Intramolecular electron transfer between doubly six sigma-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 a-bonds, sB6 sigma(+) and aB6 sigma(+), 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, lambda, and the electronic coupling, V-J, were determined by simulation, using vibronic coupling theory. The partitioning between solvent and vibrational components of lambda were made assuming that the average energy of the vibrational modes coupled to the electron transfer, hv(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 sigma(+) 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 sigma-bond-linked bis-hydrazine radical cations (J. Am. Chem. Sec. 1997, 119, XXXX). It is suggested that the significantly smaller vibronic coupling constants S = lambda(v)/h(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.