INTRAMOLECULAR ELECTRON-TRANSFER BETWEEN DOUBLY 6 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-diazab icyclo[2.2.2]octyldiazenium radical cations cations which are doubly l inked 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-se nsitive charge transfer absorption bands from which the vertical elect ron 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 wer e made assuming that the average energy of the vibrational modes coupl ed to the electron transfer, hv(v), is 3.15 kcal/ mol (1100 cm(-1)). T he observed rate constants interpolated to 298 K are factors of 4.7-5. 8 larger than those calculated from the electron transfer parameters o btained 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 tim es larger than k(cal)(350)/k(cal)(250)in CH3CN and DMF and 0.9 times l arger in CH2Cl2. The agreement with theory for the bis-diazeniums is f ar better than that obtained for doubly four sigma-bond-linked bis-hyd razine radical cations (J. Am. Chem. Sec. 1997, 119, XXXX). It is sugg ested that the significantly smaller vibronic coupling constants S = l ambda(v)/h(v) for the bis-diazeniums (6.5-7.6) compared to those of th e bis-hydrazines (13.6-17.5) might be principally responsible for the difference in agreement of theory with experiment.