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Chemistry LibreTexts

Solutions 4

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Q1

  1. T
  2. T
  3. F, the overlap integral is zero. β pertains to energy, not overlap.
  4. T
  5. F, β is experimentally found to be negative.
  6. F, the resonance integral pertains to energy, so it is β .
  7. F, non-adjacent resonance integrals are zero.
  8. T

Q2

In the ethylene system, there are two p orbitals to consider, |1 and |p, each with energy E.

The Hamiltonian, ˆH, is given by:

[αββα]

We know that ˆH|ψ=E|ψ . Factoring, we have ˆHEI|ψ=|0, where I is the identity matrix.

This implies that |ψ=(ˆHEI)1|0. For the left side to be non-zero, the inverse matrix that acts on the zero vector must be infinite. From linear algebra, the inverse of a matrix is inversely proportionate to its determinant:

(ˆHEI)1=Cdet(ˆHEI). For the inverse to be infinite, we require det(ˆHEI)=0.

Q3

a). ˆH for a 5 carbon system by the Huckel Theory, is given by:

[αβ000βαβ000βαβ000βα0000βα]

b). Given the following wavefunctions, choose two and show they are normalized:

  • |π1=+0.45|1+0.45|2++0.45|3+0.45|4+0.45|5
  • |π2=0.51|1+0.63|20.51|3+0.20|4+0.20|5
  • |π3=0.37|1+0.37|3+0.60|4+0.60|5
  • |π4=0.21|10.63|20.20|3+0.51|4+0.51|5
  • |π5=+60|10.60|3+0.37|4+0.37|5

The dot products to show normalization are:

  • 1|1 is: 1.012500e+00
  • 2|2 is: 9.971000e-01
  • 3|3 is: 9.938000e-01
  • 4|4 is: 1.001200e+00
  • 5|5 is: 9.938000e-01

c). Show that two are orthogonal to each other:

Actually calculating the dot products we see they are not perfectly orthogonal, some with great intersection.

  • 1|2 is: 4.635000e-01
  • 1|3 is: 5.400000e-01
  • 1|4 is: 1.710000e-01
  • 1|5 is: 3.330000e-01
  • 2|3 is: 6.174000e-01
  • 2|4 is: 1.620000e-02
  • 2|5 is: -4.640000e-01
  • 3|4is: 7.637000e-01
  • 3|5 is: 0
  • 4|5 is: 1.314000e-01

d). Use the Hamiltonian matrix to calculate the distinct energies using these eigenfunctions.

We can find the energies of this system by setting ˆHEI=0.

|αEβ000βαEβ000βαEβ000βαE0000βαE|=0

Dividing the matrix by β and using the variable x=αEβ, we can solve a determinant of the form:

|x10001x10001x10001x00001x|=0

Continuing to solve for x by breaking down the determinant into smaller determinants, (also known as its "minors" or Laplace's formula) you will arrive after careful factoring (see https://people.richland.edu/james/le...terminant.html):

x(x23)(x21)=0

The energies relate to the other variables via: E=αxβ

The roots are x=0,±1,±3, so the energies are α,α±3β,α±β

Q4

The allyl cation can be described by the Hamtilonian:

ˆH=[αβ0βαβ0βα]

Similar to the previous problem, we must again solve for det(ˆHEI)=0, which results in:

|x101x101x|=x(x22)=0

The roots are x=0,±2, which correspond to energies: α+2β,α,α2β which correspond to bonding, non-bonding and anti-bonding orbitals. The deolocalization of the electrons lowers the energy of the system to 0.82 β when compared to ethylene.


Solutions 4 is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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