Combined Ring Inversion and Side Group Rotation in Geminal Diphosphoryl Substituted Pyrrolinoxyl Radicals: ESR Analysis of Chemical Exchange between Four Nonequivalent Sites

The ESR spectra of three diphosphorylated pyrrolidinoxyl radicals have been studied over a large temperature range. While for the trans 2,5-disubstituted compound 1 no line width alternation was found, for the 2,2-disubstituted compounds 2 and 3, dramatic changes in the spectra were observed as a function of the temperature. These changes were explained by a four-site chemical exchange model including both ring inversion and hindered rotation around the carbon-phosphorus bonds. For radical 3, the presence of additional 5,5-dimethyl substitution can completely block rotations around carbon-phosphorus bonds for certain ring geometries, while for other ring conformations chemical exchange still occurs through combined inversionrotation processes. An effective two-site model composed of nonequivalent sites and a superposition model composed of a pair of exchanging conformers and a pair of nonexchanging conformers were used to simulate spectrum variations. A satisfactory fit was obtained over the entire temperature range investigated. From the temperature dependence of the exchange frequencies the potential barriers for ring inversion (27 kJ/mol), for combined inversion-rotation (11 kJ/mol), and for rotation around the carbon-phosphorus bonds (14 kJ/mol) were estimated