EPR in a nutshell:

• Electron paramagnetic resonance is also known as electron spin resonance spectroscopy.
• A paramagnetic substance is one which contains atoms that have uncoupled electrons in their outer shells.
• When a paramagnetic sample is placed in a magnetic field, the magnetic moments of the uncoupled electrons will cause them to align with the magnetic field.
• From quantum mechanics we were informed that for free and uncoupled electrons, there are only two possible orientations… spin up and spin down.
• The electrons we are interested in are the ones in the lower energy level.
• By subjecting the sample to an electromagnetic field, in our case microwaves, we can cause the lower energy electrons to resonate, or briefly transition from spin down to spin up before returning to equilibrium.
• This transition will only occur when the exact amount of energy is introduced. This is found from the fundamental equation for EPR: DE=hv=guH
• To obtain a spectrum using EPR, we can scan through a range of microwave frequencies or magnetic field strengths. Sweeping the magnetic field will result in a better resolution.
• If we record the signal during the sweep, when the magnetic field and microwave frequency are "just right", we should record an absorption line and if the resolution is good enough we should see some hyperfine splitting in the spectral lines.
• EPR spectroscopy will not only allow for the identification of paramagnetic species, but by studying the line widths, intensities and deviation of the g value from free electrons we can gain some insights into the environment of the uncoupled electrons.