Chapter 9, Problem 9.2IA

(a)

Interpretation Introduction

Interpretation:

The orbitals ${\psi }_1$, ${\psi }_2$ and ${\psi }_3$ has to be sketched and characterization of them as bonding, nonbonding, or antibonding has to be done.

Concept introduction:

Huckel molecular orbital theory is used for the determination of the energies of molecular orbitals of $\pi$ electrons of conjugated systems.  In Huckel molecular orbital theory, molecular orbitals are formed by a linear combination of atomic orbitals.

Molecular orbital diagrams are used for determining the bonding in molecules using a linear combination of atomic orbitals.  The number of molecular orbitals formed is equal to the number of atomic orbitals that combine with each other.  The energy of the molecular orbital is given by the expression shown below.

  $E_{\pm }=\frac{1}{2}\left({\alpha }_{\text{A}}+{\alpha }_{\text{B}}\right)\pm \frac{1}{2}\left({\alpha }_{\text{A}}-{\alpha }_{\text{B}}\right){\left\{1+{\left(\frac{2\beta }{{\alpha }_{\text{A}}-{\alpha }_{\text{B}}}\right)}^2\right\}}^{1/2}$

(b)

Interpretation Introduction

Interpretation:

The reason as to why a molecular orbital theory treatment is consistent only with a planar conformation of the peptide link has to be stated.

Concept introduction:

As mentioned in the concept of introduction in part (a).

(c)

Interpretation Introduction

Interpretation:

The diagram showing the relative energies of molecular orbitals and occupancy of the electrons in the orbitals has to be drawn.

Concept introduction:

As mentioned in the concept of introduction in part (a).

(d)

Interpretation Introduction

Interpretation:

The orbitals ${\psi }_4$, ${\psi }_5$ and ${\psi }_6$ has to be sketched and characterization of them as bonding, nonbonding, or antibonding has to be done.  The diagram showing the relative energies of molecular orbitals and occupancy of the electrons has to be drawn.

Concept introduction:

As mentioned in the concept of introduction in part (a).

(e)

Interpretation Introduction

Interpretation:

The reason as to why the corresponding molecular orbital theory treatment is consistent with a nonplanar conformation of the peptide link has to be stated.

Concept introduction:

As mentioned in the concept of introduction in part (a).

(f)

Interpretation Introduction

Interpretation:

The energy of the bonding MO and antibonding MO associated with the planar and nonplanar conformation has to be compared.  The bonding MO having lower energy has to be stated.

Concept introduction:

As mentioned in the concept of introduction in part (a).

(g)

Interpretation Introduction

Interpretation:

The arguments which support the planar model for the peptide link has to be stated.

Concept introduction:

As mentioned in the concept of introduction in part (a).