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*1/ With the BLW code calculate the relative energies of the three Lewis structures of the allyl cation at the HF level. By comparison with the energy of the allyl cation, determine the VRE and the ARE. Compare the C-C bond distances.
 
*1/ With the BLW code calculate the relative energies of the three Lewis structures of the allyl cation at the HF level. By comparison with the energy of the allyl cation, determine the VRE and the ARE. Compare the C-C bond distances.
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{| class="collapsible collapsed wikitable"
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!'''Hints'''
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* Starting from the delocalized geometry, the first iteration of the optimization of the localized structure will give the VRE.
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*2/ Repeat the first question at the B3LYP level.
 
*2/ Repeat the first question at the B3LYP level.
  
 
*3/ Repeat questions 1 and 2 for the allyl radical.
 
*3/ Repeat questions 1 and 2 for the allyl radical.
 
*4/ With XMVB, for the allyl cation put in resonance  structures '''I''' and '''II''', then add  structure '''III'''. What is the gain in energy due to the inclusion of this third structure?
 
  
 
[[VBFile 4-2 | FILES FOR THE ALLYLS]]
 
[[VBFile 4-2 | FILES FOR THE ALLYLS]]
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  don't forget to name the cube file.   
 
  don't forget to name the cube file.   
 
  '''test.cube_BLW'''  
 
  '''test.cube_BLW'''  
  see also [[VBFile_4-4#gaussiancube.com | gaussiancube file]]
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  see also [[VBFile_4-3#gaussiancube.com | gaussiancube file]]
  
 
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[[VBFile 4-4 | FILES FOR THE NH3 ... BH3]]
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[[VBFile 4-3 | FILES FOR THE NH3 ... BH3]]
  
 
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== Exercise 4 (Butadiene deconjugation without hyperconjugation) ==
 
== Exercise 4 (Butadiene deconjugation without hyperconjugation) ==
 
[[File:Rotated_butadiene.png|right|150px|alt=butadiene - Lewis alt text | planar butadiene ]]
 
[[File:Rotated_butadiene.png|right|150px|alt=butadiene - Lewis alt text | planar butadiene ]]
[[File:planar_butadiene.png|right|150px|alt=butadiene - Lewis alt text | planar butadiene ]]
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[[File:planar_butadiene.png|right|200px|alt=butadiene - Lewis alt text | planar butadiene ]]
 
Examine the conjugation in planar butadiene and the hyperconjugation in perpendicular butadiene, and explain the rotational barrier.
 
Examine the conjugation in planar butadiene and the hyperconjugation in perpendicular butadiene, and explain the rotational barrier.
  
 
Note that often people rotate one participating group to disable the conjugation and use the barrier to measure the conjugation energy. What is the inconvenience of this approach?  
 
Note that often people rotate one participating group to disable the conjugation and use the barrier to measure the conjugation energy. What is the inconvenience of this approach?  
  
BLW shall be used
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* to compute the deconjugated planar form, and compare to the conjugated form at the same planar geometry.
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* Compute the conjugated planar form with a standard B3LYP/6-31G(d) calculation
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* Using BLW, localize the pi electrons on C1=C2 and on C3=C4 double bonds. (view the geometrie to verify that the pi system is along the X axis.
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* Compare the energies to calculate the conjugaison energy.
  
 
{| class="collapsible collapsed wikitable"
 
{| class="collapsible collapsed wikitable"
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  H    1.0  0.0000000000  0.3149214640  -2.3230051413
 
  H    1.0  0.0000000000  0.3149214640  -2.3230051413
 
</pre></html>  
 
</pre></html>  
* to inhibit at will the hyperconjugaison in the perpendicular form, hence to mesure its stabilizing  contribution to the rotation barrier.
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* Use the perpendicular form given below to compute the "deconjugated" system. The comparairison with the planar standard calculation gives an estimate of the conjugaison, which is contaminated by some hyperconjugaison.
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* To inhibit hyperconjugaison in the perpendicular form, localize the electrons on the C1=C2 and on C3=C4 double bond. (note that the C3=C4 vinyl group has rotated along the XZ plane; hence its pi system is along the Y axis.
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{| class="collapsible collapsed wikitable"
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!'''Twisted (perpendicular) geometry'''
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<html><pre>
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C    6.0  0.000000  -1.086858    2.236154
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C    6.0  0.000000    0.000000    1.489418
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C    6.0  0.000000    0.000000    0.000000
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C    6.0  -1.086858    0.000000  -0.746736
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H    1.0  0.967527    0.000000  -0.478451
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H    1.0  0.000000    0.967527    1.967869
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H    1.0  -2.072275    0.000000  -0.314213
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H    1.0  -1.029400    0.000000  -1.820913
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H    1.0  0.000000  -2.072275    1.803631
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H    1.0  0.000000  -1.029400    3.310331
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$END
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</pre></html>
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[[File:Rotated_butadiene.png|right|250px|alt=butadiene - Lewis alt text | planar butadiene The C3=C4 has rotated along the XZ plane; hence its pi system is along the Y axis.]]
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[[VBFile 4-4 | FILES FOR THE BUTADIENE DECONJUGAISON ]]
 
[[VBFile 4-4 | FILES FOR THE BUTADIENE DECONJUGAISON ]]
  
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<math> \Psi_{HL-CI}=0.81\Psi_{I}+0.58\Psi_{II}</math>, hence the weights of the structures (I/II)=(66%/34%)  
 
<math> \Psi_{HL-CI}=0.81\Psi_{I}+0.58\Psi_{II}</math>, hence the weights of the structures (I/II)=(66%/34%)  
  
Note that in HL-CI <math><Psi_{I}|\Psi_{II}>=0</math>
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Note that in HL-CI <math><\Psi_{I}|\Psi_{II}>=0</math>
 
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  {| class="collapsible collapsed wikitable"
 
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Dernière version du 14 février 2013 à 08:27

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