Chief Scientist, The Institute of Physical and Chemical Research (RIKEN)
Molecular fingerprints
— Manipulating molecules via molecular vibration —
There are various states of bonding between molecules and solid surfaces on which the molecules are adsorbed. Without detailed knowledge about these states, chemical reactions between the surfaces and molecules cannot be understood clearly. Dr. Kawai has been conducting studies as a surface scientist from the stance, “Issues should be discussed along with direct observation of actual reactions.”
Dr. Kawai adds: “Molecules’ vibrations are said to be their fingerprints. Information about the vibration energy of a molecule signals that particular functional groups exist in it. Therefore, we can identify the type of molecule by measuring its vibration energy and we can also excite the vibration of a specific functional group of a molecule.” She has succeeded in converting a molecule from one type into another by exciting its vibrations.
A trans-2-butene (C4H8) molecule has four carbon atoms, two of which form a double bond at the center. When tunneling electrons are injected into a trans-2-butene molecule on a Pd(110) surface from the tip of a scanning tunneling microscope (STM) for exciting the stretching vibrations of its C-H, the quantized stretching vibrations of the C-H are amplified in a ladder-climbing manner. The C-H bond is dissociated eventually as a result of the vibrational excitation. The trans-2-butene molecule is converted into a butadiene molecule through the dehydrogenation process. The two hydrogen atoms dissociated through this process migrate to the palladium. She says, “We have confirmed through inelastic electron tunneling spectroscopy with STM that a trans-2-butene molecule is converted into butadiene molecule. If we want to give a specific function to a molecule, we may need to change its target functional group. We will be able to make only the target molecule react as long as we can identify the molecule in a specific position and excite their specific functional groups with the level of vibration energy chosen only for this purpose.”
Dr. Kawai says she was interested more in physics than chemistry until high school because in physics, phenomena that appear to be complicated can be described in terms of simple relationships between physical quantities. But at university she majored in chemistry, and studied under the supervision of Prof. Kenji Tamaru, who researched solid catalysts. Dr. Kawai says, “Rules of thumb play a key role in studying catalysts.” Why catalysts promote chemical reactions effectively is not yet clearly understood. She says, “When you want to facilitate a certain chemical reaction by using a catalyst, you can determine by testing the catalyst whether it is effective for promoting the reaction. But if there were a database for predicting possible reactions among certain materials, it would be very convenient for researchers. Such basic research may be the type of study conducted mainly by physicists. I like that type of research. I enjoy the variety in chemistry, in which there are many reaction patterns among various materials. Beyond that, I think predicting various chemical reactions is one of the goals in science.”
