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

Abstract Raman spectra of water and supercooled water were measured in the temperature range of −6 to 18°C with every 2°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°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 −6 to 18°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.