Near-Infrared Spectroscopic Investigation of the Hydrogen-Bond Network of Water Using Two-Dimensional Correlation Spectroscopy and Difference Spectral Analysis

This study investigates the near-infrared (NIR) spectra of water to gain molecular-level insight into its hydrogen-bond network using two-dimensional correlation spectroscopy (2D-COS), difference spectra, and second derivative analysis. The NIR spectra exhibit two major bands corresponding to water species with tetrahedral structure (THS) and pseudotetrahedral structure (PTHS). Both bands are notably broad, particularly the band assigned to THS, suggesting that THS species exhibit significant configurational heterogeneity, which gives rise to a range of hydrogen-bonding environments. Temperature-dependent 2D-COS analysis suggests that thermally induced distortions of the hydrogen-bond network are more sensitively reflected in the spectral features associated with THS than in those associated with PTHS. Furthermore, the combination band involving the H-O-H bending vibrational mode was found to be more sensitive to temperature changes than that involving only O-H stretching vibrations. 2D-COS provides valuable information not only for establishing correlations between NIR bands and the corresponding vibrational features in the IR region but also for characterizing the order of spectral variations. These results provide new insights into the heterogeneity and dynamics of the hydrogen-bond network in water.