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 ([hv(in)]) 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 [hv(in)] = 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 AMI 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 Gamma(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.