Research On The Structure And Dynamics Of Ions And Small Organic Molecules In Aqueous Solutions And Construction Of Optical Paths For Ultrafast Spectroscopy Experiments

Water(H2O)has always been one of the hot issues in the field of physical chemistry.Water is vital to all known forms of life,making it a "solvent for life".Water molecules possess a large amount of unique physical and chemical properties that are inextricably linked to the hydrogen bond network structure of water molecules.In addition,the change of three-dimensional hydrogen bond network structure of water molecules happens at the time scale of ps(10-12 s)or even fs(10-15 s),which cannot be observed by general techniques.When adding ions,organic small molecules and other substances into aqueous solutions,whether specific ions will affect the water microstructure,how the structural dynamics of water molecules in the organic small molecule-water binary system will change,etc.A technique with ultrafast time resolution is needed to investigate the upper scientific issues.The development of ultrafast multi-dimensional vibrational spectroscopy provides a suitable means for studying the dynamics of the above-mentioned problems taking place at ps and even fs time scales.Based on this,this dissertation uses the ultra-fast multi-dimensional vibrational spectroscopy technology setted up to investigate the structural and dynamical behaviors of water molecules after the addition of ions and organic small molecules.The dissertation mainly consists of the following three sections:In the first section,we investigated the specific cation effects on the vibrational relaxation dynamics of water molecules in the hydration shell of C104-in NaClO4 and LiClO4 aqueous solutions using polarization selective IR pump probe experiment.The reorientation of the perchlorate-bound water molecules show a bi-exponential decay and the slow component reorientation time constant is sensitive to function used to fit the rotational anisotropy decay.We showed that rotational dynamics of water molecules that hydrogen bonded to anions is restricted and cannot decay to zero in the concentrated solution.The cation effects(Li+,Na+)on the vibrational relaxation dynamics of water molecules hydrogen bonded to ClO4-are relatively negligible even at high concentration of salts.In the second section,by combing the FTIR spectroscopy and the ultrafast IR spectroscopy,the structure and dynamics of DMSO aqueous solutions were revealed by investigating the vibrational relaxation dynamics and rotational dynamics of SCN-anion serving as a local vibrational probe.Our results indicated that the rotational dynamics of SCN-anion are slowed down significantly with the presence of DMSO and the concentration dependent rotational dynamics showed a maximum response at XDMSO=0.35.By comparing the rotational dynamics of SCN-and water molecules,it is proposed that the DMSO and water molecules form complex structure and have similar.Also the SCN-is positioned at the vicinity of the complex structure and can reflect the structural dynamics of the local environment.The DMSO clustering or micro-heterogeneous structures were not supported by our data.Also,we used non-resonant second harmonic generation(SHG)to investigate the molecular structure and adsorption of DMSO at air/neat DMSO liquid and air/DMSO aqueous solution interfaces.The molecular orientation of interfacial DMSO as a function of the bulk DMSO concentration was investigated by quantitative polarization analysis.For the air/neat DMSO liquid interface,the transition dipole moment of the S=O group of DMSO is oriented 1400 from the surface normal.The S=O group of DMSO is estimated to be 300 from the surface plane and the orientation of the S=O group of interfacial DMSO is not dependent on the bulk DMSO concentration.Furthermore,the concentration-dependent SHG signal reveals that the antiparallel double layer structure does not form at the air/DMSO water interface.The free energy of adsorption of DMSO at the air/DMSO aqueous solution interface was determined to be ?Gads=-5.6±0.4 kJ/mol.Our experiment results revealed that DMSO is unlikely to associate with itself compared with other polar molecules.In the third section,the ultrafast spectroscopy experimental device that have been built in the laboratory is introduced,including:polarization selective pump probe experiment(including mid-IR pump-mid-IR probe experiment and visible pump-mid-IR probe experiment),two-dimensional infrared experiment and sum-frequency generation.We mainly described the instrument function,the optical path setting,the signal optimization and the signal acquisition.