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

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 ). 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.