Heterogeneous Electron Transfer Studies on the Reduction of Some Pyridinium Cations: Substituents and the Inner-Reorganization Energy

Abstract

Standard heterogeneous electron transfer rate constant k for some important pyridinium model compounds: N-methyl--picolinium- , N-(2,4-dinitrophenyl)-4-(4’-pyridyl) pyridinium- , N- (phenyl)-4-(4’ -pyridyl) pyridinium- , N-(cyanophenyl)-4-(4’ -pyridil) pyridinium- and N-(2, 4- dinitrophenyl) pyridinium chlorides as well as 1-1’ dimethyl 4-4’-bipyridinium (methyl viologen), were determined using cyclic voltammetric technique. Methods of Nicholson and Gileadi were used. k were computed using various forms of Marcus equations. It was found that the simple Marcus equation gave much higher values of k; while the precursor equilibrium-model gave better agreement with the experimental results. In all these cases an adiabatic electron transfer was assumed. It was inferred that the general practice of omitting i, the inner reorganization energy, can not be accepted in the present case: when strong electron withdrawing group nitro-, cyano-, carbomethoxy- etc. are the substituents near the reaction site, i should not be ignored.