14N/15N and 12C/13C Equilibrium Isotope Effects on the Electron-Transfer Reaction between N-Methylphenothiazine and Its Radical Cation

Abstract

Appreciable equilibrium isotope effects have been observed for electron-transfer process between N-methylphenothiazine (MPT) and the radical cation of its 15N- and/or N−13C-methyl-substituted analogues via electron spin resonance (ESR) line-broadening effect of the radical cation perchlorates in the presence of the corresponding parent neutral molecule. The equilibrium constants for the following electron-transfer processes were determined to be K1 = 1.19 ± 0.06, K2 = 1.17 ± 0.12, K3 = 1.06 ± 0.03, K4 = 1.06 ± 0.05, and K5 = 1.27 ± 0.14 respectively, in acetonitrile at ambient temperature: MPT + [15N]MPT•+ MPT•+ + [15N]MPT; [13C]MPT + [13C,15N]MPT•+ [13C]MPT•+ + [13C,15N]MPT; [15N]MPT + [13C,15N]MPT•+ [15N]MPT•+ + [13C,15N]MPT; MPT + [13C]MPT•+ MPT•+ + [13C]MPT; MPT + [13C,15N]MPT•+ MPT•+ + [13C,15N]MPT. In addition, infrared and Raman spectra of the N-methylphenothiazines and their radical cations were recorded and compared to assign the vibrational frequency shifts caused by the heavy-atom substitution and radical cation formation, from which the enthalpy changes of the electron-transfer processes were estimated. These results reveal that 15N- and/or 13C-substitution of methylphenothiazine increases the ionization potential of the molecule, making it more difficult to lose an electron to form the corresponding radical cation in solution.