A Flexible Water Structure : Evidence From Raman Spectroscopy

There are still large controversial ideas on water structure, mainly focused on classifying the hydrogen bonding configurations and their relative amounts. To understand water structure, the Raman spectra of H2O/D2O under different temperatures, H <-> D isotopic substitution (IS) ratios and chlorine ion concentrations. For the OH stretch bands, the bandwidth is largely reduced, the main peak blue-shifts and the relative intensity between the shoulders and the main peak changes as the temperature rises from 253 to 753 K, indicating various hydrogen bonding configurations in water. Using Gaussian deconvolution, we assign these spectral features to the five main hydrogen-bonding configurations : two tetrahedral, single donor (SD) , single hydrogen bonded water (SHW), and free water (FW). The tetrahedral configuration is the structural basis for the bending mode, librational+ bending sum mode and intermolecular couplings. The estimated tetrahedralitlly indicates a more flexible water structure, and most of the structure configurations in water are nontetrahedral. The temperature increase leads to a water structure transition from the tetrahedral to a single donor, single hydrogen-bonded water and free water. IS can reduce the relative intensity of the lower-frequency shoulder to the main peak but intensify the higher-frequency shoulder, and especially, the higher-frequency shoulder turns into the main peak of the OH/OD bands for the V-H2O/V-D2O =1/4 or 4/1 water. These spectral features strongly support a multi-structure mode of water : IS promotes the transition of hydrogen bonding structures that the primary O-D center dot center dot center dot O(O-H center dot center dot center dot O) (non)tetrahedral hydrogen bonding configurations are decreased due to the O-D center dot center dot center dot O <-> O-H center dot center dot center dot O transformation. The transition of structures in water with the temperature rise is further evidenced from the observation that the FW mode is intensified with IS so that it becomes the main peak of the OD/OH stretch band at higher temperatures. The multi-mode concept concerning the multi-structure body of water is sufficient to explain the OD/OH band features for H2O/D2O-NaCl solutions. Adding NaCl can greatly reduce the population of the tetrahedral configurations and transform them to SD and SHW at temperatures lower than 433 K, but above 513 K, NaCl slightly enhances the hydrogen bonding in water. The classification scheme on the structureal details in water derived from the Raman spectral features at wide condition ranges by this study can provide the theoretical basis for the spectroscopic and structural studies in the area of geologic aqueous fluids.