Chapter 14, Problem 14.29P

Interpretation Introduction

Interpretation:

It is to be determined how a one-electron excitation from the ${\text{π}}_2$ MO to the ${\text{π}}_3$ MO of $\text{1, 3-butadiene}$ will affect the $\text{C1}-\text{C2}$ and $\text{C2}-\text{C3}$ bond lengths.

Concept introduction:

Electron density between adjacent atoms depends on whether the electron(s) from the contributing p orbitals reside(s) in a bonding MO or in an antibonding MO. When both electrons are in a bonding MO, the electron density is high in the region between the nuclei. This increases the bond order and shortens the bond length. When one or both electrons are excited to a high energy antibonding MO, the electron density between the nuclei decreases, leading to an increase in the bond length.