ESR studies at high pressure. II. Spin exchange of 2,2,6,6-tetramethyl-4-oxo-1-piperidinyloxyl in protic and aprotic solvents

The activation volumes for the spin exchange of 2,2,6,6-tetramethyl-4-oxo-1-piperidinyloxyl (Tanone) in protic and aprotic solvents were determined from the pressure dependence of the rate constants, and were compared with those for the diffusive process. It was concluded that the rate process for the spin exchange in aprotic solvents is a diffusion-controlled one, and in protic solvents an encounter complex as intermediate is formed. The average reaction volume for the complex formation in protic solvents was estimated to be -3.6 cm³ mol^-1.