Chapter 15, Problem 15.37P

Interpretation Introduction

Interpretation:

What kind of electron transition, in propenal, does the absorption at $\text{202 nm}$ correspond to 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.