Magnetic properties of paramagnetic systems: Density functional studies

The main concern of this thesis is to investigate the magnetic properties of paramagnetic species which have unpaired electrons. The present study is based on density functional theory (DFT).

In order to simulate magnetic resonance parameters efficiently, we propose a combined quantum mechanics/molecular mechanics (QM/MM) approach. The quantum region is truncated by one-electron quantum capping potentials (QCPs) which are parameterized to duplicate all-electron molecular structures and charge distributions. The effects from the surroundings are modeled by including the electric field, due to the MM domain, in the DFT Hamiltonian. This approach shows that the electrostatic effects from the MM part mainly transfer to the QM part implicitly through the field-free Kohn-Sham orbitals and through the corresponding energies. We examine several model systems ranging from small organic molecules to biological models. Because of the simplicity of the implementation, the present approach enables us to investigate the magnetic parameters of large, realistic models of biological active sites.

For paramagnetic NMR shifts, we provide a general expression which is independent of empirical parameters and give the recipe for practical calculations. For a special case (spatially non-degenerate Kramers doublet) with no thermally accessible excited states, the working equation for the paramagnetic shifts is derived by using an effective spin Hamiltonian and Boltzmann statistics. The paramagnetic NMR shifts are decomposed into the three contributions within the equation: the orbital, Fermi contact, and pseudocontact contributions. The individual contributions are determined by the first-principles calculations of the magnetic resonance parameters. For validation, the DFT calculations are carried out for the NMR chemical shifts of some nitroxides, blue copper proteins, and ferric cyanide-imidazole complexes. The theoretical studies, compared with the experimental works, indicate mainly three things: (1) the present approach within the DFT framework provides reliable and promising simulations; (2) the orbital shifts are readily approximated by the NMR chemical shifts in similar, closed-shell environments; (3) the Fermi contact shifts dominate the total shifts and they are very sensitive to structural changes.