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Estimation of self-exchange electron transfer rate constants for organic compounds from stopped-flow studies

Nelsen, Stephen F. and Ramm, Michael T. and Ismagilov, Rustem F. and Nagy, Mark A. and Trieber, Dwight A., II and Powell, Douglas R. and Chen, Xi and Gengler, Jamie J. and Qu, Qinling and Brandt, Jennifer L. and Pladziewicz, Jack R. (1997) Estimation of self-exchange electron transfer rate constants for organic compounds from stopped-flow studies. Journal of the American Chemical Society, 119 (25). pp. 5900-5907. ISSN 0002-7863. doi:10.1021/ja970321j.

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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 k12 f12)1/2, ln f12 = (ln 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, kii(fit), are obtained that allow remarkably accurate calculation of k12(obsd); k12(obsd)/k12‘(calcd) is in the range of 0.55−1.94 for all 47 reactions. The average ΔΔGij between observed activation free energy and that calculated using kii(fit) is 0.13 kcal/mol. Simulations using Jortner vibronic coupling theory to calculate k12 using parameters which produce the wide range of kii 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 kii 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 kii, 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 kii by a factor of 17 for the doubly substituted hydrazine and by considerably less for the doubly substituted p-phenylenediamine.

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Ismagilov, Rustem F.0000-0002-3680-4399
Additional Information:Copyright © 1997 American Chemical Society. Published In Issue: June 25, 1997. Received January 30, 1997. We thank the National Science Foundation for partial financial support of this work under Grants CHE-9504133 (J.R.P.) and −9417946 (S.F.N.). Acknowledgement is also made to the donors of the Petroleum Research Fund, administered by the ACS for partial support of the research under grant ACS-PRF 29982-B4 (JRP). We thank Fred King for assistance with Fortran programming. Supporting Information Available: Experimental for determination of ET rate constants for iPr2N)20/+ by 2H and 1H NMR, and crystal data for iPr2N)2, cHx2N)2, k33)2PD+PF6-, and k33N)2 (5 pages).
Funding AgencyGrant Number
Petroleum Research FundACS-PRF 29982-B4
Subject Keywords:substituted para-phenylenediamines; activation parameters; radical cations; metal-complexes; marcus theory; kinetics; acetonitrile; quinonediimines; validity; systems
Issue or Number:25
Record Number:CaltechAUTHORS:20130821-160728161
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:40849
Deposited By: Whitney Barlow
Deposited On:29 Aug 2013 18:45
Last Modified:10 Nov 2021 00:09

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