Chapter 15, Problem 15.57P

Interpretation Introduction

Interpretation:

It is to be explained why IR spectroscopy does not distinguish very well between isomers I and J. It is to be explained how UV-vis spectroscopy could be used to distinguish between them.

Concept introduction:

In UV-vis spectroscopy, absorption of a photon promotes an electron from a lower-energy MO to a higher-energy MO. The ${\text{λ}}_{\text{max}}$ for the longest-wavelength absorption in the spectrum corresponds to the HOMO-LUMO. Conjugation decreases the energy difference between the HOMO-LUMO and thus increases the wavelength of the photon absorbed.

IR spectroscopy enables to identify different functional groups in the given molecule.

One of the greatest advantages of IR spectroscopy is that the frequency of a particular type of vibration is typically found within a characteristic range of frequencies, regardless of the functional group with which the vibration is associated. The fact that absorptions by certain vibrations appear with characteristic frequency ranges enables us to use IR spectroscopy to obtain structural information about a molecule.