N-14/N-15 and C-12/C-13 equilibrium isotope effects on the electron-transfer reaction between N-methylphenothiazine and its radical cation

Appreciable equilibrium isotope effects have been observed for electron-tra nsfer process between N-methylphenothiazine (MPT) and the radical cation of its N-15- and/or N-C-13-methyl-substituted analogues via electron spin res onance (ESR) line-broadening effect of the radical cation perchlorates in t he presence of the corresponding parent neutral molecule. The equilibrium c onstants for the following electron-transfer processes were determined to b e 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 + [N-15]MPT.+ K-1/reversible arrow MPT.+ + [N-15]- MPT; [ C-13]MPT + [C-13,N-15]MPT.+ K-2/reversible arrow[C-13]MPT.+ + [C-13,N-15]MP T; [N-15]MPT + [C-13,N-15]MPT.+ K-3/reversible arrow[N-15]- MPT.+ + [C-13,N -15]MPT; MPT + [C-13]MPT.+ K-4/reversible arrow MPT.+ +[C-13]MPT; MPT + [C- 13,N-15]MPT.+ K-5/reversible arrow MPT.+ + [C-13,N-15]MPT. In addition, inf rared and Raman spectra of the N-methylphenothiazines and their radical cat ions were recorded and compared to assign the vibrational frequency shifts caused by the heavy-atom substitution and radical cation formation, from wh ich the enthalpy changes of the electron-transfer processes were estimated. These results reveal that N-15- and/or C-13-substitution of methylphenothi azine increases the ionization potential of the molecule, making it more di fficult to lose an electron to form the corresponding radical cation in sol ution.