When light passes through a substance, a portion of the light is absorbed while the remaining light is reflected or transmitted. If the molecule absorbs light between the wavelengths of 180–400 nm range, the UV spectrum is obtained, and if it absorbs light in the 400–780 nm wavelength range, the visible spectrum is obtained.
The UV–Vis spectrum of a molecule is the plot of its absorbance versus wavelength. The plot is drawn by taking molar absorptivity (ε) or log ε on the y-axis (ordinate) and wavelength on the x-axis (abscissa). Absorbance values represent light absorption by the molecule and cannot exceed 100 percent. When the absorbance is plotted against wavelength, the wavelength at which the molecule shows the maximum absorbance is called λmax. Since absorption occurs at a broad wavelength, it is often called an absorption band rather than a single peak.
Peaks in the UV–Vis spectrum vary in height and width based on molecular structure, electronic transitions, and solvent interactions.
These factors often result in broadened bands instead of single peaks, reflecting the molecule's complex behavior in different conditions.
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 λmax. 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 λmax 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.
繁體中文
