Heterogeneous electron transfer studies on the reduction of some pyridinium cations: Substituents and the inner-reorganization energy

Standard heterogeneous electron transfer rate constant k°a for some important pyridinium model compounds: n-methyl-α-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'a were computed using various forms of marcus equations. it was found that the simple marcus equation gave much higher values of k°a; 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 λi,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,λi should not be ignored.