VBTutorial2

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VB applications on PI systems

To the Tutors

Sason remarks and prospective 2 hours talk +

Philippe's remark on the initially proposed tutorial. are included in bold.

Qualitative

• Exercices from The Book ... >PCH< (30')

Computational

• Allyl cation : VB 3 configuration (3rd configuration has a large weight).
• Benzene : question about basis of covelent structures (paper exercise, then xmvb zith str=cov), then all ionics.
• Ozone : paper exercise ; expand MO wf in VB basis, wieghts of biradical with HF, compute with XMVB (VBSCF, BOVB)
• Benzyl radical with most spin alternant determinants (2 determinants) (p228 of The Book) show spin location. Objection : this uses Heisenberg spin hamiltonian theory, a topic that we do not teach.

(trash: * O2 paradigm : compute singlet - triplet gap : too hard

• H2O lone pairs : compute H2O+ states (2 configurations mixing) Objection : did you try to do that ? If we let the orbitals optimize themselves, I guess we will converge to the MO solution with weight 1.0, the other structure with weigt 0.0. The H2O+ states would better be done as qualitative exercises.)

Exercices

Remark : in all the following exercises, <math>\pi</math> the system will be taken as active, and the <math>\sigma</math> system as inactive. In all VB calculations, the <math>\sigma</math> orbitals shall be described by MOs delocalized onto the whole molecule.

Exercise 1 : The allyl radical and cation

Subject

Here is a image example

Image example: Allyl Cation MO's

Th

Access to files :

title title

Exercise 2 : Radical character of ozone

1. Paper exercice :
   1. Propose a complete basis of non-redondant VB structures for the ozone molecule. Based on your chemical knowledge, propose a selection subset of the most chemically meaningful structures.
   2. Use the HuLis software to retrieve the Hückel MOs for ozone. Write a single-determinant MO wave function based on Hückel orbitals. Develop it into the basis of atomic orbitals, to get an expression in terms of VB structures.
   3. Compute by hand the weights of the different structures (neglecting all overlaps for simplicity). What is the radical character of ozone according to simple MO theory ?
2. Computer exercise :
   1. Compute a VBSCF wave function for ozone (6-31G* basis set) using your selected set of structures (question 1.1). Compute a VBSCF wave function including the complete set of VB structures, using the "str=all" keyword. Compare the weights and energies for both wave functions to validate your selection of structures.
   2. Compute a BOVB wave function for ozone which include only your selected set of most chemically meaningful structures (use the guess orbitals obtained at the VBSCF level).
   3. Compare the weights obtained at the VBSCF and BOVB level. Compare the BOVB diradical weight to the value predicted by simple MO theory.

>> Answer

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