Environment-Induced Changes In The Structure Of Water By Two-Dimensional Raman Correlation Spectroscopy

Water is fundamental to human existence and is widespread in nature.Despite the simplicity of its elemental composition,the study of liquid water was challenging.The mysterious nature of water is closely related to its complex hydrogen bond（H-bond）network.Binary solutions formed by the interaction of water with other substances play an important role in physical,chemical and biological processes.Complex interactions affect not only the structure of water,but also the dynamic properties of water molecules.Therefore,changes in microstructure and molecular dynamics processes in aqueous solutions have been a hot topic in the field of water-organic chemistry.This paper focuses on the structural changes in water induced by organic molecules using two-dimensional Raman correlation spectroscopy（2D Raman-COS）,and elucidates the roles of H-bond and hydrophobic effects in organic molecule-induced the enhancement of water structure.The specific results were as follows:（1）The ethylene glycol（EG）-induced water structural transformation under external perturbations of concentration and temperature were investigated by Raman spectroscopy.The spectral change in the O-H stretching mode indicated that when the molar ratio of EG/water is less than 1/28,EG-induced enhancement of water structure,resulting from the hydrophobic effect around the methylene groups of EG.Further increase in EG concentration,the H-bond structure of water was gradually weakened.Decreasing the temperature of the aqueous EG solution,the Raman peak of ice Ih was observed at 3120 cm^-1.Keeping the external temperature perturbation constant,the Raman peak of ice Ih decreases with increasing EG concentration due to the strong H-bonding between EG and water molecules,which breaks the H-bonding network structure of water,resulting in a lower freezing point of water.In addition,the decrease in temperature induces an enhancement of the Raman peak at about 3200cm^-1,representing an increase in the orderliness of the water molecules.Further analysis of the EG-induced water structural transformation by 2D Raman-COS reveals that the strong H-bond structure of water preferentially responds to external perturbations,and induces a transition to the weak H-bond structure of water.Finally,the EG-induced water structural transition was analyzed using density functional theory（DFT）to provide evidence for the spectroscopic study.（2）The role of H-bonding and hydrophobic effects in the enhancement of the H-bonding network of water induced by organic alcohols was investigated using Raman spectroscopy.The spectral changes in the H-O-H bending mode demonstrated that n-propanol（NPA）induced a blue shift and methanol caused a red shift over their entire range of compositions,while ethanol exhibited a blue shift only when the ethanol/water molar ratio was below 1/5.These results indicated that the extent of alcohol-induced water structure enhancement increased as the length of the alkyl chain increased.Further comparison with aqueous EG solution revealed that the enhanced water structure stemmed mainly from the hydrophobic effect rather than the hydrophilic effect of hydrophilic hydroxyl groups.Alcohol-induced water structural transitions were further analyzed using 2D Raman-COS,which showed that the free O-H and strong H-bond structure of water respond preferentially to changes in alcohol content,inducing a transition in the weak H-bond structure of water.In addition,the CH₃ stretching mode of alcohol responds preferentially to variations in water content compared to other C-H vibrational modes,which implies that hydrophobic effects occur predominantly around the methyl group of the alcohol.Finally,the alcohol-induced structural transitions of water were analyzed using DFT,which was consistent with the experimental results.（3）The structural changes in water induced by dimethyl sulfoxide（DMSO）,acetone,and isopropanol（IPA）were investigated by Raman spectroscopy combined with derivative spectroscopy under external perturbation of concentration.The results show that the organic molecules induce water structure enhancement mainly from the strong H-bonding of hydrophilic groups in the approximate agreement of hydrophobic effects.In addition,the second-order derivative spectral results suggested that the spectral decompositions of the O-H stretching mode into three sub-bands are more reasonable,located at 3225,3450 and 3625 cm^-1,respectively.Finally,the homospectral 2D Raman-COS in the O-H stretching mode showed that the strong H-bond structure of water responds preferentially to changes in the concentration of organic molecules than the weak H-bond structure.The heterospectral 2D Raman-COS formed by the correlation of O-H and C-H stretching modes indicated that the spectral peaks of the water structure change in preference to those of the C-H stretching mode as the DMSO concentration increases,suggesting that weak H-bond C-H…O may be present in aqueous DMSO solution.In contrast,the heterospectral 2D Raman-COS of binary solutions of water with acetone or IPA demonstrated that the methyl group does not H-bond with the surrounding water molecules.（4）Spontaneous and stimulated Raman spectra were used to study the changes in intermolecular interactions in aqueous solutions of N,N-dimethylformamide（DMF）at different concentrations.The results showed that when the DMF/water molar ratio reached 1/6.5,DMF induced a transition in the H-bond network structure of water,forming DMF-3H₂O and DMF-2H₂O complexes.The homospectral 2D Raman-COS showed that the free O-H and strong H-bond structure of water respond preferentially to the increase in DMF content than the weak H-bond structure of water.The heterospectral 2D Raman-COS indicated that the CH₃ symmetric and asymmetric stretching mode respond preferentially to the increase in DMF content,suggesting that the hydrophobic group of DMF does not H-bond with water molecules.The subjects chosen for this paper extend from unique EG molecules to a range of alcohol molecules,then to general organic molecules,and to the specific DMF molecule,confirming the universality of these results for aqueous solutions of organic molecules.Our results contribute to understanding the microphysical mechanisms of intermolecular interactions in aqueous solutions of organic molecules,thus advancing the study of other complex water-organic chemistry.