Deeper understanding of IR and Raman spectra of water including low-frequency regions and comparison between the IR...

Temperature-dependent IR and Raman spectra variations of water in the OH stretching band region (4000-3000 cm^-1) and low-frequency region (1000-50 cm^-1) were measured in the temperature range of 20-40 °C. We compared a transmission IR spectrum of water with its attenuated total reflection (ATR)-IR spectra with depth penetration correction or anomalous dispersion correction and found that the OH stretching band region of the ATR-IR spectrum with anomalous dispersion correction is close to that of the transmission spectrum in terms of the band positions and the relative intensity of two OH stretching bands. The OH stretching band regions of the second derivative spectra of the IR and Raman spectra show two bands at 3404 and 3237 cm^-1 (IR transition spectrum) or 3430 and 3200 cm^-1 (Raman). The 3404 (IR) and 3430 (Raman) cm^-1 bands are assigned to water species with pseudo tetrahedron structure (PTHS) and while those at 3237 (IR) and 3200 (Raman) cm^-1 are ascribed to water species with tetrahedron structure (THS). In the IR spectra the PTHS and THS bands show different temperature-dependent variations in band widths, band shifts, and intensity changes, suggesting that PTHS and THS yield significantly different thermal responses. The two OH bands in the second derivatives are significantly different between the IR and Raman spectra in the band frequencies, band widths, temperature-dependent shifts, and temperature-dependent intensity changes of two OH stretching bands. The spectral differences in the OH stretching region between the IR and Raman spectra probably arise from the differences in the relative contribution from the OH symmetric stretching mode and the OH antisymmetric stretching mode to the OH bands. Two-dimensional (2D) IR correlation spectroscopy (COS) of temperature-dependent variations of IR spectra of water in the low-frequency region reveals that there are three bands at 800, 660, and 310 cm^-1. The former two bands are due to the librational L2 modes of the THS species, and the last one is assigned to that of the PTHS species. These results suggest that there are three kinds of water hydrogen-bonding networks in liquid water. Two of them are THS water hydrogen-bonding networks with different three-dimensional structure and another one is that of PTHS.