Influence of Polarity and Ionic Strength on Intramolecular Spin Exchange in a Short Nitroxide Biradical, Containing Sulphur Atom in the Bridge

A short nitroxide biradical: O=S(OR 6 ) 2 ( BS ), where OR 6 is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, has been studied by electron paramagnetic resonance spectroscopy at X-, Q-, and W-band in liquid and frozen toluene, ethanol and ionic liquid solutions. Variations of the intramolecular dynamics and conformational transitions in the biradical as a function of temperature in the range of 240–420 K, polarity and the ionic strength were characterized by changes in the isotropic 14 N hyperfine splitting (hfs) constant a , values of the exchange integral | J |, and the empirical parameter γ 3 , the ratio between conformations with slow and fast transitions. Thermodynamic parameters of the conformational rearrangements are calculated. The obtained results were compared with the X-ray structural data and quantum chemical calculations of the geometries and intramolecular transitions of biradical BS . Possible mechanisms of the polarity and the ionic strength effect on the biradical behavior in solutions of different types are discussed.