Temperature-dependent structural variations of water and supercooled water and spectral analysis of Raman spectra of water in the OH-stretching band region and low-frequency region studied by two-dimensional correlation Raman spectroscopy

Raman spectra of water and supercooled water were measured in the temperature range of -6 to 18 degrees C with every 2 degrees C step. The obtained spectra were analyzed for the 3750-2850 and 400-100 cm(-1) regions by two-dimensional correlation (2D-COS) Raman spectroscopy. As previously reported, there are three bands at around 3620, 3420, and 3200 cm(-1) 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(-1) region of Raman spectra of water developed using the spectra measured in the temperature ranges of -6 to 2 degrees C, 0 to 8 degrees C, and 8 to 18 degrees 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 -6 to 18 degrees C range has a peak at 3171 cm(-1) with a broad shoulder at around 3400 cm(-1). These peaks at 3171 and 3400 cm(-1) may be assigned to the collective mode and its local mode of structured water, respectively. In the 400-100 cm(-1) region, there is a broad feature centered at 185 cm(-1) 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(-1) and a weak shoulder at around 150 cm(-1). We have assigned these two peaks at 185 and 150 cm(-1) 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(-1) regions reveals that there is a cross peak between the structured water band at around 3200 cm(-1) and the 185 cm(-1) band, confirming that the 185 cm(-1) band comes from the structured water.