What is Computational Chemistry?

Fig. 1 Cluster System of Computers

I am engaged in research in the area of “theoretical and computational chemistry” which is not taught in high school. One might think research in chemistry involves experiments, but I am tackling various themes in chemistry using a computer and without conducting any experiments. The phenomena at a molecular level happen at a size of 10-10 meter dealing in time periods of 10-15 second in this microscopic world, Newtonian mechanics does not hold up, and instead a completely different type of mechanics governs. This is called “quantum mechanics,” and it was established in 1926 thanks to the contributions from many physicists. The structures, physical properties and reactions of molecules can all be described in principle by solving the basic equations of quantum mechanics. However, these equations are hard to solve without computers. Thanks to the astonishing progress of computer technology these days, cutting-edge research can be achieved, without necessarily using supercomputers such as "K", by carrying out parallel computation with a cluster of personal computers. Both progress in molecular theory and the popularization of quantum chemistry programs improve the computation accuracy drastically and help expand research subjects in computational chemistry (1). In the chemistry field now, it is expected that experimental research is performed in conjunction with computational chemistry.

 

What Kind of Research are You Conducting?

Fig. 2 Mechanism of Photoisomerization of
Stilbene Molecule


Fig. 3 Branching Ratio of Photochemical
Reactions of Stilbene

I am engaged in the development of computational methods and programs that investigate elementary processes of photochemical reactions in order to extend the application targets in theoretical chemistry. In photochemical reactions, one molecule changes to another through the change of the molecular structure, such as cleavage or generation in bonds, which are caused by the transition to the electronically excited-state, triggered by the irradiation of UV light to the molecule. Figure 2 shows an example of the reaction of a stilbene molecule. Stilbene is a molecule in which two hydrogen atoms in ethylene are substituted with benzene rings and has a cis isomer where the two benzene rings are on the same side and a trans isomer where the two benzene rings are on different sides. These isomers are known to interconvert from the irradiation of UV light. The electronically excitation induces a structural change. A rotation around the CC bond axis occurs, which causes de-excitation to the ground state at around 90 degrees between the two benzene rings, resulting in a structural change to either a cis or trans isomer. The cis-trans photoisomerization reaction is one of the important mechanisms for the industrial development of light switch devices.
Experiments can hardly provide the details of the steps during a structural change in an electronically excited state, and therefore, the computational simulation is a powerful research tool. We are collaborating with the experimental group led by Dr. Tahei Tahara of RIKEN (2). The latest result shows that a trans isomer is dynamically generated eminently, although a cyclized isomer (DHP), which has a closed loop in the middle, is advantageously generated from the shape of a potential energy surfaces.

 

Tell Us about Research Results that will Impact Society

Since Japan is poor in natural resources, it may not thrive for long given the scarcity of elements. However, recently, the Element Strategy project is making progress to substitute precious metals or rare metals with common elements. Our group is collaborating with the experimental group led by Professor Kohei Uosaki of National Institute for Materials Science to pursue research on element strategy. Recently, we theoretically proposed and successfully verified through experiments the hypothesis: boron nitride (BN), which is basically an insulator, can function as an electrode catalyst for the oxidation-reduction reaction, which is an important reaction in fuel cells (3). For fuel cells, platinum is widely used as the catalyst in oxidation-reduction reactions, however alternative catalysts are sought and developed because of the cost and scarcity of platinum. We found that the support on a gold substrate of the BN in which defects are inserted can add conductivity and proposed the possibility for a catalyst in an oxidation-reduction reaction. Given this proposal, we actually observed the catalytic activity when checking the oxidation-reduction reaction of the newly-made specimen with BN supported on a gold surface. Going forward, combining approaches using theoretical computations and experiments will increase more and more in order to investigate and design new catalytic materials.

 

References

(1) Kimihiko Hirao, Tetsuya Taketsugu (Eds.) “An Easy Guide to Quantum Chemistry Calculations (Sugu Dekiru Ryoshi Kagaku Keisan Beginners Manuals),” Kodansha Scientific, 2006.
(2) S. Takeuchi, S. Ruhman, T. Tsuneda, M. Chiba, T. Taketsugu, and T. Tahara, "Spectroscopic Tracking of Structural Evolution in Ultrafast Stilbene Photoisomerization," Science, 322, 1073 (2008).
(3) K. Uosaki, G. Elumalai, H. Noguchi, T. Masuda, A. Lyalin, A. Nakayama, and T. Taketsugu, "Boron nitride nanosheet on gold as an electrocatalyst for oxygen reduction reaction – Theoretical suggestion and experimental proof," J. Am. Chem. Soc., 136, 6542 (2014).