191341-12-5Relevant articles and documents
Estimation of self-exchange electron transfer rate constants for organic compounds from stopped-flow studies
Nelsen, Stephen F.,Ramm, Michael T.,Ismagilov, Rustem F.,Nagy, Mark A.,Trieber II, Dwight A.,Powell, Douglas R.,Chen, Xi,Gengler, Jamie J.,Qu, Qinling,Brandt, Jennifer L.,Pladziewicz, Jack R.
, p. 5900 - 5907 (2007/10/03)
Second-order rate constants k12(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 k12(calcd) = (k11 k22 k12f12)(1/2) , Inf12 = (In K12)(2/4) ln(k11k22/Z2) works well to correlate these data. When all k12(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 k12(obsd); k12(obsd)lk12'(calcd) is in the range of 0.55-1.94 for all 47 reactions. The average ΔΔG(ii)≠ 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 k12 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. Tetracyclohexyl-hydrazine 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 CH2 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.
