The Sea is So Wide, So Great

Satoshi Yabushita (Professor, Department of Chemistry)

Chemistry is a discipline that has developed significantly through repeated trial and error in experiments and analysis. However, it is now clear that in any of chemistry's targets—chemical reactions, molecular structures, or physical properties—the leading roles are played by atoms and molecules, which are aggregates of atomic nuclei and electrons. This makes it entirely possible to adopt a research style based on the position asserted more than 80 years ago by P. A. M. Dirac, one of the founders of quantum mechanics, a fundamental law of chemistry: "The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known..." In other words, just as the date and time of an annular solar eclipse can be accurately predicted, we are now in an era where chemical phenomena can, in principle, be analyzed and predicted. From this standpoint, we conduct theoretical research to elucidate various chemical phenomena by making full use of our minds, bodies, and computers, focusing particularly on the interaction between molecules and light. Here, for the summer season, I will introduce a research example related to the color of the sea.

Many of you probably know that the blue of the sky and the red of the sunset are caused by the phenomenon of light scattering. So, what is the cause of the sea's color? Lord Rayleigh (1910) stated that it is because the blue of the sky is reflected, while C. V. Raman (1922) attributed it to light being scattered by water molecules in the water. Later, Hiroshi Tsubomura et al. (1980) observed that visible light, although very weak, is absorbed by the vibrational motion of water molecules, and that long-wavelength red light is absorbed more strongly than blue light. They announced that absorption, not light scattering, is the cause of the color.

As shown in Figure 1, the OH bond of a water molecule H ₂ O can be regarded as a simple harmonic motion that is electrically polarized as O ^δ− -H ^δ+ . For this reason, when infrared light with a frequency ν, which is the natural frequency of the simple harmonic motion, is applied to a water molecule, the light is absorbed due to a resonance phenomenon. However, the force acting between the OH atoms includes not only a force proportional to the first order of displacement from the equilibrium internuclear distance (simple harmonic motion) but also higher-order forces called anharmonic terms. Furthermore, the charges on the atoms (the values of δ+ and δ−) also change with the molecular vibration. For this reason, absorption also occurs at frequencies that are integer multiples (called overtones) of the resonant frequency, nν, and it is known that the intensity of this absorption weakens as n increases. Our research has also shown that among the overtones of the water molecule, red light in the visible region (n=4) is absorbed 100 times more strongly than blue light (n=6), although it is weak.

Furthermore, we have theoretically created the color produced by the differences in absorption intensity at each wavelength and shown it in the lower right of Figure 2. Although the shade of the color changes depending on the distance the light travels, it is thought that the water in seas and lakes appears blue because the red components of sunlight are efficiently absorbed, allowing the complementary blue color to reach our eyes.

We have entered an era where we can use computers to evaluate how charge and energy change with molecular vibration. I have presented three theories about the color of the sea above. As this shows, it is common in the developmental stages of research for several models to be considered to explain a single experimental fact. To select the correct one among them and obtain “the right answers for the right reasons,” the approach of theoretical chemistry is extremely effective. This trend is expected to become stronger year by year.

Recently, the match between a professional shogi player and a computer has become a hot topic. In connection with this, I have been pondering an article arguing that the recent global increase in unemployment is because humans are beginning to lose the competition against machines, including computers. In another 20 years, the style of chemical research will likely have changed considerably. Of course, a similar trend is sweeping through other scientific fields as well. For students who are future researchers, what and how should you learn at university? Leaving aside theoretical chemistry, I hope that you will aim for a way of learning that develops creativity and critical thinking skills—areas where humans can maintain an advantage in the future—rather than focusing on content that can be left to computers.