Application of trajectory surface hopping to the study of a symmetry-forbidden intramolecular hole transfer process in bismethyleneadamantane cation radical

A recently developed, Landau-Zener-based quasiclassical trajectory surface- hopping method for medium-sized organic molecules is used to investigate ho le transfer (HT) in the formally symmetry forbidden hole transfer process i n bismethyleneadamantane cation radical 4a and its d(4)-labeled analogue 4b . The calculations involve sets of 200 trajectories, sampled from a canonic al ensemble at 298.15 R, to directly calculate the mean first passage times , tau, for HT in both systems, together with Fourier transform analyses to identify the important modes which induce hole transfer. Very small tau val ues for hole transfer are predicted, despite the fact that the reaction is nominally symmetry forbidden. The main symmetry breaking mode is identified as the torsional vibration about the terminal methylene group on the one-e lectron, pi bond. An approximate secondary kinetic isotope effect is calcul ated, and is shown to be largely attributable to the change in frequency of the key torsional mode. The magnitude of the electronic coupling at the av oided crossing region for HT in both 4a and 4b is estimated to be 0.01 eV, placing the HT process within the nonadiabatic regime. It is found that qua litatively, the calculated tau values and the derived approximate secondary kinetic isotope effects are fairly insensitive to the method used to ident ify the avoided crossing region in the trajectory calculations. It is concl uded that the trajectory surface-hopping method described herein should pro vide useful qualitative insights into the effect of nuclear dynamics on ET and HT processes occurring in a variety of structurally complex systems of chemical interest.