Estimation of Self-Exchange Electron Transfer Rate Constants for Organic Compounds from Stopped-Flow Studies

Abstract

Second-order rate constants k 12 (obsd) measured at 25°C in acetonitrile by stopped-flow for 47 electron transfer (ET) reactions among ten tetraalkylhydrazines, four ferrocene derivatives, and three p-phenylenediamine derivatives are discussed. Marcus's adiabatic cross rate formula k 12 (calcd) ) (k 11 k 22 k 12 f 12 ) 1/2 , ln f 12 ) (ln K 12 ) 2 /4 ln(k 11 k 22 /Z 2 ) works well to correlate these data. When all k 12 (obsd) values are simultaneously fitted to this relationship, best-fit self-exchange rate constants, k ii (fit), are obtained that allow remarkably accurate calculation of k 12 (obsd); k 12 (obsd)/k 12 ′(calcd) is in the range of 0.55-1.94 for all 47 reactions. The average ∆∆G ij q between observed activation free energy and that calculated using k ii (fit) is 0.13 kcal/mol. Simulations using Jortner vibronic coupling theory to calculate k 12 using parameters which produce the wide range of k ii values observed predict that Marcus's formula should be followed even when V is as low as 0.1 kcal/mol, in the weakly nonadiabatic region. Tetracyclohexylhydrazine has a higher k ii than tetraisopropylhydrazine by a factor of ca. 10. Replacing the dimethylamino groups of tetramethyl-p-phenylenediamine by 9-azabicyclo[3.3.1]nonyl groups has little effect on k ii , demonstrating that conformations which have high intermolecular aromatic ring overlap are not necessary for large ET rate constants. Replacing a γ CH 2 group of a 9-azabicyclo[3.3.1]nonyl group by a carbonyl group lowers k ii by a factor of 17 for the doubly substituted hydrazine and by considerably less for the doubly substituted p-phenylenediamine.

Solvent Effects on Electronic Coupling between Dialkylamino Groups in Aromatic Diamine Radical Cations

“…30 TMB was prepared by the literature method. 31 It was oxidized in situ using less than one equivalent of NOBF 4 , as in our previous work.…”

A Critical Assessment of Classical and Semi-Classical Models for Electron Transfer Reactions in Solution

“…Our AM1 calculations also support the use of n ‡ = 1 in these exchanges. As mentioned before, the shape of the alkylhydrazines considered in this work approaches that of a sphere with the NN bond in the middle (r av = 393 pm for 22/u22 [249] and r av = 434 pm for iPr 2 N) 2 [220]). This restricts the approach between the neutral and cation to larger separations than the flat aromatic molecules and their electron exchange should be treated as non-adiabatic [249].…”

“…This restricts the approach between the neutral and cation to larger separations than the flat aromatic molecules and their electron exchange should be treated as non-adiabatic [249]. The value of χ r for electron exchanges involving alkylhydrazines was calculated by the method described for the other ET reported in this work, using r-r 0 = 2r av and the oxidation potentials of the relevant species [220].…”

Application of Marcus cross-relation to mixed inorganic–organic redox couples. A stopped-flow study of the oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine with various oxidantsIn memory of Lennart Eberson.

“…This is in agreement with a recently published paper, also using experimental self-exchange rates. 4…”

“…3 Reports on mixed inorganic-organic redox reactions are also rare. In an extensive study, Nelsen et al 4 (1)…”