12.8 : 紫外可见光谱

The UV–visible spectrum is the plot of absorbance versus wavelength. However, according to the Beer-Lambert law, absorbance is directly proportional to molar absorptivity, ɛ. So, the UV–visible spectrum is typically replotted with log ɛ on the y-axis and wavelength on the x-axis.

The wavelength at which the molecule has maximum absorbance is called λ_max. As absorption occurs over a broad range of wavelengths, the absorption peak is often called a band.

To obtain a fine absorption band, the solvent used in UV–visible spectroscopy should not be absorbed in the region under investigation. It should also be nonpolar and not form hydrogen bonds with the solute because this promotes the formation of solute-solvent complexes.

Consider the UV–visible spectrum of benzoic acid in cyclohexane solvent. On passing UV radiation, the electrons excite from the HOMO to the LUMO by absorbing radiation over a range of wavelengths, with absorption bands at λ_max values of 230, 272, and 282 nanometers.

The log ɛ values corresponding to these wavelengths are 4.2, 3.1, and 2.9, respectively, indicating strong absorptions, with the maximum absorption occurring at  230 nanometers.