Dynamics Calculations on Bis(hydrazine) and Bis(hydrazyl) Radical Cation Intramolecular Electron Transfer

Bis(hydrazine) [22/HEX/22] exists principally in
the unsymmetrical double nitrogen inversion form at
230 K. UHF/AM1 dynamics calculations starting from the radical
cation of the above material and syn and anti
[B/TET/B]+, each constrained to
have equivalent dinitrogen units so they represent the electron
transfer transition
states, indicate that many modes (10 or more) contribute to reaching
the electron transfer transition state, and that
the frequencies involved are predictable from those of the relaxed
radical cations. The results were analyzed to give
average barrier-crossing frequencies activated at the transition state
for electron transfer (〈hνin〉) of 827,
1403, and
1293 cm-1, respectively. A V
of 5.0 kcal/mol was obtained from the experimental data for
[22/HEX/22]+
when
used with the first-order adiabatic energy surface expression (text eq
4), and of 4.7 at 〈hνin〉 = 827
cm-1 using
Jortner's single averaged vibronic coupling treatment (text eq 6).
These V values are between that estimated
using
Hush theory on the CT band (3.9 kcal/mol) and that obtained by AM1
calculations (6.1 kcal/mol). The Holstein
tunneling coefficient expression (text eq 7) gives poor fit to our
data, but better fit is attained using a multiple mode
fit with a Hush equation (text eq 10) which employs a preexponential
factor for Γ(q) which makes tunneling of
increasing importance as the number of modes increases, and
high-frequency modes contribute significantly even
when they have tiny contributions to the barrier.