ESR-spectroscopy in ionic liquids: Dynamic linebroadening effects caused by electron-self exchange reactions within the methylviologene redox couple

Abstract

Well-resolved ESR-spectra of the methylviologene radical cation ( MV + ) are recorded in room-temperature liquid ions as solvents. The temperature dependences of the ESR-coupling constants are similar to those measured in classical organic solvents. Electron-self exchange rates are reported for the methylviologene redox couple ( M V + + / MV + ) in 1-butyl-3-methylimidazolium hexafluorophosphate (bmim +PF 6 −), 1-butlyl-3-methylimidazolium fluoroborate (bmim +BF 4 −) and 1-ethyl-3-imidazolium ethylsulfate (emim +O 3SOEt −) within a temperature range of 350 K ≤ T ≤ 460 K. The diffusion controlled rate constants observed vary between 8.2 × 10 7 M −1 s −1 and 1.2 × 10 9 M −1 s −1. From temperature-dependent measurements the activation energies obtained range from 27.4 kJ/mol in emim +O 3SOEt − to 42.1 kJ/mol in bmim +PF 6 −, respectively.