Electron transfer parameters are extracted from the optical spectra of intervalence bis(hydrazine) radical cations. Compounds with 2-tert-butyl-3-phenyl-2,3-diazabicyclo[2.2.2]octyl-containing charge-bearing units that are doubly linked by 4-sigma-bond and by 6-sigma-bond saturated bridges are compared with ones having tert-butylisopropyl- and diphenyl-substituted charge bearing units and others having the aromatic units functioning as the bridge. Solvent effect studies show that the optical transition energy (E(op)) does not behave as dielectric continuum theory predicts but that solvent reorganization energy may be usefully separated from the vibrational reorganization energy by including linear terms in both the Pekar factor (gamma) and the Gutmann donor number (DN) in correlating the solvent effect. Solvation of the bridge for these compounds is too large to ignore, which makes dielectric continuum theory fail to properly predict solvent effects on either E(op) or the free energy for comproportionation.

The radical cations of properly designed bishydrazines allow comparison of observed and calculated electron transfer rate constants. These compounds have rate constants small enough to be measured by dynamic electron spin resonance spectroscopy and show charge transfer bands corresponding to vertical excitation from the energy well for the charge occurring upon one hydrazine unit to that for the electron-transferred species. Analysis of the data for all six compounds studied indicates that the shape of the adiabatic surface on which electron transfer occurs can be obtained from the charge transfer band accurately enough to successfully predict the electron transfer rate constant and that explicit tunneling corrections are not required for these compounds.

Rate constants k ESR for intramolecular electron transfer between the reduced and oxidized diazene units of dimeric 2-tert-butyl-2,3-diazabicyclo[2.2.2]octyldiazenium radical cations cations which are doubly linked through the bicyclic units by six σ-bonds, sB6σ+ and aB6σ+, were determined from their variable temperature ESR spectra in CH3CN, dimethylformamide, and CH2Cl2. These cations show solvent−sensitive charge transfer absorption bands from which the vertical electron transfer excitation energy, λ, and the electronic coupling, V J, were determined by simulation, using vibronic coupling theory. The partitioning between solvent and vibrational components of λ were made assuming that the average energy of the vibrational modes coupled to the electron transfer, hνv, is 3.15 kcal/mol (1100 cm-1). The observed rate constants interpolated to 298 K are factors of 4.7−5.8 larger than those calculated from the electron transfer parameters obtained from vibronic coupling theory analysis of the charge transfer bands, k cal, in acetonitrile and DMF, and for sB6σ+ in CH2Cl2 the factor is 2.5. The ratios k ESR(350)/k ESR(250) are 1.0−1.6 times larger than k cal(350)/k cal(250)in CH3CN and DMF and 0.9 times larger in CH2Cl2. The agreement with theory for the bis-diazeniums is far better than that obtained for doubly four σ-bond-linked bis-hydrazine radical cations (J. Am. Chem. Soc. 1997, 119, XXXX). It is suggested that the significantly smaller vibronic coupling constants S = λv/hνv for the bis-diazeniums (6.5−7.6) compared to those of the bis-hydrazines (13.6−17.5) might be principally responsible for the difference in agreement of theory with experiment.

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.