Spin and electron correlations play important roles in electronic structure theories of unstable intermediates of chemical reactions and of large molecules with peculiar electronic, magnetic and optical properties. Professor Kizashi Yamaguchi has been developing Broken-symmetry (BS), resonating BS (RBS) and related multi-reference (MR) theories and computer programs for such strongly correlated electron systems.
A lot of first principle calculations based on the methods have been carried out for analyses and design of functional molecule-based materials as described below.
1.Theory of quantum chemistry for systems with a strong electron correlation and spin-spin interactions.
Prof. Yamaguchi started his study at early 1970s. He examined the electronic structure of the molecule whose HOMO-LUMO energy gap was smaller than coulomb repulsion energy between opposite spins (e.g. alpha and beta spins) at HOMO. In those systems, an one-electron exited triplet state became stable in comparison with a closed-shell singlet ground state. This phenomenon, which was called a triplet instability, broke spin-symmetry and caused localized spins even in singlet state. Through the studies, he pioneered a new calculation method called the broken-symmetry (BS) method in its early period. Subsequently, he applied the BS method for biradical systems and anti-aromatic systems. His analyses of spin polarization effects were summarized as a spin-alignment rule based on a magnetic group theory. The spin-alignment rule and related results already predicted that the strong electron-electron and spin-spin correlation brought an anti-ferromagnetism at the time. He also developed a general spin orbital Hartree-Fock (GSO HF) method and an approximate spin projection HF (APHF or Extended HF (EHF)) method in order to express more complicated electronic structures by using simple way. He finally sophisticated a method of quantum chemistry to examine electronic structures of systems with the strong electron correlation by taking dynamical correlation effects based on natural orbitals of the GSO HF method. In recent years, he and his coworkers succeeded to develop software for muti-reference (MR) DFT and CAS DFT.
2.Derivation of chemical bond indices and their application to chemical reactionss
Prof. Yamaguchi indicated that mechanisms of chemical reactions could be classified into four types (concerned, twitter-ionic, biradical, and CT biradical) if we focused on symmetry breakings of molecular orbitals and the triplet instability. And he pointed out that charge and spin density distributions calculated by the BS method could be utilized for the purpose of classifications. Its utility has been demonstrated by analyses of singlet oxygen, transition metal-oxo complexes, olefins and so on by his group. He also derived chemical bond indices such as a biradical index, information entropy, unpaired electron density and so on for an assessment of nature of chemical bonds. It has been shown that the indices are powerful tools to analyses of chemical bonds4). His group is applying this method to analyses of electronic structures of multinuclear transition metal complexes, active sites of metalloproteins that has temperature-dependent paramagnetism.
3.Application to molecular design.
In 1980s, Prof. Yamaguchi introduced Heisenberg model for explanations of interactions between localized spins. He developed a methodology for calculating the effective exchange integrals (J) values by using the BS method. In 1986, he found that ferromagnetic interaction between nitroxide radicals could exist according to a stacking form. Consequently, a series of his studies about molecular magnetism of the organic radicals predicted ferromagnets by pure organic molecular crystals. Furthermore, he demonstrated a possibility of the molecular design for molecular magnetic metals by quantum chemistry calculations. On the other hand, he proposed a hypothesis that critical temperatures (Tc) of superconductors could be estimated by magnitude of J values since superconductor of copper oxide, which has high-Tc involved strong anti-ferromagnetic interactions between Cu ions. He expanded his idea into a possibility of superconductor consisted of π-d and π-R conjugated systems based on spin excitation model instead of charge excitation model by Little. His idea has been verified by theoretical calculations and various model systems for organic superconductors have been proposed by his group. His group has also designed molecules involving peculiar electronic or optical properties besides magnetic properties by theoretical calculations.
4.Theoretical calculations of real systems and efforts to "Material-Oriented Quantum Chemistry"
In 1990s, Prof. Yamaguchi organized a research group of "Material-Oriented Quantum Chemistry". At that time, many syntheses of molecules, molecule assembling systems and molecular solids possessing unique electronic and magnetic properties were reported. He applied above theories, methodologies and programs to such systems in order to analyze their electronic structures and to elucidate their nature. One of the impressive studies was a series of results about electronic structures and magnetic interactions of large polynuclear metal complexes. His group demonstrated that the improved hybrid DFT method could reproduce experimental J values quantitatively even in such large complexes. From those results, he could explain chemistry in such very complicated electronic structures. He carried out his theoretical studies in collaboration with many experimental researchers.
All those achievements in his studies are praised not only domestically but also internationally. And the Chemical Society of Japan acknowledges that his achievements are worth The Chemical Society of Japan Awards for 2006.