14213-69-5Relevant academic research and scientific papers
Indirect determination of self-exchange electron transfer rate constants
Nelsen, Stephen F.,Ismagilov, Rustem F.,Gentile, Kevin E.,Nagy, Mark A.,Tran, Hieu Q.,Qu, Qinling,Halfen, DeWayne T.,Odegard, Amy L.,Pladziewicz, Jack R.
, p. 8230 - 8240 (2007/10/03)
Second-order rate constants k(ij)(obsd) measured at 25 °C in acetonitrile by stopped-flow spectrophotometry for forty-four electron transfer (ET) reactions among fourteen 0/+1 couples [three aromatic compounds (tetrathiafulvalene, tetramethyltetraselenafulvalene, and 9,10-dimethyl- 9,10-dihydrophenazine), four 2,3-disubstituted 2,3-diazabicyclo[2.2.2]octane derivatives, six acyclic hydrazines, and the bridgehead diamine 1,5- diazabicyclo[3.3.3]undecane] and seventeen compounds and forty-seven reactions from a previous study (J. Am. Chem. Soc. 1997, 119, 5900) [three p- phenylenediamine derivatives, four ferrocene derivatives, and ten tetraalkylhydrazines] are discussed. When all 91 k(ij)(obsd) values are simultaneously fitted to Marcus's adiabatic cross rate formula k(ij)(calcd) = (k(ii)k(jj)K(ij)f(ij))(1/2), ln f(ij) = (ln K(ij))(2/4) ln((kii)k(jj)/Z2), best-fit self-exchange rate constants, k(ii)(fit), are obtained that allow remarkably accurate calculation of k(ij)(obsd); k(ij)(obsd)/k(ij)(calcd) is in the range 0.5-2.0 for all 91 reactions. The average difference without regard to sign, |ΔΔG(+)(ij)|, between observed cross reaction activation free energy and that calculated using the k(ii)(fit) values and equilibrium constants is 0.13 kcal/mol. The ΔG(+)(ii)(fit) values obtained range from 2.3 kcal/mol for tetramethyltetraselenafulvalene(0/+) to 21.8 kcal/mol for tetra-n-propylhydrazine(0/+), corresponding to a factor of 2 x 1014 in k(ii)(fit). The principal factor affecting k(ii)(fit) for our data appears to be the internal vertical reorganization energy (λ(v)), but k(ii)(fit) values also incorportate the effects of changes in the electronic matrix coupling element (V). Significantly smaller V values for ferrocenes and for hydrazines with alkyl groups larger than methyl than for aromatics and tetramethylhydrazine are implied by the observed ΔG(+)(ii)(fit) values.
Kinetic Effects of an Unusually Large Neutral to Radical Cation Geometry Change. Slow Electron-Transfer Reactions between Alkylhydrazines
Nelsen, Stephen F.,Rumack, Daniel T.,Meot-Ner (Mautner), Michael
, p. 1373 - 1379 (2007/10/02)
High pressure mass spectrometry was used to measure the kinetics for electron transfer between 54 pairs of tetraalkylhydrazines containing acyclic and f-ve- to seven-membered cyclic and bicyclic rings.Rate constants for electron transfer vary between 18 and 0.03 x 10-11 cm3molecule-1s-1 at 550 K.Variable-temeprature mesuremeants were made on five pairs over a 77-120 deg range.The (Me2N)2(1+), (EtMeN2)2 pair gave Ea=2.7 kcal/mol.The association energy for (Me2N)2 was measured at ΔH0=-13.0 kcal/mol near room temperature.These data are combined to estimate an energy separation of about 15.7 kcal/mol between associated (Me2N)2(1+)/(EtMeN)2 dimer complex and the transition state for electron transfer.The observed Bronsted α value of about 0.5 suggests a large barrier to electron transfer, and the kinetics suggests that the components largely retain their original structures in the associated complex, but that significant distortion is required to reach the transition-state geometry.The effect of alkyl group changes on the electron-transfer rate and comparison of these data with solution experiments are discussed.
