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VB applications on PI systems

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.

Exercises

Main Exercises

***** INPUT FILES TO BE WRITTEN *****

Exercise 1 : The allyl radical

Paper Exercise

1. Covalent structures of the allyl radical :
   1. What are the three possible covalent structures for the allyl radical molecule ? Use the thumb rules of qualitative VB theory to compute their energy, and show that two of them are degenerate.
   2. Show that the third structure can be expressed as a linear combination of the first two structures, and thus that only two of the three covalent structures form a complete basis of non-redundant structures (Rumer basis).
2. Understanding the pattern of spin density distributions in the allyl radical as found in EPR spectroscpy.
   1. Express the wave functions of the non-redundant structures (from 1.1) of allyl radical in terms of the VB determinant. Write the wave function of the ground state of the allyl radical as a negative combination of the wave functions of the non-redundant structures. Based on the expression of the spin density <math> \rho_s </math>
      <math> {\rho_k}^s = N^2 \sum_i {c_i}^2 {\delta_{ki}}</math>
      propose the spin density distribution in the allyl radical. In the above equation N is a normalization constant, c is the coefficient of the VB determinant in the wave function and <math> \delta </math> is either +1 or -1 depending on whether the electron which is located on atom k in the ith determinant is <math> \alpha </math> or <math> \beta </math> spin, respectively.
      
   2. Show pictorially:
      1. Why your spin density is polarized.
      2. What would be the spin density pattern in pentadienyl radical?
      3. What would be the spin density in the excited state of allyl radical (taking into account that resulting wave function is a positive combination of the <math> \Phi_L </math> and <math> \Phi_R </math> wave functions?
         
3. Covalent and ionic structures of the allyl radical :
   1. What are the possible ionic structures for the allyl radical ? Based on your chemical knowledge, propose a selection subset of the most chemically meaningful covalent + ionic structures.

Computer Exercise

1. Computation of covalent state of allyl radical
   1. Compute the VBSCF wave function for the covalent state of the allyl radical (6-31G basis set) using the covalent structures you have chosen (paper Ex. 1.2). What are the weights of the different VB structures. Was that expected?
      
   2. What is the wavefunction in determinant description? What is the spin population? Do the calculated results agree with your answer for paper Ex. 2.1?
   3. Repeat the calcultions for the first excited state using "nstate=1" . What is the the wavefunction? Use determinant description. What is the spin population in this case? Was that what you expected?
2. Computation of allyl radical energy and weights:
   1. Compute a VBSCF wave function for allyl radical (6-31G basis set) using your selected set of structures (questions 1.2 and 3.1). Compute a VBSCF wave function including the complete set of VB structures (the adiabatic wavefunction), using the "str=full" keyword. Compare the weights and energies for both wave functions to validate your selection of structures.
   2. Compute for allyl radical a BOVB wave function which includes only your selected set of most chemically meaningful structures (use the guess orbitals obtained at the VBSCF level by copying the vbscf_filename.orb file after VBSCF calculation to bovb_filename.gus file in addition remember that only the ISCF=2 option can be used for BOVB). Compare the weights obtained at the VBSCF and BOVB level.
3. Computation of resonance energies :
   1. We want to build a wave-function corresponding to only one Lewis (diabatic) structure for the allyl radical. To do so, we will include in the wave-function only one covalent structure, and the ionic structures associated with this covalent bond. Propose a selection of VB structures which would describe one Lewis structure for the allyl radical.
   2. Compute this wave function at VBSCF then BOVB levels. Deduce what is the resonance energy of the allyl radical at the BOVB level. The resonance energy is calculated as the difference between the adiabatic state and the Lewis (diabatic) state of the allyl radical.

