Chemistry
How Electron Paramagnetic Resonance Detects Radicals in Chemical Reactions
Quick fact
EPR is so sensitive to unpaired electrons that it can detect radical concentrations as low as 10^-10 molar, and it can even follow the formation and decay of radicals in a reaction in real time.
Why this is interesting
You’ve probably heard of MRI and NMR, but did you know that a related technique can actually 'see' the unpaired electrons that drive many chemical reactions? How does it work?
Read the full explanation
Understanding How Electron Paramagnetic Resonance Detects Radicals in Chemical Reactions
Imagine a tiny compass needle that is an electron. In a magnetic field, this needle can point either with the field or against it, and these two orientations have slightly different energies. When we shine microwaves on the sample, the energy of the microwave photons can exactly match the energy difference between the two orientations, causing the electron to flip from one orientation to the other. This is resonance, and by measuring the microwave absorption at different magnetic field strengths, we can detect the presence of unpaired electrons. Because most molecules have paired electrons (which cancel out their magnetic effects), only radicals and certain metal ions give an EPR signal. The strength of the magnetic field at which absorption occurs, and the pattern of splitting, provide a 'fingerprint' that identifies the radical and tells us about its environment.
A deeper explanation
The fundamental principle behind EPR is the Zeeman effect. An electron has a magnetic moment due to its spin, and in an external magnetic field B, this moment can align either parallel (lower energy) or antiparallel (higher energy) to the field. The energy difference ΔE is proportional to the magnetic field strength: ΔE = gβB, where g is the electron's g-factor (approximately 2.0023 for a free electron) and β is the Bohr magneton. When microwaves of frequency ν satisfy the resonance condition hν = gβB, electrons are promoted from the lower to the higher energy level, and we observe absorption. For an isolated electron, this would give a single line, but in real radicals, the unpaired electron interacts with nearby magnetic nuclei (e.g., protons, 14N). This hyperfine coupling splits the resonance into multiple lines, each corresponding to a different nuclear spin state. The number of lines and their relative intensities reveal the number and types of nuclei interacting with the electron. This is how EPR identifies radicals. In chemical reactions, radicals are often short-lived, so EPR is frequently combined with techniques like spin trapping (where a diamagnetic compound reacts with the radical to form a more stable radical that can be detected) or used in stopped-flow setups to observe transient species. EPR provides real-time information on radical concentration, structure, and dynamics, making it invaluable for studying polymerization, enzymatic redox reactions, and oxidative stress in biological systems.