Electron spin resonance (ESR) is a spectroscopic technique used to study materials with unpaired electrons. It detects the magnetic fields produced by these electrons when they are exposed to microwave radiation in the presence of a magnetic field. This allows researchers to gain insights into the electronic structure and dynamics of various substances, including free radicals and transition metal complexes. ESR is particularly useful in fields like chemistry, physics, and biology, where it helps in understanding reaction mechanisms and the behavior of free radicals. It can also be applied in medical research to investigate oxidative stress and its effects on biological systems.