Ab-initio study of the electronic factor (Δ) conformational dependence in [CH2-(phenylene)n-CH2]− electron transfer model systems

The dependence of Δ and of the decay constant (β) on the methylene-adjacent benzene dihedral angle (θ), for thermal electron transfer in [CH2-(phenylene)n-CH2]− (n = 1, 2, 3) model systems, is studied. Closed-shell Hartree-Fock (RHF) ab initio, through use of Koopmans theorem (KT), open-shell HF (ROHF) and multiconfigurational SCF (MCSCF) level calculations are performed, with several basis sets. A non-exponential distance dependence of Δ2 between the methylene groups for some values of θ is found, due to potential energy crossover for the molecules containing one and two phenylene units and due to a remarkable different dependence of Δ2 on θ for n = 3, at MCSCF level. The effects of orbital relaxation and electronic correlation in both Δ and are also analyzed. Though Δ values depend significantly on θ, β values are fairly dependent on this angle in the ranges 0-70° (at KT level), 0-75° (at ROHF level) and 0-50° (at MCSCF level) and show a sudden increase at ∼85-90°, points at which an exponential behavior is obtained.