Chapter 15, Problem 15.5YT

Interpretation Introduction

Interpretation:

It is to be explained how the energies required for the electronic transitions in a given figure compare to the energy required for the HOMO–LUMO transition.

Concept introduction:

UV-vis spectrum of a particular compound is a plot of relative absorbance against wavelength. The band in the spectrum at which absorbance is a local maximum which corresponds to the ${\text{λ}}_{\text{max}}$ (lambda-max) which shows the absorbance at the maximum wavelength. Electromagnetic radiations have dual nature, particle as well as wave, when it behaves as a particle, then the radiation exists as photons. Each photon possesses characteristic energy that depends only on its frequency (or wavelength) and is given by the Planck’s equation: ${\text{E}}_{\text{photon}}{\text{= hυ}}_{\text{photon}}{\text{= hc/λ}}_{\text{photon}}$. Where, h: Planck’s constant (value is $\text{6}{\text{.626×10}}^{\text{-34}}\text{J}\text{.s}$), c: speed of light, universal constant (value is $\text{2}{\text{.9979 × 10}}^{\text{8}}\text{m/s}$) and ν is the frequency of the corresponding light. In UV–vis spectroscopy, the absorption of a photon typically corresponds to the promotion of an electron from an occupied MO (HOMO) to an unoccupied one (LUMO). The HOMO-LUMO transition requires the least energy, so it corresponds to the longest-wavelength UV-vis absorption. In other words, the longest-wavelength UV–vis absorption band corresponds to the HOMO–LUMO transition of the organic compound.