Chapter 14, Problem 14.13P

Interpretation Introduction

Interpretation:

The complete MO energy diagram for cyclobutadiene including the MOs of $\text{σ}$ symmetry is to be drawn.

Concept introduction:

Frost's method is used to obtain the relative energies of the $\text{π}$ MOs of a completely conjugated cyclic $\text{π}$ system. The molecule is oriented so that one vertice points directly downward. Each vertex then represents the energy of a $\text{π}$ MO. The center of the polygon (molecular ring) represents the energy of each unhybridized p atomic orbitals.

According to Frost’s method, cyclobutadiene yields four $\text{π}$ MOs in which one is bonding, two are nonbonding and one is antibonding.

In cyclobutadiene, there are four $\text{(σ})$ $\text{C-C}$ bonding interactions and four $\text{(σ})$ $\text{C-H}$ bonding interactions. So there are eight $\text{σ}$ MOs and eight ${\text{σ}}^{*}$ MOs.