COMBINED RING INVERSION AND SIDE-GROUP ROTATION IN GEMINAL DIPHOSPHORYL SUBSTITUTED PYRROLIDINOXYL RADICALS - ESR ANALYSIS OF CHEMICAL-EXCHANGE BETWEEN 4 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 w ere observed as a function of the temperature. These changes were expl ained by a four-site chemical exchange model including both ring inver sion and hindered rotation around the carbon-phosphorus bonds. For rad ical 3, the presence of additional 5,5-dimethyl substitution can compl etely block rotations around carbon-phosphorus bonds for certain ring geometries, while for other ring conformations chemical exchange still occurs through combined inversion-rotation processes. An effective tw o-site model composed of nonequivalent sites and a superposition model composed of a pair of exchanging conformers and a pair of nonexchangi ng conformers were used to simulate spectrum variations. A satisfactor y fit was obtained over the entire temperature range investigated. Fro m the temperature dependence of the exchange frequencies the potential barriers for ring inversion (27 kJ/mol), for combined inversion-rotat ion (11 kJ/mol), and for rotation around the carbon-phosphorus bonds ( 14 kJ/mol) were estimated.