Intramolecular Electron Transfer between Doubly Four σ-Bond-Linked Tetraalkylhydrazine Cationic and Neutral Units

Abstract

Rate constants kESR for intramolecular electron transfer between the reduced and oxidized hydrazine units of dimeric 2-tert-butyl-3−isopropyl-2,3-diazabicyclo[2.2.2]octylhydrazine radical cations which are doubly linked through the bicyclic units by four σ-bonds (aBIT+, sBIT+, and aBIH+) were determined by simulations of their variable temperature ESR spectra in methylene chloride to be 10.5(7), 9.6(3), and 12.4(4) × 107 s-1 at 298 K, respectively. These cations show solvent sensitive charge transfer absorption bands from which the vertical electron transfer excitation energy, λ, and the electronic coupling, VJ, were determined by simulation of the charge transfer bands using vibronic coupling theory (ref 13). Partitioning between solvent and vibrational components of λ was made assuming that the average energy of the vibrational modes coupled to the electron transfer, hνv, is 2.29 kcal/mol (800 cm-1). The ESR rate constants at 298 K for s- and aBIT+ are factors of 23 and 26, respectively, larger than kcal, calculated from λs, λv, hνv, and VJ estimated using a vibronic coupling theory analysis of the charge transfer bands. The ratio kcal(350)/kcal(250) is 7.4 and 9.4(kESR(350)/kESR(250)) for sBIT+ and aBIT+, respectively. Previously reported data for the doubly-linked four σ-bond dimeric N,N‘-bis-2,3-diazabicyclo[2.2.2]octyl hydrazine (22H+) was also reanalyzed using vibronic coupling theory. kESR/kcal in acetonitrile at 298 K is 20, but kcal(350)/kcal(250) is 25(kESR(350)/kESR(250)). Possible reasons for the rather poor agreement with theory are suggested.