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.
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The present paper considers the density effect on the water vapour absorption profile in the OH stretch range. The density evolution of the water vapour OH fundamental band shape is interpreted in terms of the monomer–dimer equilibrium. In contrast to previous works we adopt experimental values for the frequencies and IR intensities of the dimer vibrations in the vicinity of 3µm. This made it possible to reduce the number of unknown parameters required in the course of our spectral fit.
