Chapter 15, Problem 15.38P

Interpretation Introduction

Interpretation:

In the UV-vis. Spectrum of benzene, the absorption that corresponds to the HOMO-LUMO transition occurs at $\text{184 nm}$. How does this compare to the corresponding electron transition in $\text{hexa-1, 3, 5-triene}$ and the significant difference is to be explained.

Concept introduction:

In UV–vis. spectroscopy, absorption of a photon promotes an electron from a lower energy MO to a higher-energy MO. ${\text{λ}}_{\text{max}}$ for the longest-wavelength (lowest-energy) absorption in the spectrum generally corresponds to the HOMO–LUMO transition. In a $\text{π}\to \text{π*}$ transition, an electron from a $\text{π}$ MO is promoted to a $\text{π*}$ MO. In an $\text{n}\to \text{π*}$ transition, an electron from a nonbonding MO is promoted to a $\text{π*}$ MO. Conjugation decreases the energy difference between the HOMO and LUMO, and thus increases the wavelength of the photons absorbed.