Theoretical studies of long-range electron transfer reactions: Development of new methods and applications

A new method for accurate calculations of donor/acceptor electronic interactions for long-range electron transfer (ET) reactions was developed. The technique uses a Divide-and-Conquer (D&C) strategy to explore the electronic structure of large molecules, In this approach, a molecular system under study is divided into a collection of overlapping fragments and accurate ab initio calculations are performed for each fragment. The effective hamiltonian matricies of molecular fragments defined on local valence atomic orbitals are generated from the projected electron propagator matricies. The effective hamiltonian of the molecular system is constructed from the effective-hamiltonian matricies of its fragments and then electronic interactions between localized donor and acceptor states are calculated from energy splittings of eigen states of the effective hamiltonian (Heff).;To test the divide-and-conquer technique, we performed calculations of donor/acceptor energy splittings and Green function elements for model compounds. We applied the method to calculate electron transfer couplings in synthetic ET systems. These included a series of compounds with iridium-dimer donors and piridinium acceptors studied in Prof. Gray's laboratory and compounds with norbornyl bridges connecting excited state anthracene donors and cyanoethylene acceptors (C-clamp compounds), studied in the laboratories of Prof. Zimmt and Prof. Waldeck. For these systems, we also studied the dependence of the solvent reorganization energy of the ET reaction upon the molecular conformation using numerical solutions of the 3D Poisson equation. We proposed a procedure to correct for discrete solvent molecule effects in the description of the temperature dependence of the ET reorganization energy based upon the theory of Dr. Matyushov, and implemented the theory to interprete temperature dependence of ET rates in C-clamp compounds.;We applied the D&C/Heff techniques to study electron transfer reaction in cytochrome-b562 derivatives with chemically attached Ru-bpy redox groups. We found satisfactory agreement with experimental results if a proper consideration is given to the quasi-degenerate electronic states localized on the ruthenium species.;Finally, we examined ATP hydrolysis coupled electron transfer in the nitrogenase complex. We found that a redox potential of the Av2 nitrogenase compound is boosted by complex formation with the Av1 nitrogenase subunit by desolvation of the negatively charged Fe4 S4 cluster of Av2. We argued that this boost in the redox-potential is essential to ensure fast electron transfer between the Av2 and Av1 subunits.;We report an initial implementation of the Divide-and-Conquer technique to calculate the polarizability and optical rotational angles of large molecules. The method is based on the transferability of the particle-hole propagator in the basis of localized orbitals. The polarizability of a long linear alkane molecule was calculated by the reconstructing of the electrical dipole-electrical dipole tenzor from the calculations on the molecular fragments.;We implemented the Divide-and-Conquer method as well as other theoretical methods to examine biomolecules in the program package HARLEM (HAmiltonians for Response properties of LargE Molecules). The package provides a convenient user interface that will allow the methods developed here to be used by a wide community of scientists studing the properties of macromolecules.