Effect of Viscosity on the Spin Exchange of TCNE and TEMPO Radicals with Iron(III) Acetylacetonate

Abstract

The spin exchange of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and the radical anion of tetracyanoethene (TCNE), with iron acetylacetonate, Fe(acac)3, was investigated as a function of solvent viscosity and temperature by X-band EPR spectroscopy. Acetone, methanol, ethanol, 1-propanol, 1-pentanol, 1-hexanol, 1-octanol, and chloroform were used as solvents because they provide a wide range of viscosity. The spin exchange rate constants measured in the different solvents ranged from 3 × 108 to 8 × 109 M−1 s−1, and from 7 × 108 to 1 × 1010 M−1 s−1 for the TEMPO/Fe(acac)3 and the TCNE/Fe(acac)3 systems, respectively. The spin exchange rate constants, k e, are compared with the corresponding diffusion rate constants. It was found that the k e values correlate well with those of diffusion and hence the intermolecular spin exchange is strongly diffusion-controlled. The activation energies and pre-exponential factors are calculated from the Arrhenius plots.