Chapter 15, Problem 15.5P

Interpretation Introduction

Interpretation:

The diagram similar to Figure 15-5, depicting the longest-wavelength UV-vis absorption for ethane is to be constructed.

Concept introduction:

A double bond consists of two electrons residing in a sigma MO and two electrons residing in a π MO, so a double bond is said to consist of both a sigma bond and a pi bond. Thus, there are two electrons in π MO, which has one bonding and one anti-bonding MO ${\text{π}}_{\text{1}}{\text{ and π}}^{\text{*}}{}_{\text{2}}$. The ${\text{π}}_{\text{1}}$ is the bonding MO, ${\text{and π}}^{\text{*}}{}_{\text{2}}$ is an antibonding MO. In the ground state, both the electrons are in the HOMO. When an incoming photon with the appropriate energy is absorbed, the molecule undergoes ${\text{π}}_{\text{1}}\to {\text{ π}}^{\text{*}}{}_{\text{2}}$ (pi-to-pi star) transition, producing an excited state.