Raman spectra of water and supercooled water were measured in the temperature range of À6 to 18 C with every 2 C step. The obtained spectra were analyzed for the 3750-2850 and 400-100 cm À1 regions by two-dimensional correlation (2D-COS) Raman spectroscopy. As previously reported, there are three bands at around 3620, 3420, and 3200 cm À1 in the OH-stretching region.These bands were assigned to the OH-stretching modes of dangling (Dang) bonds of water, destructured, and structured water species, respectively. A pair of clear peaks appear in asynchronous 2D-COS maps in the 3750-2850 cm À1 region of Raman spectra of water developed using the spectra measured in the temperature ranges of À6 to 2 C, 0 to 8 C, and 8 to 18 C, and they are similar to each other, suggesting nonlinear (convex) temperature-dependent increase and decrease of the two kinds of water species. A power spectrum calculated along the diagonal line in the synchronous spectrum in the À6 to 18 C range has a peak at 3171 cm À1 with a broad shoulder at around 3400 cm À1 . These peaks at 3171 and 3400 cm À1 may be assigned to the collective mode and its local mode of structured water, respectively. In the 400-100 cm À1 region, there is a broad feature centered at 185 cm À1 assigned to the intermolecular stretching mode of water molecule. Close inspection of the low-frequency region by the baseline corrected spectra and the second derivative spectra shows that the broad feature consists of a major band at around 185 cm À1 and a weak shoulder at around 150 cm À1 . We have assigned these two peaks at 185 and 150 cm À1 to the structured and destructured water, respectively, based on the results of 2D-COS and the comparison with the results of multivariate curve resolution-alternative least squares reported by Hamaguchi et al. A hetero 2D correlation synchronous map between the 3750-3000 and 400-100 cm À1 regions reveals that there is a cross peak between the structured water band at Hiroto Asano and Nami Ueno contributed equally.
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