>> Answer

Exercise 2 : Radical character of ozone

Computer Exercises

1. Propose a complete basis of non-redundant VB structures for the ozone molecule. Based on your chemical knowledge, propose a selection subset of the most chemically meaningful structures. (Paper exercise)
2. Compute a VBSCF wave function for ozone (6-31G basis set) using your selected set of structures. Compute a VBSCF wave function including the complete set of VB structures, using the "str=full" keyword. Compare the weights and energies for both wave functions to validate your selection of structures.
3. Compute a BOVB wave function for ozone which includes only your selected set of most chemically meaningful structures (use the guess orbitals obtained at the VBSCF level).
4. Compare the weights obtained at the VBSCF and BOVB level. Compare the BOVB diradical weight to the value predicted by simple MO theory. For your convenience the MO wave function is as follows
   <math>

\psi_{H\ddot{u}ckel} =

       25%\Phi_1 + 
       25%\Phi_2 + 12.5%\Phi_3 +
       25%\Phi_4 + 6.25%\Phi_5 + 
       6.25%\Phi_6

</math>
(consult the following paper exercise)

Paper Exercises - Optional/Homework

1. 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 (consult the hint given below).
2. 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?
   

Hint
- It is useful to use here the electron hole equivalence for details consult: >> The electron-hole equivalence

>> Answer

Additional Optional-Exercises/Homework

Exercise 3 : Resonance energy of Benzene ***** EXERCISE AND INPUT FILES TO BE WRITTEN ***** Computer Exercise Covalent state of benzene : Propose a complete basis of non-redondant covalent VB structures for benzene. (hint there should be 5 structures). Compute benzene with str=cov (6-31G basis set), and analyze the structures obtained. Were you right in your description of the covalent structures? now repeat the calculation with only 1 of the kékulé strucures and estimate the Vertical Resonance Energy (VRE) Full description of benzene : Compute a VBSCF wave function including the complete set of VB structures, using the "str=full" keyword. How many structures are available overall? In principle in order to calcuate properly the VRE one has to compute the energy difference between the overall adiabatic state of benzene and a corresponding Kékulé diabatic state. The latter Kékulé state requires an overall of 27 structures for its proper description, which have to be hand-written(!) One solution is the use of semi-localized orbitals offered by the "Bond Distorted Orbitals" option. Calculate the energy of one Kékulé state using the BDO (see BDO procedue below). Compare the energy of the covalent Kékulé structure calculated with HAO (hybrid atomic orbitals) and BDO. What is the reason for the difference? Estimate the corresponding VRE, has it improved? BDO procedue First run standard VBSCF calculation with the following keywords (frgtyp=sao, orbtype=hao and guess=auto). Copy the file file_name.orb which was created in the previous step, to a new file new_file_name.gus. Copy the file file_name.xmi to new_file_name.xmi and modify it as follows: Use guess=read and orbtyp=bdo. frgtyp keyword is not required for this type of calculation (no need of $frag part in the input). Modify $orb part according to new_file_name.gus This BDO procedure enables inclusion of all ionic structures associated with the given Kékulé structure implicitly. >> Answer Exercise 4 : The allyl cation Paper Exercise Structures of the allyl cation, and weights from Hückel theory : Find a complete basis of covalent structures for the allyl cation. What are the possible ionic structures for the allyl cation ? Based on your chemical knowledge, propose a selection subset of the most chemically meaningful covalent + ionic structures. Use the HuLis software to retrieve the Hückel MOs for allyl cation. 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. Compute by hand the weights of the different structures (neglecting all overlaps for simplicity). Computer Exercise Computation of allyl cation energies and weights : Compute a VBSCF wave function for allyl cation (6-31G basis set) using your selected set of structures (questions 1.3 and 2.2). Compute a VBSCF wave function including the complete set of VB structures, using the "str=full" keyword. Compare the weights and energies for both wave functions to validate your selection of structures. Compute for allyl cation a BOVB wave function which include only your selected set of most chemically meaningful structures (use the guess orbitals obtained at the VBSCF level). Compare the weights obtained at the VBSCF and BOVB level. Compare the BOVB weights to the weights predicted by simple MO theory. Computation of resonance energies : We want to build a wave-function corresponding to only one Lewis structures for the allyl cation. To do so, we will include in the wave-function only one covalent structure, and the ionic structures associated to this covalent bond. Propose a selection of VB structures which would describe one Lewis structure for the allyl cation. Compute this wave function at VBSCF then BOVB levels. Deduce what is the resonance energy of the allyl cation at the BOVB level. >> Answer

>> general guidelines for BOVB calculations