The Spin Chemistry of Nitroxide-Based Organic Biradicals

The electronic spin has a crucial importance in life chemistry, although relatively rarely encountered in several processes, while the vast majority of organic molecules (including the biological systems) are based on closed-shell spinless configurations. To advocate for the incidence of the spin it is suggestive to point out that the oxygen molecule, O2, the fuel in the quasi-totality of organisms, has a spin-triplet ground state and all the processes triggered by respiration are including, then, spin interactions. In the investigation of subtle spin-related molecular and supramolecular interactions, a wise technique consists in introducing stable open-shell organic radicals as “spies” of a given environment, under electronic paramagnetic resonance measurements. The nitroxide-based spin carriers are frequently used as spin probes, the TEMPO group, (2,2,6,6-Tetramethylpiperidin-1-yl)-oxyl, being the most widely used in this view. Here the authors illustrate a case study on a series of nitroxide-based biradicals, corroborating the EPR measurements with state-of-the-art modeling.