Pulsed Electron Paramagnetic Resonance Techniques for Structure Determination

Abstract Over the past several decades pulsed electron paramagnetic resonance (EPR) spectroscopy has advanced to provide a unique and useful set of tools for structural determination in systems that contain paramagnetic centers. Pulsed EPR methods use sequences of resonant microwave pulses to excite and manipulate paramagnetic centers subjected to an external magnetic field. The signals generated through these magnetic resonance methods are analyzed to provide information about the electronic and geometric structure near the paramagnetic centers. This article provides a general overview of the theory and common experimental methods of pulsed EPR including electron spin echo envelope modulation (ESEEM), double electron–electron resonance (DEER), and hyperfine sublevel correlation spectroscopy (HYSCORE). Electron–nuclear and electron–electron interactions are discussed in detail, as well as how measurement of these interactions can be used to obtain structural insight for a range of important systems. Selected applications are presented to illustrate the utility of pulsed EPR methods for structure determination in real and model systems, with specific examples from materials science, astrogeology, and bioinorganic chemistry. The use of density functional theory (DFT) calculations to enhance experimental data analysis and interpretation is discussed.