Electron Paramagnetic Resonance Spectroscopy

This chapter summarizes the state of the art in electron paramagnetic resonance (EPR) spectroscopy. It should give the reader an overview of the tremendous methodological developments and applications, which have a huge impact on the fields of biophysics, biology, biochemistry, material sciences, solid state physics, and physical organic chemistry. The basic theory and principles of continuous wave (CW) EPR are introduced, as well as principles and common experiments of pulse EPR. We discuss, how the choice of microwave frequency or pulse sequence allow to separately address different interactions that contribute to the spin Hamiltonian and thus the EPR spectrum. An important family of advanced EPR experiments focusses on the determination of couplings between electron spins and nearby nuclear spins. EPR imaging (EPRI) is introduced as a method to add spatial resolution to EPR spectroscopy. How the use of spin labels or spin probes makes diamagnetic systems, which initially do not offer an EPR signal, accessible to EPR spectroscopy and EPR distance measurements is discussed in the last section of the chapter. Keywords: electron paramagnetic resonance (EPR) spectroscopy; electron spin resonance (ESR); electron Zeeman interaction; spin Hamiltonian; spin labels; spin probes; transition metal ions