Intramolecular Electron Transfer between Doubly Six σ-Bond-Linked Trialkyldiazenium Cation and Diazenyl Radical Units

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, sB6 σ + and aB6 σ +, were determined from their variable temperature ESR spectra in CH 3 CN, dimethylformamide, and CH 2 Cl 2 . 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, h ν v, 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 sB6 σ + in CH 2 Cl 2 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 CH 3 CN and DMF and 0.9 times larger in CH 2 Cl 2 . 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 = λ v / h ν v 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.