Density effect on infrared spectrum for supercritical water in the low- and medium-density region studied by molecular dynamics simulation

This is the authors' abstract. We don't add key points for this paper.

The origin of the line shape of the O-H stretch vibrational spectrum is analyzed for supercritical water in the low- and medium-density region by using classical molecular dynamics simulation for the flexible point-charge model, SPC/Fw. The spectrum calculated for the water model is in good agreement with the experimental one in the low-density region. The spectral origins in the low-density region of 0.01-0.04 g cm(-3) are assigned to a sharp peak due to the bond oscillation along the O-H vector and two broad bands due to the rotational coupling, by taking an isolated single molecule as a reference in the low-density limit. The bands due to the rotational coupling reduce in intensity with increasing density as the rotations are more hindered by the hydrogen-bonding interactions, and their intensities increase with increasing temperature due to the accelerated rotational motion. The O-H stretch oscillation in the time correlation function attenuates in a timescale comparable with the lifetime of the hydrogen bonds, and the spectra conditioned by the number of hydrogen bonds are dominantly controlled by the local solvation structure.