 –1 –1 1 Fall, 2008 Page 2 of 9 pages *** see p. 2002 *** T z = a “Report on Notation” R z z is axis of symmetry T x = c, R y T y = b, R x x to plane Figure 1: Correlation of the orbitals of planar H 2 XY to those of the united molecule Y 2 and to those of 2H + XY. The variable along the abscissa is the XH distance. Note that at the left, since Y 2 is homonuclear, the orbitals are σ g , u , π g , π u while at the right, since XY is heteronuclear, the g , u characteristic does not strictly apply. However, just as at the left, the orbitals 2s are mixtures (but not 50:50 mixtures) of the 2 s orbitals of X and Y and similarly for the other XY orbitals. The order of π (u) 2 p and σ (g) 2 p is reversed at the right compared to the left in accordance with the situation in CO as compared to O 2 (see Herzberg, Vol. I, p. 346). At the left, the splitting of π u and π g into b 2 ( π in-plane) and b 1 ( π plane) corresponds to breaking the cylindrical symmetry of the C + H + H = O united atom. σ
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This note was uploaded on 11/28/2011 for the course CHEM 5.74 taught by Professor Robertfield during the Spring '04 term at MIT.

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32_580ln_fa08 - MIT OpenCourseWare http:/ocw.mit.edu 5.80...

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