Electronic spectroscopy: Electronic transitions UV/VIS transitions - - PowerPoint PPT Presentation

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Electronic spectroscopy: Electronic transitions UV/VIS transitions - - PowerPoint PPT Presentation

Oct 28, 2022 1.97k likes •2.15k views

Electronic spectroscopy: Electronic transitions UV/VIS transitions between discrete electronic states. Electrons promoted from ground state to excited state. C---O e- excited e-C Oe- change bond order C---O vib ->

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SLIDE 1

Electronic spectroscopy:

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SLIDE 2

Electronic transitions

CEM 484 Molecular Spectroscopy

UV/VIS transitions between discrete electronic states.

Electrons promoted from ground state to excited state.

C---O e- excited e-C—Oe- change bond order

C---O vib -> vibrate faster

C---O rot -> rotate faster

Rotational and vibrational transitions accompany electronic transitions.

Difficult to resolve all transitions

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SLIDE 3

Energy level diagram

CEM 484 Molecular Spectroscopy 3

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SLIDE 4

Vibronic transitions

CEM 484 Molecular Spectroscopy

Total energy need to include terms from al three transitions.

Etotal/hc = ñelec + G(n) + F(J)

Etotal/hc = ñelec + (n+1/2) ñe – (n+1/2)2x̃eñe + B̃n[J(J+1)] – D̃J2(J+1)2

Rotational and vibrational terms are comparatively small and not easily resolved.

Ignoring rotations an expression for excitations between excited state

ñobs = E1,n’ – Eg,n’’=0 / hc = ṽ’e1 + (n’+1/2) ñ’e – (n’+1/2)2x̃’eñ’e - {ṽ’’e0 + ñ’’e/2 – x̃’’eñ’’e/4 }

Define: n0,0 = (ñelec1 – ñelec0 ) + ñ’e/2 – x'̃eñ’e/4 – (ñ’’e/2 – x̃’’eñ’’e/4 )

ñobs = n0,0 + n’ ñ’e – n’(n’+1)x̃’eñ’e

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SLIDE 5

Vibronic transitions

CEM 484 Molecular Spectroscopy

Electronic energy spectrum of I2.

Higher energy transitions become harder to resolve.

Vibronic progression

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SLIDE 6

Dissociation energy

CEM 484 Molecular Spectroscopy

Do is dissociation energy.

Measured from the first vibrational state

Ground state is not at bottom of potential well.

D̃o = -Etotal/hc – D̃e – ñe/2 + x̃eñe/4

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SLIDE 7

Dissociation energies: Example

The fundamental line in the infrared spectrum of 12C16O

  • ccurs at 2143.0 cm-1, and the first overtone is found at

4260 cm-1. Calculate the values of νe and νexe for 12C16O.

CEM 484 Molecular Spectroscopy 7

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SLIDE 8

Iclicker: Disociation energies

The energy difference between two arbitrary levels n and n+1 is

DG = ṽe (1 – 2x̃e(n + 1) )

The dissociation energy can be written as

D̃e = ṽe(1-x̃e

2)/4x̃e ≈

ṽe/4x̃e

Using the graph on right, estimate the dissociation energy for H2.

CEM 484 Molecular Spectroscopy y = -228.63x + 4154.7 R² = 0.9999 2500 2700 2900 3100 3300 3500 3700 3900 4100 1 2 3 4 5 6 7 DG (cm-1) v+1 8

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SLIDE 9

Electronic excitation intensity pattern

CEM 484 Molecular Spectroscopy

Intensity pattern of electronic transitions reveals molecular structure.

Franck-Condon principle

Nuclei do not move appreciably during electron excitation

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SLIDE 10

Iclicker: Label the curve

Label the energy curve with:

Vibration and rotational quantum numbers.

The zero point energy for the ground electronic state.

The fundamental vibrational frequency of the ground electronic state.

The first overtone of the second electronic state.

The dissociation energy of the second electronic state.

The P-branch transition from the J=1, n=0 level in the ground electronic state.

The R-branch transition from the J=1, n=0 level in the ground electronic state.

CEM 484 Molecular Spectroscopy 10