Slow Electron Transfer Reactions Involving Tetraisopropylhydrazine

Abstract

Marcus pointed out in 1956 that the fundamental parameter to know for predicting outer-sphere electron transfer (ET) activation free energies is λ, the vertical free energy gap between a non-interacting pair consisting of a neutral species M 0 and its own radical ion (M + or M -) in solution, and the same pair in which an electron has been transferred between the two components without allowing any relaxation. 1 If the relaxed, solvated neutral and radical cation are designated by n and c, and the charge present is shown as a superscript, this energy gap for vertical "self-ET" between n 0 and c + may be written as the sum of the relaxation energies for cationic and neutral species (eq 1). 2 λ corresponds to 4 times the thermal barrier for ET,

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

“…We have not discerned a way of accurately predicting which reactions will show unusually large |ΔΔ G ij ⧧ | values. As noted previously with much less data, varying Z 11 Z 22 in eq 1b has only small effects on the fit of eq 1 to experimental data 2 Summary of Reactions Studied entryreductantoxidant k 12 (obsd) (M -1 s -1 ) k 12 (calcd) (M -1 s -1 ) a k 12 ‘(calcd) (M -1 s -1 ) b k 12 (obsd)/ k 12 ‘(calcd)ΔΔ G ij ⧧ (kcal/mol) c 1 d 22/u22 FeCp* 2 + 2.3(2) × 10 6 6.8 × 10 6 1.5 × 10 6 1.5 −0.2 2 22/u23 " 5.7(3) × 10 6 1.0 × 10 7 5.0 × 10 6 1.1 −0.1 3 d FeCp* 2 21/u22 + 1.35(23) × 10 6 5.5 × 10 6 2.1 × 10 6 0.6 0.3 4 e,f " iPr 2 N) 2 + 7.6(5) × 10 4 2.1 × 10 6 1.1 × 10 5 0.7 0.2 5 d 21/u22 FeCp ‘ 2 + 6.3(13) × 10 6 7.5 × 10 6 6.3 × 10 6 1.0 0.0 6 d " FeCp*Cp + 3.5(5) × 10 5 4.9 × 10 6 3.3 × 10 5 1.0 −0.0 7 e,f FeCp*Cp iPr 2 N) 2 + 1.6(1) × 10 3 2.1 × 10 3 2.0 × 10 3 0.8 0.1 8 e,f TMPD " 1.2(1) × 10 4 2.9 × 10 4 7.4 × 10 3 1.6 −0.3 9 e 21/21 " 2.6(2) × 10 2 7.6 × 10 2 2.7 × 10 2 1.0 0.0 10 21/u22 " 5.5(2) × 10 1 1.1 × 10 2 6.5 × 10 1 0.8 0.1 11 e 22/22 " 3.2(2) × 10 5 6.9 × 10 5 2.5 × 10 5 1.3 −0.1 12 e 22/u22 " 1.1(1) × 10 4 4.0 × 10 4 1.3 × 10 4 0.8 0.1 13 …”

“…e From ref . f There is an error in the k 12 reported in ref ; this is the correct value. g From ref .…”

“…We employ preexponential factors Z 11 = Z 22 = 1 × 10 11 M -1 s -1 , as has been traditional. Equation 1 is followed surprisingly well by these reactions, − encouraging us to use stopped-flow intermolecular ET studies to estimate k ii values for compounds for which measurement under self-exchange conditions is not possible, and the six compounds shown in Scheme are considered in this work. Their formal potentials are also shown in Table .…”

“…The small k ii value obtained for iPr 2 N) 2 in stopped-flow experiments encouraged us to use direct measurement of k ii (self) by mixing deuterium-labeled neutral hydrazine with unlabeled radical cation . We employed material having a single isopropyl group substituted as CH(CD 3 ) 2 (see Experimental Section).…”

“…Three previous studies of intermolecular electron transfer (ET) reactions involving hydrazines using stopped-flow measurements of rate constants have been reported. − In the work discussed here a purified, isolable cation radical dissolved in acetonitrile containing 0.1 M tetrabutylammonium perchlorate is injected into a stopped-flow cell along with a neutral compound of lower formal redox potential ( E °‘) in the same solvent, and the change in absorbance resulting from the ET reaction which ensues is followed as a function of time. A series of experiments varying the concentrations, carried out under pseudo-first-order conditions if possible, is used to determine the second-order rate constant, k 12 (obsd), for the reaction.…”