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UV–Vis Spectrum

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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.

Overview

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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 λ_max. 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.

Molecular Structure: Different structures, like conjugated double bonds (alternating single and double bonds), have smaller energy gaps between their molecular orbitals. This means they absorb light at longer wavelengths, often in the visible range. As a result, the structure of the molecule can shift the peak to higher or lower wavelengths and change its intensity.
Electronic Transitions: Various electron transitions (e.g., π → π*, n → π*) absorb light differently, influencing peak strength. Molecules with π (pi) bonds or nonbonding (n) electron pairs can exhibit multiple types of transitions, leading to different peaks or bands in the spectrum.
Solvent Effects: The solvent in which the molecule is dissolved can also affect the spectrum. When a molecule interacts with a solvent, it can alter the molecule's electronic environment, sometimes stabilizing or destabilizing certain molecular orbitals. This solvent effect can cause shifts in the peak position and may alter the peak's intensity.

These factors often result in broadened bands instead of single peaks, reflecting the molecule's complex behavior in different conditions.

Transcript

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UV Vis Spectrum

Absorbance Wavelength

Molar Absorptivity

Electronic Transitions

Molecular Structure

Solvent Effects

Absorption Band

Conjugated Bonds