Theoretical Study of the Electronic Factor in Electron Transfer Reactions
The electronic factor D is calculated for a number of organic and biological electron transfer systems. Quantum chemical methods at different levels of accuracy are used: ab initio Hartree-Fock, broken-symmetry, and correlation-corrected methods such as multiconfigurational second-order perturbation theory, and also the semiempirical CNDO/S method for very large systems.
The systems studied are characterized by an electron donor-acceptor part which is separated by a rigid molecular bridge. Even if the bridge itself is not oxidized or reduced during the electron transfer process, it plays a fundamental role in promoting donor-acceptor interactions. This type of bridge-mediated electron transfer or "through-bond" mechanism is analyzed in detail and attempts are made to rationalize its variation for different systems.
Most of the bridges considered here are saturated bridges (polycyclohexanes, polynorbornanes, bicycloalkanes, etc.), however, some aromatic ones (a,w-diphenyl-stilbenes, p-benzoquinone, aromatic residues in the photosynthetic reaction center) are also studied. The dependence of the calculated D on the electronic structure and increasing length of the bridge is analyzed in detail. The influence of stereoelectronic effects such as the substitutional position and angles of donor and acceptor is also considered.
For many of the systems studied correlation effects are included in the calculation of D. The broken-symmetry correction accounts for the polarization effect around the hole or extra-electron after ionization or electron transfer, respectively. For some compounds a more extensive treatment of the electronic correlation is done using multi-configurational-SCF methods (CASSCF and RASSCF) which accounts for all-near degeneracy effects and includes full orbital relaxation. Dynamic correlation effects are also included, using second-order pertubation theory with CASSCF wavefunction as the reference state (CASPT2). These highly accurate methods, however, are very demanding in computational resources and therefore limited to the study of relatively small systems.