Unexpected Influence of Viscosity on Intramolecular Spin Exchange in Nitroxide Biradicals

The spin exchange dynamics and the rotational mobility of rigid-frame nitroxide biradicals have been studied by EPR; despite the intramolecular character of the exchange, its rate correlates with molecular mobility, being damped in viscous media, thus proving that even the simplest intramolecular reactions are controlled by molecular dynamics.