Chapter 5. Theoretical Spectroscopies of Iron-Containing Enzymes and Biomimetics

Spectroscopy methods are important tools in the study of iron-containing enzymes that make it possible to probe the electronic structure, oxidation state and geometry of metal centers in the active sites. This is especially important for short-lived species for which X-ray data are usually not available. However, satisfactory interpretation of experimental spectra and extraction of the electronic and geometric information can be challenging. Theoretical spectroscopy in combination with other computational techniques is then extremely valuable in the interpretation of spectra in ambiguous situations, and is also more broadly useful for distinguishing between proposed models for reactive species, since spectroscopic parameters are often more sensitive to the electronic structure than the total energy itself. Theoretical spectroscopy has been successfully applied to a varied set of problems of iron-containing systems. In this chapter we briefly introduce the underlying physics of the spectroscopic techniques that are most commonly used to study iron-containing enzymes and biomimetic compounds: Mossbauer spectroscopy (MB), electron paramagnetic resonance (EPR), absorption spectra (ABS) and X-ray absorption spectroscopy (XAS), and nuclear resonance vibrational spectroscopy (NRVS). Several good examples of studies of high-valent iron complexes are included to demonstrate how experimental and theoretical spectroscopy can be combined to gain as much as possible insight into the electronic and geometric structure of iron-containing systems.