He received his bachelor’s degree (BS) in chemistry in 1968 from the University of Tokyo. He then enrolled at the Graduate School of Science at the University of Tokyo to receive a master’s degree (MS) in chemistry, consequently becoming an assistant professor in 1972. He subsequently received his PhD degree (DSc) in chemistry in 1973. He then became an associate professor (1980) and a full professor (1989) at the University of Tokyo. He joined the Department of Chemistry at Tohoku University in 1994 to establish a group focusing on theoretical chemistry. After retirement, he worked as a fellow at the Toyota Physical and Chemistry Research Institute until 2013. He launched the Institute for Quantum Chemical Exploration (IQCE) and was the first director and president from 2013 until his final days.
The following three items show his highly original achievements in introducing a new paradigm in the field of theoretical chemistry based on extensive research on potential energy surfaces (PES).
Molecular vibrations: potential profile inside molecules
He invented a novel calculation method, MO/8, which can be conceived as a variation of the Hückel method for molecular vibration calculations. This method has been successfully applied to polycyclic aromatic hydrocarbons with several carbon honeycomb structures to obtain accurate information on the vibrational modes and associated vibrational excited states. The effective incorporation of anharmonicity enables it to overcome the long-standing obstacle of evaluating not only the fundamental tone but also the overtone and combination lines with high accuracy.
Frontier orbital properties of molecules: potential profile at the forefront of molecules
He proposed a unique method for analyzing the shape and reactivity of the frontier orbitals of molecules. To this end, he first developed a two-dimensional Penning ionization electron spectroscopy (2D-PIES) method using a noble gas to experimentally investigate the surface properties of frontier orbitals. His theoretical endeavors opened up a novel analysis method called the overlap expansion (OE) method, which enabled investigation of the molecular surface in more detail by determining the atom–molecule interaction occurring at the frontier orbital. In brief, the hardness and softness of the interaction potential when the probe atom and the target molecule interact in a certain geometry can be visualized in three dimensions based on both experimental and theoretical investigations. Moreover, the ionization state and collision direction can be identified by highly sensitive collision-energy-resolved PIES, providing information on the accurate profile of the spatial distribution of wave functions at the forefront of various molecules.
Global reaction route mapping (GRRM): extensive search on potential energy surfaces
He first discovered that the reaction path could be effectively selected based on the principle of anharmonic downward distortion (ADD) in quantum chemistry. He proposed the scaled hypersphere search (SHS) method based on ADD analysis for an extensive search of the potential energy surface. This method enables automatic searching for the reaction pathways between stable structures, transition states or dissociation fragments, which was previously extremely difficult, even with the fastest computers. These findings enabled the discovery of an enormous number of reaction pathways and novel molecular structures, such as prism carbon molecules and various isomers composed of five different atoms, BCNOS. An enormous extension of the exploration range in PES was realized. This provides an effective means to discover novel synthesis methods with minimal consumption of precious materials, as well as the discovery of currently unknown molecules to solve environmental issues.
He published 280 original papers, 28 review articles, and several books. For these outstanding achievements, he was awarded the Chemical Society of Japan Award for Creative Work and recognized as a fellow member of the Chemical Society of Japan and an honorary member of the Japan Society for Molecular Science. Recently, the number of GRRM user groups exceeded 300 and continues to grow globally. He devoted his entire life not only to his own research activities, but he also supported researchers and colleagues of all generations through funding for research and scholarships, notably by the IQCE. He organized many lectures and meetings to support the quantum chemistry community in Japan and to promote exchange and communication worldwide.
He is survived by Yaeko, his wife, one son, one daughter, and four grandchildren. He enjoyed gardening at home and driving his car to rural places. He is well-remembered as a gifted amateur athlete, especially at skiing and playing baseball as a pitcher.
