14N/15N ISOTOPE EFFECT ON THE ELECTRON TRANSFER PROCESS BETWEEN PHENOTHIAZINE AND ITS RADICAL CATION

An appreciable equilibrium isotope effect has been observed for electron transfer from phenothiazine (PT) to the radical cation of its N-15 substituted analogue ([N-15]PT+.), i,e, PT+[N-15]PT+.K reversible arrow PT+.+[N-15]PT via electron paramagnetic resonance analysis of the mixed radical cations formed from mixing the [N-15]phenothiazlne radical cation hexachloroantimonate and phenothiazine in acetonitrile (K=0.77+/-0.10 at 25 degrees C), and by physical separation of the neutral phenothiazines from the radical cation salts in the equilibrium mixture (K=0.83+/-0.10 at 25 degrees C), Infrared and Raman spectra of [N-14]- and [N-15]phenothiazines and their radical cations were measured to assign the vibrational frequency shifts caused by the heavy-atom substitution and radical cation formation, which gave an estimate of the enthalpy change of 441.7 J mol(-1) for the electron transfer process. These results reveal that N-15 substitution of phenothiazine decreases appreciably the ionization potential of the molecule, making it easier to lose an electron to form the corresponding radical cation in solution. (C) 1997 by John Wiley & Sons, Ltd.