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How can spectroscopy be used to study free radicals in a reaction?

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A graph showing a potential energy curve for a chemical reaction, with the y-axis labeled potential energy and the x-axis labeled progress of reaction. The curve rises from the initial state (reactants) to a peak labeled transition state (activated complex), indicating activation energy, then drops to the final state (products). — chemical kinetics In the Arrhenius equation, the activation energy (E) represents the minimum amount of energy required to transform reactants into products in a chemical reaction. On a potential energy curve, the value of the activation energy is equivalent to the difference in potential energy between particles in an intermediate configuration (known as the activated complex, or transition state) and particles of reactants in their initial state. The activation energy thus can be visualized as a barrier that must be overcome by reactants before products can be formed.

Spectroscopy is a vital tool in chemical kinetics for identifying and studying short-lived free radicals. Because these species are often transient and exist in low concentrations, specialized techniques are required for detection.12

Electron Paramagnetic Resonance (EPR)

Free radicals are paramagnetic due to the presence of an unpaired electron. When placed in a magnetic field, the energy levels of this electron split based on its orientation. By exposing the system to electromagnetic radiation, researchers can induce transitions between these energy levels. The resulting absorption spectrum allows for the identification of specific radicals and provides insights into their electronic structure. This technique is highly sensitive, capable of detecting extremely low concentrations of radicals.1

Flash Photolysis

For unstable, short-lived intermediates, flash photolysis is frequently employed. This method uses an intense, brief burst of light to dissociate molecules into radicals, followed by a second flash that enables identification via spectrophotometry. This approach is essential for studying the mechanisms of fast chemical reactions where radicals act as transient intermediates.32