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

摘要

Appreciable equilibrium isotope effects have been observed for electron-transfer process between N-methylphenothiazine (MPT) and the radical cation of its ~(15)N - and/or N-~(13)C-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 K_1 = 1.19 +- 0.06, K_2 = 1.17 +- 0.12, K_3 = 1.06 +- 0.03, K_4 = 1.06 +- 0.05, and K_5 = 1.27 +- 0.14 respectively, in acetonitrile at ambient temperature: MPT + [~(15)N]MPT~(centre dot +)K_1/reversible + [~(15)N]MPT; [~(13)C]MPT + [~(13)C,~(15)N]MPT~(centre dot +)K_2/reversible [~(13)C]MPT~(centre dot +) + [~(13)C,~(15)N]MPT; [~(15)N]MPT + [~(13)C,~(15)N]MPT~(centre dot +)K_3/reversible[~(15)N]MPT~(centre dot +) + [~(13)C,~(15)N]MPT; MPT + [~(13)C]MPT~(centre dot +)K_4/reversible MPT~(centre dot +) + [~(13)C]MPT; MPT + [~(13)C,~(15)N]MPT~(centre dot +)K_5/reversible MPT~(centre dot +) + [~(13),~(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 ~(15)N- and/or ~(13)C-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.