14N/15N Isotope Effect on the Electron Transfer Process betweenN,N,N‘,N‘-Tetramethyl-p-phenylenediamine and Wurster's Blue

Abstract

Appreciable 14N/15N equilibrium isotope effect has been observed for the electron-transfer process between N,N,N‘,N‘-tetramethyl-p-phenylenediamine (TMPD) and its radical cation perchlorate (TMPD+•ClCO4-, Wurster's blue). The equilibrium constant for the reaction, TMPD + [15N2]TMPD+• ⇌ TMPD+• + [15N2]TMPD, was determined to be 0.85 ± 0.05 at 25 °C in acetonitrile by electron spin resonance (ESR) analysis of the line-broadening effect of Wurster's blue in the presence of its parent neutral molecule. Redox potentials of TMPD and [15N2]TMPD were determined by cyclic voltammetry to be E° = 0.107 and 0.103 ± 0.002 V vs SCE respectively, corresponding to K = 0.86 ± 0.06 for the reaction. Comparison of the infrared and Raman spectra of TMPD, [15N2]TMPD and their radical cations revealed significant vibrational frequency shifts caused by the heavy-atom substitution and radical cation formation, from which the free energy change for the electron-transfer process was estimated to be 300 J mol-1, corresponding to K = 0.89. These results demonstrate that 15N-substitution of TMPD decreases appreciably the ionization potential of the molecule, making it easier to lose an electron forming the corresponding radical cation in solution.