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

Rate constants kESR 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, VJ, 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,
kcal, in acetonitrile and DMF, and for
sB6σ+ in CH2Cl2
the factor is 2.5. The ratios
kESR(350)/kESR(250)
are 1.0−1.6
times larger than
kcal(350)/kcal(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.