Solutions 4
( \newcommand{\kernel}{\mathrm{null}\,}\)
Q1
- T
- T
- F, the overlap integral is zero. β pertains to energy, not overlap.
- T
- F, β is experimentally found to be negative.
- F, the resonance integral pertains to energy, so it is β .
- F, non-adjacent resonance integrals are zero.
- 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 ˆH−EI|ψ⟩=|0⟩, where I is the identity matrix.
This implies that |ψ⟩=(ˆH−EI)−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:
(ˆH−EI)−1=Cdet(ˆH−EI). For the inverse to be infinite, we require det(ˆH−EI)=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|2⟩−0.51|3⟩+0.20|4⟩+0.20|5⟩
- |π3⟩=−0.37|1⟩+0.37|3⟩+0.60|4⟩+0.60|5⟩
- |π4⟩=−0.21|1⟩−0.63|2⟩−0.20|3⟩+0.51|4⟩+0.51|5⟩
- |π5⟩=+60|1⟩−0.60|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|4⟩is: 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 ˆH−EI=0.
|α−Eβ000βα−Eβ000βα−Eβ000βα−E0000βα−E|=0
Dividing the matrix by β and using the variable x=α−Eβ, 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(x2−3)(x2−1)=0
The energies relate to the other variables via: E=α−xβ
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(ˆH−EI)=0, which results in:
|x101x101x|=x(x2−2)=0
The roots are x=0,±√2, which correspond to energies: α+√2β,α,α−√2β 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.