First-Principles Study Of The Structures And Infrared Spectra Of Hydrated Ion Clusters

As an important hydrogen bonding system and solvent,water is ubiquitous in nature.Water is a polar molecule that can form hydrated ion clusters with many ions.Hydrated ion clusters widely exist in solution,atmosphere,and organism.Especially,hydrated ion clusters are the most important composion of haze.The study of hydrated ion clusters helps understand the microscopic mechanisms of many physical,chemical,and biological processes as salt dissolution,metal corrosion,smog formation,electrochemical reactions,ion transfer in the body,seawater desalination,and so on.To confirm the stable structures of hydrated ion clusters is crucial for the study of properties since the property of hydrated ion clusters varies with the structure of cluster.In this work,we used genetic algorithm(GA)combined with first-principles study to search the ground state structures of protonated water clusters and fluorine anion water clusters.Then we investigate the electronic,energitic,and vibrational properties of the ground state structures.The pH of solution is related to the concentration of H+.H+ interacts with water molecules to form a protonated water cluster.The study of protonated water clusters helps understand the pH and proton transport behavior of solution or material.The potential energy surface of small-sized protonated water clusters is simple.Therefore,the lowest-energy structures of small-sized protonated water clusters obtained from GA are same with the previous results.Using high-level CCSD(T)and MP2 results and experimental values as benchmark,we comprehensively evaluated the performance of seven density functional theory methods to study the geometries,vibrational frequencies,interaction energies,and relative energies of isomers of small-sized protonated water clusters H+(H2O)2-9,12.The results reveal that,among the seven methods,X3LYP is the best to describe the interaction energies,PBEO gives the best anharmonic frequencies,PBEIW,B3LYP,B97-D,and X3LYP can yield better geometries,the capability of B97-D to distinguish the relative energies between isomers is the best.Based on the results of small-sized protonated water clusters,we used GA combined with first-principles to study the structures and infrared(IR)spectra of medium-sized protonated water clusters H+(H2O)10-17.All the low-lying structures of H+(H2O)10-17 are cage-like structures.Except for the lowest-energy structure of H+(H2O)17 is a core-shell structure with a water molecule in the center,all the lowest-energy structures are monocage structures with high symmetry of oxygen framework.The simulated IR spectra with anharmonic correction reproduce the experimental results well,confirming that GA is capable for explore the structures of protonated water clusters and anharmonic correction is necessary for the vibrational property of protonated water clusters.Meanwhile,our results provide a reliable theoretical basis for the experimental study of the IR spectra of protonated water clusters.Fluoride ion is closely related to human activities since it widely exists in drinking water,air,and various industrial wastewaters.There are strong hydrogen bond interactions between fluoride ion and water molecules.Based on the first-principles methods,we studied the interaction between fluoride ion and water molecules.We used GA combined with density functional theory to search the potential energy surface of fluorine anion water clusters F-(H2O)1-10;and calculated the geometries,stabilization energies,and IR spectra with anharmonic correction of the ground state structures.The F-(H2O)n clusters tend to adopt the structures in which the F-anion remains at the surface of the cluster and coordinates with four water molecules.The strong interaction between the F-anion and adjacent H2O molecule leads to a longer O-H distance in the adjacent molecule and results in a large redshift of the adjacent O-H stretching mode.Natural bond orbital(NBO)analysis of the lowest-energy structures of the F(H2O)1-10 clusters illustrates that the charge transfer from the lone pair electron orbital of F-to the antibonding orbital of the adjacent O-H is mainly responsible for the strong interaction between the F-anion and water molecules.In this thesis,the ground state structures and IR spectra of protonated water clusters and fluorine anion water clusters are investigated comprehensively,which provide reliable microscopic models for the processes of ions in physical,chemical and biological processes.Our results provide a deep insight for the mechanism of interaction between ions and water molecules and give a reliable explanation of experimental IR spectra.